![1.12 POWER GENERATION
Writing an energy balance across the evaporator neglecting heat and blowdown
losses, we get: (T1 ⫺ T2)/(T1 ⫺ T3) ⫽ (hs ⫺ hw )/ ⫽ X, where T1 ⫽ gas
(h ⫺ h )
s ƒ
temperature in boiler, ⬚F (⬚C); T2 ⫽ pinch-point gas temperature, ⬚F (⬚C); T3 ⫽
outlet gas temperature for TEG, ⬚F (⬚C); enthalpy, h, values as listed above; X ⫽
ratio of temperature or enthalpy differences. Substituting, X ⫽ (1199.6 ⫺ 342)/
(1199.9 ⫺ 196.3) ⫽ 0.855, using enthalpy values as given above.
The critical gas inlet-temperature, T1c ⫽ (ts ⫺ /(1 ⫺ X), where ts ⫽ tem-
Xt )
ƒ
perature of saturated steam, ⬚F (⬚C); ⫽ temperature of feedwater, ⬚F (⬚C); other
tƒ
symbols as before. Using the values determined above, T1c ⫽ [390 ⫺
(0.855)(227)]/(1 ⫺ 0.855) ⫽ 1351⬚F (732.8⬚C).
2. Determine the system pinch point and gas/steam profile
Up to a gas inlet temperature of approximately 1351⬚F (732.8⬚C), the pinch point
can be arbitrarily selected. Beyond this, the feedwater inlet temperature limits the
temperature profile. Let’s then select a pinch point of 25⬚F (13.9⬚C), Fig. 4. Then,
T2, the gas-turbine gas temperature at the pinch point, ⬚F (⬚C) ⫽ t ⫹ pinch-point
ƒ
temperature difference, or 390⬚F ⫹ 25⬚F ⫽ 415⬚F (212.8⬚C).
Setting up an energy balance across the evaporator, assuming a heat loss of 2
percent and a blowdown of 3 percent, leads to: (1 ⫺ heat loss)(TEG
Q ⫽ W
evap e
heat capacity, Btu/⬚F) (T1 ⫺ T2), where We ⫽ TEG flow, lb/h; heat capacity of
TEG ⫽ 0.27 Btu/⬚F; T1 ⫽ TEG inlet temperature, ⬚F (⬚C). Substituting, ⫽
Qevap
150,000(0.98)(0.27)(950 ⫺ 415) ⫽ 21.23 ⫻ 106
Btu/h (6.22 MW).
The rate of steam generation, ⫹ blowdown percent ⫻
W ⫽ Q /[(h ⫺ h )
s evap s w
(hl ⫺ hw )], where the symbols are as given earlier. Substituting, Ws ⫽ 21.23 ⫻
106
/[(1199.6 ⫺ 342) ⫹ 0.03 ⫻ (364 ⫺ 342)] ⫽ 24,736 lb/h (11,230 kg/h).
Determine the boiler economizer duty from ⫽ (1 ⫹ blowdown)(Ws )
Qecon
where symbols are as before. Substituting, ⫽ 1.03(24,736)(342 ⫺
(h ⫺ h ), Q
w ƒ econ
196.3) ⫽ 3.71 ⫻ 106
Btu/h (1.09 MW).
The gas exit-temperature, T3 ⫽ T2 ⫺ /TEG gas flow, lb/h)(1 ⫺ heat
Qecon
loss)(heat capacity, Btu/lb ⬚F). Since all values are known, T3 ⫽ 415 ⫺ 3.71 ⫻
106
/(150,000 ⫻ 0.98 ⫻ 0.27) ⫽ 317⬚F (158⬚C). Figure 4 shows the temperature
profile for this installation.
Related Calculations. Use this procedure for heat-recovery boilers fired by
gas-turbine exhaust in any industry or utility application. Such boilers may be un-
fired, supplementary fired, or exhaust fired, depending on steam requirements.
Typically, the gas pressure drop across the boiler system ranges from 6 to 12 in
(15.2 to 30.5 cm) of water. There is an important tradeoff: a lower pressure drop
means the gas-turbine power output will be higher, while the boiler surface and the
capital cost will be higher, and vice versa. Generally, a lower gas pressure drop
offers a quick payback time.
If ⌬Pe is the additional gas pressure in the system, the power, kW, consumed in
overcoming this loss can be shown approximately from P ⫽ 5 ⫻ 10⫺8
(We ⌬Pe T
/E, where E ⫽ efficiency of compression).
To show the application of this equation and the related payback period, assume
We ⫽ 150,000 lb/g (68,100 kg/h), T ⫽ 1000⬚R (average gas temperature in the
boiler, ⌬Pe ⫽ 4 in water (10.2 cm), and E ⫽ 0.7. Then P ⫽ 5 ⫻ 10⫺8
(150,000 ⫻
4 ⫻ 1000/0.7) ⫽ 42 kW.
If the gas turbine output is 4000 kW, nearly 1 percent of the power is lost due
to the 4-in (10.2-cm) pressure drop. If electricity costs 7 cent/kWh, and the gas
turbine runs 8000 h/yr, the annual loss will be 8000 ⫻ 0.07 ⫻ 42 ⫽ $23,520. If
the incremental cost of a boiler having a 4-in (10.2-cm) lower pressure drop is, say
$22,000, the payback period is about one year.
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![1.14 POWER GENERATION
FIGURE 6 Ideal gas-turbine cycle, 1-2-3-4-1. Actual compression takes place along 1-2⬘; actual
heat added 2⬘-3⬘; ideal expansion 3⬘-4⬘.
to 56 lb/in2
(abs) (385.8 kPa); the isentropic efficiency of the compressor is 82
percent. Sufficient fuel is injected to give the mixture of fuel vapor and air a heating
value of 200 Btu/lb (466 kJ/kg). Assume complete combustion of the fuel. The
expander reduces the flow pressure to 14.9 lb/in2
(abs), with an engine efficiency
of 85 percent. Assuming that the combustion products have the same thermody-
namic properties as air, cp ⫽ 0.24, and is constant. The isentropic exponent may
be taken as 1.4. (a) Find the temperature after compression, after combustion, and
at the exhaust. (b) Determine the Btu/lb (kJ/kg) of air supplied, the work delivered
by the expander, the net work produced by the gas turbine, and its thermal effi-
ciency.
Calculation Procedure:
1. Plot the ideal and actual cycles
Draw the ideal cycle as 1-2-3-4-1, Figs. 6 and 7. Actual compression takes place
along 1-2⬘. Actual heat added lies along 2⬘-3⬘. The ideal expansion process path is
3⬘-4⬘. Ideal work ⫽ cp (ideal temperature difference). Actual work ⫽ cp (actual
temperature difference).
2. Find the temperature after compression
Use the relation (T2 /T1) ⫽ where T1 ⫽ entering air temperature, ⬚R;
(k⫺1) / k
(P /P ) ,
2 1
T2 ⫽ temperature after adiabatic compression, ⬚R; P1 ⫽ entering air pressure, in
units given above; P2 ⫽ pressure after compression, in units given above; k ⫽
isentropic exponent ⫽ 1.4. With an entering air temperature, T1 of 60⬚F (15.6⬚C),
or 60 ⫹ 460 ⫽ 520⬚R, and using the data given, ⫽
(1.4⫺1) / 1.4
T ⫽ 520[(56/14)]
2
772.7⬚R, or 772.7 ⫺ 520 ⫽ 252.7⬚F (122.6⬚C).
(a) Here we have isentropic compression in the compressor with an effi-
ciency of 85 percent. Using the equation, Efficiency, isentropic ⫽ (cp )(T2 ⫺ T1)/
(cp ) and solve for the temperature after isentropic compression. Solv-
(T ⫺ T ), T ,
2⬘ 1 2⬘
ing, ⫽ 0.82 ⫽ 0.24(772.7 ⫺ 520)/0.24 ⫽ 828.4⬚R, or 368⬚F. This
T (T ⫺ 520)
2⬘ 2⬘
is the temperature after compression.
3. Determine the temperature after combustion
To find the temperature after combustion, use the relation Heating value of fuel ⫽
Q ⫽ cp where ⫽ temperature after combustion, ⬚R. Substituting,
(T ⫺ T ), T
3⬘ 2⬘ 3⬘
200 ⫽ 0.24 Solving, ⫽ 1661.3⬚R; 1201.3⬚F (649.6⬚C).
(T ⫺ 828). T
3⬘ 3⬘
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.15
FIGURE 7 Ideal gas-turbine cycle T-S diagram with the same processes as in Fig. 6; complete-
cycle gas turbine shown below the T-S diagram.
4. Find the temperature at the exhaust of the gas turbine
Using an approach similar to that above, determine T4 from ⫽
(T /T )
4⬘ 3⬘
Substituting and solving for ⫽ 1661 ⫽
k⫺1 / k. (1.4⫺1) / 1.4
[(P /P )] T [(14.9/56)]
4⬘ 3⬘ 4⬘
1137.9⬚R, or 677.8⬚F (358.8⬚C).
Now use the equation for gas-turbine efficiency, namely, Turbine efficiency ⫽
cp ⫽ 0.85, and solve for the temperature after expan-
(T ⫺ T )/c (T ⫺ T ) T ,
3⬘ 4ⴖ p 3⬘ 4⬘ 4ⴖ
sion, at the exhaust. Substituting as earlier, ⫽ 1218.2⬚R, 758.2⬚F (403.4⬚C). This
T4ⴖ
is the temperature after expansion, i.e., at the exhaust of the gas turbine.
5. Determine the work of compression, expander work, and thermal efficiency
(b) The work of compression ⫽ cp ⫽ 0.24(828 ⫺ 520) ⫽ 74.16 Btu (78.23
(T ⫺ T )
2⬘ 1
J).
The work delivered by the expander ⫽ cp ⫽ 0.24 (1661 ⫺ 1218) ⫽
(T ⫺ T )
2⬘ 1
106.32 Btu (112.16 J).
The net work ⫽ 106.3 ⫺ 74.2 ⫽ 32.1 Btu (33.86 J). Then, the thermal
efficiency ⫽ net work/heat supplied ⫽ 32.1/200 ⫽ 0.1605, 16.6 percent thermal
efficiency.
Related Calculations. With the widespread use today of gas turbines in a va-
riety of cycles in industrial and central-station plants, it is important that an engineer
be able to analyze this important prime mover. Because gas turbines can be quickly
installed and easily hooked to heat-recovery steam generators (HRSG), they are
more popular than ever before in history.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.19
The life-cycle cost model (CT ) consists of annual investment cost (Cp ) ⫹ annual
fuel cost ⫹ annual maintenance cost (Cm ). Equations for these values are:
(C )
ƒ
⫺n
l{i /[1 ⫺ (1 ⫺ i) ]}
C ⫽
p
(A)(kW)(8760)(G)
where l ⫽ initial capital cost of equipment, dollars
i ⫽ interest rate
n ⫽ number of payment periods
A ⫽ availability (expressed as decimal)
kW ⫽ kilowatts of electricity produced
8760 ⫽ total hours in year
G ⫽ efficiency of electric generator
C ⫽ E(293)
ƒ
where E ⫽ thermal efficiency of gas turbine
293 ⫽ conversion of Btu to kWh
C ⫽ M/kW
m
where M ⫽ maintenance cost, dollars per operating (fired) hour.
Thus, the life-cycle working model can be expressed as
⫺n
l{i/[1 ⫺ (1 ⫺ i) ]}
C ⫽ ⫹ F/E(293) ⫹ M/kW
T
(A)(kW)(8760)(G)
where F ⫽ fuel cost, dollars per million Btu (higher heating value)
To evaluate the comparative capital cost of a gas-turbine electrical generating
package the above model uses the capital-recovery factor technique. This approach
spreads the initial investment and interest costs for the repayment period into an
equal annual expense using the time value of money. The approach also allows for
the comparison of other periodic expenses, like fuel and maintenance costs.
3. Perform the computation for each of the gas turbines being considered
Using the compiled data shown above, compute the values for Cp, and Cm, and
C ,
ƒ
sum the results. List for each of the units as shown below.
Results from Cost Model
Unit Mils/kWh produced
A 48.3
B 47.5
C 48.3
D 46.6
E 51.9
4. Analyze the findings of the life-cycle model
Note that the initial investment cost for the turbines being considered ranges be-
tween $200 and $320/kW. On a $/kW basis, only unit E at the $200 level, would
be considered. However, the life-cycle cost model, above, shows the cost per kWh
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(21,000 kW) ⫽ $974,988; this
was rounded to $975,000. Since the difference in the initial cost between units D
and E is $6,720,000 ⫺ $4,200,000 ⫽ $2,520,000, this cost difference will be re-
covered in $2,520,000/974,988 ⫽ 2.58 years, or about one-eighth of the 20-year
life span of the equipment.
Also, note that the 20-year differential in cost/kWh produced between units D
and E is equivalent to over 4.6 times the initial equipment cost of unit E. When
considering the values output of a life-cycle model, remember that such values are
only as valid as the data input. So take precautions to input both reasonable and
accurate data to the life-cycle cost model. Be careful in attempting to distinguish
model outputs that vary less than 0.5 mil from one another.
Since the predictions of this life-cycle cost model cannot be compared to actual
measurements at this time, a potential shortcoming of the model lies with the va-
lidity of the data and assumptions used for input. For this reason, the model is best
applied to establish comparisons to differentiate between several pieces of com-
peting equipment.
Related Calculations. The first gas turbines to enter industrial service in the
early 1950s represented a blend of steam-turbine and aerothermodynamic design.
In the late 1950s/early-1960s, lightweight industrial gas turbines derived directly
from aircraft engines were introduced into electric power generation, pipeline com-
pression, industrial power generation, and a variety of other applications. These
machines had performance characteristics similar to their steam-turbine counter-
parts, namely pressure ratios of about 12⬊1, firing temperatures of 1200–1500⬚F
(649–816⬚C), and thermal efficiencies in the 23–27 percent range.
In the 1970s, a new breed of aeroderivative gas turbines entered industrial ser-
vice. These units, with simple-cycle thermal efficiencies in the 32–37 percent
bracket, represented a new technological approach to aerothermodynamic design.
Today, these second-generation units are joined by hybrid designs that incor-
porate some of the aeroderivative design advances but still maintain the basic struc-
tural concepts of the heavy-frame machines. These hybrid units are not approaching
the simple-cycle thermal-efficiency levels reached by some of the early second-
generation aeroderivative units first earmarked for industrial use.
Traditionally, the major focus has been on first cost of industrial gas-turbine
units, not on operating cost. Experience with higher-technology equipment, how-
ever, reveals that a low first cost does not mean a lower total cost during the
expected life of the equipment. Conversely, reliable, high-quality equipment with
demonstrated availability will be remembered long after the emotional distress as-
sociated with high initial cost is forgotten.
The life-cycle cost model presented here uses 10 independent variables. A sin-
gle-point solution can easily be obtained, but multiple solutions require repeated
calculations. Although curves depicting simultaneous variations in all variables
would be difficult to interpret, simplified diagrams can be constructed to illustrate
the relative importance of different variables.
Thus, the simplified diagrams shown in Fig. 11, all plot production cost, mils/
kWh, versus investment cost. All the plots are based on continuous operation of
8760 h/yr at 21-MW capacity with an equipment life expectancy of 20 years.
The curves shown depict the variation in production cost of electricity as a
function of initial investment cost for various levels of thermal efficiency, loan
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.23
FIGURE 12 Strouhl number, S, for inline tube banks. Each curve
represents a different longitudinal pitch/diameter ratio (Chen).
2. Find the natural frequency of the tube bundle
Use the relation, ƒn ⫽ 90C[ Substituting, with C ⫽ 22.37,
4 4 2 0.5
d ⫺ d ]/(L ⫺ M ).
o i
ƒn ⫽ (90)(22.37)[24
⫺ 1.844
]0.5
/(13.52
⫺ 1.670.5
) ⫽ 18.2 cycles per second (cps).
In Mode 2, ƒn ⫽ 50.2, as C ⫽ 61.67.
3. Compute the vortex shedding frequency
To compute the vortex shedding frequency we must know several factors, the first
of which is the Strouhl Number, S. Using Fig. 12 with a transverse pitch/diameter
of 1.75 and a longitudinal pitch diameter of 1.5 we find S ⫽ 0.33. Then, the air
density ⫽ 40/(460 ⫺ 219) ⫽ 0.059 lb/ft3
(0.95 kg/m3
); free gas area ⫽ 40(3.5 ⫺
2)(13.5/12) ⫽ 67.5 ft2
(6.3 m2
); gas velocity, V ⫽ 300,000/(67.5)(0.059)(3600) ⫽
21 ft/s (6.4 m/s).
Use the relation, ƒc ⫽ 12(S)(V)/do ⫽ 12(0.33)(21)/2 ⫽ 41.6 cps, where ƒc ⫽
vortex shedding frequency, cps.
4. Determine the acoustic frequency
As with vortex frequency, we must first determine several variables, namely: ab-
solute temperature ⫽ ⬚R ⫽ 219 ⫹ 460 ⫽ 679⬚R; sonic velocity, Vs ⫽ 49(679)0.5
⫽
1277 ft/s (389.2 m/s); wave length, ⫽ 2(w)/n, where w ⫽ width of tube bank,
ft (m); n ⫽ mode of vibration ⫽ 1 for this tube bank; then ⫽ 2(11.7)/1 ⫽ 23.4
ft (7.13 m).
The acoustic frequency, ƒa ⫽ (Vs )/, where Vs ⫽ velocity of sound at the gas
temperature in the duct or shell, ft/s (m/s); Vs ⫽ [(g)()(RT)]0.5
, where R ⫽ gas
constant ⫽ 1546/molecular weight of the gas; T ⫽ gas temperature, ⬚R; ⫽ ratio
of gas specific heats, typically 1.4 for common flue gases; the molecular weight ⫽
29. Simplifying, we get Vs ⫽ 49(T)0.5
, as shown above. Substituting, ƒa ⫽ 1277/
23.4 ⫽ 54.5 cps. For n ⫽ 2; ƒa ⫽ 54.4(2) ⫽ 109 cps. The results for Modes 1 and
2 are summarized in the tabulation below.
Mode of vibration
n
1 2
ƒn, cps 18.2 50.2
ƒc, cps 41.6 41.6
ƒa (without baffles) 54.5 109
ƒa (with baffles) 109 218
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.25
DETERMINING OXYGEN AND FUEL INPUT IN
GAS-TURBINE PLANTS
In a gas-turbine HRSG (heat-recovery steam generator) it is desired to raise the
temperature of 150,000 lb/h (68,100 kg/h) of exhaust gases from 950⬚F (510⬚C)
to 1575⬚F (857.2⬚C) in order to nearly double the output of the HRSG. If the
exhaust gases contain 15 percent oxygen by volume, determine the fuel input and
oxygen consumed, using the gas specific-heat method.
Calculation Procedure:
1. Determine the air equivalent in the exhaust gases
In gas-turbine based cogeneration/combined-cycle projects the HRSG may be fired
to generate more steam than that produced by the gas-turbine exhaust gases. Typ-
ically, the gas-turbine exhaust gas contains 14 to 15 percent oxygen by volume. So
the question arises: How much fuel can be fired to generate more steam? Would
the oxygen in the exhaust gases run out if we fired to a desired temperature? These
questions are addressed in this procedure.
If 0 percent oxygen is available in Wg lb/h (kg/h) of exhaust gases, the air-
equivalent Wa in lb/h (kg/h) is given by: Wa ⫽ 100(Wg )(32Ox )/[23(100)(29.5)] ⫽
0.0417 Wg (O). In this relation, we are converting the oxygen from a volume basis
to a weight basis by multiplying by its molecular weight of 32 and dividing by the
molecular weight of the exhaust gases, namely 29.5. Then multiplying by (100/
23) gives the air equivalent as air contains 23 percent by weight of oxygen.
2. Relate the air required with the fuel fired using the MM Btu (kJ) method
Each MM Btu (kJ) of fuel fired (HHV basis) requires a certain amount of air, A.
If Q ⫽ amount of fuel fired in the turbine exhaust gases on a LHV basis (calcu-
lations for turbine exhaust gases fuel input are done on a low-heating-value basis)
then the fuel fired in lb/h (kJ/h) ⫽ W ⫽ Q /LHV.
ƒ
The heat input on an HHV basis ⫽ (HHV)/(106
) ⫽ (Q /LHV)(HHV)/106
Wƒ
Btu/h (kJ/h). Air required lb/h (kg/h) ⫽ (Q /LHV)(HHV)(A), using the MM Btu,
where A ⫽ amount of air required, lb (kg) per MM Btu (kJ) fired. The above
quantity ⫽ air available in the exhaust gases, Wa ⫽ 0.0417 Wg (O).
3. Simplify the gas relations further
From the data in step 2, (Q /LHV)(HHV)(A)/106
⫽ 0.0417 Wg (O). For natural gas
and fuel oils it can be shown that (LHV/Ax HHV) ⫽ 0.00124. For example, LHV
of methane ⫽ 21,520 Btu/lb (50,055.5 kJ/kg); HHV ⫽ 23,879 Btu/lb (55,542.6
kJ/kg), and A ⫽ 730 lb (331.4 kg). Hence, (LHV/Ax HHV) ⫽ 21,520/(730 ⫻
23,879) ⫽ 0.00124. By substituting in the equation in step 1, we have Q ⫽ 58.4
(Wg )(O). This is an important equation because it relates the oxygen consumption
from the exhaust gases to the burner fuel consumption.
4. Find the fuel input to the HRSG
The fuel input is given by Wg ⫹ hg 1 ⫹ Q ⫽ (Wg ⫹ )(hg 2), where hg 1 and hg 2
Wƒ
are the enthalpies of the exhaust gas before and after the fuel burner; ⫽ fuel
Wƒ
input, lb/h (kg/h); Q ⫽ fuel input in Btu/h (kJ/h).
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.29
HRSGs operate at different exhaust-gas conditions. For example, variations in
ambient temperature or gas-turbine load affect exhaust-gas flow and temperature.
This, in turn, affects HRSG performance, temperature profiles, efficiency, and steam
generation. The tool consultants use for evaluating HRSG performance under dif-
ferent operating conditions is simulation. With this tool you can: (1) Predict off-
design performance of an HRSG; (2) Predict auxiliary fuel consumption for periods
when the gas-turbine exhaust-gas flow is insufficient to generate the required steam
flow; (3) Evaluate options for improving an HRSG system; (4) Evaluate field data
for validating an HRSG design; (5) Evaluate different HRSG configurations for
maximizing efficiency.
In this HRSG, using steam-table data, the saturation temperature of 200-lb/in2
(gage) (1378-kPa) steam ⫽ 388⬚F (197.8⬚C). The gas temperature leaving the evap-
orator with the 20⬚F (11.1⬚C) pinch point ⫽ 388 ⫹ 20 ⫽ 408⬚F (208.9⬚C). Water
temperature entering the evaporator ⫽ saturated-steam temperature ⫺ the approach
point temperature difference, or 388 ⫺ 15 ⫽ 373⬚F (189.4⬚C).
Then, the energy absorbed by the evaporator, Q1 ⫽ (gas flow, lb/h)(1.0 ⫺ heat
loss)(gas specific heat, Btu/lb ⬚F)(gas-turbine exhaust gas HRSG entering temper-
ature, ⬚F ⫺ gas temperature leaving evaporator, ⬚F). Or, Q1 ⫽
(140,000)(0.99)(0.27)(980 ⫺ 408) ⫽ 21.4 MM Btu/h (6.26 MW). The enthalpy
absorbed by the steam in the evaporator, Btu/lb (kJ/kg) ⫽ (enthalpy of the saturated
steam in the HRSG outlet ⫺ enthalpy of the feedwater entering the evaporator at
373⬚F) ⫹ (blowdown percentage)(enthalpy of the saturated liquid of the outlet
steam ⫺ enthalpy of the water entering the evaporator, all in Btu/lb). Or, enthalpy
absorbed in the evaporator ⫽ (1199.3 ⫺ 345) ⫹ (0.05)(362.2 ⫺ 345) ⫽ 855.2
Btu/lb (1992.6 kJ/kg). The quantity of steam generated ⫽ (Q1, energy absorbed
by the evaporator, Btu/h)/(enthalpy absorbed by the steam in the evaporator, Btu
/lb) ⫽ (21.4 ⫻ 106
)/855.2 ⫽ 25,023 lb/h (11,360 kg/h).
2. Determine the economizer duty and exit gas temperature
The economizer duty ⫽ (steam generated, lb/h)(enthalpy of water entering the
economizer, Btu/lb ⫺ enthalpy ⫺ enthalpy of the feedwater at 230⬚F, Btu/lb)(1 ⫹
blowdown percentage) ⫽ (25,023)(345 ⫺ 198.5)(1.05) ⫽ 3.849 MM Btu/h (1.12
MW).
The gas temperature drop through the economizer ⫽ (economizer duty)/(gas
flow rate, lb/h)(1 ⫺ heat loss percentage)(specific heat of gas, Btu/lb ⬚F) ⫽
(3.849 ⫻ 106
)/(140,000)(0.99)(0.253) ⫽ 109.8⬚F (60.9⬚C). Hence, the exit-gas tem-
perature from the economizer ⫽ (steam saturation temperature, ⬚F ⫺ exit-gas tem-
perature from the economizer, ⬚F) ⫽ (408 ⫺ 109) ⫽ 299⬚F (148.3⬚C).
3. Calculate the constant K for evaporator performance
In simulating evaporator performance the constant K1 is used to compute revised
performance under differing flow conditions. In equation form, K1 ⫽
ln[(temperature of gas-turbine exhaust gas entering the HRSG, ⬚F ⫺ HRSG satu-
rated steam temperature, ⬚F)/(gas temperature leaving the evaporator, ⬚F ⫺ HRSG
saturate steam temperature, ⬚F)]/(gas flow, lb/h). Substituting, K1 ⫽ ln[(980 ⫺
388)/(408 ⫺ 388)]/140,000 ⫽ 387.6, where the temperatures used reflect design
condition.
4. Compute the revised evaporator performance
Under the revised performance conditions, using the given data and the above value
of K1 and solving for Tg 2, the evaporator exit gas temperature, ln[(880 ⫺ 366)/
(Tg 2 ⫺ 366)] ⫽ 387.6 Tg 2 ⫽ 388⬚F (197.8⬚C). Then, the evaporator
⫺0.4
(165,000) ;
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![1.30 POWER GENERATION
duty, using the same equation as in step 1 above ⫽ (165,000)(0.99)(0.27)(880 ⫺
388) ⫽ 21.7 MM Btu/h (6.36 MW).
In this calculation, we assumed that the exhaust-gas analysis had not changed.
If there are changes in the exhaust-gas analysis, then the gas properties must be
evaluated and corrections made for variations in the exhaust-gas temperature. See
Waste Heat Boiler Deskbook by V. Ganapathy for ways to do this.
5. Find the assumed duty, Qa, for the economizer
Let the economizer leaving-water temperature ⫽ 360⬚F (182.2⬚C). The enthalpy of
the feedwater ⫽ 332 Btu/lb (773.6 kJ/kg); saturated-steam enthalpy ⫽ 1195.7 Btu/
lb (2785.9 kJ/kg); saturated liquid enthalpy ⫽ 338.5 Btu/lb (788.7 kJ/kg). Then,
the steam flow, as before, ⫽ (21.5 ⫻ 106
)/[(1195.7 ⫺ 332) ⫹ 0.05 (338.5 ⫺
332)] ⫽ 25,115.7 lb/h (11,043 kg/h). Then, the assumed duty for the economizer,
Qa ⫽ (25,115.7)(1.05)(332 ⫺ 198.5) ⫽ 3.52 MM Btu/h (1.03 MW).
6. Determine the UA value for the economizer in both design and off-design
conditions
For the design conditions, UA ⫽ Q /(⌬T), where Q ⫽ economizer duty from step
2, above; ⌬T ⫽ design temperature conditions from the earlier data in this proce-
dure. Solving, UA ⫽ (3.84 ⫻ 106
)/{[(299 ⫺ 230) ⫺ (408 ⫺ 373)]/ln(69/35)} ⫽
76,800 Btu/h ⬚F (40.5 kW). For off-design conditions, UA ⫽ (UA at design con-
ditions)(gas flow at off-design/gas flow at design conditions) ⫽
0.65
(76,800)(165,000/140,000) ⫽ 85,456 Btu/h F (45.1 kW).
0.65
7. Calculate the economizer duty
The energy transferred ⫽ Qt ⫽ (UA)(⌬T). Based on 360⬚F (182.2⬚C) water leaving
the economizer, Qa ⫽ 3.52 MM Btu/h (1.03 MW). Solving for tg 2 as before ⫽
382 ⫺ [(3.52 ⫻ 106
)/(165,000)(0.9)(0.253)] ⫽ 388 ⫺ 85 ⫽ 303⬚F (150.6⬚C). Then,
⌬T ⫽ [(303 ⫺ 230) ⫺ (388 ⫺ 360)]/ln(73/28) ⫽ 47⬚F (26.1⬚C). The energy
transferred ⫽ Qt ⫽ (UA)(⌬T) ⫽ (85,456)(47) ⫽ 4.01 MM Btu/h (1.18 MW).
Since the assumed and transferred duty do not match, i.e., 3.52 MM Btu/h vs.
4.01 MM Btu/h, another iteration is required. Continued iteration will show that
when Qa ⫽ Qt ⫽ 3.55 MM Btu/h (1.04 MW), and the temperature of the water
leaving the economizer ⫽ 366⬚F (185.6⬚C) (saturation) and exit-gas temperature ⫽
301⬚F (149.4⬚C), the amount of steam generated ⫽ 25,310 lb/h (11,491 kg/h).
Related Calculations. Studying the effect of gas inlet temperature and gas
flows on HRSG performance will show that at lower steam generation rates or at
lower pressures that the economizer water temperature approaches saturation tem-
perature, a situation called ‘‘steaming’’ in the economizer. This steaming condition
should be avoided by generating more steam by increasing the inlet gas temperature
or through supplementary firing, or by reducing exhaust-gas flow.
Supplementary firing in an HRSG also improves the efficiency of the HRSG in
two ways: (1) The economizer acts as a bigger heat sink as more steam and hence
more feedwater flows through the economizer. This reduces the exit gas tempera-
ture. So with a higher gas inlet temperature to the HRSG we have a lower exit gas
temperature, thanks to the economizer. (2) Additional fuel burned in the HRSG
reduces the excess air as more air is not added; instead, the excess oxygen is used.
In conventional boilers we know that the higher the excess air, the lower the boiler
efficiency. Similarly, in the HRSG, the efficiency increases with more supplemen-
tary firing. HRSGs used in combined-cycle steam cycles, Fig. 17, may use multiple
pressure levels, gas-turbine steam injection, reheat, selective-catalytic-reduction
(SCR) elements for NOx control, and feedwater heating. Such HRSGs require ex-
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.35
TABLE 3 HRSG Exit Gas Temperatures Versus Steam Parameters*
Pressure
lb/in2
(gage) (kPa)
Steam temp
⬚F (⬚C)
Saturation temp
⬚F (⬚C)
Exit gas
⬚F (⬚C)
100 (689) sat (170) 338 (170) 300 (149)
150 (1034) sat (186) 366 (186) 313 (156)
250 (1723) sat (208) 406 (208) 332 (167)
400 (2756) sat (231) 448 (231) 353 (178)
400 (2756) 600 (316) 450 (232) 367 (186)
600 (4134) sat (254) 490 (254) 373 (189)
600 (4134) 750 (399) 492 (256) 398 (203)
*Pinch point ⫽ 20⬚F (11.1⬚C); approach ⫽ 15⬚F (8.3⬚C); gas inlet temperature ⫽ 900⬚F (482.2⬚C);
blowdown ⫽ 0; feedwater temperature ⫽ 230⬚F (110⬚C).
5. Determine the economizer duty and exit-gas temperature
The economizer duty, Q3 ⫽ (rate of steam generation, lb/h)(1 ⫹ blowdown ex-
pressed as a decimal)(enthalpy of water leaving the economizer ⫺ enthalpy of
feedwater at 240⬚F) ⫽ (18,502)(1.02)(431.2 ⫺ 209.6) ⫽ 4.182 MM Btu/h (1.225
MW).
The HRSG exit gas temperature ⫽ (480, the exit gas temperature at the evap-
orator computed in step 1, above) ⫺ (economizer duty)/(gas-turbine exhaust-gas
flow, lb/h)(1.0 ⫺ heat loss)(exhaust gas specific heat) ⫽ 371.73⬚F (188.9⬚C); round
to 372⬚F (188.9⬚C). Note that you must compute the gas specific heat at the average
gas temperature of each of the heat-transmission surfaces.
6. Compute the ASME HRSG efficiency
The ASME Power Test Code PTC 4.4 defines the efficiency of an HRSG as: E ⫽
efficiency ⫽ (energy absorbed by the steam and fluids)/[gas flow ⫻ inlet
enthalpy ⫹ fuel input to HRSG on LHV basis]. In the above case, E ⫽ (16.65 ⫹
4.182)(106
)/(150,000 ⫻ 220) ⫽ 0.63, or 63 percent. In this computation, 220 Btu
/lb (512.6 kJ/kg) is the enthalpy of the exhaust gas at 900⬚F (482.2⬚C) and
(16.65 ⫹ 4.182) is the total energy absorbed by the steam in MM Btu/h (MW).
Related Calculations. Note that the exit gas temperature is high. Further, with-
out having done this analytical mathematical analysis, the results could not have
been guessed correctly. Minor variations in the efficiency will result if one assumes
different pinch and approach points. Hence, it is obvious that one cannot assume a
value for the exit gas temperature—say 300⬚F (148.9⬚C)—and compute the steam
generation.
The gas/steam temperature profile is also dependent on the steam pressure and
steam temperature. The higher the steam temperature, the lower the steam gener-
ation rate and the higher the exit gas temperature. Arbitrary assumption of the exit
gas temperature or pinch point can lead to temperature cross situations. Table 3
shows the exit gas temperatures for several different steam parameters. From the
table, it can be seen that the higher the steam pressure, the higher the saturation
temperature, and hence, the higher the exit gas temperature. Also, the higher the
steam temperature, the higher the exit gas temperature. This results from the re-
duced steam generation, resulting in a smaller heat sink at the economizer.
This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO
Industries, who is the author of several works listed in the references for this sec-
tion.
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![1.36 POWER GENERATION
FIGURE 19 T-S diagrams
for steam turbine.
STEAM TURBOGENERATOR EFFICIENCY AND
STEAM RATE
A 20,000-kW turbogenerator is supplied with steam at 300 lb/in2
(abs) (2067.0
kPa) and a temperature of 650⬚F (343.3⬚C). The backpressure is 1 in (2.54 cm) Hg
absolute. At best efficiency, the steam rate is 10 lb (25.4 kg) per kWh. (a) What is
the combined thermal efficiency (CTE) of this unit? (b) What is the combined
engine efficiency (CEE)? (c) What is the ideal steam rate?
Calculation Procedure:
1. Determine the combined thermal efficiency
(a) Combined thermal efficiency, CTE ⫽ (3413/wr )(1/[h1 ⫺ h2]), where wr ⫽
combined steam rate, lb/kWh (kg/kWh); h1 ⫽ enthalpy of steam at throttle pressure
and temperature, Btu/lb (kJ/kg); h2 ⫽ enthalpy of steam at the turbine backpres-
sure, Btu/lb (kJ/kg). Using the steam tables and Mollier chart and substituting in
this equation, CTE ⫽ (3413/10)(1/[1340.6 ⫺ 47.06]) ⫽ 0.2638, or 26.38 percent.
2. Find the combined engine efficiency
(b) Combined engine efficiency, CEE ⫽ (wi )/(we ) ⫽ (weight of steam used by
ideal engine, lb/kWh)/(weight of steam used by actual engine, lb/kWh). The
weights of steam used may also be expressed as Btu/lb (kJ/kg). Thus, for the ideal
engine, the value is 3413 Btu/lb (7952.3 kJ/kg). For the actual turbine, h1 ⫺ h2x
is used, h2x is the enthalpy of the wet steam at exhaust conditions; h1 is as before.
Since the steam expands isentropically into the wet region below the dome of
the T-S diagram, Fig. 19, we must first determine the quality of the steam at point
2 either from a T-S diagram or Mollier chart or by calculation. By calculation using
the method of mixtures and the entropy at each point: S1 ⫽ S2 ⫽ 0.0914 ⫹
(x2)(1.9451). Then x2 ⫽ (1.6508 ⫺ 0.0914)/1.9451 ⫽ 0.80, or 80 percent quality.
Substituting and summing, using steam-table values, h2x ⫽ 47.06 ⫹ 0.8(1047.8) ⫽
885.3 Btu/lb (2062.7 kJ/kg).
(c) To find the CEE we first must obtain the ideal steam rate, wi ⫽ 3413/(h1 ⫺
h2x ) ⫽ 3413/(1340.6 ⫺ 885.3) ⫽ 7.496 lb/kWh (3.4 kg/kWh).
Now, CEE ⫽ (7.496/10)(100) ⫽ 74.96 percent. This value is excellent for such
a plant and is in a range being achieved today.
Related Calculations. Use this approach to analyze the efficiency of any tur-
bogenerator used in central-station, industrial, marine, and other plants.
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![1.44 POWER GENERATION
(a) (b)
356,000 lb/hr (161,624 kg/hr) 194,000 lb/hr (88,076 kg/hr)
FIGURE 28 (a) Straight-line rating characteristic. (b) T-S diagram.
wk ⫽ the actual steam rate, lb/kWh (kg/kWh) ⫽ w ⫽ ws /kW output, where the
symbols are as defined earlier.
Substituting ws ⫽ 32,000 no-load throttle flow ⫹ (difference between full-load
and half-load throttle flow rate/kW output at half load)(kW output) ⫽ 32,000 ⫹
(162,000/20,000)(kW) ⫽ 32,000 ⫹ 8.1 kW for this turbine-generator set. Also
wk ⫽ (32,000/kW) ⫹ 8.1.
Using the steam tables, we find H1 ⫽ 1398 Btu/lb (3257.3 kJ/kg); ⫽ 83
Hƒ
Btu/lb (193.4 kJ/kg). Then, H1 ⫺ ⫽ 1315 Btu/lb (3063.9 kJ/kg). Substituting,
Hƒ
heat rate ⫽ [(1315)(32,000)/(kW)] ⫹ (1316)(8.1) ⫽ 10.651.5 ⫹ (42,080,000/kW).
Computing the steam rate and heat rate for the quarter-load points for this
turbine-generator we find:
At full load, w ⫽ 8.9 lb/kWh (4.04 kg/kWh)
s
3
At ⁄4 load, w ⫽ 9.17 lb/kWh (4.16 kg/kWh)
s
1
At ⁄2 load, w ⫽ 9.7 lb/kWh (4.4 kg/kWh)
s
1
At ⁄4 load, w ⫽ 11.3 lb/kWh (5.13 kg/kWh)
s
At full load, heat rate ⫽ 11,700 Btu/kWh (27,261 kJ/kWh)
3
At ⁄4 load, heat rate ⫽ 12,080 Btu/kWh (28,146 kJ/kWh)
1
At ⁄2 load, heat rate ⫽ 12,770 Btu/kWh (29,754 kJ/kWh)
1
At ⁄4 load, heat rate ⫽ 14,870 Btu/kWh (34,647 kJ/kWh)
3. Determine the internal steam rate of the turbine
(c) For the turbine and generator combined, Ee ⫽ 3413/(wk)(H1 ⫺ Hc), where Ee
⫽ turbine engine efficiency; Hc ⫽ enthalpy of the steam at the condenser; other
symbols as given earlier. Since, from the steam tables, H1 ⫽ 1398 Btu/lb (3257.3
kJ/kg); Hc ⫽ 912 Btu/lb (2124.9 kJ/kg); then (H1 ⫺ Hc) ⫽ 486 Btu/lb (1132.4
kJ/kg).
From earlier steps, ws ⫽ 356,000 lb/h (161,624 kg/h) at full-load; ws ⫽ 32,000
lb/h (14,528 kg/h) at no-load. For the full-load range the total change is 356,000
⫺ 32,000 ⫽ 324,000 lb/h (147,096 kg/h). Then, ws at 30 percent load ⫽ [(32,000)
⫹ 0.30(324,000)]/0.30(40,000) ⫽ 10.77 lb/kWh (4.88 kg/kWh). Then, Ee ⫽
3413/(10.77)(486) ⫽ 0.652 for combined turbine and generator.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.47
DETERMINING TURBOGENERATOR STEAM RATE
AT VARIOUS LOADS
A 100-MW turbogenerator is supplied steam at 1250 lb/in2
(abs) (8612.5 kPa) and
1000⬚F (537.8⬚C) with a condenser pressure of 2 in (5.08 cm) Hg (abs). At rated
load, the turbine uses 1,000,000 lb (454,000 kg) of steam per hour; at zero load,
steam flow is 50,000 lb/h (22,700 kg/h). What is the steam rate in pounds (kg)
per kWh at 4
⁄4, 3
⁄4, 2
⁄4, and 1
⁄4 load?
Calculation Procedure:
1. Write the steam-flow equation for this turbogenerator
The curve of steam consumption, called the Willian’s line, is practically a straight
line for steam turbines operating without overloads. Hence, we can assume a
straight line for this turbogenerator. If the Willian’s line is extended to intercept the
Y (vertical) axis for total steam flow per hour, this intercept represents the steam
required to operate the turbine when delivering no power. This no-load steam
flow—50,000 lb/h (22,700 kg/h) for this turbine—is the flow rate required to
overcome the friction of the turbine and the windage, governor and oil-pump drive
power, etc., and for meeting the losses caused by turbulence, leakage, and radiation
under no-load conditions.
Using the data provided, the steam rate equation can be written as [(50/L) ⫹
9.5] ⫽ (F/L) ⫽ [50 ⫹ (1000 ⫺ 50)/100(L)]/(L), where F ⫽ full-load steam flow,
lb/h (kg/h); L ⫽ load percent.
2. Compute the steam flow at various loads
Use the equation above thus:
Load fraction Load, MW
Steam rate
lb/kWh (kg/kWh)
1
⁄4 100 ⫻ 1
⁄4 ⫽ 25 50
⁄25 ⫹ 9.5 ⫽ 11.5 (5.22)
2
⁄4 100 ⫻ 2
⁄4 ⫽ 50 50
⁄50 ⫹ 9.5 ⫽ 10.5 (4.77)
3
⁄4 100 ⫻ 3
⁄4 ⫽ 75 50
⁄75 ⫹ 9.5 ⫽ 10.17 (4.62)
4
⁄4 100 ⫻ 4
⁄4 ⫽ 100 50
⁄100 ⫹ 9.5 ⫽ 10.00 (4.54)
Related Calculations. The Willian’s line is a useful tool for analyzing steam-
turbine steam requirements. As a check on its validity, compare actual turbine
performance steam conditions with those computed using this procedure. The agree-
ment is startlingly accurate.
ANALYSIS OF REHEATING-REGENERATIVE
TURBINE CYCLE
An industrial turbogenerator operates on the reheating-regenerative cycle with one
reheat and one regenerative feedwater heater. Throttle steam at 400 lb/in2
(abs)
(2756 kPa) and 700⬚F (371⬚C) is used. Exhaust is at 2 in (5.1 cm) Hg (abs). Steam
is taken from the turbine at a pressure of 63 lb/in2
(abs) (434 kPa) for both reheating
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![1.48 POWER GENERATION
(a) (b)
400 psia (2756 kPa) 700˚F (371˚C) 63 psia (434 kPa)
2 in. Hg (5.1 cm Hg)
FIGURE 30 (a) Cycle diagram. (b) T-S diagram for cycle in (a).
and feedwater heating. Reheat is to 700⬚F (371⬚C). For the ideal turbine working
under these conditions find: (a) percentage of throttle steam bled for feedwater
heating, (b) heat converted to work per pound (kg) of throttle steam, (c) heat sup-
plied per pound (kg) of throttle steam, (d) ideal thermal efficiency, (e) T-S, tem-
perature-entropy, diagram and layout of cycle.
Calculation Procedure:
1. Determine the cycle enthalpies, pressures, and entropies
Using standard steam tables and a Mollier chart, draw the cycle and T-S plot, Fig.
30a and b. Then, P1 ⫽ 400 lb/in2
(abs) (2756 kPa); t1 ⫽ 700⬚F (371⬚C); H1 ⫽
1362.3 Btu/lb (3174 kJ/kg); S1 ⫽ 1.6396; H2 ⫽ 1178 Btu/lb (2745 kJ/kg); Hg ⫽
1380.1 Btu/lb (3216 kJ/kg); Sg ⫽ 1.8543.
(a) Percent throttle steam bled ⫽ (H6 ⫺ H5)/(H2 ⫺ H5) ⫽ (196.15/1107.9) ⫽
0.1771, or 17.71 percent.
2. Find the amount of heat converted to work per pound (kg) of throttle steam
(b) The amount of heat converted to work per pound (kg) of throttle steam ⫽
(H1 ⫺ H2) ⫹ (1 ⫺ 0.1771)(Hg ⫺ H4) ⫽ 467.3 Btu/lb (1088.8 kJ/kg).
3. Compute the heat supplied per pound (kg) of throttle steam
(c) The heat supplied per pound (kg) of throttle steam ⫽ (H1 ⫺ H6) ⫹ (Hg ⫺ H2)
⫽ 1299.1 Btu/lb (3026.9 kJ/kg).
4. Determine the ideal thermal efficiency
(d) The ideal thermal efficiency ⫽ (heat recovered per pound [kg] of throttle steam)/
(heat supplied per pound [kg] of throttle steam) ⫽ 467.3/1299.13 ⫽ 0.3597, or
35.97 percent. The T-S diagram and cycle layout can be drawn as shown in Fig.
30a and b.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.49
Related Calculations. This general procedure can be used for any turbine cycle
where reheating and feedwater heating are part of the design. Note that the enthalpy
and entropy values read from the Mollier chart, or interpolated from the steam
tables, may differ slightly from those given here. This is to be expected where
judgement comes into play. The slight differences are unimportant in the analysis
of the cycle.
The procedure outlined here is valid for industrial, utility, commercial, and ma-
rine turbines used to produce power.
STEAM RATE FOR REHEAT-REGENERATIVE
CYCLE
Steam is supplied at 600 lb/in2
(abs) (4134 kPa) and 740⬚F (393⬚C) to a steam
turbine operating on the reheat-regenerative cycle. After expanding to 100 lb/in2
(abs) (689 kPa), the steam is reheated to 700⬚F (371⬚C). Expansion then continues
to 10 lb/in2
(abs) (68.9 kPa) but at 30 lb/in2
(abs) (207 kPa) some steam is extracted
for feedwater heating in a direct-contact heater. Assuming ideal operation with no
losses, find: (a) steam extracted as a percentage of steam supplied to the throttle,
(b) steam rate in pounds (kg) per kWh; (c) thermal efficiency of the turbine, (d)
quality or superheat of the exhaust if in the actual turbine combined efficiency is
72 percent, generator efficiency is 94 percent, and actual extraction is the same as
the ideal.
Calculation Procedure:
1. Assemble the key enthalpies, entropies, and pressures for the cycle
Using the steam tables and a Mollier chart, list the following pressures, tempera-
tures, enthalpies, and entropies for the cycle, Fig. 31a: Pl ⫽ 600 lb/in2
(abs) (4134
kPa); t1 ⫽ 740⬚F (393⬚C); P2 ⫽ 100 lb/in2
(abs) (689 kPa); t3 ⫽ 700⬚F (371⬚C);
Px ⫽ 30 lb/in2
(abs) (207 kPa); pc ⫽ 1 lb/in2
(abs) (6.89 kPa); H1 ⫽ 1372 Btu/lb
(3197 kJ/kg); S1 ⫽ entropy ⫽ 1.605; H2 ⫽ 1188 Btu/lb (2768 kJ/kg); H3 ⫽ 1377
Btu/lb (3208 kJ/kg); S3 ⫽ 1.802; Hx ⫽ 1245 Btu/lb (2901 kJ/kg); Hc ⫽ 1007
Btu/lb (2346 kJ/kg); ⫽ 70 Btu/lb (163 kJ/kg); ⫽ 219 Btu/lb (510 kJ/kg).
H H
ƒ ƒx
Plot Fig. 31b as a skeleton Mollier chart to show the cycle processes.
2. Compute the percent steam extracted for the feedwater heater
(a) The steam extracted for the feedwater heater, x, ⫽ ⫽ (219
(H ⫺ H )(H ⫺ H )
ƒx ƒ x ƒ
⫺ 70)/(1245 ⫺ 70) ⫽ 0.1268, or 12.68 percent.
3. Find the turbine steam rate
(b) For the Rankine-cycle steam rate, ws ⫽ 3413/(H1 ⫺ Hc). For this cycle, ws ⫽
3413/[(H1 ⫺ H2) ⫹ x(H3 ⫺ Hx) ⫹ (1 ⫺ x)(H3 ⫺ Hc)]. Or, ws ⫽ 3413/[1372 ⫺
1188) ⫹ 0.1268(1377 ⫺ 1245) ⫹ (1 ⫺ 0.1268)(1377 ⫺ 1007)] ⫽ 6.52 lb/kWh
(2.96 kg/kWh).
4. Calculate the turbine thermal efficiency
(c) The thermal efficiency, Et ⫽ [(H1 ⫺ H2) ⫹ x(H3 ⫺ Hx) ⫹ (1 ⫺ x)(H3 ⫺ Hc)]/
[H3 ⫺ H2) ⫹ (H1 ⫺ Or, Et ⫽ [(1372 ⫺ 1188) ⫹ (0.1268)(1377 ⫺ 1245) ⫹
H )].
ƒx
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![1.50 POWER GENERATION
(a)
(b)
600 psia (4134 kPa) 740˚F (393˚C) 700˚F (371˚C) 100 psia (68.9 kPa)
1188 Btu/lb (2768 kJ/kg) 1372 Btu/lb (3197 kJ/kg) 1245 Btu/lb (2901 kJ/kg)
1 psia (6.89 kPa) 30 psia (207 kPa) 1007 Btu/lb (2346 kJ/kg) 70 Btu/lb (163 kJ/kg)
FIGURE 31 (a) Cycle diagram. (b) H-S chart for cycle in (a).
(1 ⫺ 0.1268)(1377 ⫺ 1007)]/[(1377 ⫺ 1188) ⫹ (1372 ⫺ 219) ⫽ 0.3903, or 39
percent. It is interesting to note that in an ideal cycle the thermal efficiency of the
turbine is the same as that of the cycle.
5. Determine the condition of the exhaust
(d) The engine efficiency of the turbine alone ⫽ (actual turbine combined
efficiency)/actual generator efficiency). Or, using the given data, engine efficiency
of the turbine alone ⫽ 0.72/0.94 ⫽ 0.765.
Using the computed engine efficiency of the turbine alone and the Mollier chart,
(H3 ⫺ ⫽ 0.765(H3 ⫺ HC) ⫽ 283. Solving, ⫽ H3 ⫺ 283 ⫽ 1094 Btu/lb
H ) H
c⬘ c⬘
(2549 kJ/kg). From the Mollier chart, the condition at is 1.1 percent moisture.
Hc⬘
The exhaust steam quality is therefore 100 ⫺ 1.1 ⫽ 98.9 percent.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.59
FIGURE 36 Bleed-regenerative steam cycle.
Related Calculations. The procedure given here can be used for all regener-
ative cycles, provided that the equations are altered to allow for more, or fewer,
heaters and pumps. The following calculation procedure shows the application of
this method to an actual regenerative cycle.
BLEED REGENERATIVE STEAM CYCLE ANALYSIS
Analyze the bleed regenerative cycle shown in Fig. 36, determining the heat balance
for each heater, plant thermal efficiency, turbine or engine thermal efficiency, plant
heat rate, turbine or engine heat rate, and turbine or engine steam rate. Throttle
steam pressure is 2000 lb/in2
(abs) (13,790.0 kPa) at 1000⬚F (537.8⬚C); steam-
generator efficiency ⫽ 0.88; station auxiliary steam consumption (excluding pump
work) ⫽ 6 percent of the turbine or engine output; engine efficiency of each turbine
or engine section ⫽ 0.80; turbine or engine cycle has three feedwater heaters and
bleed-steam pressures as shown in Fig. 36; exhaust pressure to condenser is 1 inHg
(3.4 kPa) absolute.
Calculation Procedure:
1. Determine the enthalpy of the steam at the inlet of each heater
and the condenser
From a superheated-steam table, find the throttle enthalpy H2 ⫽ 1474.5 Btu/lb
(3429.7 kJ/kg) at 2000 lb/in2
(abs) (13,790.0 kPa) and 1000⬚F (537.8⬚C). Next
find the throttle entropy S2 ⫽ 1.5603 Btu/(lb ⬚F) [6.5 kJ/(kg ⬚C)], at the same
conditions in the superheated-steam table.
Plot the throttle steam conditions on a Mollier chart, Fig. 37. Assume that the
steam expands from the throttle conditions at constant entropy ⫽ constant S to the
inlet of the first feedwater heater, 1, Fig. 36. Plot this constant S expansion by
drawing the straight vertical line 2-3 on the Mollier chart, Fig. 37, between the
throttle condition and the heater inlet pressure of 750 lb/in2
(abs) (5171.3 kPa).
Read on the Mollier chart H3 ⫽ 1346.7 Btu/lb (3132.4 kJ/kg). Since the engine
or turbine efficiency ee ⫽ H2 ⫺ H3 /(H2 ⫺ H3) ⫽ 0.8 ⫽ 1474.5 ⫺ H3 /(1474.5 ⫺
1346.7); H3 ⫽ actual enthalpy of the steam at the inlet to heater 1 ⫽ 1474.5 ⫺
0.8(1474.5 ⫺ 1346.7) ⫽ 1372.2 Btu/lb (3191.7 kJ/kg). Plot this enthalpy point on
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![1.60 POWER GENERATION
FIGURE 37 Mollier-chart plot of the cycle in Fig. 36.
the 750-lb/in2
(abs) (5171.3-kPa) pressure line of the Mollier chart, Fig. 37. Read
the entropy at the heater inlet from the Mollier chart as ⫽ 1.5819 Btu/(lb ⬚F)
S3⬘
[6.6 kJ/(kg ⬚C)] at 750 lb/in2
(abs) (5171.3 kPa) and 1372.2 Btu/lb (3191.7 kJ/
kg).
Assume constant-S expansion from to H4 at 200 lb/in2
(abs) (1379.0 kPa),
H3⬘
the inlet pressure for feedwater heater 2. Draw the vertical straight line 3⬘-4 on the
Mollier chart, Fig. 37. By using a procedure similar to that for heater 1, ⫽
H4⬘
⫺ ee ⫺ H4) ⫽ 1372.2 ⫺ 0.8(1372.2 ⫺ 1230.0) ⫽ 1258.4 Btu/lb (2927.0
H (H
3⬘ 3⬘
kJ/kg). This is the actual enthalpy of the steam at the inlet to heater 2. Plot this
enthalpy on the 200-lb/in2
(abs) (1379.0-kPa) pressure line of the Mollier chart,
and find ⫽ 1.613 Btu/(lb ⬚F) [6.8 kJ/(kg ⬚C)], Fig. 37.
S4⬘
Using the same procedure with constant-S expansion from we find H5 ⫽
H ,
4⬘
1059.5 Btu/lb (2464.4 kJ/kg) at 16 lb/in2
(abs) (110.3 kPa), the inlet pressure to
heater 3. Next find ⫺ ee ⫺ H5) ⫽ 1258.4 ⫺ 0.8(1258.4 ⫺ 1059.5) ⫽
H ⫽ H (H
5⬘ 4⬘ 4⬘
1099.2 Btu/lb (2556.7 kJ/kg). From the Mollier chart find ⫽ 1.671 Btu/(lb
S5⬘
⬚F) [7.0 kJ/(kg ⬚C)], Fig. 37.
Using the same procedure with constant-S expansion from to H6, find H6 ⫽
H5⬘
898.2 Btu/lb (2089.2 kJ/kg) at 1 inHg absolute (3.4 kPa), the condenser inlet
pressure. Then ⫺ ⫺ H6) ⫽ 1099.2 ⫺ 0.8(1099.2 ⫺ 898.2) ⫽ 938.4
H ⫽ H e (H
6⬘ 5⬘ e 5⬘
Btu/lb (2182.7 kJ/kg), the actual enthalpy of the steam at the condenser inlet. Find,
on the Mollier chart, the moisture in the turbine exhaust ⫽ 15.1 percent.
2. Determine the overall engine efficiency
Overall engine efficiency ee is higher than the engine-section efficiency because
there is partial available-energy recovery between sections. Constant-S expansion
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![1.62 POWER GENERATION
5. Check the turbine work output
The heat added to the cycle Qa Btu/lb ⫽ H2 ⫺ H1 ⫽ 1474.5 ⫺ 505.5 ⫽ 969.0 Btu
(1022.3 kJ). The heat rejected from the cycle Qr Btu/lb ⫽ ⫺ H7)(1 ⫺ m1 ⫺
(H6⬘
m2 ⫺ m3) ⫽ (938.4 ⫺ 47.1)(0.6277) ⫽ 559.5 Btu (590.3 kJ). Then We ⫺ Wp ⫽
Qa ⫺ Qr ⫽ 969.0 ⫺ 559.5 ⫽ 409.5 Btu (432.0 kJ).
Compare this with We ⫺ Wp computed earlier, or 415.9 ⫺ 7.0 ⫽ 408.9 Btu
(431.4 kJ), or a difference of 409.5 ⫺ 408.9 ⫽ 0.6 Btu (0.63 kJ). This is an accurate
check; the difference of 0.6 Btu (0.63 kJ) comes from errors in Mollier chart and
calculator readings. Assume 408.9 Btu (431.4 kJ) is correct because it is the lower
of the two values.
6. Compute the plant and turbine efficiencies
Plant energy input ⫽ Qa /eb, where eb ⫽ boiler efficiency. Then plant energy
input ⫽ 969.0/0.88 ⫽ 1101.0 Btu (1161.6 kJ). Plant thermal efficiency ⫽ W /(Qa
/eb ) ⫽ 383.9 ⫽ 1101.0 ⫽ 0.3486. Turbine thermal efficiency ⫽ We /Qa ⫽ 415.9/
969.0 ⫽ 0.4292. Plant heat rate ⫽ 3413/0.3486 ⫽ 9970 Btu/kWh (10,329.0 kJ/
kWh), where 3413 ⫽ Btu/kWh. Turbine heat rate ⫽ 3413/0.4292 ⫽ 7950 Btu/
kWh (8387.7 kJ/kWh). Turbine throttle steam rate ⫽ (turbine heat rate)/(H2 ⫺
H1) ⫽ 7950/(1474.5 ⫺ 505.5) ⫽ 8.21 lb/kWh (3.7 kg/kWh).
Related Calculations. By using the procedures given, the following values can
be computed for any actual steam cycle: engine or turbine efficiency ee ; steam
enthalpy at the main-condenser inlet; bleed-steam flow to a feedwater heater; tur-
bine or engine work output per section; total turbine or engine work output; station
auxiliary power consumption; net station work output; plant energy input; plant
thermal efficiency; turbine or engine thermal efficiency; plant heat rate; turbine or
engine heat rate; turbine throttle heat rate. To compute any of these values, use the
equations given and insert the applicable variables.
REHEAT-STEAM CYCLE PERFORMANCE
A reheat-steam cycle has a 2000 lb/in2
(abs) (13,790-kPa) throttle pressure at the
turbine inlet and a 400-lb/in2
(abs) (2758-kPa) reheat pressure. The throttle and
reheat temperature of the steam is 1000⬚F (537.8⬚C); condenser pressure is 1 inHg
absolute (3.4 kPa); engine efficiency of the high-pressure and low-pressure turbines
is 80 percent. Find the cycle thermal efficiency.
Calculation Procedure:
1. Sketch the cycle layout and cycle T-S diagram
Figures 38 and 39 show the cycle layout and T-S diagram with each important point
numbered. Use a cycle layout and T-S diagram for every calculation of this type
because it reduces the possibility of errors.
2. Determine the throttle-steam properties from the steam tables
Use the superheated steam tables, entering at 2000 lb/in2
(abs) (13,790 kPa) and
1000⬚F (537.8⬚C) to find throttle-steam properties. Applying the symbols of the T-
S diagram in Fig. 39, we get H2 ⫽ 1474.5 Btu/lb (3429.7 kJ/kg); S2 ⫽ 1.5603
Btu/(lb ⬚F) [6.5 kJ/(kg ⬚C)].
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.63
FIGURE 38 Typical steam reheat cycle.
FIGURE 39 Irreversible expansion
in reheat cycle.
3. Find the reheat-steam enthalpy
Assume a constant-entropy expansion of the steam from 2000 to 400 lb/in2
(13,790
to 2758 kPa). Trace this expansion on a Mollier (H-S) chart, where a constant-
entropy process is a vertical line between the initial [2000 lb/in2
(abs) or 13,790
kPa] and reheat [400 lb/in2
(abs) or 2758 kPa] pressures. Read on the Mollier chart
H3 ⫽ 1276.8 Btu/lb (2969.8 kJ/kg) at 400 lb/in2
(abs) (2758 kPa).
4. Compute the actual reheat properties
The ideal enthalpy drop, throttle to reheat ⫽ H2 ⫺ H3 ⫽ 1474.5 ⫺ 1276.8 ⫽ 197.7
Btu/lb (459.9 kJ/kg). The actual enthalpy drop ⫽ (ideal drop)(turbine efficiency) ⫽
H2 ⫺ ⫽ 197.5(0.8) ⫽ 158.2 Btu/lb (368.0 kJ/kg) ⫽ We 1 ⫽ work output in the
H3⬘
high-pressure section of the turbine.
Once We 1 is known, can be computed from ⫽ H2 ⫺ We 1 ⫽ 1474.5 ⫺
H H
3⬘ 3⬘
158.2 ⫽ 1316.3 Btu/lb (3061.7 kJ/kg).
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![1.64 POWER GENERATION
FIGURE 40 Energy-flow diagram for reheat cycle in Fig. 38.
The steam now returns to the boiler and leaves at condition 4, where P4 ⫽ 400
lb/in2
(abs) (2758 kPa); T4 ⫽ 1000⬚F (537.8⬚C); S4 ⫽ 1.7623 Btu/(lb ⬚F) [7.4
kJ/(kg ⬚C)]; H4 ⫽ 1522.4 Btu/lb (3541.1 kJ/kg) from the superheated-steam table.
5. Compute the exhaust-steam properties
Use the Mollier chart and an assumed constant-entropy expansion to 1 inHg (3.4
kPa) absolute to determine the ideal exhaust enthalpy, or H5 ⫽ 947.4 Btu/lb (2203.7
kJ/kg). The ideal work of the low-pressure section of the turbine is then H4 ⫺
H5 ⫽ 1522.4 ⫺ 947.4 ⫽ 575.0 Btu/lb (1338 kJ/kg). The actual work output of
the low-pressure section of the turbine is We 2 ⫽ H4 ⫺ ⫽ 575.0(0.8) ⫽ 460.8
H5⬘
Btu/lb (1071.1 kJ/kg).
Once We 2 is known, can be computed from ⫽ H4 ⫺ We 2 ⫽ 1522.4 ⫺
H H
5⬘ 5⬘
460.0 ⫽ 1062.4 Btu/lb (2471.1 kJ/kg).
The enthalpy of the saturated liquid at the condenser pressure is found in the
saturation-pressure steam table at 1 inHg absolute (3.4 kPa) ⫽ H6 ⫽ 47.1 Btu/lb
(109.5 kJ/kg).
The pump work Wp from the compressed-liquid table diagram in the steam tables
is Wp ⫽ 5.5 Btu/lb (12.8 kJ/kg). Then the enthalpy of the water entering the boiler
H1 ⫽ H6 ⫹ Wp ⫽ 47.1 ⫹ 5.5 ⫽ 52.6 Btu/lb (122.3 kJ/kg).
6. Compute the cycle thermal efficiency
For any reheat cycle,
e ⫽ cycle thermal efficiency
(H ⫺ H ) ⫹ (H ⫺ H ) ⫺ W
2 3⬘ 4 5⬘ p
⫽
(H ⫺ H ) ⫹ (H ⫺ H )
2 1 4 3⬘
(1474.5 ⫺ 1316.3) ⫹ (1522.4 ⫺ 1062.4) ⫺ 5.5
⫽
(1474.5 ⫺ 52.6) ⫹ (1522.4 ⫺ 1316.3)
⫽ 0.3766, or 37.66 percent
Figure 40 is an energy-flow diagram for the reheat cycle analyzed here. This
diagram shows that the fuel burned in the steam generator to produce energy flow
Qa 1 is the largest part of the total energy input. The cold-reheat line carries the
major share of energy leaving the high-pressure turbine.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.69
FIGURE 47 Available energy in turbine depends on the initial
steam state and the exhaust pressure.
ft lb/(s hp)]. [Note: 550 ft lb/(s hp) ⫽ 1 N m/(W s).] For this turbine, work
output ⫽ (205 Btu/lb)(1000 lb/s)(40/100)/550 ⫽ 149 hp (111.1 kW).
Related Calculations. Use the steps given here to analyze single-stage non-
condensing mechanical-drive turbines for stationary, portable, or marine applica-
tions. Performance curves such as Fig. 47 are available from turbine manufacturers.
Single-stage noncondensing turbines are for feed-pump, draft-fan, and auxiliary-
generator drive.
CONDENSING STEAM-TURBINE POWER-OUTPUT
ANALYSIS
What is the available energy in steam supplied to a 5000-kW turbine if the inlet
steam conditions are 1000 lb/in2
(abs) (6895 kPa) and 800⬚F (426.7⬚C) and the
turbine exhausts at 1 inHg absolute (3.4 kPa)? Determine the theoretical and actual
heat rate of this turbine if its engine efficiency is 74 percent. What are the full-load
output and steam rate of the turbine?
Calculated Procedure:
1. Determine the available energy in the steam
Enter Fig. 48 at the bottom at 1000-lb/in2
(abs) (6895.0-kPa) inlet pressure, and
project vertically upward to the 800⬚F (426.7⬚C) 1-in (3.4-kPa) exhaust-pressure
curve. At the left, read the available energy as 545 Btu/lb (1267.7 kJ/kg) of steam.
2. Determine the heat rate of the turbine
Enter Fig. 49 at an initial steam temperature of 800⬚F (426.7⬚C), and project ver-
tically upward to the 1000-lb/in2
(abs) (6895.0-kPa) 1-in (3.4-kPa) curve. At the
left, read the theoretical heat rate as 8400 Btu/kWh (8862.5 kJ/kWh).
When the theoretical heat rate is known, the actual heat rate is found from:
actual heat rate HR, Btu/kWh ⫽ (theoretical heat rate, Btu/kWh)/(engine effi-
ciency). Or, actual HR ⫽ 8400/0.74 ⫽ 11,350 Btu/kWh (11,974.9 kJ/kWh).
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.73
FIGURE 52 Reduction in straight-condensing heat rate obtained by
regenerative heating.
3. Apply the heat-rate and heater-number correction factors
Full-load regenerative-cycle heat rate, Btu/kWh ⫽ (straight-condensing heat rate,
Btu/kWh) [1 ⫺ (heat-rate correction factor)(heater-number correction factor)] ⫽
(13,350)[1 ⫺ (0.148)(0.71)] ⫽ 10,160 Btu/kWh (10,719.4 kJ/kWh).
4. Find and apply the correction factors for the larger number of heaters
Enter Fig. 53 at 90 percent of the maximum possible enthalpy rise, and project
vertically to the 10-heater curve. At the left, read the heat-rate reduction for the
number of heaters and actual enthalpy rise as 0.89.
Using the heat-rate correction factor from step 2 and 0.89, found above, we see
that the full-load 10-heater regenerative-cycle heat rate ⫽ (11,350)[1 ⫺
(0.148)(0.89)] ⫽ 9850 Btu/kWh (10,392.3 kJ/kWh), by using the same procedure
as in step 3. Thus, adding 10 ⫺ 3 ⫽ 7 heaters reduces the heat rate by 10,160 ⫺
9850 ⫽ 310 Btu/kWh (327.1 kJ/kWh). This is a reduction of 3.05 percent.
To determine whether this reduction in heat rate is appreciable, the carrying
charges on the extra heaters, piping, and pumps must be compared with the reduc-
tion in annual fuel costs resulting from the lower heat rate. If the fuel saving is
greater than the carrying charges, the larger number of heaters can usually be jus-
tified. In this case, tripling the number of heaters would probably increase the
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.77
FIGURE 56 Combined gas turbine–steam turbine cycle.
one stage of economizer feedwater heating. Compute the thermal efficiency and
heat rate of the combined cycle.
Calculation Procedure:
1. Sketch the cycle layout
Figure 56 shows the cycle. Since the gas-turbine exhaust-gas temperature is usually
higher than the bleed-steam temperature, the economizer is placed after the regen-
erative feedwater heater. The feedwater will be progressively heated to a higher
temperature during passage through the regenerative heater and the gas-turbine
economizer. The cycle shown here is only one of many possible combinations of
a steam plant and a gas turbine.
2. Sketch the T-S diagram
Figure 57 shows the T-S diagram for the combined gas turbine–steam turbine cycle.
There is irreversible heat transfer QT from the gas-turbine exhaust to the feedwater
in the economizer, which helps reduce the required energy input Qa 2.
3. Sketch the energy-flow chart
Choose a suitable scale for the energy input, and proportion the energy flow to
each of the other portions of the cycle. Use a single line when the flow is too small
to plot to scale. Figure 58 shows the energy-flow chart.
4. Determine the thermal efficiency of the cycle
Since e ⫽ W/Qa, e ⫽ Qa ⫺ Qr /Qa ⫽ 1 ⫺ [Qr 1 ⫹ Qr 2 /(Qa 1 ⫹ Qa 2)], given the
notation in Figs. 56, 57, and 58.
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.79
FIGURE 58 Energy-flow chart of the gas turbine–steam turbine cycle in Fig. 56.
2. Establish the chemical equation for complete combustion with 100 percent
air
With 100 percent air: C8H18 ⫹ 12.5 O2 ⫹ (3.784 ⫻ 12.5)N2 → 8CO2 ⫹ 9H2O ⫹
47.3N2, where 3.784 is a derived volumetric ratio of atmospheric nitrogen, (N2), to
oxygen, O2, in dry air. The (N2) includes small amounts of inert and inactive gases.
See Related Calculations of this procedure.
3. Establish the chemical equation for complete combustion with 400 percent
of the stoichiometric air quantity, or 300 percent excess air
With 400 percent air: C8H18 ⫹ 50 O2 ⫹ (4 ⫻ 47.3)N2 → 8CO2 ⫹ 9H2O ⫹
189.2N2 ⫹ (3 ⫻ 12.5)O2.
4. Compute the molecular weights of the components in the combustion
process
Molecular weight of C8H18 ⫽ [(12 ⫻ 8) ⫹ (1 ⫻ 18)] ⫽ 114; O2 ⫽ 16 ⫻ 2 ⫽ 32;
N2 ⫽ 14 ⫻ 2 ⫽ 28; CO2 ⫽ [(12 ⫻ 1) ⫹ (16 ⫻ 2)] ⫽ 44; H2O ⫽ [(1 ⫻ 2) ⫹
(16 ⫻ 1)] ⫽ 18.
5. Compute the relative weights of the reactants and products of the
combustion process
Relative weight ⫽ moles ⫻ molecular weight. Coefficients of the chemical equation
in step 3 represent the number of moles of each component. Hence, for the reac-
tants, the relative weights are: C8H18 ⫽ 1 ⫻ 114 ⫽ 114; O2 ⫽ 50 ⫻ 32 ⫽ 1600;
N2 ⫽ 189.2 ⫻ 28 ⫽ 5298. Total relative weight of the reactants is 7012. For the
products, the relative weights are: CO2 ⫽ 8 ⫻ 44 ⫽ 352; H2O ⫽ 9 ⫻ 18 ⫽ 162;
N2 ⫽ 189.2 ⫻ 28 ⫽ 5298; O2 ⫽ 37.5 ⫻ 32 ⫽ 1200. Total relative weight of the
products is 7012, also.
6. Compute the enthalpy of the products of the combustion process
Enthalpy of the products of combustion, hp ⫽ mp where mp ⫽ number
(h ⫺ h ),
1600 77
of moles of the products; ⫽ enthalpy of the products at 1600⬚F (871⬚C); h77 ⫽
h1600
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![1.80 POWER GENERATION
enthalpy of the products at 77⬚F (25⬚C). Thus, hp ⫽ (8 ⫹ 9 ⫹ 189.2 ⫹
37.5)(15,400 ⫺ 3750) ⫽ 2,839,100 Btu [6,259,100 Btu (SI)].
7. Compute the air supply temperature at the combustion chamber inlet
Since the combustion process is adiabatic, the enthalpy of the reactants hr ⫽ hp,
where hr ⫽ (relative weight of the fuel ⫻ its heating value) ⫹ [relative weight of
the air ⫻ its specific heat ⫻ (air supply temperature ⫺ air source temperature)].
Therefore, hr ⫽ (114 ⫻ 19,100) ⫹ [(1600 ⫹ 5298) ⫻ 0.24 ⫻ (ta ⫺ 77)] ⫹ 2,839,100
Btu [6,259,100 Btu (SI)]. Solving for the air supply temperature, ta ⫽ [(2,839,100 ⫺
2,177,400/1655.5] ⫹ 77 ⫽ 477⬚F (247⬚C).
Related Calculations. This procedure, appropriately modified, may be used to
deal with similar questions involving such things as other fuels, different amounts
of excess air, and variations in the condition(s) being sought under certain given
circumstances.
The coefficient, (?) ⫽ 3.784 in step 2, is used to indicate that for each unit of
volume of oxygen, O2, 12.5 in this case, there will be 3.784 units of nitrogen, N2.
This equates to an approximate composition of air as 20.9 percent oxygen and 79.1
percent ‘‘atmospheric nitrogen,’’ (N2). In turn, this creates a paradox, because page
200 of Principles of Engineering Thermodynamics, by Kiefer, et al., John Wiley
Sons, Inc., states air to be 20.99 percent oxygen and 79.01 percent atmospheric
nitrogen, where the ratio (N2)/O2 ⫽ (?) ⫽ 79.01/20.99 ⫽ 3.764.
Also, page 35 of Applied Energy Conversion, by Skrotski and Vopat, McGraw-
Hill, Inc., indicates an assumed air analysis of 79 percent nitrogen and 21 percent
oxygen, where (?) ⫽ 3.762. On that basis, a formula is presented for the amount
of dry air chemically necessary for complete combustion of a fuel consisting of
atoms of carbon, hydrogen, and sulfur, or C, H, and S, respectively. That formula
is: Wa ⫽ 11.5C ⫹ 34.5[H ⫺ (0/8)] ⫹ 4.32S, lb air/lb fuel (kg air/kg fuel).
The following derivation for the value of (?) should clear up the paradox and
show that either 3.784 or 3.78 is a sound assumption which seems to be wrong,
but in reality is not. In the above equation for Wa, the carbon hydrogen, or sulfur
coefficient, Cx ⫽ (MO2 /DO2)Mx, where MO2 is the molecular weight of oxygen,
O2; DO2 is the decimal fraction for the percent, by weight, of oxygen, O2, in dry
air containing ‘‘atmospheric nitrogen,’’ (N2), and small amounts of inert and inactive
gases: Mx is the formula weight of the combustible element in the fuel, as indicated
by its relative amount as a reactant in the combustion equation. The alternate eval-
uation of Cx is obtained from stoichiometric chemical equations for burning the
combustible elements of the fuel, i.e., C ⫹ O2 ⫹ (?)N2 → CO2 ⫹ (?)N2; 2H2 ⫹
O2 ⫹ (?)N2 → 2H2O ⫹ (?)N2; S ⫹ O2 ⫹ (?)N2 → SO2 ⫹ (?)N2. Evidently, Cx ⫽
[MO2 ⫹ (?) ⫻ MN2)]/Mx, where MN2 is the molecular weight of nitrogen, N2, and
the other items are as before.
Equating the two expressions, Cx ⫽ [MO2 ⫹ (? ⫻ MN2)]/Mx ⫽ (MO2 /DO2)Mx,
reveals that the Mx terms cancel out, indicating that the formula weight(s) of com-
bustible components are irrelevant in solving for (?). Then, (?) ⫽ (1 ⫺ DO2)[M
O2 /(MN2 ⫻ DO2)]. From the above-mentioned book by Kiefer, et al., DO2 ⫽
0.23188. From Marks’ Standard Handbook for Mechanical Engineers, McGraw-
Hill, Inc., MO2 ⫽ 31.9988 and MN2 ⫽ 28.0134. Thus, (?) ⫽ (1 ⫺
0.23188)[31.9988/(28.0134 ⫻ 0.23188)] ⫽ 3.7838. This demonstrates that the use
of (?) ⫽ 3.784, or 3.78, is justified for combustion equations.
By using either of the two evaluation equations for Cx, and with accurate values
for Mx, i.e., MC ⫽ 12.0111; MH ⫽ 2 ⫻ 2 ⫻ 1.00797 ⫽ 4.0319; MS ⫽ 32.064, from
Marks’ M.E. Handbook, the more precise values for CC, CH, and CS are found out
to be 11.489, 34.227, and 4.304, respectively. However, the actual Cx values, 11.5,
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.81
FIGURE 59 Gas turbine flow diagram.
34.5, and 4.32, used in the formula for Wa are both brief for simplicity and rounded
up to be on the safe side.
GAS TURBINE COMBUSTION CHAMBER INLET
AIR TEMPERATURE
A gas turbine combustion chamber is well insulated so that heat losses to the
atmosphere are negligible. Octane, C8H18, is to be used as the fuel and 400 percent
of the stoichiometric air quantity is to be supplied. The air first passes through a
regenerative heater and the air supply temperature at the combustion chamber inlet
is to be set so that the exit temperature of the combustion gases is 1600⬚F (871⬚C).
(See Fig. 59.) Fuel supply temperature is 77⬚F (25⬚C) and its heating value is to
be taken as 19,000 Btu/lbm (44,190 kJ/kg) relative to a base of 77⬚F (15⬚C).
The air may be treated in calculations as a perfect gas with a constant-pressure
specific heat of 0.24 Btu/(lb ⬚F) [1.005 kJ/(kg ⬚C)]. The products of combustion
have an enthalpy of 15,400 Btu/lb mol) [33,950 Btu/(kg mol)] at 1600⬚F (871⬚C)
and an enthalpy of 3750 Btu/(lb mol) [8270 Btu/(kg mol)] at 77⬚F (24⬚C). De-
termine, assuming complete combustion and neglecting dissociation, the required
air temperature at the inlet of the combustion chamber.
Calculation Procedure:
1. Find the amount of oxygen required for complete combustion of the fuel
Eight atoms of carbon in C8 combine with 8 molecules of oxygen, O2, and produce
8 molecules of carbon dioxide, 8CO2. Similarly, 9 molecules of hydrogen, H2, in
H18 combine with 9 atoms of oxygen, O, or 4.5 molecules of oxygen, to form 9
molecules of water, 9H2O. Thus, 100 percent, or the stoichiometric, air quantity
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![1.82 POWER GENERATION
required for complete combustion of a mole of fuel, C8H18, is proportional to 8 ⫹
12.5 mol of oxygen, O2.
2. Establish the chemical equation for complete combustion with 100 percent
air
With 100 percent air: C8H18 ⫹ 12.5O2 ⫹ (3.784 ⫻ 12.5)N2 → 8CO2 ⫹ 9H2O ⫹
47.3N2, where 3.784 is a derived volumetric ratio of atmospheric nitrogen (N2) to
oxygen O2 in dry air. The N2 includes small amounts of inert and inactive gases.
See Related Calculations of this procedure.
3. Establish the chemical equation for complete combustion with 400 percent
of the stoichiometric air quantity, or 300 percent excess air
With 400 percent air: C8H18 ⫹ 50O2 ⫹ (4 ⫻ 47.3)N2 → 8CO2 ⫹ 9H2O ⫹ 189.2N2
⫹ (3 ⫻ 12.5)O2.
4. Compute the molecular weights of the components in the combustion proc-
ess
Molecular weight of C8H18 ⫽ [(12 ⫻ 8) ⫹ (1 ⫻ 18)] ⫽ 114; O2 ⫽ 16 ⫻ 2 ⫽ 32;
N2 ⫽ 14 ⫻ 2 ⫽ 28; CO2 ⫽ [(12 ⫻ 1) ⫹ (16 ⫻ 2)] ⫽ 44; H2O ⫽ [(1 ⫻ 2) ⫹
(16 ⫻ 1)] ⫽ 18.
5. Compute the relative weights of the reactants and products of the combustion
process
Relative weight ⫽ moles ⫻ molecular weight. Coefficients of the chemical equation
in step 3 represent the number of moles of each component. Hence, for the reac-
tants, the relative weights are C8H18 ⫽ 1 ⫻ 114 ⫽ 114; O2 ⫽ 50 ⫻ 32 ⫽ 1600;
N2 ⫽ 189.2 ⫻ 28 ⫽ 5298. Total relative weight of the reactants is 7012. For the
products, the relative weights are CO2 ⫽ 8 ⫻ 44 ⫽ 352; H2O ⫽ 9 ⫻ 18 ⫽ 162;
N2 ⫽ 189.2 ⫻ 28 ⫽ 5298; O2 ⫽ 37.5 ⫻ 32 ⫽ 1200. Total relative weight of the
products is 7012 also.
6. Compute the enthalpy of the products of the combustion process
Enthalpy of the products of combustion, hp ⫽ mp(h1600 ⫺ h77), where mp ⫽ number
of moles of the products; h1600 ⫽ enthalpy of the products at 1600⬚F (871⬚C); h77 ⫽
enthalpy of the products at 77⬚F (25⬚C). Thus, hp ⫽ (8 ⫹ 9 ⫹ 189.2 ⫹
37.5)(15,400 ⫺ 3750) ⫽ 2,839,100 Btu [6,259,100 Btu (SI)].
7. Compute the air supply temperature at the combustion chamber inlet
Since the combustion process is adiabatic, the enthalpy of the reactants hr ⫽ hp,
where hr ⫽ (relative weight of the fuel ⫻ its heating value) ⫹ [relative weight of
the air ⫻ its specific heat ⫻ (air supply temperature ⫺ air source temperature)].
Therefore, hr ⫽ (114 ⫻ 19,100) ⫹ [(1600 ⫹ 5298) ⫻ 0.24 ⫻ (ta ⫺ 77)] ⫹ 2,839,100
Btu [6,259,100 Btu (SI)]. Solving for the air supply temperature, ta ⫽ [(2,839,100 ⫺
2,177,400/1655.5] ⫹ 77 ⫽ 477⬚F (247⬚C).
Related Calculations. This procedure, appropriately modified, may be used to
deal with similar questions involving such things as other fuels, different amounts
of excess air, and variations in the condition(s) being sought under certain given
circumstances.
The coefficient, (?) ⫽ 3.784 in step 2, is used to indicate that for each unit of
volume of oxygen, O2, 12.5 in this case, there will be 3.784 units of nitrogen, N2.
This equates to an approximate composition of air as 20.9 percent oxygen and 79.1
percent ‘‘atmospheric nitrogen’’ (N2). In turn, this creates a paradox, because page
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.83
200 of Principles of Engineering Thermodynamics, by Kiefer et al., John Wiley
Sons, Inc., states air to be 20.99 percent oxygen and 79.01 percent atmospheric
nitrogen, where the ratio N2 /O2 ⫽ (?) ⫽ 79.01/20.99 ⫽ 3.764.
Also, page 35 of Applied Energy Conversion, by Skrotski and Vopat, McGraw-
Hill, Inc., indicates an assumed air analysis of 79 percent nitrogen and 21 percent
oxygen, where (?) ⫽ 3.762. On that basis, a formula is presented for the amount
of dry air chemically necessary for complete combustion of a fuel consisting of
atoms of carbon, hydrogen, and sulfur, or C, H, and S, respectively. That formula
is Wa ⫽ 11.5C ⫹ 34.5[H ⫺ (0/8)] ⫹ 4.32S, lb air/lb fuel (kg air/kg fuel).
The following derivation for the value of (?) should clear up the paradox and
show that either 3.784 or 3.78 is a sound assumption which seems to be wrong,
but in reality is not. In the above equation for Wa, the carbon, hydrogen, or sulfur
coefficient, Cx ⫽ (MO2 /DO2)Mx, where MO2 is the molecular weight of oxygen,
O2; DO2 is the decimal fraction for the percent, by weight, of oxygen, O2, in dry
air containing ‘‘atmospheric nitrogen,’’ (N2), and small amounts of inert and inactive
gases: Mx is the formula weight of the combustible element in the fuel, as indicated
by its relative amount as a reactant in the combustion equation. The alternate eval-
uation of Cx is obtained from stoichiometric chemical equations for burning the
combustible elements of the fuel, i.e., C ⫹ O2 ⫹ (?)N2 → CO2 ⫹ (?)N2; 2H2 ⫹
O2 ⫹ (?)N2 → 2H2O ⫹ (?)N2; S ⫹ O2 ⫹ (?)N2 → SO2 ⫹ (?)N2. Evidently, Cx ⫽
[MO2 ⫹ (? ⫻ MN2)]/Mx, where MN2 is the molecular weight of nitrogen, N2, and
the other items are as before.
Equating the two expressions, Cx ⫽ [MO2 ⫹ (? ⫻ MN2)]/Mx ⫽ (MO2 /DO2)Mx,
reveals that the Mx terms cancel out, indicating that the formula weight(s) of com-
bustible components are irrelevant in solving for (?). Then, (?) ⫽ (1 ⫺
DO2)[MO2 /(MN2 ⫻ DO2)]. From the above-mentioned book by Kiefer, et al.,
DO2 ⫽ 0.23188. From Marks’ Standard Handbook for Mechanical Engineers,
McGraw-Hill, Inc., MO2 ⫽ 31.9988 and MN2 ⫽ 28.0134. Thus, (?) ⫽ (1 ⫺
0.23188)[31.9988/(28.0134 ⫻ 0.23188)] ⫽ 3.7838. This demonstrates that the use
of (?) ⫽ 3.784, or 3.78, is justified for combustion equations.
By using either of the two evaluation equations for Cx, and with accurate values
of Mx, i.e., MC ⫽ 12.0111; MH ⫽ 2 ⫻ 2 ⫻ 1.00797 ⫽ 4.0319; MS ⫽ 32.064, from
Marks’ M.E. Handbook, the more precise values for CC, CH, and CS are found out
to be 11.489, 34.227, and 4.304, respectively. However, the actual Cx values, 11.5,
34.5, and 4.32, used in the formula for Wa are both brief for simplicity and rounded
up to be on the safe side.
REGENERATIVE-CYCLE GAS-TURBINE ANALYSIS
What is the cycle air rate, lb/kWh, for a regenerative gas turbine having a pressure
ratio of 5, an air inlet temperature of 60⬚F (15.6⬚C), a compressor discharge tem-
perature of 1500⬚F (815.6⬚C), and performance in accordance with Fig. 60? Deter-
mine the cycle thermal efficiency and work ratio. What is the power output of a
regenerative gas turbine if the work input to the compressor is 4400 hp (3281.1
kW)?
Calculation Procedure:
1. Determine the cycle rate
Use Fig. 60, entering at the pressure ratio of 5 in Fig. 60c and projecting to the
1500⬚F (815.6⬚C) curve. At the left, read the cycle air rate as 52 lb/kWh (23.6
kg/kWh).
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.89
2. Find the steam properties from the steam tables
(a) Steam tables normally list absolute pressures or temperature in degrees Fahren-
heit as one of their arguments. Therefore, when the steam pressure is given in terms
of a gage reading, it must be converted to an absolute pressure before the table can
be entered. To convert gage pressure to absolute pressure, add 14.7 to the gage
pressure, or pa ⫽ pg ⫹ 14.7. In this instance, pa ⫽ 145.3 ⫹ 14.7 ⫽ 160.0 lb/in2
(abs) (1103.2 kPa). Once the absolute pressure is known, enter the saturation pres-
sure table of the steam table at this value, and project horizontally to the desired
values. For 160-lb/in2
(abs) (1103.2-kPa) steam, using the ASME or Keenan and
Keyes—Thermodynamic Properties of Steam, we see that the enthalpy of evapo-
ration ⫽ 859.2 Btu/lb (1998.5 kJ/kg), and the enthalpy of saturated vapor hg ⫽
hƒg
1195.1 Btu/lb (2779.8 kJ/kg), read from the respective columns of the steam tables.
The specific volume vg of the saturated vapor of 160-lb/in2
(abs) (1103.2-kPa)
steam is, from the tables, 2.834 ft3
/lb (0.18 m3
/kg), and the entropy sg is 1.5640
Btu/(lb ⬚F) [6.55 kJ/(kg ⬚C)].
(c) Every steam table contains a separate tabulation of properties of superheated
steam. To enter the superheated steam table, two arguments are needed—the ab-
solute pressure and the temperature of the steam. To determine the properties of
1100-lb/in2
(abs) (7584.5-kPa) 600⬚F (315.6⬚C) steam, enter the superheated steam
table at the given absolute pressure and project horizontally from this absolute
pressure [1100 lb/in2
(abs) or 7584.5 kPa] to the column corresponding to the
superheated temperature (600⬚F or 315.6⬚C) to read the enthalpy of the superheated
vapor as h ⫽ 1236.7 Btu/lb (2876.6 kJ/kg) and the specific volume of the super-
heated vapor v ⫽ 0.4532 ft3
/lb (0.03 m3
/kg).
(c) For high-pressure steam use the ASME—Steam Table, entering it in the
same manner as the superheated steam table. Thus, for 7500-lb/in2
(abs) (51,712.5
kPa) 1200⬚F (648.9⬚C) steam, the enthalpy of the superheated vapor is 1474.9
Btu/lb (3430.6 kJ/kg), and the specific volume of the superheated vapor is 0.1060
ft3
/lb (0.0066 m3
/kg).
(d) To determine the enthalpy, specific volume, and the entropy of wet steam
having y percent moisture by using steam tables instead of the Mollier diagram,
apply these relations: h ⫽ hg ⫺ 100; v ⫽ vg ⫺ /100; s ⫽ sg ⫺ /100,
yh / yv ys
ƒg ƒg ƒg
where y ⫽ percentage of moisture expressed as a whole number. For 10-lb/in2
(abs)
(68.9-kPa) steam containing 40 percent moisture, obtain the needed values—hg,
vg, sg, and —from the saturation-pressure steam table and substitute in
h , v , s
ƒg ƒg ƒg
the above relations. Thus,
Note that Keenan and Keyes, in Thermodynamic Properties of Steam, do not
tabulate Therefore, this value must be obtained by subtraction of the tabulated
v .
ƒg
values, or ⫽ vg ⫺ The value thus obtained is used in the relation for the
v v . v
ƒg ƒ ƒg
volume of the wet steam. For 10-lb/in2
(abs) (68.9-kPa) steam containing 40 per-
cent moisture, vg ⫽ 38.42 ft3
/lb (2.398 m3
/kg) and ⫽ 0.017 ft3
/lb (0.0011 m3
/
vƒ
kg). Then ⫽ 38.42 ⫺ 0.017 ⫽ 28.403 ft3
/lb (1.773 m3
/kg).
vƒg
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![1.90 POWER GENERATION
In some instances, the quality of steam may be given instead of its moisture
content in percentage. The quality of steam is the percentage of vapor in the mix-
ture. In the above calculation, the quality of the steam is 60 percent because 40
percent is moisture. Thus, quality ⫽ 1 ⫺ m, where m ⫽ percentage of moisture,
expressed as a decimal.
INTERPOLATION OF STEAM TABLE VALUES
(1) Determine the enthalpy, specific volume, entropy, and temperature of saturated
steam at 151 lb/in2
(abs) (1041.1 kPa). (2) Determine the enthalpy, specific volume,
entropy, and pressure of saturated steam at 261⬚F (127.2⬚C). (3) Find the pressure
of steam at 1000⬚F (537.8⬚C) if its specific volume is 2.6150 ft3
/lb (0.16 m3
/kg).
(4) Calculate the enthalpy, specific volume, and entropy of 300-lb/in2
(abs) (2068.5-
kPa) steam at 567.22⬚F (297.3⬚C).
Calculation Procedure:
1. Use the saturation-pressure table
Study of the saturation-pressure table shows that there is no pressure value for 151
lb/in2
(abs) (1041.1 kPa) listed. So it will be necessary to interpolate between the
next higher and next lower tabulated pressure values. In this instance, these values
are 152 and 150 lb/in2
(abs) (1048.0 and 1034.3 kPa), respectively. The pressure
for which properties are being found [151 lb/in2
(abs) or 1041.1 kPa] is called the
intermediate pressure. At 152 lb/in2
(abs) (1048.0 kPa), hg ⫽ 1194.3 Btu/lb
(2777.5 kJ/kg); vg ⫽ 2.977 ft3
/lb (0.19 m3
/kg); sg ⫽ 1.5683 Btu/(lb ⬚F) [6.67
kJ/(kg ⬚C)/ t ⫽ 359.46⬚F (181.9⬚C). At 150 lb/in2
(abs) (1034.3 kPa), hg ⫽ 1194.1
Btu/lb (2777.5 kJ/kg); vg ⫽ 3.015 ft3
/lb (0.19 m3
/kg); sg ⫽ 1.5694 Btu/(lb ⬚F)
[6.57 kJ/(kg ⬚C); t ⫽ 358.42⬚F (181.3⬚C).
For the enthalpy, note that as the pressure increases, so does hg . Therefore, the
enthalpy at 151 lb/in2
(abs) (1041.1 kPa), the intermediate pressure, will equal the
enthalpy at 150 lb/in2
(abs) (1034.3 kPa) (the lower pressure used in the interpo-
lation) plus the proportional change (difference between the intermediate pressure
and the lower pressure) for a 1-lb/in2
(abs) (6.9-kPa) pressure increase. Or, at any
higher pressure, hgi ⫽ hgl ⫹ [(pi ⫺ pl )/(ph ⫺ pl )](hh ⫺ hl ), where hgi ⫽ enthalpy
at the intermediate pressure; hgl ⫽ enthalpy at the lower pressure used in the in-
terpolation; hh ⫽ enthalpy at the higher pressure used in the interpolation; pi ⫽
intermediate pressure; ph and pl ⫽ higher and lower pressures, respectively, used
in the interpolation. Thus, from the enthalpy values obtained from the steam table
for 150 and 152 lb/in2
(abs) (1034.3 and 1048.0 kPa), hgi ⫽ 1194.1 ⫹ [(151 ⫺
150)/(152 ⫺ 150)](1194.3 ⫺ 1194.1) ⫽ 1194.2 Btu/lb (2777.7 kJ/kg) at 151 lb/
in2
(abs) (1041.1 kPa) saturated.
Next study the steam table to determine the direction of change of specific
volume between the lower and higher pressures. This study shows that the specific
volume decreases as the pressure increases. Therefore, the specific volume at 151
lb/in2
(abs) (1041.1 kPa) (the intermediate pressure) will equal the specific volume
at 150 lb/in2
(abs) (1034.3 kPa) (the lower pressure used in the interpolation) minus
the proportional change (difference between the intermediate pressure and the lower
interpolating pressure) for a 1-lb/in2
(abs) pressure increase. Or, at any pressure,
vgi ⫽ vgl ⫺ [(pi ⫺ pl )/(ph ⫺ pl )](vl ⫺ vh ), where the subscripts are the same as
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 ⫽ 2.996 ft3
/lb (0.19
m3
/kg) and 151 lb/in2
(abs) (1041.1 kPa) saturated.
Study of the steam table for the direction of entropy change shows that entropy,
like specific volume, decreases as the pressure increases. Therefore, the entropy at
151 lb/in2
(abs) (1041.1 kPa) (the intermediate pressure) will equal the entropy at
150 lb/in2
(abs) (1034.3 kPa) (the lower pressure used in the interpolation) minus
the proportional change (difference between the intermediate pressure and the lower
interpolating pressure) for a 1-lb/in2
(abs) (6.9-kPa) pressure increase. Or, at any
higher pressure, sgi ⫽ sgl ⫺ [(pi ⫺ pl )/(ph ⫺ pl )](sl ⫺ sh ) ⫽ 1.5164 ⫺ [(151 ⫺
150)/(152 ⫺ 150)](1.5694 ⫺ 1.5683) ⫽ 1.56885 Btu/(lb ⬚F) [6.6 kJ/(kg ⬚C)] at
151 lb/in2
(abs) (1041.1 kPa) saturated.
Study of the steam table for the direction of temperature change shows that the
saturation temperature, like enthalpy, increases as the pressure increases. Therefore,
the temperature at 151 lb/in2
(abs) (1041.1 kPa) (the intermediate pressure) will
equal the temperature at 150 lb/in2
(abs) (1034.3 kPa) (the lower pressure used in
the interpolation) plus the proportional change (difference between the intermediate
pressure and the lower interpolating pressure) for a 1-lb/in2
(abs) (6.9-kPa) increase.
Or, at any higher pressure, tgi ⫽ tgl ⫹ [(pi ⫺ pl )/(ph ⫺ pl )](th ⫺ tl ) ⫽ 358.42 ⫹
[(151 ⫺ 150)/(152 ⫺ 150)](359.46 ⫺ 358.42) ⫽ 358.94⬚F (181.6⬚C) at 151 lb/in2
(abs) (1041.1 kPa) saturated.
2. Use the saturation-temperature steam table
Study of the saturation-temperature table shows that there is no temperature value
of 261⬚F (127.2⬚C) listed. Therefore, it will be necessary to interpolate between the
next higher and next lower tabulated values. In this instance these values are 262
and 260⬚F (127.8 and 126.7⬚C), respectively. The temperature for which properties
are being found (261⬚F or 127.2⬚C) is called the intermediate temperature.
For enthalpy, note that as the temperature increases, so does hg . Therefore, the
enthalpy at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the enthalpy
at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) plus the pro-
portional change (difference between the intermediate temperature and the lower
temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature,
hgi ⫽ hgl ⫹ [(ti ⫺ tl )/(th ⫺ tl )](hh ⫺ hl ), where hgl ⫽ enthalpy at the lower tem-
perature used in the interpolation; hh ⫽ enthalpy at the higher temperature used in
the interpolation; ti ⫽ intermediate temperature; th and tl ⫽ higher and lower tem-
peratures, respectively, used in the interpolation. Thus, from the enthalpy values
obtained from the steam table for 260 and 262⬚F (126.7 and 127.8⬚C), hgi ⫽
1167.3 ⫹ [(261 ⫺ 260)/(262 ⫺ 260)](1168.0 ⫺ 1167.3) ⫽ 1167.65 Btu/lb (2716.0
kJ/kg) at 260⬚F (127.2⬚C) saturated.
Next, study the steam table to determine the direction of change of specific
volume between the lower and higher temperatures. This study shows that the
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 ⫽ 11.763 ⫺ [(261 ⫺ 260)/
(262 ⫺ 260)](11.763 ⫺ 11.396) ⫽ 11.5795 ft3
/lb (0.7 m3
/kg) at 261⬚F (127.2⬚C)
saturated.
Study of the steam table for the direction of entropy change shows that entropy,
like specific volume, decreases as the temperature increases. Therefore, the entropy
at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the entropy at 260⬚F
(126.7⬚C) (the lower temperature used in the interpolation) minus the proportional
change (difference between the intermediate temperature and the lower temperature)
for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature, sgi ⫽ sgl ⫺
[(ti ⫺ tl )/(hh ⫺ tl )](sl ⫺ sh ) ⫽ 1.6860 ⫺ [(261 ⫺ 260)/(262 ⫺ 260)](1.6860 ⫺
1.6833) ⫽ 1.68465 Btu/(lb ⬚F) [7.1 kJ/(kg ⬚C)] at 261⬚F (127.2⬚C).
Study of the steam table for the direction of pressure change shows that the
saturation pressure, like enthalpy, increases as the temperature increases. Therefore,
the pressure at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the pres-
sure at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) plus the
proportional change (difference between the intermediate temperature and the lower
interpolating temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher
temperature, pgi ⫽ pgl ⫹ [(ti ⫺ tl )/(th ⫺ tl )](ph ⫺ pl ) ⫽ 35.429 ⫹ [(261 ⫺
260)(262 ⫺ 260)](36.646 ⫺ 35.429) ⫽ 36.0375 lb/in2
(abs) (248.5 kPa) at 261⬚F
(127.2⬚C) saturated.
3. Use the superheated steam table
Choose the superheated steam table for steam at 1000⬚F (537.9⬚C) and 2.6150
ft3
/lb (0.16 m3
/kg) because the highest temperature at which saturated steam can
exist is 705.4⬚F (374.1⬚C). This is also the highest temperature tabulated in some
saturated-temperature tables. Therefore, the steam is superheated when at a tem-
perature of 1000⬚F (537.9⬚C).
Look down the 1000⬚F (537.9⬚C) columns in the superheated steam table until
a specific volume value of 2.6150 (0.16) is found. This occurs between 325 lb/in2
(abs) (2240.9 kPa, v ⫽ 2.636 or 0.16) and 330 lb/in2
(abs) (2275.4 kPa, v ⫽ 2.596
or 0.16). Since there is no volume value exactly equal to 2.6150 tabulated, it will
be necessary to interpolate. List the values from the steam table thus:
Note that as the pressure rises, at constant temperature, the volume decreases.
Therefore, the intermediate (or unknown) pressure is found by subtracting from the
higher interpolating pressure [330 lb/in2
(abs) or 2275.4 kPa in this instance] the
product of the proportional change in the specific volume and the difference in the
pressures used for the interpolation. Or, pgi ⫽ ph ⫺ [vi ⫺ vh )/(vl ⫺ vh )](ph ⫺ pl ),
where the subscripts h, l, and i refer to the high, low, and intermediate (or unknown)
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 ⫽ 327.62 lb/in2
(abs) (2258.9 kPa) at 1000⬚F (537.9 kPa) and
a specific volume of 2.6150 ft3
/lb (0.16 m3
/kg).
4. Use the superheated steam table
When a steam pressure and temperature are given, determine, before performing
any interpolation, the state of the steam. Do this by entering the saturation-pressure
table at the given pressure and noting the saturation temperature. If the given tem-
perature exceeds the saturation temperature, the steam is superheated. In this in-
stance, the saturation-pressure table shows that at 300 lb/in2
(abs) (2068.5 kPa) the
saturation temperature is 417.33⬚F (214.1⬚C). Since the given temperature of the
steam is 567.22⬚F (297.3⬚C), the steam is superheated because its actual temperature
is greater than the saturation temperature.
Enter the superheated steam table at 300 lb/in2
(abs) (2068.5 kPa), and find the
next temperature lower than 567.22⬚F (297.3⬚C); this is 560⬚F (293.3⬚C). Also find
the next higher temperature; this is 580⬚F (304.4⬚C). Tabulate the enthalpy, specific
volume, and entropy for each temperature thus:
Use the same procedures for each property—enthalpy, specific volume, and
entropy—as given in step 2 above; but change the sign between the lower volume
and entropy and the proportional factor (temperature in this instance), because for
superheated steam the volume and entropy increase as the steam temperature in-
creases. Thus
Note: Also observe the direction of change of a property before interpolating.
Use a plus or minus sign between the higher interpolating value and the proportional
change depending on whether the tabulated value increases (⫹) or decreases (⫺).
CONSTANT-PRESSURE STEAM PROCESS
Three pounds of wet steam, containing 15 percent moisture and initially at a pres-
sure of 400 lb/in2
(abs) (2758.0 kPa), expands at constant pressure (P ⫽ C) to
600⬚F (315.6⬚C). Determine the initial temperature T1, enthalpy H1, internal energy
E1, volume V1, entropy S1, final entropy H2, internal energy E2, volume V2, entropy
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![1.94 POWER GENERATION
FIGURE 64 Constant-pressure process.
S2, heat added to the steam Q1, work output W2, change in initial energy ⌬E, change
in specific volume ⌬V, change in entropy ⌬S.
Calculation Procedure:
1. Determine the initial steam temperature from the steam tables
Enter the saturation-pressure table at 400 lb/in2
(abs) (2758.0 kPa), and read the
saturation temperature as 444.59⬚F (229.2⬚C).
2. Correct the saturation values for the moisture of the steam in the initial
state
Sketch the process on a pressure-volume (P-V), Mollier (H-S), or temperature en-
tropy (T-S) diagram, Fig. 64. In state 1, y ⫽ moisture content ⫽ 15 percent. Using
the appropriate values from the saturation-pressure steam table for 40 lb/in2
(abs)
(2758.0 kPa), correct them for a moisture content of 15 percent:
3. Determine the steam properties in the final state
Since this is a constant-pressure process, the pressure in state 2 is 400 lb/in2
(abs)
(2758.0 kPa), the same as state 1. The final temperature is given as 600⬚F (315.6⬚C).
This is greater than the saturation temperature of 444.59⬚F (229.2⬚C). Hence, the
steam is superheated when in state 2. Use the superheated steam tables, entering at
400 lb/in2
(abs) (2758.8 kPa) and 600⬚F (315.6⬚C). At this condition, H2 ⫽ 1306.9
Btu/lb (3039.8 kJ/kg); V2 ⫽ 1.477 ft3
/lb (0.09 m3
/kg). Then E2 ⫽ h2g ⫺ P2V2 /
J ⫽ 1306.9 ⫺ 400(144)(1.477)/778 ⫽ 1197.5 Btu/lb (2785.4 kJ/kg). In this equa-
tion, the constant 144 converts pounds per square inch to pounds per square foot,
absolute, and J ⫽ mechanical equivalent of heat ⫽ 778 ft lb/Btu (1 N m/J). From
the steam tables, S2 ⫽ 1.5894 Btu/(lb ⬚F) [6.7 kJ/(kg ⬚C)].
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(3) ⫽ 108.1 Btu
(114.1 kJ). In this equation, m ⫽ weight of steam used in the process ⫽ 3 lb (1.4
kg). Then
5. Check the computations
The work output W2 should equal the change in internal energy plus the heat input,
or W2 ⫽ E1 ⫺ E2 ⫹ Q1 ⫽ ⫺550.2 ⫹ 658.5 ⫽ 108.3 Btu (114.3 kJ). This value
very nearly equals the computed value of W2 ⫽ 108.1 Btu (114.1 kJ) and is close
enough for all normal engineering computations. The difference can be traced to
calculator input errors. In computing the work output, the internal-energy change
has a negative sign because there is a decrease in E during the process.
Related Calculations. Use this procedure for all constant-pressure steam pro-
cesses.
CONSTANT-VOLUME STEAM PROCESS
Five pounds (2.3 kg) of wet steam initially at 120 lb/in2
(abs) (827.4 kPa) with 30
percent moisture is heated at constant volume (V ⫽ C) to a final temperature of
1000⬚F (537.8⬚C). Determine the initial temperature T1, enthalpy H1, internal energy
E1, volume V1, final pressure P2, enthalpy H2, internal energy E2, volume V2, heat
added Q1, work output W, change in internal energy ⌬E, volume ⌬V, and entropy
⌬S.
Calculation Procedure:
1. Determine the initial steam temperature from the steam tables
Enter the saturation-pressure table at 120 lb/in2
(abs) (827.4 kPa), the initial pres-
sure, and read the saturation temperature T1 ⫽ 341.25⬚F (171.8⬚C).
2. Correct the saturation values for the moisture in the steam in the initial
state
Sketch the process on P-V, H-S, or T-S diagrams, Fig. 65. Using the appropriate
values from the saturation-pressure table for 120 lb/in2
(abs) (827.4 kPa), correct
them for a moisture content of 30 percent:
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 ⫽ 327.62 lb/in2
(abs) (2258.9 kPa). In this equation,
the volume values correspond to the upper [330 lb/in2
(abs) or 2275.4 kPa], lower
[325 lb/in2
(abs) or 2240.9 kPa], and unknown pressures.
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 ⫽ 1524.45 Btu/lb (3545.8 kJ/kg). Since the
difference in enthalpy between the two pressures is only 0.1 Btu/lb (0.23 kJ/kg)
(⫽ 1524.5 ⫺ 1524.4), the enthalpy at 327.62 lb/in2
(abs) could have been assumed
equal to the enthalpy at the lower pressure [325 lb/in2
(abs) or 2240.9 kPa], or
1524.4 Btu/lb (3545.8 kJ/kg), and the error would have been only 0.05 Btu/lb
(0.12 kJ/kg), which is negligible. However, where the enthalpy values vary by more
than 1.0 Btu/lb (2.3 kJ/kg), interpolate as shown, if accurate results are desired.
Find S2 by interpolating between pressures, or
6. Compute the changes resulting from the process
Here Q1 ⫽ (E2 ⫺ E1)m ⫽ (1365.9 ⫺ 868.9)(5) ⫽ 2485 Btu (2621.8 kJ); ⌬S ⫽
(S2 ⫺ S1)m ⫽ (1.7854 ⫺ 1.2589)(5) ⫽ 2.6325 Btu/⬚F (5.0 kJ/⬚C).
By definition, W ⫽ 0; ⌬V ⫽ 0; ⌬E ⫽ Q1. Note that the curvatures of the constant-
volume line on the T-S chart, Fig. 65, are different from the constant-pressure line,
Fig. 64. Adding heat Q1 to a constant-volume process affects only the internal
energy. The total entropy change must take into account the total steam mass m ⫽
5 lb (2.3 kg).
Related Calculations. Use this general procedure for all constant-volume
steam processes.
CONSTANT-TEMPERATURE STEAM PROCESS
Six pounds (2.7 kg) of wet steam initially at 1200 lb/in2
(abs) (8274.0 kPa) and
50 percent moisture expands at constant temperature (T ⫽ C) to 300 lb/in2
(abs)
(2068.5 kPa). Determine the initial temperature T1, enthalpy H1, internal energy E1,
specific volume V1, entropy S1, final temperature T2, enthalpy H2, internal energy
E2, volume V2, entropy S2, heat added Q1, work output W2, change in internal energy
⌬E, volume ⌬V, and entropy ⌬S.
Calculation Procedure:
1. Determine the initial steam temperature from the steam tables
Enter the saturation-pressure table at 1200 lb/in2
(abs) (8274.0 kPa), and read the
saturation temperature T1 ⫽ 567.22⬚F (297.3⬚C).
2. Correct the saturation values for the moisture in the steam in the initial
state
Sketch the process on P-V, H-S, or T-S diagrams, Fig. 66. Using the appropriate
values from the saturation-pressure table for 1200 lb/in2
(abs) (8274.0 kPa), correct
them for the moisture content of 50 percent:
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.99
4. Compute the process changes
Here Q1 ⫽ T(S2 ⫺ S1)m, where T1 ⫽ absolute initial temperature, ⬚R. So Q1 ⫽
(567.22 ⫹ 460)(1.6093 ⫺ 1.0689)(6) ⫽ 3330 Btu (3513.3 kJ). Then
Related Calculations. Use this procedure for any constant-temperature steam
process.
CONSTANT-ENTROPY STEAM PROCESS
Ten pounds (4.5 kg) of steam expands under two conditions—nonflow and steady
flow—at constant entropy (S ⫽ C) from an initial pressure of 2000 lb/in2
(abs)
(13,790.0 kPa) and a temperature of 800⬚F (426.7⬚C) to a final pressure of 2 lb/
in2
(abs) (13.8 kPa). In the steady-flow process, assume that the initial kinetic
energy Ek 1 ⫽ the final kinetic energy Ek 2. Determine the initial enthalpy H1, internal
energy E1, volume V1, entropy S1, final temperature T2, percentage of moisture y,
enthalpy H2, internal energy E2, volume V2, entropy S2, change in internal energy
⌬E, enthalpy ⌬H, entropy ⌬S, volume ⌬V, heat added Q1, and work output W2.
Calculation Procedure:
1. Determine the initial enthalpy, volume, and entropy from the steam tables
Enter the superheated-vapor table at 2000 lb/in2
(abs) (13,790.0 kPa) and 800⬚F
(427.6⬚C), and read H1 ⫽ 1335.5 Btu/lb (3106.4 kJ/kg); V1 ⫽ 0.3074 ft3
/lb (0.019
m3
/kg); S1 ⫽ 1.4576 Btu/(lb ⬚F) [6.1 kJ/(kg ⬚C)].
2. Compute the initial energy
3. Determine the vapor properties on the final state
Sketch the process on P-V, H-S, or T-S diagrams, Fig. 67. Note that the expanded
steam is wet in the final state because the 2-lb/in2
(abs) (13.8-kPa) pressure line is
under the saturation curve on the H-S and T-S diagrams. Therefore, the vapor prop-
erties in the final state must be corrected for the moisture content. Read, from the
saturation-pressure steam table, the liquid and vapor properties at 2 lb/in2
(abs)
(13.8 kPa). Tabulate these properties thus:
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.101
FIGURE 68 Irreversible adiabatic process.
Note: In a constant-entropy process, the nonflow work depends on the change
in internal energy. The steady-flow work depends on the change in enthalpy and is
larger than the nonflow work by the amount of the change in the flow work.
IRREVERSIBLE ADIABATIC EXPANSION OF
STEAM
Ten pounds (4.5 kg) of steam undergoes a steady-flow expansion from an initial
pressure of 2000 lb/in2
(abs) (13,790.0 kPa) and a temperature 800⬚F (426.7⬚C) to
a final pressure of 2 lb/in2
(abs) (13.9 kPa) at an expansion efficiency of 75 percent.
In this steady flow, assume Ek 1 ⫽ Ek 2. Determine ⌬E, ⌬H, ⌬S, ⌬V, Q, and W2.
Calculation Procedure:
1. Determine the initial vapor properties from the steam tables
Enter the superheated-vapor tables at 2000 lb/in2
(abs) (13,790.0 kPa) and
800⬚F (426.7⬚C), and read H1 ⫽ 1335.5 Btu/lb (3106.4 kJ/kg); V1 ⫽ 0.3074 ft3
/lb
(0.019 m3
/kg); E1 ⫽ 1221.6 Btu/lb (2840.7 kJ/kg); S1 ⫽ 1.4576 Btu/(lb ⬚F)
[6.1 kJ/(kg ⬚C)].
2. Determine the vapor properties in the final state
Sketch the process on P-V, H-S, or T-S diagram, Fig. 68. Note that the expanded
steam is wet in the final state because the 2-lb/in2
(abs) (13.9-kPa) pressure line is
under the saturation curve on the H-S and T-S diagram. Therefore, the vapor prop-
erties in the final state must be corrected for the moisture content. However,
the actual final enthalpy cannot be determined until after the expansion efficiency
[H1 ⫺ H2(H1 ⫺ H2s )] is evaluated.
To determine the final enthalpy H2, another enthalpy H2s must be computed by
assuming a constant-entropy expansion to 2 lb/in2
(abs) (13.8 kPa) and a temper-
ature of 126.08⬚F (52.3⬚C). Enthalpy H2s will then correspond to a constant-entropy
expansion into the wet region, and the percentage of moisture will correspond to
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.103
FIGURE 69 Irreversible adiabatic com-
pression process.
IRREVERSIBLE ADIABATIC STEAM
COMPRESSION
Two pounds (0.9 kg) of saturated steam at 120 lb/in2
(abs) (827.4 kPa) with 80
percent quality undergoes nonflow adiabatic compression to a final pressure of 1700
lb/in2
(abs) (11,721.5 kPa) at 75 percent compression efficiency. Determine the
final steam temperature T2, change in internal energy ⌬E, change in entropy ⌬S,
work input W, and heat input Q.
Calculation Procedure:
1. Determine the vapor properties in the initial state
From the saturation-pressure steam tables, T1 ⫽ 341.25⬚F (171.8⬚C) at a pressure
of 120 lb/in2
(abs) (827.4 kPa) saturated. With x1 ⫽ 0.8, E1 ⫽ ⫽
u ⫹ x u
ƒ 1 ƒg
312.05 ⫹ 0.8(795.6) ⫽ 948.5 Btu/lb (2206.5 kJ/kg), from internal-energy values
from the steam tables. The initial entropy is S1 ⫽ ⫽ 0.4916 ⫹
s ⫹ x s
ƒ 1 ƒg
0.8(1.0962) ⫽ 1.3686 Btu/(lb ⬚F) [5.73 kJ/(kg ⬚C)].
2. Determine the vapor properties in the final state
Sketch a T-S diagram of the process, Fig. 69. Assume a constant-entropy compres-
sion from the initial to the final state. Then S2s ⫽ S1 ⫽ 1.3686 Btu/(lb ⬚F) [5.7
kJ/(kg ⬚C)].
The final pressure, 1700 lb/in2
(abs) (11,721.5 kPa), is known, as is the final
entropy, 1.3686 Btu/(lb ⬚F) [5.7 kJ/(kg ⬚C)] with constant-entropy expansion. The
T-S diagram (Fig. 69) shows that the steam is superheated in the final state. Enter
the superheated steam table at 1700 lb/in2
(abs) (11,721.5 kPa), project across to
an entropy of 1.3686, and read the final steam temperature as 650⬚F (343.3⬚C). (In
most cases, the final entropy would not exactly equal a tabulated value, and it would
be necessary to interpolate between tabulated entropy values to determine the in-
termediate pressure value.)
From the same table, at 1700 lb/in2
(abs) (11.721.5 kPa) and 650⬚F (343.3⬚C),
H2s ⫽ 1214.4 Btu/lb (2827.4 kJ/kg); V2s ⫽ 0.2755 ft3
/lb (0.017 m3
/lb). Then
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![1.104 POWER GENERATION
E2s ⫽ H2s ⫺ P2V2s /J ⫽ 1214.4 ⫺ 1700(144)(0.2755)/788 ⫽ 1127.8 Btu/lb (2623.3
kJ/kg). Since E1 and E2s are known, the ideal work W can be computed. Or, W ⫽
E2s ⫺ E1 ⫽ 1127.8 ⫺ 948.5 ⫽ 179.3 Btu/lb (417.1 kJ/kg).
3. Compute the vapor properties of the actual compression
Since the compression efficiency is known, the actual final internal energy can be
found from compression efficiency ⫽ ideal W /actual W ⫽ E2s ⫺ E1 /(E2 ⫺ E1), or
0.75 ⫽ 1127.8 ⫺ 948.5/(E2 ⫺ 948.5); E2 ⫽ 1187.6 Btu/lb (2762.4 kJ/kg). Then
E ⫽ (E2 ⫺ E2)m ⫽ (1187.6 ⫺ 948.5)(2) ⫽ 478.2 Btu (504.5 kJ) for 2 lb (0.9 kg)
of steam. The actual work input W ⫽ ⌬E ⫽ 478.2 Btu (504.5 kJ). By definition,
Q ⫽ 0.
Last, the actual final temperature and entropy must be computed. The final actual
internal energy E2 ⫽ (1187.6 Btu/lb (2762.4 kJ/kg) is known. Also, the T-S dia-
gram shows that the steam is superheated. However, the superheated steam tables
do not list the internal energy of the steam. Therefore, it is necessary to assume a
final temperature for the steam and then compute its internal energy. The computed
value is compared with the known internal energy, and the next assumption is
adjusted as necessary. Thus, assume a final temperature of 720⬚F (382.2⬚C). This
assumption is higher than the ideal final temperature of 650⬚F (343.3⬚C) because
the T-S diagram shows that the actual final temperature is higher than the ideal final
temperature. Using values from the superheated steam table for 1700 lb/in2
(abs)
(11,721.5 kPa) and 720⬚F (382.2⬚C), we find
PV 1700(144)(0.3283)
E ⫽ H ⫺ ⫽ 1288.4 ⫺ ⫽ 1185.1 Btu/lb (2756.5 kJ/kg)
J 778
This value is less than the actual internal energy of 1187.6 Btu/lb (2762.4 kJ/
kg). Therefore, the actual temperature must be higher than 720⬚F (382.2⬚C), since
the internal energy increases with temperature. To obtain a higher value for the
internal energy to permit interpolation between the lower, actual, and higher values,
assume a higher final temperature—in this case, the next temperature listed in the
steam table, or 740⬚F (393.3⬚C). Then, for 1700 lb/in2
(abs) (11,721.5 kPa) and
740⬚F (393.3⬚C),
1700(144)(0.3410)
E ⫽ 1305.8 ⫺ ⫽ 1198.5 Btu/lb (2757.7 kJ/kg)
778
This value is greater than the actual internal energy of 1187.6 Btu/lb (2762.4
kJ/kg). Therefore, the actual final temperature of the steam lies somewhere between
720 and 740⬚F (382.2 and 393.3⬚C). Interpolate between the known internal ener-
gies to determine the final steam temperature and final entropy. Or,
1178.6 ⫺ 1185.1
T ⫽ 720 ⫹ (740 ⫺ 720) ⫽ 723.7⬚F (384.3⬚C)
2
1198.5 ⫺ 1185.1
1187.6 ⫺ 1185.1
S ⫽ 1.4333 ⫹ (1.4480 ⫺ 1.4333)
1
1198.5 ⫺ 1185.1
⫽ 1.4360 Btu/(lb ⬚F) [6.0 kJ/(kg ⬚C)]
⌬S ⫽ (S ⫺ S )m ⫽ (1.4360 ⫺ 1.3686)(2) ⫽ 0.1348 Btu/⬚F (0.26 kJ/⬚C)
2 1
Note that the final actual steam temperature is 73.7⬚F (40.9⬚C) higher than that
(650⬚F or 343.3⬚C) for the ideal compression.
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![1.108 POWER GENERATION
saturated steam, (3) to superheat the steam to 1000⬚F (537.8⬚C), and (4) Q1, ⌬V,
and ⌬S from state 1 to state 4.
Calculation Procedure:
1. Sketch the T-S diagram for this process
Figure 71 is typical of a steam boiler and superheater. Feedwater fed to a boiler is
usually subcooled liquid. If the feedwater pressure is relatively high, subcooling
must be taken into account, if accurate results are desired. Some authorities rec-
ommend that at pressures below 400 lb/in2
(abs) (2758.0 kPa) subcooling be ig-
nored and values from the saturated-steam table be used. This means that the en-
thalpies and other properties listed in the steam table corresponding to the actual
water temperature are sufficiently accurate. But above 400 lb/in2
(abs) 2758.0 kPa),
the compressed-liquid table should be used.
2. Determine the initial properties of the liquid
In the saturation-temperature steam table read, at 140⬚F (60.0⬚C), ⫽ 107.89
hƒ
Btu/lb (251.0 kJ/kg); ⫽ 2.889 lb/in2
(19.9 kPa); ⫽ 0.01629 ft3
/lb (0.0010
p v
ƒ ƒ
m3
/kg); ⫽ 0.1984 Btu/(lb ⬚F) [0.83 kJ/(kg ⬚C)].
sƒ
Next, the enthalpy, volume, and entropy of the water at 1500 lb/in2
(abs)
(10,342.5 kPa) and 140⬚F (60.0⬚C) must be found. Since the water is at a much
higher pressure than that corresponding to its temperature [1500 versus 2.889 lb/
in2
(abs)], the compressed-liquid portion of the steam table must be used. This table
shows that three desired properties are plotted for 32, 100, and 200⬚F (0.0, 37.8,
and 93.3⬚C) and higher temperatures. However, 140⬚F (60.0⬚C) is not included.
Therefore, it is necessary to interpolate between 100 and 200⬚F (37.8 and 93.3⬚C).
Thus, at 1500 lb/in2
(abs) (10,342.5 kPa) in the compressed-liquid table:
Each property is interpolated in the following way:
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![1.112 POWER GENERATION
Economic Analyses of
Alternative Energy Sources
CHOICE OF MOST ECONOMIC ENERGY SOURCE
USING THE TOTAL-ANNUAL-COST METHOD
A penal institution needs 50,000 MWh of electric energy per year with a maximum
demand of 10 MW. This can be purchased from the local utility for $480,000
annually. As an alternate scheme, the penal institution is considering installing a
10-MW steam-turbine plant using equipment purchased at reduced prices. Three
different plants are being proposed, as shown in Table 4. The plant will run 24 h/
day, with inmate operators working an 8-h day, 5 days per week. The average
annual salary for the inmate operators, which includes their prison costs, is $5200.
Maintenance costs are estimated at 90 cents per ton of coal burned for all alternative
plants. From past experience, the designers found that installed costs for penal
institutions exceed estimates by 20 percent; so this allowance is made as a contin-
gency. General operating supplies are estimated as $10,000 annually for all plants
being considered. The useful life of each plant is taken as 15 years; the cost of
money is 6 percent. Real estate taxes on the real estate for the plant are estimated
to be 4 percent; various operating taxes add 1 percent to annual operating costs.
Annual insurance costs are 0.2 percent of all equipment costs. Coal fuel costs $6.50
per short ton and has a higher heating value (HHV) of 14,200 Btu/lb (33,086 kJ/
kg). Determine which scheme is the economic one, using the total-annual-cost
method, assuming equal reliability.
Calculation Procedure:
1. Determine the required boiler capacity for each proposed plant
Use the relation: Required boiler capacity, lb/h (kg/h) ⫽ (plant heat rate, lb/kWh
[kg/kWh])(plant capacity, kW). Find the full-load steam rate and plant capacity in
Table 4. Then the required boiler capacity, C, for each plant is
Plan A: C ⫽ 11.8(10,000) ⫽ 118,000, say 120,000 lb/h (54,480 kg/h)
Plan B: C ⫽ 10.5(10,000) ⫽ 105,000, say 110,000 lb/h (49,940 kg/h)
Plan C: C ⫽ 8.5(10,000) ⫽ 85,000, say 90,000 lb/h (40,860 kg/h)
The boiler capacity was rounded off to the next-higher standard boiler rating for
each plan. This permits purchase of a standardized boiler while providing a nominal
amount of extra capacity. Do not, in general, round off to the next-lower capacity
because this can result in choosing a boiler with too small a capacity for the full-
load output of the turbine.
2. Compute the cost of the boiler and related equipment
As noted earlier, this is a plant for a penal institution, and the equipment is being
obtained at reduced cost. However, the procedure for computing the cost of the
boiler and its related equipment is the same, regardless of the equipment cost and
the type of use of the equipment. Hence, use the relation: Installed boiler cost, $
⫽ (required boiler capacity, lb/h [kg/h])(boiler cost, $/lb h capacity). Using the
cost data in Table 4, installed boiler cost is
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![1.114 POWER GENERATION
5. Determine the depreciation and fixed-charge rates
The depreciation rate ⫽ i/[(1 ⫹ i)n
⫺ 1], where i ⫽ earnings rate or interest that
the investment would yield (6 percent for this plant); n ⫽ expected life of the capital
equipment, years. Or, depreciation rate ⫽ 0.06/[(1.06)15
⫺ 1] ⫽ 0.0431.
The fixed-charge rate ⫽ 兺 (interest rate, depreciation rate, tax rate, and insurance
rate). Or, fixed-charge rate ⫽ 0.06 ⫹ 0.0431 ⫹ 0.04 ⫹ 0.002 ⫽ 0.1451, or 14.51%.
6. Find the unit and total fuel costs
Unit fuel cost, $/ton(10)6
/2000 (HHV) ⫽ $6.50(10)6
/2000(14,200) ⫽ 22.9 cents
per 106
Btu. Then total annual fuel cost ⫽ (unit fuel cost, cents/106
Btu)(annual
kWh energy consumption)(average plant heat rate, Btu/kWh). For each plan, total
annual fuel cost is
Plan A: 0.229(50)(16,000) ⫽ $183,200
Plan B: 0.229(50)(13,500) ⫽ $154,575
Plan C: 0.229(50)(12,000) ⫽ $137,400
7. Compute the annual operating and maintenance costs
There are 21 shifts per week (⫽ 3 shifts per day ⫻ 7 days per week). Any operator
works 5 shifts per week (⫽ 1 shift per day ⫻ 5 working days per week). Then
each position requires 21/5 ⫽ 4.2 operators to keep the post continuously staffed.
Assume that the operators are interchangeable in their posts. Then all plans will
need (4 operators per shift)(4.2 people per position) ⫽ 16.8, or 17, operators. Since
each operator earns prison wages and keep costs of $5200 annually, the annual cost
of operating labor ⫽ (17 operators)($5200) ⫽ $88,400, say, $88,000 per year, av-
erage, over the life of the plant.
Average annual maintenance cost ⫽ (tons of coal burned per year)(maintenance
cost per ton of coal) ⫽ (annual total fuel cost/cost per ton of coal)(maintenance
cost per ton of coal). For each plan, average annual maintenance cost is
$183,200
Plan A: (0.90) ⫽ $25,366
冉 冊
$6.50
$154,575
Plan B: (0.90) ⫽ $21,403
冉 冊
$6.50
$137,400
Plan C: (0.90) ⫽ $19,025
冉 冊
$6.50
8. Calculate the total annual operating cost and the total annual cost
Tabulate each operating cost as shown in Table 6. Then add the investment charges
to obtain the total annual cost. This gives the results shown at the bottom of Ta-
ble 6.
9. Analyze the costs and choose the best alternative plan
For the given conditions of fixed-charge rate (14.51 percent) and fuel costs, plan
B incurs the least annual cost of the three plans. However, since the utility can
supply electric power for less than any of these private plants, the utility supply
would be the logical choice. In certain circumstances, even if the utility service
cost $520,000, it might be selected in preference to the private plant plan B. If no
utility service were available, plan B would undoubtedly be chosen for this instal-
lation.
Related Calculations. Use this procedure to determine the most economic en-
ergy source for any type of power plant—steam, diesel, gas-turbine, windmill—
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![1.116 POWER GENERATION
plan because it is considerably less costly than that of the utility service. From this
result an important principle is clear: As the fixed-charge rate decreases, the alter-
natives involving higher capital investment are favored as optimum choices. This
means that as the cost of money use decreases, or the life span of the investment
increases, more money can be invested to secure greater overall economy. This
principle holds only when the annual operating cost of each alternative remains
unchanged.
2. Analyze the effect of increased fuel costs
In thermal stations, fuel prices control the general magnitude of the annual operating
costs. Thus, the fuel purchase price will have a controlling effect on the economic
conclusions drawn from financial comparisons. Assume that the fuel cost doubles
to $13 per short ton. Compute the fuel costs as in step 6 of the previous calculation
procedure. Tabulate the total annual costs as follows:
Total annual costs Plan A Plan B Plan C
Investment charges at 14.5 percent . . . . . . . . . . . . $221,000 $233,000 $260,000
Operating costs with $13 coal . . . . . . . . . . . . . . . . 492,000 432,000 396,000
Total annual cost . . . . . . . . . . . . . . . . . . . . . . . . . $713,000 $665,000 $656,000
For this condition, the plan requiring the highest investment again becomes the
optimum one. From this a corollary principle may be deduced: As the cost of fuel
increases, plans involving higher capital investment are favored as optimum. Thus,
as the cost of fuel increases, more money may be invested to gain increased effi-
ciency. This holds only when the fixed-charge rate is common to all plans and
remains constant.
3. Determine the optimum investment based on the earned rate of return
Assume the same data as previously, except that the utility service costs $530,000
annually. This price could be obtained by getting a direct quote from the utility.
Compute all the items of annual cost except interest and depreciation. Then the
difference between the utility service cost of $530,000 and the computed annual
costs represents a saving available for the annual interest on the investment and its
amortization. The sum of the differences, $A, and the investment from step 4 of
the previous calculation procedure will be the principal, $P, for each plan. Solve
for the interest rate, I, using a calculator, or trial and error, using A/P ⫽ i/[1 ⫺
(1 ⫹ i)⫺n
]. Tabulate the results thus:
Plan A Plan B Plan C
Utility
service
Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000
Annual taxes on capital . . . . . . . . . . . . 61,000 64,000 72,000
Annual insurance costs . . . . . . . . . . . . . 3,000 3,000 4,000
Annual costs excluding interest and
depreciation . . . . . . . . . . . . . . . . . . . . $373,000 $344,000 $334,000 $530,000
Difference available for annuity A . . . $157,000 $186,000 $196,000 Base
Investment P . . . . . . . . . . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000
Ratio A/P. . . . . . . . . . . . . . . . . . . . . . . . 0.1033 0.1155 0.1094
Rate of return i, percent . . . . . . . . . . . . 6.4 7.8 7.0
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![1.118 POWER GENERATION
Plan A Plan B Plan C
Utility
service
Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000 $530,000
Savings available for investment
charges . . . . . . . . . . . . . . . . . . . . . . . . $221,000 $253,000 $272,000 Base
Investment. . . . . . . . . . . . . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000 0
Ratio of savings to investment,a
percent . . . . . . . . . . . . . . . . . . . . . . . . 14.6 15.7 15.2
Increment savings . . . . . . . . . . . . . . . . . $32,000 $19,000
Increment investment . . . . . . . . . . . . . . $90,000 $180,000
Ratio of increment savings to
increment investment, percenta
. . . . 35.6
10.6
a
Rates comparable to fixed-charge rate ⫽ 14.51 percent.
Again, Plan B is the optimum one. This is the same conclusion as drawn from
the previous methods of comparison.
5. Determine the maximum justifiable investment
In this method, the savings available for investment charges are divided by the
fixed-charge rate to arrive at a justifiable maximum investment. Compare the jus-
tifiable maximum investment with the actual investment required for the given plan.
If the actual investment is equal to, or less than, the justifiable investment, then the
plan may prove satisfactory. Compute the actual increment investment and tabulate
thus:
Plan A Plan B Plan C
Utility
service
Annual operating costs . . . . . . . . . . . . . . $309,000 $277,000 $258,000 $530,000
Savings available for investment
charges. . . . . . . . . . . . . . . . . . . . . . . . . . $220,000 $253,000 $272,000 Base
Fixed-charge rate, percent . . . . . . . . . . . . 14.5 14.5 14.5
Justifiable investment . . . . . . . . . . . . . . . . $1,525,000 $1,745,000 $1,877,000
Active investment needed . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000
Increment savings. . . . . . . . . . . . . . . . . . . $32,000 $19,000
Fixed-charge rate, percent . . . . . . . . . . . . 14.5 14.5
Justifiable added investment . . . . . . . . . . $221,000 $131,000
Actual increment investment needed . . . $90,000 $180,000
Actual investments for the three plans, A, B, and C, are less than the maximum
justifiable investments and hence appear to be satisfactory. Again, plan B is the
optimum choice.
6. Compute the total present worth of each alternative
The plan having the lowest present worth involves a minimum overall expenditure
during the life of the investment and therefore is the desirable, or optimum, plan.
Compute the annual taxes on capital and the annual insurance cost. Determine the
present-worth factor, PWF, from the relation in step 3 above for 6 percent interest
and a 15-year life. Then PWF ⫽ 0.103. Solve for the present worth of the costs
excluding interest and depreciation and add the investment, $P, as in Wp ⫽ [P ⫹
(t ⫹ j)P ⫹ Co]/PWF, where Wp ⫽ total present worth of a series of annual expen-
ditures, t ⫽ tax rate on capital equipment, j ⫽ insurance rate, Co ⫽ annual operating
expenses. Tabulate the results thus:
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![1.120 POWER GENERATION
Kilowatts Hours
5000 200
4000 4000
2000 2000
1000 1000
500 1560
Two plants, a steam-turbine plant and a diesel-engine plant, are being considered.
For the steam plant, coal at $4.50 per short ton with a heating value of 14,000 Btu/
lb (32,620 kJ/kg) is available. For the diesel plant, oil at $2.10/bbl containing 358
lb (162.5 kg) with a heating value of 18,500 Btu/lb (43,105 kJ/kg) is available.
The performance characteristics of the plants are as follows: Steam: I ⫽ 106
(6 ⫹
8L ⫹ 0.111L3
); diesel: I ⫽ 106
(9 ⫹ 4L ⫹ [0.52L2
⫺ 0.016L3
]). The steam plant
requires three more operators at a cost of $4000 per person than does the diesel
plant. The fixed-charge rate is 12 percent for each plant. Which plant should be
selected if the steam plant costs $230/kW installed? No reserve capacity is required.
Use the total annual costs as the basis of comparison of the plants.
Calculation Procedure:
1. Compute the unit fuel cost for each prime mover
The steam-plant unit fuel cost, cents per million Btu ⫽ 106
(fuel cost, cents/lb)/
(lb/ton)(heating value, Btu/lb) ⫽ 106
(450)/(2000)(14,000) ⫽ 16.07 cents per
106
/Btu. The diesel unit fuel cost, cents per million Btu ⫽ 31.71 cents per 106
,
using the same method of calculation.
2. Find the total energy input for each plant
Use the input-output characteristic for each plant, and compute the total energy for
each load tabulated above. Thus, for the steam plant and the load of 5000 kW ⫽
5 MW, I ⫽ 106
(6 ⫹ 8 ⫻ 5 ⫹ 0.11 ⫻ 125) ⫽ 59.9 ⫻ 106
Btu/h (17.55 MW).
Tabulate the computed input for each plant for each load on a per-hour basis.
Multiply the per-hour input by the number of hours at this load to obtain the total
input at each load. Thus, for 200 h at 5 MW, total input ⫽ (200 h)(59.9 ⫻ 106
Btu/
h) ⫽ 11.98 ⫻ 109
Btu (12.58 ⫻ 109
kJ). Tabulate these values:
Calculation of Fuel Requirements
MW h MW
Steam plant
Input rate,
106
Btu/h
Total input,
109
Btu
Diesel plant
Input rate,
106
Btu/h
Total input,
109
Btu
5 200 1,000 59.90 11.98 40.00 8.00
4 4,000 16,000 45.12 180.48 32.30 129.20
2 2,000 4,000 22.89 45.78 18.95 37.90
1 1,000 1,000 14.11 14.11 13.50 13.50
1
⁄2 1,560 780 10.01 15.62 11.13 17.36
Total . . . 8,760 22,780 . . . . . . . . . 267.97 . . . . . . . . . 205.96
Average heat rate, Btu/kWh. . . . . . . . . . . 11,800 . . . . . . . . . 9050
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.121
Find the heat rate, HR, for each plant by dividing the tabulated total heat input
by the total-load energy, MWh. For the steam plant, HR ⫽ 11,800 Btu/kWh
(11,210 kL/kWh). Tabulate the values for each plant.
3. Calculate the total annual cost for each plant
The total annual cost is Co ⫽ Cc ⫹ Cf ⫹ Cl, where the subscripts are c ⫽ capital-
cost write off, f ⫽ fuel cost, and l ⫽ labor cost. For the steam plant, Cc ⫽
($230)(5000)(0.12) ⫽ $318,000. And Cf ⫽ (0.1607)(267,970 ⫻ 109
) ⫽ $43,063; Cl
⫽ 3($4000) ⫽ $12,000. Then Co ⫽ $193,063 per year. For the diesel plant, Co ⫽
$197,310 per year.
4. Choose the most economical prime mover
Since the steam plant has a lower total annual cost, choose it in preference to the
diesel plant, despite the higher first cost and greater relative labor cost of the steam
plant. These costs are outweighed by the lower fuel cost of the steam plant.
5. Evaluate the effect of relative fuel prices
Since fuel prices vary, particularly the price of oil, total-annual-cost comparisons
depend on the relative fuel prices at the time of the study. To give a complete
picture of the importance of fuel prices in determining plant economics, the cost
comparison can be set up to determine the relationship of fuel prices for given
conditions of labor costs, thermal efficiency, and loan conditions.
From the foregoing, the plants are equally desirable when their annual costs are
equal, Co,steam ⫽ Co,diesel. Or, (Cc ⫹ Cf)steam ⫽ (Cc ⫹ Cf)diesel. Substituting previously
computed values, ($138,000) ⫹ [(267,970 ⫻ 106
⫻ S)/(2000 ⫻ 14,000)] ⫹ $12,000
⫽ ($132,000) ⫹ [(205,960 ⫻ 106
⫻ D)/(358 ⫻ 18,500)], where S ⫽ price of coal,
$/ton; D ⫽ price of oil, $/bbl. Solving gives D ⫽ 0.578 ⫹ 0.308S. Thus, for coal
at $4.50/ton, oil would have to be D ⫽ 0.578 ⫹ 0.308(4.50) ⫽ $1.96/bbl to
produce equal fuel costs. With oil at $2.10/bbl, coal would have to be $4.94/ton
for equal fuel costs.
Related Calculations. Use this annual-cost procedure to compare any type of
prime mover—steam, diesel, natural-gas, gas-turbine, windmill—serving any type
of load—utility, industrial, institutional, or marine. Note that the relative current
prices of fuel do not change the steps or methods given here. Hence, use the same
steps, inserting today’s fuel prices. This procedure is the work of B. G. A. Skrotzki
and William Vopat, as reported in their book Power Station Engineering and Econ-
omy, McGraw-Hill.
DETERMINING IF A PRIME MOVER SHOULD BE
OVERHAULED
A 20-MW steam turbine has been in use for 4 years and generates 105,000,000
kWh annually. The first year this energy was produced at an average heat rate of
15,000 Btu/kWh (14,250 kJ/kWh). Because of blade erosion the average heat rate
increases at the rate of 0.05 percent per 1000 h of operation. This increase is
expected to continue for the life of the machine despite adequate routine mainte-
nance. To reblade the turbine and restore its original efficiency will cost $35,000.
The total life of the turbine is 15 years, and it will be required to produce the same
amount of energy annually in the future. If fuel costs 20 cents per million Btu, a
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![MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.123
3. Determine the reblading annual interest and amortization rates
The factor for computing the annual interest and amortization is i/(1 ⫺ [1 ⫹ I]⫺m
),
where I ⫽ desired rate of return; m ⫽ life of reblading investment, years. Thus,
with a reblading investment life of 4 years and n ⫽ 11 years, factor ⫽ 0.2886, after
substituting the above equation. To find the annual interest and amortization on the
$35,000 reblading investment, take the product of the factor and the reblading cost,
or (0.2886)($35,000) ⫽ $10,101. The rate of return on the $35,000 investment is
found by setting up the ratio of annual savings, A, to the investment, P, and solving
by calculator, or trial and error, the relation A/P ⫽ i/(1 ⫺ [1 ⫹ i]⫺n
) for I, the rate
of return. For n ⫽ 9 years, I ⫽ 8.0 percent. Enter this value in the table above.
Computing the other rates of return in the same manner shows that the maximum
rate of return occurs when reblading is done in the ninth year. The 8 percent return
exceeds the minimum desired 6 percent rate of return.
Related Calculations. Use this procedure to compute the timing of and return
on investment for any expenditure which improves the efficiency of any type of
prime mover—steam turbine, diesel engine, gasoline engine, gas turbine, hydraulic
turbine, windmill, etc., in a stationary, portable or marine installation. This proce-
dure is the work of B. G. A. Skrotzki and William Vopat.
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![STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.13
TABLE 3 Standard Condenser Tube Data
FIGURE 9 Heat-transfer and correction curves for calculating surface-condenser performances.
690 Btu/(ft2
䡠 ⬚F) [14,104.8 kJ/(m2
䡠 ⬚C)] LMTD (log mean temperature difference).
Also read from Fig. 9 the temperature correction factor for an inlet-water temper-
ature of 80⬚F (26.7⬚C) by entering at the bottom at 80⬚F (26.7⬚C) and projecting
vertically upward to the temperature-correction curve. From the intersection with
this curve, project to the right to read the correction as 1.04. Correct U for tem-
perature and cleanliness by multiplying the value obtained from the chart by the
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![2.14 POWER GENERATION
correction factors, or U ⫽ 690(1.04)(0.80) ⫽ 574 Btu/(ft2
䡠 h 䡠 ⬚F) [11,733.6 kJ/
(m2
䡠 h 䡠 ⬚C)] LMTD.
3. Compute the tube constant
Read from Table 3, for two passes through 3
⁄4-in (1.9-cm) OD 18 BWG tubes, k ⫽
a constant ⫽ 0.377. Then kL/V ⫽ 0.377(14)/6.5 ⫽ 0.812.
4. Compute the outlet-water temperature
The equation for outlet-water temperature is t2 ⫽ ts ⫺ (ts ⫺ t1 /ex
), where x ⫽ (kL
/V)(U/500), or x ⫽ 0.812(574/500) ⫽ 0.932. Then ex
⫽ 2.71830.932
⫽ 2.54. With
this value known, t2 ⫽ 101.1 ⫺ (101.1 ⫺ 80/2.54) ⫽ 92.8⬚F (33.8⬚C). Check to
see that ⌬t(ts ⫺ t2) is less than the minimum 5⬚F (2.8⬚C) terminal difference. Or,
101.1 ⫺ 92.8 ⫽ 8.3⬚F (4.6⬚C), which is greater than 5⬚F (2.8⬚C).
5. Compute the required tube surface area
The required cooling-water flow, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(25,000)/
[500(92.8 ⫺ 80)] ⫽ 3700 gal/min (233.4 L/s). This equation assumes that 950 Btu
is to be removed from each pound (2209.7 kJ/kg) of steam condensed. When a
different quantity of heat must be removed, use the actual quantity in place of the
950 in this equation.
With the tube constant kL/V and cooling-water flow rate known, the required
area is computed from A ⫽ (kL/V)(gpm) ⫽ (0.812)(3700) ⫽ 3000 ft2
(278.7 m2
).
Since the value of U was not corrected for condenser loading, it is necessary to
check whether such a correction is needed. Condenser loading ⫽ S/A ⫽ 25,000/
3000 ⫽ 8.33 lb/ft2
(40.7 kg/m2
). Figure 9 shows that no correction (correction
factor ⫽ 1.0) is necessary for loadings greater than 8.0 lb/ft2
(39.1 kg/m2
). There-
fore, the loading for this condenser is satisfactory without correction.
This step concludes the general calculation procedure for a surface condenser
serving any steam turbine. The next procedure shows the method to follow when
a higher cooling-water inlet temperature prevails.
6. Compute the cooling-water outlet temperature
(b) Higher cooling water temperature. From Fig. 9 for 85⬚F (29.4⬚C) cooling-water
inlet temperature and a 0.80 cleanliness factor, U ⫽ 690(1.06)(0.80) ⫽ 585 Btu/
(ft2
䡠 h 䡠 ⬚F) [3.3 kJ/(m2
䡠 ⬚C 䡠 s)] LMTD.
Given data from Table 3, the tube constant kL/V ⫽ 0.377(14)/6.5 ⫽ 0.812.
Then x ⫽ (kL/V)(U/500) ⫽ 0.812(585/500) ⫽ 0.950. Using this exponent, we get
ex
⫽ 2.81830.950
⫽ 2.586. The cooling-water outlet temperature is then t2 ⫽ ts ⫺
(ts ⫺ t1 /ex
) ⫽ 101.1 ⫺ (101.1 ⫺ 85)/2.586 ⫽ 94.9⬚F (34.9⬚C). Check to see that
⌬t(ts ⫺ t2) is greater than the minimum 5⬚F (2.8⬚C) terminal temperature difference.
Or, 101.1 ⫺ 94.9 ⫽ 6.5⬚F (3.6⬚C), which is greater than 5⬚F (2.8⬚C).
7. Compute the water flow rate, required area, and loading
The required cooling-water flow, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(25,000)/
[500(94.9 ⫺ 85)] ⫽ 4800 gal/min (302.8 L/s).
With the tube constant kL/V and cooling-water flow rate known, the required
area is computed from A ⫽ (kL/V)(gpm) ⫽ 0.812(4800) ⫽ 3900 ft2
(362.3 m2
).
Then loading ⫽ S/A ⫽ 25,000/3900 ⫽ 6.4 lb/ft2
(31.2 kg/m2
).
Since the loading is less than 8 lb/ft2
(39.1 kg/m2
), refer to Fig. 9 to obtain the
loading correction factor. Enter at the bottom at 6.4 lb/ft2
(31.2 kg/m2
), and project
vertically to the loading curve. At the right, read the loading correction factor as
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![STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.15
0.95. Now the value of U already computed must be corrected, and all dependent
quantities recalculated.
8. Recalculate the condenser proportions
First, correct U for loading. Or, U ⫽ 585(0.95) ⫽ 555. Then x ⫽ 0.812(555/500)
⫽ 0.90; ex
⫽ 2.71830.90
⫽ 2.46; t2 ⫽ 101.1 ⫺ (101.1 ⫺ 85/2.46) ⫽ 94.6⬚F (34.8⬚C).
Check ⌬t ⫽ ts ⫺ t2 ⫽101.1 ⫺ 94.6 ⫽ 6.5⬚F (3.6⬚C), which is greater than 5⬚F
(2.8⬚C). The cooling-water flow rate, gal/min ⫽ 950(25,000)/[500(94.6 ⫺ 85)] ⫽
4950 gal/min (312.3 L/s). Then A ⫽ 0.812(4950) ⫽ 4020 ft2
(373.5 m2
), and
loading ⫽ 25,000/4020 ⫽ 6.23 lb/ft2
(30.4 kg/m2
).
Check the correction factor for this loading in Fig. 9. The correction factor is
0.94, compared with 0.95 for the first calculation. Since the value of U would be
changed only about 1 percent by using the lower factor, the calculations need not
be revised further. Where U would change by a larger amount—say 5 percent or
more—it would be necessary to repeat the procedure just detailed, applying the
new correction factor.
Note that the 5⬚F (2.8⬚C) increase in cooling-water temperature (from 80 to 85⬚F
or 26.7 to 29.4⬚C) requires an additional 1020 ft2
(94.8 m2
) of condenser surface
and 125 gal/min (7.9 L/s) of cooling-water flow to maintain the same back pres-
sure. These increments will vary, depending on the temperature level at which the
increase occurs. The effect of reduced steam flow on the steam pressure in the
condenser shell will not be computed because the recalculation above is the last
step in part (b) of this procedure.
(c) Reduced steam flow to condenser.
9. Determine the condenser loading
From procedure (a) above, the cooling-water flow ⫽ 3700 gal/min (233.4 L/s);
condenser surface A ⫽ 3000 ft2
(278.7 m2
). Then, with a 15,000-lb/h (1.9-kg/s)
steam flow, loading ⫽ S/A ⫽ 15,000/3000 ⫽ 5 lb/ft2
(24.4 kg/m2
).
10. Compute the heat-transfer coefficient
Correct the previous heat-transfer rate U ⫽ 690 Btu/(ft2
䡠 h 䡠 ⬚F) [3.9 kJ/(m2
䡠 ⬚C 䡠
s)] LMTD for temperature, cleanliness, and loading. Or, U ⫽ 690(1.04)(0.80)(0.89)
⫽ 511 Btu/(ft2
䡠 h 䡠 ⬚F) [2.9 kJ/(m2
䡠 ⬚C 䡠 s)] LMTD, given the correction factors from
Fig. 9.
11. Compute the final steam temperature
As before, x ⫽ (kL/V)(U/500 ) ⫽ (0.377)(14/6.5)(511/500) ⫽ 0.830. Then ⌬t ⫽
t2 ⫺ t1 ⫽ 950S/(500gpm) ⫽ 950(15,000)/[500(3700)] ⫽ 7.7⬚F (4.3⬚C). With t1 ⫽
80⬚F (26.7⬚C), t2 ⫽ ⌬t ⫹ t1 ⫽ 7.7 ⫹ 80 ⫽ 87.7⬚F (30.9⬚C). Since t2 ⫽ ts ⫺ t1)/e
x
, ex
⫽ ts ⫺ t1 /(ts ⫺ t2), or 2.71830.830
⫽ ts ⫺ 80/(ts ⫺ 87.7). Solve for ts; or, ts ⫽
201.1 ⫺ 80/1.294 ⫽ 93.6⬚F (34.2⬚C).
At a saturation temperature of 93.6⬚F (34.2⬚C), the steam table (saturation tem-
perature) shows that the steam pressure in the condenser shell is 1.59 inHg (5.4
kPa).
Check the ⌬t terminal temperature difference. Or, ⌬t ⫽ ts ⫺ t2 ⫽ 93.6 ⫺ 87.7
⫽ 5.9⬚F (3.3⬚C). Since the terminal temperature difference is greater than 5⬚F
(2.8⬚C), the calculated performance can be realized.
Related Calculations. The procedures and data given here can be used to com-
pute the required cooling-water flow, cooling-water temperature rise, quantity of
steam condensed by a given cooling-water flow rate and temperature rise, required
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![2.16 POWER GENERATION
condenser surface area, tube length per pass, water velocity, steam temperature in
condenser, cleanliness factor, and heat-transfer rate. Whereas Fig. 9 is suitable for
all usual condenser calculations for the ranges given, check the Heat Exchange
Institute for any new curves that might have been made available before you make
the final selection of very large condensers (more than 100,000 lb/h or 12.6 kg/s
of steam flow).
Note: The design water temperature used for condensers is either the average
summer water temperature or the average annual water temperature, depending on
which is higher. The design steam load is the maximum steam flow expected at the
full-load rating of the turbine or engine. Usual shell-and-tube condensers have tubes
that vary in length from about 8 ft (2.4 m) in the smallest sizes to about 40 ft (12.2
m) or more in the largest sizes. Each square foot of tube surface will condense 7
to 20 lb/h (0.88 to 2.5 g/s) of steam with a cooling-water circulating rate of 0.1
to 0.25 gal/(lb 䡠 min) [0.014 to 0.035 L/(kg 䡠 s)] of steam condensed. The method
presented here is the work of Glenn C. Boyer.
STEAM-CONDENSER AIR LEAKAGE
The air leakage into a condenser is estimated to be 12 ft3
/min (0.34 m3
/min) of
70⬚F (21⬚C) air at 14.7 lb/in2
(101 kPa). At the air outlet connection on the con-
denser, the temperature is 84⬚F (29⬚C) and the total (mixture) pressure is 1.80 inHg
absolute (6.1 kPa). Determine the quantity of steam, lbm /h (kg/h), lost from the
condenser.
Calculation Procedure:
1. Compute the mass rate of flow per hour of the air leakage
The mass rate of flow per hour of the estimated dry air leakage into the condenser,
wa ⫽ pV/RaT, where the air pressure, p ⫽ 14.7 ⫻ 144 lbƒ /ft2
(101 kPa); volumetric
flow rate, V ⫽ 12 ⫻ 60 ⫽ 720 ft3
/h (20.4 m3
/h); gas constant for air, Ra ⫽ 53.34
ft 䡠 lb/(lb 䡠 ⬚R) [287(m 䡠 N/kg 䡠 K)]; air temperature, T ⫽ 70 ⫹ 460 ⫽ 530⬚R (294
K). Then, wa ⫽ (14.7 ⫻ 144)(720)/(53.34 ⫻ 530) ⫽ 53.9 lb/h (24.4 kg/h).
2. Determine the partial pressure of the air in the mixture
The partial pressure of the air in the mixture of air and steam, pa ⫽ pm ⫺ pv, where
the mixture pressure, pm ⫽ 1.80 ⫻ 0.491 ⫽ 0.884 lb/in2
(6.09 kPa); partial vapor
pressure, pv ⫽ 0.577 lb/in2
(3.98 kPa), as found in the Steam Tables mentioned
under Related Calculations of this procedure. Then, pa ⫽ 0.884 ⫺ 0.577 ⫽ 0.307
lb/in2
(2.1 kPa).
3. Compute the humidity ratio of the mixture
The humidity ratio of the mixture, wv ⫽ Rapv /(Rvpa), where the gas constant for
steam vapor, Rv ⫽ 85.8 ft 䡠 lb/(lbm 䡠 ⬚R) [462(J/kg 䡠 K)], as found in a reference
mentioned under Related Calculations of this procedure. Then, wv ⫽ 53.34 ⫻
0.577/(85.8 ⫻ 0.307) ⫽ 1.17 lb vapor/lb dry air (0.53 kg/kg).
4. Compute the rate of steam lost from the condenser
Steam is lost from the condenser at the rate of wh ⫽ wv ⫻ wa ⫽ 1.17 ⫻ 53.9 ⫽
63.1 lb/h (28.6 kg/h).
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![2.18 POWER GENERATION
Were ts ⫺ t2 less than 5⬚F (2.8⬚C), another ratio value would be assumed and
the difference computed again. You would continue doing this until a value of
ts ⫺ t2 greater than 5⬚F (2.8⬚C) were obtained. Then LMTD ⫽ (t2 ⫺ t1)/ln[(ts ⫺
t1)/(ts ⫺ t2)]; LMTD ⫽ 15.68/ln (26.1/10.46) ⫽ 17.18⬚F (9.5⬚C).
3. Determine the heat-transfer coefficient
From the Heat Exchange Institute or manufacturer’s data U is 740 Btu/(ft2
䡠 h 䡠 ⬚F)
[4.2 kJ/(m2
䡠 ⬚C 䡠 s)] LMTD for a water velocity of 8 ft/s (2.4 m/s). If these data
are not available, Fig. 9 can be used with complete safety for all preliminary se-
lections.
Now U must be corrected for the inlet-water temperature, 75⬚F (23.9⬚C), and
the condition factor, 0.85, which is a term used in place of the correction factor by
some authorities. From Fig. 9, the correction for 75⬚F (23.9⬚C) inlet water ⫽ 1.04.
Then actual U⫽ 740(1.04)(0.85) ⫽ 655 Btu/(ft2
䡠 h 䡠 ⬚F) [3.7 kJ/(m2
䡠 ⬚C 䡠 s)] LMTD.
4. Compute the steam condensation rate
The heat-transfer rate per square foot of condenser surface with a 17.18⬚F (9.5⬚C)
LMTD is U(LMTD) ⫽ 655(17.18) ⫽ 11,252.9 Btu/(ft2
䡠 h) [35.5 kJ/(m2
䡠 s)].
Condensers serving steam turbines are assumed, for design purposes, to remove
950 Btu/lb (2209.7 kJ/kg) of steam condensed. Therefore, the steam condensation
rate for any condenser is [Btu/(ft2
䡠 h)]/950, or 1252.9/950 ⫽ 11.25 lb/(ft2
䡠 h) [15.3
g/(m2
䡠 s)].
5. Compute the required surface area and water flow
The required surface area ⫽ steam flow (lb/h)/[condensation rate, lb/(ft2
䡠 h)], or
with a 150,000-lb/h (18.9-kg/s) flow, 150,000/11.25 ⫽ 13,320 ft2
(1237.4 m2
).
The water flow rate, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(150,000)/
[500(15.68)] ⫽ 18,200 gal/min (1148.1 L/s).
Related Calculations. See the previous calculation procedure for steps in de-
termining the water-pressure loss through a surface condenser.
To choose a surface condenser for a steam engine, use the same procedures as
given above, except that the heat removed from the exhaust steam is 1000 Btu/lb
(2326.9 kJ/kg). Use a condition (cleanliness) factor of 0.65 for steam engines be-
cause the oil in the exhaust steam fouls the condenser tubes, reducing the rate of
heat transfer. The condition (cleanliness) factor for steam turbines is usually as-
sumed to be 0.8 to 0.9 for relatively clean, oil-free cooling water.
At loads greater than 50 percent of the design load, ts ⫺ t1 follows a straight-
line relationship. Thus, in the above condenser, ts ⫺ t1 ⫽ 26.14⬚F (14.5⬚C) at the
full load of 150,000 lb/h (18.9 kg/s). If the load falls to 60 percent (90,000 lb/h
or 11.3 kg/s), then ts ⫺ t1 ⫽ 26.14(0.60) ⫽ 15.7⬚F (8.7⬚C). At 120 percent load
(180,00 lb/h or 22.7 kg/s), ts ⫺ t1 ⫽ 26.14(1.20) ⫽ 31.4⬚F (17.4⬚C). This straight-
line law is valid with constant inlet-water temperature and cooling-water flow rate.
It is useful in analyzing condenser operating conditions at other than full load.
Single- or multiple-pass surface condensers may be used in power services.
When a liberal supply of water is available, the single-pass condenser is often
chosen. With a limited water supply, a two-pass condenser is often chosen.
AIR-EJECTOR ANALYSIS AND SELECTION
Choose a steam-jet air ejector for a condenser serving a 250,000-lb/h (31.5-kg/s)
steam turbine exhausting at 2 inHg absolute (6.8 kPa). Determine the number of
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![STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.21
FIGURE 11 Steam-jet ejector characteristics.
TABLE 5 Air-Ejector Capacities for Surface Condensers for Steam Turbines*
Calculation Procedure:
1. Determine the water flow rate per tube
Use a tabulation of condenser-tube engineering data available from the manufacturer
or the Heat Exchange Institute, or complete the water flow rate from the physical
dimensions of the tube thus: 3
⁄4-in (1.9 cm) no. 16 BWG tube ID ⫽ 0.620 in (1.6
cm) from a tabulation of condenser-tube data, such as Table 3. Assume a water
velocity of 1 ft/s (0.3 m/s). Then a 1-ft (0.3-m) length of the tube will contain
(12)(0.620)2
/4 ⫽ 3.62 in3
(59.3 cm3
) of water. This quantity of water will flow
through the tube for each foot of length per second of water velocity [194.6 cm3
/
(m 䡠 s)]. The flow per minute will be 3.62 (60 s/min) ⫽ 217.2 in3
/min (3559.3
cm3
/min). Since 1 U.S. gal ⫽ 231 in3
(3.8 L), the gal/min flow at a 1 ft/s (0.3
m/s) velocity ⫽ 217.2/231 ⫽ 0.94 gal/min (0.059 L/s).
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![2.22 POWER GENERATION
With an actual velocity of 7 ft/s (2.1 m/s), the water flow rate per tube is 7(0.94)
⫽ 6.58 gal/min (0.42 L/s).
2. Determine the number of tubes and length of water travel
Since the water flow rate through the condenser is 10,000 gal/min (630.8 L/s) and
each tube conveys 6.58 gal/min (0.42 L/s), the number of tubes ⫽ 10,000/6.58 ⫽
1520 tubes per pass.
Next, the total length of water travel for a condenser having A ft2
of condensing
surface is computed from A(number of tubes)(outside area per linear foot, ft2
). The
outside area of each tube can be obtained from a table of tube properties, such as
Table 3; or computed from (OD, in)()(12)/144, or (0.75)()(12)/144 ⫽ 0.196
ft2
/lin ft (0.06 m2
/m). Then, total length of travel ⫽ 12,000/[(1520)(0.196)] ⫽ 40.2
ft (12.3 m). Since the condenser has two passes, the length of tube per pass ⫽
40.2/2 ⫽ 20.1 ft (6.1 m). Since each pass has an equal number of tubes and there
are two passes, the total number of tubes in the condenser ⫽ 2 passes (1520 tubes
per pass) ⫽ 3040 tubes.
3. Compute the friction loss in the system
Use the Heat Exchange Institute or manufacturer’s curves to find the friction loss
per foot of condenser tube. At 7 ft/s (2.1 m/s), the Heat Exchange Institute curve
shows the head loss is 0.4 ft of head per foot (3.9 kPa/m) of travel for 3
⁄4-in (1.9-
cm) no. 16 BWG tubes. With a total length of 40.2 ft (12.3 m), the tube head loss
is 0.4(40.2) ⫽ 16.1 ft (48.1 kPa).
Use the Heat Exchange Institute or manufacturer’s curves to find the head loss
through the condenser waterboxes. From the first reference, for a velocity of 7 ft/
s (2.1 m/s), head loss ⫽ 1.4 ft (4.2 kPa) of water for a single-pass condenser. Since
this is a two-pass condenser, the total waterbox head loss ⫽ 2(1.4) ⫽ 2.8 ft (8.4
kPa).
The total condenser friction loss is then the sum of the tube and waterbox losses,
or 16.1 ⫹ 2.8 ⫽ 18.9 ft (56.5 kPa) of water. With an external friction loss of 20 ft
(59.8 kPa) in the circulating-water piping, the total loss in the system, without
siphonic assistance, is 18.9 ⫹ 20 ⫽ 38.9 ft (116.3 kPa). Since there is 10 ft (29.9
kPa) of siphonic assistance, the total friction loss in the system with siphonic as-
sistance is 38.9 ⫺ 10 ⫽ 28.9 ft (86.3 kPa). In choosing a pump to serve this system,
the frictional resistance of 28.9 ft (86.3 kPa) would be rounded to 30 ft (89.7 kPa),
and any factor of safety added to this value of head loss.
Note: The most economical cooling-water velocity in condenser tubes is 6 to 7
ft/s (1.8 to 2.1 m/s); a velocity greater than 8 ft/s (2.4 m/s) should not be used,
unless warranted by special conditions.
SURFACE-CONDENSER WEIGHT ANALYSIS
A turbine exhaust nozzle can support a weight of 100,000 lb (444,822.2 N). De-
termine what portion of the total weight of a surface condenser must be supported
by the foundation if the weight of the condenser is 275,000 lb (1,223,261.1 N), the
tubes and waterboxes have a capacity of 8000 gal (30,280.0 L), and the steam space
has a capacity of 30,000 gal (113,550.0 L) of water.
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![2.24 POWER GENERATION
steam is 0.5067 psia (3.39 kPa), from the steam tables. In the condensing condition,
there are 0.622 lb (0.28 kg) of water per pound (kg) of dry air. Since the water
content of the air is a function of the partial pressures, (0.622) (0.5067)/[(2 ⫻
0.5067)] ⫽ 0.673 lb of water per lb of dry air (0.305 kg).
3. Compute the percent of air by weight
Use the relation, percent of air by weight ⫽ (100)(1)/(1 ⫹ 0.672) ⫽ (100)(1)/(1
⫹ 0.672) ⫽ 59.8 percent by weight of air.
Related Calculations. Use this general procedure for analyzing the air in sur-
face condensers serving steam turbines of all types.
BAROMETRIC-CONDENSER ANALYSIS AND
SELECTION
Select a countercurrent barometric condenser to serve a steam turbine exhausting
25,000 lb/h (3.1 kg/s) of steam at 5 inHg absolute (16.9 kPa). Determine the
quantity of cooling water required if the water inlet temperature is 50⬚F (10.0⬚C).
What is the required dry-air capacity of the ejector? What is the required pump
head if the static head is 40 ft (119.6 kPa) and the pipe friction is 15 ft of water
(44.8 kPa)?
Calculation Procedure:
1. Find the steam properties from the steam tables
At 5 inHg absolute (16.9 kPa), hg ⫽ 1119.4 Btu/lb (2603.7 kJ/kg), from the sat-
uration-pressure table. If the condensing water were to condense the steam without
subcooling the condensate, the final temperature of the condensate, from the steam
tables, would be 133.76⬚F (56.5⬚C), corresponding to the saturation temperature.
However, subcooling almost always occurs, and the usual practice in selecting a
countercurrent barometric condenser is to assume the final condensate temperature
tc will be 5⬚F (2.8⬚C) below the saturation temperature corresponding to the absolute
pressure in the condenser. Given a 5⬚F (2.8⬚C) difference, tc ⫽ 133.76 ⫺ 5 ⫽
128.76⬚F (53.7⬚C). Interpolating in the saturation-temperature steam table, we find
the enthalpy of the condensate hƒ at 128.76⬚F (53.7⬚C) is 96.6 Btu/lb (224.8 kJ/
kg).
2. Compute the quantity of condensing water required
In any countercurrent barometric condenser, the quantity of cooling water Q lb/h
required is Q ⫽ W(hg ⫺ ht)/(tc ⫺ t1), where W ⫽ weight of steam condensed, lb/
h; t1
⫽ cooling-water inlet temperature, ⬚F. Then Q ⫽ 25,000(1119.4 ⫺ 96.66)/
(128.76 ⫺ 50) ⫽ 325,000 lb/h (40.9 kg/s). By converting to gallons per minute,
Q ⫽ 325,000/500 ⫽ 650 gal/min (41.0 L/s).
3. Determine the required ejector dry-air capacity
Use the Heat Exchanger Institute or a manufacturer’s tabulation of free, dry-air
leakage and the allowance for air in the cooling water to determine the required
dry-air capacity. Thus, from Table 6, the free, dry-air leakage for a barometric
condenser serving a turbine is 3.0 ft3
/min (0.08 m3
/min) of air and vapor. The
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![STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.25
TABLE 6 Free, Dry-Air Leakage
[ft3
/min (m3
/s) at 70⬚F or 21.1⬚C air and vapor mixture, 71
⁄2⬚ below vacuum temperature or
4.2⬚ for Celsius]
FIGURE 12 Allowance for air in condenser injection
water.
allowance for air in the 50⬚F (10.0⬚C) cooling water is 3.3 ft3
/min (0.09 m3
/min)
of air at 70⬚F (21.1⬚C) per 1000 gal/min (63.1 L/s) of cooling water, Fig 12. The
total dry-air leakage is the sum, or 3.0 ⫹ 3.3 ⫽ 6.3 ft3
/min (0.18 m3
/min). Thus,
the ejector must be capable of handling at least 6.3 ft3
/min (0.18 m3
/min) of dry
air to serve this barometric condenser at its rated load of 25,000 lb/h (3.1 kg/s) of
steam.
Where the condenser will operate at a lower vacuum (i.e., a higher absolute
pressure), overloads up to 50 percent may be met. To provide adequate dry-air
handling capacity at this overload with the same cooling-water inlet temperature,
find the free, dry-air leakage at the higher condensing rate from Table 6 and add
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![STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.27
FIGURE 13 Spray-pond nozzle and piping layout.
2. Determine the surface area required
Usual design practice is to provide 1 ft2
(0.09 m2
) of pond area per 250 lb (113.4
kg) of water cooled for water quantities exceeding 1000 gal/min (63.1 L/s). Thus,
in this pond, the weight of water cooled ⫽ (10,000 gal/min)(8.33 lb/gal)(60 min/
h) ⫽ 4,998,000, say 5,000,000 lb/h (630.0 kg/s). Then, the area required, given 1
ft2
of pond area per 250 lb of water (0.82 m2
per 1000 kg) cooled ⫽ 5,000,000/
250 ⫽ 20,000 ft2
(1858.0 m2
).
As a cross-check, use another commonly accepted area value: 125 Btu/(ft2
䡠 ⬚F)
[2555.2 kJ/(m2
䡠 ⬚C)] is the difference between the air wet-bulb temperature and the
warm entering-water temperature. This is the equivalent of (120 ⫺ 60)(125) ⫽ 7500
Btu/ft2
(85,174 kJ/m2
) in this spray pond, because the air wet-bulb temperature is
60⬚F (15.6⬚C) and the warm-water temperature is 120⬚F (48.9⬚C). The heat removed
from the water is (lb/h of water)(temperature decrease, ⬚F)(specific heat of water)
⫽ (5,000,000)(120 ⫺ 90)(1.0) ⫽ 150,000,000 Btu/h (43,960.7 kW). Then, area
required ⫽ (heat removed, Btu/h)/(heat removal, Btu/ft2
) ⫽ 150,000,000/7500 ⫽
20,000 ft2
(1858.0 m2
). This checks the previously obtained area value.
3. Determine the spray-pond dimensions
Spray groups on the same header or pipe main are usually arranged on about 12-
ft (3.7-m) centers with the headers or pipe mains spaced on about 25-ft (7.6-m)
centers, Fig. 13. Assume that 34 spray groups are used, instead of the required
331
⁄3, to provide an equal number of groups in two headers and a small extra
capacity.
Sketch the spray pond and headers, Fig. 13. This shows that the length of each
header will be about 204 ft (62.2 m) because there are seventeen 12-ft (3.7-m)
spaces between spray groups in each header. Allowing 3 ft (0.9 m) at each end of
a header for fittings and clean-outs gives an overall header length of 210 ft (64.0
m). The distance between headers is 25 ft (7.6 m). Allow 25 ft (7.6 m) between
the outer sprays and the edge of the pond. This gives an overall width of 85 ft
(25.9 m) for the pond, if we assume the width of each arm in a spray group is 10
ft (3.0 m). The overall length will then be 210 ⫹ 25 ⫹ 25 ⫽ 260 ft (79.2 m). A
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![3.2 POWER GENERATION
Combustion Constants for Fuels
Fuel
Constant, lb dry air per
million Btu (kg/MW)
Blast furnace gas 575 (890.95)
Bagasse 650 (1007.2)
Carbon monoxide gas 670 (1038.2)
Refinery and oil gas 720 (1115.6)
Natural gas 730 (1131.1)
Furnace oil and lignite 745–750 (1154.4–1162.1)
Bituminous coals 760 (1177.6)
Anthracite coal 780 (1208.6)
Coke 800 (1239.5)
To determine the energy input to the boiler, use the relation Qf ⫽ (Qs)/Eh, where
energy input by the fuel, Btu/hr (W); Qs ⫽ energy absorbed by the steam in the
boiler, Btu/Hr (W); Qs ⫽ energy absorbed by the steam, Btu/hr (W); Eh ⫽ effi-
ciency of the boiler on an HHV basis. Substituting for this boiler, Qf ⫽ 100/
0.83 ⫽ 120.48 million Btu/hr on an HHV basis (35.16 MW).
2. Estimate the quantity of dry air required by this boiler
The total air required Ta ⫽ (Qf)(Fuel constant from list above). For natural gas,
Ta ⫽ (120.48)(730) ⫽ 87,950 lb/hr (39,929 kg/hr). With 15 percent excess air,
total air required ⫽ (1.15)(87,950) ⫽ 101,142.5 lb/hr (45,918.7 kg/hr).
3. Compute the quantity of wet air required
Air has some moisture because of its relative humidity. Estimate the amount of
moisture in dry air in M lb/lb (kg/kg) from, M ⫽ 0.622 (pw)/(14.7 ⫺ pw), where
0.622 is the ratio of the molecular weights of water vapor and dry air; pw ⫽ partial
pressure of water vapor in the air, psia (kPa) ⫽ saturated vapor pressure (SVP) ⫻
relative humidity expressed as a decimal; 14.7 ⫽ atmospheric pressure of air at sea
level (101.3 kPa). From the steam tables, at 80 F (26.7 C), SVP ⫽ 0.5069 psia
(3.49 kPa). Substituting, M ⫽ 0.622 (0.5069 ⫻ 0.65)/(14.7 ⫺ [0.5069 ⫻ 0.65]) ⫽
0.01425 lb of moisture/lb of dry air (0.01425 kg/kg).
The total flow rate of the wet air then ⫽ 1.0142 (101,142.5) ⫽ 102,578.7 lb/hr
(46,570.7 kg/hr). To convert to a volumetric-flow basis, recall that the density of
air at 80⬚F (26.7⬚C) and 14.7 psia (101.3 kPa) ⫽ 39/(480 ⫹ 80) ⫽ 0.0722 lb/cu
ft (1.155 kg/cu m). In this relation, 39 ⫽ a constant and the temperature of the air
is converted to degrees Rankine. Hence, the volumetric flow ⫽ 102,578.7/(60
min/hr)(0.0722) ⫽ 23,679.3 actual cfm (670.1 cm m/min).
4. Estimate the rate of fuel firing and flue-gas produced
The rate of fuel firing ⫽ Qf /HHV ⫽ (120.48 ⫻ 106
)/23,000 ⫽ 5238 lb/hr (2378
kg/hr). Hence, the total flue gas produced ⫽ 5238 ⫹ 102,578 ⫽ 107,816 lb/hr
(48,948 kg/hr).
If the temperature of the flue gas is 400⬚F (204.4⬚C) (a typical value for a natural-
gas fired boiler), then the density, as in Step 3 is: 39/(400 ⫹ 460) ⫽ 0.04535 lb/
cu ft (0.7256 kg/cu m). Hence, the volumetric flow ⫽ (107,816)/(60 min/hr ⫻
0.04535) ⫽ 39,623.7 actual cfm (1121.3 cu m/min).
Related Calculations. Detailed combustion calculations based on actual fuel
gas analysis can be performed to verify the constants given in the list above. For
example, let us say that the natural-gas analysis was: Methane ⫽ 83.4 percent;
Ethane ⫽ 15.8 percent; Nitrogen ⫽ 0.8 percent by volume. First convert the analysis
to a percent weight basis:
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![COMBUSTION 3.3
Fuel Percent volume MW Col. 2 ⫻ Col. 3 Percent weight
Methane 83.4 16 1334.4 72.89
Ethane 15.8 30 474 25.89
Nitrogen 0.8 28 22.4 1.22
Note that the percent weight in the above list is calculated after obtaining the sum
under Column 2 ⫻ Column 3. Thus, the percent methane ⫽ (1334.4)/(1334.4 ⫹
474 ⫹ 22.4) ⫽ 72.89 percent.
From a standard reference, such as Ganapathy, Steam Plant Calculations Man-
ual, Marcel Dekker, Inc., find the combustion constants, K, for various fuels and
use them thus: For the air required for combustion, Ac ⫽ (K for methane)(percent
by weight methane from above list) ⫹ (K for ethane)(percent by weight ethane);
or Ac ⫽ (17.265)(0.7289) ⫹ (16.119)(0.2589) ⫽ 16.76 lb/lb (16.76 kg/kg).
Next, compute the higher heating value of the fuel (HHV) using the air constants
from the same reference mentioned above. Or HHV ⫽ (heat of combustion for
methane)(percent by weight methane) ⫹ (heat of combustion of ethane)(percent by
weight ethane) ⫽ (23,879)(0.7289) ⫹ (22,320)(0.2589) ⫽ 23,184 Btu/lb (54,018.7
kJ/kg). Then, the amount of fuel equivalent to 1,000,000 Btu (1,055,000 kJ) ⫽
(1,000,000)/23,184 ⫽ 43.1 lb (19.56 kg), which requires, as computed above,
(43.1)(16.76) ⫽ 722.3 lb dry air (327.9 kg), which agrees closely with the value
given in Step 1, above.
Similarly, if the fuel were 100 percent methane, using the steps given above,
and suitable constants from the same reference work, the air required for combus-
tion is 17.265 lb/lb (7.838 kg/kg) of fuel. HHV ⫽ 23,879 Btu/lb (55,638 kJ/kg).
Hence, the fuel in 1,000,000 Btu (1,055,000 kJ) ⫽ (1,000,000)/(23,879) ⫽ 41.88
lb (19.01 kg). Then, the dry air per million Btu (1.055 kg) fired ⫽ (17.265)
(41.88) ⫽ 723 lb (328.3 kg).
Likewise, for propane, using the same procedure, 1 lb (0.454 kg) requires 15.703
lb (7.129 kg) air and 1 million Btu (1,055,000 kJ) has (1,000,000)/21,661 ⫽ 46.17
lb (20.95 kg) fuel. Then, 1 million Btu (1,055,000 kJ) requires (15.703)(46.17) ⫽
725 lb (329.2 kg) air. This general approach can be used for various fuel oils and
solid fuels—coal, coke, etc.
Good estimates of excess air used in combustion processes may be obtained if
the oxygen and nitrogen in dry flue gases are measured. Knowledge of excess air
amounts helps in performing detailed combustion and boiler efficiency calculations.
Percent excess air, EA ⫽ 100(O2–CO2)/[0.264 ⫻ N2–(O2–CO/2)], where O2 ⫽
oxygen in the dry flue gas, percent volume; CO ⫽ percent volume carbon mon-
oxide; N2 ⫽ percent volume nitrogen.
You can also estimate excess air from oxygen readings. Use the relation, EA ⫽
(constant from list below)((O2)/(21–O2).
Constants for Excess Air Calculations
Fuel Constant
Carbon 100
Hydrogen 80
Carbon monoxide 121
Sulfur 100
Methane 90
Oil 94.5
Coal 97
Blast furnace gas 223
Coke oven gas 89.3
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![3.4 POWER GENERATION
If the percent volume of oxygen measured is 3 on a dry basis in a natural-gas
(methane) fired boiler, the excess air, EA ⫽ (90)[3/(21–3)] ⫽ 15 percent.
This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO
Industries.
SAVINGS PRODUCED BY PREHEATING
COMBUSTION AIR
A 20,000 sq ft (1858 sq m) building has a calculated total seasonal heating load
of 2,534,440 MBH (thousand Btu) (2674 MJ). The stack temperature is 600⬚F
(316⬚C) and the boiler efficiency is calculated to be 75 percent. Fuel oil burned has
a higher heating value of 140,000 Btu/gal (39,018 MJ/L). A preheater can be
purchased and installed to reduce the breeching discharge combustion air temper-
ature by 250⬚F (139⬚C) to 350⬚F (177⬚C) and provide the burner with preheated
air. How much fuel oil will be saved? What will be the monetary saving if fuel oil
is priced at 80 cents per gallon?
Calculation Procedure:
1. Compute the total combustion air required by this boiler
A general rule used by design engineers is that 1 cu ft (0.0283 cu m) of combustion
air is required for each 100 Btu (105.5 J) released during combustion. To compute
the combustion air required, use the relation CA ⫽ H/100 ⫻ Boiler efficiency,
expressed as a decimal, where CA ⫽ annual volume of combustion air, cu ft (cu
m); H ⫽ total seasonal heating load, Btu/yr (kJ/yr). Substituting for this boiler,
CA ⫽ (2,534,400)(1000)/100 ⫻ 0.75 ⫽ 33,792,533 cu ft/yr (956.329 cu m).
2. Calculate the annual energy savings
The energy savings, ES ⫽ (stack temperature reduction, deg F)(cu ft air per
yr)(0.018), where the constant 0.018 is the specific heat of air. Substituting, ES ⫽
(250)(33,792,533)(0.018) ⫽ 152,066,399 Btu/yr (160,430 kJ/yr).
With a boiler efficiency of 75 percent, each gallon of oil releases 0.75 ⫻ 140,000
Btu/gal ⫽ 105,000 Btu (110.8 jk). Hence, the fuel saved, FS ⫽ ES/usuable heat
in fuel, Btu/gal. Or, FS ⫽ 152,066,399/105,000 ⫽ 1448.3 gal/yr (5.48 cu m/yr).
With fuel oil at $1.10 per gallon, the monetary savings will be $1.10 (1448.3) ⫽
$1593.13. If the preheater cost $6000, the simple payoff time would be $6000/
1593.13 ⫽ 3.77 years.
Related Calculations. Use this procedure to determine the potential savings
for burning any type of fuel—coal, oil, natural gas, landfill gas, catalytic cracker
offgas, hydrogen purge gas, bagesse, sugar cane, etc. Other rules of thumb used by
designers to estimate the amount of combustion air required for various fuels are:
10 cu ft of air (0.283 cu m) per 1 cu ft (0.0283 cu m) of natural gas; 1300 cu ft
of air (36.8 cu m) per gal (0.003785 cu m) of No. 2 fuel oil; 1450 cu ft of air (41
cu m) per gal of No. 5 fuel oil; 1500 cu ft of air (42.5 cu m) per gal of No. 6 fuel
oil. These values agree with that used in the above computation—i.e. 100 cu ft per
100 Btu of 140,000 Btu per gal oil ⫽ 140,000/100 ⫽ 1400 cu ft per gal (39.6 cu
m/0.003785 cu m).
This procedure is the work of Jerome F. Mueller, P.E. of Mueller Engineering
Corp.
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![3.8 POWER GENERATION
FIGURE 1 Energy Independence transports coal to central stations.
(Power.)
tomers more than $60-million annually by displacing about 14-million bbl (2.2
million cu m) of oil per year.
To reduce costs, the company built the first coal-fired collier, Fig. 1, in more
than 50 years in the United States, and assumed responsibility for coal transpor-
tation to its stations, cutting operating costs by more than $2-million per year. The
collier makes economic sense because the utility stations in the system are not
accessible by rail. This ship, the Energy Independence, has been an economic suc-
cess for the utility. Measuring 665 ft (203 m) long by 95 ft (29 m) wide by 56 ft
(17 m) deep with a 34-ft (10-m) draft, the vessel discharges a typical 40,000-ton
load in 12 hours.
Data in these two paragraphs and Fig. 1 are from Power magazine.
PERCENT EXCESS AIR WHILE BURNING COAL
A certain coal has the following composition by weight percentages: carbon 75.09,
nitrogen 1.56, ash 3.38, hydrogen, 5.72, oxygen 13.82, sulfur 0.43. When burned
in an actual furnace, measurements showed that there was 8.93 percent combustible
in the ash pit refuse and the following Orsat analysis in percentages was obtained:
carbon dioxide 14.2, oxygen 4.7, carbon monoxide 0.3. If it can be assumed that
there was no combustible in the flue gas other that the carbon monoxide reported,
calculate the percentage of excess air used.
Calculation Procedure:
1. Compute the amount of theoretical air required per lbm (kg) of coal
Theoretical air required per pound (kilogram) of coal, wta ⫽ 11.5C⬘ ⫹ 34.5[H⬘2 ⫺
O⬘2 /8)] ⫹ 4.32S⬘, where C⬘, H⬘2, O⬘2, and S⬘ represent the percentages by weight,
expressed as decimal fractions, of carbon, hydrogen, oxygen, and sulfur, respec-
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![COMBUSTION 3.9
tively. Thus, wta ⫽ 11.5(0.7509) ⫹ 34.5[0.0572 ⫺ (0.1382/8)] ⫹ 4.32(0.0043) ⫽
10.03 lb (4.55 kg) of air per lb (kg) of coal. The ash and nitrogen are inert and do
not burn.
2. Compute the correction factor for combustible in the ash
The correction factor for combustible in the ash, C1 ⫽ (wfCf ⫺ wrCr)/(wf ⫻ 100),
where the amount of fuel, wf ⫽ 1 lb (0.45 kg) of coal; percent by weight, expressed
as a decimal fraction, of carbon in the coal, Cf ⫽ 75.09; percent by weight of the
ash and refuse in the coal, wr ⫽ 0.0338; percent by weight of combustible in the
ash, Cr ⫽ 8.93. Hence, C1 ⫽ [(1 ⫻ 75.09) ⫺ (0.0338 ⫻ 8.93)]/(1 ⫻ 100) ⫽ 0.748.
3. Compute the amount of dry flue gas produced per lb (kg) of coal
The lb (kg) of dry flue gas per lb (kg) of coal, wdg ⫽ C1(4CO2 ⫹ O2 ⫹ 704)/
[3(CO2 ⫹ CO)], where the Orsat analysis percentages are for carbon dioxide, CO2
⫽ 14.2; oxygen, O2 ⫽ 4.7; carbon monoxide, CO ⫽ 0.3. Hence, wdg ⫽ 0.748 ⫻
[(4 ⫻ 14.2) ⫹ 4.7 ⫹ 704)]/[3(14.2 ⫹ 0.3)] ⫽ 13.16 lb/lb (5.97 kg/kg).
4. Compute the amount of dry air supplied per lb (kg) of coal
The lb (kg) of dry air supplied per lb (kg) of coal, wda ⫽ wdg ⫺ C1 ⫹ 8[H⬘2 ⫺
(O⬘2 /8)] ⫺ (N⬘2 /N), where the percentage by weight of nitrogen in the fuel, N⬘2 ⫽
1.56, and ‘‘atmospheric nitrogen’’ in the supply air, N2 ⫽ 0.768; other values are
as given or calculated. Then, wda ⫽ 13.16 ⫺ 0.748 ⫹ 8[0.0572 ⫺ (0.1382/8)] ⫺
(0.0156/0.768) ⫽ 12.65 lb/lb (5.74 kg/kg).
5. Compute the percent of excess air used
Percent excess air ⫽ (wda ⫺ wta)/wta ⫽ (12.65 ⫺ 10.03)/10.03 ⫽ 0.261, or 26.1
percent.
Related Calculations. The percentage by weight of nitrogen in ‘‘atmospheric
air’’ in step 4 appears in Principles of Engineering Thermodynamics, 2nd edition,
by Kiefer et al., John Wiley Sons, Inc.
COMBUSTION OF FUEL OIL IN A FURNACE
A fuel oil has the following ultimate analysis: C ⫽ 0.8543; H2 ⫽ 0.1131; O2 ⫽
0.0270; N2 ⫽ 0.0022; S ⫽ 0.0034; total ⫽ 1.0000. This fuel oil is burned in a
steam-boiler furnace. Determine the weight of air required for theoretically perfect
combustion, the weight of gas formed per pound (kilogram) of oil burned, and the
volume of flue gas, at the boiler exit temperature of 600⬚F (316⬚C), per pound
(kilogram) of oil burned; the air required with 20 percent excess air, and the volume
of gas formed with this excess; the CO2 percentage in the flue gas on a dry and
wet basis.
Calculation Procedure:
1. Compute the weight of oxygen required per pound (kilogram) of oil
The same general steps as given in the previous calculation procedure will be
followed. Consult that procedure for a complete explanation of each step.
Using the molecular weight of each element, we find
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, where the heating value of carbon, HVC ⫽ 14,087
Btu/lbm (32.74 ⫻ 103
kJ/kg); the constant-pressure specific heat of carbon dioxide,
oxygen, and nitrogen are cpCO2 ⫽ 0.300 Btu/lb (0.697 kJ/kg), cpO2 ⫽ 0.240 Btu
/lb (0.558 kJ/kg), and cpN2 ⫽ 0.285 Btu/lb (0.662 kJ/kg), respectively; final prod-
uct temperature is t2; other values as before. Then, 1 ⫻ 14,087 ⫽ [(3.667 ⫻
0.30) ⫹ (0.533 ⫻ 0.24) ⫹ (10.58 ⫻ 0.285)(t2 ⫺ 77)]. Solving, t2 ⫽ 3320 ⫹ 77 ⫽
3397⬚F (1869⬚C).
Related Calculations. In the above procedure it is assumed that the carbon is
burned in dry air. Also, the nitrogen coefficient of 3.78 used in the chemical equa-
tion in step 1 is based on a theoretical composition of dry air as 79.1 percent
nitrogen and 20.9 percent oxygen by volume, so that 79.1/20.9 ⫽ 3.78. For a more
detailed description of this coefficient see the Related Calculations under the pro-
cedure for ‘‘Gas Turbine Combustion Chamber Inlet Air Temperature’’ elsewhere
in this handbook.
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![4.8 POWER GENERATION
superheated steam is desuperheated continuously by injecting water at 500⬚F
(260⬚C), how many pounds (kg) of saturated steam will be produced per pound
(kg) of superheated steam?
Calculation Procedure:
1. Using the steam tables, determine the steam and water properties
Rounding off the enthalpy and temperature values we find that: Enthalpy of the
superheated steam at 1350 lb/in2
(abs) (9301.5 kPa) and 950⬚F (510⬚C) ⫽ H1 ⫽
1465 Btu/lb (3413.5 kJ/kg); Enthalpy of saturated steam at 1000 lb/in2
(abs) (6890
kPa) ⫽ H2 ⫽ 1191 Btu/lb (2775 kJ/kg); Enthalpy of water at 500⬚F (260⬚C) ⫽
(500 ⫺ 32) ⫽ H3 ⫽ 488 Btu/lb (1137 kJ/kg).
2. Set up a heat-balance equation and solve it
L X ⫽ lb (kg) of water at 500⬚F (260⬚C) required to desuperheat the superheated
steam. Then, using the symbols given above, H1 ⫹ X(H3) ⫽ (1 ⫹ X)H2. Solving
for X ⫽ (H1 ⫺ H2)/(H2 ⫺ H3) ⫽ (1465 ⫺ 1191)/(1191 ⫺ 488) ⫽ 0.39. Then,
1.0 ⫹ 0.39 ⫽ 1.39 lb (0.63 kg) of saturated steam produced per lb (kg) of super-
heated steam. Thus, if the process used 1000 lb (454 kg) of saturated steam at 1000
lb/in2
(abs) (689 kPa), the amount of superheated steam needed to produce this
saturated steam would be 1000/1.39 ⫽ 719.4 lb (326.6 kg).
Related Calculations. Desuperheating superheated steam for process and other
use is popular because it can save purchase and installation of a separate steam
generator for the lower pressure steam. While there is a small loss of energy in
desuperheating (from heat losses in the piping and desuperheater), this loss is small
compared to the savings made. That’s why you’ll find desuperheating being used
in central stations, industrial, commercial and marine plants throughout the world.
DETERMINING FURNACE-WALL HEAT LOSS
A furnace wall consists of 9-in (22.9-cm) thick fire brick, 4.5-in (11.4-cm) Sil-O-
Cel brick, 4-in (10.2-cm) red brick, and 0.25-in (0.64-cm) transite board. The ther-
mal conductivity, k, values, Btu/(ft2
)(⬚F)(ft) [kJ/(m2
)(⬚C)(m)] are as follows: 0.82
at 1800⬚F (982⬚C) for fire brick; 0.125 at 1800⬚F (982⬚C) for Sil-O-Cel; 0.52 at
500⬚F (260⬚C) for transite. A temperature of 1800⬚F (982⬚C) exists on the inside
wall of the furnace and 200⬚F (93.3⬚C) on the outside wall. Determine the heat loss
per hour through each 10 ft2
(0.929 m2
) of furnace wall. What is the temperature
of the wall at the joint between the fire brick and Sil-O-Cel?
Calculation Procedure:
1. Find the heat loss through a unit area of the furnace wall
Use the relation Q ⫽ ⌬t/R, where Q ⫽ heat transferred, Btu/h (W); ⌬t ⫽ temper-
ature difference between the inside of the furnace wall and the outside, ⬚F (⬚C);
R ⫽ resistance of the wall to heat flow ⫽ L/(kXA), where L ⫽ length of path
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.9
through which the heat flow, ft (m); k ⫽ thermal conductivity, as defined above;
A ⫽ area of path of heat flow, ft2
(m2
). Where there is more than one resistance to
heat flow, add them to get the total resistance.
Substituting, the above values for this furnace wall, remembering that there are
three resistances in series and solving for the heat flow through one square ft
(0.0.0929 m2
), Q ⫽ (1800 ⫺ 200)/{[(1/0.82)(9/12)] ⫹ [(1/0.125)(4.5/12)] ⫹ [(1
/052)(4/12)] ⫹ [(1/0.23)(0.25/12)]} ⫽ 344 Btu/h ft2
(1083.6 W/m2
), or 10 (344)
⫽ 34400 Btu/h for 10 ft2
(10,836 W/10 m2
).
2. Compute the temperature within the wall at the stated joint
Use the relation, (⌬t)/(⌬t1) ⫽ (R/R1), where ⌬t ⫽ temperature difference across
the wall, ⬚F (⬚C); ⌬t1 ⫽ temperature at the joint being considered, ⬚F (⬚C); R ⫽
total resistance of the wall; R1 ⫽ resistance of the first portion of the wall between
the inside and the joint in question.
Substituting, (1800 ⫺ 200)/(⌬t1) ⫽ 4.646/0.915); ⌬t1 ⫽ 315⬚F (157.2⬚C). Then
the interface temperature at the between the fire brick and the Sil-O-Cel is 1800 ⫺
315 ⫽ 1485⬚F (807.2⬚C)
Related Calculations. The coefficient of thermal conductivity given here,
Btu/(ft2
)(⬚F)(ft) is sometimes expressed in terms of per inch of thickness, instead
of per foot. Either way, the conversion is simple. In SI units, this coefficient is
expressed in kJ/(m2
)(⬚C)(m), or cm2
and cm.
The exterior temperature of a furnace wall is an important considered in boiler
and process unit design from a human safety standpoint. Excess exterior tempera-
tures can cause injury to plant workers. Further, the higher the exterior temperature
of a furnace wall, the larger the heat loss from the fired vessel. Therefore, both
safety and energy conservation considerations are important in furnace design.
Typical interior furnace temperatures encountered in modern steam boilers range
from 2400⬚F (1316⬚C) near the fuel burners to 1600⬚F (871⬚C) in the superheater
interior. With today’s emphasis on congeneration and energy conservation, many
different fuels are being burned in boilers. Thus, a plant in Louisiana burns rice to
generate electricity while disposing of a process waste material.
Rice hulls, which comprise 20 percent of harvested rice, are normally processed
in a hammermill to increase their bulk from about 11 lb/ft3
(176 kg/m3
) to 20
lb/ft3
(320 kg/m3
). Then they are spread or piled on land adjacent to the rice mill.
The hulls often smolder in the fields, like mine tailings from coal production. Con-
tinuous, uncontrolled burning may result, creating an environmental hazard and
problem.
Burning rice hulls in a boiler furnace may create unexpected temperatures both
inside and outside the furnace. Hence, it is important that the designer be able to
analyze both the interior and exterior furnace temperatures using the procedure
given here.
Another modern application of waste usage for power generation is the burning
of sludge in a heat-recovery boiler to generate electricity. Sludge from a wastewater
plant is burned in a combustor to generate steam for a turbogenerator. Not only are
fuel requirements for the boiler reduced, there is also significant savings of fuel
used to incinerate the sludge in earlier plants. Again, the furnace temperature is an
important element in designing such plants.
The data present in these comments on new fuels for boilers is from Power
magazine.
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![4.10 POWER GENERATION
CONVERTING POWER-GENERATION
POLLUTANTS FROM MASS TO
VOLUMETRIC UNITS
In the power-generation industry, emission levels of pollutants such as CO and NOx
are often specified in mass units such as pounds per million Btu (kg per 1.055 MJ)
and volumetric units such as ppm (parts per million) volume. Show how to relate
these two measures for a gaseous fuel having this analysis: Methane ⫽ 97 percent;
Ethane ⫽ 2 percent; Propane ⫽ 1 percent by volume, and excess air ⫽ 10 percent.
Ambient air temperature during combustion ⫽ 80⬚F (26.7⬚C) and relative humid-
ity ⫽ 60 percent; fuel higher heating value, HHV ⫽ 23,759 Btu/lb (55,358 kJ/
kg); 100 moles of fuel gas is the basis of the flue gas analysis.
Calculation Procedure:
1. Find the theoretical dry air required, and the moisture in the actual air
The theoretical dry air requirements, in M moles, can be computed from the sum
of (ft3
of air per ft3
of combustible gas)(percent of combustible in the fuel) using
data from Ganapathy, Steam Plant Calculations Manual, Marcel Dekker, Inc. thus:
M ⫽ (9.528 ⫻ 97) ⫹ (16.675 ⫻ 2) ⫹ (23.821 ⫻ 1) ⫽ 981.4 moles. Then, with 10
percent excess air, excess air, EA ⫽ 0.1(981.4) ⫽ 98.1 moles.
The excess oxygen, O2 ⫽ (98.1 moles)(0.21) ⫽ 20.6 moles, where 0.21 ⫽ moles
of oxygen in 1 mole of air. The nitrogen, N2, produced by combustion ⫽ (1.1 for
excess air)(981.4 moles)(0.79 moles of nitrogen in 1 mole of air) ⫽ 852.8 moles;
round to 853 moles for additional calculations.
The moisture in the air ⫽ (981.4 ⫹ 98.1)(29 ⫻ 0.0142/18) ⫽ 24.69, say 24.7
moles. In this computation the values 29 and 18 are the molecular weights of dry
air and water vapor, respectively, while 0.0142 is the lb (0.0064 kg) moisture per
lb of dry air as shown in the previous procedure.
2. Compute the flue gas analysis for the combustion
Using the given data, CO2 ⫽ (1 ⫻ 97) ⫹ (2 ⫻ 2) ⫹ (3 ⫻ 1) ⫽ 104 moles. For
H2O ⫽ (2 ⫻ 97) ⫹ (3 ⫻ 2) ⫹ (4 ⫻ 1) ⫹ 24.7 ⫽ 228.7 moles. From step 1,
N2 ⫽ 853 moles; O2 ⫽ 20.6 moles.
Now, the total moles ⫽ 104 ⫹ 228.7 ⫹ 853 ⫹ 20.6 ⫽ 1206.3 moles. The percent
volume of CO2 ⫽ (104/1206.3)(100) ⫽ 8.6; the percent H2O ⫽ (228.7/
1206.1)(100) ⫽ 18.96; the percent N2 ⫽ (853/1206.3)(100) ⫽ 70.7; the percent
O2 ⫽ (20.6/1206.3)(100) ⫽ 1.71.
3. Find the amount of flue gas produced per million Btu (1.055 MJ)
To relate the pounds per million Btu (1.055 MJ) of NOx or CO produced to ppmv,
we must know the amount of flue gas produced per million Btu (1.055 MJ). From
step 2, the molecular weight of the flue gases ⫽ [(8.68 ⫻ 44) ⫹ (18.96 ⫻ 18) ⫹
(70.7 ⫻ 28) ⫹ (1.71 ⫻ 32)]/100 ⫽ 27.57.
The molecular weight of the fuel ⫽ [(97 ⫻ 16) ⫹ (2 ⫻ 30) ⫹ (1 ⫻ 44)]/100 ⫽
16.56. Now the ratio of flue gases/fuel ⫽ (1206.3 ⫻ 27.57)/(100 ⫻ 16.56) ⫽ 20.08
lb flue gas/lb fuel (9.12 kg/kg). Hence, 1 million Btu fired produces (1,000,000)/
23,789 ⫽ 42 lb (19.1 kg) fuel ⫽ (42)(20.08) ⫽ 844 lb (383 kg) wet flue gases.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.11
4. Calculate ppm values for the gases
Let 1 million Btu fired generate N lb (kg) of NOx. For emission calculations, NOx
is considered to have a molecular weight of 46. Also, the reference for NOx or CO
regulations is 3 percent dry oxygen by volume for steam generators. Hence, we
have the relation, NN ⫽ 106
(Yx)(N/46)(MWƒg)[(21 ⫺ 3)/(21 ⫺ O2XY)], where VN ⫽
ppm dry NOx; Y ⫽ 100/(100 ⫺ percent H2O), where percent H2O is the percent
volume water vapor in the flue gases; N ⫽ lb (kg) of NOx per million Btu (1.055
MJ) fired on an HHV basis, MWƒg ⫽ molecular weight of wet flue gases; Wgm ⫽
amount of wet flue gas produced per million Btu (1.055 MJ) fired.
Substituting in the above relation, VN ⫽ 106
(Nx)[100/(100 ⫺ 18.96)] (27.57/
844)(21 ⫺ 3)/[(21 ⫺ 1.71)(100/(100 ⫺ 18.96)] ⫽ 832N.
Similarly, Vc ⫽ ppmv CO2 generated per million Btu (1.055 MJ) fired ⫽ 1367⬚C,
where C ⫽ lb (kg) of CO generated per million Btu (1.055 MJ) and Vc ⫽ amount
in ppmvd (dry). The effect of excess air on these calculations is not at all significant.
One may perform these calculations at 30 percent excess air and still show that VN
⫽ 832N and Vc ⫽ 1367 for natural gas.
Related Calculations. These calculations for oil fuels also to show that VN ⫽
783N and Vc ⫽ 1286C.
This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO
Industries, Inc.
STEAM BOILER HEAT BALANCE
DETERMINATION
A steam generator having a maximum rated capacity of 60,000 lb/h (27,000 kg/
h) is operating at 45,340 lb/h (20,403 kg/h), delivering 125-lb/in2
(gage) 400⬚F
(862-kPa, 204⬚C) steam with a feedwater temperature of 181⬚F (82.8⬚C). At this
generating rate, the boiler requires 4370 lb/h (1967 kg/h) of West Virginia bitu-
minous coal having a heating value of 13,850 Btu/lb (32,215 kJ/kg) on a dry basis.
The ultimate fuel analysis is: C ⫽ 0.7757; H2 ⫽ 0.0507; O2 ⫽ 0.0519; N2 ⫽ 0.0120;
S ⫽ 0.0270; ash ⫽ 0.0827; total ⫽ 1.0000. The coal contains 1.61 percent moisture.
The boiler-room intake air and the fuel temperature ⫽ 79⬚F (26.1⬚C) dry bulb, 71⬚F
(21.7⬚C) wet bulb. The flue-gas temperature is 500⬚F (260⬚C), and the analysis of
the flue gas shows these percentages: CO2 ⫽ 12.8; CO ⫽ 0.4; O2 ⫽ 6.1; N2 ⫽
80.7; total ⫽ 100.0. Measured ash and refuse ⫽ 9.42 percent of dry coal; combus-
tible in ash and refuse ⫽ 32.3 percent. Compute a heat balance for this boiler based
on these test data. The boiler has four water-cooled furnace walls.
Calculation Procedure:
1. Determine the heat input to the boiler
In a boiler heat balance the input is usually stated in Btu per pound of fuel as fired.
Therefore, input ⫽ heating value of fuel ⫽ 13,850 Btu/lb (32,215 kJ/kg).
2. Compute the output of the boiler
The output of any boiler ⫽ Btu/lb (kJ/kg) of fuel ⫹ the losses. In this step the
first portion of the output, Btu/lb (kJ/kg) of fuel will be computed. The losses will
be computed in step 3.
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![4.12 POWER GENERATION
First find Ws, lb of steam produced per lb of fuel fired. Since 45,340 lb/h (20,403
kg/h) of steam is produced when 4370 lb/h (1967 kg/h) of fuel is fired, Ws ⫽
45,340/4370 ⫽ 10.34 lb of steam per lb (4.65 kg/kg) of fuel.
Once Ws is known, the output h1 Btu/lb of fuel can be found from h1 ⫽ Ws(hs
⫺ hw), where hs ⫽ enthalpy of steam leaving the superheater, or boiler if a super-
heater is not used; hw ⫽ enthalpy of feedwater, Btu/lb. For this boiler with steam
at 125 lb/in2
(gage) [⫽ 139.7 lb/in2
(abs)] and 400⬚F (930 kPa, 204⬚C), hs ⫽ 1221.2
Btu/lb (2841 kJ/kg), and hw ⫽ 180.92 Btu/lb (420.8 kJ/kg), from the steam tables.
Then h1 ⫽ 10.34(1221.2 ⫺ 180.92) ⫽ 10,766.5 Btu/lb (25,043 kJ/kg) of coal.
3. Compute the dry flue-gas loss
For any boiler, the dry flue-gas loss h2 Btu/lb (kJ/kg) of fuel is given by h2 ⫽
0.24Wg ⫻ (Tg ⫺ Ta), where Wg ⫽ lb of dry flue gas per lb of fuel; Tg ⫽ flue-gas
exit temperature,⬚F; Ta ⫽ intake-air temperature,⬚F.
Before Wg can be found, however, it must be determined whether any excess
air is passing through the boiler. Compute the excess air, if any, from excess air,
percent ⫽ 100 (O2 ⫺ 1
⁄2CO)/[0.264N2 ⫺ (O2 ⫺ 1
⁄2CO)], where the symbols refer
to the elements in the flue-gas analysis. Substituting values from the flue-gas anal-
ysis gives excess air ⫽ 100(6.1 ⫺ 0.2)/[0.264 ⫻ 80.7 ⫺ (6.1 ⫺ 0.2)] ⫽ 38.4
percent.
Using the method given in earlier calculation procedures, find the air required
for complete combustion as 10.557 lb/lb (4.571 kg/kg) of coal. With 38.4 percent
excess air, the additional air required ⫽ (10.557)(0.384) ⫽ 4.053 lb/lb (1.82 kg/
kg) of fuel.
From the same computation in which the air required for complete combustion
was determined, the lb of dry flue gas per lb of fuel ⫽ 11.018 (4.958 kg/kg). Then,
the total flue gas at 38.4 percent excess air ⫽ 11.018 ⫹ 4.053 ⫽ 15.071 lb/lb
(6.782 kg/kg) of fuel.
With a flue-gas temperature of 500⬚F (260⬚C), and an intake-air temperature of
79⬚F (26.1⬚C), h2 ⫽ 0.24(15.071)(500 ⫺ 70) ⫽ 1524 Btu/lb (3545 kJ/kg) of fuel.
4. Compute the loss due to evaporation of hydrogen-formed water
Hydrogen in the fuel is burned in forming H2O. This water is evaporated by heat
in the fuel, and less heat is available for producing steam. This loss is h3 Btu/lb
of fuel ⫽ 9H(1089 ⫺ Tƒ ⫹ 0.46Tg), where H ⫽ percent H2 in the fuel ⫼ 100;
Tƒ ⫽ temperature of fuel before combustion,⬚F; other symbols as before. For this
fuel with 5.07 percent H2, h3 ⫽ 9(5.07/100)(1089 ⫺ 79 ⫹ 0.46 ⫻ 500) ⫽ 565.8
Btu/lb (1316 kJ/kg) of fuel.
5. Compute the loss from evaporation of fuel moisture
This loss is h4 Btu/lb of fuel ⫽ Wmƒ(1089 ⫺ Tƒ ⫹ 0.46Tg), where Wmƒ ⫽ lb of
moisture per lb of fuel; other symbols as before. Since the fuel contains 1.61 percent
moisture, in terms of dry coal this is (1.61)/(100 ⫺ 1.61) ⫽ 0.0164, or 1.64 percent.
Then h4 ⫽ (1.64/100)(1089 ⫺ 79 ⫹ 0.46 ⫻ 500) ⫽ 20.34 Btu/lb (47.3 kJ/kg) of
fuel.
6. Compute the loss from moisture in the air
This loss is h5 Btu/lb of fuel ⫽ 0.46Wma(Tg ⫺ Ta), Wma ⫽ (lb of water per lb of
dry air)(lb air supplied per lb fuel). From a psychrometric chart, the weight of
moisture per lb of air at a 79⬚F (26.1⬚C) dry-bulb and 71⬚F (21.7⬚C) wet-bulb
temperature is 0.014 (0.006 kg). The combustion calculation, step 3, shows that the
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(10.190), where CO and CO2
are the percent by volume of these compounds in the flue gas by Orsat analysis;
C ⫽ lb carbon per lb of coal. With the given flue-gas analysis and the coal ultimate
analysis, h6 ⫽ 0.4/(0.4 ⫹ 12.8)[(77.57)/(100)](10.190) ⫽ 239.5 Btu/lb (557
kJ/kg) of fuel.
8. Compute the loss due to unconsumed carbon in the refuse
This loss is h7 Btu/lb of fuel ⫽ Wc(14,150), where Wc ⫽ lb of unconsumed carbon
in refuse per lb of fuel fired. With an ash and refuse of 9.42 percent of the dry
coal and combustible in the ash and refuse of 32.3 percent, h7 ⫽ (9.42/100)
(32.3/100)(14,150) ⫽ 430.2 Btu/lb (1006 kJ/kg) of fuel.
9. Find the radiation loss in the boiler furnace
Use the American Boiler and Affiliated Industries (ABAI) chart, or the manufac-
turer’s engineering data to approximate the radiation loss in the boiler. Either source
will show that the radiation loss is 1.09 percent of the gross heat input. Since the
gross heat input is 13,850 Btu/lb (32,215 kJ/kg) of fuel, the radiation loss ⫽
(13,850)(1.09/100) ⫽ 151.0 Btu/lb (351.2 kJ/kg) of fuel.
10. Summarize the losses; find the unaccounted-for loss
Set up a tabulation thus, entering the various losses computed earlier.
The unaccounted-for loss is found by summing all the other losses, 3 through
9, and subtracting from 100.00.
Related Calculations. Use this method to compute the heat balance for any
type of boiler—watertube or firetube—in any kind of service—power, process, or
heating—using any kind of fuel—coal, oil, gas, wood, or refuse. Note that step 3
shows how to compute excess air from an Orsat flue-gas analysis.
More stringent environmental laws are requiring larger investments in steam-
boiler pollution-control equipment throughout the world. To control sulfur emis-
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![4.16 POWER GENERATION
Ma[G ⫺ Cb ⫺ N2 ⫺ 7.94(H2 ⫺ O2 /8)]}, where G ⫽ {[11CO2 ⫹ 8O2 ⫹ 7(N2 ⫹
CO)]/[3(CO2 ⫹ CO)]}(Cb ⫹ S/2.67) ⫹ S/1.60; Mƒ ⫽ lb of moisture per lb fuel
burned; Ma ⫽ lb of moisture per lb of dry air to furnace; Cb ⫽ lb of carbon burned
per lb of fuel burned ⫽ C ⫽ RCr; Cr ⫽ lb of combustible per lb of refuse; R ⫽ lb
of refuse per lb of fuel; H2, N2, C, O2, S ⫽ lb of each element per lb of fuel, as
fired; CO2, CO, O2, N2 ⫽ percentage parts of volumetric analysis of dry combustion
gas entering the economizer. Substituting gives Cb ⫽ 0.685 ⫺ (0.093)(0.07) ⫽ 0.678
lb/lb (0.678 kg/kg) fuel; G ⫽ [11(0.158) ⫹ 8(0.028) ⫹ 7(0.814)]/[3(0.158)] ⫻
(0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60; G ⫽ 11.18 lb/lb (11.18 kg/kg) fuel. Hg ⫽
(800 ⫺ 300)(0.24) ⫻ (11.18) ⫹ (800 ⫺ 300)(0.46){0.045 ⫹ (8.9)(0.05) ⫹ 56/
7000[11.18 ⫺ 0.678 ⫺ 0.012 ⫺ 7.94 ⫻ (0.05 ⫺ 0.089/8)]}; Hg ⫽ 1473 Btu/lb
(3426.2 kJ/kg) fuel. Heat available ⫽ HgF ⫽ (1473)(5958) ⫽ 8,770,000 Btu/h
(2570.2 kW).
6. Compute the economizer efficiency
The economizer efficiency ⫽ (heat absorbed, Btu/h)/(heat available, Btu/h) ⫽
4,283,000/8,770,000 ⫽ 0.488, or 48.8 percent.
7. Compute the heat absorbed by air heater
The heat absorbed by the air heater, Btu/lb of fuel, ⫽ Ah(t2 ⫺ t1)(0.24 ⫹ 0.46Ma),
where Ah ⫽ air flow through heater, lb/lb fuel ⫽ A ⫺ Am; A ⫽ total air to furnace,
lb/lb fuel ⫽ G ⫺ Cb ⫺ N2 ⫺ 7.94(H2 ⫺ O2 /8); G ⫽ similar to economizer but
based on gas at the furnace exit; Am ⫽ external air supplied by the mill fan or other
source, lb/lb of fuel. Substituting shows G ⫽ [11(0.16) ⫹ 8(0.26) ⫹ 7(0.184)]/
[3(0.16)](0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60; G ⫽ 11.03 lb/lb (11.03 kg/kg) fuel;
A ⫽ 11.03 ⫺ 0.69 ⫺ 0.012 ⫺ 7.94(0.05 ⫺ 0.089/8); A ⫽ 10.02 lb/lb (10.02 kg/
kg) fuel. Heat absorbed ⫽ (1 ⫺ 0.15)(10.02)(480 ⫺ 63)(0.24 ⫹ 56/7000) ⫽ 865.5
Btu/lb (2013.2 kJ/kg fuel.
8. Compute the heat available to the air heater
The heat available to the air heater, Btu/h ⫽ (t5 ⫺ t1)0.24G ⫹ (t5 ⫺ t1)0.46(Mƒ ⫹
8.94H2 ⫹ Ma A). In this relation, all symbols are the same as for the economizer
except that G and A are based on the gas entering the heater. Substituting gives G
⫽ [11(0.15) ⫹ 8(0.036) ⫹ 7(0.814)]/[3(0.15)](0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60;
G ⫽ 11.72 lb/lb (11.72 kg/kg) fuel. And A ⫽ 11.72 ⫺ 0.69 ⫺ 0.012 ⫺ 7.94(0.05
⫺ 0.089/8) ⫽ 10.71 lb/lb (10.71 kg/kg) fuel. Heat available ⫽ (570 ⫺
3)(0.24)(11.72) ⫹ (570 ⫺ 63)(0.46)[0.045 ⫹ 8.94(0.05) ⫹ 56/7000(10.71)] ⫽ 1561
Btu/lb (3630.9 kJ/kg).
9. Compute the air-heater efficiency
The air-heater efficiency ⫽ (heat absorbed, Btu/lb fuel)/(heat available, Btu/lb
fuel) ⫽ 865.5/1561 ⫽ 0.554, or 55.4 percent.
Related Calculations. The above procedure is valid for all types of steam gen-
erators, regardless of the kind of fuel used. Where oil or gas is the fuel, alter the
combustion calculations to reflect the differences between the fuels. Further, this
procedure is also valid for marine and portable boilers.
FIRE-TUBE BOILER ANALYSIS AND SELECTION
Determine the heating surface in an 84-in (213.4-cm) diameter fire-tube boiler 18
ft (5.5 m) long having 84 tubes of 4-in (10.2-cm) ID if 25 percent of the upper
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.17
shell ends are heat-insulated. How much steam is generated if the boiler evaporates
34.5 lb/h of water per 12 ft2
[3.9 g/(m2
䡠 s)] of heating surface? How much heat is
added by the boiler if it operates at 200 lb/in2
(abs) (1379.0 kPa) with 200⬚F
(93.3⬚C) feedwater? What is the factor of evaporation for this boiler? How much
hp is developed by the boiler if 7,000,000 Btu/h (2051.4 kW) is delivered to the
water?
Calculation Procedure:
1. Compute the shell area exposed to furnace gas
Shell area ⫽ DL(1 ⫺ 0.25), where D ⫽ boiler diameter, ft; L ⫽ shell length, ft;
1 ⫺ 0.25 is the portion of the shell in contact with the furnace gas. Then shell
area ⫽ (84/12)(18)(0.75) ⫽ 297 ft2
(27.0 m2
).
2. Compute the tube area exposed to furnace gas
Tube area ⫽ dLN, where ⫽ tube ID, ft; L ⫽ tube length, ft; N ⫽ number of tubes
in boiler. Substituting gives tube area ⫽ (4/12)(18)(84) ⫽ 1583 ft2
(147.1 m2
).
3. Compute the head area exposed to furnace gas
The area exposed to furnace gas is twice (since there are two heads) the exposed
head area minus twice the area occupied by the tubes. The exposed head area is
(total area)(1 ⫺ portion covered by insulation, expressed as a decimal). Substituting,
we get 2D2
/4 ⫺ (2)(84)d2
/4 ⫽ 2/4(84/12)2
(0.75) ⫺ (2)(84)(4/12)2
/4 ⫽
head area ⫽ 43.1 ft2
(4.0 m2
).
4. Find the total heating surface
The total heating surface of any fire-tube boiler is the sum of the shell, tube, and
head areas, or 297.0 ⫹ 1583 ⫹ 43.1 ⫽ 1923 ft2
(178.7 m2
), total heating surface.
5. Compute the quantity of steam generated
Since the boiler evaporates 34.5 lb/h of water per 12 ft2
[3.9 g/(m2
䡠 s)] of heating
surface, the quantity of steam generated ⫽ 34.5 (total heating surface, ft2
)/12 ⫽
34.5(1923.1)/12 ⫽ 5200 lb/h (0.66 kg/s).
Note: Evaporation of 34.5 lb/h (0.0043 kg/s) from and at 212⬚F (100.0⬚C) is
the definition of the now-discarded term boiler hp. However, this term is still met
in some engineering examinations and is used by some manufacturers when com-
paring the performance of boilers. A term used in lieu of boiler horsepower, with
the same definition, is equivalent evaporation. Both terms are falling into disuse,
but they are included here because they still find some use today.
6. Determine the heat added by the boiler
Heat added, Btu/lb of steam ⫽ hg ⫺ hƒ1; from steam table values 1198.4 ⫺
167.99 ⫽ 1030.41 Btu/lb (2396.7 kJ/kg). An alternative way of computing heat
added is hg ⫺ (feedwater temperature,⬚F, ⫺ 32), where 32 is the freezing temper-
ature of water on the Fahrenheit scale. By this method, heat added ⫽ 1198.4 ⫺
(200 ⫺ 32) ⫽ 1030.4 Btu/lb (2396.7 kJ/kg). Thus, both methods give the same
results in this case. In general, however, use of steam table values is preferred.
7. Compute the factor of evaporation
The factor of evaporation is used to convert from the actual to the equivalent evap-
oration, defined earlier. Or, factor of evaporation ⫽ (heat added by boiler,
Btu/lb)/970.3, where 970.3 Btu/lb (2256.9 kJ/kg) is the heat added to develop 1
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![4.18 POWER GENERATION
boiler hp (bhp) (0.75 kW). Thus, the factor of evaporation for this boiler ⫽
1030.4/970.3 ⫽ 1.066.
8. Compute the boiler hp output
Boiler hp ⫽ (actual evaporation, lb/h) (factor of evaporation)/34.5. In this relation,
the actual evaporation must be computed first. Since the furnace delivers 7,000,000
Btu/h (2051.5 kW) to the boiler water and the water absorbs 1030.4 Btu/lb (2396.7
kJ/kg) to produce 200-lb/in2
(abs) (1379.0-kPa) steam with 200⬚F (93.3⬚C) feed-
water, the steam generated, lb/h ⫽ (total heat delivered, Btu/h)/(heat absorbed,
Btu/lb) ⫽ 7,000,000/1030.4 ⫽ 6670 lb/h (0.85 kg/s). Then boiler hp ⫽
(6760)(1.066)/34.5 ⫽ 209 hp (155.9 kW).
The rated hp output of horizontal fire-tube boilers with separate supporting walls
is based on 12 ft2
(1.1 m2
) of heating surface per boiler hp. Thus, the rated hp of
the boiler ⫽ 1923.1/12 ⫽ 160 hp (119.3 kW). When producing 209 hp (155.9
kW), the boiler is operating at 209/160, or 1,305 times its normal rating, or
(100)(1.305) ⫽ 130.5 percent of normal rating.
Note: Today most boiler manufacturers rate their boilers in terms of pounds per
hour of steam generated at a stated pressure. Use this measure of boiler output
whenever possible. Inclusion of the term boiler hp in this handbook does not in-
dicate that the editor favors or recommends its use. Instead, the term was included
to make the handbook as helpful as possible to users who might encounter the term
in their work.
SAFETY-VALVE STEAM-FLOW CAPACITY
How much saturated steam at 150 lb/in2
(abs) (1034.3 kPa) can a 2.5-in (6.4-cm)
diameter safety valve having a 0.25-in (0.6-cm) lift pass if the discharge coefficient
of the valve cd is 0.75? What is the capacity of the same valve if the steam is
superheated 100⬚F (55.6⬚C) above its saturation temperature?
Calculation Procedure:
1. Determine the area of the valve annulus
Annulus area, in2
⫽ A ⫽ DL, where D ⫽ valve diameter, in; L ⫽ valve lift, in.
Annulus area ⫽ (2.5)(0.25) ⫽ 1.966 in2
(12.7 cm2
).
2. Compute the ideal flow for this safety valve
Ideal flow Fi lb/s for any safety valve handling saturated steam is Fi ⫽ A/60,
0.97
ps
where ps ⫽ saturated-steam pressure, lb/in2
(abs). For this valve, Fi ⫽ (150)0.97
(1.966)/60 ⫽ 4.24 lb/s (1.9 kg/s).
3. Compute the actual flow through the valve
Actual flow Fa ⫽ Fi cd ⫽ (4.24)(0.75) ⫽ 3.18 lb/s (1.4 kg/s) ⫽ (3.18)(3600 s/h) ⫽
11,448 lb/h (1.44 kg/s).
4. Determine the superheated-steam flow rate
The ideal superheated-steam flow Fis lb/s is Fis ⫽ A/[60(1 ⫹ 0.0065ts)], where
0.97
ps
ts ⫽ superheated temperature, above saturation temperature,⬚F. The Fis ⫽
(150)0.97
(1.966)/[60(1 ⫹ 0.0065 ⫻ 100)] ⫽ 3.96 lb/s (1.8 kg/s). The actual flow
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.19
is Fas ⫽ Fiscd ⫽ (3.96)(0.75) ⫽ 2.97 lb/s (1.4 kg/s) ⫽ (2.97)(3600) ⫽ 10,700 lb/
h (1.4 kg/s).
Related Calculations. Use this procedure for safety valves serving any type of
stationary or marine boiler.
SAFETY-VALVE SELECTION FOR A WATERTUBE
STEAM BOILER
Select a safety valve for a watertube steam boiler having a maximum rating of
100,000 lb/h (12.6 kg/s) at 800 lb/in2
(abs) (5516.0 kPa) and 900⬚F (482.2⬚C).
Determine the valve diameter, size of boiler connection for the valve, opening
pressure, closing pressure, type of connection, and valve material. The boiler is oil-
fired and has a total heating surface of 9200 ft2
(854.7 m2
) of which 1000 ft2
(92.9
m2
) is in waterwall surface. Use the ASME Boiler and Pressure Vessel Code rules
when selecting the valve. Sketch the escape-pipe arrangement for the safety valve.
Calculation Procedure:
1. Determine the minimum valve relieving capacity
Refer to the latest edition of the Code for the relieving-capacity rules. Recent edi-
tions of the Code require that the safety valve have a minimum relieving capacity
based on the pounds of steam generated per hour per square foot of boiler heating
surface and waterwall heating surface. In the edition of the Code used in preparing
this handbook, the relieving requirement for oil-fired boilers was 10 lb/(ft2
䡠 h) of
steam [13.6 g/(m2
䡠 s)] of boiler heating surface, and 16 lb/(ft2
䡠 h) of steam [21.9
g/(m2
䡠 s)] of waterwall surface. Thus, the minimum safety-valve relieving capacity
for this boiler, based on total heating surface, would be (8200)(10) ⫹ (1000)(16) ⫽
92,000 lb/h (11.6 kg/s). In this equation, 1000 ft2
(92.9 m2
) of waterwall surface
is deducted from the total heating surface of 9200 ft2
(854.7 m2
) to obtain the boiler
heating surface of 8200 ft2
(761.8 m2
).
The minimum relieving capacity based on total heating surface is 92,000 lb/h
(11.6 kg/s); the maximum rated capacity of the boiler is 100,000 lb/h (12.6
kg/s). Since the Code also requires that ‘‘the safety valve or valves will discharge
all the steam that can be generated by the boiler,’’ the minimum relieving capacity
must be 100,000 lb/h (12.6 kg/s), because this is the maximum capacity of the
boiler and it exceeds the valve capacity based on the heating-surface calculation.
If the valve capacity based on the heating-surface steam generation were larger than
the stated maximum capacity of the boiler, the Code heating-surface valve capacity
would be used in safety-valve selection.
2. Determine the number of safety valves needed
Study the latest edition of the Code to determine the requirements for the number
of safety valves. The edition of the Code used here requires that ‘‘each boiler shall
have at least one safety valve and if it [the boiler] has more than 500 ft2
(46.5 m2
)
of water heating surface, it shall have two or more safety valves.’’ Thus, at least
two safety valves are needed for this boiler. The Code further specifies, in the
edition used, that ‘‘when two or more safety valves are used on a boiler, they may
be mounted either separately or as twin valves made by placing individual valves
on Y bases or duplex valves having two valves in the same body casing. Twin
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![4.20 POWER GENERATION
valves made by placing individual valves on Y bases, or duplex valves having two
valves in the same body, shall be of equal sizes.’’ Also, ‘‘when not more than
two valves of different sizes are mounted singly, the relieving capacity of the
smaller valve shall not be less than 50 percent of that of the larger valve.’’
Assume that two equal-size valves mounted on a Y base will be used on the
steam drum of this boiler. Two or more equal-size valves are usually chosen for
the steam drum of a watertube boiler.
Since this boiler handles superheated steam, check the Code requirements re-
garding superheaters. The Code states that ‘‘every attached superheater shall have
one or more safety valves near the outlet.’’ Also, ‘‘the discharge capacity of the
safety valve, or valves, on an attached superheater may be included in determining
the number and size of the safety valves for the boiler, provided there are no
intervening valves between the superheater safety valve and the boiler, and provided
the discharge capacity of the safety valve, or valves, on the boiler, as distinct from
the superheater, is at least 75 percent of the aggregate valve capacity required.’’
Since the safety valves used must handle 100,000 lb/h (12.6 kg/s), and one or
more superheater safety valves are required by the Code, assume that the two steam-
drum valves will handle, in accordance with the above requirement, 80,000 lb/h
(10.1 kg/s). Assume that one superheater safety valve will be used. Its capacity
must then be at least 100,000 ⫺ 80,000 ⫽ 20,000 lb/h (2.5 kg/s). (Use a few
superheater safety valves as possible, because this simplifies the installation and
reduces cost.) With this arrangement, each steam-drum valve must handle 80,000/
2 ⫽ 40,000 lb/h (5.0 kg/s) of steam, since there are two safety valves on the steam
drum.
3. Determine the valve pressure settings
Consult the Code. It requires that ‘‘one or more safety valves on the boiler proper
shall be set at or below the maximum allowable working pressure.’’ For modern
boilers, the maximum allowable working pressure is usually 1.5, or more, times
the rated operating pressure in the lower [under 1000 lb/in2
(abs) or 6895.0 kPa]
pressure ranges. To prevent unnecessary operation of the safety valve and to reduce
steam losses, the lowest safety-valve setting is usually about 5 percent higher than
the boiler operating pressure. For this boiler, the lowest pressure setting would be
800 ⫹ 800(0.05) ⫽ 840 lb/in2
(abs) (5791.8 kPa). Round this to 850 lb/in2
(abs)
(5860.8 kPa, or 6.25 percent) for ease of selection from the usual safety-valve rating
tables. The usual safety-valve pressure setting is between 5 and 10 percent higher
than the rated operating pressure of the boiler.
Boilers fitted with superheaters usually have the superheater safety valve set at
a lower pressure than the steam-drum safety valve. This arrangement ensures that
the superheater safety valve opens first when overpressure occurs. This provides
steam flow through the superheater tubes at all times, preventing tube burnout.
Therefore, the superheater safety valve in this boiler will be set to open at 850 lb
/in2
(abs) (5860.8 kPa), the lowest opening pressure for the safety valves chosen.
The steam-drum safety valves will be set to open at a higher pressure. As decided
earlier, the superheater safety valve will have a capacity of 20,000 lb/h (2.5 kg/s).
Between the steam drum and the superheater safety valve, there is a pressure
loss that varies from one boiler to another. The boiler manufacturer supplies a
performance chart showing the drum outlet pressure for various percentages of the
maximum continuous steaming capacity of the boiler. This chart also shows the
superheater outlet pressure for the same capacities. The difference between the drum
and superheater outlet pressure for any given load is the superheater pressure loss.
Obtain this pressure loss from the performance chart.
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![4.26 POWER GENERATION
0.9278 ft3
/lb (0.058 m3
/kg) at 500 lb/in2
(abs) (3447.5 kPa) saturated. The mass-
flow ratio ws /As ⫽ 200,000/0.500 ⫽ 400,000.
2. Compute the superheater entrance and exit pressure loss
Entrance and exit pressure loss pE lb/in2
⫽ vƒ /8(0.00001ws /As) ⫽ 0.9278/
8[(0.00001) ⫻ (400,000)]2
⫽ 1.856 lb/in2
(12.8 kPa).
3. Compute the pressure loss in the straight tubes
Straight-tube pressure loss ps lb/in2
⫽ vƒƒ/ID(0.00001ws /As)2
⫽ 0.9278(150) ⫻
(0.025)/2.125[(0.00001)(400,000)]2
⫽ 26.2 lb/in2
(abs) (180.6 kPa).
4. Compute the pressure loss in the superheater bends
Bend pressure loss pb ⫽ 0.0833Bƒ(0.00001ws /As)2
⫽ 0.0833(12.0)[(0.00001) ⫻
(400,000)]2
⫽ 16.0 lb/in2
(110.3 kPa).
5. Compute the total pressure loss
The total pressure loss in any superheater is the sum of the entrance, straight-tube,
bend, and exit-pressure losses. These losses were computed in steps 2, 3, and 4
above. Therefore, total pressure loss pt ⫽ 1.856 ⫹ 26.2 ⫹ 16.0 ⫽ 44.056 lb/in2
(303.8 kPa).
Note: Data for superheater pressure-loss calculations are best obtained from the
boiler manufacturer. Several manufacturers have useful publications discussing su-
perheater pressure losses. These are listed in the references at the beginning of this
section.
SELECTION OF A STEAM BOILER FOR A
GIVEN LOAD
Choose a steam boiler, or boilers, to deliver up to 250,000 lb/h (31.5 kg/s) of
superheated steam at 800 lb/in2
(abs) (5516 kPa) and 900⬚F (482.2⬚C). Determine
the type or types of boilers to use, the capacity, type of firing, feedwater-quality
requirements, and best fuel if coal, oil, and gas are all available. The normal con-
tinuous steam requirement is 200,000 lb/h (25.2 kg/s).
Calculation Procedure:
1. Select type of steam generator
Use Fig. 5 as a guide to the usual types of steam generators chosen for various
capacities and different pressure and temperature conditions. Enter Fig. 5 at the left
at 800 lb/in2
(abs) (5516 kPa), and project horizontally to the right, along AB, until
the 250,000-lb/h (31.5-kg/s) capacity ordinate BC is intersected. At B, the oper-
ating point of this boiler, Fig. 5 shows that a watertube boiler should be used.
Boiler units presently available can deliver steam at the desired temperature of
900⬚F (482.2⬚C). The required capacity of 250,000 lb/h (31.5 kg/s) is beyond the
range of packaged watertube boilers—defined by the American Boiler Manufac-
turer Association as ‘‘a boiler equipped and shipped complete with fuel-burning
equipment, mechanical-draft equipment, automatic controls, and accessories.’’
Shop-assembled boilers are larger units, where all assembly is handled in the
builder’s plant but with some leeway in the selection of controls and auxiliaries.
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![4.32 POWER GENERATION
Determine from combustion calculations for the boiler the density of the flue
gas. Assume that the combustion calculations for this boiler show that the flue-gas
density is 0.045 lb/ft3
(0.72 kg/m3
) at the exit-gas temperature. To determine the
exit-gas temperature, apply a 10 percent factor of safety to the given exit temper-
ature, 350⬚F (176.6⬚C). Hence, exit-gas temperature ⫽ 350 ⫹ 350(0.10) ⫽ 385⬚F
(196.1⬚C). Then flue-gas flow, ft3
/min ⫽ (flue-gas flow, lb/h)/(60)(flue-gas density,
lb/ft3
) ⫽ 343,600/[(60)(0.045)] ⫽ 127,000 ft3
/min (59.9 m3
/s). Apply a 10 to 25
percent factor of safety to the flue-gas quantity to allow for increased gas flow.
With a 20 percent factor of safety, the actual flue-gas flow the fan must handle ⫽
127,000 ⫹ 127,000(0.20) ⫽ 152,400 ft3
/min (71.8 m3
/s), say 152,500 ft3
/min (71.9
m3
/s) for fan-selection purposes.
6. Compute the induced-draft fan discharge pressure
Find the sum of the draft losses from the burner outlet to the induced-draft fan
inlet. These losses are as follows for this boiler:
Allow a 10 to 25 percent factor of safety to ensure adequate pressure during all
boiler loads and furnace conditions. With a 20 percent factor of safety for this fan,
the total actual pressure loss ⫽ 8.95 ⫹ 8.95(0.20) ⫽ 10.74 inH2O (2.7 kPa). Round
this to 11.0 inH2O (2.7 kPa) for fan-selection purposes.
7. Compute the power required to drive the induced-draft fan
As with the forced-draft fan, air hp ⫽ 0.0001753HƒC ⫽ 0.0001753(11.0) ⫻
(127,000) ⫽ 245 hp (182.7 kW). If the combined efficiency of the fan and its
driver, assumed to be an electric motor, is 68 percent, the motor hp required ⫽
245/0.68 ⫽ 360.5 hp (268.8 kW). A 375-hp (279.6-kW) motor would be chosen
for the fan driver.
8. Choose the fans from a manufacturer’s engineering data
Use the next calculation procedure to select the fans from the engineering data of
an acceptable manufacturer. For larger boiler units, the forced-draft fan is usually
a backward-curved blade centrifugal-type unit. Where two fans are chosen to op-
erate in parallel, the pressure curve of each fan should decrease at the same rate
near shutoff so that the fans divide the load equally. Be certain that forced-draft
fans are heavy-duty units designed for continuous operations with well-balanced
rotors. Choose high-efficiency units with self-limiting power characteristics to pre-
vent overloading the driving motor. Airflow is usually controlled by dampers on
the fan discharge.
Induced-draft fans handle hot, dusty combustion products. For this reason, ex-
treme care must be taken to choose units specifically designed for induced-draft
service. The usual choice for large boilers is a centrifugal-type unit with forward-
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![4.34 POWER GENERATION
TABLE 5 Typical Fan Capacities
capacity-table ft3
/min. Or, capacity-table ft3
/min ⫽ 69,400/1.147 ⫽ 62,250 ft3
/
min (29.4 m3
/s).
2. Choose the fan size from the capacity table
Turn to the fan-capacity table in the engineering data, and look for a fan delivering
62,250 ft3
/min (29.4 m3
/s) at 6.3-inH2O (1.6-kPa) static pressure. Inspection of the
table shows that the capacities are tabulated for 6.0- and 6.5-inH2O (1.5- and 1.6-
kPa) static pressure. There is no tabulation for 6.3-inH2O (1.57-kPa) static pressure.
Enter the table at the nearest capacity to that required, 62,250 ft3
/min (29.4
m3
/s), as shown in Table 5. This table, excerpted with permission from the Amer-
ican Standard Inc. engineering data, shows that the nearest capacity of this particular
type of fan is 62,595 ft3
/min (29.5 m3
/s). The difference, or 62,595 ⫺ 62,250 ⫽
345 ft3
/min (0.16 m3
/s), is only 345/62,250 ⫽ 0.0055, or 0.55 percent. This is a
negligible difference, and the 62,595-ft3
/min (29.5-m3
/s) fan is well suited for its
intended use. The extra static pressure of 6.5 ⫺ 6.3 ⫽ 0.2 inH2O (0.05 kPa) is
desirable in a forced-draft fan because furnace or duct resistance may increase
during the life of the boiler. Also, the extra static pressure is so small that it will
not markedly increase the fan power consumption.
3. Compute the fan speed and power input
Multiply the capacity-table rpm and brake hp (bhp) by the composite factor to
determine the actual rpm and bhp. Thus, with data from Table 5, the actual rpm ⫽
(1096)(1.1147) ⫽ 1221.7 r/min. Actual bhp ⫽ (99.08)(1.1147) ⫽ 110.5 bhp (82.4
kW). This is the hp input required to drive the fan and is close to the 113.2 hp
(84.4 kW) computed in the previous calculation procedure. The actual motor hp
would be the same in each case because a standard-size motor would be chosen.
The difference of 113.2 ⫺ 110.5 ⫽ 2.7 hp (2.0 kW) results from the assumed
efficiencies that depart from the actual values. Also, a sea-level attitude was as-
sumed in the previous calculation procedure. However, the two methods used show
how accurately fan capacity and hp input can be estimated by judicious evaluation
of variables.
4. Compute the correction factors for the induced-draft fan
The flue-gas density is 0.045 lb/ft3
(0.72 kg/m3
) at 385⬚F (196.1⬚C). Interpolate in
the temperature correction-factor table because a value of 385⬚F (196.1⬚C) is not
tabulated. Find the correction factor for 285⬚F (196.1⬚C) thus: [(Actual temperature
⫺ lower temperature)/(higher temperature ⫺ lower temperature)] ⫻ (higher tem-
perature correction factor ⫺ lower temperature correction factor) ⫹ lower temper-
ature correction factor. Or, [(385 ⫺ 375)/(400 ⫺ 375)](1.273 ⫺ 1.255) ⫹ 1.255 ⫽
1.262.
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![4.38 POWER GENERATION
EFFECT OF BOILER RELOCATION ON DRAFT
FAN PERFORMANCE
An acceptance test of a boiler shows that its induced-draft fan handles 600,000
(272,400 kg) lb per hour of flue gas at 290⬚F (143.3⬚C) against total friction of 10
in (25.4 cm) water gage (WG). The boiler is relocated to a higher elevation where
the barometric pressure is 24 inHg (60.96 cmHg), as compared to the original 30
inHg (76.2 cmHg) at sea level. If no changes are made to the equipment except
adjustments to the fan and with gas weights and temperatures as before, what are
the new volume and suction conditions for the fan design and relocation?
Calculation Procedure:
1. Determine the new inlet condition for the fan
When a fan is required to handle air or gas at conditions other than standard, a
correction must be made in the static pressure and hp (kW). Since a fan is essen-
tially a constant-volume machine, the ft3
/min (m3
/min) delivered will not change
materially if the speed and system configuration do not change, regardless of the
air or gas density.
The static pressure, however, changes directly with density. Hence, the static
pressure must be carefully calculated for specified conditions. For the situation
described here, assume a gas molecular weight of 28, a typical value. The density
correction factor can be computed from the ratio of the new-location barometric
pressure to the first-location barometric pressure, both expressed inHg (cmHg). Or,
density correction factor ⫽ 24 in/30 in ⫽ 0.80.
There is no temperature correction factor because the air temperature remains
the same. Therefore, at the new elevated location of the boiler the intake condition
for the fan will be (10 in WG)(0.8 correction factor) ⫽ 8 in (20.3 cm) WG.
2. Compute the new volume condition
Use the relation Volume flow ⫽ (lb/h)(molecular weight of gas)(cfm at inlet con-
ditions)[(absolute temperature of flue gas)/(‘‘standard’’ air temperature of 60⬚F in
absolute terms)][(reduced barometric pressure)/(reduced barometric pressure ⫺ new
suction condition)]. Substituting, new volume condition ⫽ (600,000/28)(379)[(460
⫹ 290)/(60 ⫹ 460)][(24)/(24 ⫺ 8)] ⫽ 17.57 ⫻ 106
ft3
/h (0.497 m3
/h ⫻ 106
).
Related Calculations. With used power-plant equipment becoming more pop-
ular throughout the world (see the classified section of any major engineering mag-
azine) it is important that the engineer be able to determine the performance of re-
used equipment at different locations.
ANALYSIS OF BOILER AIR DUCTS AND
GAS UPTAKES
Three oil-fired boilers are supplied air through the breeching shown in Fig. 7a.
Each boiler will burn 13,600 lb/h (1.71 kg/s) of fuel oil at full load. The draft loss
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.41
permits easy review of the calculations for detection of errors. Assign letters to
each point of the duct where a change in section dimensions or directions, or both,
occurs. Use these letters:
Point A: Assume that 70⬚F (21.1⬚C) air having a density of 0.075 lb/ft3
(1.2
kg/m3
) enters the system when the ambient barometric pressure is 29.92 inHg
(101.3 kPa). Compute the velocity pressure at point A, in inH2O, from pv ⫽ v2 /
[3.06(104
)(460 ⫹ t)], where t ⫽ air temperature,⬚F. Since the velocity of the air at
A is 1545 ft/min (11.7 m/s), pv ⫽ (1545)2
/[3.06(104
)(530)] ⫽ 0.147 inH2O (0.037
kPa) at 70⬚F (21.1⬚C).
The entrance loss at A, where there is a sharp-edged duct, is 0.5pv, or 0.5(0.147)
⫽ 0.0735 inH2O (0.018 kPa). With rounded inlet, the loss in velocity pressure
would be negligible.
Section AB: There is a pressure loss due to duct friction between A and B, and
B and C. Also, there is a bend loss at points B and C. Compute the duct friction
first.
For any circular duct, the static pressure loss due to friction ps inH2O ⫽
(0.03L/d1.24
)(v/1000)1.84
, where L ⫽ duct length, ft; d ⫽ duct diameter, in. To
convert any rectangular or square duct with sides a and b high and wide, respec-
tively, to an equivalent round duct of D-ft diameter, use the relation D ⫽ 2ab/
(a ⫹ b). For this duct, d ⫽ 2(10.5)(10.5)/(10.5 ⫹ 10.5) ⫽ 10.5 ft (3.2 m) ⫽ 126
in (320 cm) ⫽ d. Since this duct is 12 ft (3.7 m) long between A and B, ps ⫽
[0.03(12)/1261.24
](1.545/1000)1.84
⫽ 0.002 inH2O (0.50 kPa).
Point B: The 45⬚ bend at B has, from Baumeister and Marks—Standards Hand-
book for Mechanical Engineers, a pressure drop of 60 percent of the velocity head
in the duct, for (0.60)(0.147) ⫽ 0.088 inH2O (20.5 Pa) loss.
Section BC: Duct friction in the 14-ft (4.3-m) long downcomer BC is ps ⫽
[0.03(14)/1261.24
](1545/1000)1.84
⫽ 0.0023 inH2O (0.56 Pa). Point C: The 45⬚
bend at C has a velocity head loss of 60 percent of the velocity pressure. Determine
the velocity pressure in this duct in the same manner as for point A, or pv ⫽
(1545)2
/[3.06(104
)(530)] ⫽ 0.147 inH2O (36.1 Pa), since the velocity at points B
and C is the same. Then the velocity head loss ⫽ (0.60)(0.147) ⫽ 0.088 inH2O
(21.9 Pa).
Section CD: The equivalent round-duct diameter is D ⫽ (2)(10.25)(13)/(10.25
⫹ 13) ⫽ 11.45 ft (3.5 m) ⫽ 137.3 in (348.7 cm). Duct friction is then ps ⫽
[0.03(9)/137.31.24
](1275/1000)1.84
⫽ 0.000934 inH2O (0.23 Pa). Velocity pressure
in the duct is pv ⫽ (1275)2
/[3.06(104
)(530)] ⫽ 0.100 inH2O (24.9 Pa). Since there
is no room for a transition piece—that is, a duct providing a gradual change in
flow area between points C and D–the decrease in velocity pressure from 0.147 to
0.100 in (36.6 to 24.9 Pa), or 0.147 ⫺ 0.10 ⫽ 0.047 inH2O (11.7 Pa), is not
converted to static pressure and is lost.
Point E: The pressure loss in the right-angle bend at E is, from Baumeister and
Marks—Standard Handbook for Mechanical Engineers, 1.2 times the velocity head,
or (1.2)(0.1) ⫽ 0.12 inH2O (29.9 Pa). Also, since this is a sharp-edged elbow, there
is an additional loss of 50 percent of the velocity head, or (0.5)(0.10) ⫽ 0.05 inH2O
(12.4 Pa).
The velocity pressure at point E is pv ⫽ (2360)2
/[3.06(104
)(530)] ⫽ 0.343 inH2O
(85.4 Pa).
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1.84
⫽ 0.0247 inH2O (6.2 Pa).
Air entering the large plenum chamber at F loses all its velocity. There is no
static-pressure regain; therefore, the velocity-head loss ⫽ 0.348 ⫺ 0.0 ⫽ 0.348
inH2O (86.6 Pa).
4. Compute the losses in the uptake and stack
Convert the airflow of 250 ft3
/lb (15.6 m3
/kg) of fuel oil to pounds of air per pound
of fuel oil by multiplying by the density, or 250(0.075) ⫽ 18.75 lb of air per pound
of oil. The flue gas will contain 18.75 lb of air ⫹ 1 lb of oil per pound of fuel
burned, or (18.75 ⫹ 1)/18.75 ⫽ 1.052 times as much gas leaves the boiler as air
enters; this can be termed the flue-gas factor.
Point G: The quantity of flue gas entering the stack from each boiler (corrected
to a 400⬚F or 204.4⬚C outlet temperature) is, in ⬚R (cfm air to furnace)(stack,
⬚R/air,⬚R)(flue-gas factor). Or stack flue-gas flow ⫽ (56,700)[(400 ⫹ 460)/(70 ⫹
460)](1.052) ⫽ 97,000 ft3
/min (45.8 m3
/s) per boiler.
The total duct area available for the uptake leading to the stack is 9 ⫻ 10 ft (2.7
⫻ 3.0 m) ⫽ 90 ft2
(8.4 m2
), based on the clearance above the boilers. The flue-gas
velocity for three boilers is v ⫽ (3)(97,000)/90 ⫽ 3235 ft/min (16.4 m/s). The
velocity pressure in the uptake is p ⫽ (3235)2
/[3.06(104
)(460 ⫹ 400)] ⫽ 0.397
inH2O (98.8 Pa).
Point H: The flue-gas flow from all the boilers is divided equally between three
ducts. HG, IG, JG, Fig. 7. It is desirable to maintain the same gas velocity in each
duct and have this velocity equal to that in the uptake. The same velocity can be
obtained in each duct by making each duct one-third the area of the uptake, or
90/3 ⫽ 30 ft2
(2.8 m2
). Then v ⫽ 97,000/30 ⫽ 3235 ft/min (16.4 m/s) in each
duct. Since the velocity in each duct equals the velocity in the uptake, the velocity
pressure in each duct equals that in the uptake, or 0.397 inH2O (98.8 Pa).
Ducts HG and JG have two 45⬚ bends in them, or the equivalent of one 90⬚
bend. The velocity-pressure loss in a 90⬚ bend is 1.20 times the velocity head in
the duct; or, for either HG or JG, (1.20)(0.397) ⫽ 0.476 inH2O (118.5 Pa).
Section HG: The equivalent duct diameter for a 30-ft2
(2.8-m2
) duct is D ⫽
2(30/)0.5
⫽ 6.19 ft (1.9 m) ⫽ 74.2 in (188.4 cm). The duct friction in HG, which
equals that in JG, is ps ⫽ [0.03(20)/74.21.24
](530/860)(3235/1000)1.84
⫽ 0.01536
inH2O (3.8 Pa), if we correct for the flue-gas temperature with the ratio (70 ⫹
460)/(400 ⫹ 460) ⫽ 530/860.
Section GK: The stack joins the uptake at point G. Assume that this installation
is designed for a stack-gas area of 500 lb of oil per square foot (2441.2 kg/m2
) of
stack; for three boilers, stack area ⫽ (3)(13,600 lb/h oil)/500 ⫽ 81.5 ft2
(7.6 m2
).
The stack diameter will then be D ⫽ 2(8.15/)0.5
⫽ 10.18 ft (3.1 m) ⫽ 122 in
(309.9 cm).
The gas velocity in the stack is v ⫽ (3)(97,000)/81.5 ⫽ 3570 ft/min (18.1 m/
s). The friction in the stack is ps ⫽ [0.03(50)1221.24
](3570/1000)1.84
(503/860) ⫽
0.0194 inH2O (4.8 Pa).
5. Compute the total losses in the system
Tabulate the individual losses and find the sum as follows:
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.47
metric pressure is 29.92 inHg (101.3 kPa); the boiler flue gas enters the stack at
500⬚F (260.0⬚C); the flue-gas flow rate is 100 lb/s (45.4 kg/s); The flue-gas density
is 0.045 lb/ft3
(0.72 kg/m3
); and the flue-gas velocity is 30 ft/s (9.1 m/s). What
diameter and height would be required for this stack if it were located 5000 ft
(1524.0 m) above sea level?
Calculation Procedure:
1 Compute the required stack height
The required stack height Sh ft ⫽ ds /0.256pK, where ds ⫽ stack draft, inH2O; p ⫽
barometric pressure, inHg; K ⫽ 1/Ta ⫺ 1/Tg, where Ta ⫽ air temperature,⬚R; Tg
⫽ average temperature of stack gas,⬚R. In applying this equation, the temperature
of the gas at the stack outlet must be known to determine the average temperature
of the gas in the stack. Since the outlet temperature cannot be measured until after
the stack is in use, an assumed outlet temperature must be used for design calcu-
lations. The outlet temperature depends on the inlet temperature, ambient air tem-
perature, and materials used in the stack construction. For usual smokestacks, the
gas temperature will decrease 100 to 200⬚F (55.6 to 111.1⬚C) between the stack
inlet and outlet. Using a 100⬚F (55.6⬚C) gas-temperature decrease for this stack, we
get Sh ⫽ (1.0) ⫹ 0.256(29.92)(1/520 ⫺ 1/910) ⫽ 159 ft (48.5 m). Apply a 10
percent factor of safety. Then the stack height ⫽ (159)(1.10) ⫽ 175 ft (53.3 m).
2. Compute the required stack diameter
Stack diameter ds ft is found from ds ⫽ 0.278(WgTg /Vdgp)0.5
, where Wg ⫽ flue-gas
flow rate in stack, lb/s; V ⫽ flue-gas velocity in stack, ft/s; dg ⫽ flue-gas density,
lb/ft3
. For this stack, ds ⫽ 0.278{(100)(910)/[(30)(0.045)(29.92)]}0.5
⫽ 13.2 ft (4.0
m), or 13 ft 3 in (4 m 4 cm), rounding to the nearest inch diameter.
Note: Use this calculation procedure for any stack material—masonary, steel,
brick, or plastic. Most boiler and stack manufacturers use charts based on the equa-
tions above to determine the economical height and diameter of a stack. Thus, the
Babcock Wilcox Company, New York, Inc., also presents four charts for stack
sizing, in Steam: Its Generation and Use. Combustion Engineering, Inc., also pre-
sents four charts for stack sizing, in Combustion Engineering. The equations used
in the present calculation procedure are adequate for a quick, first approximation
of stack height and diameter.
3. Compute the required stack height and diameter at 5000-ft (1524.0-m)
elevation
Fuels require the same amount of oxygen for combustion regardless of the altitude
at which they are burned. Therefore, this stack must provide the same draft as at
sea level. But as the altitude above sea level increases, more air must be supplied
to the fuel to sustain the same combustion rate, because air above sea level contains
less oxygen per cubic foot than at sea level. To accommodate the larger air and
flue-gas flow rate without an increase in the stack friction loss, the stack diameter
must be increased.
To determine the required stack height Se ft at an elevation above sea level,
multiply the sea-level height Sh by the ratio of the sea-level and elevated height
barometric pressures inHg. Since the barometric pressure at 5000 ft (1524.0 m) is
24.89 inHg (84.3 kPa) and the sea-level barometric pressure is 29.92 inHg (101.3
kPa), Se ⫽ (175)(29.92/24.89) ⫽ 210.2 ft (64.1 m).
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(0.08) ⫽ 480 lb/min (3.6 kg/s).
2. Compute the airflow required through the dryer
Air enters the dryer at 70⬚F (21.1⬚C). Assume that evaporation of the moisture on
the coal takes place at 125⬚F (51.7⬚C)—this is about midway in the usual evapo-
ration temperature range of 110 to 145⬚F (43.3 to 62.8⬚C). Determine the moisture
content of saturated air at each temperature, using the psychrometric chart for air.
Thus, for saturated air at 70⬚F (21.1⬚C) dry-bulb temperature, the weight of the
moisture it contains is wm lb (kg) of water per pound (kilogram) of dry air ⫽ 0.0159
(0.00721), whereas at 125⬚F (51.7⬚C), wm ⫽ 0.09537 lb of water per pound (0.04326
kg/kg) of dry air. The weight of water removed per pound of air passing through
the dryer is the difference between the moisture content at the leaving temperature,
125⬚F (51.7⬚C), and the entering temperature, 70⬚F (21.1⬚C), or 0.09537 ⫺ 0.01590
⫽ 0.07947 lb of water per pound (0.03605 kg/kg) of dry air.
Since air at 70⬚F (21.1⬚C) has a density of 0.075 lb/ft3
(1.2 kg/m3
), 1/0.075 ⫽
13.3 ft3
(0.4 m3
) of air at 70⬚F (21.1⬚C) must be supplied to absorb 0.07947 lb of
water per pound (0.03605 kg/kg) of dry air. With 480 lb/min (3.6 kg/s) of water
to be evaporated in the dryer, each cubic foot of air will absorb 0.7947/13.3 ⫽
0.005945 lb (0.095 kg/m3
), of moisture, and the total airflow must be (480 lb/
min)/(0.005945) ⫽ 80,800 ft3
/min (38.1 m3
/s), given a dryer efficiency of 100
percent. However, the usual dryer efficiency is about 75 percent, not 100 percent.
Therefore, the total actual airflow through the dryer should be 80,800/0.75 ⫽
107,700 ft3
/min (50.8 m3
/s).
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. The heat required to evaporate
the water, Btu ⫽ (weight of water, lb/min)(hƒg at evaporation temperature). The
heat required to raise the temperature of the coal and water from ambient to the
evaporation temperature, Btu ⫽ (weight of coal, lb/min)(evaporation temperature
⫺ ambient temperature)[specific heat of coal, Btu/(lb 䡠 ⬚F)] ⫹ (weight of water, lb
/min)(evaporation temperature ⫺ ambient temperature)[specific heat of water, Btu
/(lb 䡠 ⬚F)]. The heat required to make up for radiation losses, Btu ⫽ {(area of dryer
insulated surfaces, ft2
)[heat-transfer coefficient, Btu/(ft2
䡠 ⬚F 䡠 h)](t ⫺ ambient tem-
perature) ⫹ (area of dryer uninsulated surfaces, ft2
)[heat-transfer coefficient, Btu/
(ft2
䡠 ⬚F 䡠 h)](t ⫺ ambient temperature)}/60.
Compute the heat given up by the air, Btu, as (107,700)(0.075)(0.24)(t ⫺ 70),
where 0.075 is the air density and 0.24 is the specific heat of air.
Compute the heat required to evaporate the water, Btu, as (480)(1022.9), where
1022.9 ⫽ hƒg at 125⬚F (51.7⬚C) from the steam tables.
Compute the heat required to raise the temperature of the coal and water from
ambient to the evaporation temperature, Btu, as (6000)(t ⫺ 70)(0.30) ⫹ (480)(t ⫺
70)(1.0), where 0.30 is the specific heat of the coal and 1.0 is the specific heat of
water.
Compute the heat required to make up the radiation losses, assuming 3000 ft2
(278.7 m2
) of insulated and 1500 ft2
(139.4 m2
) of uninsulated surface in the dryer,
with coefficients of heat transfer of 0.35 and 3.0 for the insulated and uninsulated
surfaces, respectively. Then radiation heat loss, Btu ⫽ (3000)(0.35)(t ⫺ 70) ⫹
(1500)(3.0)(t ⫺ 70).
Set up the heat balance thus and solve for t: (107,7000)(0.075)(0.24)(t ⫺ 70) ⫽
(480)(1022.9) ⫹ (6000)(125 ⫺ 70)(0.30) ⫹ (480)(125 ⫺ 70)(1.0) ⫹ [(3000)(0.35)(t
⫺ 70) ⫹ (1500)(3.0)(t ⫺ 70)]/60; so t ⫽ 406⬚F (207.8⬚C). In this heat balance, the
factor 60 is divided into the radiation heat loss to convert flow in Btu/h to Btu/
min because all the other expressions are in Btu/min.
4. Determine the total heat required by the dryer
Using the equation of step 3 with t ⫽ 406⬚F (207.8⬚C), we find the total heat ⫽
(107,770)(0.075)(0.24)(406 ⫺ 70) ⫽ 651,000 Btu/min, or 60(651,000) ⫽
39,060,000 Btu/h (11,439.7 kW)
5. Compute the dryer-furnace grate area
Assume that heat for the dryer is produced from coal having a lower heating value
of 13,000 Btu/lb (30,238 kJ/kg) and that 40 lb/h of coal is burned per square foot
[0.05 kg/(m2
䡠 s)] of grate area with a combustion efficiency of 70 percent.
The rate of coal firing ⫽ (Btu/min to dryer)/(coal heating value, Btu/
lb)(combustion efficiency) ⫽ 651,000/(13,000)(0.70) ⫽ 71.5 lb/min, or 60(71.5)
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.51
For this conical pile, volume ⫽ (3.4)2
(25)/3 ⫽ 31,000 ft3
(877.8 m3
). Since
50 lb of bituminous coal occupies about 1 ft3
of volume (800.9 kg/m3
), the weight
of coal in the conical pile ⫽ (31,000 ft3
)(50 lb/ft3
) ⫽ 1,550,000 lb, or (1,550,000
lb)/(2000 lb/ton) ⫽ 775 tons (703.1 t).
For the triangular pile, base length ⫽ 2h/tan 36⬚ ⫽ (2)(25)/0.727 ⫽ 68.8 ft
(21.0 m). Then volume ⫽ (68.8)(25)(100)/2 ⫽ 86,000 ft3
(2435.2 m3
). The weight
of bituminous coal in the pile is, as for the conical pile, (86,000)(50) ⫽ 4,300,000
lb, or (4,300,000 lb)/(2000 lb/ton) ⫽ 2150 tons (1950.4 t).
Related Calculations. Use this general procedure to compute the weight of
coal in piles of all shapes, and in bunkers, silos, bins, and similar storage com-
partments. The procedure can be used for other materials also—grain, sand, gravel,
coke, etc. Be sure to use the correct density when converting the total storage
volume to total weight. Refer to Baumeister and Marks—Standard Handbook for
Mechanical Engineers for a comprehensive tabulation of the densities of various
materials.
PROPERTIES OF A MIXTURE OF GASES
A 10-ft3
(0.3-m3
) tank holds 1 lb (0.5 kg) of hydrogen (H2), 2 lb (0.9 kg) of nitrogen
(N2), and 3 lb (1.4 kg) of carbon dioxide (CO2) at 70⬚F (21.1⬚C). Find the specific
volume, pressure, specific enthalpy, internal energy, and specific entropy of the
individual gases and of the mixture and the mixture density. Use Avogadro’s and
Dalton’s laws and Keenan and Kaye—Thermodynamic Properties of Air, Products
of Combustion and Component Gases, Krieger, commonly termed the Gas Tables.
Calculation Procedure:
1. Compute the specific volume of each gas
Using H, N, and C as subscripts for the respective gases, we see that the specific
volume of any gas v ft3
/lb ⫽ total volume of tank, ft3
weight of gas in tank, lb.
Thus, vH ⫽ 10/1 ⫽ 10 ft3
/lb (0.6 m3
/kg); vN ⫽ 10/2 ⫽ 5 ft3
/lb (0.3 m3
/kg); vC
⫽ 10/3 ⫽ 3.33 ft3
/lb (0.2 m3
/dg). Then the specific volume of the mixture of
gases is vt ft3
/lb ⫽ total volume of gas in tank, ft3
/sum of weight of individual
gases, lb ⫽ 10/(1 ⫹ 2 ⫹ 3) ⫽ 1.667 ft3
/lb (0.1 m3
/kg).
2. Determine the absolute pressure of each gas
Using P ⫽ RTw/vtM, where P ⫽ absolute pressure of the gas, lb/ft2
(abs); R ⫽
universal gas constant ⫽ 1545; T ⫽ absolute temperature of the gas,⬚R ⫽ ⬚F ⫹
459.9, usually taken as 460; w ⫽ weight of gas in the tank, lb; vt ⫽ total volume
of the gas in the tank, ft3
; M ⫽ molecular weight of gas. Thus, PH ⫽ (1545)(70 ⫹
460)(1.0)/[(10)(2.0)] ⫽ 40,530 lb/ft2
(abs) (1940.6 kPa); PN ⫽ (1545)(70 ⫹
460)(2.0)/[(10)(28)] ⫽ 5850 lb/ft2
(abs) (280.1 kPa); PC ⫽ (1545)(70 ⫹
460)(3.0)/[(10)(44)] ⫽ 5583 lb/ft2
(abs) (267.3 kPa); Pt ⫽ 兺PH, PN, PC ⫽
40,530 ⫹ 5850 ⫹ 5583 ⫽ 51,963 lb/ft2
(abs) (2488.0 kPa).
3. Determine the specific enthalpy of each gas
Refer to the Gas Tables, entering the left-hand column of the table at the absolute
temperature, 530⬚F (294 K), for the gas being considered. Opposite the temperature,
read the specific enthalpy in the h column. Thus, hH ⫽ 1796.1 Btu/lb (4177.7 kJ/
kg); hN ⫽ 131.4 Btu/lb (305.6 kJ/kg); hC ⫽ 90.17 Btu/lb (209.7 kJ/kg). The total
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![4.52 POWER GENERATION
enthalpy of the mixture of the gases is the sum of the products of the weight of
each gas and its specific enthalpy, or (1)(1796.1) ⫹ (2)(131.4) ⫹ (3)(90.17) ⫹
2329.4 Btu (2457.6 kJ) for the 6 lb (2.7 kg) or 10 ft3
(0.28 m3
) of gas. The specific
enthalpy of the mixture is the total enthalpy/gas weight, lb, or 2329.4/(1 ⫹ 2 ⫹
3) ⫽ 388.2 Btu/lb (903.0 kJ/kg) of gas mixture.
4. Determine the internal energy of each gas
Using the Gas Tables as in step 3, we find EH ⫽ 1260.0 Btu/lb (2930.8 kJ/kg);
EN ⫽ 93.8 Btu/lb (218.2 kJ/kg); EC ⫽ 66.3 Btu/lb (154.2 kJ/kg). The total energy
⫽ (1)(1260.0) ⫹ (2)(93.8) ⫹ (3)(66.3) ⫽ 1646.5 Btu (1737.2 kJ). The specific
enthalpy of the mixture ⫽ 1646.5/(1 ⫹ 2 ⫹ 3) ⫽ 274.4 Btu/lb (638.3 kJ/kg) of
gas mixture.
5. Determine the specific entropy of each gas
Using the Gas Tables as in step 3, we get SH ⫽ 15.52 Btu/(lb 䡠 ⬚F) [65.0 kJ/(kg 䡠
⬚C)]; SN ⫽ 1.558 Btu/(lb 䡠 ⬚F) [4.7 kJ/(kg 䡠 ⬚C)]. The entropy of the mixture ⫽
(1)(12.52) ⫹ (2)(1.558) ⫹ (3)(1.114) ⫽ 18.978 Btu/⬚F (34.2 kJ/⬚C). The specific
entropy of the mixture ⫽ 18.978/(1 ⫹ 2 ⫹ 3) ⫽ 3.163 Btu/(lb 䡠 ⬚F) [13.2 kJ/
(kg 䡠 ⬚C) of the gas mixture.
6. Compute the density of the mixture
For any gas, the total density dt ⫽ sum of the densities of he individual gases. And
since density of a gas ⫽ 1/specific volume, dt ⫽ 1/vt ⫽ 1/vH ⫹ 1/vN ⫹ 1/vC ⫽
1/10 ⫹ 1/5 ⫹ 1/3.33 ⫽ 0.6 lb/ft3
(9.6 kg/m3
) of mixture. This checks with step
1, where vt ⫽ 1.667 ft3
/lb (0.1 m3
/kg), and is based on the principle that all gases
occupy the same volume.
Related Calculations. Use this method for any gases stored in any type of
container—steel, plastic, rubber, canvas, etc.—under any pressure from less than
atmospheric to greater than atmospheric at any temperature.
STEAM INJECTION IN AIR SUPPLY
In a certain manufacturing process, a mixture of air and steam at a total mixture
pressure of 300 lb/in2
absolute (2068 kPa) and 400⬚F (204⬚C) is desired. The rel-
ative humidity of the mixture is to be 60 percent. For a required mixture flow rate
of 500 lb/h (3.78 kg/s) determine (a) the volume flow rate of dry air in ft3
/min
(m3
/s) of free air, where air is understood to be air at 14.7 lb/in2
(101 kPa) and
70⬚F (21⬚C); and (b) the required rate of steam injection in lb/h (kg/s).
Calculation Procedure:
1. Determine the partial pressure of the vapor and that of the air
From Table 1, Saturation: Temperatures of the Steam Tables mentioned under Re-
lated Calculations of this procedure, at 400⬚F (204⬚C) the steam saturation pressure,
Pvs ⫽ 247.31 lb/in2
(1705 kPa), by interpolation. Since the vapor pressure is ap-
proximately proportional to the grains of moisture in the mixture, the partial pres-
sure of vapor in the mixture, Pvp ⫽ Pvs ⫽ 0.6 ⫻ 247.31 ⫽ 148.4 lb/in2
absolute
(1023 kPa), where is the relative humidity as a decimal. Then, the partial pressure
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.53
of the air in the mixture, Pa ⫽ Pm ⫺ Pvs ⫽ 300 ⫺ 148.4 ⫽ 151.6 lb/in2
absolute
(1045 kPa), where Pm is the total mixture pressure.
2. Compute the density of air in the mixture
The air density, a ⫽ Pa /(RaTa), where Pa ⫽ 151.6 ⫻ 144 ⫽ 21.83 ⫻ 103
lb/ft2
(1045 kPa); the gas constant for air, Ra ⫽ 53.3 ft 䡠 lb/(lb 䡠 ⬚R) [287 J/(kg 䡠 K)];
absolute temperature of the air Ta ⫽ 400 ⫹ 460 ⫽ 860⬚R (478 K), then, a ⫽
21.83 ⫻ 103
/(53.3 ⫻ 860) ⫽ 0.4762 lb/ft3
(7.63 kg/m3
).
3. Find the specific volume of the vapor in the mixture
From Table 3, Vapor of the Steam Tables, at 148.4 lb/in2
absolute (1023 kPa) and
400⬚F (204⬚C), the specific volume of the vapor, vv ⫽ 3.261 ft3
/lb (0.2036 m3
/kg),
by interpolation.
4. Compute the density of the vapor and that of the mixture
The density of the vapor, v ⫽ 1/vv ⫽ 1/3.261 ⫽ 0.3066 lb/ft 3
(4.91 kg/m3
). The
density of the mixture, m ⫽ a ⫹ v ⫽ 0.4762 ⫹ 0.3066 ⫽ 0.7828 lb/ft3
(12.54
kg/m3
).
5. Compute the amount of air in 500 lb/h (3.78 kg/s) of mixture
In 500 lb/h (3.78 kg/s) of mixture, wm, the amount of air, wa ⫽ a ⫻ wm /m ⫽
0.4762 ⫻ 500/0.7828 ⫽ 304 lb/h (2.30 kg/s).
6. Compute the flow rate of dry air
(a) The flow rate of dry air at 14.7 lb/in2
(101 kPa) and 70⬚F (21⬚C), Va ⫽ wa ⫻
Ra ⫻ T/P, where the free air temperature, T ⫽ 70 ⫹ 460 ⫽ 530⬚R (294 K); free
air pressure, P ⫽ 14.7 ⫻ 144 ⫽ 2.117 ⫻ 103
lb/ft2
(101 kPa); other values as
before. Hence, Va ⫽ 304 ⫻ 53.3 ⫻ 530/(2.117 ⫻ 103
) ⫽ 4060 ft3
/h ⫽ 67.67 ft3
/
min (1.92 ⫻ 10⫺3
m3
/s).
7. Compute the rate of steam injection
(b) The rate of steam injection, ws ⫽ ws ⫺ wa ⫽ 500 ⫺ 304 ⫽ 196 lb/h (1.48
kg/s).
Related Calculations. The Steam Tables appear in Thermodynamic Properties
of Water Including Vapor, Liquid, and Solid Phases, 1969, Keenan, et al., John
Wiley Sons, Inc. This procedure considers the air and steam as ideal gases which
behave in accordance with the Gibbs-Dalton law of gas mixtures having complete
homogeneous molecular dispersion and additive pressures. Also, calculations in
steps 2 and 6 are based on Boyle’s law and Charles’ law, which relate pressure,
volume, and temperature of a gas, or gas mixture. Clear and concise presentations
of these and other significant definitions appear in Thermodynamics and Heat
Power, 4th edition, by Irving Granet, Regents/Prentice-Hall, Englewood Cliffs, NJ
07632.
BOILER AIR-HEATER ANALYSIS AND SELECTION
A boiler manufacturer proposes these two alternatives to a prospective purchaser:
(a) A steam-generating unit equipped with a small air heater which results in an
overall steam-generating efficiency of 83 percent; (b) A similar steam-generating
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![4.54 POWER GENERATION
unit equipped with a larger air heater which results in an overall steam-generating
unit efficiency of 87 percent. It is anticipated that coal delivered to the furnace will
cost $60.00/ton (0.907 metric ton). The boiler is intended to operate 8000 hours
per year and is to deliver one million pounds of steam per hour (126 kg/s) with
an enthalpy rise of 1200 Btu per pound mass (2.791 ⫻ 106
J/kg). If the total
investment charges are 20 percent, what additional cost can be paid for the larger
heater? Indicate the Btu content of coal upon which the selection is based.
Calculation Procedure:
1. Compute boiler heat output during one year of intended operation
Boiler heat output per year of operation, Q ⫽ H ⫻ S ⫻ ⌬h, where the time used
per year, H ⫽ 8000 h (2.88 ⫻ 106
s); rate of steam delivery, S ⫽ 106
lbm /h (126
kg/s); enthalpy rise, ⌬h ⫽ 1200 Btu/lbm (2.791 ⫻ 106
J/kg). Then, Q ⫽ 8000 ⫻
1200 ⫽ 9.6 ⫻ 1012
Btu/year (22.31 ⫻ 1015
J/year).
2. Compute the annual mass of coal input for each Proposal
Proposal (a): Annual coal input, Ca ⫽ Q/(B ⫻ Ea), where the assumed Btu content
of coal, B ⫽ 13,500 Btu/lbm (31.4 ⫻ 106
J/kg); efficiency of Proposal (a), Ea ⫽
0.83. Then, Ca ⫽ 9.6 ⫻ 1012
/(13,500 ⫻ 0.83) ⫽ 711 ⫻ 106
/0.83 ⫽ 857 ⫻ 106
lbm /year (389 ⫻ 106
kg/year).
Proposal (b): Annual coal input, Cb ⫽ Q/(B ⫻ Eb), where efficiency of Proposal
(b), Eb ⫽ 0.87. Then, Cb ⫽ 711 ⫻ 106
/0.87 ⫽ 817 ⫻ 106
lbm /year (371 ⫻ 106
kg/year).
3. Compute the annual cost of coal for each Proposal
Proposal (a): Annual cost of coal, Aa ⫽ (Ca /2000) ⫻ Ct, where the cost per ton
(0.907 metric ton) of coal, Ct ⫽ $60.00. Then, Aa ⫽ (857 ⫻ 106
/2000) ⫻ 60 ⫽
$25,710,000/year.
Proposal (b): Annual cost of coal, Ab ⫽ (Cb /2000)Ct ⫽ (817 ⫻ 106
/2000) ⫻
60 ⫽ $24,510,000/year.
4. Compute the additional investment that can be made for the larger heater
Additional investment, L, for the larger heater can be found by setting proposed
annual costs for Proposal (a) equal to those for Proposal (b) in a ‘‘Break Even’’
equation. Thus, (dc ⫻ Ia) ⫹ Om ⫹ Aa ⫽ [dc ⫻ (Ia ⫹ L)] ⫹ Om ⫹ Ab, where the
decimal fraction for total investment charges, dc ⫽ 0.20; total investment charges
for Proposal (a) ⫽ Ia, and for Proposal (b) ⫽ (Ia ⫹ L); operating and maintenance
charges for either Proposal ⫽ Om; other items as before. Then, (0.20 ⫻ Ia) ⫹ Om
⫹ $25,710,000 ⫽ [0.20 ⫻ (Ia ⫹ L)] ⫹ Om ⫹ $24,510,000. This reduces to, L ⫽
($25,710,000 ⫺ $24,510,000)/0.20 ⫽ $6,000,000, the additional investment.
Note: The $60-per-ton price for coal used here was for example purposes only.
Since coal prices vary widely with source region, transport distance, and quality, a
high price was used to highlight the importance of investment decisions while
reflecting the effects of inflation and the possible demand for specialty coal. The
procedural steps remain the same, regardless of the dollar, franc, yen, pound, or
other monetary unit price of the coal.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.55
FIGURE 10 Steam plant containing deaerator and
boiler blowdown.
EVALUATION OF BOILER BLOWDOWN,
DEAERATION, STEAM AND WATER QUALITY
A boiler generates 50,000 lb/h (22,700 kg/h) of saturated steam at 300 lb/in2
(abs)
(2067 kPa), out of which 10,000 lb/h (4540 kg/h) is taken for process and is
returned to the deaerator, Fig. 10, as condensate at 180⬚F (82.2⬚C); the remainder
is consumed. Makeup water enters the deaerator at 70⬚F (21.1⬚C) and steam is
available at 300 lb/in2
(abs) (2067 kPa) for deaeration. The deaerator operates at
25 lb/in2
(abs) (172.3 kPa). The blowdown has total dissolved solids (TDS) of
1500 ppm (parts per million by weight) and makeup has a TDS of 100 ppm.
Evaluate the blowdown and deaeration steam quantities.
Calculation Procedure:
1. Understand steam quality and steam purity
Steam purity refers to the impurities in steam in ppm. A typical value in low-
pressure boilers is 1 ppm. Steam quality, by contrast, refers to the moisture in steam.
For example, the operator of a boiler plant will maintain a certain concentration
of solids in the boiler drum, depending on either the American Boiler Manufacturers
Association or the American Society of Mechanical Engineers recommendations,
which can be found in publications of these organizations or in Ganapathy—Steam
Plant Calculations Manual, Marcel Dekker, Inc. At 500 lb/in2
(abs) (3445 kPa),
for instance, if the boiler water concentration is 2500 ppm and steam purity is 0.5
ppm solids, the steam quality is obtained from: Percent moisture in steam ⫽ (steam
purity, ppm)/(boiler water concentration, ppm)(100) ⫽ (0.5/2500)(100) ⫽ 0.02
percent; steam quality ⫽ 100.0 ⫺ 0.02 ⫽ 99.98 percent.
2. Set up the deaerator mass and energy balance
From the Fig. 10, the mass balance around the deaerator gives, using the data
provided: 10,000 ⫹ D ⫹ M ⫽ F ⫽ 50,000 ⫹ B [Eq. (1)], where D ⫽ deaeration
steam flow, lb/h (kg/h); B ⫽ blowdown, lb/h (kg/h); M ⫽ makeup flow, lb/h
(kg/h); F ⫽ feedwater flow, lb/h (kg/h).
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.61
FIGURE 16 Furnace size comparisons. (Combustion.)
combustion. Oil produces only small amounts of ash; natural gas produces no ash.
Coal must be stored, conveyed, and pulverized before being introduced into a fur-
nace. Oil and gas require little preparation. For these reasons, a boiler designed to
burn oil as its primary fuel makes a poor conversion candidate for coal firing.
2. Evaluate the coal properties from various sources
Table 7 shows coal properties from many parts of the United States. Note that the
heating values range from 12,000 Btu/lb (27,960 kJ/kg) to 6800 Btu/lb (15,844
kJ/kg). For a 600-MW unit, the coal firing rates [450,000 to 794,000 lb/h (56.7
or 99.9 kg/s)] to yield comparable heat inputs provide an appreciation of the coal
storage yard and handling requirements for the various coals. On an hourly usage
ratio alone, the lower-heating-value coal required 1.76 times more fuel to be han-
dled.
Pulverizer requirements are shown in Table 8 while furnace sizes needed for the
various coals are shown in Fig. 17.
3. Evaluate conversion to coal fuel
Most gas-fired boilers can readily be converted to oil at reasonable cost. Little or
no derating (reduction of steam or electricity output) is normally required.
From an industrial or utility view, conversion of oil- or gas-fired boilers not
initially designed to fire coal is totally impractical from an economic viewpoint.
Further, the output of the boiler would be severely reduced.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.65
FIGURE 19 Effects of scale on boiler operation. (Chemical En-
gineering.)
tubes, determine what savings can be made if 144,000-Btu/gal (40.133-MJ/m3
)
fuel oil costs $1 per gallon ($1 per 3.8 L) and the boiler uses 16.74 million Btu/
h (4.9 MW) operating 8000 h/year.
Calculation Procedure:
1. Determine the annual energy usage
Compute the annual energy usage from (million Btu/h) (hours of operation
annually)/efficiency. For this boiler, annual energy usage ⫽ (16.74)(8000)/0.75 ⫽
178,560 million Btu (188,380 kJ).
2. Find the energy loss caused by scale on the tubes
Enter Fig. 19 at the scale thickness, 1
⁄32 in (0.79 mm), and project vertically upward
to the ‘‘normal’’ scale (salts of Ca and Mg) curve. At the left read the energy loss
as 2 percent. Hence, the annual energy loss in heat units ⫽ (178,560 million Btu/
year)(0.02) ⫽ 3571 million Btu/year (130.8 kW)
3. Compute the annual savings if the scale is removed
If the scale is removed, then the energy lost, computed in step 2, will be saved.
Thus, the annual dollar savings after scale removal ⫽ (heat loss in energy units)
(fuel price, $/gal)/(fuel heating value, Btu/gal). Or, savings ⫽ (3571 ⫻
106
)($1.00)/144,000 ⫽ $26,049.
Related Calculations. This approach can be used with any type of
boiler—waterturbe, firetube, etc. The data are also applicable to tubed water heaters
which are directly fired.
Note that when the scale is high in iron and silica that the energy loss is much
greater. Thus, with scale of the same thickness [1
⁄32 in (0.79 mm)], the energy loss
for scale high in iron and silica is 7 percent, from Fig. 19. Then the annual loss ⫽
178,560(0.07) ⫽ 12,500 million Btu/year (3.63 MW). Removing the scale and
preventing its reformation will save, assuming the same heating value and cost for
the fuel oil, (12,500 ⫻ 106
Btu/year) ($1.00)/144,000 ⫽ $86,805 per year.
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![4.66 POWER GENERATION
While this calculation gives the energy savings from reduced boiler scale, the
results also can be used to determine the amount that can be invested in a water-
treatment system to prevent scale formation in a boiler, water heater, or other heat
exchanger. Thus, the initial investment in treating equipment can at least equal the
projected annual savings produced by the removal of scale.
This procedure is the work of Walter A. Hendrix and Guillermo H. Hoyos,
Engineering Experiment Station, Georgia Institute of Technology, as reported in
Chemical Engineering magazine.
GROUND AREA AND UNLOADING CAPACITY
REQUIRED FOR COAL BURNING
An industrial plant is considering switching from oil to coal firing to reduce fuel
costs. Determine the ground are required for 60 days’ coal storage if the plant
generates 100,000 lb/h (45,360 kg/h) of steam at a 60 percent winter load factor
with a steam pressure of 150 lb/in2
(gage) (1034 kPa), average boiler evaporation
is 9.47 steam/lb coal (4.3 kg/kg), coal density ⫽ 50 lb/ft3
(800 kg/m3
), boiler
efficiency is 83 percent with an economizer, and the average storage pile height for
the coal is 20 ft (6.096 m).
Calculation Procedure:
1. Determine the storage area required for the coal
The storage area, A ft2
, can be found from A ⫽ 24WFN/EdH, where H ⫽ steam
generation rate, lb/h; F ⫽ load factor, expressed as a decimal; N ⫽ number of days
storage required; E ⫽ average boiler evaporation rate, lb/h; d ⫽ density of coal,
lb/ft3
; H ⫽ height of coal pile allowed, ft. Substituting yields A ⫽
24(100,000)(0.6)(60)/[(9.47)(50)(20)] ⫽ 9123 ft2
(847 m2
).
2. Find the maximum hourly burning rate of the boiler
The maximum hourly burning rate in tons per hour is given by B ⫽ W/2000E,
where the symbols are as defined earlier. Substituting, we find B ⫽ 100,000/
2000(9.47) ⫽ 5.28 tons/h (4.79 t/h). With 24-h use in any day, maximum daily
use ⫽ 24 ⫻ 5.28 ⫽ 126.7 tons/day (115 t/day).
3. Find the required unloading rate for this plant
As a general rule, the unloading rate should be about 9 times the maximum total
plant burning rate. Higher labor and demurrage costs justify higher unloading rates
and less manual supervision of coal handling. Find the unloading rate in tons per
hour from U ⫽ 9W/2000E, where the symbols are as defined earlier. Substituting
gives U ⫽ 9(100,000)/2000(8.47) ⫽ 47.5 tons/h (43.1 t/h).
Related Calculations. With the price of oil, gas, wood, and waste fuels rising
to ever-higher levels, coal is being given serious consideration by industrial, central-
station, commercial, and marine plants. Factors which must be included in any study
of conversion to (or original use of) coal include coal delivery to the plant, storage
before use, and delivery to the boiler.
For land installations, coal is usually received in railroad hopper-bottom cars in
net capacities ranging between 50 and 100 tons with 50- and 70-tons (45.4- and
63.5-t) capacity cars being most common.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.67
FIGURE 20 Typical blowdown heat-recovery sys-
tem. (Combustion.)
Because cars require time for spotting and moving on the railroad siding, coal
is actually delivered to storage for only a portion of the unloading time. Thawing
of frozen coal and car shaking also tend to reduce the actual delivery. True un-
loading rate may be as low as 50 percent of the continuous-flow capacity of the
handling system. Hence, the design coal-handling rate of the conveyor system serv-
ing the unloading station should be twice the desired unloading rate. So, for the
installation considered in this procedure, the conveyor system should be designed
to handle 2(47.5) ⫽ 95 tons/h (86.2 t/h). This will ensure that at least six rail cars
of 60-ton (54.4-t) average capacity will be emptied in an 8-h shift, or about 360
tons/day (326.7 t/day).
With a maximum daily usage of 126.7 tons/day (115 t/day), as computed in
step 2 above, the normal handling of coal, from rail car delivery during the day
shift, will accumulate about 3 days’ peak use during an 8-h shift. If larger than
normal shipments arrive, the conveyor system can be operated more than 8 h/day
to reduce demurrage charges.
This procedure is the work of E. R. Harris, Department Head, G. F. Connell,
and F. Dengiz, all of the Environmental and Energy Systems, Argonaut Realty
Division, General Motors Corporation, as reported in Combustion magazine.
HEAT RECOVERY FROM BOILER
BLOWDOWN SYSTEMS
Determine the heat lost per day from sewering the blowdown from a 600-lb/in2
(gage) (4137-kPa) boiler generating 1 million lb/day (18,939.4 kg/h) of steam at
80 percent efficiency. Compare this loss to the saving from heat recovery if the
feedwater has 20 cycles of concentration (that is, 5 percent blowdown), ambient
makeup water temperature is 70⬚F (21⬚C), flash tank operating pressure is 10 lb/
in2
(gage) (69 kPa) with 28 percent of the blowdown flashed, blowdown heat ex-
changer effluent temperature is 120⬚F (49⬚C), fuel cost is $2 per 106
Btu [$2 per
(9.5)6
J], and the piping is arranged as shown in Fig. 20.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.69
years, the annual saving will increase. Of course, the cost of the blowdown heat-
recovery equipment must be offset against this saving. In general, the savings war-
rant the added investment for the extra equipment.
Related Calculations. This procedure is valid for any type of steam-generating
equipment for residential, commercial, industrial, central-station, or marine instal-
lations. (In the latter installation the ‘‘sewer’’ is the sea.) The typical range of
blowdown heat recovery is in the 80 to 90 percent area. In view of the rapid rise
in fuel prices, this range of heat recovery is significant. Hence, much wider use of
blowdown heat recovery can be expected in all types of steam-generating plants.
To reduce scale buildup in boilers, low cycles of boiler water concentration are
preferred. This means that high blowdown rates will be used. To prevent wasting
expensive heat present in the blowdown, heat-recovery equipment such as that dis-
cussed above is used. In industrial plants (which are subject to many sources of
condensate contamination), cycles of concentration are seldom allowed to exceed
50 (2 percent blowdown). In the above application, the cycles of concentration ⫽
20, or 5 percent blowdown.
To prevent boiler scale buildup, good pretreatment of the makeup is recom-
mended. Typical current selections for pretreatment equipment, by using the boiler
operating pressure as the main criterion, are thus:
This procedure is the work of A. A. Askew, Betz Laboratories, Inc., as reported in
Combustion magazine.
BOILER BLOWDOWN PERCENTAGE
The allowable concentration in a certain drum is 2000 ppm. Pure condensate is fed
to the drum at the rate of 85,000 gal/h (89.4 L/s). Make-up, containing 50 grains
(gr)/gal (856 mg/L) of sludge-producing impurities, is also delivered to the drum
at the rate of 1500 gal/h (1.58 L/s). Calculate the blowdown as a percentage of
the boiler steaming capacity.
Calculation Procedure:
1. Compute the ppm of impurities per gallon of make-up water
There are 58,410 gr/gal (106
mg/L). See the Related Calculations of this procedure
for the basis of this factor. Parts per million of impurities, ppmi ⫽ [(gr/gal)i /(gr/
gal)i /(gr/gal)] ⫻ 106
, where (gr/gal)i ⫽ quality of, or impurities in, the make-up
water, gr/gal (mg/L); (gr/gal) ⫽ grains/gal (mg/L). Hence, ppmi ⫽ (50/58,410) ⫻
106
⫽ 50 ⫻ 17.12 ⫽ 856.
2. Compute the blowdown rate
To maintain impurities in the drum at a certain concentration, the parts fed to the
drum ⫽ parts discharged by the blowdown. Thus, ppmi ⫽ (gal/h)m ⫽ ppma ⫻
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![4.70 POWER GENERATION
(gal/h)a, where the subscripts stand for i ⫽ impurities; m ⫽ make-up; a ⫽ allow-
able; b ⫽ blowdown. Then, 856 ⫻ 1500 ⫽ 2000 ⫻ (gal/h)b. Solving: (gal/h)b ⫽
856 ⫻ 1500/2000 ⫽ 642 (0.675 L/s).
3. Compute the boiler steaming capacity
The boiler steaming capacity (gal/h)s ⫽ (gal/h)ƒ ⫹ (gal/h)m ⫺ (gal/h)b, where
(gal/h)ƒ ⫽ feedwater flow rate. Then, (gal/h)s ⫽ 85,000 ⫹ 1500 ⫺ 642 ⫽ 85,858
(90.3 L/s).
4. Compute the blowdown percentage
Blowdown percentage ⫽ [(gal/h)b /(gal/h)s] ⫻ 100 ⫽ (642/85,858) ⫻ 100 ⫽ 0.747
percent.
Related Calculations. The gr/gal factor in step 1 is based on the density of
impurities being considered as equal to the maximum density of clean fresh water,
8.3443 lb/gal (1.0 kg/L). Since 1 lb ⫽ 7000 gr, then 8.3443 ⫻ 7000 ⫽ 58,410
gr/gal (106
mg/L).
SIZING FLASH TANKS TO CONSERVE ENERGY
Determine the dimensions required for a commercial flash tank if the flash tank
pressure is 5 lb/in2
(gage) (34.5 kPa) and 14,060 lb/h (1.77 kg/s) of flash steam
is available. Would the flash tank be of the centrifugal or top-inlet type?
Calculation Procedure:
Two major types of flash tanks are in use today: top-inlet and centrifugal-inlet tanks,
as shown in Fig. 21. Tank and overall height and outside diameter are also shown
in Fig. 21.
1. Determine the rating and type of flash tank required
Refer to Table 9. Locate the 5-lb/in2
(gage) (34.5-kPa) flash tank pressure column,
and project downward to the minimum value that exceeds 14,060 lb/h (1.77 kg/
s). Note that a no. 5 centrifugal flash tank with a maximum rating of 20,000 lb/h
(2.5 kg/s) of flash steam is appropriate, and no standard top-inlet type has sufficient
capacity at this pressure for this flow rate.
2. Determine the dimensions of the tank
In Table 9 locate tank no. 5, and read the dimensions horizontally to the right.
Hence, the dimensions required for the tank are 60-in (152.4-cm) OD, 78-in (198.1-
cm) tank height, 88-in (223.5-cm) overall height, inlet pipe size of 6 in (15.2 cm),
steam outlet pipe of 8 in (20.3 cm), and a water outlet pipe of 6 in (15.2 cm).
Related Calculations. Use this procedure for choosing a flash tank for a variety
of applications—industrial power plants, central stations, marine steam plants, and
nuclear stations. Flash tanks can conserve energy by recovering steam that might
otherwise be wasted. This steam can be used for space heating, feedwater heating,
industrial processes, etc. Condensate remaining after the flashing can be used as
boiler feedwater because it is usually pure and contains valuable heat. Or the con-
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.73
FIGURE 22 (a) Boiler blowdown flow diagram. (b) Temperature-entropy schematic for blow-
down.
saturated steam tables mentioned under Related Calculations, an enthalpy hd ⫽
598.7 Btu/lb ⫽ (1392 kJ/kg).
3. Find the enthalpy of the blowdown fluid at the flash tank
The blowdown fluid is assumed to undergo an isenthalpic, or constant-enthalpy,
throttling process from point d to the point e on Fig. 22b where, at the flash tank,
he ⫽ hd, found above.
4. Find the enthalpy of saturated liquid within the flash tank
From the saturated steam tables, at pe ⫽ 15 lb/in2
(103 kPa), slightly above at-
mospheric pressure, the enthalpy of the saturated liquid at point ƒ on Fig. 22b,
hƒ ⫽ 181.1 Btu/lb (421 kJ/kg).
5. Find the enthalpy of evaporation within the flash tank
From the saturated steam tables, the heat required to evaporate 1 lb (0.45 kg) of
water under the pressure pe within the flash tank is hg ⫺ hƒ ⫽ hƒg ⫽ 969.7 Btu/lb
(2254 kJ/kg).
6. Calculate the amount of steam flashed per hour
The tank flashes steam at the rate of F ⫽ B[(he ⫺ hƒ)] ⫽ 4000[598.7 ⫺ 181.1)/
969.7] ⫽ 1723 lb/h (13 kg/s).
Related Calculations. Saturation steam tables appear in Thermodynamic Prop-
erties of Water Including Vapor, Liquid, and Solid Phases. 1969, John Wiley
Sons, Inc.
The equation for F in step 6 stems from the presumption of an adiabatic heat
balance where F ⫻ hƒg ⫽ B(he ⫺ hƒ).
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.75
FIGURING FLUE-GAS REYNOLDS NUMBER BY
SHORTCUTS
A low-sulfur No. 2 distillate fuel oil has a chemical composition of 87.4 percent
carbon and 12.6 percent hydrogen by weight, ignoring sulfur. The fuel’s higher
heating value (HHV), or ⌬Hgross, the standard (60⬚F) (15.5⬚C) heat of combustion
(based on stoichiometric air usage), is 18,993 Btu/lb (44.148 kJ/kg) of the fuel.
With a volumetric proportion of 79 percent atmospheric nitrogen, including rare
gases, to 21 percent oxygen, the molar ratio of N2 to O2 in air is 3.76:1. What is
the Reynolds number for the flow of the flue gas produced by that fuel if it is
completely burned in 50 percent excess air at the rate of 25.3 lb/h (11.5 kg/h) and
the flue gas leaves a 1-ft (0.3-m) diameter duct at 2000⬚F (1093⬚C).
Calculation Procedure:
1. Compute the volume flow rate of the flue gas
Based on stoichiometric air usage, the standard (32⬚F, 1-atm) (0⬚C, 101.3-kPa) vol-
ume of flue gas (std ft3
/lb) (std m3
/kg) of fuel burned is Vstd ⫽ (⌬Hgross /100)[1 ⫹
(percent excess air)/100 percent]. Then, Vstd ⫽ (18,993/100)[1 ⫹ (50/100)] ⫽ 285
std ft3
/lb (17.79 std m3
/kg).
Adjust for temperature expansion by using the ideal-gas law to get the per lb
(kg) of fuel actual amount of flue gas, V⬘ ⫽ Vstd(460 ⫹ T)/(460 ⫹ 32), where T is
the flue-gas temperature in⬚F. Thus, V⬘ ⫽ 285(460 ⫹ 2000)/(492) ⫽ 1425 ft3
/lb
(89 m3
/kg). The metric result can be verified as follows: Vstd ⫽ 17.79[273 ⫹ (2000
⫺ 32)/1.8]/273 ⫽ 89 m3
/kg.
The flue-gas approximate flow rate, V ⫽ V⬘W/3600, where W is the given hourly
burning rate of the fuel. Hence, V ⫽ 1425 ⫻ 25.3/3600 ⫽ 10.0 ft3
/s (0.28 m3
/s).
2. Determine the viscosity of the flue gas
Boiler and incinerator flue gases are composed of several gases, hence a precise
calculation of the Reynolds number can be cumbersome. By assuming that the flue
gas behaves like nitrogen, it is possible to obtain fast and accurate preliminary
approximations for both boilers and incinerators. Then, by means of a graph in Fig.
23, read the dynamic viscosity of nitrogen as ⬘ ⫽ 0.054 cp (54 ⫻ 10⫺6
Pa).
3. Compute the Reynolds number of the flue gas
By algebraic manipulations, as mentioned under Related Calculations of this pro-
cedure, the shortcut formula for an estimate of the Reynolds number is found to
be Re ⫽ 73,700/[D⬘(460 ⫹ T)], where the given duct diameter, D ⫽ 1.0 ft (0.3
m) and the other values are as previously determined. Thus, Re ⫽ 72,700 ⫻
10.0[(1)(0.054)(460 ⫹ 2000)] ⫽ 5470.
Related Calculations. The shortcut formula for the value of Re is derived by
algebraic manipulation of four expressions listed below. Symbols adequately de-
fined previously are not redefined.
(1) Reynolds number Re ⫽ DU/, where D ⫽ diameter of a circular cross-
section, or equivalent diameter of some other cross-section, ft (m); ⫽ density of
the gas, lb/ft3
(kg/m3
); U ⫽ average linear velocity of the gas, ft/s (m/s); ⫽
viscosity of the gas, lb/(ft 䡠 s) (Pa 䡠 s).
(2) At high temperatures, the average densities and viscosities of flue gas closely
approximate those of nitrogen alone. Thus, and for nitrogen can be used to
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![4.76 POWER GENERATION
FIGURE 23 Dynamic viscosity of nitrogen gas. (Chemical Engineering.)
TABLE 10 Flue-gas components
estimate Re with no significant error. Hence, from the ideal-gas law an estimate of
the density of nitrogen, ⫽ (28)(460 ⫹ 32)/[(359)(460 ⫹ T)], where 28 is the
molecular weight of nitrogen; 359 is the volume, ft3
, of 1 lb 䡠 mol of gas at 32⬚F
(0⬚C) and at atmospheric pressure, 14.7 lb/in2
(6.89 kPa). In SI units the factor is
22.41 m3
/kg 䡠 mol.
(3) U ⫽ 4V/D2
(4) ⫽ ⬘/1488
Turbulent flow in a boiler or incinerator assures adequate mixing and near-
complete or complete combustion. Flow is turbulent when its Reynolds number is
greater than 2000 or 3000, and is more turbulent when Re is greater. This reduces
the amount of excess air required for complete combustion and hence increases
boiler and incinerator efficiency.
A review of the shortcut equations reveals that, for a given set of values for D,
M, and T and that ⬘ depends on T, the degree of accuracy of the shortcut value
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.77
of Re is reflected by the precision of the value of Vstd found in step 1 of this
procedure. This can be done by checking the stoichiometry of the combustion
process per lb (kg) of the fuel with the given percentages of C and H2, as follows:
lb 䡠 mol C/lb oil ⫽ 0.87/12 ⫽ 0.0728; lb 䡠 mol H2 /lb ⫽ 0.126/2 ⫽ 0.0630.
Then, 0.0728 C ⫹ 0.0630 H2 ⫹ [1 ⫹ (percent excess air/100 percent]
(0.0728 ⫹ 0.0630/s) O2 ⫹ [1 ⫹ (percent excess air/100 percent](3.76)(0.0728 ⫹
0.0630/s) N2 → 0.0728 CO2 ⫹ 0.0630 H2O ⫹ (percent excess air/100 per-
cent)(0.1043) O2 ⫹ [1 ⫹ (percent excess air/100 percent)](3.76)(0.1043) N2. Table
11 indicates that the lb 䡠 mol/lb oil is 0.7763; hence Vstd ⫽ 0.7763 ⫻ 359 ⫽ 279
std ft3
/lb (17.42 std m3
/kg) of oil. This shows the error in the shortcut estimate
for Vstd to be about 2 percent in this case.
Also, Table 10 shows that this flue gas has a composition of 9.4 percent CO2;
8.1 percent H2O; 6.7 percent O2; 75.8 percent N2; and has an average molecular
weight of 28.96. By the method shown on page 3.279 in Perry’s Chemical Engi-
neers’ Handbook, 6th edition, McGraw-Hill, this flue gas has a calculated mixture
viscosity of ⬘ ⫽ 0.0536 cP (53.6 ⫻ 10⫺6
Pa). Using this value and the average
molecular weight of 28.96 instead of 28 in the gas density formula in step 2, the
Re estimate would be 5700. This indicates the shortcut estimate of 5470 to be in
error by about 4 percent in this case. There are other factors that could contribute
to errors in the shortcut calculations. In practice, wood and municipal sold waste
contain considerable amounts of moisture, which reduces their heating values that
refer to dry conditions, only. The shortcut calculations are very accurate for fossil
fuels, such as coal, fuel oil, and natural gas, and wood. They are useful for wastes
or waste-fuel mixtures. Errors by shortcut calculations seldom exceed Ⳳ10 percent
when excess air is less than 150 to 200 percent.
Though errors for fossil fuels with 100 percent or less excess air are generally
5 percent or less, there are factors that increase the error of the shortcut method:
(1) High water content in the fuel or waste; (2) high halogen content; (3) excess
air above 100 percent. However, the shortcut method can still be used to give a
quick approximation even when these factors are present.
This shortcut method is based on two articles written by Irwin Frankel of The
Mitrer Corp., Metrek Div., 1820 Dolley Madison Blvd., McLean, VA 22102. The
articles, ‘‘Shortcut calculations for fluegas volume’’ and ‘‘Figure fluegas Reynolds
number,’’ appeared in the Chemical Engineering magazine issues of June 1, 1981,
and August 24, 1981, respectively.
DETERMINING THE FEASIBILITY OF FLUE-GAS
RECIRCULATION FOR NOx CONTROL IN
PACKAGED BOILERS
Determine if it is feasible to recirculate flue gas in packaged boilers* to reduce
NOx emissions to comply with EPA and local environmental requirements. Find the
ranges of applicable parameters for packaged boilers in their typical applications.
*Defined as shop-assembled steam generators usually designed for oil and/or natural-gas firing in either
watertube or firetube types. Packaged watertube boilers have capacities ranging up to 600,000 lb/h (75.6
kg/s) at pressures from 125 to 2000 lb/in2
(gage) (860 to 13,800 kPa) with temperatures from 353 to 950⬚F
(78 to 510⬚C). Higher capacities, pressures, and temperatures are possible. Firetube packaged boiler capac-
ities can range up to some 50,000 lb/h (6.3 kg/s) at pressures up to 250 lb/in2
(gage) (1720 kPa) with
possible higher capacities, pressures, and temperatures.
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![STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.79
FIGURE 25 Choice of FGR rate is important for reducing NOx emis-
sions. Smaller units (b) require less recirculation to achieve desired NOx
reduction than larger units (a). (Power.)
kW). These two plots show that, in general, the NOx emission decreases as the
FGR rate, as a percentage of the total flue-gas flow, is increased. Certain limitations
on the FGR rate apply, as discussed in this calculation procedure.
3. Compute the reduction in NOx produced by FGR
Using Fig. 25 for the 350-hp (261.1-kW) packaged boiler at rated capacity, we see
that the NOx is reduced from 89 ppm to 60 ppm when the FGR rate is increased
from 0 to 10 percent of the total flue-gas flow. This is a reduction of ([89 ⫺ 60]/
80)(100) ⫽ 32.6 percent. A reduction of this magnitude in NOx emission is signif-
icant.
4. Evaluate recirculation rates and burners to use
Recirculation rates can vary, depending on a particular unit’s NOx production and
size. On natural gas, FGR is usually limited to 20 percent; and on oil-fired units
to 10 to 12 percent at high fire. These limits are set to eliminate two common
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![FEEDWATER HEATING METHODS 5.3
(37.8⬚C). Exhaust steam at 10.3 lb/in2
(gage) (71.0 kPa) saturated flows to the
heater at the rate of 25,000 lb/h (31.5 kg/s). What saving is obtained by using this
heater if the boiler pressure is 250 lb/in2
(abs) (1723.8 kPa)?
Calculation Procedure:
1. Compute the water outlet temperature
Assume the heater is 90 percent efficient. Then to ⫽ tiww ⫹ 0.9wshg /(ww ⫹ 0.9ws),
where to ⫽ outlet water temperature, ⬚F; ti ⫽ inlet water temperature, ⬚F; ww ⫽
weight of water flowing through heater, lb/h; 0.9 ⫽ heater efficiency, expressed as
a decimal; ws ⫽ weight of steam flowing to the heater, lb/h; hg ⫽ enthalpy of the
steam flowing to the heater, Btu/lb.
For saturated steam at 10.3 lb/in2
(gage) (71.0 kPa), or 10.3 ⫹ 14.7 ⫽ 25 lb/
in2
(abs) (172.4 kPa), hg ⫽ 1160.6 Btu/lb (2599.6 kJ/kg), from the saturation
pressure steam tables. Then
100(250,000) ⫹ 0.9(25,000)(1160.6)
t ⫽ ⫽ 187.5⬚F (86.4⬚C)
o
250,000 ⫹ 0.9(25,000)
2. Compute the savings obtained by feed heating
The percentage of saving, expressed as a decimal, obtained by heating feedwater
is (ho ⫺ hi)/( hb ⫺ hi) where ho and hi ⫽ enthalpy of the water leaving and entering
the heater, respectively, Btu/lb; hb ⫽ enthalpy of the steam at the boiler operating
pressure, Btu/lb. For this plant from the steam tables ho ⫺ hi /(hb ⫺ hi) ⫽ 155.44
⫺ 67.97/(1201.1 ⫺ 67.97) ⫽ 0.077, or 7.7 percent.
A popular rule of thumb states that for every 11⬚F (6.1⬚C) rise in feedwater
temperature in a heater, there is approximately a 1 percent saving in the fuel that
would otherwise be used to heat the feedwater. Checking the above calculation with
this rule of thumb shows reasonably good agreement.
3. Determine the heater volume
With a capacity of W lb/h of water, the volume of a direct-contact or open-type
heater can be approximated from v ⫽ W/10,000, where v ⫽ heater internal volume,
ft3
. For this heater v ⫽ 250,000/10,000 ⫽ 25 ft3
(0.71 m3
).
Related Calculations. Most direct-contact or open feedwater heaters store in
2-min supply of feedwater when the boiler load is constant, and the feedwater
supply is all makeup. With little or no makeup, the heater volume is chosen so that
there is enough capacity to store 5 to 30 min feedwater for the boiler.
CLOSED FEEDWATER HEATER ANALYSIS AND
SELECTION
Analyze and select a closed feedwater heater for the third stage of a regenerative
steam-turbine cycle in which the feedwater flow rate is 37,640 lb/h (4.7 kg/s), the
desired temperature rise of the water during flow through the heater is 80⬚F (44.4⬚C)
(from 238 to 318⬚F or, 114.4 to 158.9⬚C), bleed heating steam is at 100 lb/in2
(abs)
(689.5 kPa) and 460⬚F (237.8⬚C), drains leave the heater at the saturation temper-
ature corresponding to the heating steam pressure [110 lb/in2
(abs) or 689.5 kPa],
and 5
⁄8-in (1.6-cm) OD admiralty metal tubes with a maximum length of 6 ft (1.8
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![5.4 POWER GENERATION
m) are used. Use the Standards of the Bleeder Heater Manufacturers Association,
Inc., when analyzing the heater.
Calculation Procedure:
1. Determine the LMTD across heater
When heat-transfer rates in feedwater heaters are computed, the average film tem-
perature of the feedwater is used. In computing this the Standards of the Bleeder
Heater Manufacturers Association specify that the saturation temperature of the
heating steam be used. At 100 lb/in2
(abs) (689.5 kPa), ts ⫽ 327.81⬚F (164.3⬚C).
Then
(t ⫺ t ) ⫺ (t ⫺ t )
s i s o
LMTD ⫽ t ⫽
m
ln [t ⫺ t /(t ⫺ t )]
s i s o
where the symbols are as defined in the previous calculation procedure. Thus,
(327.81 ⫺ 238) ⫺ (327.81 ⫺ 318)
t ⫽
m
ln [327.81 ⫺ 238/(327.81 ⫺ 318)]
⫽ 36.5⬚F (20.3⬚C)
The average film temperature tf for any closed heater is then
t ⫽ t ⫺ 0.8t
f s m
⫽ 327.81 ⫺ 29.2 ⫽ 298.6⬚F (148.1⬚C)
2. Determine the overall heat-transfer rate
Assume a feedwater velocity of 8 ft/s (2.4 m/s) for this heater. This velocity value
is typical for smaller heaters handling less than 100,000-lb/h (12.6-kg/s) feedwater
flow. Enter Fig. 2 at 8 ft/s (2.4 m/s) on the lower horizontal scale, and project
vertically upward to the 250⬚F (121.1⬚C) average film temperature curve. This curve
is used even though tf ⫽ 298.6⬚F (148.1⬚C), because the standards recommend that
heat-transfer rates higher than those for a 250⬚F (121.1⬚C) film temperature not be
used. So, from the 8-ft/s (2.4 m/s) intersection with the 250⬚F (121.1⬚C) curve in
Fig. 2, project to the left to read U ⫽ the overall heat-transfer rate ⫽ 910 Btu/
(ft2
䡠 ⬚F 䡠 h) [5.2 k]/m2
䡠 ⬚C 䡠 s)].
Next, check Table 1 for the correction factor for U. Assume that no. 18 BWG
5
⁄8-in (1.6-cm) OD arsenical copper tubes are used in this exchanger. Then the
correction factor from Table 1 is 1.00, and Ucorr ⫽ 910(1.00) ⫽ 910. If no. 9 BWG
tubes are chosen, Ucorr ⫽ 910(0.85) ⫽ 773.5 Btu/(ft2
䡠 ⬚F 䡠 h) [4.4 kJ/(m2
䡠 ⬚C 䡠 s)],
given the correction factor from Table 1 for arsenical copper tubes.
3. Compute the amount of heat transferred by the heater
The enthalpy of the entering feedwater at 238⬚F (114.4⬚C) is, from the saturation-
temperature steam table, hfi ⫽ 206.32 Btu/lb (479.9 kJ/kg). The enthalpy of the
leaving feedwater at 318⬚F (158.9⬚C) is, from the same table, hfo ⫽ 288.20 Btu/lb
(670.4 kJ/kg). Then the heater transferred Ht Btu/h is Ht ⫽ ww(hfo ⫺ hfi), where
ww ⫽ feedwater flow rate, lb/h. Or, Ht ⫽ 37,640(288.20 ⫺ 206.32) ⫽ 3,080,000
Btu/h (902.7 kW).
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![FEEDWATER HEATING METHODS 5.5
FIGURE 2 Heat-transfer rates for closed feedwater heaters. (Standards of
Bleeder Heater Manufacturers Association, Inc.)
TABLE 1 Multipliers for Base Heat-Transfer Rates
[For tube OD 5
⁄8 to 1 in (1.6 to 2.5 cm) inclusive]
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![5.6 POWER GENERATION
4. Compute the surface area required in the exchanger
The surface area required A ft2
⫽ Ht /Utm. Then A ⫽ 3,080,000/[910)(36.5)] ⫽
92.7 ft2
(8.6 m2
).
5. Determine the number of tubes per pass
Assume the heater has only one pass, and compute the number of tubes required.
Once the number of tubes is known, a decision can be made about the number of
passes required. In a closed heater, number of tubes ⫽ ww (passes) (ft3
/s per
tube)/[v(ft2
per tube open area)], where ww ⫽ lb/h of feedwater passing through
heater; v ⫽ feedwater velocity in tubes, ft/s.
Since the feedwater enters the heater at 238⬚F (114.4⬚C) and leaves at 318⬚F
(158.9⬚C), its specific volume at 278⬚F (136.7⬚C), midway between ti and to, can
be considered the average specific volume of the feedwater in the heater. From the
saturation-pressure steam table, vf ⫽ 0.01691 ft3
/lb (0.0011 m3
/kg) at 278⬚F
(136.7⬚C). Convert this to cubic feet per second per tube by dividing this specific
volume by 3600 (number of seconds in 1 h) and multiplying by the pounds per
hour of feedwater per tube. Or, ft3
/s per tube ⫽ (0.01691/3600)(lb/h per tube).
Since no. 18 BWG 5
⁄8-in (1.6-cm) OD tubes are being used, ID ⫽ 0.625 ⫺
2(thickness) ⫽ 0.625 ⫺ 2(0.049) ⫽ 0.527 in (1.3 cm). Then, open area per tube
ft2
⫽ (d2
/4)/144 ⫽ 0.7854(0.527)2
/144 ⫽ 0.001525 ft2
(0.00014 m2
) per tube.
Alternatively, this area could be obtained from a table of tube properties.
With these data, compute the total number of tubes from number of tubes ⫽
[(37,640)(1)(0.01681/3600)]/[(8)(0.001525)] ⫽ 14.29 tubes.
6. Compute the required tube length
Assume that 14 tubes are used, since the number required is less than 14.5. Then,
tube length l, ft ⫽ A/(number of tubes per pass)(passes)(area per ft of tube). Or,
tube length for 1 pass ⫽ 92.7/[(14)(1)(0.1636)] ⫽ 40.6 ft (12.4 m). The area per
ft of tube length is obtained from a table of tube properties or computed from
12(OD)/144 ⫽ 12(0.625)/155 ⫽ 0.1636 ft2
(0.015 m2
).
7. Compute the actual number of passes and the actual tube length
Since the tubes in this heater cannot exceed 6 ft (1.8 m) in length, the number of
passes required ⫽ (length for one pass, ft)/(maximum allowable tube length, ft) ⫽
40.6/6 ⫽ 6.77 passes. Since a fractional number of passes cannot be used and an
even number of passes permit a more convenient layout of the heater, choose eight
passes.
From the same equation for tube length as in step 6, l ⫽ tube length ⫽ 92.7/
[(14)(8)(0.1636)] ⫽ 5.06 ft (1.5 m).
8. Determine the feedwater pressure drop through heater
In any closed feedwater heater, the pressure loss ⌬p lb/in2
is ⌬p ⫽ F1F2(L ⫹
5.5D)N/D1.24
, where ⌬p ⫽ pressure drop in the feedwater passing through the
heater, lb/in2
; F1 and F2 ⫽ correction factors from Fig. 3; L ⫽ total lin ft of tubing
divided by the number of tube holes in one tube sheet; D ⫽ tube ID; N ⫽ number
of passes. In finding F2, the average water temperature is taken as ts ⫺ tm.
For this heater, using correction factors from Fig. 3,
5.06(8)(14) 8
⌬p ⫽ (0.136)(0.761) ⫹ 5.5(0.527)
冋 册 1.24
(8)(14) 0.527
2
⫽ 14.6 lb/in (100.7 kPa)
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![INTERNAL-COMBUSTION ENGINES 6.7
DIESEL GENERATING UNIT EFFICIENCY
A 3000-kW diesel generating unit performs thus: fuel rate, 1.5 bbl (238.5 L) of
25⬚ API fuel for a 900-kWh output; mechanical efficiency, 82.0 percent; generator
efficiency, 92.0 percent. Compute engine fuel rate, engine-generator fuel rate, in-
dicated thermal efficiency, overall thermal efficiency, brake thermal efficiency.
Calculation Procedure:
1. Compute the engine fuel rate
The fuel rate of an engine driving a generator is the weight of fuel, lb, used to
generate 1 kWh at the generator input shaft. Since this engine burns 1.5 bbl (238.5
L) of fuel for 900 kW at the generator terminals, the total fuel consumption is (1.5
bbl)(42 gal/bbl) ⫽ 63 gal (238.5 L), at a generator efficiency of 92.0 percent.
To determine the weight of this oil, compute its specific gravity s from s ⫽
141.5/(131.5 ⫹ ⬚API), where ⬚API ⫽ API gravity of the fuel. Hence, s ⫽
141.5(131.5 ⫹ 25) ⫽ 0.904. Since 1 gal (3.8 L) of water weighs 8.33 lb (3.8 kg)
at 60⬚F (15.6⬚C), 1 gal (3.8 L) of this oil weighs (0.904)(8.33) ⫽ 7.529 lb (3.39
kg). The total weight of fuel used when burning 63 gal is (63 gal)(7.529 lb/gal) ⫽
474.5 lb (213.5 kg).
The generator is 92 percent efficient. Hence, the engine actually delivers enough
power to generate 900/0.92 ⫽ 977 kWh at the generator terminals. Thus, the engine
fuel rate ⫽ 474.5 lb fuel/977 kWh ⫽ 0.485 lb/kWh (0.218 kg/kWh).
2. Compute the engine-generator fuel rate
The engine-generator fuel rate takes these two units into consideration and is the
weight of fuel required to generate 1 kWh at the generator terminals. Using the
fuel-consumption data from step 1 and the given output of 900 kW, we see that
engine-generator fuel rate ⫽ 474.5 lb fuel/900 kWh output ⫽ 0.527 lb/kWh (0.237
kg/kWh).
3. Compute the indicated thermal efficiency
Indicated thermal efficiency is the thermal efficiency based on the indicated horse-
power of the engine. This is the horsepower developed in the engine cylinder. The
engine fuel rate, computed in step 1, is the fuel consumed to produce the brake or
shaft horsepower output, after friction losses are deducted. Since the mechanical
efficiency of the engine is 82 percent, the fuel required to produce the indicated
horsepower is 82 percent of that required for the brake horsepower, or (0.82)(0.485)
⫽ 0.398 lb/kWh (0.179 kg/kWh).
The indicated thermal efficiency of an internal-combustion engine driving a gen-
erator is ei ⫽ 3413/ƒi(HHV), where ei ⫽ indicated thermal efficiency, expressed as
a decimal; ƒi ⫽ indicated fuel consumption, lb/kWh; HHV ⫽ higher heating value
of the fuel, Btu/lb.
Compute the HHV for a diesel fuel from HHV ⫽ 17,680 ⫹ 60 ⫻ ⬚API. For this
fuel, HHV ⫽ 17,680 ⫹ 60(25) ⫽ 19,180 Btu/lb (44,612.7 kJ/kg).
With the HHV known, compute the indicated thermal efficiency from ei ⫽
3,413/[(0.398)(19,180)] ⫽ 0.447 or 44.7 percent.
4. Compute the overall thermal efficiency
The overall thermal efficiency eo is computed from eo ⫽ 3413/ƒo(HHV), where
ƒo ⫽ overall fuel consumption, Btu/kWh; other symbols as before. Using the
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![6.8 POWER GENERATION
engine-generator fuel rate from step 2, which represents the overall fuel consump-
tion eo ⫽ 3413/[(0.527)(19,180)] ⫽ 0.347, or 34.7 percent.
5. Compute the brake thermal efficiency
The engine fuel rate, step 1, corresponds to the brake fuel rate ƒb. Compute the
brake thermal efficiency from eb ⫽ 3413/ƒb(HHV), where ƒb ⫽ brake fuel rate,
Btu/kWh; other symbols as before. For this engine-generator set, eb ⫽ 3413/
[(0.485)(19,180)] ⫽ 0.367, or 36.7 percent.
Related Calculations. Where the fuel consumption is given or computed in
terms of lb/(hp 䡠 h), substitute the value of 2545 Btu/(hp 䡠 h) (1.0 kW/kWh) in
place of the value 3413 Btu/kWh (3600.7 kJ/kWh) in the numerator of the ei, eo,
and eb equations. Compute the indicated, overall, and brake thermal efficiencies as
before. Use the same procedure for gas and gasoline engines, except that the higher
heating value of the gas or gasoline should be obtained from the supplier or by
test.
ENGINE DISPLACEMENT, MEAN EFFECTIVE
PRESSURE, AND EFFICIENCY
A 12 ⫻ 18 in (30.5 ⫻ 44.8 cm) four-cylinder four-stroke single-acting diesel engine
is rated at 200 bhp (149.2 kW) at 260 r/min. Fuel consumption at rated load is
0.42 lb/(bhp 䡠 h) (0.25 kg/kWh). The higher heating value of the fuel is 18,920
Btu/lb (44,008 kJ/kg). What are the brake mean effective pressure, engine dis-
placement in ft3
/(min 䡠 bhp), and brake thermal efficiency?
Calculation Procedure:
1. Compute the brake mean effective pressure
Compute the brake mean effective pressure (bmep) for an internal-combustion en-
gine from bmep ⫽ 33,000 bhpn /LAn, where bmep ⫽ brake mean effective pressure,
lb/in2
; bhpn ⫽ brake horsepower output delivered per cylinder, hp; L ⫽ piston
stroke length, ft; a ⫽ piston area, in2
; n ⫽ cycles per minute per cylinder ⫽ crank-
shaft rpm for a two-stroke cycle engine, and 0.5 the crankshaft rpm for a four-
stroke cycle engine.
For this engine at its rated hbp, the output per cylinder is 200 bhp/4 cylinders
⫽ 50 bhp (37.3 kW). Then bmep ⫽ 33,000(50)/[(18/12)(12)2
(/4)(260/2)] ⫽ 74.8
lb/in2
(516.1 kPa). (The factor 12 in the denominator converts the stroke length
from inches to feet.)
2. Compute the engine displacement
The total engine displacement Vd ft3
is given by Vd ⫽ LAnN, where A ⫽ piston
area, ft2
; N ⫽ number of cylinders in the engine; other symbols as before. For this
engine, Vd ⫽ (18/12)(12/12)2
(/4)(260/2)(4) ⫽ 614 ft3
/min (17.4 m3
/min). The
displacement is in cubic feet per minute because the crankshaft speed is in r/min.
The factor of 12 in the denominators converts the stroke and area to ft and ft2
,
respectively. The displacement per bhp ⫽ (total displacement, ft3
/min)/bhp output
of engine ⫽ 614/200 ⫽ 3.07 ft3
/(min 䡠 bhp) (0.12 m3
/kW).
3. Compute the brake thermal efficiency
The brake thermal efficiency eb of an internal-combustion engine is given by eb ⫽
2545/(sfc)(HHV), where sfc ⫽ specific fuel consumption, lb/(bhp 䡠 h); HHV ⫽
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![INTERNAL-COMBUSTION ENGINES 6.9
higher heating value of fuel, Btu/lb. For this engine, eb ⫽ 2545/[(0.42)(18,920)]
⫽ 0.32, or 32.0 percent.
Related Calculations. Use the same procedure for gas and gasoline engines.
Obtain the higher heating value of the fuel from the supplier, a tabulation of fuel
properties, or by test.
ENGINE MEAN EFFECTIVE PRESSURE
AND HORSEPOWER
A 500-hp (373-kW) internal-combustion engine has a brake mean effective pressure
of 80 lb/in2
(551.5 kPa) at full load. What are the indicated mean effective pressure
and friction mean effective pressure if the mechanical efficiency of the engine is
85 percent? What are the indicated horsepower and friction horsepower of the
engine?
Calculation Procedure:
1. Determine the indicated mean effective pressure
Indicated mean effective pressure imep lb/in2
for an internal-combustion engine is
found from imep ⫽ bmep/em, where bmep ⫽ brake mean effective pressure, lb/
in2
; em ⫽ mechanical efficiency, percent, expressed as a decimal. For this engine,
imep ⫽ 80/0.85 ⫽ 94.1 lb/in2
(659.3 kPa).
2. Compute the friction mean effective pressure
For an internal-combustion engine, the friction mean effective pressure ƒmep lb/
in2
is found from ƒmep ⫽ imep ⫺ bmep, or ƒmep ⫽ 94.1 ⫺ 80 ⫽ 14.1 lb/in2
(97.3
kPa).
3. Compute the indicated horsepower of the engine
For an internal-combustion engine, the mechanical efficiency em ⫽ bhp/ihp, where
ihp ⫽ indicated horsepower. Thus, ihp ⫽ bhp/em, or ihp ⫽ 500/0.85 ⫽ 588 ihp
(438.6 kW).
4. Compute the friction hp of the engine
For an internal-combustion engine, the friction horsepower is ƒhp ⫽ ihp ⫺ bhp. In
this engine, ƒhp ⫽ 588 ⫺ 500 ⫽ 88 fhp (65.6 kW).
Related Calculations. Use a similar procedure to determine the indicated en-
gine efficiency eei ⫽ ei /e, where e ⫽ ideal cycle efficiency; brake engine efficiency,
eeb ⫽ ebe; combined engine efficiency or overall engine thermal efficiency eeo ⫽
eo⫽ eoe. Note that each of these three efficiencies is an engine efficiency and cor-
responds to an actual thermal efficiency, ei, eb, and eo.
Engine efficiency ee ⫽ et /e, where et ⫽ actual engine thermal efficiency. Where
desired, the respective actual indicated brake, or overall, output can be substituted
for ei, eb, and eo in the numerator of the above equations if the ideal output is
substituted in the denominator. The result will be the respective engine efficiency.
Output can be expressed in Btu per unit time, or horsepower. Also, ee ⫽ actual
mep/ideal mep, and eei ⫽ imep/ideal mep; eeb ⫽ bmep/ideal mep; eeo ⫽ overall
mep/ideal mep. Further, eb ⫽ emei, and bmep ⫽ em(imep). Where the actual heat
supplied by the fuel, HHV Btu/lb, is known, compute eieb and eo by the method
given in the previous calculation procedure. The above relations apply to any re-
ciprocating internal-combustion engine using any fuel.
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![6.10 POWER GENERATION
TABLE 1 Internal-Combustion Engine Rating Table
SELECTION OF AN INDUSTRIAL
INTERNAL-COMBUSTION ENGINE
Select an internal-combustion engine to drive a centrifugal pump handling 2000
gal/min (126.2 L/s) of water at a total head of 350 ft (106.7 m). The pump speed
will be 1750 r/min, and it will run continuously. The engine and pump are located
at sea level.
Calculation Procedure:
1. Compute the power input to the pump
The power required to pump water is hp ⫽ 8.33GH/33,000e, where G ⫽ water
flow, gal/min; H ⫽ total head on the pump, ft of water; e ⫽ pump efficiency,
expressed as a decimal. Typical centrifugal pumps have operating efficiencies rang-
ing from 50 to 80 percent, depending on the pump design and condition and liquid
handled. Assume that this pump has an efficiency of 70 percent. Then hp ⫽
8.33(2000)/(350)/[(33,000)(0.70)] ⫽ 252 hp (187.9 kW). Thus, the internal-
combustion engine must develop at least 252 hp (187.9 kW) to drive this pump.
2. Select the internal-combustion engine
Since the engine will run continuously, extreme care must be used in its selection.
Refer to a tabulation of engine ratings, such as Table 1. This table shows that a
diesel engine that delivers 275 continuous brake horsepower (205.2 kW) (the near-
est tabulated rating equal to or greater than the required input) will be rated at 483
bhp (360.3 kW) at 1750 r/min.
The gasoline-engine rating data in Table 1 show that for continuous full load at
a given speed, 80 percent of the tabulated power can be used. Thus, at 1750 r/min,
the engine must be rated at 252/0.80 ⫽ 315 bhp (234.9 kW). A 450-hp (335.7-
kW) unit is the only one shown in Table 1 that would meet the needs. This is too
large; refer to another builder’s rating table to find an engine rated at 315 to 325
bhp (234.9 to 242.5 kW) at 1750 r/min.
The unsuitable capacity range in the gasoline-engine section of Table 1 is a
typical situation met in selecting equipment. More time is often spent in finding a
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![INTERNAL-COMBUSTION ENGINES 6.15
using the heat rejection quantities computed in step 1. When the engine is devel-
oping its full rated output of 1000 bhp (746 kW), the jacket water must absorb
[939 Btu/(bhp 䡠 h)(1000 bhp) ⫽ 939,000 Btu/h (275,221 W).
Apply a safety factor to allow for scaling of the heat-transfer surfaces and other
unforeseen difficulties. Most designers use a 10 percent safety factor. Applying this
value of the safety factor for this engine, we see the total jacket-water heat load ⫽
939,000 ⫹ (0.10)(939,000) ⫽ 1,032,900 Btu/h (302.5 kW).
Find the required jacket-water flow from G ⫽ H/500⌬t, where G ⫽ jacket-water
flow, gal/min; H ⫽ heat absorbed by jacket water, Btu/h; ⌬t ⫽ temperature rise
of the water during passage through the jackets, ⬚F. The usual temperature rise of
the jacket water during passage through a diesel engine is 10 to 20⬚F (5.6 to 11.1⬚C).
Using 10⬚F for this engine we find G ⫽ 1,032,900/[(500)(10)] ⫽ 206.58 gal/min
(13.03 L/s), say 207 gal/min (13.06 L/s).
3. Determine the water quantity for radiator cooling
In the usual radiator cooling system for large engines, a portion of the cooling
water is passed through a horizontal or vertical radiator. The remaining water is
recirculated, after being tempered by the cooled water. Thus, the radiator must
dissipate the jacket, turbocharger, and lube-oil cooler heat, Fig. 5.
The lube oil gives off 264 Btu/(bhp 䡠 h) (103.8 W/kWh). With a 10 percent
safety factor, the total heat flow is 264 ⫹ (0.10)(264) ⫽ 290.4 Btu/(bhp 䡠 h) (114.1
W/kWh). At the rated output of 1000 bhp (746 kW), the lube-oil heat load ⫽
[290.4 Btu/(bhp 䡠 h)](1000 bhp) ⫽ 290,400 Btu/h (85.1 kW). Hence, the total heat
load on the radiator ⫽ jacket ⫹ lube-oil heat load ⫽ 1,032,900 ⫹ 290,400 ⫽
1,323,300 Btu/h (387.8 kW)
Radiators (also called fan coolers) serving large internal-combustion engines are
usually rated for a 35⬚F (19.4⬚C) temperature reduction of the water. To remove
1,323,300 Btu/h (387.8 kW) with a 35⬚F (19.4⬚C) temperature decrease will
require a flow of G ⫽ H/(500⌬t) ⫽ 1,323,300/[(500)(35)] ⫽ 76.1 gal/min
(4.8 L/s).
4. Determine the aftercooler cooling-water quantity
The aftercooler must dissipate 278 Btu/(bhp 䡠 h) (109.2 W/kWh). At an output of
1000 bhp (746 kW), the heat load ⫽ [278 Btu/(bhp 䡠 h)](1000 bhp) ⫽ 278,000
Btu/h (81.5 kW). In general, designers do not use a factor of safety for the after-
cooler because there is less chance of fouling or other difficulties.
With a 5⬚F (2.8⬚C) temperature rise of the cooling water during passage through
the after-cooler, the quantity of water required G ⫽ H/(500⌬t) ⫽ 278,000/
[(500)(5)] ⫽ 111 gal/min (7.0 L/s).
5. Compute the quantity of steam generated by the exhaust
Find the heat available in the exhaust by using He ⫽ Wc⌬te, where He ⫽ heat
available in the exhaust, Btu/h; W ⫽ exhaust-gas flow, lb/h; c ⫽ specific heat of
the exhaust gas ⫽ 0.252 Btu/(lb 䡠 ⬚F) (2.5 kJ/kg); ⌬te ⫽ exhaust-gas temperature
at the boiler inlet, ⬚F ⫺ exhaust-gas temperature at the boiler outlet, ⬚F.
The exhaust-gas flow from a four-cycle turbocharged diesel is about 12.5 lb/
(bhp 䡠 h) (7.5 kg/kWh). At full load this engine will exhaust [12.5 lb/(bhp 䡠 h)](1000
bhp) ⫽ 12,500 lb/h (5625 kg/h).
The temperature of the exhaust gas will be about 750⬚F (399⬚C) at the boiler
inlet, whereas the temperature at the boiler outlet is generally held at 75⬚F (41.7⬚C)
higher than the steam temperature to prevent condensation of the exhaust gas. Steam
at 30 lb/in2
(abs) (206.8 kPa) has a temperature of 250.33⬚F (121.3⬚C). Thus, the
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![6.18 POWER GENERATION
The total heat supplied to an engine ⫽ (engine output, bhp)[heat rate, Btu/
(bhp 䡠 h)]. A jacket-water flow rate of 0.25 to 0.60 gal/(min 䡠 bhp) (0.02 to 0.05
kg/kW) is usually recommended. The normal jacket-water temperature rise is 10⬚F
(5.6⬚C); with a jacket-water outlet temperature of 180⬚F (82.2⬚C) or higher, the
temperature rise of the jacket water is usually held to 7⬚F (3.9⬚C) or less.
To keep the cooling-water system pressure loss within reasonable limits, some
designers recommend a pipe velocity equal to the nominal pipe size used in the
system, or 2ft/s for 2-in pipe (0.6 m/s for 50.8-mm); 3 ft/s for 3-in pipe (0.9 m/
s for 76.2-mm); etc. The maximum recommended velocity is 10 ft/s for 10 in (3.0
m/s for 254 mm) and larger pipes. Compute the actual pipe diameter from d ⫽
(G/2.5v)0.5
, where G ⫽ cooling-water flow, gal/min; v ⫽ water velocity, ft/s.
Air needed for a four-cycle high-output turbocharged diesel engine is about 3.5
ft3
/(min 䡠 bhp) (0.13 m3
/kW); 4.5 ft3
/(min 䡠 bhp)(0.17 m3
/kW) for two-cycle en-
gines. Exhaust-gas flow is about 8.4 ft3
/(min 䡠 bhp) (0.32 m3
/kW) for a four-cycle
diesel engine; 13 ft3
/(min 䡠 bhp) (0.49 m3
/kW) for two-cycle engines. Air velocity
in the turbocharger blower piping should not exceed 3300 ft/min (1006 m/min);
gas velocity in the exhaust system should not exceed 6000 ft/min (1828 m/min).
The exhaust-gas temperature should not be reduced below 275⬚F (135⬚C), to prevent
condensation.
The method presented here is the work of W. M. Kauffman, reported in Power.
DESIGN OF A VENT SYSTEM FOR AN
ENGINE ROOM
A radiator-cooled 60-kW internal-combustion engine generating set operates in an
area where the maximum summer ambient temperature of the inlet air is 100⬚F
(37.8⬚C). How much air does this engine need for combustion and for the radiator?
What is the maximum permissible temperature rise of the room air? How much
heat is radiated by the engine-alternator set if the exhaust pipe is 25 ft (7.6 m)
long? What capacity exhaust fan is needed for this engine room if the engine room
has two windows with an area of 30 ft2
(2.8 m2
) each, and the average height
between the air inlet and the outlet is 5 ft (1.5 m)? Determine the rate of heat
dissipation by the windows. The engine is located at sea level.
Calculation Procedure:
1. Determine engine air-volume needs
Table 4 shows typical air-volume needs for internal-combustion engines installed
indoors. Thus, a 60-kW set requires 390 ft3
/min (11.0 m3
/min) for combustion and
6000 ft3
/min (169.9 m3
/min) for the radiator. Note that in the smaller ratings, the
combustion air needed is 6.5 ft3
/(min 䡠 kW)(0.18 m3
/kW), and the radiator air re-
quirement is 150 ft3
/(min 䡠 kW)(4.2 m3
/kW).
2. Determine maximum permissible air temperature rise
Table 4 also shows that with an ambient temperature of 95 to 105⬚F (35 to 40.6⬚C),
the maximum permissible room temperature rise is 15⬚C (8.3⬚C). When you deter-
mine this value, be certain to use the highest inlet air temperature expected in the
engine locality.
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![6.20 POWER GENERATION
TABLE 6 Range of Discharge Temperature*
5. Compute the cooling airflow required
This 60-kW internal-combustion engine generating set radiates 2625 Btu/min (12.8
W), step 3. Compute the cooling airflow required from C ⫽ HK/⌬ta, where C ⫽
cooling airflow required, ft3
/min; H ⫽ heat radiated by the engine, Btu/min; K ⫽
constant from Table 6; other symbols as before. Thus, for this engine with a fan
discharge temperature of 111 to 120⬚F (43.9 to 48.9⬚C), Table 6, K ⫽ 60; ⌬ta ⫽
15⬚F (8.3⬚C) from step 4. Then C ⫽ (2625)(60)/15 ⫽ 10,500 ft3
/min (297.3 m3
/
min).
The windows provide 2445 ft3
/min (69.2 m3
/min), step 4, and the engine ra-
diator gives 6000 ft3
/min (169.9 m3
/min), step 1, or a total of 2445 ⫹ 6000 ⫽
8445 ft3
/min (239.1 m3
/min). Thus, 10,500 ⫺ 8445 ⫽ 2055 ft3
/min (58.2 m3
/min)
must be removed from the room. The usual method employed to remove the air is
an exhaust fan. An exhaust fan with a capacity of 2100 ft3
/min (59.5 m3
/min)
would be suitable for this engine room.
Related Calculations. Use this procedure for engines burning any type of
fuel—diesel, gasoline, kerosene, or gas—in any type of enclosed room at sea level
or elevations up to 1000 ft (304.8 m). Where windows or the fan outlet are fitted
with louvers, screens, or intake filters, be certain to compute the net free area of
the opening. When the radiator fan requires more air than is needed for cooling the
room, an exhaust fan is unnecessary.
Be certain to select an exhaust fan with a sufficient discharge pressure to over-
come the resistance of exhaust ducts and outlet louvers, if used. A propeller fan is
usually chosen for exhaust service. In areas having high wind velocity, an axial-
flow fan may be needed to overcome the pressure produced by the wind on the fan
outlet.
Table 6 shows the pressure developed by various wind velocities. When the
engine is located above sea level, use the multiplying factor in Table 7 to correct
the computed air quantities for the lower air density.
An engine radiates 2 to 5 percent of its total heat input. The total heat input ⫽
(engine output, bhp) [heat rate, Btu/(bhp 䡠 h)]. Provide 12 to 20 air changes per
hour for the engine room. The most effective ventilators are power-driven exhaust
fans or roof ventilators. Where the heat load is high, 100 air changes per hour may
be provided. Auxiliary-equipment rooms require 10 air changes per hour. Windows,
louvers, or power-driven fans are used. A four-cycle engine requires 3 to 3.5 ft3
/
min of air per bhp (0.11 to 0.13 m3
/kW); a two-cycle engine, 4 to 5 ft3
/(min 䡠 bhp)
(0.15 to 0.19 m3
/kW).
The method presented here is the work of John P. Callaghan, reported in Power.
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![INTERNAL-COMBUSTION ENGINES 6.23
hp) [heat dissipated, Btu/(bhp 䡠 h)]; ⌬t ⫽ temperature rise of the water during pas-
sage through the engine, ⬚F. Thus, for this engine, G ⫽ (402)(3500)/[500(10)] ⫽
281 gal/min (17.7 L/s).
2. Choose the cooling-system valve and pipe size
Obtain the friction head-loss data for the engine, the heat exchanger, and the three-
way valve from the manufacturers of the respective items. Most manufacturers have
curves or tables available for easy use. Plot the head losses, as shown in Fig. 8,
for the engine and heat exchanger.
Before the three-way valve head loss can be plotted, a valve size must be chosen.
Refer to a three-way valve capacity tabulation to determine a suitable valve size to
handle a flow of 281 gal/min (17.7 L/s). Once such tabulation recommends a 3-
in (76.2-mm) valve for a flow of 281 gal/min (17.7 L/s). Obtain the head-loss data
for the valve, and plot it as shown in Fig. 8.
Next, assume a size for the cooling-water piping. Experience shows that a water
velocity of 300 to 600 ft/min (91.4 to 182.9 m/min) is satisfactory for internal-
combustion engine cooling systems. Using the Hydraulic Institute’s Pipe Friction
Manual or Cameron’s Hydraulic Data, enter at 280 gal/min (17.6 L/s), the ap-
proximate flow, and choose a pipe size to give a velocity of 400 to 500 ft/min
(121.9 to 152.4 m/min), i.e., midway in the recommended range.
Alternatively, compute the approximate pipe diameter from d ⫽ 4.95 [gpm/
velocity, ft/min]0.5
. With a velocity of 450 ft/min (137.2 m/min), d ⫽ 4.95(281/
450)0.5
⫽ 3.92, say 4 in (101.6 mm). The Pipe Friction Manual shows that the
water velocity will be 7.06 ft/s (2.2 m/s), or 423.6 ft/min (129.1 m/min), in a 4-
in (101.6 mm) schedule 40 pipe. This is acceptable. Using a 31
⁄2-in (88.9-mm) pipe
would increase the cost because the size is not readily available from pipe suppliers.
A 3-in (76.2-mm) pipe would give a velocity of 720 ft/min (219.5 m/min), which
is too high.
3. Compute the piping-system head loss
Examine Fig. 7, which shows the cooling system piping layout. Three flow con-
ditions are possible: (a) all the jacket water passes through the heat exchanger, (b)
a portion of the jacket water passes through the heat exchanger, and (c) none of
the jacket water passes through the heat exchanger—instead, all the water passes
through the bypass circuit. The greatest head loss usually occurs when the largest
amount of water passes through the longest circuit (or flow condition a). Compute
the head loss for this situation first.
Using the method given in the piping section of this handbook, compute the
equivalent length of the cooling-system fitting and piping, as shown in Table 8.
Once the equivalent length of the pipe and fittings is known, compute the head loss
in the piping system, using the method given in the piping section of this handbook
with a Hazen-Williams constant of C ⫽ 130 and a rounded-off flow rate of 300
gal/min (18.9 L/s). Summarize the results as shown in Table 8.
The total head loss is produced by the water flow through the piping, fittings,
engine, three-way valve, and heat exchanger. Find the head loss for the last com-
ponents in Fig. 8 for a flow of 300 gal/min (18.9 L/s). List the losses in Table 8,
and find the sum of all the losses. Thus, the total circuit head loss is 57.61 ft (17.6
m) of water.
Compute the head loss for 0, 0.2, 0.4, 0.6, and 0.8 load on the engine, using
the same procedure as in steps 1, 2, and 3 above. Plot on the pump characteristic
curve, Fig. 9, the system head loss for each load. Draw a curve A through the points
obtained, Fig. 9.
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![6.24 POWER GENERATION
TABLE 8 Sample Calculation for Full Flow through Cooling Circuit*
(Fittings and Piping in Circuit)
Compute the system head loss for condition b with half the jacket water [150
gal/min (9.5 L/s)] passing through the heat exchanger and half [150 gal/min (9.5
L/s)] through the bypass circuit. Make the same calculation for 0, 0.2, 0.4, 0.6,
and 0.8 load on the engine. Plot the result as curve B, Fig 9.
Perform a similar calculation for condition c—full flow through the bypass cir-
cuit. Plot the results as curve C, Fig. 9.
4. Compute the actual cooling-water flow rate
Find the points of intersection of the pump total-head curve and the three system
head-loss curves A, B, and C, Fig. 9. These intersections occur at 314, 325, and
330 gal/min (19.8, 20.5, and 20.8 L/s), respectively.
The initial design assumed a 10⬚F (5.6⬚C) temperature rise through the engine
with a water flow rate of 281 gal/min (17.7 L/s). Rearranging the equation in step
1 gives ⌬t ⫽ H/(400G). Substituting the flow rate for condition a gives an actual
temperature rise of ⌬t ⫽ (402)(3500)/[(500)(314)] ⫽ 8.97⬚F (4.98⬚C). If a 180⬚F
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![6.26 POWER GENERATION
FIGURE 10 Internal-combustion engine cooling system.
14.4 ⫹ 7.0 ⫽ 21.4 lb/in2
(abs) (147.5 kPa). The steam tables show that water at
this pressure boils at 232⬚F (111.1⬚C). Checking the NPSH at this pressure shows
that NPSH ⫽ (10 ⫺ 1.02) ⫹ 2.31(21.4 ⫺ 21.4)/0.0954 ⫽ 8.98 ft (2.7 m). This is
close to the required 8-ft (2.4-m) head. However, the engine could be safely op-
erated at a slightly lower temperature, say 225⬚F (107.2⬚C).
8. Compute the pressure at the pump suction flange
The pressure at the pump suction flange P lb/in2
(gage) ⫽ 0.433s(Hs ⫺ Hf) ⫽
(0.433)(0.974)(10.00 ⫺ 1.02) ⫽ 3.79 lb/in2
(gage) (26.1 kPa).
A positive pressure at the pump suction is needed to prevent the entry of air
along the shaft. To further ensure against air entry, a mechanical seal can be used
on the pump shaft in place of packing.
Related Calculations. Use this general procedure in designing the cooling sys-
tem for any type of reciprocating internal-combustion engine—gasoline, diesel, gas,
etc. Where a coolant other than water is used, follow the same procedure but change
the value of the constant in the denominator of the equation of step 1. Thus, for a
mixture of 50 percent glycol and 50 percent water, the constant ⫽ 436, instead of
500.
The method presented here is the work of Duane E. Marquis, reported in Me-
chanical Engineering.
HOT-WATER HEAT-RECOVERY
SYSTEM ANALYSIS
An internal-combustion engine fitted with a heat-recovery silencer and a jacket-
water cooler is rated at 1000 bhp (746 kW). It exhausts 13.0 lb/(bhp 䡠 h) [5.9 kg/
(bhp 䡠 h)] of exhaust gas at 700⬚F (371.1⬚C). To what temperature can hot water be
heated when 500 gal/min (31.5 L/s) of jacket water is circulated through the
hookup in Fig. 10 and 100 gal/min (6.3 L/s) of 60⬚F (15.6⬚C) water is heated?
The jacket water enters the engine at 170⬚F (76.7⬚C) and leaves at 180⬚F (82.2⬚C).
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![6.28 POWER GENERATION
141.5/(131.5 ⫹ 24) ⫽ 0.910. Since water at 60⬚F (15.6⬚C) weighs 8.33 lb/gal (3.75
kg/L), the weight of this oil is (0.910)(8.33) ⫽ 7.578 lb/gal (3.41 kg/L). For the
28⬚ API oil, s ⫽ 141.5/(131.5 ⫹ 28) ⫽ 0.887, and the weight of this oil is
(0.887)(8.33) ⫽ 7.387 lb/gal (3.32 kg/L). Using the lighter oil, since this will give
a larger gal/h consumption, we get the fuel rate ⫽ (2320 lb/h)/(7.387 lb/gal) ⫽
315 gal/h (1192 L/h).
The daily fuel consumption is then (24 h/day)(315 gal/h) ⫽ 7550 gal/day
(28,577 L/day). In 7 days the engines will use (7 days)(7550 gal/day) ⫽ 52,900,
say 53,000 gal (200,605 L).
2. Select the tank capacity
The actual fuel consumption is 53,000 gal (200,605 L) in 7 days. If fuel is delivered
exactly on time every 7 days, a fuel-tank capacity of 53,000 gal (200,605 L) would
be adequate. However, bad weather, transit failures, strikes, or other unpredictable
incidents may delay delivery. Therefore, added capacity must be provided to prevent
engine stoppage because of an inadequate fuel supply.
Where sufficient space is available, and local regulations do not restrict the
storage capacity installed, use double the required capacity. The reason is that the
additional storage capacity is relatively cheap compared with the advantages gained.
Where space or storage capacity is restricted, use 11
⁄2 times the required capacity.
Assuming double capacity is used in this plant, the total storage capacity will
be (2)(53,000) ⫽ 106,000 gal (401,210 L). At least two tanks should be used, to
permit cleaning of one without interrupting engine operation.
Consult the National Board of Fire Underwriters bulletin Storage Tanks for
Flammable Liquids for rules governing tank materials, location, spacing, and fire-
protection devices. Refer to a tank capacity table to determine the required tank
diameter and length or height depending on whether the tank is horizontal or ver-
tical. Thus, the Buffalo Tank Corporation Handbook shows that a 16.5-ft (5.0-m)
diameter 33.5-ft (10.2-m) long horizontal tank will hold 53,600 gal (202,876 L)
when full. Two tanks of this size would provide the desired capacity. Alternatively,
a 35-ft (10.7-m) diameter 7.5-ft (2.3-m) high vertical tank will hold 54,000 gal
(204,390 L) when full. Two tanks of this size would provide the desired capacity.
Where a tank capacity table is not available, compute the capacity of a cylin-
drical tank from capacity ⫽ 5.87D2
L, where D ⫽ tank diameter, ft; L ⫽ tank length
or height, ft. Consult the NBFU or the tank manufacturer for the required tank wall
thickness and vent size.
3. Select the day-tank capacity
Day tanks supply filtered fuel to an engine. The day tank is usually located in the
engine room and holds enough fuel for a 4- to 8-h operation of an engine at full
load. Local laws, insurance requirements, or the NBFU may limit the quantity of
oil that can be stored in the engine room or a day tank. One day tank is usually
used for each engine.
Assume that a 4-h supply will be suitable for each engine. Then the day tank
capacity for a 1000-hp (746-kW) engine ⫽ (1000 hp) [0.35 lb/(bhp 䡠 h) fuel] (4 h)
⫽ 1400 lb (630 kg), or 1400/7.387 ⫽ 189.6 gal (717.6 L), given the lighter-weight
fuel, step 1. Thus, one 200-gal (757-L) day tank would be suitable for each of the
1000-hp (746-kW) engines.
For the 600-hp (448-kW) engines, the day-tank capacity should be (600 hp)[0.35
lb/(bhp 䡠 h) fuel](4 h) ⫽ 840 lb (378 kg), or 840/7.387 ⫽ 113.8 gal (430.7 L).
Thus, one 125-gal (473-L) day tank would be suitable for each of the 600-hp (448-
kW) engines.
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. For the 24⬚ API oil, cost per 10,000 Btu (10,550 kJ) ⫽
(cost, $/gal)/[(HHV, Btu/lb)/10,000](oil weight, lb/gal). For the 24⬚ API oil, cost
per 10,000 Btu (10,550 kJ) ⫽ $0.0825/[(18.941/10,000)(7.578)] ⫽ $0.00574, or
0.574 cent per 10,000 Btu (10,550 kJ). For the 28⬚ API oil, cost per 10,000 Btu ⫽
$0.0910/[(19,157/10,000)(7387)] ⫽ $0.00634, or 0.634 cent per 10,000 Btu
(10,550 kJ). Thus, the 24⬚ API is the better buy because it costs less per 10,000
Btu (10,550 kJ).
Related Calculations. Use this method for engines burning any liquid fuel. Be
certain to check local laws and the latest NBFU recommendations before ordering
fuel storage or day tanks.
Low-sulfur diesel amendments were added to the federal Clean Air Act in 1991.
These amendments required diesel engines to use low-sulfur fuel to reduce atmos-
pheric pollution. Reduction of fuel sulfur content will not require any change in
engine operating procedures. If anything, the lower sulfur content will reduce en-
gine maintenance requirements and costs.
The usual distillate fuel specification recommends a sulfur content of not more
than 1.5 percent by weight, with 2 percent by weight considered satisfactory. Re-
fineries are currently producing diesel fuel that meets federal low-sulfur require-
ments. While there is a slight additional cost for such fuel at the time of this writing,
when the regulations went into effect, predictions are that the price of low-sulfur
fuel will decline as more is manufactured.
Automobiles produce 50 percent of the air pollution throughout the developed
world. The Ozone Transport Commission, set up by Congress as part of the 1990
Clear Air Act, is enforcing emission standards for new automobiles and trucks. To
date, the cost of meeting such standards has been lower than anticipated. By the
year 2003, all new automobiles will be pollution-free—if they comply with the
requirements of the act. Stationary diesel plants using low-sulfur fuel will emit
extremely little pollution.
POWER INPUT TO COOLING-WATER AND
LUBE-OIL PUMPS
What is the required power input to a 200-gal/min (12.6-L/s) jacket-water pump
if the total head on the pump is 75 ft (22.9 m) of water and the pump has an
efficiency of 70 percent when it handles freshwater and saltwater? What capacity
lube-oil pump is needed for a four-cycle 500-hp (373-kW) turbocharged diesel
engine having oil-cooled pistons? What is the required power input to this pump
if the discharge pressure is 80 lb/in2
(551.5 kPa) and the efficiency of the pump is
68 percent?
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![6.30 POWER GENERATION
Calculation Procedure:
1. Determine the power input to the jacket-water pump
The power input to jacket-water and raw-water pumps serving internal-combustion
engines is often computed from the relation hp ⫽ Gh/Ce, where hp ⫽ hp input;
G ⫽ water discharged by pump, gal/min; h ⫽ total head on pump, ft of water; C
⫽ constant ⫽ 3960 for freshwater having a density of 62.4 lb/ft3
(999.0 kg/m3
);
3855 for saltwater having a density of 64 lb/ft3
(1024.6 kg/m3
).
For this pump handling freshwater, hp ⫽ (200)(75)/(3960)(0.70) ⫽ 5.42 hp (4.0
kW). A 7.5-hp (5.6-kW) motor would probably be selected to handle the rated
capacity plus any overloads.
For this pump handling saltwater, hp ⫽ (200)(75/[(3855)(0.70)] ⫽ 5.56 hp (4.1
kW). A 7.5-hp (5.6-kW) motor would probably be selected to handle the rated
capacity plus any overloads. Thus, the same motor could drive this pump whether
it handles freshwater or saltwater.
2. Compute the lube-oil pump capacity
The lube-oil pump capacity required for a diesel engine is found from G ⫽ H/
200⌬t, where G ⫽ pump capacity, gal/min; H ⫽ heat rejected to the lube oil,
Btu/(bhp 䡠 h); ⌬t ⫽ lube-oil temperature rise during passage through the engine, ⬚F.
Usual practice is to limit the temperature rise of the oil to a range of 20 to 25⬚F
(11.1 to 13.9⬚C), with a maximum operating temperature of 160⬚F (71.1⬚C). The
heat rejection to the lube oil can be obtained from the engine heat balance, the
engine manufacturer, or Standard Practices for Stationary Diesel Engines, pub-
lished by the Diesel Engine Manufacturers Association. With a maximum heat
rejection rate of 500 Btu/(bhp 䡠 h) (196.4 W/kWh) from Standard Practices and
an oil-temperature rise of 20⬚F (11.1⬚C), G ⫽ [500 Btu/(bhp 䡠 h)](1000 hp)/
[(200)(20)] ⫽ 125 gal/min (7.9 L/s).
By using the lowest temperature rise and the highest heat rejection rate, a safe
pump capacity is obtained. Where the pump cost is a critical factor, use a higher
temperature rise and a lower heat rejection rate. Thus, with a heat rejection, the
above pump would have a capacity of G ⫽ (300)(1000)/[(200)(25)] ⫽ 60 gal/min
(3.8 L/s).
3. Compute the lube-oil pump power input
The power input to a separate oil pump serving a diesel engine is given by hp ⫽
Gp/1720e, where G ⫽ pump discharge rate, gal/min; p ⫽ pump discharge pressure,
lb/in2
, e ⫽ pump efficiency. For this pump, hp ⫽ (125)(80)/[(1720)(0.68)] ⫽ 8.56
hp (6.4 kW). A 10-hp (7.5-kW) motor would be chosen to drive this pump.
With a capacity of 60 gal/min (3.8 L/s), the input is hp ⫽ (60)(80)/
[(1720)(0.68)] ⫽ 4.1 hp (3.1 kW). A 5-hp (3.7-kW) motor would be chosen to
drive this pump.
Related Calculations. Use this method for any reciprocating diesel engine,
two- or four-cycle. Lube-oil pump capacity is generally selected 10 to 15 percent
oversize to allow for bearing wear in the engine and wear of the pump moving
parts. Always check the selected capacity with the engine builder. Where a bypass-
type lube-oil system is used, be sure to have a pump of sufficient capacity to handle
both the engine and cooler oil flow.
Raw-water pumps are generally duplicates of the jacket-water pump, having the
same capacity and head ratings. Then the raw-water pump can serve as a standby
jacket-water pump, if necessary.
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![INTERNAL-COMBUSTION ENGINES 6.31
LUBE-OIL COOLER SELECTION AND
OIL CONSUMPTION
A 500-hp (373-kW) internal-combustion engine rejects 300 to 600 Btu/(bhp 䡠 h)
(118 to 236 W/kWh) to the lubricating oil. What capacity and type of lube-oil
cooler should be used for this engine if 10 percent of the oil is bypassed? If this
engine consumes 2 gal (7.6 L) of lube oil per 24 h at full load, determine its lube-
oil consumption rate.
Calculation Procedure:
1. Determine the required lube-oil cooler capacity
Base the cooler capacity on the maximum heat rejection rate plus an allowance for
overloads. The usual overload allowance is 10 percent of the full-load rating for
periods of not more than 2 h in any 24 h period.
For this engine, the maximum output with a 10 percent overload is 500 ⫹
(0.10)(500) ⫽ 550 hp (410 kW). Thus, the maximum heat rejection to the lube oil
would be (500 hp)[600 Btu/(bhp 䡠 h)] ⫽ 330,000 Btu/h (96.7 kW).
2. Choose the type and capacity of lube-oil cooler
Choose a shell-and-tube type heat exchanger to serve this engine. Long experience
with many types of internal-combustion engines shows that the shell-and-tube heat
exchanger is well suited for lube-oil cooling.
Select a lube-oil cooler suitable for a heat-transfer load of 330,000 Btu/h (96.7
kW) at the prevailing cooling-water temperature difference, which is usually as-
sumed to be 10⬚F (5.6⬚C). See previous calculation procedures for the steps in
selecting a liquid cooler.
3. Determine the lube-oil consumption rate
The lube-oil consumption rate is normally expressed in terms of bhp 䡠 h/gal. Thus,
if this engine operates for 24 h and consumes 2 gal (7.6 L) of oil, its lube-oil
consumption rate ⫽ (24 h)(500 bhp)/2 gal ⫽ 6000 bhp 䡠 h/gal (1183 kWh/L).
Related Calculations. Use this procedure for any type of internal-combustion
engine using any fuel.
QUANTITY OF SOLIDS ENTERING AN
INTERNAL-COMBUSTION ENGINE
What weight of solids annually enters the cylinders of a 1000-hp (746-kW) internal-
combustion engine if the engine operates 24 h/day, 300 days/year in an area having
an average dust concentration of 1.6 gr per 1000 ft3
of air (28.3 m3
)? The engine
air rate (displacement) is 3.5 ft3
/(min 䡠 bhp) (0.13 m3
/kW). What would the dust
load be reduced to if an air filter fitted to the engine removed 80 percent of the
dust from the air?
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![6.32 POWER GENERATION
Calculation Procedure:
1. Compute the quantity of air entering the engine
Since the engine is rated at 1000 hp (746 kW) and uses 3.5 ft3
/(min 䡠 bhp) [0.133
m3
/(min 䡠 kW)], the quantity of air used by the engine each minute is (1000 hp)[3.5
ft3
/(min 䡠 hp)] ⫽ 3500 ft3
/min (99.1 m3
/min).
2. Compute the quantity of dust entering the engine
Each 1000 ft3
(28.3 m3
) of air entering the engine contains 1.6 gr (103.7 mg) of
dust. Thus, during every minute of engine operation, the quantity of dust entering
the engine is (3500/1000)(1.6) ⫽ 5.6 gr (362.8 mg). The hourly dust intake ⫽ (60
min/h)(5.6 gr/min) ⫽ 336 gr/h (21,772 mg/h).
During the year the engine operates 24 h/day for 300 days. Hence, the annual
intake of dust is (24 h/day)(300 days/year)(336 gr/h) ⫽ 2,419,200 gr (156.8 kg).
Since there is 7000 gr/lb, the weight of dust entering the engine per year ⫽
2,419,200 gr/(7000 gr/lb) ⫽ 345.6 lb/year (155.5 kg/year).
3. Compute the filtered dust load
With the air filter removing 80 percent of the dust, the quantity of dust reaching
the engine is (1.00⫺ 0.80)(345.6 lb/year) ⫽ 69.12 lb/year (31.1 kg/year). This
shows the effectiveness of an air filter in reducing the dust and dirt load on an
engine.
Related Calculations. Use this general procedure to compute the dirt load on
an engine from any external source.
INTERNAL-COMBUSTION ENGINE
PERFORMANCE FACTORS
Discuss and illustrate the important factors in internal-combustion engine selection
and performance. In this discussion, consider both large and small engines for a
full range of usual applications.
Calculation Procedure:
1. Plot typical engine load characteristics
Figure 11 shows four typical load patterns for internal-combustion engines. A con-
tinuous load, Fig. 11a, is generally considered to be heavy-duty and is often met
in engines driving pumps or electric generators.
Intermittent heavy-duty loads, Fig. 11b, are often met in engines driving concrete
mixers, batch machines, and similar loads. Variable heavy-duty loads, Fig. 11c, are
encountered in large vehicles, process machinery, and similar applications. Variable
light-duty loads, Fig. 11d, are met in small vehicles like golf carts, lawn mowers,
chain saws, etc.
2. Compute the engine output torque
Use the relation T ⫽ 5250 bhp/(r/min) to compute the output torque of an internal-
combustion engine. In this relation, bhp ⫽ engine bhp being developed at a crank-
shaft speed having rotating speed of rpm.
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![6.36
TABLE
10
Performance
Table
for
Small
Internal-Combustion
Engines
[Less
than
7
hp
(5
kW)]*
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![PUMPS AND PUMPING SYSTEMS 7.11
FIGURE 7 Upper limits of specific speeds of single-stage, single- and double-suction
centrifugal pumps handling clear water at 85⬚F (29.4⬚C) at sea level. (Hydraulic Institute.)
The next lower induction-motor speed is 585 r/min. But we could buy a lower-
cost pump and motor if it could be run at the next higher full-load induction motor
speed of 700 r/min. The specific speed of such a pump would be: Ns ⫽ [700
⫽ 4592. Referring to Fig. 7, the maximum suction lift with a
0.5 0.75
(20,000) ]/60
specific speed of 4592 is 13 ft (3.96 m) when the total head is 60 ft (18.3). If the
pump setting or location could be lowered 2 ft (0.6 m), the less expensive pump
and motor could be used, thereby saving on the investment cost.
Related Calculations. Use this general procedure to choose the driver and
pump rpm for centrifugal pumps used in boiler feed, industrial, marine, HVAC, and
similar applications. Note that the latest Hydraulic Institute curves should be used.
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![PUMPS AND PUMPING SYSTEMS 7.23
Hts ⫽ 10 ⫹ 100 ⫽ 110 ft (33.5 m). Note that the static discharge head is computed
between the pump centerline and the water level with an underwater discharge, Fig.
10a; to the pipe outlet with a free discharge, Fig. 10b; and to the maximum water
level in the discharge tank, Fig. 10c. When a pump is discharging into a closed
compression tank, the total discharge head equals the static discharge head plus the
head equivalent, ft of liquid, of the internal pressure in the tank, or 2.31 ⫻ tank
pressure, lb/in2
.
Where the pump has a static suction head, as in Fig. 10d, e, and ƒ, the total
static head Hts ft ⫽ hsd ⫺ static suction head hsh ft. In these installations,
Ht ⫽ 100 ⫺ 15 ⫽ 85 ft (25.9 m).
The total static head, as computed above, refers to the head on the pump without
liquid flow. To determine the total head on the pump, the friction losses in the
piping system during liquid flow must be also determined.
3. Compute the piping friction losses
Mark the length of each piece of straight pipe on the piping drawing. Thus, in Fig.
10a, the total length of straight pipe Lt ft ⫽ 8 ⫹ 10 ⫹ 5 ⫹ 102 ⫹ 5 ⫽ 130 ft (39.6
m), if we start at the suction tank and add each length until the discharge tank is
reached. To the total length of straight pipe must be added the equivalent length of
the pipe fittings. In Fig. 10a there are four long-radius elbows, one swing check
valve, and one globe valve. In addition, there is a minor head loss at the pipe inlet
and at the pipe outlet.
The equivalent length of one 8-in (203.2-mm) long-radius elbow is 14 ft (4.3
m) of pipe, from Table 9. Since the pipe contains four elbows, the total equivalent
length ⫽ 4(14) ⫽ 56 ft (17.1 m) of straight pipe. The open gate valve has an
equivalent resistance of 4.5 ft (1.4 m); and the open swing check valve has an
equivalent resistance of 53 ft (16.2 m).
The entrance loss he ft, assuming a basket-type strainer is used at the suction-
pipe inlet, is he ft ⫽ Kv2
/2g, where K ⫽ a constant from Fig. 11; v ⫽ liquid
velocity, ft/s; g ⫽ 32.2 ft/s2
(980.67 cm/s2
). The exit loss occurs when the liquid
passes through a sudden enlargement, as from a pipe to a tank. Where the area of
the tank is large, causing a final velocity that is zero, hex ⫽ v2
/2g.
The velocity v ft/s in a pipe ⫽ gpm /2.448d2
. For this pipe, v ⫽ 2000/
[(2.448)(7.98)2
] ⫽ 12.82 ft/s (3.91 m/s). Then he ⫽ 0.74(12.82)2
/[2(32.2)] ⫽ 1.89
ft (0.58 m), and hex ⫽ (12.82)2
/[(2)(32.2)] ⫽ 2.56 ft (0.78 m). Hence, the total
length of the piping system in Fig. 10a is 130 ⫹ 56 ⫹ 4.5 ⫹ 53 ⫹ 1.89 ⫹
2.56 ⫽ 247.95 ft (75.6 m), say 248 ft (75.6 m).
Use a suitable head-loss equation, or Table 10, to compute the head loss for the
pipe and fittings. Enter Table 10 at an 8-in (203.2-mm) pipe size, and project
horizontally across to 2000 gal/min (126.2 L/s) and read the head loss as 5.86 ft
of water per 100 ft (1.8 m/30.5 m) of pipe.
The total length of pipe and fittings computed above is 248 ft (75.6 m). Then
total friction-head loss with a 2000 gal/min (126.2-L/s) flow is ft ⫽
Hƒ
(5.86)(248/100) ⫽ 14.53 ft (4.5 m).
4. Compute the total head on the pump
The total head on the pump Ht ⫽ Hts ⫹ For the pump in Fig. 10a,
H .
ƒ
Ht ⫽ 110 ⫹ 14.53 ⫽ 124.53 ft (37.95 m), say 125 ft (38.1 m). The total head on
the pump in Fig. 10b and c would be the same. Some engineers term the total head
on a pump the total dynamic head to distinguish between static head (no-flow
vertical head) and operating head (rated flow through the pump).
The total head on the pumps in Fig. 10d, c, and ƒ is computed in the same way
as described above, except that the total static head is less because the pump has
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![PUMPS AND PUMPING SYSTEMS 7.41
Related Calculations. Use the techniques given here for any type of
pump—centrifugal, reciprocating, or rotary—handling any type of liquid—oil, wa-
ter, chemicals, etc. The methods given here are the work of Melvin Mann, as
reported in Chemical Engineering, and Peerless Pump Division of FMC Corp.
NET POSITIVE SUCTION HEAD FOR
HOT-LIQUID PUMPS
What is the maximum capacity of a double-suction condensate pump operating at
1750 r/min if it handles 100⬚F (37.8⬚C) water from a hot well in a condenser having
an absolute pressure of 2.0 in (50.8 mm) Hg if the pump centerline is 10 ft (30.5
m) below the hot-well liquid level and the friction-head loss in the suction piping
and fitting is 5 ft (1.52 m) of water?
Calculation Procedure:
1. Compute the net positive suction head on the pump
The net positive suction head hn on a pump when the liquid supply is above the
pump inlet ⫽ pressure on liquid surface ⫹ static suction head ⫺ friction-head loss
in suction piping and pump inlet ⫺ vapor pressure of the liquid, all expressed in
ft absolute of liquid handled. When the liquid supply is below the pump
centerline—i.e., there is a static suction lift—the vertical distance of the lift is
subtracted from the pressure on the liquid surface instead of added as in the above
relation.
The density of 100⬚F (37.8⬚C) water is 62.0 lb/ft3
(992.6 kg/m3
), computed as
shown in earlier calculation procedures in this handbook. The pressure on the liquid
surface, in absolute ft of liquid ⫽ (2.0 inHg)(1.133)(62.4/62.0) ⫽ 2.24 ft (0.68 m).
In this calculation, 1.133 ⫽ ft of 39.2⬚F (4⬚C) water ⫽ 1 inHg; 62.4 ⫽ lb/ft3
(999.0
kg/m3
) of 39.2⬚F (4⬚C) water. The temperature of 39.2⬚F (4⬚C) is used because at
this temperature water has its maximum density. Thus, to convert inHg to ft absolute
of water, find the product of (inHg)(1.133)(water density at 39.2⬚F)/(water density
at operating temperature). Express both density values in the same unit, usually
lb/ft3
.
The static suction head is a physical dimension that is measured in ft (m) of
liquid at the operating temperature. In this installation, hsh ⫽ 10 ft (3 m) absolute.
The friction-head loss is 5 ft (1.52 m) of water. When it is computed by using
the methods of earlier calculation procedures, this head loss is in ft (m) of water
at maximum density. To convert to ft absolute, multiply by the ratio of water den-
sities at 39.2⬚F (4⬚C) and the operating temperature, or (5)(62.4/62.0) ⫽ 5.03 ft
(1.53 m).
The vapor pressure of water at 100⬚F (37.8⬚C) is 0.949 lb/in2
(abs) (6.5 kPa)
from the steam tables. Convert any vapor pressure to ft absolute by finding the
result of [vapor pressure, lb/in2
(abs)] (144 in2
/ft2
)/liquid density at operating tem-
perature, or (0.949)(144)/62.0 ⫽ 2.204 ft (0.67 m) absolute.
With all the heads known, the net positive suction head is hn ⫽
2.24 ⫹ 10 ⫺ 5.03 ⫺ 2.204 ⫽ 5.01 ft (1.53 m) absolute.
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![PUMPS AND PUMPING SYSTEMS 7.43
tion. In general, pump manufacturers who are members of the Hydraulic Institute
rate their pumps in accordance with the Standards, and a pump chosen from a
catalog capacity table or curve will deliver the stated capacity. A similar procedure
is used for computing the capacity of pumps handling volatile petroleum liquids.
When you use this procedure, be certain to refer to the latest edition of the Stan-
dards.
CONDENSATE PUMP SELECTION FOR A STEAM
POWER PLANT
Select the capacity for a condensate pump serving a steam power plant having a
140,000 lb/h (63,000 kg/h) exhaust flow to a condenser that operates at an absolute
pressure of 1.0 in (25.4 mm) Hg. The condensate pump discharges through 4-in
(101.6-mm) schedule 40 pipe to an air-ejector condenser that has a frictional resis-
tance of 8 ft (2.4 m) of water. From here, the condensate flows to and through a
low-pressure heater that has a frictional resistance of 12 ft (3.7 m) of water and is
vented to the atmosphere. The total equivalent length of the discharge piping, in-
cluding all fittings and bends, is 400 ft (121.9 m), and the suction piping total
equivalent length is 50 ft (15.2 m). The inlet of the low pressure heater is 75 ft
(22.9 m) above the pump centerline, and the condenser hot-well water level is 10
ft (3 m) above the pump centerline. How much power is required to drive the pump
if its efficiency is 70 percent?
Calculation Procedure:
1. Compute the static head on the pump
Sketch the piping system as shown in Fig. 29. Mark the static elevations and equiv-
alent lengths as indicated.
The total head on the pump Ht ⫽ Hts ⫹ where the symbols are the same as
H ,
ƒ
in earlier calculation procedures. The total static head Hts ⫽ hsd ⫺ hsh . In this
installation, hsd ⫽ 75 ft (22.9 m). To make the calculation simpler, convert all the
heads to absolute values. Since the heater is vented to the atmosphere, the pressure
acting on the surface of the water in it ⫽ 14.7 lb/in2
(abs) (101.3 kPa), or 34 ft
(10.4 m) of water. The pressure acting on the condensate in the hot well is 1 in
(25.4 mm) Hg ⫽ 1.133 ft (0.35 m) of water. [An absolute pressure of 1 in (25.4
mm) Hg ⫽ 1.133 ft (0.35 m) of water.] Thus, the absolute discharge static
head ⫽ 75 ⫹ 34 ⫽ 109 ft (33.2 m), whereas the absolute suction head ⫽
10 ⫹ 1.13 ⫽ 11.13 ft (3.39 m). Then Hts ⫽ hhd ⫺ hsh ⫽ 109.00 ⫺ 11.13 ⫽
97.87 ft (29.8 m), say 98 ft (29.9 m) of water.
2. Compute the friction head in the piping system
The total friction head ⫽ pipe friction ⫹ heater friction. The pipe friction loss
Hƒ
is found first, as shown below. The heater friction loss, obtained from the manu-
facturer or engineering data, is then added to the pipe-friction loss. Both must be
expressed in ft (m) of water.
To determine the pipe friction, use Fig. 30 of this section and Table 17 and Fig.
6 of the Piping section of this handbook in the following manner. Find the product
of the liquid velocity, ft/s, and the pipe internal diameter, in, or vd. With an exhaust
flow of 140,000 lb/h (63,636 kg/h) to the condenser, the condensate flow is the
same, or 140,000 lb/h (63,636 kg/h) at a temperature of 79.03⬚F (21.6⬚C), corre-
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![7.44 PLANT AND FACILITIES ENGINEERING
FIGURE 29 Condensate pump serving a steam power plant.
sponding to an absolute pressure in the condenser of 1 in (25.4 mm) Hg, obtained
from the steam tables. The specific volume of the saturated liquid at this temperature
and pressure is 0.01608 ft3
/lb (0.001 m3
/kg). Since 1 gal (0.26 L) of liquid occupies
0.13368 ft3
(0.004 m3
), specific volume, gal/lb, is (0.01608/0.13368) ⫽ 0.1202
(1.01 L/kg). Therefore, a flow of 140,000 lb/h (63,636 kg/h) ⫽ a flow of
(140,000)(0.1202) ⫽ 16,840 gal/h (63,739.4 L/h), or 16,840/60 ⫽ 281 gal/min
(17.7 L/s). Then the liquid velocity v ⫽ gpm /2.448d2
⫽ 281/2.448(4.026)2
⫽ 7.1
ft/s (2.1 m/s), and the product vd ⫽ (7.1)(4.026) ⫽ 28.55.
Enter Fig. 30 at a temperature of 79⬚F (26.1⬚C), and project vertically upward
to the water curve. From the intersection, project horizontally to the right to
vd ⫽ 28.55 and then vertically upward to read R ⫽ 250,000. Using Table 17 and
Fig. 6 of the Piping section and R ⫽ 250,000, find the friction factor ƒ ⫽ 0.0185.
Then the head loss due to pipe friction ⫽ (L/D)(v2
/2g) ⫽ 0.0185 (450/4.026/
Hƒ
12)/[(7.1)2
/2(32.2)] ⫽ 19.18 ft (5.9 m). In this computation, L ⫽ total equivalent
length of the pipe, pipe fittings, and system valves, or 450 ft (137.2 m).
3. Compute the other head losses in the system
There are two other head losses in this piping system: the entrance loss at the
square-edged hot-well pipe leading to the pump and the sudden enlargement in the
low-pressure heater. The velocity head v2
/2g ⫽ (7.1)2
/2(32.2) ⫽ 0.784 ft (0.24 m).
Using k values from Fig. 11 in this section, he ⫽ kv2
/2g ⫽ (0.5)(0.784) ⫽ 0.392
ft (0.12 m); hex ⫽ v2
/2g ⫽ 0.784 ft (0.24 m).
4. Find the total head on the pump
The total head on the pump Ht ⫽ Hts ⫹ ⫽ 97.87 ⫹ 19.18 ⫹ 8 ⫹ 12 ⫹
Hƒ
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![7.46 PLANT AND FACILITIES ENGINEERING
0.392 ⫹ 0.784 ⫽ 138.226 ft (42.1 m), say 140 ft (42.7 m) of water. In this calcu-
lation, the 8-(2.4-m) and 12-ft (3.7-m) head losses are those occurring in the heaters.
With a 25 percent safety factor, total head ⫽ (1.25)(140) ⫽ 175 ft (53.3 m).
5. Compute the horsepower required to drive the pump
The brake horsepower input bhpi ⫽ (gpm)(Ht )(s)/3960e, where the symbols are
the same as in earlier calculation procedures. At 1 in (25.4 mm) Hg, 1 lb (0.45 kg)
of the condensate has a volume of 0.01608 ft3
(0.000455 m3
). Since density ⫽ 1/
specific volume, the density of the condensate ⫽ 1/0.01608 ⫽ 62.25 ft3
/lb (3.89
m3
/kg). Water having a specific gravity of unity weighs 62.4 lb/ft3
(999 kg/m3
).
Hence, the specific gravity of the condensate is 62.25/62.4 ⫽ 0.997. Then, assum-
ing that the pump has an operating efficiency of 70 percent, we get bhpi ⫽
(281)(175) ⫻ (0.997)/[3960(0.70)] ⫽ 17.7 bhp (13.2 kW).
6. Select the condensate pump
Condensate or hot-well pumps are usually centrifugal units having two or more
stages, with the stage inlets opposed to give better axial balance and to subject the
sealing glands to positive internal pressure, thereby preventing air leakage into the
pump. In the head range developed by this pump, 175 ft (53.3 m), two stages are
satisfactory. Refer to a pump manufacturer’s engineering data for specific stage head
ranges. Either a turbine or motor drive can be used.
Related Calculations. Use this procedure to choose condensate pumps for
steam plants of any type—utility, industrial, marine, portable, heating, or
process—and for combined steam-diesel plants.
MINIMUM SAFE FLOW FOR A
CENTRIFUGAL PUMP
A centrifugal pump handles 220⬚F (104.4⬚C) water and has a shutoff head (with
closed discharge valve) of 3200 ft (975.4 m). At shutoff, the pump efficiency is 17
percent and the input brake horsepower is 210 (156.7 kW). What is the minimum
safe flow through this pump to prevent overheating at shutoff? Determine the min-
imum safe flow if the NPSH is 18.8 ft (5.7 m) of water and the liquid specific
gravity is 0.995. If the pump contains 500 lb (225 kg) of water, determine the rate
of the temperature rise at shutoff.
Calculation Procedure:
1. Compute the temperature rise in the pump
With the discharge valve closed, the power input to the pump is converted to heat
in the casing and causes the liquid temperature to rise. The temperature rise
t ⫽ (1 ⫺ e) ⫻ Hs /778e, where t ⫽ temperature rise during shutoff, ⬚F; e ⫽ pump
efficiency, expressed as a decimal; Hs ⫽ shutoff head, ft. For this pump,
t ⫽ (1 ⫺ 0.17)(3200)/[778(0.17)] ⫽ 20.4⬚F (36.7⬚C).
2. Compute the minimum safe liquid flow
For general-service pumps, the minimum safe flow M gal/min ⫽ 6.0(bhp input at
shutoff)/t. Or, M ⫽ 6.0(210)/20.4 ⫽ 62.7 gal/min (3.96 L/s). This equation in-
cludes a 20 percent safety factor.
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![PUMPS AND PUMPING SYSTEMS 7.47
Centrifugal boiler-feed pumps usually have a maximum allowable temperature
rise of 15⬚F (27⬚C). The minimum allowable flow through the pump to prevent the
water temperature from rising more than 15⬚F (27⬚C) is 30 gal/min (1.89 L/s) for
each 110-bhp (74.6-kW) input at shutoff.
3. Compute the temperature rise for the operating NPSH
An NPSH of 18.8 ft (5.73 m) is equivalent to a pressure of
18.8(0.433)(0.995) ⫽ 7.78 lb/in2
(abs) (53.6 kPa) at 220⬚F (104.4⬚C), where the
factor 0.433 converts ft of water to lb/in2
. At 220⬚F (104.4⬚C), the vapor pressure
of the water is 17.19 lb/in2
(abs) (118.5 kPa), from the steam tables. Thus, the
total vapor pressure the water can develop before flashing occurs ⫽ NPSH
pressure ⫹ vapor pressure at operating temperature ⫽ 7.78 ⫹ 17.19 ⫽ 24.97
lb/in2
(abs) (172.1 kPa). Enter the steam tables at this pressure, and read the cor-
responding temperature as 240⬚F (115.6⬚C). The allowable temperature rise of the
water is then 240 ⫺ 220 ⫽ 20⬚F (36.0⬚C). Using the safe-flow relation of step 2,
we find the minimum safe flow is 62.9 gal/min (3.97 L/s).
4. Compute the rate of temperature rise
In any centrifugal pump, the rate of temperature rise tr ⬚F/min ⫽ 42.4(bhp input
at shutoff)/wc, where w ⫽ weight of liquid in the pump, lb; c ⫽ specific heat of
the liquid in the pump, Btu/(lb ⬚F). For this pump containing 500 lb (225 kg) of
water with a specific heat, c ⫽ 1.0, tr ⫽ 42.4(210)/[500(1.0)] ⫽ 17.8⬚F/min
(32⬚C/min). This is a very rapid temperature rise and could lead to overheating in
a few minutes.
Related Calculations. Use this procedure for any centrifugal pump handling
any liquid in any service—power, process, marine, industrial, or commercial. Pump
manufacturers can supply a temperature-rise curve for a given model pump if it is
requested. This curve is superimposed on the pump characteristic curve and shows
the temperature rise accompanying a specific flow through the pump.
SELECTING A CENTRIFUGAL PUMP TO HANDLE
A VISCOUS LIQUID
Select a centrifugal pump to deliver 750 gal/min (47.3 L/s) of 1000-SSU oil at a
total head of 100 ft (30.5 m). The oil has a specific gravity of 0.90 at the pumping
temperature. Show how to plot the characteristic curves when the pump is handling
the viscous liquid.
Calculation Procedure:
1. Determine the required correction factors
A centrifugal pump handling a viscous liquid usually must develop a greater ca-
pacity and head, and it requires a larger power input than the same pump handling
water. With the water performance of the pump known—from either the pump
characteristic curves or a tabulation of pump performance parameters—Fig. 31,
prepared by the Hydraulic Institute, can be used to find suitable correction factors.
Use this chart only within its scale limits; do not extrapolate. Do not use the chart
for mixed-flow or axial-flow pumps or for pumps of special design. Use the chart
only for pumps handling uniform liquids; slurries, gels, paper stock, etc., may cause
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![PUMPS AND PUMPING SYSTEMS 7.49
incorrect results. In using the chart, the available net positive suction head is as-
sumed adequate for the pump.
To use Fig. 31, enter at the bottom at the required capacity, 750 gal/min (47.3
L/s), and project vertically to intersect the 100-ft (30.5-m) head curve, the required
head. From here project horizontally to the 1000-SSU viscosity curve, and then
vertically upward to the correction-factor curves. Read CE ⫽ 0.635; CQ ⫽ 0.95;
CH ⫽ 0.92 for 1.0QNW . The subscripts E, Q, and H refer to correction factors for
efficiency, capacity, and head, respectively; and NW refers to the water capacity at
a particular efficiency. At maximum efficiency, the water capacity is given as
1.0QNW ; other efficiencies, expressed by numbers equal to or less than unity, give
different capacities.
2. Compute the water characteristics required
The water capacity required for the pump Qw ⫽ /CQ where ⫽ viscous ca-
Q Q
v v
pacity, gal/min. For this pump, Qw ⫽ 750/0.95 ⫽ 790 gal/min (49.8 L/s). Like-
wise, water head Hw ⫽ /CH, where ⫽ viscous head. Or, Hw ⫽ 100/
H H
v v
0.92 ⫽ 108.8 (33.2 m), say 109 ft (33.2 m) of water.
Choose a pump to deliver 790 gal/min (49.8 L/s) of water at 109-ft (33.2-m)
head of water, and the required viscous head and capacity will be obtained. Pick
the pump so that it is operating at or near its maximum efficiency on water. If
the water efficiency Ew ⫽ 81 percent at 790 gal/min (49.8 L/s) for this pump,
the efficiency when handling the viscous liquid ⫽ Ew CE . Or, ⫽
E E
v v
0.81(0.635) ⫽ 0.515, or 51.5 percent.
The power input to the pump when handling viscous liquids is given by P ⫽
v
where s ⫽ specific gravity of the viscous liquid. For this pump,
Q H s /3960E ,
v v v
⫽ (750) ⫻ (100)(0.90)/[3960(0.515)] ⫽ 33.1 hp (24.7 kW).
Pv
3. Plot the characteristic curves for viscous-liquid pumping
Follow these eight steps to plot the complete characteristic curves of a centrifugal
pump handling a viscous liquid when the water characteristics are known: (a) Se-
cure a complete set of characteristic curves (H, Q, P, E) for the pump to be used.
(b) Locate the point of maximum efficiency for the pump when handling water.
(c) Read the pump capacity, Q gal/min, at this point. (d) Compute the values of
0.6Q, 0.8Q, and 1.2Q at the maximum efficiency. (e) Using Fig. 31, determine the
correction factors at the capacities in steps c and d. Where a multistage pump is
being considered, use the head per stage (⫽ total pump head, ft/number of stages),
when entering Fig. 31. (ƒ) Correct the head, capacity, and efficiency for each of
the flow rates in c and d, using the correction factors from Fig. 31. (g) Plot the
corrected head and efficiency against the corrected capacity, as in Fig. 32. (h)
Compute the power input at each flow rate and plot. Draw smooth curves through
the points obtained, Fig. 32.
Related Calculations. Use the method given here for any uniform viscous
liquid—oil, gasoline, kerosene, mercury, etc—handled by a centrifugal pump. Be
careful to use Fig. 31 only within its scale limits; do not extrapolate. The method
presented here is that developed by the Hydraulic Institute. For new developments
in the method, be certain to consult the latest edition of the Hydraulic Institute
Standards.
PUMP SHAFT DEFLECTION AND CRITICAL SPEED
What are the shaft deflection and approximate first critical speed of a centrifugal
pump if the total combined weight of the pump impellers is 23 lb (10.4 kg) and
the pump manufacturer supplies the engineering data in Fig. 33?
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![PUMPS AND PUMPING SYSTEMS 7.63
FIGURE 40 Performance of a pump at constant speed in pump duty and
turbine duty. (Chemical Engineering.)
Using the steam tables, find the saturation temperature Ts corresponding to this
absolute pressure as Ts ⫽ 226.1⬚F (107.8⬚C). Then the permissible temperature rise
is Tp ⫽ Ts ⫺ Top, where Top ⫽ water temperature in the pump inlet. Or,
Tp ⫽ 226.1 ⫺ 220 ⫽ 6.1⬚F (3.4⬚C).
2. Compute the recirculation flow rate at the shutoff head
From the pump characteristic curve with recirculation, Fig. 43, the continuous-
recirculation flow QB for an 11.5-in (29.2-cm) impeller at an operating head of 450
ft (137.2 m) is 48.6 gal/min (177.1 L/min). Find the continuous-recirculation flow
at shutoff head Hs ft (m) of 540 ft (164.6 m) from Qs ⫽ QB (Hs /Hop )0.5
, where
Hop ⫽ operating head, ft (m). Or Qs ⫽ 48.6(540/450) ⫽ 53.2 gal/min (201.4
L/min).
3. Find the minimum safe flow for this pump
The minimum safe flow, lb/h, is given by wmin ⫽ 2545bhp /[Cp Tp ⫹
(1.285 ⫻ 10⫺3
)Hs ], where Cp ⫽ specific head of the water; other symbols as before.
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![7.64 PLANT AND FACILITIES ENGINEERING
FIGURE 41 System-head curves without recirculation flow.
(Chemical Engineering.)
FIGURE 42 Pumping system with a con-
tinuous-recirculation line. (Chemical Engi-
neering.)
Substituting, we find wmin ⫽ 2545(60)/[1.0(6.1) ⫹ (1.285 ⫻ 10⫺3
)(540)] ⫽ 22,476
lb/h (2.83 kg/s). Converting to gal/min yields Qmin ⫽ wmin /[(ft3
/h)(gal/min)(lb/
ft3
)] for the water flowing through the pump. Or, Qmin ⫽ 22,476/[(8.021)(59.6)] ⫽
47.1 gal/min (178.3 L/min).
4. Compare the shutoff recirculation flow with the safe recirculation flow
Since the shutoff recirculation flow Qs ⫽ 53.2 gal/min (201.4 L/min) is greater
than Qmin ⫽ 47.1 gal/min (178.3 L/min), the 11.5-in (29.2-cm) impeller is adequate
to provide safe continuous recirculation. An 11.25-in (28.6-cm) impeller would not
be adequate because Qmin ⫽ 45 gal/min (170.3 L/min) and Qs ⫽ 25.6 gal/min
(96.9 L/min).
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![PUMPS AND PUMPING SYSTEMS 7.67
TABLE 17 Type of Pump for Highest Energy Efficiency*
Area 1 of Fig. 44 is the densest, crowded both with pumps operating at 1750
and 3500 r/min, because years ago 3500-r/min pumps were not thought to be as
durable as 1750-r/min ones. Since the adoption of the AVS standard in 1960 (su-
perseded by ANSI B73.1), pumps with stiffer shafts have been proved reliable.
Also responsible for many 1750-r/min pumps in area 1 has been the impression
that the higher (3500-r/min) speed causes pumps to wear out faster. However,
because impeller tip speed is the same at both 3500 and 1750 r/min [as, for ex-
ample, a 6-in (15-cm) impeller at 3500 r/min and a 12-in (30-cm) one at 1750 r/
min], so is the fluid velocity, and so should be the erosion of metal surface. Another
reason for not limiting operating speed is that improved impeller inlet design allows
operation at 3500 r/min to capacities of 5000 gal/min (19,000 L/min) and higher.
Choice of operating speed also may be indirectly limited by specifications per-
taining to suction performance, such as that fixing the top suction specific speed S
directly or indirectly by choice of the sigma constant or by reliance on Hydraulic
Institute charts.
Values of S below 8000 to 10,000 have long been accepted for avoiding cavi-
tation. However, since the development of the inducer, S values in the range of
20,000 to 25,000 have become commonplace, and values as high as 50,000 have
become practical.
The sigma constant, which relates NPSH to total head, is little used today, and
Hydraulic Institute charts (which are being revised) are conservative.
In light of today’s designs and materials, past restrictions resulting from suction
performance limitations should be reevaluated or eliminated entirely.
Even if the most efficient pump has been selected, there are a number of cir-
cumstances in which it may not operate at peak efficiency. Today’s cost of energy
has made these considerations more important.
A centrifugal pump, being a hydrodynamic machine, is designed for a single
peak operating-point capacity and total head. Operation at other than this best ef-
ficiency point (bep) reduces efficiency. Specifications now should account for such
factors as these:
1. A need for a larger number of smaller pumps. When a process operates over a
wide range of capacities, as many do, pumps will often work at less than full
capacity, hence at lower efficiency. This can be avoided by installing two or
three pumps in parallel, in place of a single large one, so that one of the smaller
pumps can handle the flow when operations are at a low rate.
2. Allowance for present capacity. Pump systems are frequently designed for full
flow at some time in the future. Before this time arrives, the pumps will operate
far from their best efficiency points. Even if this interim period lasts only 2 or
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![8.2 PLANT AND FACILITIES ENGINEERING
Friction Loss in Pipes Handling Solids
in Suspension 8.111
Desuperheater Water Spray Quantity
8.112
Sizing Condensate Return Lines for
Optimum Flow Conditions 8.114
Estimating Cost of Steam Leaks from
Piping and Pressure Vessels 8.116
Quick Sizing of Restrictive Orifices in
Piping 8.117
Steam Tracing a Vessel Bottom to
Keep Its Contents Fluid 8.118
Designing Steam-Transmission Lines
Without Steam Traps 8.119
Line Sizing for Flashing Steam
Condensate 8.124
Determining the Friction Factor for
Flow of Bingham Plastics 8.127
Time Needed to Empty a Storage
Vessel with Dished Ends 8.130
Time Needed to Empty a Vessel
Without Dished Ends 8.133
Time Needed to Drain a Storage Tank
Through Attached Piping 8.134
Pressure Surge in Fluid Piping Systems
PRESSURE SURGE IN A PIPING SYSTEM FROM
RAPID VALVE CLOSURE
Oil, with a specific weight of 52 lb/ft3
(832 kg/m3
) and a bulk modulus of 250,000
lb/in2
(1723 MPa), flows at the rate of 40 gal/min (2.5 L/s) through stainless steel
pipe. The pipe is 40 ft (12.2 m) long, 1.5 in (38.1 mm) O.D., 1.402 in (35.6 mm)
I.D., 0.049 in (1.24 mm) wall thickness, and has a modulus of elasticity, E, of
29 ⫻ 106
lb/in2
(199.8 kPa ⫻ 106
). Normal static pressure immediately upstream
of the valve in the pipe is 500 lb/in2
(abs) (3445 kPa). When the flow of the oil
is reduced to zero in 0.015 s by closing a valve at the end of the pipe, what is: (a)
the velocity of the pressure wave; (b) the period of the pressure wave; (c) the
amplitude of the pressure wave; and (d) the maximum static pressure at the valve?
Calculation Procedure:
1. Find the velocity of the pressure wave when the valve is closed
(a) Use the equation
68.094
a ⫽
兹␥[(1/K) ⫹ (D/Et)]
where the symbols are as given in the notation below. Substituting,
68.094
a ⫽ 4 6
兹52 [(1/25 ⫻ 10 ) ⫹ (1.402/29 ⫻ 10 ⫻ 0.049]
⫽ 4228 ft/s (1288.7 m/s)
An alternative solution uses Fig. 1. With a D/t ratio ⫽ 1.402/0.049 ⫽ 28.6 for
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![8.4 PLANT AND FACILITIES ENGINEERING
The velocity of flow, V ⫽ [(40)(231)(4)]/[(60)()(1.4022
)(12)] using the stan-
dard pipe flow relation, or V ⫽ 8.3 ft/s (2.53 m/s).
Then, the amplitude of the pressure wave, using the equation above is:
52 ⫻ 4228 ⫻ 8.3 2
⌬p ⫽ ⫽ 393.5 lb/in (2711.2 kPa).
144 ⫻ 32.2
4. Determine the resulting maximum static press in the pipe
d) The resulting maximum static pressure in the line, pmax ⫽ p ⫹ ⌬p ⫽ 500 ⫹
393.5 ⫽ 893.5 lb/in2
(abs) (6156.2 kPa).
Related Calculations. In an industrial hydraulic system, such as that used in
machine tools, hydraulic lifts, steering mechanisms, etc., when the velocity of a
flowing fluid is changed by opening or closing a valve, pressure surges result. The
amplitude of the pressure surge is a function of the rate of change in the velocity
of the mass of fluid. This procedure shows how to compute the amplitude of the
pressure surge with rapid valve closure.
The procedure is the work of Nils M. Sverdrup, Hydraulic Engineer, Aerojet-
General Corporation, as reported in Product Engineering magazine. SI values were
added by the handbook editor.
Notation
a ⫽ velocity of pressure wave, ft/s (m/s)
aE ⫽ effective velocity of pressure wave, ft/s (m/s)
A ⫽ cross-sectional area of pipe, in2
(mm2
)
Ao ⫽ area of throttling orifice before closure, in2
(mm2
)
c ⫽ velocity of sound, ft/s (m/s)
CD ⫽ coefficient of discharge
D ⫽ inside diameter of pipe, in (mm)
E ⫽ modulus of elasticity of pipe material, lb/in2
(kPa)
F ⫽ force, lb (kg)
g ⫽ gravitational acceleration, 32.2 ft/s2
K ⫽ bulk modulus of fluid medium, lb/in2
(kPa)
L ⫽ length of pipe, ft (m)
m ⫽ mass, slugs
N ⫽ T/(2L/a) ⫽ number of pressure wave intervals during time of valve clo-
sure
p ⫽ normal static fluid pressure immediately upstream of valve when the fluid
velocity is V, lb/in2
(absolute) (kPa)
⌬p ⫽ amplitude of pressure wave, lb/in2
(kPa)
pmax ⫽ maximum static pressure immediately upstream of valve, lb/in2
(absolute)
(kPa)
pd ⫽ static pressure immediately downstream of the valve, lb/in2
(absolute)
(kPa)
Q ⫽ volume rate of flow, ft3
/s (m3
/s)
t ⫽ wall thickness of pipe, in (mm)
T ⫽ time in which valve is closed, s
v ⫽ fluid volume, in3
(mm3
)
vA ⫽ air volume, in3
(mm3
)
V ⫽ normal velocity of fluid flow in pipe with valve wide open, ft/s (m/s)
VE ⫽ equivalent fluid velocity, ft/s (m/s)
Vn ⫽ velocity of fluid flow during interval n, ft/s (m/s)
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![8.12 PLANT AND FACILITIES ENGINEERING
TABLE 1 Recommended Fluid Velocities in Piping
2. Determine the pipe schedule number
The ANSA Code for Pressure Piping, commonly called the Piping Code, defines
schedule number as SN ⫽ 1000 Pi /S, where Pi ⫽ internal pipe pressure, lb/in2
(gage); S ⫽ allowable stress in the pipe, lb/in2
, from Piping Code. Table 2 shows
typical allowable stress values for pipe in power piping systems. For this pipe,
assuming that seamless ferritic alloy steel (1% Cr, 0.55% Mo) pipe is used with
the steam at 900⬚F (482⬚C), SN ⫽ (1000)(1014.7)/13,100 ⫽ 77.5. Since pipe is
not ordinarily made in this schedule number, use the next highest readily available
schedule number, or SN ⫽ 80. [Where large quantities of pipe are required, it is
sometimes economically wise to order pipe of the exact SN required. This is not
usually done for orders of less than 1000 ft (304.8 m) of pipe.]
3. Determine the pipe-wall thickness
Enter a tabulation of pipe properties, such as in Crocker and King—Piping Hand-
book, and find the wall thickness for 4-in (101.6-mm) SN 80 pipe as 0.337 in (8.56
mm).
Related Calculations. Use the method given here for any type of pipe—steam,
water, oil, gas, or air—in any service—power, refinery, process, commercial, etc.
Refer to the proper section of B31.1 Code for Pressure Piping when computing
the schedule number, because the allowable stress S varies for different types of
service.
The Piping Code contains an equation for determining the minimum required
pipe-wall thickness based on the pipe internal pressure, outside diameter, allowable
stress, a temperature coefficient, and an allowance for threading, mechanical
strength, and corrosion. This equation is seldom used in routine piping-system de-
sign. Instead, the schedule number as given here is preferred by most designers.
PIPE-WALL THICKNESS DETERMINATION BY
PIPING CODE FORMULA
Use the ANSA B31.1 Code for Pressure Piping wall-thickness equation to deter-
mine the required wall thickness for an 8.625-in (219.1-mm) OD ferritic steel plain-
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![8.14 PLANT AND FACILITIES ENGINEERING
end pipe if the pipe is used in 900⬚F (482⬚C) 900-lb/in2
(gage) (6205-kPa) steam
service.
Calculation Procedure:
1. Determine the constants for the thickness equation
Pipe-wall thickness to meet ANSA Code requirements for power service is com-
puted from tm ⫽ {DP/[2(S ⫹ YP)]} ⫹ C, where tm ⫽ minimum wall thickness, in;
D ⫽ outside diameter of pipe, in; P ⫽ internal pressure in pipe, lb/in2
(gage);
S ⫽ allowable stress in pipe material, lb/in2
; Y ⫽ temperature coefficient; C ⫽ end-
condition factor, in.
Values of S, Y, and C are given in tables in the Code for Pressure Piping in the
section on Power Piping. Using values from the latest edition of the Code, we get
S ⫽ 12,500 lb/in2
(86.2 MPa) for ferritic-steel pipe operating at 900⬚F (482⬚C);
Y ⫽ 0.40 at the same temperature; C ⫽ 0.065 in (1.65 mm) for plain-end steel
pipe.
2. Compute the minimum wall thickness
Substitute the given and Code values in the equation in step 1, or tm ⫽
[(8.625)(900)]/[2(12,500 ⫹ 0.4 ⫻ 900)] ⫹ 0.065 ⫽ 0.367 in (9.32 mm).
Since pipe mills do not fabricate to precise wall thicknesses, a tolerance above
or below the computed wall thickness is required. An allowance must be made in
specifying the wall thickness found with this equation by increasing the thickness
by 121
⁄2 percent. Thus, for this pipe, wall thickness ⫽ 0.367 ⫹ 0.125(0.367) ⫽
0.413 in (10.5 mm).
Refer to the Code to find the schedule number of the pipe. Schedule 60 8-in
(203-mm) pipe has a wall thickness of 0.406 in (10.31 mm), and schedule 80 has
a wall thickness of 0.500 in (12.7 mm). Since the required thickness of 0.413 in
(10.5 mm) is greater than schedule 60 but less than schedule 80, the higher schedule
number, 80, should be used.
3. Check the selected schedule number
From the previous calculation procedure, SN ⫽ 1000 Pi /S. From this pipe,
SN ⫽ 1000(900)/12,500 ⫽ 72. Since piping is normally fabricated for schedule
numbers 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160, the next larger schedule
number higher than 72, that is 80, will be used. This agrees with the schedule
number found in step 2.
Related Calculations. Use this method in conjunction with the appropriate
Code equation to determine the wall thickness of pipe conveying air, gas, steam,
oil, water, alcohol, or any other similar fluids in any type of service. Be certain to
use the correct equation, which in some cases is simpler than that used here. Thus,
for lead pipe, tn ⫽ Pd/2S, where P ⫽ safe working pressure of the pipe, lb/in2
(gage); d ⫽ inside diameter of pipe, in; other symbols as before.
When a pipe will operate at a temperature between two tabulated Code values,
find the allowable stress by interpolating between the tabulated temperature and
stress values. Thus, for a pipe operating at 680⬚F (360⬚C), find the allowable stress
at 650⬚F (343⬚C) [⫽ 9500 lb/in2
(65.5 MPa)] and 700⬚F (371⬚C) [⫽ 9000 lb/in2
(62.0 MPa)]. Interpolate thus: allowable stress at 680⬚F (360⬚C) ⫽ [(700⬚F ⫺
680⬚F)/(700⬚F ⫺ 650⬚F)](9500 ⫺ 9000) ⫹ 9000 ⫽ 200 ⫹ 9000 ⫽ 9200 lb/in2
(63.4 MPa). The same result can be obtained by interpolating downward from 9500
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 ⫽ 9200 lb/in2
(63.4 MPa).
DETERMINING THE PRESSURE LOSS IN
STEAM PIPING
Use a suitable pressure-loss chart to determine the pressure loss in 510 ft (155.5
m) of 4-in (101.6-mm) flanged steel pipe containing two 90⬚ elbows and four 45⬚
bends. The schedule 40 piping conveys 13,000 lb/h (5850 kg/h) of 20-lb/in2
(gage)
(275.8-kPa) 350⬚F (177⬚C) superheated steam. List other methods of determining
the pressure loss in steam piping.
Calculation Procedure:
1. Determine the equivalent length of the piping
The equivalent length of a pipe Le ft ⫽ length of straight pipe, ft ⫹ equivalent
length of fittings, ft. Using data from the Hydraulic Institute, Crocker and
King—Piping Handbook, earlier sections of this handbook, or Fig. 4, find the
equivalent length of a 90⬚ 4-in (101.6-mm) elbow as 10 ft (3 m) of straight pipe.
Likewise, the equivalent length of a 45⬚ bend is 5 ft (1.5 m) of straight pipe.
Substituting in the above relation and using the straight lengths and the number of
fittings of each type, we get Le ⫽ 510 ⫹ (2)(10) ⫹ 4(5) ⫽ 550 ft (167.6 m) of
straight pipe.
2. Compute the pressure loss, using a suitable chart
Figure 2 presents a typical pressure-loss chart for steam piping. Enter the chart at
the top left at the superheated steam temperature of 350⬚F (177⬚C), and project
vertically downward until the 40-lb/in2
(gage) (275.8-kPa) superheated steam pres-
sure curve is intersected. From here, project horizontally to the right until the outer
border of the chart is intersected. Next, project through the steam flow rate, 13,000
lb/h (5900 kg/h) on scale B, Fig. 5, to the pivot scale C. From this point, project
through 4-in (101.6-mm) schedule 40 pipe on scale D, Fig. 5. Extend this line to
intersect the pressure-drop scale, and read the pressure loss as 7.25 lb/in2
(50
kPa)/100 ft (30.4 m) of pipe.
Since the equivalent length of this pipe is 550 ft (167.6 m), the total pressure
loss in the pipe is (550/100)(7.25) ⫽ 39.875 lb/in2
(274.9 kPa), say 40 lb/in2
(275.8 kPa).
3. List the other methods of computing pressure loss
Numerous pressure-loss equations have been developed to compute the pressure
drop in steam piping. Among the better known are those of Unwin, Fritzche, Spitz-
glass, Babcock, Guttermuth, and others. These equations are discussed in some
detail in Crocker and King—Piping Handbook and in the engineering data pub-
lished by valve and piping manufacturers.
Most piping designers use a chart to determine the pressure loss in steam piping
because a chart saves time and reduces the effort involved. Further, the accuracy
obtained is sufficient for all usual design practice.
Figure 3 is a popular flowchart for determining steam flow rate, pipe size, steam
pressure, or steam velocity in a given pipe. Using this chart, the designer can
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![8.18 PLANT AND FACILITIES ENGINEERING
determine any one of the four variables listed above when the other three are known.
In solving a problem on the chart in Fig. 6, use the steam-quantity lines to intersect
pipe sizes and the steam-pressure lines to intersect steam velocities. Here are two
typical applications of this chart.
Example. What size schedule 40 pipe is needed to deliver 8000 lb/h (3600
kg/h) of 120-lb/in2
(gage) (827.3-kPa) steam at a velocity of 5000 ft/min (1524
m/min)?
Solution. Enter Fig. 6 at the upper left at a velocity of 5000 ft/min (1524 m/
min), and project along this velocity line until the 120-lb/in2
(gage) (827.3-kPa)
pressure line is intersected. From this intersection, project horizontally until the
8000-lb/h (3600-kg/h) vertical line is intersected. Read the nearest pipe size as 4
in (101.6 mm) on the nearest pipe-diameter curve.
Example. What is the steam velocity in a 6-in (152.4-mm) pipe delivering
20,000 lb/h (9000 kg/h) of steam at 85 lb/in2
(gage) (586 kPa)?
Solution. Enter the bottom of the chart, Fig. 6, at the flow rate of 20,000
lb/h (9000 kg/h), and project vertically upward until the 6-in (152.4-mm) pipe
curve is intersected. From this point, project horizontally to the 85-lb/in2
(gage)
(586-kPa) curve. At the intersection, read the velocity as 7350 ft/min (2240.3 m/
min).
Table 3 shows typical steam velocities for various industrial and commercial
applications. Use the given values as guides when sizing steam piping.
PIPING WARM-UP CONDENSATE LOAD
How much condensate is formed in 5 min during warm-up of 500 ft (152.4 m) of
6-in (152.4-mm) schedule 40 steel pipe conveying 215-lb/in2
(abs) (1482.2-kPa)
saturated steam if the pipe is insulated with 2 in (50.8 mm) of 85 percent magnesia
and the minimum external temperature is 35⬚F (1.7⬚C)?
Calculation Procedure:
1. Compute the amount of condensate formed during pipe warm-up
For any pipe, the condensate formed during warm-up Ch lb/h ⫽ 60(Wp )(⌬t)(s)/
, where Wp ⫽ total weight of pipe, lb; ⌬t ⫽ difference between final and initial
h N
ƒg
temperature of the pipe, ⬚F; s ⫽ specific heat of pipe material, Btu/(lb 䡠 ⬚F);
⫽ enthalpy of vaporization of the steam, Btu/lb; N ⫽ warm-up time, min.
hƒg
A table of pipe properties shows that this pipe weighs 18.974 lb/ft (28.1 kg/
m). The steam table shows that the temperature of 215-lb/in2
(abs) (1482.2-kPa)
saturated steam is 387.89⬚F (197.7⬚C), say 388⬚F (197.8⬚C); the enthalpy
⫽ 837.4 Btu/lb (1947.8 kJ/kg). The specific heat of steel pipe s ⫽ 0.144 Btu
hƒg
/(lb 䡠 ⬚F) [0.6 kJ/(kg 䡠 ⬚C)]. Then Ch ⫽ 60(500 ⫻ 18.974)(388 ⫺ 35)(0.114)/
[(837.4)(5)] ⫽ 5470 lb/h (2461.5 kg/h).
2. Compute the radiation-loss condensate load
Condensate is also formed by radiation of heat from the pipe during warm-up and
while the pipe is operating. The warm-up condensate load decreases as the radiation
load increases, the peak occurring midway (21
⁄2 min in this case) through the warm-
up period. For this reason, one-half the normal radiation load is added to the warm-
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![8.20 PLANT AND FACILITIES ENGINEERING
TABLE 3 Steam Velocities Used in Pipe Design
up load. Where the radiation load is small, it is often disregarded. However, the
load must be computed before its magnitude can be determined.
For any pipe, Cr ⫽ (L)(A)(⌬t)(H)/ , where L ⫽ length of pipe, ft; A ⫽ external
hƒg
area of pipe, ft2
/ft of length; H ⫽ heat loss through bare pipe or pipe insulation,
Btu/(ft2
䡠 h 䡠 ⬚F), from the piping or insulation tables. This 6-in (152.4-mm) schedule
40 pipe has an external area A ⫽ 1.73 ft2
/ft (0.53 m2
/m) of length. The heat loss
through 2 in (50.8 mm) of 85 percent magnesia, from insulation tables, is H ⫽
0.286 Btu/(ft2
䡠 h 䡠 ⬚F) [1.62 W/(m2
䡠 ⬚C)]. Then
Cr ⫽ (500) ⫻ (1.73)(388 ⫺ 35)(0.286)/837.4 ⫽ 104.2 lb/h (46.9 kg/h). Adding
half the radiation load to the warm-up load gives 5470 ⫹ 52.1 ⫽ 5522.1 lb/h
(2484.9 kg/h).
3. Apply a suitable safety factory to the condensate load
Trap manufacturers recommend a safety factor of 2 for traps installed between a
boiler and the end of a steam main; traps at the end of a long steam main or ahead
of pressure-regulating or shutoff valves usually have a safety factor of 3. With a
safety factor of 3 for this pipe, the steam trap should have a capacity of at least
3(5522.1) ⫽ 16,566.3 lb/h (7454.8 kg/h), say 17,000 lb/h (7650.0 kg/h).
Related Calculations. Use this method to find the warm-up condensate load
for any type of steam pipe—main or auxiliary—in power, process, heating, or
vacuum service. The same method is applicable to other vapors that form
condensate—Dowtherm, refinery vapors, process vapors, and others.
STEAM TRAP SELECTION FOR INDUSTRIAL
APPLICATIONS
Select steam traps for the following four types of equipment: (1) the steam directly
heats solid materials as in autoclaves, retorts, and sterilizers; (2) the steam indirectly
heats a liquid through a metallic surface, as in heat exchangers and kettles, where
the quantity of liquid heated is known and unknown; (3) the steam indirectly heats
a solid through a metallic surface, as in dryers using cylinders or chambers and
platen presses; and (4) the steam indirectly heats air through metallic surfaces, as
in unit heaters, pipe coils, and radiators.
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![8.24 PLANT AND FACILITIES ENGINEERING
TABLE 7 Condensate Formed in Submerged Steel* Heating Elements, lb/(ft2
䡠 h) [kg/(m2
䡠
min)]
TABLE 8 Steam Condensed by Air, lb/h at 1000 ft3
/min (kg/h at 28.3
m3
/min)*
1000)(122) ⫽ 1220 lb/h (549 kg/h). With a safety factor of 3, the trap
capacity ⫽ (3)(1220) ⫽ 3660 lb/h (1647 kg/h), say 3700 lb/h (1665 kg/h).
Table 9 shows the condensate formed by radiation from bare iron and steel pipes
in still air and with forced-air circulation. Thus, with a steam pressure of 100 lb/
in2
(gage) (689.4 kPa) and an initial air temperature of 75⬚F (23.9⬚C), 1.05 lb/h
(0.47 kg/h) of condensate will be formed per ft2
(0.09 m2
) of heating surface in
still air. With forced-air circulation, the condensate rate is (5)(1.05) ⫽ 5.25 lb/(h 䡠
ft2
) [25.4 kg/(h 䡠 m2
)] of heating surface.
Unit heaters have a standard rating based on 2-lb/in2
(gage) (13.8-kPa) steam
with entering air at 60⬚F (15.6⬚C). If the steam pressure or air temperature is dif-
ferent from these standard conditions, multiply the heater Btu/h capacity rating by
the appropriate correction factor form, Table 10. Thus, a heater rated at 10,000
Btu/h (2931 W) with 2-lb/in2
(gage) (13.8-kPa) steam and 60⬚F (15.6⬚C) air would
have an output of (1.290)(10,000) ⫽ 12,900 Btu/h (3781 W) with 40⬚F (4.4⬚C)
inlet air and 10-lb/in2
(gage) (68.9-kPa) steam. Trap manufacturers usually list
heater Btu ratings and recommend trap model numbers and sizes in their trap en-
gineering data. This allows easier selection of the correct trap.
2. Select the trap size based on the load and steam pressure
Obtain a chart or tabulation of trap capacities published by the manufacturer whose
trap will be used. Figure 7 is a capacity chart for one type of bucket trap manu-
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![PIPING AND FLUID FLOW 8.25
TABLE 9 Condensate Formed by Radiation from Bare Iron and Steel, lb/(ft2
䡠 h)
[kg/(m2
䡠 h)]
TABLE 10 Unit-Heater Correction Factors
factured by Armstrong Machine Works. Table 11 shows typical capacities of im-
pulse traps manufactured by the Yarway Company.
To select a trap from Fig. 7, when the condensate rate is uniform and the pressure
across the trap is constant, enter at the left at the condensation rate, say 8000
lb/h (3600 kg/h) (as obtained from step 1). Project horizontally to the right to the
vertical ordinate representing the pressure across the trap [⫽ ⌬p ⫽ steam-line pres-
sure, lb/in2
(gage) ⫺ return-line pressure with with trap valve closed, lb/in2
(gage)].
Assume ⌬p ⫽ 20 lb/in2
(gage) (138 kPa) for this trap. The intersection of the
horizontal 8000-lb/h (3600-kg/h) projection and the vertical 20-lb/in2
(gage)
(137.9-kPa) projection is on the sawtooth capacity curve for a trap having a 9
⁄16-in
(14.3-mm) diameter orifice. If these projections intersected beneath this curve, a
9
⁄16-in (14.3-mm) orifice would still be used if the point were between the verticals
for this size orifice.
The dashed lines extending downward from the sawtooth curves show the ca-
pacity of a trap at reduced ⌬p. Thus, the capacity of a trap with a 3
⁄8-in (9.53-mm)
orifice at ⌬p ⫽ 30 lb/in2
(gage) (207 kPa) is 6200 lb/h (2790 kg/h), read at the
intersection of the 30-lb/in2
(gage) (207-kPa) ordinate and the dashed curve ex-
tended from the 3
⁄8-in (9.53-mm) solid curve.
To select an impulse trap from Table 11, enter the table at the trap inlet pressure,
say 125 lb/in2
(gage) (862 kPa), and project to the desired capacity, say 8000 lb/
h (3600 kg/h), determined from step 1. Table 11 shows that a 2-in (50.8-mm) trap
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![8.26 PLANT AND FACILITIES ENGINEERING
FIGURE 7 Capacities of one type of bucket steam trap.
(Armstrong Machine Works.)
TABLE 11 Capacities of Impulse Traps, lb/h (kg/h)
[Maximum continuous discharge of condensate, based on
condensate at 30⬚F (16.7⬚C) below steam temperature.]
having an 8530-lb/h (3839-kg/h) capacity must be used because the next smallest
size has a capacity of 5165 lb/h (2324 kg/h). This capacity is less than that re-
quired.
Some trap manufacturers publish capacity tables relating various trap models to
specific types of equipment. Such tables simplify trap selection, but the condensate
rate must still be computed as given here.
Related Calculations. Use the procedure given here to determine the trap ca-
pacity required for any industrial, commercial, or domestic application including
acid vats, air dryers, asphalt tanks, autoclaves, baths (dyeing), belt presses, bleach
tanks, blenders, bottle washers, brewing kettles, cabinet dryers, calenders, can wash-
ers, candy kettles, chamber dryers, chambers (reaction), cheese kettles, coils (cook-
ing, kettle, pipe, tank, tank-car), confectioners’ kettles, continuous dryers, conveyor
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![PIPING AND FLUID FLOW 8.27
dyers, cookers (nonpressure and pressure), cooking coils, cooking kettles, cooking
tanks, cooking vats, cylinder dryers, cylinders (jacketed), double-drum dryers, drum
dryers, drums (dyeing), dry cans, dry kilns, dryers (cabinet, chamber, continuous,
conveyor, cylinder, drum, festoon, jacketed, linoleum, milk, paper, pulp, rotary,
shelf, stretch, sugar, tray, tunnel), drying rolls, drying rooms, drying tables, dye
vats, dyeing baths and drums, dryers (package), embossing-press platens, evapo-
rators, feed waterheaters, festoon dryers, fin-type heaters, fourdriniers, fuel-oil pre-
heaters, greenhouse coils, heaters (steam), heat exchangers, heating coils and ket-
tles, hot-break tanks, hot plates, kettle coils, kettles (brewing, candy, cheese,
confectioners’, cooking, heating, process), kiers, kilns (dry), liquid heaters, mains
(steam), milk-bottle washers, milk-can washers, milk dryers, mixers, molding press
platens, package dryers, paper dryers, percolators, phonograph-record press platens,
pipe coils (still- and circulating-air), platens, plating tanks, plywood press platens,
preheaters (fuel-oil), preheating tanks, press platens, pressure cookers, process ket-
tles, pulp dryers, purifiers, reaction chambers, retorts, rotary dryers, steam mains
(risers, separators), stocking boarders, storage-tank coils, storage water heaters,
stretch dryers, sugar dryers, tank-car coils, tire-mold presses, tray dryers, tunnel
dryers, unit heaters, vats, veneer press platens, vulcanizers, and water stills. Hospital
equipment—such as autoclaves and sterilizers—can be analyzed in the same way,
as can kitchen equipment—bain marie, compartment cooker, egg boiler, kettles,
steam table, and urns; and laundry equipment—blanket dryers, curtain dryers, flat-
work ironers, presses (dry-cleaning, laundry) sock forms, starch cookers, tumblers,
etc.
When using a trap capacity diagram or table, be sure to determine the basis on
which it was prepared. Apply any necessary correction factors. Thus, cold-water
capacity ratings must be corrected for traps operating at higher condensate tem-
peratures. Correction factors are published in trap engineering data. The capacity
of a trap is greater at condensate temperatures less than 212⬚F (100⬚C) because at
or above this temperature condensate forms flash steam when it flows into a pipe
or vessel at atmospheric [14.7 lb/in2
(abs) (101.3 kPa)] pressure. At altitudes above
sea level, condensate flashes into steam at a lower temperature, depending on the
altitude.
The method presented here is the work of L. C. Campbell, Yarway Corporation,
as reported in Chemical Engineering.
SELECTING HEAT INSULATION FOR HIGH-
TEMPERATURE PIPING
Select the heat insulation for a 300-ft (91.4-m) long 10-in (254-mm) turbine lead
operating at 570⬚F (299⬚C) for 8000 h/year in a 70⬚F (21.1⬚C) turbine room. How
much heat is saved per year by this insulation? The boiler supplying the turbine
has an efficiency of 80 percent when burning fuel having a heating value of 14,000
Btu/lb (32.6 MJ/kg). Fuel costs $6 per ton ($5.44 per metric ton). How much
money is saved by the insulation each year? What is the efficiency of the insulation?
Calculation Procedure:
1. Choose the type of insulation to use
Refer to an insulation manufacturer’s engineering data or Crocker and King—
Piping Handbook for recommendations about a suitable insulation for a pipe op-
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![8.28 PLANT AND FACILITIES ENGINEERING
TABLE 12 Recommended Insulation Thickness
erating in the 500 to 600⬚F (260 to 316⬚C) range. These references will show that
calcium silicate is a popular insulation for this temperature range. Table 12 shows
that a thickness of 3 in (76.2 mm) is usually recommended for 10-in (254-mm)
pipe operating at 500 to 599⬚F (260 to 315⬚C).
2. Determine heat loss through the insulation
Refer to an insulation manufacturer’s engineering data to find the heat loss through
3-in (76.2-mm) thick calcium silicate as 0.200 Btu/(h 䡠 ft2
䡠 ⬚F) [1.14 W/(m2
䡠 ⬚C)].
Since 10-in (254-mm) pipe has an area of 2.817 ft2
/ft (0.86 m2
/m) of length and
since the temperature difference across the pipe is 570 ⫺ 70 ⫽ 500⬚F (260⬚C), the
heat loss per hour ⫽ (0.200)(2.817)(50)⫽ 281.7 Btu/(h 䡠 ft) (887.9 W/m2
). The
heat loss from bare 10-in (254-mm) pipe with a 500⬚F (260⬚C) temperature differ-
ence is, from an insulation manufacturer’s engineering data, 4.640 Btu/(h 䡠 ft2
䡠 ⬚F)
[26.4 W/(m2
䡠 ⬚C)], or (4.64)(2.817)(500) ⫽ 6510 Btu/(h 䡠 ft) (6.3 kW/m).
3. Determine annual heat saving
The heat saved ⫽ bare-pipe loss, Btu/h ⫺ insulated-pipe loss, Btu/
h ⫽ 6510 ⫺ 281.7 ⫽ 6228.3 Btu/(h 䡠 ft) (5989 W/m) of pipe. Since the pipe is
300 ft (91.4 m) long and operates 8000 h per year, the annual heat
saving ⫽ (300)(8000)(6228.3) ⫽ 14,940,000,000 Btu/year (547.4 kW).
4. Compute the money saved by the heat insulation
The heat saved in fuel as fired ⫽ (annual heat saving, Btu/year)/(boiler
efficiency) ⫽ 14,940,000,000/0.80 ⫽ 18,680,000,000 Btu/year (5473 MW). Weight
of fuel saved ⫽ (annual heat saving, Btu/year)/(heating value of fuel, Btu/lb)(2000
lb/ton) ⫽ 18,680,000,000/[(14,000)(2000)] ⫽ 667 tons (605 t). At $6 per ton
($5.44 per metric ton), the monetary saving is ($6)(667) ⫽ $4002 per year.
5. Determine the insulation efficiency
Insulation efficiency ⫽ (bare-pipe loss ⫺ insulated-pipe loss)/bare pipe loss, all
expressed in Btu/h, or bare-pipe loss ⫽ (6510.0 ⫺ 281.7)/6510.0 ⫽ 0.957, or 95.7
percent.
Related Calculations. Use this method for any type of insulation—magnesia,
fiber-glass, asbestos, felt, diatomaceous, mineral wool, etc.—used for piping at el-
evated temperatures conveying steam, water, oil, gas, or other fluids or vapors. To
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![PIPING AND FLUID FLOW 8.29
coordinate and simplify calculations, become familiar with the insulation tables in
a reliable engineering handbook or comprehensive insulation catalog. Such famil-
iarity will simplify routine calculations.
ORIFICE METER SELECTION FOR A STEAM PIPE
Steam is metered with an orifice meter in a 10-in (254-mm) boiler lead having an
internal diameter of dp ⫽ 9.760 in (247.9 mm). Determine the maximum rate of
steam flow that can be measured with a steel orifice plate having a diameter of
d0 ⫽ 5.855 in (148.7 mm) at 70⬚F (21.1⬚C). The upstream pressure tap is 1D ahead
of the orifice, and the downstream tap is 0.5D past the orifice. Steam pressure at
the orifice inlet pp ⫽ 250 lb/in2
(gage) (1724 kPa), temperature is 640⬚F (338⬚C).
A differential gage fitted across the orifice has a maximum range of 120 in (304.8
cm) of water. What is the steam flow rate when the observed differential pressure
is 40 in (101.6 cm) of water? Use the ASME Research Committee on Fluid Meters
method in analyzing the meter. Atmospheric pressure is 14.696 lb/in2
(abs) (101.3
kPa).
Calculation Procedure:
1. Determine the diameter ratio and steam density
For any orifice, meter, diameter ratio ⫽  ⫽ meter orifice diameter, in/pipe internal
diameter, in ⫽ 5.855/9.760 ⫽ 0.5999.
Determine the density of the steam by entering the superheated steam table at
250 ⫹ 14.696 ⫽ 264.696 lb/in2
(abs) (1824.8 kPa) and 640⬚F (338⬚C) and reading
the specific volume as 2.387 ft3
/lb (0.15 m3
/kg). For steam, the density ⫽ 1/
specific volume ⫽ ds ⫽ 1/2.387 ⫽ 0.4193 lb/ft3
(6.7 kg/m3
).
2. Determine the steam viscosity and meter flow coefficient
From the ASME publication, Fluid Meters—Their Theory and Application, the
steam viscosity gu1 for a steam system operating at 640⬚F (338⬚C) is
gu1 ⫽ 0.0000141 in 䡠 lb/(⬚F 䡠 s 䡠 ft2
) [0.000031 N 䡠 m/(⬚C 䡠 s 䡠 m2
)].
Find the flow coefficient K from the same ASME source by entering the 10-in
(254-mm) nominal pipe diameter table at  ⫽ 0.5999 and projecting to the appro-
priate Reynolds number column. Assume that the Reynolds number ⫽ 107
, ap-
proximately, for the flow conditions in this pipe. Then K ⫽ 0.6486. Since the
Reynolds number for steam pressures above 100 lb/in2
(689.4 kPa) ranges from
106
to 107
, this assumption is safe because the value of K does not vary appreciably
in this Reynolds number range. Also, the Reynolds number cannot be computed
yet because the flow rate is unknown. Therefore, assumption of the Reynolds num-
ber is necessary. The assumption will be checked later.
3. Determine the expansion factor and the meter area factor
Since steam is a compressible fluid, the expansion factor Y1 must be determined.
For superheated steam, the ratio of the specific heat at constant pressure cp to the
specific heat at constant volume is k ⫽ ⫽ 1.3. Also, the ratio of the
c c /c
v p v
differential maximum pressure reading hw, in of water, to the maximum pressure
in the pipe, lb/in2
(abs) ⫽ 120/246.7 ⫽ 0.454. From the expansion-factor curve in
the ASME Fluid Meters, Y1 ⫽ 0.994 for  ⫽ 0.5999 and the pressure ratio ⫽ 0.454.
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![8.30 PLANT AND FACILITIES ENGINEERING
And, from the same reference, the meter area factor Fa ⫽ 1.0084 for a steel meter
operating at 640⬚F (338⬚C).
4. Compute the rate of steam flow
For square-edged orifices, the flow rate, lb/s ⫽ w ⫽ 0.0997Fa Kd2
Y1(hw ds )0.5
⫽
(0.0997)(1.0084)(0.6486)(5.855)2
(0.994)(120 ⫻ 0.4188)0.5
⫽ 15.75 lb/s (7.1 kg/s).
5. Compute the Reynolds number for the actual flow rate
For any steam pipe, the Reynolds number R ⫽ 48w/(dp gu1) ⫽ 48(15.75)/
[(3.1416)(0.760)(0.0000141)] ⫽ 1,750,000.
6. Adjust the flow coefficient for the actual Reynolds number
In step 2, R ⫽ 107
was assumed and K ⫽ 0.6486. For R ⫽ 1,750,000, K ⫽ 0.6489,
from ASME Fluid Meters, by interpolation. Then the actual flow rate
wh ⫽ (computed flow rate)(ratio of flow coefficients based on assumed and actual
Reynolds numbers) ⫽ (15.75)(0.6489/0.6486)(3.600) ⫽ 56,700 lb/h (25,515 kg/
h), closely, where the value 3600 is a conversion factor for changing lb/s to lb/h.
7. Compute the flow rate for a specific differential gage deflection
For a 40-in (101.6-cm) H2O deflection, Fa is unchanged and equals 1.0084. The
expansion factor changes because hw /pp ⫽ 40/264.7 ⫽ 0.151. From the ASME
Fluid Meters, Y1 ⫽ 0.998. By assuming again that R ⫽ 107
, K ⫽ 0.6486, as before,
w ⫽ (0.0997) (1.0084)(0.6486)(5.855)2
(0.998)(40 ⫻ 0.4188)0.5
⫽ 9.132 lb/s (4.1
kg/s). Computing the Reynolds number as before, gives R ⫽ (40)(0.132)/
[(3.1416)(0.76)(0.0000141)] ⫽ 1,014,000. The value of K corresponding to this
value, as before, is from ASME—Fluid Meters: K ⫽ 0.6497. Therefore, the flow
rate for a 40 in (101.6 cm) H2O reading, in lb/h ⫽ wh ⫽ (0.132)(0.6497/
0.6486)(3600) ⫽ 32,940 lb/h (14,823 kg/h).
Related Calculations. Use these steps and the ASME Fluid Meters or com-
prehensive meter engineering tables giving similar data to select or check an orifice
meter used in any type of steam pipe—main, auxiliary, process, industrial, marine,
heating, or commercial, conveying wet, saturated, or superheated steam.
SELECTION OF A PRESSURE-REGULATING
VALVE FOR STEAM SERVICE
Select a single-seat spring-loaded diaphragm-actuated pressure-reducing valve to
deliver 350 lb/h (158 kg/h) of steam at 50 lb/in2
(gage) (344.7 kPa) when the
initial pressure is 225 lb/in2
(gage) (1551 kPa). Also select an integral pilot-
controlled piston-operated single-seat pressure-regulating valve to deliver 30,000
lb/h (13,500 kg/h) of steam at 40 lb/in2
(gage) (275.8 kPa) with an initial pressure
of 225 lb/in2
(gage) (1551 kPa) saturated. What size pipe must be used on the
downstream side of the valve to produce a velocity of 10,000 ft/min (3048 m/
min)? How large should the pressure-regulating valve be if the steam entering the
valve is at 225 lb/in2
(gage) (1551 kPa) and 600⬚F (316⬚C)?
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![PIPING AND FLUID FLOW 8.33
from the inlet pressure. Data in this calculation procedure are based on valves built
by the Clark-Reliance Corporation, Cleveland, Ohio.
Some valve manufacturers use the valve flow coefficient for valve sizing.
Cv
This coefficient is defined as the flow rate, lb/h, through a valve of given size when
the pressure loss across the valve is 1 lb/in2
(6.89 kPa). Tabulations like Tables 13
and 14 incorporate this flow coefficient and are somewhat easier to use. These tables
make the necessary allowances for downstream pressure less than the critical pres-
sure (⫽ 0.55 ⫻ absolute upstream pressure, lb/in2
, for superheated steam and hy-
drocarbon vapors; and 0.58 ⫻ absolute upstream pressure, lb/in2
, for saturated
steam). The accuracy of these tabulations equals that of valve sizes determined by
using the flow coefficient.
HYDRAULIC RADIUS AND LIQUID VELOCITY IN
WATER PIPES
What is the velocity of 1000 gal/min (63.1 L/s) of water flowing through a 10-in
(254-mm) inside-diameter cast-iron water main? What is the hydraulic radius of
this pipe when it is full of water? When the water depth is 8 in (203.2 mm)?
Calculation Procedure:
1. Compute the water velocity in the pipe
For any pipe conveying water, the liquid velocity is v ft/s ⫽ gal/min/(2.448d2
),
where d ⫽ internal pipe diameter, in. For this pipe, v ⫽ 1000/[2.448(10)] ⫽ 4.08
ft/s (1.24 m/s), or (60)(4.08) ⫽ 244.8 ft/min (74.6 m/min).
2. Compute the hydraulic radius for a full pipe
For any pipe, the hydraulic radius is the ratio of the cross-sectional area of the pipe
to the wetted perimeter, or d /4. For this pipe, when full of water, the hydraulic
radius ⫽ 10/4 ⫽ 2.5.
3. Compute the hydraulic radius for a partially full pipe
Use the hydraulic radius tables in King and Brater—Handbook of Hydraulics, or
compute the wetted perimeter by using the geometric properties of the pipe, as in
step 2. From the King and Brater table, the hydraulic radius ⫽ Fd, where F ⫽ table
factor for the ratio of the depth of water, in/diameter of channel, in ⫽ 8/10 ⫽ 0.8.
For this ratio, F ⫽ 0.304. Then, hydraulic radius ⫽ (0.304)(10) ⫽ 3.04 in (77.2
mm).
Related Calculations. Use this method to determine the water velocity and
hydraulic radius in any pipe conveying cold water—water supply, plumbing, pro-
cess, drain, or sewer.
FRICTION-HEAD LOSS IN WATER PIPING OF
VARIOUS MATERIALS
Determine the friction-head loss in 2500 ft (762 m) of clean 10-in (254-mm) new
tar-dipped cast-iron pipe when 2000 gal/min (126.2 L/s) of cold water is flowing.
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![8.34 PLANT AND FACILITIES ENGINEERING
TABLE 16 Values of C in Hazen-Williams Formula
What is the friction-head loss 20 years later? Use the Hazen-Williams and Manning
formulas, and compare the results.
Calculation Procedure:
1. Compute the friction-head loss by the Hazen-Williams formula
The Hazen-Williams formula is ⫽ [v /(1.318 where ⫽ friction-
0.63 1.85
h CR )] , h
ƒ h ƒ
head loss per ft of pipe, ft of water; v ⫽ water velocity, ft/s; C ⫽ a constant
depending on the condition and kind of pipe; Rh ⫽ hydraulic radius of pipe, ft.
For a water pipe, v ⫽ gal/min/(2.44d2
); for this pipe, v ⫽ 2000/
[2.448(10)2
] ⫽ 8.18 ft/s (2.49 m/s). From Table 16 or Crocker and King—Piping
Handbook, C for new pipe ⫽ 120; for 20-year-old pipe, C ⫽ 90; Rh ⫽ d /4 for a
full-flow pipe ⫽ 10/4 ⫽ 2.5 in, or 2.5/12 ⫽ 0.208 ft (63.4 mm). Then
⫽ [8.18/(1.318 ⫻ 120 ⫻ 0.208 ⫽ 0.0263 ft (8.0 mm) of water per ft (m)
0.63 1.85
h )]
ƒ
of pipe. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽
2500(0.0263) ⫽ 65.9 ft (20.1 m) of water for the new pipe.
For 20-year-old pipe and the same formula, except with C ⫽ 90, ⫽ 0.0451
hƒ
ft (13.8 mm) of water per ft (m) of pipe. For 2500 ft (762 m) of pipe, the total
friction-head loss ⫽ 2500(0.0451) ⫽ 112.9 ft (34.4 m) of water. Thus, the friction-
head loss nearly doubles [from 65.9 to 112.9 ft (20.1 to 34.4 m)] in 20 years. This
shows that it is wise to design for future friction losses; otherwise, pumping equip-
ment may become overloaded.
2. Compute the friction-head loss from the Manning formula
The Manning formula is where n ⫽ a constant depending on
2 2 4 / 3
h ⫽ n v /2.208R ,
ƒ h
the condition and kind of pipe, other symbols as before.
Using n ⫽ 0.011 for new coated cast-iron pipe from Table 17 or Crocker and
King—Piping Handbook, we find ⫽ (0.011)2
(8.18)2
/[2.208 ⫽ 0.0295
4 / 3
h (0.208) ]
ƒ
ft (8.9 mm) of water per ft (m) of pipe. For 2500 ft (762 m) of pipe, the total
friction-head loss ⫽ 2500(0.0295) ⫽ 73.8 ft (22.5 m) of water, as compared with
65.9 ft (20.1 m) of water computed with the Hazen-Williams formula.
For coated cast-iron pipe in fair condition, n ⫽ 0.013, and ⫽ 0.0411 ft (12.5
hƒ
mm) of water. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽
2500(0.0411) ⫽ 102.8 ft (31.3 m) of water, as compared with 112.9 ft (34.4 m) of
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![PIPING AND FLUID FLOW 8.43
FIGURE 12 Complex series pipeline.
2. Determine the throat-to-pipe diameter ratio
The throat-to-pipe diameter ratio  ⫽ 4.023/6.094 ⫽ 0.6602. Then 1/(1 ⫺ 4
)0.5
and 1/(1 ⫺ 0.66024
)0.5
⫽ 1.1111.
3. Assume a Reynolds number value, and compute the flow rate
The flow equation for a Venturi tube is w lb/h ⫽ 359.0(Cd2
/ ,
4 0.5
兹1 ⫺  )(w h )
dp w
where C ⫽ meter discharge coefficient, expressed as a function of the Reynolds
number; wdp ⫽ density of the water at the pipe temperature, lb/ft3
. With a Reynolds
number greater than 250,000, C is a constant. As a first trial, assume R ⬎ 250,000
and C ⫽ 0.984 from Fluid Meters. Then w ⫽ 359.0(0.984)(4.023)2
(1.1111)
(62.426 ⫻ 126.8)0.5
⫽ 565,020 lb/h (254,259 kg/h), or 565,020/8.33 lb/gal ⫽
67,800 gal/h (71.3 L/s), or 67,800/60 min/h ⫽ 1129 gal/min (71.23 L/s).
4. Check the discharge coefficient by computing the Reynolds numbers
For a water pipe, R ⫽ 48ws /(dp gu), where ws ⫽ flow rate, lb/s ⫽ w /3600;
u ⫽ coefficient of absolute viscosity. Using Fluid Meters data for water at 41⬚F
(5⬚C), we find R ⫽ 48(156.95)/[( ⫻ 6.094)(0.001004)] ⫽ 391,900. Since C is
constant for R ⬎ 250,000, use of C ⫽ 0.984 is correct, and no adjustment in the
computations is necessary. Had the value of C been incorrect, another value would
be chosen and the Reynolds number recomputed. Continue this procedure until a
satisfactory value for C is obtained.
5. Use an alternative solution to check the results
Fluid Meters gives another equation for Venturi meter flow rate, that is w lb/
s ⫽ 0.525(Cd2
/ where p1 ⫺ p2 is the manometer dif-
4 0.5
兹1 ⫺  )[w (p ⫺ p )] ,
dp 1 2
ferential pressure in lb/in2
. Using the conversion factor in Fluid Meters for con-
verting in of mercury under water at 77⬚F (25⬚C) to lb/in2
(kPa), we get p1 ⫺
p2 ⫽ (10.1)(0.4528) ⫽ 4.573 lb/in2
(31.5 kPa). Then w ⫽ (0.525)(0.984)
(4.023)2
(1.1111)(62.426 ⫻ 4.573)0.5
⫽ 156.9 lb/s (70.6 kg/s), or (156.9)(3600 s/
h) ⫽ 564,900 lb/h (254,205 kg/h), or 564,900/8.33 lb/gal ⫽ 67,800 gal/h (71.3
L/s), or 67,800/60 min/h ⫽ 1129 gal/min (71.2 L/s). This result agrees with that
computed in step 3 within 1 part in 5600. This is much less than the probable
uncertainties in the values of the discharge coefficient and the differential pressure.
Related Calculations. Use this method for any Venturi tube serving cold-water
piping in process, industrial, water-supply, domestic, or commercial service.
EQUIVALENT LENGTH OF A COMPLEX
SERIES PIPELINE
Figure 12 shows a complex series pipeline made up of four lengths of different
size pipe. Determine the equivalent length of this pipe if each size of pipe has the
same friction factor.
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![PIPING AND FLUID FLOW 8.45
FIGURE 13 Parallel piping system.
its value need not be exact or even approximate. Assume a total head loss of 50 ft
of water for each pipe in this system.
2. Compute the flow rate in each pipe in the system
Assume that the roughness coefficient C in the Hazen-Williams formula is equal
for each of the pipes in the system. This is a valid assumption. Using the assumed
value of C, compute the flow rate in each pipe. To allow for possible tuberculation
of the pipe, assume that C ⫽ 100.
The Hazen-Williams formula is given in a previous calculation procedure and
can be used to solve for the flow rate in each pipe. A more rapid way to make the
computation is to use the friction-loss tabulations for the Hazen-Williams formula
in Crocker and King—Piping Handbook, the Hydraulic Institute—Pipe Friction
Manual, or a similar set of tables.
Using such a set of tables, enter at the friction-head loss equal to 50 ft (15.2 m)
per 5000 ft (1524 m) of pipe for the 6-in (152.4-mm) line. Find the corresponding
flow rate Q gal/min. Using the Hydraulic Institute tables, Qa ⫽ 270 gal/min (17.0
L/s); Qb ⫽ 580 gal/min (36.6 L/s); Qc ⫽ 1000 gal/min (63.1 L/s). Hence, the
total flow ⫽ 270 ⫹ 580 ⫹ 1000 ⫽ 1850 gal/min (116.7 L/s).
3. Find the equivalent size and length of the pipe
Using the Hydraulic Institute tables again, look for a pipe having a 50-ft (15.2 m)
head loss with a flow of 1850 gal/min (116.7 L/s). Any pipe having a discharge
equal to the sum of the discharge rates for all the pipes, at the assumed friction
head, is an equivalent pipe.
Interpolating friction-head values in the 14-in (355.6-mm) outside-diameter
[13.126-in (333.4-mm) inside-diameter] table shows that 5970 ft (1820 m) of this
pipe is equivalent to the system in Fig. 13. This equivalent size can be used in any
calculations related to this system—selection of a pump, determination of head loss
with longer or shorter mains, etc. If desired, another equivalent-size pipe could be
found by entering a different pipe-size table. Thus, 5310 ft (1621.5 m) of 14-in
(355.6-mm) pipe [12.814-in (326.5-mm) inside diameter] is also equivalent to this
system.
Related Calculations. Use this procedure for any liquid—water, oil, gasoline,
brine—flowing through a parallel piping system. The pipes are assumed to be full
at all times.
MAXIMUM ALLOWABLE HEIGHT FOR A
LIQUID SIPHON
What is the maximum height h ft (m), Fig. 14, that can be used for a siphon in a
water system if the length of the pipe from the water source to its highest point is
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![8.46 PLANT AND FACILITIES ENGINEERING
FIGURE 14 Liquid siphon piping system.
500 ft (152.4 m), the water velocity is 13.0 ft/s (3.96 m/s), the pipe diameter is
10 in (254 mm), and the water temperature is 70⬚F (21.1⬚C) if 3200 gal/min (201.9
L/s) is flowing?
Calculation Procedure:
1. Compute the velocity of the water in the pipe
From an earlier calculation procedure, v ⫽ gpm /(2.448d2
). With an internal
diameter of 10.020 in (254.5 mm), v ⫽ 3200/[(2.448)(10.02)2
] ⫽ 13.0 ft/s (3.96
m/s).
2. Determine the vapor pressure of the water
Using a steam table, we see that the vapor pressure of water at 70⬚F (21.1⬚C) is
⫽ 0.3631 lb/in2
(abs) (2.5 kPa), or (0.3631) (144 in2
/ft2
) ⫽ 52.3 lb/ft2
(2.5
pv
kPa). The specific volume of water at 70⬚F (21.1⬚C) is, from a steam table, 0.01606
ft3
/lb (0.001 m3
/kg). Converting this to density at 70⬚F (21.1⬚C), density ⫽
1/0.01606 ⫽ 62.2 lb/ft3
(995.8 kg/m3
). The vapor pressure in ft of 70⬚F (21.1⬚C)
water is then ⫽ (52.3 lb/ft2
)/(62.2 lb/ft3
) ⫽ 0.84 ft (0.26 m) of water.
ƒv
3. Compute or determine the friction-head loss and velocity head
From the reservoir to the highest point of the siphon, B, Fig. 14, the friction head
in the pipe must be overcome. Use the Hazen-Williams or a similar formula to
determine the friction head, as given in earlier calculation procedures or a pipe-
friction table. From the Hydraulic Institute Pipe Friction Manual, ⫽ 4.59 ft per
hƒ
100 ft (1.4 m per 3.5 m), or (500/100)(4.59) ⫽ 22.95 ft (7.0 m). From the same
table, velocity head ⫽ 2.63 ft/s (0.8 m/s).
4. Determine the maximum height for the siphon
For a siphon handling water, the maximum allowable height h at sea level with an
atmospheric pressure of 14.7 lb/in2
(abs) (101.3 kPa) ⫽ [14.7 ⫻ (144 in2
/ft2
)/
(density of water at operating temperature, lb/ft3
) ⫺ (vapor pressure of water at
operating temperature, ft ⫹ 1.5 ⫻ velocity head, ft ⫹ friction head, ft)]. For this
pipe, h ⫽ 14.7 ⫻ 144/62.2 ⫺ (0.84 ⫹ 1.5 ⫻ 2.63 ⫹ 22.95) ⫽ 11.32 ft (3.45 m).
In actual practice, the value of h is taken as 0.75 to 0.8 the computed value. Using
0.75 gives h ⫽ (0.75)(11.32) ⫽ 8.5 ft (2.6 m).
Related Calculations. Use this procedure for any type of siphon conveying a
liquid—water, oil, gasoline, brine, etc. Where the liquid has a specific gravity dif-
ferent from that of water, i.e., less than or greater than 1.0, proceed as above,
expressing all heads in ft of liquid handled. Divide the resulting siphon height by
the specific gravity of the liquid. At elevations above atmospheric, use the actual
atmospheric pressure instead of 14.7 lb/in2
(abs) (101.3 kPa).
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![PIPING AND FLUID FLOW 8.47
WATER-HAMMER EFFECTS IN LIQUID PIPELINES
What is the maximum pressure developed in a 200-lb/in2
(1378.8-kPa) water pipe-
line if a valve is closed nearly instantly or pumps discharging into the line are all
stopped at the same instant? The pipe is 8-in (203.2-mm) schedule 40 steel, and
the water flow rate is 2800 gal/min (176.7 L/s). What maximum pressure is de-
veloped if the valve closes in 5 s and the line is 5000 ft (1524 m) long?
Calculation Procedure:
1. Determine the velocity of the pressure wave
For any pipe, the velocity of the pressure wave during water hammer is found from
vw ⫽ 4720/(1 ⫹ Kd/Et)0.5
, where vw ⫽ velocity of the pressure wave in the pipe-
line, ft/s; K ⫽ bulk modulus of the liquid in the pipeline ⫽ 300,000 for water;
d ⫽ internal diameter of pipe, in; E ⫽ modulus of elasticity of pipe material, lb/
in2
⫽ 30 ⫻ 106
lb/in2
(206.8 Gpa) for steel; t ⫽ pipe-wall thickness, in. For 8-in
(203.2-mm) schedule 40 steel pipe and data from a table of pipe properties,
vw ⫽ 4720/[1 ⫹ 300,000 ⫻ 7.981/(30 ⫻ 106
⫻ 0.322)]0.5
⫽ 4225.6 ft/s (1287.9
m/s).
2. Compute the pressure increase caused by water hammer
The pressure increase p1 lb/in2
due to water hammer ⫽ vw v /[32.2(2.31)], where
v ⫽ liquid velocity in the pipeline, ft/s; 32.2 ⫽ acceleration due to gravity, ft/s2
;
2.31 ft of water ⫽ 1-lb/in2
(6.9-kPa) pressure.
For this pipe, v ⫽ 0.4085 gpm/d2
⫽ 0.4085(2800)/(7.981)2
⫽ 18.0 ft/s (5.5
m/s). Then pi ⫽ (4225.6)(18)/[32.2(2.31)] ⫽ 1022.56 lb/in2
(7049.5 kPa). The
maximum pressure developed in the pipe is then p1 ⫹ pipe operating
pressure ⫽ 1022.56 ⫹ 200 ⫽ 1222.56 lb/in2
(8428.3 kPa).
3. Compute the hammer pressure rise caused by valve closure
The hammer pressure rise caused by valve closure lb/in2
⫽ 2pi L /vw T, where
pv
L ⫽ pipeline length, ft; T ⫽ valve closing time, s. For this pipeline,
⫽ 2(1022.56)(5000)/[(4225.6)(5)] ⫽ 484 lb/in2
(3336.7 kPa). Thus, the maxi-
pv
mum pressure in the pipe will be 484 ⫹ 200 ⫽ 648 lb/in2
(4467.3 kPa).
Related Calculations. Use this procedure for any type of liquid—water, oil,
etc.—in a pipeline subject to sudden closure of a valve or stoppage of a pump or
pumps. The effects of water hammer can be reduced by relief valves, slow-closing
check valves on pump discharge pipes, air chambers, air spill valves, and air in-
jection into the pipeline.
SPECIFIC GRAVITY AND VISCOSITY OF LIQUIDS
An oil has a specific gravity of 0.8000 and a viscosity of 200 SSU (Saybolt Seconds
Universal) at 60⬚F (15.6⬚C). Determine the API gravity and Bé gravity of this oil
and its weight in lb/gal (kg/L). What is the kinematic viscosity in cSt? What is
the absolute viscosity in cP?
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![8.48 PLANT AND FACILITIES ENGINEERING
Calculation Procedure:
1. Determine the API gravity of the liquid
For any oil at 60⬚F (15.6⬚C), its specific gravity S, in relation to water at 60⬚F
(15.6⬚C), is S ⫽ 141.5/(131.5 ⫹ ⬚API); or ⬚API ⫽ (141.5 ⫺ 131.5S)/S. For this
oil, ⬚API ⫽ [141.5 ⫺ 131.5(0.80)]/0.80 ⫽ 45.4 ⬚API.
2. Determine the Bé gravity of the liquid
For any liquid lighter than water, S ⫽ 140/(130 ⫹ Bé); or Bé ⫽ (140 ⫺ 130S)/S.
For this oil, Bé ⫽ [140 ⫺ 130(0.80)]/0.80 ⫽ 45 Bé.
3. Compute the weight per gal of liquid
With a specific gravity of S, the weight of 1 ft3
of oil ⫽ (S)[weight of 1 ft3
(1 m3
)
of fresh water at 60⬚F (15.6⬚C)] ⫽ (0.80)(62.4) ⫽ 49.92 lb/ft3
(799.2 kg/m3
). Since
1 gal (3.8 L) of liquid occupies 0.13368 ft3
the weight of this oil is
(49.92)(0.13368) ⫽ 6.66 lb/gal (0.79 kg/L).
4. Compute the kinematic viscosity of the liquid
For any liquid having an SSU viscosity greater than 100 s, the kinematic viscosity
k ⫽ 0.220 (SSU) ⫽ 135/SSU cSt. For this oil, k ⫽ 0.220(200) ⫺ 135/200 ⫽ 43.325
cSt.
5. Convert the kinematic viscosity to absolute viscosity
For any liquid, the absolute viscosity, cP ⫽ (kinematic viscosity, cSt)(density).
Thus, for this oil, the absolute viscosity ⫽ (43.325)(49.92) ⫽ 2163 cP.
Related Calculations. For liquids heavier than water, S ⫽ 145/(145 ⫺ Bé).
When the SSU viscosity is between 32 and 99 SSU, k ⫽ 0.226 (SSU) ⫺ 195/SSU
cSt. Modern terminology for absolute viscosity is dynamic viscosity. Use these
relations for any liquid—brine, gasoline, crude oil, kerosene, Bunker C, diesel oil,
etc. Consult the Pipe Friction Manual and Crocker and King—Piping Handbook
for tabulations of typical viscosities and specific gravities of various liquids.
PRESSURE LOSS IN PIPING HAVING
LAMINAR FLOW
Fuel oil at 300⬚F (148.9⬚C) and having a specific gravity of 0.850 is pumped
through a 30,000-ft (9144-m) long 24-in (609.6-mm) pipe at the rate of 500 gal/
min (31.6 L/s). What is the pressure loss if the viscosity of the oil is 75 cP (0.075
Pa 䡠 s)?
Calculation Procedure:
1. Determine the type of flow that exists
Flow is laminar (also termed viscous) if the Reynolds number R for the liquid in
the pipe is less than 1200. Turbulent flow exists if the Reynolds number is greater
than 2500. Between these values is a zone in which either condition may exist,
depending on the roughness of the pipe wall, entrance conditions, and other factors.
Avoid sizing a pipe for flow in this critical zone because excessive pressure drops
result without a corresponding increase in the pipe discharge.
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![PIPING AND FLUID FLOW 8.49
TABLE 20 Reynolds Number
Compute the Reynolds number from R ⫽ 3.162G/kd, where G ⫽ flow rate gal/
min (L/s); k ⫽ kinematic viscosity of liquid, cSt ⫽ viscosity z, cP/specific gravity
of the liquid S; d ⫽ inside diameter of pipe, in (cm). From a table of pipe properties,
d ⫽ 22.626 in (574.7 mm). Also, k ⫽ z/S ⫽ 75/0.85 ⫽ 88.2 cSt. Then
R ⫽ 3162(500)/[88.2(22.626)] ⫽ 792. Since R ⬍ 1200, laminar flow exists in this
pipe.
2. Compute the pressure loss by using the Poiseuille formula
The Poiseuille formula gives the pressure drop pd lb/in2
(kPa) ⫽ 2.73(10⫺4
)luG/
d4
, where l ⫽ total length of pipe, including equivalent length of fittings, ft;
u ⫽ absolute viscosity of liquid, cP (Pa 䡠 s); G ⫽ flow rate gal/min (L/s); d ⫽ inside
diameter of pipe, in (cm). For this pipe, pd ⫽ 2.73(10⫺4
)(10,000)(75)(500)/
262,078 ⫽ 1.17 lb/in2
(8.1 kPa).
Related Calculations. Use this procedure for any pipe in which there is laminar
flow of the liquid. Other liquids for which this method can be used include water,
molasses, gasoline, brine, kerosene, and mercury. Table 20 gives a quick summary
of various ways in which the Reynolds number can be expressed. The symbols in
Table 20, in the order of their appearance, are D ⫽ inside diameter of pipe, ft (m);
v ⫽ liquid velocity, ft/s (m/s); p ⫽ liquid density, lb/ft3
(kg/m3
); ⫽ absolute
viscosity of liquid, lb mass/(ft 䡠 s) [kg/(m 䡠 s)]; d ⫽ inside diameter of pipe, in (cm).
From a table of pipe properties, d ⫽ 22.626 in (574.7 mm). Also, k ⫽ z/S liquid
flow rate, lb/h (kg/h); B ⫽ liquid flow rate, bbl/h (L/s); k ⫽ kinematic viscosity
of the liquid, cSt; q liquid flow rate, ft3
(m3
/s); Q ⫽ liquid flow rate, ft3
/min (m3
/min). Use Table 20 to find the Reynolds number for any liquid flowing through a
pipe.
DETERMINING THE PRESSURE LOSS IN
OIL PIPES
What is the pressure drop in a 5000-ft (1524-m) long 6-in (152.4-mm) oil pipe
conveying 500 bbl/h (22.1 L/s) of kerosene having a specific gravity of 0.813 at
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![8.50 PLANT AND FACILITIES ENGINEERING
65⬚F (18.3⬚C), which is the temperature of the liquid in the pipe? The pipe is
schedule 40 steel.
Calculation Procedure:
1. Determine the kinematic viscosity of the oil
Use Fig. 15 and Table 21 or the Hydraulic Institute—Pipe Friction Manual kine-
matic viscosity and Reynolds number chart to determine the kinematic viscosity of
the liquid. Enter Table 12 at kerosene, and find the coordinates as X ⫽ 10.2,
Y ⫽ 16.9. Using these coordinates, enter Fig. 15 and find the absolute viscosity of
kerosene at 65⬚F (18.3⬚C) as 2.4 cP. By the method of a previous calculation pro-
cedure, the kinematic viscosity ⫽ absolute viscosity, cP/specific gravity of the
liquid ⫽ 2.4/0.813 ⫽ 2.95 cSt. This value agrees closely with that given in the
Pipe Friction Manual.
2. Determine the Reynolds number of the liquid
The Reynolds number can be found from the Pipe Friction Manual chart mentioned
in step 1 or computed from R ⫽ 2214B /(dk) ⫽ 2214(500)/[(6.065)(2.95)] ⫽
61,900.
To use the Pipe Friction Manual chart, compute the velocity of the liquid in the
pipe by converting the flow rate to ft3
/s. Since there is 42 gal/bbl (0.16 L) and 1
gal (0.00379 L) ⫽ 0.13368 ft3
(0.00378 m3
), 1 bbl ⫽ (42)(0.13368) ⫽ 5.6 ft3
(0.16
m3
). With a flow rate of 500 bbl/h (79.5 m3
/h) the equivalent
flow ⫽ (500)(5.6) ⫽ 2800 ft3
/h (79.3 m3
/h), or 2800/3600 s/h ⫽ 0.778 ft3
/s (0.02
m3
/s). Since 6-in (152.4-mm), schedule 40 pipe has a cross-sectional area of 0.2006
ft2
(0.02 m2
) internally, the liquid velocity ⫽ 0.778/0.2006 ⫽ 3.88 ft/s (1.2 m/s).
Then, the product (velocity, ft/s)(internal diameter, in) ⫽ (3.88)(6.065) ⫽ 23.75
ft/s. In the Pipe Friction Manual, project horizontally from the kerosene specific-
gravity curve to the vd product of 23.75, and read the Reynolds number as 61,900,
as before. In general, the Reynolds number can be found more quickly by com-
puting it using the appropriate relation given in an earlier calculation procedure,
unless the flow velocity is already known.
3. Determine the friction factor of this pipe
Enter Fig. 16 at the Reynolds number value of 61,900, and project to the curve 4
as indicated by Table 22. Read the friction factor as 0.0212 at the left. Alternatively,
the Pipe Friction Manual friction-factor chart could be used, if desired.
4. Compute the pressure loss in the pipe
Use the Fanning formula pd ⫽ 1.06(10⫺4
)ƒlB2
/d5
. In this formula, ⫽ density of
the liquid, lb/ft3
. For kerosene, ⫽ (density of water, lb/ft3
)(specific gravity of the
kerosene) ⫽ (62.4)(0.813) ⫽ 50.6 lb/ft3
(810.1 kg/m3
). Then pd ⫽ 1.06(10⫺4
) ⫻
(0.0212)(50.6)(5000)(500)2
/8206 ⫽ 17.3 lb/in2
(119.3 kPa).
Related Calculations. The Fanning formula is popular with oil-pipe designers
and can be stated in various ways: (1) with velocity v ft/s, pd ⫽ 1.29(10⫺3
)ƒV2
l/
d; (2) with velocity V ft/min, pd ⫽ 3.6(10⫺7
)ƒV2
l/d; (3) with flow rate in G gal
/min, pd ⫽ 2.15(10⫺4
)ƒlG2
/d2
; (4) with the flow rate in W lb/h,
pd ⫽ 3.36(10⫺6
)ƒlW2
/d5
.
Use this procedure for any petroleum product—crude oil, kerosene, benzene,
gasoline, naphtha, fuel oil, Bunker C, diesel oil, toluene, etc. The tables and charts
presented here and in the Pipe Friction Manual save computation time.
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![8.56 PLANT AND FACILITIES ENGINEERING
TABLE 23 Gas Constants
FLOW RATE AND PRESSURE LOSS IN
COMPRESSED-AIR AND GAS PIPING
Dry air at 80⬚F (26.7⬚C) and 150 lb/in2
(abs) (1034 kPa) flows at the rate of 500
ft3
/min (14.2 m3
/min) through a 4-in (101.6-mm) schedule 40 pipe from the dis-
charge of an air compressor. What are the flow rate in lb/h and the air velocity in
ft/s? Using the Fanning formula, determine the pressure loss if the total equivalent
length of the pipe is 500 ft (152.4 m).
Calculation Procedure:
1. Determine the density of the air or gas in the pipe
For air or a gas, pV ⫽ MRT, where p ⫽ absolute pressure of the gas, lb/ft2
(abs);
V ⫽ volume of M lb of gas, ft3
; M ⫽ weight of gas, lb; R ⫽ gas constant, ft 䡠 lb/
(lb 䡠 ⬚F); T ⫽ absolute temperature of the gas, ⬚R. For this installation, using 1 ft3
of air, M ⫽ pV /(RT), M ⫽ (150)(144)/[(53.33)(80 ⫹ 459.7)] ⫽ 0.750 lb/ft3
(12.0
kg/m3
). The value of R in this equation was obtained from Table 23.
2. Compute the flow rate of the air or gas
For air or a gas, the flow rate Wh lb/h ⫽ (60) (density, lb/ft3
)(flow rate, ft3
/min);
or Wh ⫽ (60)(0.750)(500) ⫽ 22,500 lb/h (10,206 kg/h).
3. Compute the velocity of the air or gas in the pipe
For any air or gas pipe, velocity of the moving fluid v ft/s ⫽ 183.4 Wh /3600 d2
,
where d ⫽ internal diameter of pipe, in; ⫽ density of fluid, lb/ft3
. For this system,
v ⫽ (183.4)(22,500)/[(3600)(4.026)2
(0.750)] ⫽ 94.3 ft/s (28.7 m/s).
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![PIPING AND FLUID FLOW 8.57
4. Compute the Reynolds number of the air or gas
The viscosity of air at 80⬚F (26.7⬚C) is 0.0186 cP, obtained from Crocker and
King—Piping Handbook, Perry et al.—Chemical Engineers’ Handbook, or a sim-
ilar reference. Then, by using the Reynolds number relation given in Table 20,
R ⫽ 6.32W /(dz) ⫽ (6.32)(22,500)/[(4.026)(0.0186)] ⫽ 1,899,000.
5. Compute the pressure loss in the pipe
Using Fig. 16 or the Hydraulic Institute Pipe Friction Manual, we get ƒ ⫽ 0.0142
to 0.0162 for a 4-in (101.6-mm) schedule 40 pipe when the Reynolds
number ⫽ 3,560,000. From the Fanning formula from an earlier calculation pro-
cedure and the higher value of ƒ, pd ⫽ 3.36(10⫺6
)ƒlW2
/d5
, or
pd ⫽ 3.36(10⫺6
)(0.0162)(500)(22,500)2
/[(4.026)5
(0.750)] ⫽ 17.37 lb/in2
(119.8
kPa).
Related Calculations. Use this procedure to compute the pressure loss, veloc-
ity, and flow rate in compressed-air and gas lines of any length. Gases for which
this procedure can be used include ammonia, carbon dioxide, carbon monoxide,
ethane, ethylene, hydrogen, hydrogen sulfide, isobutane, methane, nitrogen,
n-butane, oxygen, propane, propylene, and sulfur dioxide.
Alternate relations for computing the velocity of air or gas in a pipe are
v ⫽ 144Ws /a; v ⫽ 183.4Ws /d2
; v ⫽ 0.0509 Ws vg /d2
, where Ws ⫽ flow rate,
lb/s; a ⫽ cross-sectional area of pipe, in2
, vg ⫽ specific volume of the air or gas
at the operating pressure and temperature, ft3
/lb.
FLOW RATE AND PRESSURE LOSS IN
GAS PIPELINES
Using the Weymouth formula, determine the flow rate in a 10-mi (16.1-km) long
4-in (101.6-mm) schedule 40 gas pipeline when the inlet pressure is 200 lb/in2
(gage) (1378.8 kPa), the outlet pressure is 20 lb/in2
(gage) (137.9 kPa), the gas
has a specific gravity of 0.80, a temperature of 60⬚F (15.6⬚C), and the atmospheric
pressure is 14.7 lb/in2
(abs) (101.34 kPa).
Calculation Procedure:
1. Compute the flow rate from the Weymouth formula
The Weymouth formula for flow rate is Q ⫽ 28.05[ where
2 2 5.33 0.5
( p ⫺ p )d /sL] ,
i 0
pi ⫽ inlet pressure, lb/in2
(abs); p0 ⫽ outlet pressure, lb/in2
(abs); d ⫽ inside
diameter of pipe, in; s ⫽ specific gravity of gas; L ⫽ length of pipeline, mi. For
this pipe, Q ⫽ 28.05 ⫻ [(214.72
⫺ 34.72
)4.026 ⫽ 86,500 lb/h
5.33 0.5
/0.8 ⫻ 10]
(39,925 kg/h).
2. Determine if the acoustic velocity limits flow
If the outlet pressure of a pipe is less than the critical pressure pc lb/in2
(abs), the
flow rate in the pipe cannot exceed that obtained with a velocity equal to the criti-
cal or acoustic velocity, i.e., the velocity of sound in the gas. For any gas, pc ⫽
Q(Ti )0.5
/d2
C, where Ti ⫽ inlet temperature, ⬚R; C ⫽ a constant for the gas being
considered.
Using C ⫽ 2070 from Table 23, or Crocker and King—Piping Handbook,
pc ⫽ (86,500)(60 ⫹ 460)0.5
/[(4.026)2
(2070)] ⫽ 58.8 lb/in2
(abs) (405.4 kPa). Since
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Handbook_of_Mechanical_Engineering_calcc
- 3. P • A • R • T 1 POWER GENERATION Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 4. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. POWER GENERATION
- 5. 1.3 SECTION 1 MODERN POWER-PLANT CYCLES AND EQUIPMENT CYCLE ANALYSES 1.4 Choosing Best Options for Boosting Combined-Cycle Plant Output 1.4 Selecting Gas-Turbine Heat-Recovery Boilers 1.10 Gas-Turbine Cycle Efficiency Analysis and Output Determination 1.13 Determining Best-Relative-Value of Industrial Gas Turbines Using a Life- Cycle Cost Model 1.18 Tube Bundle Vibration and Noise Determination in HRSGs 1.22 Determining Oxygen and Fuel Input in Gas-Turbine Plants 1.25 Heat-Recovery Steam Generator (HRSG) Simulation 1.28 Predicting Heat-Recovery Steam Generator (HRSG) Temperature Profiles 1.33 Steam Turbogenerator Efficiency and Steam Rate 1.36 Turbogenerator Reheat-Regenerative Cycle Alternatives Analysis 1.37 Turbine Exhaust Steam Enthalpy and Moisture Content 1.42 Steam Turbine No-Load and Partial- Load Steam Flow 1.43 Power Plant Performance Based on Test Data 1.45 Determining Turbogenerator Steam Rate at Various Loads 1.47 Analysis of Reheating-Regenerative Turbine Cycle 1.48 Steam Rate for Reheat-Regenerative Cycle 1.49 Binary Cycle Plant Efficiency Analysis 1.51 CONVENTIONAL STEAM CYCLES 1.53 Finding Cogeneration System Efficiency vs a Conventional Steam Cycle 1.53 Bleed-Steam Regenerative Cycle Layout and T-S Plot 1.55 Bleed Regenerative Steam Cycle Analysis 1.59 Reheat-Steam Cycle Performance 1.62 Mechanical-Drive Steam-Turbine Power-Output Analysis 1.67 Condensing Steam-Turbine Power- Output Analysis 1.69 Steam-Turbine Regenerative-Cycle Performance 1.71 Reheat-Regenerative Steam-Turbine Heat Rates 1.74 Steam Turbine-Gas Turbine Cycle Analysis 1.76 Gas Turbine Combustion Chamber Inlet Air Temperature 1.81 Regenerative-Cycle Gas-Turbine Analysis 1.83 Extraction Turbine kW Output 1.86 STEAM PROPERTIES AND PROCESSES 1.87 Steam Mollier Diagram and Steam Table Use 1.87 Interpolation of Steam Table Values 1.90 Constant-Pressure Steam Process 1.93 Constant-Volume Steam Process 1.95 Constant-Temperature Steam Process 1.97 Constant-Entropy Steam Process 1.99 Irreversible Adiabatic Expansion of Steam 1.101 Irreversible Adiabatic Steam Compression 1.103 Throttling Processes for Steam and Water 1.105 Reversible Heating Process for Steam 1.107 Determining Steam Enthalpy and Quality Using the Steam Tables 1.109 Maximizing Cogeneration Electric- Power and Process-Steam Output 1.110 ECONOMIC ANALYSES OF ALTERNATIVE ENERGY SOURCES 1.112 Choice of Most Economic Energy Source Using the Total-Annual-Cost Method 1.112 Seven Comparison Methods for Energy Source Choice 1.115 Selection of Prime Mover Based on Annual Cost Analyses 1.120 Determining If a Prime Mover Should Be Overhauled 1.122 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 6. 1.4 POWER GENERATION Deaerator H-p turbine H-p steam Fuel I-p turbine L-p turbine I-p steam Generator Gas turbine Air H-p superheater Blowdown Blowdown H-p evaporator I-p suprerheater H-p economizer I-p suprerheater I-p evaporator I-p economizer L-p evaporator L-p economizer I-p pump I-p pump Reheater Hot reheat Cold reheat steam Feedwater pumps L-p steam Generator Cooling tower Makeup water Condensate pumps Deaerator FIGURE 1 155-MW natural-gas-fired gas turbine featuring a dry low NOx combustor (Power). Cycle Analyses CHOOSING BEST OPTION FOR BOOSTING COMBINED-CYCLE PLANT OUTPUT Select the best option to boost the output of a 230-MW facility based on a 155- MW natural-gas-fired gas turbine (GT) featuring a dry low NOx combustor (Fig. 1). The plant has a heat-recovery steam generator (HRSG) which is a triple-pressure design with an integral deaerator. A reheat condensing steam turbine (ST) is used and it is coupled to a cooling-tower/surface-condenser heat sink turbine inlet. Steam conditions are 1450-lb/in2 (gage)/1000⬚F (9991-kPa/538⬚C). Unit ratings are for operation at International Standard Organization (ISO) conditions. Evaluate the var- ious technologies considered for summer peaking conditions with a dry bulb (DB) temperature of 95⬚F and 60 percent RH (relative humidity) (35⬚C and 60 percent RH). The plant heat sink is a four-cell, counterflow, mechanical-draft cooling tower optimized to achieve a steam-turbine exhaust pressure of 3.75 inHg absolute (9.5 cmHg) for all alternatives considered in this evaluation. Base circulating-water sys- tem includes a surface condenser and two 50 percent-capacity pumps. Water- treatment, consumption, and disposal-related O&M (operating & maintenance) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 7. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.5 TABLE 1 Performance Summary for Enhanced-Output Options Measured change from base case Case 1 Evap. cooler Case 2 Mech. chiller Case 3 Absorp. chiller Case 4 Steam injection Case 5 Water injection Case 61 Supp.- fired HRSG Case 72 Supp.- fired HRSG GT output, MW 5.8 20.2 20.2 21.8 15.5 0 0 ST output, MW 0.9 2.4 ⫺2.1 ⫺13 3.7 8 35 Plant aux. load, MW 0.05 4.5 0.7 400 0.2 0.4 1 Net plant output, MW 6.65 18.1 17.4 8.4 19 7.6 34 Net heat rate, Btu/kWh3 15 55 70 270 435 90 320 Incremental costs Change in total water cost, $/h 15 35 35 115 85 35 155 Change in wastewater cost, $/h 1 17 17 2 1 1 30 Change in capital cost/ net output, $/kW 180 165 230 75 15 70 450 1 Partial supplementary firing. 2 Full supplementary firing. 3 Based on lower heating value of fuel. costs for the zero-discharge facility are assumed to be $3/1000 gal ($3/3.8 m3 ) of raw water, $6/1000 gal ($6/3.8 m3 ) of treated demineralized water, and $5/1000 gal ($5/3.8 m3 ) of water disposal. The plant is configured to burn liquid distillate as a backup fuel. Calculation Procedure: 1. List the options available for boosting output Seven options can be developed for boosting the output of this theoretical reference plant. Although plant-specific issues will have a significant effect on selecting an option, comparing performance based on a reference plant, Fig. 1, can be helpful. Table 1 shows the various options available in this study for boosting output. The comparisons shown in this procedure illustrate the characteristics, advantages, and disadvantages of the major power augmentation technologies now in use. Amidst the many advantages of gas turbine (GT) combined cycles (CC) popular today from various standpoints (lower investment than for new greenfield plants, reduced environmental impact, and faster installation and startup), one drawback is that the achievable output decreases significantly as the ambient inlet air tempera- ture increases. The lower density of warm air reduces mass flow through the GT. And, unfortunately, hot weather typically corresponds to peak power loads in many areas. So the need to meet peak-load and power-sales contract requirements causes many power engineers and developers to compensate for ambient-temperature- output loss. The three most common methods of increasing output include: (1) injecting water or steam into the GT, (2) precooling GT inlet air, and/or (3) supplementary firing of the heat-recovery steam generator (HRSG). All three options require sig- nificant capital outlays and affect other performance parameters. Further, the options Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 8. 1.6 POWER GENERATION may uniquely impact the operation and/or selection of other components, including boiler feedwater and condensate pumps, valves, steam turbine/generators, con- densers, cooling towers, and emissions-control systems. 2. Evaluate and analyze inlet-air precooling Evaporative cooling, Case 1, Table 1, boosts GT output by increasing the density and mass flow of the air entering the unit. Water sprayed into the inlet-air stream cools the air to a point near the ambient wet-bulb temperature. At reference con- ditions of 95⬚F (35⬚C) DB and 60 percent RH, an 85 percent effective evaporative cooler can alter the inlet-air temperature and moisture content to 85⬚F (29⬚C) and 92 percent RH, respectively, using conventional humidity chart calculations, page 16.79. This boosts the output of both the GT and—because of energy added to the GT exhaust—the steam turbine/generator. Overall, plant output for Case 1 is in- creased by 5.8 MW GT output ⫹ 0.9 MW ST output—plant auxiliary load of 0.9 MW ⫽ 6.65 MW, or 3.3 percent. The CC heat rate is improved 0.2 percent, or 15 Btu/kWh (14.2 kJ/kWh). The total installed cost for the evaporative cooling sys- tem, based on estimates provided by contractors and staff, is $1.2-million. The incremental cost is $1,200,000/6650 kW ⫽ $180.45/kW for this ambient condition. The effectiveness of the same system operating in less-humid conditions—say 95⬚F DB (35⬚C) and 40 percent RH—is much greater. In this case, the same evap- orative cooler can reduce inlet-air temperature to 75⬚F DB (23.9⬚C) by increasing RH to 88 percent. Here, CC output is increased by 7 percent, heat rate is improved (reduced) by 1.9 percent, and the incremental installed cost is $85/kW, computed as above. As you can clearly see, the effectiveness of evaporative cooling is directly related to reduced RH. Water-treatment requirements must also be recognized for this Case, No. 1. Be- cause demineralized water degrades the integrity of evaporative-cooler film media, manufacturers may suggest that only raw or filtered water be used for cooling purposes. However, both GT and evaporative-cooler suppliers specify limits for turbidity, pH, hardness, and sodium (Na) and potassium (K) concentrations in the injected water. Thus, a nominal increase in water-treatment costs can be expected. In particular, the cooling water requires periodic blowdown to limit solids buildup and system scaling. Overall, the evaporation process can significantly increase a plant’s makeup-water feed rate, treatment, and blowdown requirements. Compared to the base case, water supply costs increase by $15/h of operation for the first approach, and $20/h for the second, lower RH mode. Disposal of evaporative- cooler blowdown costs $1/h in the first mode, $2/h in the second. Evaporative cooling has little or no effect on the design of the steam turbine. 3. Evaluate the economics of inlet-air chilling The effectiveness of evaporative cooling is limited by the RH of the ambient air. Further, the inlet air cannot be cooled below the wet-bulb (WB) temperature of the inlet air. Thus, chillers may be used for further cooling of the inlet air below the wet-bulb temperature. To achieve this goal, industrial-grade mechanical or absorp- tion air-conditioning systems are used, Fig. 2. Both consist of a cooling medium (water or a refrigerant), an energy source to drive the chiller, a heat exchanger for extracting heat from the inlet air, and a heat-rejection system. A mechanical chilling system, Case 2, Table 1, is based on a compressor-driven unit. The compressor is the most expensive part of the system and consumes a significant amount of energy. In general, chillers rated above 12-million Btu/h (3.5 MW) (1000 tons of refrigeration) (3500 kW) employ centrifugal compressors. Units smaller than this may use either screw-type or reciprocating compressors. Overall, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 9. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.7 Ambient air (95F, 60% RH) Chilled air (60F, 100% RH) Gas turbine/ generator Cooling water Cooling tower Condensate return 25-psia steam from HRSG Chilled-water loop 2-stage lithium bromide adsorption chiller Electric- driven centrifugal chiller Cooling tower HRSG Chilled- water coils Circulating water pump Chilled water FIGURE 2 Inlet-air chilling using either centrifugal or absorption-type chillers, boosts the achieveable mass flow and power output during warm weather (Power). compressor-based chillers are highly reliable and can handle rapid load changes without difficulty. A centrifugal-compressor-based chiller can easily reduce the temperature of the GT inlet air from 95⬚F (35⬚C) to 60⬚F (15.6⬚C) DB—a level that is generally ac- cepted as a safe lower limit for preventing icing on compressor inlet blades—and achieve 100 percent RH. This increases plant output by 20.2 MW for GT ⫹ 2.4 MW for ST ⫺ 4.5 MW plant auxiliary load ⫽ 18.1 MW, or 8.9 percent. But it degrades the net CC heat rate by 0.8 percent and results in a 1.5-in-(3.8-cm)-H2O inlet-air pressure drop because of heat-exchanger equipment located in the inlet-air stream. Cooling requirements of the chilling system increase the plant’s required cir- culating water flow by 12,500 gal/min (47.3 m3 /min). Combined with the need for increased steam condensing capacity, use of a chiller may necessitate a heat sink 25 percent larger than the base case. The total installed cost for the mechanical chilling system for Case 2 is $3-million, or about $3,000,000/18,100 kW ⫽ $165.75/kW of added output. Again, costs come from contractor and staff studies. Raw-water consumption increase the plant’s overall O&M costs by $35/h when the chiller is operating. Disposal of additional cooling-tower blowdown costs $17/ h. The compressor used in Case 2 consumes about 4 MW of auxiliary power to handle the plant’s 68-million Btu/h (19.9 MW) cooling load. 4. Analyze an absorption chilling system Absorption chilling systems are somewhat more complex than mechanical chillers. They use steam or hot water as the cooling motive force. To achieve the same inlet- air conditions as the mechanical chiller (60⬚F DB, 100 percent RH) (15.6⬚C, 100 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 10. 1.8 POWER GENERATION percent RH), an absorption chiller requires about 111,400 lb/h (50,576 kg/h) of 10.3-lb/in2 (gage) (70.9-kPa) saturated steam, or 6830 gal/min (25.9 m3 /min) of 370⬚F (188⬚C) hot water. Cost-effective supply of this steam or hot water requires a redesign of the ref- erence plant. Steam is extracted from the low-pressure (l-p) steam turbine at 20.3 lb/in2 (gage) (139.9 kPa) and attemperated until it is saturated. In this case, the absorption chiller increases plant output by 8.7 percent or 17.4 MW but degrades the plant’s heat rate by 1 percent. Although the capacity of the absorption cooling system’s cooling-water loop must be twice that of the mechanical chiller’s, the size of the plant’s overall heat sink is identical—25 percent larger than the base case—because the steam extracted from the l-p turbine reduces the required cooling capacity. Note that this also re- duces steam-turbine output by 2 MW compared to the mechanical chiller, but has less effect on overall plant output. Cost estimates summarized in Table 1 show that the absorption chilling system required here costs about $4-million, or about $230/kW of added output. Compared to the base case, raw-water consumption increases O&M costs by $35/h when the chiller is operating. Disposal of additional cooling-water blowdown adds $17/h. Compared to mechanical chillers, absorption units may not handle load changes as well; therefore they may not be acceptable for cycling or load-following oper- ation. When forced to operate below their rated capacity, absorption chillers suffer a loss in efficiency and reportedly require more operator attention than mechanical systems. Refrigerant issues affect the comparison between mechanical and absorption chilling. Mechanical chillers use either halogenated or nonhalogenated fluorocar- bons at this time. Halogenated fluorocarbons, preferred by industry because they reduce the compressor load compared to nonhalogenated materials, will be phased out by the end of the decade because of environmental considerations (destruction of the ozone layer). Use of nonhalogenated refrigerants is expected to increase both the cost and parasitic power consumption for mechanical systems, at least in the near term. However, absorption chillers using either ammonia or lithium bromide will be unaffected by the new environmental regulations. Off-peak thermal storage is one way to mitigate the impact of inlet-air chilling’s major drawback: high parasitic power consumption. A portion of the plant’s elec- trical or thermal output is used to make ice or cool water during off-peak hours. During peak hours, the chilling system is turned off and the stored ice and/or cold water is used to chill the turbine inlet air. A major advantage is that plants can maximize their output during periods of peak demand when capacity payments are at the highest level. Thermal storage and its equipment requirements are analyzed elsewhere in this handbook—namely at page 18.70. 5. Compare steam and water injection alternatives Injecting steam or water into a GT’s combustor can significantly increase power output, but either approach also degrades overall CC efficiency. With steam injec- tion, steam extracted from the bottoming cycle is typically injected directly into the GT’s combustor, Fig. 3. For advanced GTs, the steam source may be extracted from either the high-pressure (h-p) turbine exhaust, an h-p extraction, or the heat recovery steam generator’s (HRSG) h-p section. Cycle economics and plant-specific considerations determine the steam extrac- tion point. For example, advanced, large-frame GTs require steam pressures of 410 to 435 lb/in2 (gage) (2825 to 2997 kPa). This is typically higher than the econom- ically optimal range of h-p steam turbine exhaust pressures of 285 to 395 lb/in2 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 11. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.9 Water-injection power sugmentation Steam-injection power sugmentation Attemperating station Water injection skid HRSG Gas turbine/ generator High-pressure superheater Demin. storage FIGURE 3 Water or steam injection can be used for both power augmentation and NOx control (Power). (gage) (1964 to 2722 kPa). Thus, steam must be supplied from either the HRSG or an h-p turbine extraction ahead of the reheat section. Based on installed-cost considerations alone, extracting steam from the HRSG is favored for peaking service and may be accomplished without altering the reheat steam turbine. But if a plant operates in the steam-injection mode for extended periods, extracting steam from the turbine or increasing the h-p turbine exhaust pressure becomes more cost-effective. Injecting steam from the HRSG superheat section into the GT increases unit output by 21.8 MS, Case 4 Table 1, but decreases the steam turbine/generator’s output by about 12.8 MW. Net gain to the CC is 8.4 MW. But CC plant heat rate also suffers by 4 percent, or 270 Btu/kWh (256.5 kJ/kWh). Because the steam-injection system requires makeup water as pure as boiler feedwater, some means to treat up to 350 gal/min (22.1 L/s) of additional water is necessary. A dual-train demineralizer this size could cost up to $1.5-million. However, treated water could also be bought from a third party and stored. Or portable treatment equipment could be rented during peak periods to reduce capital costs. For the latter case, the average expected cost for raw and treated water is about $130/h of operation. This analysis assumes that steam- or water-injection equipment is already in place for NOx control during distillate-fuel firing. Thus, no additional capital cost is incurred. When water injection is used for power augmentation or NOx control, the rec- ommended water quality may be no more than filtered raw water in some cases, provided the source meets pH, turbidity, and hardness requirements. Thus, water- treatment costs may be negligible. Water injection, Case 5 Table 1, can increase the GT output by 15.5 MW. In Case 5, the bottoming cycle benefits from increased GT-exhaust mass flow, increasing steam turbine/generator output by about 3.7 MW. Overall, the CC output increases by 9.4 percent or 19 MW, but the net plant heat rate suffers by 6.4 percent, or 435 Btu/kWh (413.3 kJ/kWh). Given the higher increase in the net plant heat rate and lower operating expenses, water injection is preferred over steam injection in this case. 6. Evaluate supplementary-fired HRSG for this plant The amount of excess O2 in a GT exhaust gas generally permits the efficient firing of gaseous and liquid fuels upstream of the HRSG, thereby increasing the output Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 12. 1.10 POWER GENERATION from the steam bottoming cycle. For this study, two types of supplementary firing are considered—(1) partial supplementary firing, Case 6 Table 1, and (2) full sup- plementary firing, Case 7 Table 1. There are three main drawbacks to supplementary firing for peak power en- hancement, including 910 lower cycle efficiency, (2) higher NOx and CO emissions, (3) higher costs for the larger plant equipment required. For this plant, each 100-million Btu/h (29.3 MW) of added supplementary firing capacity increases the net plant output by 5.5 percent, but increases the heat rate by 2 percent. The installed cost for supplementary firing can be significant because all the following equipment is affected: (1) boiler feed pumps, (2) condensate pumps, (3) steam turbine/generator, (4) steam and water piping and valves, and (5) selective-catalytic reduction (SCR) system. Thus, a plant designed for supplemen- tary firing to meet peak-load requirements will operate in an inefficient, off-design condition for most of the year. 7. Compare the options studied and evaluate results Comparing the results in Table 1 shows that mechanical chilling, Case 2, gives the largest increase in plant output for the least penalty on plant heat rate—i.e., 18.1 MW output for a net heat rate increase of 55 Btu/kWh (52.3 kJ/kWh). However, this option has the highest estimated installed cost ($3-million), and has a relatively high incremental installed cost. Water injection, Case 5 Table 1, has the dual advantage of high added net output and low installed cost for plants already equipped with water-injection skids for NOx control during distillate-fuel firing. Steam injection, Case 4 Table 1, has a significantly higher installed cost because of water-treatment requirements. Supplementary firing, Cases 6 and 7 Table 1, proves to be more acceptable for plants requiring extended periods of increased output, not just seasonal peaking. This calculation procedure is the work of M. Boswell, R. Tawney, and R. Narula, all of Bechtel Corporation, as reported in Power magazine, where it was edited by Steven Collins. SI values were added by the editor of this handbook. Related Calculations. Use of gas turbines for expanding plant capacity or for repowering older stations is a popular option today. GT capacity can be installed quickly and economically, compared to conventional steam turbines and boilers. Further, the GT is environmentally acceptable in most areas. So long as there is a supply of combustible gas, the GT is a viable alternative that should be considered in all plant expansion and repowering today, and especially where environmental conditions are critical. SELECTING GAS-TURBINE HEAT-RECOVERY BOILERS Choose a suitable heat-recovery boiler equipped with an evaporator and economizer to serve a gas turbine in a manufacturing plant where the gas flow rate is 150,000 lb/h (68,040 kg/h) at 950⬚F (510⬚C) and which will generate steam at 205 lb/in2 (gage) (1413.5 kPa). Feedwater enters the boiler at 227⬚F (108.3⬚C). Determine if supplementary firing of the exhaust is required to generate the needed steam. Use an approach temperature of 20⬚F (36⬚C) between the feedwater and the water leav- ing the economizer. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 13. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.11 Top Numbers: Example 1 Bottom Numbers: Example 2 Approach point Pinch point T1 950 1,550 T2 415 440 T3 317 296 Tw 370 325 Tt 227 227 Tl 390 390 950˚F (510˚C) 1550˚F (843˚C) 390˚F (199˚C) 390˚F (199˚C) 415˚F (213˚C) 440˚F (227˚C) 370˚F (188˚C) 325˚F (163˚C) 317˚F (158˚C) 296˚F (147˚C) 227˚F (108˚C) 227˚F (108˚C) FIGURE 4 Gas/steam profile and data (Chemical Engineering). Calculation Procedure: 1. Determine the critical gas inlet-temperature Turbine exhaust gas (TEG) typically leaves a gas turbine at 900–1000⬚F (482–538⬚C) and has about 13 to 16 percent free oxygen. If steam is injected into the gas turbine for NOx control, the oxygen content will decrease by 2 to 5 percent by volume. To evaluate whether supplementary firing of the exhaust is required to generate needed steam, a knowledge of the temperature profiles in the boiler is needed. Prepare a gas/steam profile for this heat-recovery boiler as shown in Fig. 4. TEG enters on the left at 950⬚F (510⬚C). Steam generated in the boiler at 205 lb/ in2 (gage) (1413.5 kPa) has a temperature of 390⬚F (198.9⬚C), from steam tables. For steam to be generated in the boiler, two conditions must be met: (1) The ‘‘pinch point’’ temperature must be greater than the saturated steam temperature of 390⬚F (198.9⬚C), and (2) the temperature of the saturated steam leaving the boiler econ- omizer must be greater than that of the feedwater. The pinch point occurs some- where along the TEG temperature line, Fig. 4, which starts at the inlet temperature of 950⬚F (510⬚C) and ends at the boiler gas outlet temperature, which is to be determined by calculation. A pinch-point temperature will be assumed during the calculation and its suitability determined. To determine the critical gas inlet-temperature, T1, get from the steam tables the properties of the steam generated by this boiler: ts ⫽ 390⬚F (198.9⬚C); hl, heat of saturated liquid ⫽ 364 Btu/lb (846.7 kJ/kg); hs, total heat of saturated vapor ⫽ 1199.6 Btu/lb (2790.3 kJ/kg; hw, heat of saturated liquid of feedwater leaving the economizer at 370⬚F (187.8⬚C) ⫽ 342 Btu/lb (795.5 kJ/kg); and heat of satu- h , ƒ rated liquid of the feedwater at 227⬚F (108.3⬚C) ⫽ 196.3 Btu/lb (456.6 kJ/kg). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 14. 1.12 POWER GENERATION Writing an energy balance across the evaporator neglecting heat and blowdown losses, we get: (T1 ⫺ T2)/(T1 ⫺ T3) ⫽ (hs ⫺ hw )/ ⫽ X, where T1 ⫽ gas (h ⫺ h ) s ƒ temperature in boiler, ⬚F (⬚C); T2 ⫽ pinch-point gas temperature, ⬚F (⬚C); T3 ⫽ outlet gas temperature for TEG, ⬚F (⬚C); enthalpy, h, values as listed above; X ⫽ ratio of temperature or enthalpy differences. Substituting, X ⫽ (1199.6 ⫺ 342)/ (1199.9 ⫺ 196.3) ⫽ 0.855, using enthalpy values as given above. The critical gas inlet-temperature, T1c ⫽ (ts ⫺ /(1 ⫺ X), where ts ⫽ tem- Xt ) ƒ perature of saturated steam, ⬚F (⬚C); ⫽ temperature of feedwater, ⬚F (⬚C); other tƒ symbols as before. Using the values determined above, T1c ⫽ [390 ⫺ (0.855)(227)]/(1 ⫺ 0.855) ⫽ 1351⬚F (732.8⬚C). 2. Determine the system pinch point and gas/steam profile Up to a gas inlet temperature of approximately 1351⬚F (732.8⬚C), the pinch point can be arbitrarily selected. Beyond this, the feedwater inlet temperature limits the temperature profile. Let’s then select a pinch point of 25⬚F (13.9⬚C), Fig. 4. Then, T2, the gas-turbine gas temperature at the pinch point, ⬚F (⬚C) ⫽ t ⫹ pinch-point ƒ temperature difference, or 390⬚F ⫹ 25⬚F ⫽ 415⬚F (212.8⬚C). Setting up an energy balance across the evaporator, assuming a heat loss of 2 percent and a blowdown of 3 percent, leads to: (1 ⫺ heat loss)(TEG Q ⫽ W evap e heat capacity, Btu/⬚F) (T1 ⫺ T2), where We ⫽ TEG flow, lb/h; heat capacity of TEG ⫽ 0.27 Btu/⬚F; T1 ⫽ TEG inlet temperature, ⬚F (⬚C). Substituting, ⫽ Qevap 150,000(0.98)(0.27)(950 ⫺ 415) ⫽ 21.23 ⫻ 106 Btu/h (6.22 MW). The rate of steam generation, ⫹ blowdown percent ⫻ W ⫽ Q /[(h ⫺ h ) s evap s w (hl ⫺ hw )], where the symbols are as given earlier. Substituting, Ws ⫽ 21.23 ⫻ 106 /[(1199.6 ⫺ 342) ⫹ 0.03 ⫻ (364 ⫺ 342)] ⫽ 24,736 lb/h (11,230 kg/h). Determine the boiler economizer duty from ⫽ (1 ⫹ blowdown)(Ws ) Qecon where symbols are as before. Substituting, ⫽ 1.03(24,736)(342 ⫺ (h ⫺ h ), Q w ƒ econ 196.3) ⫽ 3.71 ⫻ 106 Btu/h (1.09 MW). The gas exit-temperature, T3 ⫽ T2 ⫺ /TEG gas flow, lb/h)(1 ⫺ heat Qecon loss)(heat capacity, Btu/lb ⬚F). Since all values are known, T3 ⫽ 415 ⫺ 3.71 ⫻ 106 /(150,000 ⫻ 0.98 ⫻ 0.27) ⫽ 317⬚F (158⬚C). Figure 4 shows the temperature profile for this installation. Related Calculations. Use this procedure for heat-recovery boilers fired by gas-turbine exhaust in any industry or utility application. Such boilers may be un- fired, supplementary fired, or exhaust fired, depending on steam requirements. Typically, the gas pressure drop across the boiler system ranges from 6 to 12 in (15.2 to 30.5 cm) of water. There is an important tradeoff: a lower pressure drop means the gas-turbine power output will be higher, while the boiler surface and the capital cost will be higher, and vice versa. Generally, a lower gas pressure drop offers a quick payback time. If ⌬Pe is the additional gas pressure in the system, the power, kW, consumed in overcoming this loss can be shown approximately from P ⫽ 5 ⫻ 10⫺8 (We ⌬Pe T /E, where E ⫽ efficiency of compression). To show the application of this equation and the related payback period, assume We ⫽ 150,000 lb/g (68,100 kg/h), T ⫽ 1000⬚R (average gas temperature in the boiler, ⌬Pe ⫽ 4 in water (10.2 cm), and E ⫽ 0.7. Then P ⫽ 5 ⫻ 10⫺8 (150,000 ⫻ 4 ⫻ 1000/0.7) ⫽ 42 kW. If the gas turbine output is 4000 kW, nearly 1 percent of the power is lost due to the 4-in (10.2-cm) pressure drop. If electricity costs 7 cent/kWh, and the gas turbine runs 8000 h/yr, the annual loss will be 8000 ⫻ 0.07 ⫻ 42 ⫽ $23,520. If the incremental cost of a boiler having a 4-in (10.2-cm) lower pressure drop is, say $22,000, the payback period is about one year. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 15. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.13 Burner Fuel TEG We, T1´ (We ⫹ Wf),T1´ (Weh1 ´ ⫹ LHV ⫻ Wt) ⫽ (We ⫹ Wf)h1 F, Wf FIGURE 5 Gas/steam profile for fired mode (Chemical Engineering). If steam requirements are not stated for a particular gas inlet condition, and maximum steaming rate is desired, a boiler can be designed with a low pinch point, a large evaporator, and an economizer. Check the economizer for steaming. Such a choice results in a low gas exit temperature and a high steam flow. Then, the incremental boiler cost must be evaluated against the additional steam flow and gas-pressure drop. For example, Boiler A generates 24,000 lb/h (10,896 kg/h), while Boiler B provides 25,000 lb/h (11,350 kg/h) for the same gas pres- sure-drop but costs $30,000 more. Is Boiler B worth the extra expense? To answer this question, look at the annual differential gain in steam flow. As- suming steam costs $3.50/1000 lb (3.50/454 kg), the annual differential gain in steam flow ⫽ 1000 ⫻ 3.5 ⫻ 8000/1000 ⫽ $28,000. Thus, the simple payback is about a year ($30,000 vs $28,000), which is attractive. You must, however, be certain you assess payback time against the actual amount of time the boiler will operate. If the boiler is likely to be used for only half this period, then the payback time is actually two years. The general procedure presented here can be used for any type industry using gas-turbine heat-recovery boilers—chemical, petroleum, power, textile, food, etc. This procedure is the work of V. Ganapathy, Heat-Transfer Specialist, ABCO In- dustries, Inc., and was presented in Chemical Engineering magazine. When supplementary fuel is added to the turbine exhaust gas before it enters the boiler, or between boiler surfaces, to increase steam production, one has to perform an energy balance around the burner, Fig. 5, to evaluate accurately the gas temperature increase that can be obtained. V. Ganapathy, cited above, has a computer program he developed to speed this calculation. GAS-TURBINE CYCLE EFFICIENCY ANALYSIS AND OUTPUT DETERMINATION A gas turbine consisting of a compressor, combustor, and an expander has air entering at 60⬚F (15.6⬚C) and 14.0 lb/in2 (abs) (96.5 kPa). Inlet air is compressed Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 16. 1.14 POWER GENERATION FIGURE 6 Ideal gas-turbine cycle, 1-2-3-4-1. Actual compression takes place along 1-2⬘; actual heat added 2⬘-3⬘; ideal expansion 3⬘-4⬘. to 56 lb/in2 (abs) (385.8 kPa); the isentropic efficiency of the compressor is 82 percent. Sufficient fuel is injected to give the mixture of fuel vapor and air a heating value of 200 Btu/lb (466 kJ/kg). Assume complete combustion of the fuel. The expander reduces the flow pressure to 14.9 lb/in2 (abs), with an engine efficiency of 85 percent. Assuming that the combustion products have the same thermody- namic properties as air, cp ⫽ 0.24, and is constant. The isentropic exponent may be taken as 1.4. (a) Find the temperature after compression, after combustion, and at the exhaust. (b) Determine the Btu/lb (kJ/kg) of air supplied, the work delivered by the expander, the net work produced by the gas turbine, and its thermal effi- ciency. Calculation Procedure: 1. Plot the ideal and actual cycles Draw the ideal cycle as 1-2-3-4-1, Figs. 6 and 7. Actual compression takes place along 1-2⬘. Actual heat added lies along 2⬘-3⬘. The ideal expansion process path is 3⬘-4⬘. Ideal work ⫽ cp (ideal temperature difference). Actual work ⫽ cp (actual temperature difference). 2. Find the temperature after compression Use the relation (T2 /T1) ⫽ where T1 ⫽ entering air temperature, ⬚R; (k⫺1) / k (P /P ) , 2 1 T2 ⫽ temperature after adiabatic compression, ⬚R; P1 ⫽ entering air pressure, in units given above; P2 ⫽ pressure after compression, in units given above; k ⫽ isentropic exponent ⫽ 1.4. With an entering air temperature, T1 of 60⬚F (15.6⬚C), or 60 ⫹ 460 ⫽ 520⬚R, and using the data given, ⫽ (1.4⫺1) / 1.4 T ⫽ 520[(56/14)] 2 772.7⬚R, or 772.7 ⫺ 520 ⫽ 252.7⬚F (122.6⬚C). (a) Here we have isentropic compression in the compressor with an effi- ciency of 85 percent. Using the equation, Efficiency, isentropic ⫽ (cp )(T2 ⫺ T1)/ (cp ) and solve for the temperature after isentropic compression. Solv- (T ⫺ T ), T , 2⬘ 1 2⬘ ing, ⫽ 0.82 ⫽ 0.24(772.7 ⫺ 520)/0.24 ⫽ 828.4⬚R, or 368⬚F. This T (T ⫺ 520) 2⬘ 2⬘ is the temperature after compression. 3. Determine the temperature after combustion To find the temperature after combustion, use the relation Heating value of fuel ⫽ Q ⫽ cp where ⫽ temperature after combustion, ⬚R. Substituting, (T ⫺ T ), T 3⬘ 2⬘ 3⬘ 200 ⫽ 0.24 Solving, ⫽ 1661.3⬚R; 1201.3⬚F (649.6⬚C). (T ⫺ 828). T 3⬘ 3⬘ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 17. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.15 FIGURE 7 Ideal gas-turbine cycle T-S diagram with the same processes as in Fig. 6; complete- cycle gas turbine shown below the T-S diagram. 4. Find the temperature at the exhaust of the gas turbine Using an approach similar to that above, determine T4 from ⫽ (T /T ) 4⬘ 3⬘ Substituting and solving for ⫽ 1661 ⫽ k⫺1 / k. (1.4⫺1) / 1.4 [(P /P )] T [(14.9/56)] 4⬘ 3⬘ 4⬘ 1137.9⬚R, or 677.8⬚F (358.8⬚C). Now use the equation for gas-turbine efficiency, namely, Turbine efficiency ⫽ cp ⫽ 0.85, and solve for the temperature after expan- (T ⫺ T )/c (T ⫺ T ) T , 3⬘ 4ⴖ p 3⬘ 4⬘ 4ⴖ sion, at the exhaust. Substituting as earlier, ⫽ 1218.2⬚R, 758.2⬚F (403.4⬚C). This T4ⴖ is the temperature after expansion, i.e., at the exhaust of the gas turbine. 5. Determine the work of compression, expander work, and thermal efficiency (b) The work of compression ⫽ cp ⫽ 0.24(828 ⫺ 520) ⫽ 74.16 Btu (78.23 (T ⫺ T ) 2⬘ 1 J). The work delivered by the expander ⫽ cp ⫽ 0.24 (1661 ⫺ 1218) ⫽ (T ⫺ T ) 2⬘ 1 106.32 Btu (112.16 J). The net work ⫽ 106.3 ⫺ 74.2 ⫽ 32.1 Btu (33.86 J). Then, the thermal efficiency ⫽ net work/heat supplied ⫽ 32.1/200 ⫽ 0.1605, 16.6 percent thermal efficiency. Related Calculations. With the widespread use today of gas turbines in a va- riety of cycles in industrial and central-station plants, it is important that an engineer be able to analyze this important prime mover. Because gas turbines can be quickly installed and easily hooked to heat-recovery steam generators (HRSG), they are more popular than ever before in history. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 18. 1.16 POWER GENERATION FIGURE 8 With further gas-turbine cycle refinement, the specific fuel consumption declines. These curves are based on assumed efficiencies with T3 ⫽ 1400 F (760 C). Further, as aircraft engines become larger—such as those for the Boeing 777 and the Airbus 340—the power output of aeroderivative machines increases at little cost to the power industry. The result is further application of gas turbines for topping, expansion, cogeneration and a variety of other key services throughout the world of power generation and energy conservation. With further refinement in gas-turbine cycles, specific fuel consumption, Fig. 8, declines. Thus, the complete cycle gas turbine has the lowest specific fuel con- sumption, with the regenerative cycle a close second in the 6-to-1 compression- ratio range. Two recent developments in gas-turbine plants promise much for the future. The first of these developments is the single-shaft combined-cycle gas and steam turbine, Fig. 9. In this cycle, the gas turbine exhausts into a heat-recovery steam generator (HRSG) that supplies steam to the turbine. This cycle is the most significant electric generating system available today. Further, its capital costs are significantly lower than competing nuclear, fossil-fired steam, and renewable-energy stations. Other advantages include low air emissions, low water consumption, smaller space re- quirements, and a reduced physical profile, Fig. 10. All these advantages are im- portant in today’s strict permitting and siting processes. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 19. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.17 Stack H-p I-p L-p HRSG L-p I-p H-p Steam turbine Generator Inlet air Gas turbine Fuel Synchronous clutch FIGURE 9 Single-shaft combined-cycle technology can reduce costs and increase thermal effi- ciency over multi-shaft arrangements. This concept is popular in Europe (Power). 68.5 ft (20.9 m) (51.9 m) 170.6 ft 29.5 ft 95 ft 152 ft (8.99 m) (46.33 m) (28.95 m) FIGURE 10 Steam turbine, electric generator, and gas turbine fit into one compact building when all three machines are arranged on a single shaft. Net result: Reduced site footprint and civil- engineering work (Power). Having the gas turbine, steam turbine, and generator all on one shaft simplifies plant design and operation, and may lower first costs. When used for large reheat cycles, as shown here, separate high-pressure (h-p), intermediate-pressure (i-p), and low-pressure (l-p) turbine elements are all on the same shaft as the gas turbine and generator. Modern high-technology combined-cycle single-shaft units deliver a simple-cycle net efficiency of 38.5 percent for a combine-cycle net efficiency of 58 percent on a lower heating value (LHV) basis. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 20. 1.18 POWER GENERATION The second important gas-turbine development worth noting is the dual-fueled turbine located at the intersection of both gas and oil pipelines. Being able to use either fuel gives the gas turbine greater opportunity to increase its economy by switching to the lowest-cost fuel whenever necessary. Further developments along these lines is expected in the future. The data in the last three paragraphs and the two illustrations are from Power magazine. DETERMINING BEST-RELATIVE-VALUE OF INDUSTRIAL GAS TURBINES USING A LIFE-CYCLE COST MODEL An industrial application requires a 21-MW continuous electrical output year-round. Five different gas turbines are under consideration. Determine which of these five turbines is the best choice, using a suitable life-cycle cost analysis. Calculation Procedure: 1. Assemble the cost data for each gas turbine being considered Assemble the cost data as shown below for each of the five gas turbines identified by the letters A through E. Contact the gas-turbine manufacturers for the initial cost, $/kW, thermal efficiency, availability, fuel consumption, generator efficiency, and maintenance cost, $/kWh. List these data as shown below. The loan period, years, will be the same for all the gas turbines being considered, and is based on an equipment life-expectancy of 20 years. Interest rate on the capital investment for each turbine will vary, depending on the amount invested and the way in which the loan must be repaid and will be provided by the accounting department of the firm considering gas-turbine purchase. Equipment Attributes for Typical Candidates* Parameter Gas-turbine candidates A B C D E Initial cost, $/kW 205 320 275 320 200 Thermal efficiency, % 32.5 35.5 34.0 36.5 30.0 Loan period, yr 20 20 20 20 20 Availability 0.96 0.94 0.95 0.94 0.96 Fuel cost, $/million Btu 4 4 4 4 4 Interes, % 6.5 8.0 7.0 8.5 7.5 Generator efficiency, % 98.0 98.5 98.5 98.0 98.5 Maintenance cost, $/kWh 0.004 0.005 0.005 0.005 0.004 *Assuming an equipment life of 20 years, an output of 21 MW. 2. Select a life-cycle cost model for the gas turbines being considered A popular and widely used life-cycle cost model for gas turbines has three parts: (1) the annual investment cost, Cp ; (2) annual fuel cost, (3) annual maintenance C ; ƒ cost, Cm. Summing these three annual costs, all of which are expressed in mils/ kWh, gives CT , the life-cycle cost model. The equations for each of the three components are given below, along with the life-cycle working model, CT : Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 21. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.19 The life-cycle cost model (CT ) consists of annual investment cost (Cp ) ⫹ annual fuel cost ⫹ annual maintenance cost (Cm ). Equations for these values are: (C ) ƒ ⫺n l{i /[1 ⫺ (1 ⫺ i) ]} C ⫽ p (A)(kW)(8760)(G) where l ⫽ initial capital cost of equipment, dollars i ⫽ interest rate n ⫽ number of payment periods A ⫽ availability (expressed as decimal) kW ⫽ kilowatts of electricity produced 8760 ⫽ total hours in year G ⫽ efficiency of electric generator C ⫽ E(293) ƒ where E ⫽ thermal efficiency of gas turbine 293 ⫽ conversion of Btu to kWh C ⫽ M/kW m where M ⫽ maintenance cost, dollars per operating (fired) hour. Thus, the life-cycle working model can be expressed as ⫺n l{i/[1 ⫺ (1 ⫺ i) ]} C ⫽ ⫹ F/E(293) ⫹ M/kW T (A)(kW)(8760)(G) where F ⫽ fuel cost, dollars per million Btu (higher heating value) To evaluate the comparative capital cost of a gas-turbine electrical generating package the above model uses the capital-recovery factor technique. This approach spreads the initial investment and interest costs for the repayment period into an equal annual expense using the time value of money. The approach also allows for the comparison of other periodic expenses, like fuel and maintenance costs. 3. Perform the computation for each of the gas turbines being considered Using the compiled data shown above, compute the values for Cp, and Cm, and C , ƒ sum the results. List for each of the units as shown below. Results from Cost Model Unit Mils/kWh produced A 48.3 B 47.5 C 48.3 D 46.6 E 51.9 4. Analyze the findings of the life-cycle model Note that the initial investment cost for the turbines being considered ranges be- tween $200 and $320/kW. On a $/kW basis, only unit E at the $200 level, would be considered. However, the life-cycle cost model, above, shows the cost per kWh Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 22. 1.20 POWER GENERATION produced for each of the gas-turbine units being considered. This gives a much different perspective of the units. From a life-cycle standpoint, the choice of unit E over unit D would result in an added expenditure of about $975,000 annually during the life span of the equip- ment, found from [(51.9 ⫺ 46.6)/1000](8760 hr/yr)(21,000 kW) ⫽ $974,988; this was rounded to $975,000. Since the difference in the initial cost between units D and E is $6,720,000 ⫺ $4,200,000 ⫽ $2,520,000, this cost difference will be re- covered in $2,520,000/974,988 ⫽ 2.58 years, or about one-eighth of the 20-year life span of the equipment. Also, note that the 20-year differential in cost/kWh produced between units D and E is equivalent to over 4.6 times the initial equipment cost of unit E. When considering the values output of a life-cycle model, remember that such values are only as valid as the data input. So take precautions to input both reasonable and accurate data to the life-cycle cost model. Be careful in attempting to distinguish model outputs that vary less than 0.5 mil from one another. Since the predictions of this life-cycle cost model cannot be compared to actual measurements at this time, a potential shortcoming of the model lies with the va- lidity of the data and assumptions used for input. For this reason, the model is best applied to establish comparisons to differentiate between several pieces of com- peting equipment. Related Calculations. The first gas turbines to enter industrial service in the early 1950s represented a blend of steam-turbine and aerothermodynamic design. In the late 1950s/early-1960s, lightweight industrial gas turbines derived directly from aircraft engines were introduced into electric power generation, pipeline com- pression, industrial power generation, and a variety of other applications. These machines had performance characteristics similar to their steam-turbine counter- parts, namely pressure ratios of about 12⬊1, firing temperatures of 1200–1500⬚F (649–816⬚C), and thermal efficiencies in the 23–27 percent range. In the 1970s, a new breed of aeroderivative gas turbines entered industrial ser- vice. These units, with simple-cycle thermal efficiencies in the 32–37 percent bracket, represented a new technological approach to aerothermodynamic design. Today, these second-generation units are joined by hybrid designs that incor- porate some of the aeroderivative design advances but still maintain the basic struc- tural concepts of the heavy-frame machines. These hybrid units are not approaching the simple-cycle thermal-efficiency levels reached by some of the early second- generation aeroderivative units first earmarked for industrial use. Traditionally, the major focus has been on first cost of industrial gas-turbine units, not on operating cost. Experience with higher-technology equipment, how- ever, reveals that a low first cost does not mean a lower total cost during the expected life of the equipment. Conversely, reliable, high-quality equipment with demonstrated availability will be remembered long after the emotional distress as- sociated with high initial cost is forgotten. The life-cycle cost model presented here uses 10 independent variables. A sin- gle-point solution can easily be obtained, but multiple solutions require repeated calculations. Although curves depicting simultaneous variations in all variables would be difficult to interpret, simplified diagrams can be constructed to illustrate the relative importance of different variables. Thus, the simplified diagrams shown in Fig. 11, all plot production cost, mils/ kWh, versus investment cost. All the plots are based on continuous operation of 8760 h/yr at 21-MW capacity with an equipment life expectancy of 20 years. The curves shown depict the variation in production cost of electricity as a function of initial investment cost for various levels of thermal efficiency, loan Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 23. 1.21 FIGURE 11 Economic study plots for life-cycle costs (Power). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 24. 1.22 POWER GENERATION repayment period, gas-turbine availability, and fuel cost. Each of these factors is an element in the life-cycle cost model presented here. This procedure is the work of R. B. Spector, General Electric Co., as reported in Power magazine. TUBE BUNDLE VIBRATION AND NOISE DETERMINATION IN HRSGs A tubular air heater 11.7 ft (3.57 m) wide, 12.5 ft (3.81 m) deep and 13.5 ft (4.11 m) high is used in a boiler plant. Carbon steel tubes 2 in (5.08 cm) in outer diameter and 0.08 in (0.20 cm) thick are used in inline fashion with a traverse pitch of 3.5 in (8.89 cm) and a longitudinal pitch of 3 in (7.62 m). There are 40 tubes wide and 60 tubes deep in the heater; 300,000 lb (136,200 kg) of air flows across the tubes at an average temperature of 219⬚F (103.9⬚C). The tubes are fixed at both ends. Tube mass per unit length ⫽ 1.67 lb/ft (2.49 kg/m). Check this air heater for possible tube vibration problems. Calculation Procedure: 1. Determine the mode of vibration for the tube bundle Whenever a fluid flows across a tube bundle such as boiler tubes in an evaporator, economizer, HRSG, superheater, or air heater, vortices are formed and shed in the wake beyond the tubes. This shedding on alternate sides of the tubes causes a harmonically varying force on the tubes perpendicular to the normal flow of the fluid. It is a self-excited vibration. If the frequency of the Von Karman vortices, as they are termed, coincides with the natural frequency of vibration of the tubes, then resonance occurs and the tubes vibrate, leading to possible damage of the tubes. Vortex shedding is most prevalent in the range of Reynolds numbers from 300 to 200,000, the range in which most boilers operate. Another problem encountered with vortex shedding is acoustic vibration, which is normal to both the fluid flow and tube length observed in only gases and vapors. This occurs when the vortex shedding frequency is close to the acoustic frequency. Excessive noise is generated, leading to large gas pressure drops and bundle and casing damage. The starting point in the evaluation for noise and vibration is the estimation of various frequen- cies. Use the listing of C values shown below to determine the mode of vibration. Note that C is a factor determined by the end conditions of the tube bundle. End conditions Mode of vibration 1 2 3 Both ends clamped 22.37 61.67 120.9 One end clamped, one end hinged 15.42 49.97 104.2 Both hinged 9.87 39.48 88.8 Since the tubes are fixed at both ends, i.e., clamped, select the mode of vibration as 1, with C ⫽ 22.37. For most situations, Mode 1 is the most important case. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 25. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.23 FIGURE 12 Strouhl number, S, for inline tube banks. Each curve represents a different longitudinal pitch/diameter ratio (Chen). 2. Find the natural frequency of the tube bundle Use the relation, ƒn ⫽ 90C[ Substituting, with C ⫽ 22.37, 4 4 2 0.5 d ⫺ d ]/(L ⫺ M ). o i ƒn ⫽ (90)(22.37)[24 ⫺ 1.844 ]0.5 /(13.52 ⫺ 1.670.5 ) ⫽ 18.2 cycles per second (cps). In Mode 2, ƒn ⫽ 50.2, as C ⫽ 61.67. 3. Compute the vortex shedding frequency To compute the vortex shedding frequency we must know several factors, the first of which is the Strouhl Number, S. Using Fig. 12 with a transverse pitch/diameter of 1.75 and a longitudinal pitch diameter of 1.5 we find S ⫽ 0.33. Then, the air density ⫽ 40/(460 ⫺ 219) ⫽ 0.059 lb/ft3 (0.95 kg/m3 ); free gas area ⫽ 40(3.5 ⫺ 2)(13.5/12) ⫽ 67.5 ft2 (6.3 m2 ); gas velocity, V ⫽ 300,000/(67.5)(0.059)(3600) ⫽ 21 ft/s (6.4 m/s). Use the relation, ƒc ⫽ 12(S)(V)/do ⫽ 12(0.33)(21)/2 ⫽ 41.6 cps, where ƒc ⫽ vortex shedding frequency, cps. 4. Determine the acoustic frequency As with vortex frequency, we must first determine several variables, namely: ab- solute temperature ⫽ ⬚R ⫽ 219 ⫹ 460 ⫽ 679⬚R; sonic velocity, Vs ⫽ 49(679)0.5 ⫽ 1277 ft/s (389.2 m/s); wave length, ⫽ 2(w)/n, where w ⫽ width of tube bank, ft (m); n ⫽ mode of vibration ⫽ 1 for this tube bank; then ⫽ 2(11.7)/1 ⫽ 23.4 ft (7.13 m). The acoustic frequency, ƒa ⫽ (Vs )/, where Vs ⫽ velocity of sound at the gas temperature in the duct or shell, ft/s (m/s); Vs ⫽ [(g)()(RT)]0.5 , where R ⫽ gas constant ⫽ 1546/molecular weight of the gas; T ⫽ gas temperature, ⬚R; ⫽ ratio of gas specific heats, typically 1.4 for common flue gases; the molecular weight ⫽ 29. Simplifying, we get Vs ⫽ 49(T)0.5 , as shown above. Substituting, ƒa ⫽ 1277/ 23.4 ⫽ 54.5 cps. For n ⫽ 2; ƒa ⫽ 54.4(2) ⫽ 109 cps. The results for Modes 1 and 2 are summarized in the tabulation below. Mode of vibration n 1 2 ƒn, cps 18.2 50.2 ƒc, cps 41.6 41.6 ƒa (without baffles) 54.5 109 ƒa (with baffles) 109 218 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 26. 1.24 POWER GENERATION Moment-connected corners Main wall beams Main roof beams Roof cross-tie beams Main frame Tube-bundle support beam Suspension bolt Vibration stopper Heat- transfer tube bundle Casing Vibration stopper Insulation Liner Roof pressure-part supports Pressure-part expansion guides Upper header Tube restraint Tubes Lower header Lower header cradle Floor pressure-part supports Main floor beams Floor cross-tie beams Moment-connected corners Gas flow Wall cross-tie beams 1/2-in. dia. liner stud Tube restraint supports 1/4-in. casing FIGURE 13 Tube bundles in HRSGs require appropriate support mechanisms; thermal cycling in combined-cycle units makes this consideration even more important (Power). The tube natural frequency and the vortex shedding frequency are far apart. Hence, the tube bundle vibration problem is unlikely to occur. However, the vortex shedding and acoustic frequencies are close. If the air flow increases slightly, the two frequencies will be close. By inserting a baffle in the tube bundle (dividing the ductwork into two along the gas flow direction) we can double the acoustic fre- quency as the width of the gas path is now halved. This increases the difference between vortex shedding and acoustic frequencies and prevents noise problems. Noise problems arise when the acoustic and vortex shedding frequencies are close—usually within 20 percent. Tube bundle vibration problems arise when the vortex shedding frequency and natural frequency of the bundle are close—within 20 percent. Potential noise problems must also be considered at various turndown conditions of the equipment. Related Calculations. For a thorough analysis of a plant or its components, evaluate the performance of heat-transfer equipment as a function of load. Analyze at various loads the possible vibration problems that might occur. At low loads in the above case, tube bundle vibration is likely, while at high loads acoustic vibration is likely without baffles. Hence, a wide range of performance must be reviewed before finalizing any tube bundle design, Fig. 13. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 27. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.25 DETERMINING OXYGEN AND FUEL INPUT IN GAS-TURBINE PLANTS In a gas-turbine HRSG (heat-recovery steam generator) it is desired to raise the temperature of 150,000 lb/h (68,100 kg/h) of exhaust gases from 950⬚F (510⬚C) to 1575⬚F (857.2⬚C) in order to nearly double the output of the HRSG. If the exhaust gases contain 15 percent oxygen by volume, determine the fuel input and oxygen consumed, using the gas specific-heat method. Calculation Procedure: 1. Determine the air equivalent in the exhaust gases In gas-turbine based cogeneration/combined-cycle projects the HRSG may be fired to generate more steam than that produced by the gas-turbine exhaust gases. Typ- ically, the gas-turbine exhaust gas contains 14 to 15 percent oxygen by volume. So the question arises: How much fuel can be fired to generate more steam? Would the oxygen in the exhaust gases run out if we fired to a desired temperature? These questions are addressed in this procedure. If 0 percent oxygen is available in Wg lb/h (kg/h) of exhaust gases, the air- equivalent Wa in lb/h (kg/h) is given by: Wa ⫽ 100(Wg )(32Ox )/[23(100)(29.5)] ⫽ 0.0417 Wg (O). In this relation, we are converting the oxygen from a volume basis to a weight basis by multiplying by its molecular weight of 32 and dividing by the molecular weight of the exhaust gases, namely 29.5. Then multiplying by (100/ 23) gives the air equivalent as air contains 23 percent by weight of oxygen. 2. Relate the air required with the fuel fired using the MM Btu (kJ) method Each MM Btu (kJ) of fuel fired (HHV basis) requires a certain amount of air, A. If Q ⫽ amount of fuel fired in the turbine exhaust gases on a LHV basis (calcu- lations for turbine exhaust gases fuel input are done on a low-heating-value basis) then the fuel fired in lb/h (kJ/h) ⫽ W ⫽ Q /LHV. ƒ The heat input on an HHV basis ⫽ (HHV)/(106 ) ⫽ (Q /LHV)(HHV)/106 Wƒ Btu/h (kJ/h). Air required lb/h (kg/h) ⫽ (Q /LHV)(HHV)(A), using the MM Btu, where A ⫽ amount of air required, lb (kg) per MM Btu (kJ) fired. The above quantity ⫽ air available in the exhaust gases, Wa ⫽ 0.0417 Wg (O). 3. Simplify the gas relations further From the data in step 2, (Q /LHV)(HHV)(A)/106 ⫽ 0.0417 Wg (O). For natural gas and fuel oils it can be shown that (LHV/Ax HHV) ⫽ 0.00124. For example, LHV of methane ⫽ 21,520 Btu/lb (50,055.5 kJ/kg); HHV ⫽ 23,879 Btu/lb (55,542.6 kJ/kg), and A ⫽ 730 lb (331.4 kg). Hence, (LHV/Ax HHV) ⫽ 21,520/(730 ⫻ 23,879) ⫽ 0.00124. By substituting in the equation in step 1, we have Q ⫽ 58.4 (Wg )(O). This is an important equation because it relates the oxygen consumption from the exhaust gases to the burner fuel consumption. 4. Find the fuel input to the HRSG The fuel input is given by Wg ⫹ hg 1 ⫹ Q ⫽ (Wg ⫹ )(hg 2), where hg 1 and hg 2 Wƒ are the enthalpies of the exhaust gas before and after the fuel burner; ⫽ fuel Wƒ input, lb/h (kg/h); Q ⫽ fuel input in Btu/h (kJ/h). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 28. 1.26 POWER GENERATION Exhaust To process HRSG Load L-p steam L-p turbine H-p turbine Combustor Fuel Air L-p compressor H-p compressor H-p steam Power turbine FIGURE 14 Steam injection systems offer substantial improvement in both capacity and ef- ficiency (Power). The relation above requires enthalpies of the gases before and after the burner, which entails detailed combustion calculations. However, considering that the mass of fuel is a small fraction of the total gas flow through the HRSG, the fuel flow can be neglected. Using a specific heat for the gases of 0.31 Btu/lb ⬚F (1297.9 J/ kg K), we have, Q ⫽ 150,000(0.31)(1575 ⫺ 950) ⫽ 29 ⫻ 106 Btu/h (8.49 kW). The percent of oxygen by volume, O ⫽ (29 ⫻ 106 )/(58.4 ⫻ 150,000) ⫽ 3.32 percent. That is, only 3.32 percent oxygen by volume is consumed and we still have 15.00 ⫺ 3.32 ⫽ 11.68 percent left in the flue gases. Thus, more fuel can be fired and the gases will not run out of oxygen for combustion. Typically, the final oxygen content of the gases can go as low as 2 to 3 percent using 3 percent final oxygen, the amount of fuel that can be fired ⫽ (150,000)(58.4)(15 ⫺ 3) ⫽ 105 MM Btu/h (110.8 MMJ/h). It can be shown through an HRSG simulation program (contact the author for more information) that all of the fuel energy goes into steam. Thus, if the unfired HRSG were gen- erating 23,000 lb/h (10,442 kg/h) of steam with an energy absorption of 23 MM Btu/h (24.3 MM J/h), approximately, the amount of steam that can be generated by firing fuel in the HRSG ⫽ 23 ⫹ 105 ⫽ 128 MM Btu/h (135 MM J/h), or 128,000 lb/h (58,112 kg/h) of steam. This is close to a firing temperature of 3000 to 3100⬚F (1648 to 1704⬚C). Related Calculations. Using the methods given elsewhere in this handbook, one may make detailed combustion calculations and obtain a flue-gas analysis after combustion. Then compute the enthalpies of the exhaust gas before and after the burner. Using this approach, you can check the burner duty more accurately than using the gas specific-heat method presented above. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. Power magazine recently commented on the place of gas turbines in today’s modern power cycles thus: Using an HRSG with a gas turbine enhances the overall efficiency of the cycle by recovering heat in the gas-turbine’s hot exhaust gases. The recovered heat can be used to generate steam in the HRSG for either (1) injection back into the gas turbine, Fig. 14, (2) use in district heating or an industrial process, (3) driving a steam turbine-generator in a combined-cycle arrangement, or (4) any combination of the first three. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 29. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.27 To existing 11.5 kV switchyard Existing generator Steam turbine L-p steam H-p steam Condenser Deaerator Boiler feed pump H-p drum L-p drum Stack L-p economizer H-p economizer L-p superheater H-p superheater HRSG Existing circ-water system Existing condensate pump Fuel oil Fuel gas To 115-kV switchyard Combustion turbine Existing fuel-oil storage tank FIGURE 15 HRSG and gas turbine used in repowering (Power). Steam injection into the gas turbine has many benefits, including: (1) achievable output is increased by 25 percent or more, depending on the gas-turbine design, (2) part-load gas-turbine efficiency can be significantly improved, (3) gas-fired NOx emissions can be markedly reduced—up to the 15–45 ppm range in many cases, (4) operating flexibility is improved for cogeneration plants because electrical and thermal outputs can be balanced to optimize overall plant efficiency and profit- ability. Combined-cycle gas-turbine plants are inherently more efficient than simple- cycle plants employing steam injection. Further, combined-cycle plants may also be considered more adaptable to cogeneration compared to steam-injected gas tur- bines. The reason for this is that the maximum achievable electrical output de- creases significantly for steam-injected units in the cogeneration mode because less steam is available for use in the gas turbine. In contrast, the impact of cogeneration on electrical output is much less for combined-cycle plants. Repowering in the utility industry can use any of several plant-revitalization schemes. One of the most common repowering options employed or considered today by utilities consists of replacing an aging steam generator with a gas- turbine/generator and HRSG, Fig. 15. It is estimated that within the next few years, more than 3500 utility power plants will have reached their 30th birthdays. A significant number of these facilities—more than 20 GW of capacity by some estimates—are candidates for repowering, an option that can cut emissions and boost plant efficiency, reliability, output, and service life. And repowering often proves to be more economical, per cost of kilowatt gen- erated, compared to other options for adding capacity. Further, compared to building a new power plant, the permitting process for repowering its typically much shorter and less complex. The HRSG will often have a separate firing capability such as that discussed in this calculation procedure. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 30. 1.28 POWER GENERATION To steam turbine H-p drum L-p evaporator H-p economizer Gas flow Super- heater H-p evaporator H-p feedwater pump L-p drum To steam turbine L-p economizer Condensate pump FIGURE 16 HRSG circuit shown is used by at least one manufacturer to prevent steaming in the economizer during startup and low-load operation (Power). These comments from Power magazine were prepared by Steven Collins, As- sistant Editor of the publication. HEAT-RECOVERY STEAM GENERATOR (HRSG) SIMULATION A gas turbine exhausts 140,000 lb/h (63,560 kg/h) of gas at 980⬚F (526.7⬚C) to an HRSG generating saturated steam at 200 lb/in2 (gage) (1378 kPa). Determine the steam-generation and design-temperature profiles if the feedwater temperature is 230⬚F (110⬚C) and blowdown ⫽ 5 percent. The average gas-turbine exhaust gas specific heat is 0.27 Btu/lb ⬚F (1.13 kJ/kg ⬚C) at the evaporator and 0.253 Btu/lb ⬚F (1.06 kJ/kg ⬚C) at the economizer. Use a 20⬚F (11.1⬚C) pinch point, 15⬚F (8.3⬚C) approach point and 1 percent heat loss. Evaluate the evaporator duty, steam flow, economizer duty, and exit-gas temperature for normal load conditions. Then deter- mine how the HRSG off-design temperature profile changes when the gas-turbine exhaust-gas flow becomes 165,000 lb/h (74,910 kg/h) at 880⬚F (471⬚C) with the HRSG generating 150-lb/in2 (gage) (1033.5 kPa) steam with the feedwater tem- perature remaining the same. Calculation Procedure: 1. Compute the evaporator duty and steam flow Engineers should be able to predict both the design and off-design performance of an HRSG, such as that in Fig. 16, under different conditions of exhaust flow, tem- perature, and auxiliary firing without delving into the mechanical design aspects of tube size, length, or fin configuration. This procedure shows how to make such predictions for HRSGs of various sizes by using simulation techniques. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 31. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.29 HRSGs operate at different exhaust-gas conditions. For example, variations in ambient temperature or gas-turbine load affect exhaust-gas flow and temperature. This, in turn, affects HRSG performance, temperature profiles, efficiency, and steam generation. The tool consultants use for evaluating HRSG performance under dif- ferent operating conditions is simulation. With this tool you can: (1) Predict off- design performance of an HRSG; (2) Predict auxiliary fuel consumption for periods when the gas-turbine exhaust-gas flow is insufficient to generate the required steam flow; (3) Evaluate options for improving an HRSG system; (4) Evaluate field data for validating an HRSG design; (5) Evaluate different HRSG configurations for maximizing efficiency. In this HRSG, using steam-table data, the saturation temperature of 200-lb/in2 (gage) (1378-kPa) steam ⫽ 388⬚F (197.8⬚C). The gas temperature leaving the evap- orator with the 20⬚F (11.1⬚C) pinch point ⫽ 388 ⫹ 20 ⫽ 408⬚F (208.9⬚C). Water temperature entering the evaporator ⫽ saturated-steam temperature ⫺ the approach point temperature difference, or 388 ⫺ 15 ⫽ 373⬚F (189.4⬚C). Then, the energy absorbed by the evaporator, Q1 ⫽ (gas flow, lb/h)(1.0 ⫺ heat loss)(gas specific heat, Btu/lb ⬚F)(gas-turbine exhaust gas HRSG entering temper- ature, ⬚F ⫺ gas temperature leaving evaporator, ⬚F). Or, Q1 ⫽ (140,000)(0.99)(0.27)(980 ⫺ 408) ⫽ 21.4 MM Btu/h (6.26 MW). The enthalpy absorbed by the steam in the evaporator, Btu/lb (kJ/kg) ⫽ (enthalpy of the saturated steam in the HRSG outlet ⫺ enthalpy of the feedwater entering the evaporator at 373⬚F) ⫹ (blowdown percentage)(enthalpy of the saturated liquid of the outlet steam ⫺ enthalpy of the water entering the evaporator, all in Btu/lb). Or, enthalpy absorbed in the evaporator ⫽ (1199.3 ⫺ 345) ⫹ (0.05)(362.2 ⫺ 345) ⫽ 855.2 Btu/lb (1992.6 kJ/kg). The quantity of steam generated ⫽ (Q1, energy absorbed by the evaporator, Btu/h)/(enthalpy absorbed by the steam in the evaporator, Btu /lb) ⫽ (21.4 ⫻ 106 )/855.2 ⫽ 25,023 lb/h (11,360 kg/h). 2. Determine the economizer duty and exit gas temperature The economizer duty ⫽ (steam generated, lb/h)(enthalpy of water entering the economizer, Btu/lb ⫺ enthalpy ⫺ enthalpy of the feedwater at 230⬚F, Btu/lb)(1 ⫹ blowdown percentage) ⫽ (25,023)(345 ⫺ 198.5)(1.05) ⫽ 3.849 MM Btu/h (1.12 MW). The gas temperature drop through the economizer ⫽ (economizer duty)/(gas flow rate, lb/h)(1 ⫺ heat loss percentage)(specific heat of gas, Btu/lb ⬚F) ⫽ (3.849 ⫻ 106 )/(140,000)(0.99)(0.253) ⫽ 109.8⬚F (60.9⬚C). Hence, the exit-gas tem- perature from the economizer ⫽ (steam saturation temperature, ⬚F ⫺ exit-gas tem- perature from the economizer, ⬚F) ⫽ (408 ⫺ 109) ⫽ 299⬚F (148.3⬚C). 3. Calculate the constant K for evaporator performance In simulating evaporator performance the constant K1 is used to compute revised performance under differing flow conditions. In equation form, K1 ⫽ ln[(temperature of gas-turbine exhaust gas entering the HRSG, ⬚F ⫺ HRSG satu- rated steam temperature, ⬚F)/(gas temperature leaving the evaporator, ⬚F ⫺ HRSG saturate steam temperature, ⬚F)]/(gas flow, lb/h). Substituting, K1 ⫽ ln[(980 ⫺ 388)/(408 ⫺ 388)]/140,000 ⫽ 387.6, where the temperatures used reflect design condition. 4. Compute the revised evaporator performance Under the revised performance conditions, using the given data and the above value of K1 and solving for Tg 2, the evaporator exit gas temperature, ln[(880 ⫺ 366)/ (Tg 2 ⫺ 366)] ⫽ 387.6 Tg 2 ⫽ 388⬚F (197.8⬚C). Then, the evaporator ⫺0.4 (165,000) ; Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 32. 1.30 POWER GENERATION duty, using the same equation as in step 1 above ⫽ (165,000)(0.99)(0.27)(880 ⫺ 388) ⫽ 21.7 MM Btu/h (6.36 MW). In this calculation, we assumed that the exhaust-gas analysis had not changed. If there are changes in the exhaust-gas analysis, then the gas properties must be evaluated and corrections made for variations in the exhaust-gas temperature. See Waste Heat Boiler Deskbook by V. Ganapathy for ways to do this. 5. Find the assumed duty, Qa, for the economizer Let the economizer leaving-water temperature ⫽ 360⬚F (182.2⬚C). The enthalpy of the feedwater ⫽ 332 Btu/lb (773.6 kJ/kg); saturated-steam enthalpy ⫽ 1195.7 Btu/ lb (2785.9 kJ/kg); saturated liquid enthalpy ⫽ 338.5 Btu/lb (788.7 kJ/kg). Then, the steam flow, as before, ⫽ (21.5 ⫻ 106 )/[(1195.7 ⫺ 332) ⫹ 0.05 (338.5 ⫺ 332)] ⫽ 25,115.7 lb/h (11,043 kg/h). Then, the assumed duty for the economizer, Qa ⫽ (25,115.7)(1.05)(332 ⫺ 198.5) ⫽ 3.52 MM Btu/h (1.03 MW). 6. Determine the UA value for the economizer in both design and off-design conditions For the design conditions, UA ⫽ Q /(⌬T), where Q ⫽ economizer duty from step 2, above; ⌬T ⫽ design temperature conditions from the earlier data in this proce- dure. Solving, UA ⫽ (3.84 ⫻ 106 )/{[(299 ⫺ 230) ⫺ (408 ⫺ 373)]/ln(69/35)} ⫽ 76,800 Btu/h ⬚F (40.5 kW). For off-design conditions, UA ⫽ (UA at design con- ditions)(gas flow at off-design/gas flow at design conditions) ⫽ 0.65 (76,800)(165,000/140,000) ⫽ 85,456 Btu/h F (45.1 kW). 0.65 7. Calculate the economizer duty The energy transferred ⫽ Qt ⫽ (UA)(⌬T). Based on 360⬚F (182.2⬚C) water leaving the economizer, Qa ⫽ 3.52 MM Btu/h (1.03 MW). Solving for tg 2 as before ⫽ 382 ⫺ [(3.52 ⫻ 106 )/(165,000)(0.9)(0.253)] ⫽ 388 ⫺ 85 ⫽ 303⬚F (150.6⬚C). Then, ⌬T ⫽ [(303 ⫺ 230) ⫺ (388 ⫺ 360)]/ln(73/28) ⫽ 47⬚F (26.1⬚C). The energy transferred ⫽ Qt ⫽ (UA)(⌬T) ⫽ (85,456)(47) ⫽ 4.01 MM Btu/h (1.18 MW). Since the assumed and transferred duty do not match, i.e., 3.52 MM Btu/h vs. 4.01 MM Btu/h, another iteration is required. Continued iteration will show that when Qa ⫽ Qt ⫽ 3.55 MM Btu/h (1.04 MW), and the temperature of the water leaving the economizer ⫽ 366⬚F (185.6⬚C) (saturation) and exit-gas temperature ⫽ 301⬚F (149.4⬚C), the amount of steam generated ⫽ 25,310 lb/h (11,491 kg/h). Related Calculations. Studying the effect of gas inlet temperature and gas flows on HRSG performance will show that at lower steam generation rates or at lower pressures that the economizer water temperature approaches saturation tem- perature, a situation called ‘‘steaming’’ in the economizer. This steaming condition should be avoided by generating more steam by increasing the inlet gas temperature or through supplementary firing, or by reducing exhaust-gas flow. Supplementary firing in an HRSG also improves the efficiency of the HRSG in two ways: (1) The economizer acts as a bigger heat sink as more steam and hence more feedwater flows through the economizer. This reduces the exit gas tempera- ture. So with a higher gas inlet temperature to the HRSG we have a lower exit gas temperature, thanks to the economizer. (2) Additional fuel burned in the HRSG reduces the excess air as more air is not added; instead, the excess oxygen is used. In conventional boilers we know that the higher the excess air, the lower the boiler efficiency. Similarly, in the HRSG, the efficiency increases with more supplemen- tary firing. HRSGs used in combined-cycle steam cycles, Fig. 17, may use multiple pressure levels, gas-turbine steam injection, reheat, selective-catalytic-reduction (SCR) elements for NOx control, and feedwater heating. Such HRSGs require ex- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 33. 1.31 Reheat steam to i-p turbine H-p steam to h-p turbine To GT steam injection To I-p turbine To condenser Condensate makeup To I-p superheater I-p steam for deaerator I-p steam H-p drum SCR Spray for reheater and superheater desuperheating H-p economizer bypass From deaerator Reheater 1 H-p superheater 1 Reheater 2 H-p superheater 2 H-p evaporator I-p superheater H-p economizer 1 L-p superheater I-p evaporator H-p economizer 2 I-p economizer 3 H-p economizer 3 L-p evaporator Feedwater heater I-p drum L-p drum Deaerator Steam for GT injection For reheater H-p steam Reheat steam from h-p turbine FIGURE 17 HRSGs in combined-cycle steam cycles are somewhat more involved when multiple pressure levels, gas-turbine steam injection, reheat, SCR, and feedwater heating are used (Power). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 34. 1.32 TABLE 2 HRSG Performance in Fired Mode Item Case 1 Case 2 Case 3 Gas flow, lb/h (kg/h) 150,000 (68,100) 150,000 (68,100) 150,000 (68,100) Inlet gas temp, ⬚F (⬚C) 900 (482.2) 900 (482.2) 900 (482.2) Firing temperature, ⬚F (⬚C) 900 (482.2) 1290 (698.9) 1715 (935.0) Burner duty, MM Btu/h (LHV)* 0 (0) 17.3 (5.06) 37.6 (11.01) Steam flow, lb/h (kg/hr) 22,780 (10,342) 40,000 (18,160) 60,000 (27,240) Steam pressure, lb/in2 (gage) (kPa) 200 (1378.0) 200 (1378.0) 200 (1378.0) Feed water temperature, ⬚F (⬚C) 240 (115.6) 240 (115.6) 240 (115.6) Exit gas temperature, ⬚F (⬚C) 327 (163.9) 315 (157.2) 310 (154.4) System efficiency, % 68.7 79.2 84.90 Steam duty, MM Btu/h (MW) 22.67 (6.64) 39.90 (11.69) 59.90 (17.55) *(MW) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 35. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.33 tensive analysis to determine the best arrangement of the various heat-absorbing surfaces. For example, an HRSG generates 22,780 lb/h (10.342 kg/h) of steam in the unfired mode. The various parameters are shown in Table 2. Studying this table shows that as the steam generation rate increases, more and more of the fuel energy goes into making steam. Fuel utilization is typically 100 percent in an HRSG. The ASME efficiency is also shown in the table. This simulation was done using the HRSG simulation software developed by the author, V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. PREDICTING HEAT-RECOVERY STEAM GENERATOR (HRSG) TEMPERATURE PROFILES A gas turbine exhausts 150,000 lb/h (68,100 kg/h) of gas at 900⬚F (482.2⬚C) to an HRSG generating steam at 450 lb/in2 (gage) (3100.5 kPa) and 650⬚F (343.3⬚C). Feedwater temperature to the HRSG is 240⬚F (115.6⬚C) and blowdown is 2 percent. Exhaust gas analysis by percent volume is: CO2 ⫽ 3; H2O ⫽ 7; O2 ⫽ 15. Determine the steam generation and temperature profiles with a 7-lb/in2 (48.2-kPa) pressure drop in the superheater, giving an evaporator pressure of 450 ⫹ 7 ⫽ 457 lb/in2 (gage) (3148.7 kPa) for a saturation temperature of the steam of 460⬚F (237.8⬚C). There is a heat loss of 1 percent in the HRSG. Find the ASME efficiency for this HRSG unit. Calculation Procedure: 1. Select the pinch and approach points for the HRSG Gas turbine heat recovery steam generators (HRSGs) are widely used in cogener- ation and combined-cycle plants. Unlike conventionally fired steam generators where the rate of steam generation is predetermined and can be achieved, steam- flow determination in an HRSG requires an analysis of the gas/steam temperature profiles. This requirement is mainly because we are starting at a much lower gas temperature—900 to 1100⬚F—(482.2 to 593.3⬚C) at the HRSG inlet, compared to 3000 to 3400⬚F (1648.9 to 1871.1⬚C) in a conventionally fired boiler. As a result, the exit gas temperature from an HRSG cannot be assumed. It is a function of the operating steam pressure, steam temperature, and pinch and approach points used, Fig. 18. Typically, the pinch and approach points range from 10 to 30⬚F (5.56 to 16.6⬚C). Higher values may be used if less steam generation is required. In this case, we will use 20⬚F (11.1⬚C) pinch point (⫽ Tg 3 ⫺ ts ) and 10⬚F (5.56⬚C) approach (⫽ ts ⫺ tw 2). Hence, the gas temperature leaving the evaporator ⫽ 460 ⫹ 20 ⫽ 480⬚F (248.9⬚C), and the water temperature leaving the economizer ⫽ 460 ⫺ 10 ⫽ 450⬚F (232.2⬚C). 2. Compute the steam generation rate The energy transferred to the superheater and evaporator ⫽ Q1 ⫹ Q2 ⫽ (rate of gas flow, lb/h)(gas specific heat, Btu/lb ⬚F)(entering gas temperature, ⬚F ⫺ temperature of gas leaving evaporator, ⬚F)(1.0 percent heat loss) ⫽ (150,000)(0.267)(900 ⫺ 480)(0.99) ⫽ 16.65 MM Btu/h (4.879 MW). The enthalpy absorbed by the steam in the evaporator and superheater ⫽ (en- thalpy of the superheated steam at 450 lb/in2 (gage) and 650⬚F ⫺ enthalpy of the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 36. 1.34 POWER GENERATION (a) (b) 900˚F (482˚C) 843˚F (450˚C) 481˚F (249˚C) 372˚F (189˚C) 650˚F (343˚C) 461˚F (238˚C) 451˚F (233˚C) 240˚F (115˚C) FIGURE 18 Gas/steam temperature profiles. water entering the evaporator at 450⬚F) ⫹ (blowdown percentage)(enthalpy of the saturated liquid at the superheated condition ⫺ enthalpy of the water entering the evaporator, all expressed in Btu/lb). Or, enthalpy absorbed in the evaporator and superheater ⫽ (1330.8 ⫺ 431.2) ⫹ (0.02)(442.3 ⫺ 431.2) ⫽ 899.8 Btu/lb (2096.5 kJ/kg). To compute the steam generation rate, set up the energy balance, 899.8(Ws ) ⫽ 16.65 MM Btu/h, where Ws ⫽ steam generation rate. 3. Calculate the energy absorbed by the superheater and the exit gas temperature Q1, the energy absorbed by the superheater ⫽ (steam generation rate, lb/h)(enthalpy of superheated steam, Btu/lb ⫺ enthalpy of saturated steam at the superheater pressure, Btu/lb) ⫽ (18,502)(1330.8 ⫺ 1204.4) ⫽ 2.338 MM Btu/h (0.685 MW). The superheater gas-temperature drop ⫽ (Q1)/(rate of gas-turbine exhaust-gas flow, lb/h)(1.0 ⫺ heat loss)(gas specific heat) ⫽ (2,338,000)/ (150,000)(0.99)(0.273) ⫽ 57.67⬚F, say 58⬚F (32.0⬚C). Hence, the superheater exit gas temperature ⫽ 900 ⫺ 58 ⫽ 842⬚F (450⬚C). In this calculation the exhaust-gas specific heat is taken as 0.273 because the gas temperature in the superheater is different from the inlet gas temperature. 4. Compute the energy absorbed by the evaporator The total energy absorbed by the superheater and evaporator, from the above, is 16.65 MM Btu/h (4.878 MW). Hence, the evaporator duty ⫽ Q2 ⫽ 16.65 ⫺ 2.34 ⫽ 14.31 MM Btu/h (4.19 MW). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 37. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.35 TABLE 3 HRSG Exit Gas Temperatures Versus Steam Parameters* Pressure lb/in2 (gage) (kPa) Steam temp ⬚F (⬚C) Saturation temp ⬚F (⬚C) Exit gas ⬚F (⬚C) 100 (689) sat (170) 338 (170) 300 (149) 150 (1034) sat (186) 366 (186) 313 (156) 250 (1723) sat (208) 406 (208) 332 (167) 400 (2756) sat (231) 448 (231) 353 (178) 400 (2756) 600 (316) 450 (232) 367 (186) 600 (4134) sat (254) 490 (254) 373 (189) 600 (4134) 750 (399) 492 (256) 398 (203) *Pinch point ⫽ 20⬚F (11.1⬚C); approach ⫽ 15⬚F (8.3⬚C); gas inlet temperature ⫽ 900⬚F (482.2⬚C); blowdown ⫽ 0; feedwater temperature ⫽ 230⬚F (110⬚C). 5. Determine the economizer duty and exit-gas temperature The economizer duty, Q3 ⫽ (rate of steam generation, lb/h)(1 ⫹ blowdown ex- pressed as a decimal)(enthalpy of water leaving the economizer ⫺ enthalpy of feedwater at 240⬚F) ⫽ (18,502)(1.02)(431.2 ⫺ 209.6) ⫽ 4.182 MM Btu/h (1.225 MW). The HRSG exit gas temperature ⫽ (480, the exit gas temperature at the evap- orator computed in step 1, above) ⫺ (economizer duty)/(gas-turbine exhaust-gas flow, lb/h)(1.0 ⫺ heat loss)(exhaust gas specific heat) ⫽ 371.73⬚F (188.9⬚C); round to 372⬚F (188.9⬚C). Note that you must compute the gas specific heat at the average gas temperature of each of the heat-transmission surfaces. 6. Compute the ASME HRSG efficiency The ASME Power Test Code PTC 4.4 defines the efficiency of an HRSG as: E ⫽ efficiency ⫽ (energy absorbed by the steam and fluids)/[gas flow ⫻ inlet enthalpy ⫹ fuel input to HRSG on LHV basis]. In the above case, E ⫽ (16.65 ⫹ 4.182)(106 )/(150,000 ⫻ 220) ⫽ 0.63, or 63 percent. In this computation, 220 Btu /lb (512.6 kJ/kg) is the enthalpy of the exhaust gas at 900⬚F (482.2⬚C) and (16.65 ⫹ 4.182) is the total energy absorbed by the steam in MM Btu/h (MW). Related Calculations. Note that the exit gas temperature is high. Further, with- out having done this analytical mathematical analysis, the results could not have been guessed correctly. Minor variations in the efficiency will result if one assumes different pinch and approach points. Hence, it is obvious that one cannot assume a value for the exit gas temperature—say 300⬚F (148.9⬚C)—and compute the steam generation. The gas/steam temperature profile is also dependent on the steam pressure and steam temperature. The higher the steam temperature, the lower the steam gener- ation rate and the higher the exit gas temperature. Arbitrary assumption of the exit gas temperature or pinch point can lead to temperature cross situations. Table 3 shows the exit gas temperatures for several different steam parameters. From the table, it can be seen that the higher the steam pressure, the higher the saturation temperature, and hence, the higher the exit gas temperature. Also, the higher the steam temperature, the higher the exit gas temperature. This results from the re- duced steam generation, resulting in a smaller heat sink at the economizer. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, who is the author of several works listed in the references for this sec- tion. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 38. 1.36 POWER GENERATION FIGURE 19 T-S diagrams for steam turbine. STEAM TURBOGENERATOR EFFICIENCY AND STEAM RATE A 20,000-kW turbogenerator is supplied with steam at 300 lb/in2 (abs) (2067.0 kPa) and a temperature of 650⬚F (343.3⬚C). The backpressure is 1 in (2.54 cm) Hg absolute. At best efficiency, the steam rate is 10 lb (25.4 kg) per kWh. (a) What is the combined thermal efficiency (CTE) of this unit? (b) What is the combined engine efficiency (CEE)? (c) What is the ideal steam rate? Calculation Procedure: 1. Determine the combined thermal efficiency (a) Combined thermal efficiency, CTE ⫽ (3413/wr )(1/[h1 ⫺ h2]), where wr ⫽ combined steam rate, lb/kWh (kg/kWh); h1 ⫽ enthalpy of steam at throttle pressure and temperature, Btu/lb (kJ/kg); h2 ⫽ enthalpy of steam at the turbine backpres- sure, Btu/lb (kJ/kg). Using the steam tables and Mollier chart and substituting in this equation, CTE ⫽ (3413/10)(1/[1340.6 ⫺ 47.06]) ⫽ 0.2638, or 26.38 percent. 2. Find the combined engine efficiency (b) Combined engine efficiency, CEE ⫽ (wi )/(we ) ⫽ (weight of steam used by ideal engine, lb/kWh)/(weight of steam used by actual engine, lb/kWh). The weights of steam used may also be expressed as Btu/lb (kJ/kg). Thus, for the ideal engine, the value is 3413 Btu/lb (7952.3 kJ/kg). For the actual turbine, h1 ⫺ h2x is used, h2x is the enthalpy of the wet steam at exhaust conditions; h1 is as before. Since the steam expands isentropically into the wet region below the dome of the T-S diagram, Fig. 19, we must first determine the quality of the steam at point 2 either from a T-S diagram or Mollier chart or by calculation. By calculation using the method of mixtures and the entropy at each point: S1 ⫽ S2 ⫽ 0.0914 ⫹ (x2)(1.9451). Then x2 ⫽ (1.6508 ⫺ 0.0914)/1.9451 ⫽ 0.80, or 80 percent quality. Substituting and summing, using steam-table values, h2x ⫽ 47.06 ⫹ 0.8(1047.8) ⫽ 885.3 Btu/lb (2062.7 kJ/kg). (c) To find the CEE we first must obtain the ideal steam rate, wi ⫽ 3413/(h1 ⫺ h2x ) ⫽ 3413/(1340.6 ⫺ 885.3) ⫽ 7.496 lb/kWh (3.4 kg/kWh). Now, CEE ⫽ (7.496/10)(100) ⫽ 74.96 percent. This value is excellent for such a plant and is in a range being achieved today. Related Calculations. Use this approach to analyze the efficiency of any tur- bogenerator used in central-station, industrial, marine, and other plants. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 39. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.37 TURBOGENERATOR REHEAT-REGENERATIVE CYCLE ALTERNATIVES ANALYSIS A turbogenerator operates on the reheating-regenerative cycle with one stage of reheat and one regenerative feedwater heater. Throttle steam at 400 lb/in2 (abs) (2756.0 kPa) and 700⬚F (371.1⬚C) is used. Exhaust at 2-in (5.08-cm) Hg is taken from the turbine at a pressure of 63 lb/in2 (abs) (434 kPa) for both reheating and feedwater heating with reheat to 700⬚F (371.1⬚C). For an ideal turbine working under these conditions, find: (a) Percentage of throttle steam bled for feedwater heating; (b) Heat converted to work per pound (kg) of throttle steam; (3) Heat supplied per pound (kg) of throttle steam; (d) Ideal thermal efficiency; (e) Other ways to heat feedwater and increase the turbogenerator output. Figure 20 shows the layout of the cycle being considered, along with a Mollier chart of the steam conditions. Calculation Procedure: 1. Using the steam tables and Mollier chart, list the pertinent steam conditions Using the subscript 1 for throttle conditions, list the key values for the cycle thus: 2 P ⫽ 400 lb/in (abs) (2756.0 kPa) 1 t ⫽ 700⬚F (371.1⬚C) 1 H ⫽ 1362.2 Btu/lb (3173.9 kJ/kg) 1 S ⫽ 1.6396 1 H ⫽ 1178 Btu/lb (2744.7 kJ/kg) 2 H ⫽ 1380.1 Btu/lb (3215.6 kJ/kg) 3 H ⫽ 1035.8 (2413.4 kJ/kg) 4 H ⫽ 69.1 Btu/lb (161.0 kJ/kg) 5 H ⫽ 265.27 Btu/lb (618.07 kJ/kg) 6 2. Determine the percentage of throttle steam bled for feedwater heating (a) Set up the ratio for the feedwater heater of (heat added in the feedwater heater)/(heat supplied to the heater)(100). Or, using the enthalpy data from step 1 above, (H6 ⫺ H5)/(H2 ⫺ H5)(100) ⫽ (265.26 ⫺ 69.1)/(1178 ⫺ 69.1)(100) ⫽ 17.69 percent of the throttle steam is bled for feedwater heating. 3. Find the heat converted to work per pound (kg) of throttle steam (b) The heat converted to work is the enthalpy difference between the throttle steam and the bleed steam at point 2 plus the enthalpy difference between points 3 and 4 times the percentage of throttle flow between these points. In equation form, heat converted to work ⫽ H1 ⫺ H2 ⫹ (1.00 ⫺ 0.1769)(H3 ⫺ H4) ⫽ (1362.2 ⫺ 1178) ⫹ (0.0823)(1380.1 ⫺ 1035.8) ⫽ 467.55 Btu/lb (1089.39 kJ/kg). 4. Calculate the heat supplied per pound (kg) of throttle steam (c) The heat supplied per pound (kg) of throttle steam ⫽ (H1 ⫺ H6) ⫹ (H3 ⫺ H2) ⫽ (1362.3 ⫺ 265.27) ⫹ (1380.1 ⫺ 1178) ⫽ 1299.13 Btu/lb (3026.97 kJ/kg). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 40. 1.38 FIGURE 20 Cycle layout and TS chart of steam conditions. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 41. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.39 FIGURE 21 Heat input to the economizer may be increased by the addition of induct burners, by bypassing hot furnace gases into the gas path ahead of the economizer, or by recirculation (Power). 5. Compute the ideal thermal efficiency (d) Use the relation, ideal thermal efficiency ⫽ (heat converted to work)/(heat supplied) ⫽ 467.55/1299.13 ⫽ 0.3598, or 35.98 percent. 6. Show other ways to heat feedwater while increasing the turbogenerator output For years, central stations and large industrial steam-turbine power plants shut off feedwater heaters to get additional kilowatts out of a turbogenerator during periods of overloaded electricity demand. When more steam flows through the turbine, the electrical power output increases. While there was a concurrent loss in efficiency, this was ignored because the greater output was desperately needed. Today steam turbines are built with more heavily loaded exhaust ends so that the additional capacity is not available. Further, turbine manufacturers place restric- tions on the removal of feedwater heaters from service. However, if the steam output of the boiler is less than the design capacity of the steam turbine, because of a conversion to coal firing, additional turbogenerator capacity is available and can be regained at a far lower cost than by adding new generator capacity. Compensation for the colder feedwater can be made, and the lost efficiency regained, by using a supplementary fuel source to heat feedwater. This can be done in one of two ways: (1) increase heat input to the existing boiler economizer, or (2) add a separately fired external economizer. Additional heat input to a boiler’s existing economizer can be supplied by in- duct burners, Fig. 21, from slagging coal combustors, Fig. 22, or from the furnace itself. Since the economizer in a coal-fired boiler is of sturdier construction than a heat-recovery steam generator (HRSG) with finned tubing, in-duct burners can be placed closer to the economizer, Fig. 21. Burner firing may be by coal or oil. Slagging coal combustors are under intense development. A low-NOx, low-ash combustor, Fig. 22, supplying combustion gases at 3000⬚F (1648.9⬚C) may soon be commercially available. To accommodate any of the changes shown in Fig. 21, a space from 12 (3.66 m) to 15 ft (4.57 m) is needed between the bottom of the primary superheater and the top of the economizer. This space is required for the installation of the in-duct Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 42. 1.40 POWER GENERATION FIGURE 22 Slagging combustors can be arranged to inject hot combustion gases into gas pas- sages ahead of economizer (Power). Generator L-p turbine H-p turbine I-p turbine FIGURE 23 Gas-turbine exhaust gases can be used in place of high-pressure heaters, using a compact heat exchanger (Power). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 43. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.41 FIGURE 24 Gas-turbine heat-recovery finned-tube heat exchanger is simple and needs no elab- orate controls (Power). FIGURE 25 Reheat expansion line is moved to the left on the T-S chart, increasing power output (Power). burners or for the adequate mixing of gas streams if the furnace or an external combustor is used to supply the additional heat. Another approach is to install a separately fired external economizer in series or parallel with the existing economizer, which could be fired by a variety of fuels. The most attractive possibility is to use waste heat from a gas-turbine exhaust, Fig. 23. The output of this simple combined-cycle arrangement would actually be higher than the combined capabilities of the derated plant and the gas turbine. The steam-cycle arrangement for the combined plant is shown in Fig. 23. Feed- water is bypassed around the high-pressure regenerative heaters to an external low- cost, finned-tube heat exchanger, Fig. 24, where waste heat from the gas turbine is recovered. When high-pressure feedwater heaters are shut off, steam flow through the in- termediate- and low-pressure turbine sections increases and becomes closer to the full-load design flow. The reheat expansion line moves left on the Mollier chart from its derated position, Fig. 25. With steam flow closer to the design value, the exhaust losses per pound (kg) of steam, Fig. 26, are lower than at the derated load. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 44. 1.42 POWER GENERATION FIGURE 26 Heat loss in steam-turbine exhaust is reduced when operating at rated flow (Power). 1453 Btu/lb (3385 kJ/kg) 898 Btu/lb (2092 kJ/kg) 850 psia (5856 kPa) 1000 Btu/lb (2330 kJ/kg) 1.5 in. Hg (3.8 cm Hg) FIGURE 27 Mollier chart for turbine exhaust conditions. The data and illustrations in this step 6 are based on the work of E. S. Miliares and P. J. Kelleher, Energotechnology Corp., as reported in Power magazine. TURBINE-EXHAUST STEAM ENTHALPY AND MOISTURE CONTENT What is the enthalpy and percent moisture of the steam entering a surface condenser from the steam turbine whose Mollier chart is shown in Fig. 27? The turbine is delivering 20,000 kW and is supplied steam at 850 lb/in2 (abs) (5856.5 kPa) and 900⬚F (482.2⬚C); the exhaust pressure is 1.5 in (3.81 cm) Hg absolute. The steam rate, when operating straight condensing, is 7.70 lb/delivered kWh (3.495 kg/kWh) and the generator efficiency is 98 percent. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 45. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.43 Calculation Procedure: 1. Compute the engine efficiency of the turbine Use the relation Ee ⫽ 3413/(ws)(H1 ⫺ Hc), where Ee ⫽ engine efficiency; ws ⫽ steam rate of the turbine when operating straight condensing in the units given above; enthalpies H1 and Hc are as shown in the Mollier chart. Substituting, Ee ⫽ 3413/(7.7)(1453 ⫺ 898) ⫽ 0.7986 for ideal conditions. 2. Find the Rankine engine efficiency for the actual turbine The Rankine engine efficiency for this turbine is: (0.7986/0.98) ⫽ 0.814 ⫽ (H1 ⫺ /(H1 ⫺ Hc). Solving, (H1 ⫺ ⫽ 0.814(555) ⫽ 452.3 Btu/lb (1053.8 kJ/kg). H ) H ) c⬘ c⬘ At the end of the actual expansion of the steam in the turbine, ⫽ 1453 ⫺ Hc⬘ 452.3 ⫽ 1000.7 Btu/lb (2331.6 kJ/kg) enthalpy. 3. Determine the moisture of the steam Referring to the Mollier chart where crosses the pressure line of 1.5 in (3.81 Hc⬘ cm) Hg, the moisture percent is found to be 9.6 percent. Related Calculations. The Mollier chart can be a powerful and quick reference for solving steam expansion problems in plants of all types—utility, industrial, commercial, and marine. STEAM TURBINE NO-LOAD AND PARTIAL-LOAD STEAM FLOW RATES A 40,000-kW straight-flow condensing industrial steam turbogenerator unit is sup- plied steam at 800 lb/in2 (abs) (5512 kPa) and 800⬚F (426.7⬚C) and is to exhaust at 3 in (76 cm) Hg absolute. The half-load and full-load throttle steam flows are estimated to be 194,000 lb/h (88,076 kg/h) and 356,000 lb/h (161,624 kg/h), respectively. The mechanical efficiency of the turbine is 99 percent and the gen- erator efficiency is 98 percent. Find (a) the no-load throttle steam flow; (b) the heat rate of the unit expressed as a function of the kW output; (c) the internal steam rate of the turbine at 30 percent of full load. Calculation Procedure: 1. Find the difference between full-load and half-load steam rates and the no-load rate (a) Assume a straight-line rating characteristic and plot Fig. 28a. This assumption is a safe one for steam turbines in this capacity range. Then, the difference between full-load and half-load steam rates is 356,000 ⫺ 194,000 ⫽ 162,000 lb/h (73,548 kg/h). The no-load steam rate will then be ⫽ (half-load rate) ⫺ (difference between full-load and half-load rates) ⫽ 194,000 ⫺ 162,000 ⫽ 32,000 lb/h (14,528 kg/h). 2. Determine the steam rate and heat rate at quarter-load points (b) Using Fig. 28b, we see that the actual turbine efficiency, Et ⫽ 3413/(ws)(H1 ⫺ ws ⫽ steam flow, lb/kWh (kg/kWh); H1 ⫽ enthalpy of entering steam, Btu/ H ), ƒ lb (kJ/kg); ⫽ enthalpy of condensate at the exhaust pressure, Btu/lb (kJ/kg). Hƒ Further, turbine heat rate ⫽ 3413/Et Btu/kWh (kJ/kWh) ⫽ wk(H1 ⫺ where H ), ƒ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 46. 1.44 POWER GENERATION (a) (b) 356,000 lb/hr (161,624 kg/hr) 194,000 lb/hr (88,076 kg/hr) FIGURE 28 (a) Straight-line rating characteristic. (b) T-S diagram. wk ⫽ the actual steam rate, lb/kWh (kg/kWh) ⫽ w ⫽ ws /kW output, where the symbols are as defined earlier. Substituting ws ⫽ 32,000 no-load throttle flow ⫹ (difference between full-load and half-load throttle flow rate/kW output at half load)(kW output) ⫽ 32,000 ⫹ (162,000/20,000)(kW) ⫽ 32,000 ⫹ 8.1 kW for this turbine-generator set. Also wk ⫽ (32,000/kW) ⫹ 8.1. Using the steam tables, we find H1 ⫽ 1398 Btu/lb (3257.3 kJ/kg); ⫽ 83 Hƒ Btu/lb (193.4 kJ/kg). Then, H1 ⫺ ⫽ 1315 Btu/lb (3063.9 kJ/kg). Substituting, Hƒ heat rate ⫽ [(1315)(32,000)/(kW)] ⫹ (1316)(8.1) ⫽ 10.651.5 ⫹ (42,080,000/kW). Computing the steam rate and heat rate for the quarter-load points for this turbine-generator we find: At full load, w ⫽ 8.9 lb/kWh (4.04 kg/kWh) s 3 At ⁄4 load, w ⫽ 9.17 lb/kWh (4.16 kg/kWh) s 1 At ⁄2 load, w ⫽ 9.7 lb/kWh (4.4 kg/kWh) s 1 At ⁄4 load, w ⫽ 11.3 lb/kWh (5.13 kg/kWh) s At full load, heat rate ⫽ 11,700 Btu/kWh (27,261 kJ/kWh) 3 At ⁄4 load, heat rate ⫽ 12,080 Btu/kWh (28,146 kJ/kWh) 1 At ⁄2 load, heat rate ⫽ 12,770 Btu/kWh (29,754 kJ/kWh) 1 At ⁄4 load, heat rate ⫽ 14,870 Btu/kWh (34,647 kJ/kWh) 3. Determine the internal steam rate of the turbine (c) For the turbine and generator combined, Ee ⫽ 3413/(wk)(H1 ⫺ Hc), where Ee ⫽ turbine engine efficiency; Hc ⫽ enthalpy of the steam at the condenser; other symbols as given earlier. Since, from the steam tables, H1 ⫽ 1398 Btu/lb (3257.3 kJ/kg); Hc ⫽ 912 Btu/lb (2124.9 kJ/kg); then (H1 ⫺ Hc) ⫽ 486 Btu/lb (1132.4 kJ/kg). From earlier steps, ws ⫽ 356,000 lb/h (161,624 kg/h) at full-load; ws ⫽ 32,000 lb/h (14,528 kg/h) at no-load. For the full-load range the total change is 356,000 ⫺ 32,000 ⫽ 324,000 lb/h (147,096 kg/h). Then, ws at 30 percent load ⫽ [(32,000) ⫹ 0.30(324,000)]/0.30(40,000) ⫽ 10.77 lb/kWh (4.88 kg/kWh). Then, Ee ⫽ 3413/(10.77)(486) ⫽ 0.652 for combined turbine and generator. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 47. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.45 If the internal efficiency of the turbine (not including the friction loss) Ei, then Ei ⫽ 2545/(wa)(H1 ⫺ Hc). Thus Ei ⫽ Ee /(turbine mechanical efficiency)(generator efficiency). Or Ei ⫽ 0.652/(0.99)(0.98) ⫽ 0.672. Then, the actual steam rate per horsepower (kW) is wa ⫽ 2545/(Ei)(H1 ⫺ Hc) ⫽ 2545/(0.672)(486) ⫽ 7.79 lb/hp (4.74 kg/kW). Related Calculations. Use this approach to analyze any steam turbine—utility, industrial, commercial, marine, etc.—to determine the throttle steam flow and heat rate. POWER PLANT PERFORMANCE BASED ON TEST DATA A test on an industrial turbogenerator gave these data: 29,760 kW delivered with a throttle flow of 307,590 lb/h (139,646 kg/h) of steam at 245 lb/in2 (abs) (1688 kPa) with superheat at the throttle of 252⬚F (454⬚C); exhaust pressure 0.964 in (2.45 cm) Hg (abs); pressure at the one bleed point, Fig. 29a, 28.73 in (72.97 cm) Hg (abs); temperature of feedwater leaving bleed heater 163⬚F (72.8⬚C). For the corresponding ideal unit, find: (a) percent throttle steam bled, (b) net work for each pound of throttle steam, (c) ideal steam rate, and (d) cycle efficiency. For the actual unit find, (e) the combined steam rate, (ƒ) combined thermal efficiency, and (g) combined engine efficiency. Calculation Procedure: 1. Determine the steam properties at key points in the cycle Using a Mollier chart and the steam tables, plot the cycle as in Fig. 29b. Then, S1 ⫽ 1.676; H1 ⫽ 1366 Btu/lb (3183 kJ/kg); H2 ⫽ 1160 Btu/lb (2577 kJ/kg); P2 ⫽ 14.11 lb/in2 (abs) (97.2 kPa); H3 ⫽ 130.85 Btu/lb (304.9 kJ/kg); P3 ⫽ 5.089 lb/ in2 (abs) (35.1 kPa); H4 ⫽ 46.92 Btu/lb (109.3 kJ/kg); P4 ⫽ 0.4735 lb/in2 (abs) (3.3 kPa); H5 ⫽ 177.9 Btu/lb (414.5 kJ/kg). (a) The percent throttle steam bled is found from: 100 ⫻ (H5 ⫺ H4)/(H2 ⫺ H4) ⫽ 100 ⫻ (177.9 ⫺ 46.92)/(1106 ⫺ 46.92) ⫽ 12.41 percent. 2. Find the amount of heat converted to work (b) Use the relation, heat converted to work, hw ⫽ H1 ⫺ H2 ⫹ (1 ⫺ m2)(H2 ⫺ H7), where m2 ⫽ percent throttle steam bled, H7 ⫽ enthalpy of exhaust steam in the condenser. Substituting, heat converted to work, hw ⫽ (1366 ⫺ 1106) ⫹ (1 ⫺ 0.1241)(1106 ⫺ 924.36) ⫽ 419.1 Btu/lb (976.5 kJ/kg). 3. Compute the ideal steam rate (c) Use the relation, ideal steam rate, lr ⫽ 3413 Btu/kWhr/hw. Or, lr ⫽ 3413/419.1 ⫽ 8.14 lb/kWh (3.69 kg/kWh). 4. Find the cycle efficiency of the ideal cycle (d) Cycle efficiency, Ce ⫽ (heat converted into work/heat supplied). Or hw /(H1 ⫺ H3); substituting, Ce ⫽ 419.1/(1366 ⫺ 130.85) ⫽ 0.3393, or 33.9 percent. 5. Determine the combined steam rate (e) The combined steam rate for the actual unit is Rc ⫽ lb steam consumed/kWh generated. Or Rc ⫽ 307,590/29,760 ⫽ 10.34 lb/kWh (4.69 kg/kWh). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 48. 1.46 POWER GENERATION (a) (b) 245 psia (1688 kPa) 28.73 in. Hg (72.97 cm Hg) 252˚F (454˚C) 163˚F (72.8˚C) 0.963 in. Hg (2.45 cm Hg) FIGURE 29 (a) Cycle diagram with test conditions. (b) T-S diagram for cycle. 6. Find the combined thermal efficiency of the actual unit (ƒ) The combined thermal efficiency, TEc ⫽ 3413/heat supplied. Or TEc ⫽ 3413/ 10.34(H1 ⫺ H3) ⫽ 35413/10.34(1366 ⫺ 130.85) ⫽ 0.267, or 26.7 percent. 7. Compute the combined engine efficiency (g) The combined engine efficiency TEc /Cc, or 26.7/33.9 ⫽ 0.7876, or 78.76 per- cent. Related Calculations. Use this general procedure to determine the percent bleed steam, net work of each pound of throttle steam, ideal steam rate, cycle efficiency, combined thermal efficiency and combined engine efficiency for steam- turbine installations in central stations, industrial, municipal and marine installa- tions. Any standard set of steam tables and a Mollier chart are sufficiently accurate for usual design purposes. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 49. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.47 DETERMINING TURBOGENERATOR STEAM RATE AT VARIOUS LOADS A 100-MW turbogenerator is supplied steam at 1250 lb/in2 (abs) (8612.5 kPa) and 1000⬚F (537.8⬚C) with a condenser pressure of 2 in (5.08 cm) Hg (abs). At rated load, the turbine uses 1,000,000 lb (454,000 kg) of steam per hour; at zero load, steam flow is 50,000 lb/h (22,700 kg/h). What is the steam rate in pounds (kg) per kWh at 4 ⁄4, 3 ⁄4, 2 ⁄4, and 1 ⁄4 load? Calculation Procedure: 1. Write the steam-flow equation for this turbogenerator The curve of steam consumption, called the Willian’s line, is practically a straight line for steam turbines operating without overloads. Hence, we can assume a straight line for this turbogenerator. If the Willian’s line is extended to intercept the Y (vertical) axis for total steam flow per hour, this intercept represents the steam required to operate the turbine when delivering no power. This no-load steam flow—50,000 lb/h (22,700 kg/h) for this turbine—is the flow rate required to overcome the friction of the turbine and the windage, governor and oil-pump drive power, etc., and for meeting the losses caused by turbulence, leakage, and radiation under no-load conditions. Using the data provided, the steam rate equation can be written as [(50/L) ⫹ 9.5] ⫽ (F/L) ⫽ [50 ⫹ (1000 ⫺ 50)/100(L)]/(L), where F ⫽ full-load steam flow, lb/h (kg/h); L ⫽ load percent. 2. Compute the steam flow at various loads Use the equation above thus: Load fraction Load, MW Steam rate lb/kWh (kg/kWh) 1 ⁄4 100 ⫻ 1 ⁄4 ⫽ 25 50 ⁄25 ⫹ 9.5 ⫽ 11.5 (5.22) 2 ⁄4 100 ⫻ 2 ⁄4 ⫽ 50 50 ⁄50 ⫹ 9.5 ⫽ 10.5 (4.77) 3 ⁄4 100 ⫻ 3 ⁄4 ⫽ 75 50 ⁄75 ⫹ 9.5 ⫽ 10.17 (4.62) 4 ⁄4 100 ⫻ 4 ⁄4 ⫽ 100 50 ⁄100 ⫹ 9.5 ⫽ 10.00 (4.54) Related Calculations. The Willian’s line is a useful tool for analyzing steam- turbine steam requirements. As a check on its validity, compare actual turbine performance steam conditions with those computed using this procedure. The agree- ment is startlingly accurate. ANALYSIS OF REHEATING-REGENERATIVE TURBINE CYCLE An industrial turbogenerator operates on the reheating-regenerative cycle with one reheat and one regenerative feedwater heater. Throttle steam at 400 lb/in2 (abs) (2756 kPa) and 700⬚F (371⬚C) is used. Exhaust is at 2 in (5.1 cm) Hg (abs). Steam is taken from the turbine at a pressure of 63 lb/in2 (abs) (434 kPa) for both reheating Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 50. 1.48 POWER GENERATION (a) (b) 400 psia (2756 kPa) 700˚F (371˚C) 63 psia (434 kPa) 2 in. Hg (5.1 cm Hg) FIGURE 30 (a) Cycle diagram. (b) T-S diagram for cycle in (a). and feedwater heating. Reheat is to 700⬚F (371⬚C). For the ideal turbine working under these conditions find: (a) percentage of throttle steam bled for feedwater heating, (b) heat converted to work per pound (kg) of throttle steam, (c) heat sup- plied per pound (kg) of throttle steam, (d) ideal thermal efficiency, (e) T-S, tem- perature-entropy, diagram and layout of cycle. Calculation Procedure: 1. Determine the cycle enthalpies, pressures, and entropies Using standard steam tables and a Mollier chart, draw the cycle and T-S plot, Fig. 30a and b. Then, P1 ⫽ 400 lb/in2 (abs) (2756 kPa); t1 ⫽ 700⬚F (371⬚C); H1 ⫽ 1362.3 Btu/lb (3174 kJ/kg); S1 ⫽ 1.6396; H2 ⫽ 1178 Btu/lb (2745 kJ/kg); Hg ⫽ 1380.1 Btu/lb (3216 kJ/kg); Sg ⫽ 1.8543. (a) Percent throttle steam bled ⫽ (H6 ⫺ H5)/(H2 ⫺ H5) ⫽ (196.15/1107.9) ⫽ 0.1771, or 17.71 percent. 2. Find the amount of heat converted to work per pound (kg) of throttle steam (b) The amount of heat converted to work per pound (kg) of throttle steam ⫽ (H1 ⫺ H2) ⫹ (1 ⫺ 0.1771)(Hg ⫺ H4) ⫽ 467.3 Btu/lb (1088.8 kJ/kg). 3. Compute the heat supplied per pound (kg) of throttle steam (c) The heat supplied per pound (kg) of throttle steam ⫽ (H1 ⫺ H6) ⫹ (Hg ⫺ H2) ⫽ 1299.1 Btu/lb (3026.9 kJ/kg). 4. Determine the ideal thermal efficiency (d) The ideal thermal efficiency ⫽ (heat recovered per pound [kg] of throttle steam)/ (heat supplied per pound [kg] of throttle steam) ⫽ 467.3/1299.13 ⫽ 0.3597, or 35.97 percent. The T-S diagram and cycle layout can be drawn as shown in Fig. 30a and b. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 51. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.49 Related Calculations. This general procedure can be used for any turbine cycle where reheating and feedwater heating are part of the design. Note that the enthalpy and entropy values read from the Mollier chart, or interpolated from the steam tables, may differ slightly from those given here. This is to be expected where judgement comes into play. The slight differences are unimportant in the analysis of the cycle. The procedure outlined here is valid for industrial, utility, commercial, and ma- rine turbines used to produce power. STEAM RATE FOR REHEAT-REGENERATIVE CYCLE Steam is supplied at 600 lb/in2 (abs) (4134 kPa) and 740⬚F (393⬚C) to a steam turbine operating on the reheat-regenerative cycle. After expanding to 100 lb/in2 (abs) (689 kPa), the steam is reheated to 700⬚F (371⬚C). Expansion then continues to 10 lb/in2 (abs) (68.9 kPa) but at 30 lb/in2 (abs) (207 kPa) some steam is extracted for feedwater heating in a direct-contact heater. Assuming ideal operation with no losses, find: (a) steam extracted as a percentage of steam supplied to the throttle, (b) steam rate in pounds (kg) per kWh; (c) thermal efficiency of the turbine, (d) quality or superheat of the exhaust if in the actual turbine combined efficiency is 72 percent, generator efficiency is 94 percent, and actual extraction is the same as the ideal. Calculation Procedure: 1. Assemble the key enthalpies, entropies, and pressures for the cycle Using the steam tables and a Mollier chart, list the following pressures, tempera- tures, enthalpies, and entropies for the cycle, Fig. 31a: Pl ⫽ 600 lb/in2 (abs) (4134 kPa); t1 ⫽ 740⬚F (393⬚C); P2 ⫽ 100 lb/in2 (abs) (689 kPa); t3 ⫽ 700⬚F (371⬚C); Px ⫽ 30 lb/in2 (abs) (207 kPa); pc ⫽ 1 lb/in2 (abs) (6.89 kPa); H1 ⫽ 1372 Btu/lb (3197 kJ/kg); S1 ⫽ entropy ⫽ 1.605; H2 ⫽ 1188 Btu/lb (2768 kJ/kg); H3 ⫽ 1377 Btu/lb (3208 kJ/kg); S3 ⫽ 1.802; Hx ⫽ 1245 Btu/lb (2901 kJ/kg); Hc ⫽ 1007 Btu/lb (2346 kJ/kg); ⫽ 70 Btu/lb (163 kJ/kg); ⫽ 219 Btu/lb (510 kJ/kg). H H ƒ ƒx Plot Fig. 31b as a skeleton Mollier chart to show the cycle processes. 2. Compute the percent steam extracted for the feedwater heater (a) The steam extracted for the feedwater heater, x, ⫽ ⫽ (219 (H ⫺ H )(H ⫺ H ) ƒx ƒ x ƒ ⫺ 70)/(1245 ⫺ 70) ⫽ 0.1268, or 12.68 percent. 3. Find the turbine steam rate (b) For the Rankine-cycle steam rate, ws ⫽ 3413/(H1 ⫺ Hc). For this cycle, ws ⫽ 3413/[(H1 ⫺ H2) ⫹ x(H3 ⫺ Hx) ⫹ (1 ⫺ x)(H3 ⫺ Hc)]. Or, ws ⫽ 3413/[1372 ⫺ 1188) ⫹ 0.1268(1377 ⫺ 1245) ⫹ (1 ⫺ 0.1268)(1377 ⫺ 1007)] ⫽ 6.52 lb/kWh (2.96 kg/kWh). 4. Calculate the turbine thermal efficiency (c) The thermal efficiency, Et ⫽ [(H1 ⫺ H2) ⫹ x(H3 ⫺ Hx) ⫹ (1 ⫺ x)(H3 ⫺ Hc)]/ [H3 ⫺ H2) ⫹ (H1 ⫺ Or, Et ⫽ [(1372 ⫺ 1188) ⫹ (0.1268)(1377 ⫺ 1245) ⫹ H )]. ƒx Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 52. 1.50 POWER GENERATION (a) (b) 600 psia (4134 kPa) 740˚F (393˚C) 700˚F (371˚C) 100 psia (68.9 kPa) 1188 Btu/lb (2768 kJ/kg) 1372 Btu/lb (3197 kJ/kg) 1245 Btu/lb (2901 kJ/kg) 1 psia (6.89 kPa) 30 psia (207 kPa) 1007 Btu/lb (2346 kJ/kg) 70 Btu/lb (163 kJ/kg) FIGURE 31 (a) Cycle diagram. (b) H-S chart for cycle in (a). (1 ⫺ 0.1268)(1377 ⫺ 1007)]/[(1377 ⫺ 1188) ⫹ (1372 ⫺ 219) ⫽ 0.3903, or 39 percent. It is interesting to note that in an ideal cycle the thermal efficiency of the turbine is the same as that of the cycle. 5. Determine the condition of the exhaust (d) The engine efficiency of the turbine alone ⫽ (actual turbine combined efficiency)/actual generator efficiency). Or, using the given data, engine efficiency of the turbine alone ⫽ 0.72/0.94 ⫽ 0.765. Using the computed engine efficiency of the turbine alone and the Mollier chart, (H3 ⫺ ⫽ 0.765(H3 ⫺ HC) ⫽ 283. Solving, ⫽ H3 ⫺ 283 ⫽ 1094 Btu/lb H ) H c⬘ c⬘ (2549 kJ/kg). From the Mollier chart, the condition at is 1.1 percent moisture. Hc⬘ The exhaust steam quality is therefore 100 ⫺ 1.1 ⫽ 98.9 percent. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 53. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.51 Related Calculations. This procedure is valid for a variety of cycle arrange- ments for industrial, central-station, commercial and marine plants. By using a combination of the steam tables, Mollier chart and cycle diagram, a full analysis of the plant can be quickly made. BINARY CYCLE PLANT EFFICIENCY ANALYSIS A binary cycle steam and mercury plant is being considered by a public utility. Steam and mercury temperature will be 1000⬚F (538⬚C). The mercury is condensed in the steam boiler, Fig. 32a at 10 lb/in2 (abs) (68.9 kPa) and the steam pressure is 1200 lb/in2 (abs) (8268 kPa). Condenser pressure is 1 lb/in2 (abs) (6.89 kPa). Expansions in both turbines are assumed to be at constant entropy. The steam cycle has superheat but no reheat. Find the efficiency of the proposed binary cycle. Find the cycle efficiency without mercury. Calculation Procedure: 1. Tabulate the key enthalpies and entropies for the cycle Set up two columns, thus: Mercury cycle Steam cycle Hm1 ⫽ 151.1 Btu/lb (352 kJ/kg) Hs1 ⫽ 1499.2 Btu/lb (3493 kJ/kg) Sm1 ⫽ 0.1194 Ss1 ⫽ 1.6293 Sme ⫽ 0.0094 ⫽ 69.7 Btu/lb (162.4 kJ/kg) Hsf ⫽ 22.6 Btu/lb (52.7 kJ/kg) Hmf 2. Compute the quality of the exhaust for each vapor Since expansion in each turbine is at constant entropy, Fig. 32b, the quality for the mercury exhaust, xm is: 0.1194 ⫽ 0.0299 ⫹ xm(0.1121); xm ⫽ 0.798. For the steam cycle, the quality, xs is: 1.6293 ⫽ 0.1326 ⫹ xs(1.8456); xs ⫽ 0.81. 3. Find the exhaust enthalpy for each vapor Using the properties of mercury from a set of tables, the enthalpy of the mercury exhaust, Hme ⫽ 22.6 ⫹ 0.798(123) ⫽ 120.7 Btu/lb (281.2 kJ/kg). The enthalpy of the condensed mercury, ⫽ 22.6 Btu/lb (52.7 kJ/kg). Hmf For the exhaust steam, the enthalpy Hse ⫽ 69.7 ⫹ 0.81(1036.3) ⫽ 909.1 Btu/ lb (2118 kJ/kg), using steam-table data. The enthalpy of the condensed steam, ⫽ 69.7 Btu/lb (162.4 kJ/kg). Hsf Assuming 98 percent quality steam leaving the mercury condenser, then the enthalpy of the wet steam leaving the mercury condenser, Hsw ⫽ 571.7 ⫹ 0.98(611.7) ⫽ 1171.2 Btu/lb (2728 kJ/kg). 4. Write the heat balance around the mercury condenser The steam heat gain ⫽ Hsw ⫺ ⫽ 1171.2 ⫺ 69.7 ⫽ 1101.5 Btu/lb (2566.5 kJ/ Hsf kg). Now, the mercury heat loss ⫽ Hme ⫺ ⫽ 120.7 ⫺ 98.1 ⫽ 98.1 Btu/lb Hmf Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 54. 1.52 POWER GENERATION FIGURE 32 (a) Binary cycle. (b) T-S diagram for binary cycle. (228.6 kJ/kg). The weight of mercury per pound (kg) of steam ⫽ steam heat gain/mercury heat loss ⫽ 1101.5/98.1 ⫽ 11.23. 5. Determine the heat input and work done per pound (kg) of steam The heat input per pound of steam is: For mercury ⫽ (lb Hg/lb steam)(Hm1 ⫺ ⫽ 11.23(151.1 ⫺ 22.6) ⫽ 1443.05 Btu (1522.5 J). For steam ⫽ (Hs1 ⫺ Hsw) H ) mf ⫽ 1499.2 ⫺ 1171.7 ⫽ 327.5 Btu (345.5 J). Summing these two results gives 1443.05 ⫹ 327.5 ⫽ 1770.55 Btu (1867.9 J) as the heat input per pound (kg) of steam. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 55. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.53 The work done per pound (kg) of steam is: For mercury ⫽ (lb Hg/lb steam)(hm1 ⫺ Hme) ⫽ 11.23(151.1 ⫺ 120.7) ⫽ 341.4 Btu (360.2 J). For steam ⫽ Hc1 ⫺ Hse ⫽ 1499.2 ⫺ 909.1 ⫽ 590.1 Btu (622.6 J). Summing, as before, the total work done per pound (kg) of steam ⫽ 931.5 Btu (982.7 J). 6. Compute the binary cycle efficiency The binary cycle efficiency ⫽ (work done per pound (kg) of steam)/(heat input per pound (kg) of steam). Or binary cycle efficiency ⫽ 931.5/1770.55 ⫽ 0.526, or 52.6 percent. 7. Calculate the steam cycle efficiency without the mercury topping turbine The steam cycle efficiency without the mercury topping turbine ⫽ (work done per pound (kg) of steam)/(Hs1 ⫺ ⫽ 590.1/(1499.2 ⫺ 69.7) ⫽ 0.4128, or 41.3 H ) sf percent. Related Calculations. Any binary cycle being considered for an installation depends on the effects of the difference in thermodynamic properties of the two pure fluids involved. For example, steam works under relatively high pressures with an attendant relatively low temperature. Mercury, by comparison, has the vapor characteristic of operating under low pressures with attendant high temperature. In a mercury-vapor binary cycle, the pressures are selected so the mercury vapor condenses at a temperature higher than that at which steam evaporates. The pro- cesses of mercury vapor condensation and steam evaporation take place in a com- mon vessel called the condenser-boiler, which is the heart of the cycle. In the steam portion of this cycle, condenser water carries away the heat of steam condensation; in the mercury portion of the cycle it is the steam which picks up the heat of condensation of the mercury vapor. Hence, there is a great saving in heat and the economies effected reflect the consequent improvement in cycle efficiency. The same furnace serves the mercury boiler and the steam superheater. Mercury vapor is only condensed, not superheated. And if the condenser-boiler is physically high enough above the mercury boiler, the head of mercury is great enough to return the liquid mercury to the boiler by gravity, making the use of a mercury feed pump unnecessary. To avoid the high cost entailed with using mercury, a number of man-made solutions have been developed for binary vapor cycles. Their use, however, has been limited because the conventional steam cycle is usually lower in cost. And with the advent of the aero-derivative gas turbine, which is relatively low cost and can be installed quickly in conjunction with heat-recovery steam generators, binary cycles have lost popularity. But it is useful for engineers to have a comprehension of such cycles. Why? Because they may return to favor in the future. Conventional Steam Cycles FINDING COGENERATION SYSTEM EFFICIENCY VS. A CONVENTIONAL STEAM CYCLE An industrial plant has 60,000 lb/h (27,240 kg/h) of superheated steam at 1000 lb/in2 (abs) (6890 kPa) and 900⬚F (482.2⬚C) available. Two options are being con- sidered for use of this steam: (1) expanding the steam in a steam turbine having a 70 percent efficiency to 1 lb/in2 (abs) (6.89 kPa), and (2) expand the steam in a turbine to 200 lb/in2 (abs) (1378 kPa) generating electricity and utilizing the low- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 56. 1.54 POWER GENERATION pressure exhaust steam for process heating. Evaluate the two schemes for energy efficiency when the boiler has an 82 percent efficiency on a HHV basis. Calculation Procedure: 1. Determine the enthalpies of the steam at the turbine inlet and after isentropic expansion Cogeneration systems generate power and process steam from the same fuel source. Process plants generating electricity from steam produced in a boiler and using the same steam after expansion in a steam turbine for process heating of some kind are examples of cogeneration systems. Conventional steam-turbine power plants have a maximum efficiency of about 40 percent as most of the energy is wasted in the condensing-system cooling water. In a typical cogeneration system the exhaust steam from the turbine is used for process purposes after expansion through the steam turbine; hence, its enthalpy is fully utilized. Thus, cogeneration schemes are more efficient. At 1000 lb/in2 (abs) (6890 kPa) and 900⬚F (482.2⬚C), the enthalpy, h1 ⫽ 1448 Btu/lb (3368 kJ/kg) from the steam tables. The entropy of steam at this condition, from the steam tables, is 1.6121 Btu/lb ⬚F (6.748 kJ/kg K). At 1 lb/in2 (abs) (6.89 kPa), the entropy of the saturated liquid, is 0.1326 Btu/lb ⬚F (0.555 kJ/kg K), s , ƒ and the entropy of the saturated vapor, sg is 1.9782 Btu/lb ⬚F (8.28 kJ/kg K), again from the steam tables. Now we must determine the quality of the steam, X, at the exhaust of the steam turbine at 1 lb/in2 (abs) (6.89 kPa) from (entropy at turbine inlet condition) ⫽ (entropy at outlet condition)(X) ⫹ (1 ⫺ X)(entropy of the saturated fluid at the outlet condition); or X ⫽ 0.80. The enthalpy of steam corresponding to this quality condition is h2s ⫽ (enthalpy of the saturated steam at 1 lb/in2 (abs))(X) ⫹ (enthalpy of the saturated liquid at 1 lb/in2 (abs))(1 ⫺ X) ⫽ (1106)(0.80) ⫹ (1 ⫺ 0.80)(70) ⫽ 900 Btu/lb (2124 kJ/kg). 2. Compute the power output of the turbine Use the equation P ⫽ (Ws)(et)(h1 ⫺ h2s)/3413, where P ⫽ electrical power gener- ated, kW; Ws ⫽ steam flow through the turbine, lb/h (kg/h); et ⫽ turbine efficiency expressed as a decimal; h1 ⫽ enthalpy of the steam at the turbine inlet, Btu/lb (kJ/kg); h2s ⫽ enthalpy of the steam after isentropic expansion through the turbine, Btu/lb (kJ/kg). Substituting, P ⫽ (60,000)(0.70)(1448 ⫺ 900)/3413 ⫽ 6743 kW ⫽ (6743)(3413) ⫽ 23 MM Btu/h (24.26 MM kJ). 3. Find the steam enthalpy after expansion in the cogeneration scheme The steam is utilized for process heating after expansion to 200 lb/in2 (abs) (1378 kPa) in the backpressure turbine. We must compute the enthalpy of the steam after expansion in order to find the energy available. At 200 lb/in2 (abs) (1378 kPa), using the same procedure as in step 1 above, h2s ⫽ 1257.7 Btu/lb (2925.4 kJ/kg). Since we know the turbine efficiency we can use the equation, (et)(h1 ⫺ h2s) ⫽ (h1 ⫺ h2); or (0.70)(1448 ⫺ 1257.7) ⫽ (1448 ⫺ h2); h2 ⫽ 1315 Btu/lb (3058.7 kJ/kg), h2 ⫽ actual enthalpy after expansion, Btu/lb (kJ/kg). 4. Determine the electrical output of the cogeneration plant Since the efficiency of the turbine is already factored into the exhaust enthalpy of the cogeneration turbine, use the relation, P ⫽ Ws(h1 ⫺ h2)/3413, where the sym- bols are as defined earlier. Or, P ⫽ 60,000(1448 ⫺ 1315)/3413 ⫽ 2338 kW. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 57. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.55 5. Compute the total energy output of the cogeneration plant Assuming that the latent heat of the steam at 200 lb/in2 (abs) (1378 kPa) is available for industrial process heating, the total energy output of the cogeneration scheme ⫽ electrical output ⫹ (steam flow, lb/h)(latent heat of the exhaust steam, Btu/lb). Since, from the steam tables, the latent heat of steam at 200 lb/in2 (abs) (1378 kPa) ⫽ 834 Btu/lb (1939.9 kJ/kg), total energy output of the cogeneration cycle ⫽ (2338 kW)(3413) ⫹ (60,000)(834) ⫽ 58 MM Btu/h (61.2 MM kJ/h). Since the total energy output of the conventional cycle was 23 MM Btu/h (24.3 MM kJ/h), the ratio of the cogeneration output vs. the conventional output ⫽ 58/23 ⫽ 2.52. Thus, about 2.5 times as much energy is derived from the cogen- eration cycle as from the conventional cycle. 6. Find the comparative efficiencies of the two cycles The boiler input ⫽ (weight of steam generated, lb/h)(enthalpy of superheated steam at boiler outlet, Btu/lb ⫺ enthalpy of feedwater entering the boiler, Btu/lb)/(boiler efficiency, expressed as a decimal). Or, boiler input ⫽ (60,000)(1448 ⫺ 200)/0.82 ⫽ 91.3 MM Btu/h (96.3 MM kJ/h). The efficiency of the conventional cycle is therefore (23/91.3)(100) ⫽ 25 percent. For the cogeneration cycle, the efficiency (58/91.3)(10) ⫽ 63.5 percent. Related Calculations. This real-life example shows why cogeneration is such a popular alternative in today’s world of power generation. In this study the co- generation scheme is more than twice as efficient as the conventional cycle—63.5 percent vs. 25 percent. Higher efficiencies could be obtained if the boiler outlet steam pressure were higher than 1000 lb/in2 (abs) (6890 kPa). However, the pres- sure used here is typical of today’s industrial installations using cogeneration to save energy and conserve the environment. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. BLEED-STEAM REGENERATIVE CYCLE LAYOUT AND T-S PLOT Sketch the cycle layout, T-S diagram, and energy-flow chart for a regenerative bleed-steam turbine plant having three feedwater heaters and four feed pumps. Write the equations for the work-output available energy and the energy rejected to the condenser. Calculation Procedure: 1. Sketch the cycle layout Figure 33 shows a typical practical regenerative cycle having three feedwater heat- ers and four feedwater pumps. Number each point where steam enters and leaves the turbine and where steam enters or leaves the condenser and boiler. Also number the points in the feedwater cycle where feedwater enters and leaves a heater. In- dicate the heater steam flow by m with a subscript corresponding to the heater number. Use Wp and a suitable subscript to indicate the pump work for each feed pump, except the last, which is labeled WpF . The heat input to the steam generator is Qa ; the work output of the steam turbine is We ; the heat rejected by the condenser is Qr . Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 58. 1.56 POWER GENERATION FIGURE 33 Regenerative steam cycle uses bleed steam. 2. Sketch the T-S diagram for the cycle To analyze any steam cycle, trace the flow of 1 lb (0.5 kg) of steam through the system. Thus, in this cycle, 1 lb (0.5 kg) of steam leaves the steam generator at point 2 and flows to the turbine. From state 2 to 3, 1 lb (0.5 kg) of steam expands at constant entropy (assumed) through the turbine, producing work output W1 ⫽ H2 ⫺ H3, represented by area 1-a-2-3 on the T-S diagram, Fig. 34a. At point 3, some steam is bled from the turbine to heat the feedwater passing through heater 1. The quantity of steam bled, m1 lb is less than the 1 lb (0.5 kg) flowing between points 2 and 3. Plot stages 2 and 3 on the T-S diagram, Fig. 34a. From point 3 to 4, the quantity of steam flowing through the turbine is 1 ⫺ m1 lb. This steam produces work output W2 ⫽ H3 ⫺ H4. Plot point 4 on the T-S diagram. Then, area 1-3-4-12 represents the work output W2, Fig. 34a. At point 4, steam is bled to heater 2. The weight of this steam is m2 lb. From point 4, the steam continues to flow through the turbine to point 5, Fig. 34a. The weight of the steam flowing between points 4 and 5 is 1 ⫺ m1 ⫺ m2 lb. Plot point 5 on the T-S diagram, Fig. 34a. The work output between points 4 and 5, W3 ⫽ H4 ⫺ H5, is represented by area 4-5-10-11 on the T-S diagram. At point 5, steam is bled to heater 3. The weight of this bleed steam is m3 lb. From point 5, steam continues to flow through the turbine to exhaust at point 6, Fig. 34a. The weight of steam flowing between points 5 and 6 is 1 ⫺ m1 ⫺ m2 ⫺ m3 lb. Plot point 6 on the T-S diagram, Fig. 34a. The work output between points 5 and 6 is W4 ⫽ H5 ⫺ H6, represented by area 5-6-7-9 on the T-S diagram, Fig. 34a. Area Qr represents the heat given up by 1 lb (0.5 kg) of exhaust steam. Similarly, the area marked Qa represents the heat absorbed by 1 lb (0.5 kg) of water in the steam generator. 3. Alter the T-S diagram to show actual cycle conditions As plotted in Fig. 34a, Qa is true for this cycle since 1 lb (0.5 kg) of water flows through the steam generator and the first section of the turbine. But Qr is much too large; only 1 ⫺ m1 ⫺ m2 ⫺ m3 lb of steam flows through the condenser. Likewise, the net areas for W2, W3, and W4, Fig. 34a, are all too large, because less than 1 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 59. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.57 FIGURE 34 (a) T-S chart for the bleed-steam regenera- tive cycle in Fig. 10; (b) actual fluid flow in the cycle; (c) alternative plot of (b). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 60. 1.58 POWER GENERATION FIGURE 35 Energy-flow chart of cycle in Fig. 33. lb (0.5 kg) of steam flows through the respective turbine sections. The area for W1, however, is true. A true proportionate-area diagram can be plotted by applying the factors for actual flow, as in Fig. 34b. Here W2, outlined by the heavy lines, equals the similarly labeled area in Fig. 34a, multiplied by 1 ⫺ m1. The states marked 11⬘ and 12⬘, Fig. 34b, are not true state points because of the ratioing factor applied to the area for W2. The true state points 11 and 12 of the liquid before and after heater pump 3 stay as shown in Fig. 34a. Apply 1 ⫺ m1 ⫺ m2 to W3 of Fig. 34a to obtain the proportionate area of Fig. 34b; to obtain W4, multiply by 1 ⫺ m1 ⫺ m2 ⫺ m3. Multiplying by this factor also gives Qr . Then all the areas in Fig. 34b will be in proper proportion for 1 lb (0.5 kg) of steam entering the turbine throttle but less in other parts of the cycle. In Fig. 34b, the work can be measured by the difference of the area Qa and the area Qr . There is no simple net area left, because the areas coincide on only two sides. But area enclosed by the heavy lines is the total net work W for the cycle, equal to the sum of the work produced in the various sections of the turbine, Fig. 34b. Then Qa is the alternate area Qr ⫹ W1 ⫹ W2 ⫹ W3 ⫹ W4, as shaded in Fig. 34c. The sawtooth approach of the liquid-heating line shows that as the number of heaters in the cycle increases, the heating line approaches a line of constant entropy. The best number of heaters for a given cycle depends on the steam state of the turbine inlet. Many medium-pressure and medium-temperature cycles use five to six heaters. High-pressure and high-temperature cycles use as many as nine heaters. 4. Draw the energy-flow chart Choose a suitable scale for the heat content of 1 lb (0.5 kg) of steam leaving the steam generator. A typical scale is 0.375 in per 1000 Btu/lb (0.41 cm per 1000 kJ/kg). Plot the heat content of 1 lb (0.5 kg) of steam vertically on line 2-2, Fig. 35. Using the same scale, plot the heat content in energy streams m1, m2, m3, We, W, Wp, WpF, and so forth. In some cases, as Wp 1, Wp 2, and so forth, the energy stream may be so small that it is impossible to plot it to scale. In these instances, a single thin line is used. The completed diagram, Fig. 35, provides a useful concept of the distribution of the energy in the cycle. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 61. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.59 FIGURE 36 Bleed-regenerative steam cycle. Related Calculations. The procedure given here can be used for all regener- ative cycles, provided that the equations are altered to allow for more, or fewer, heaters and pumps. The following calculation procedure shows the application of this method to an actual regenerative cycle. BLEED REGENERATIVE STEAM CYCLE ANALYSIS Analyze the bleed regenerative cycle shown in Fig. 36, determining the heat balance for each heater, plant thermal efficiency, turbine or engine thermal efficiency, plant heat rate, turbine or engine heat rate, and turbine or engine steam rate. Throttle steam pressure is 2000 lb/in2 (abs) (13,790.0 kPa) at 1000⬚F (537.8⬚C); steam- generator efficiency ⫽ 0.88; station auxiliary steam consumption (excluding pump work) ⫽ 6 percent of the turbine or engine output; engine efficiency of each turbine or engine section ⫽ 0.80; turbine or engine cycle has three feedwater heaters and bleed-steam pressures as shown in Fig. 36; exhaust pressure to condenser is 1 inHg (3.4 kPa) absolute. Calculation Procedure: 1. Determine the enthalpy of the steam at the inlet of each heater and the condenser From a superheated-steam table, find the throttle enthalpy H2 ⫽ 1474.5 Btu/lb (3429.7 kJ/kg) at 2000 lb/in2 (abs) (13,790.0 kPa) and 1000⬚F (537.8⬚C). Next find the throttle entropy S2 ⫽ 1.5603 Btu/(lb ⬚F) [6.5 kJ/(kg ⬚C)], at the same conditions in the superheated-steam table. Plot the throttle steam conditions on a Mollier chart, Fig. 37. Assume that the steam expands from the throttle conditions at constant entropy ⫽ constant S to the inlet of the first feedwater heater, 1, Fig. 36. Plot this constant S expansion by drawing the straight vertical line 2-3 on the Mollier chart, Fig. 37, between the throttle condition and the heater inlet pressure of 750 lb/in2 (abs) (5171.3 kPa). Read on the Mollier chart H3 ⫽ 1346.7 Btu/lb (3132.4 kJ/kg). Since the engine or turbine efficiency ee ⫽ H2 ⫺ H3 /(H2 ⫺ H3) ⫽ 0.8 ⫽ 1474.5 ⫺ H3 /(1474.5 ⫺ 1346.7); H3 ⫽ actual enthalpy of the steam at the inlet to heater 1 ⫽ 1474.5 ⫺ 0.8(1474.5 ⫺ 1346.7) ⫽ 1372.2 Btu/lb (3191.7 kJ/kg). Plot this enthalpy point on Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 62. 1.60 POWER GENERATION FIGURE 37 Mollier-chart plot of the cycle in Fig. 36. the 750-lb/in2 (abs) (5171.3-kPa) pressure line of the Mollier chart, Fig. 37. Read the entropy at the heater inlet from the Mollier chart as ⫽ 1.5819 Btu/(lb ⬚F) S3⬘ [6.6 kJ/(kg ⬚C)] at 750 lb/in2 (abs) (5171.3 kPa) and 1372.2 Btu/lb (3191.7 kJ/ kg). Assume constant-S expansion from to H4 at 200 lb/in2 (abs) (1379.0 kPa), H3⬘ the inlet pressure for feedwater heater 2. Draw the vertical straight line 3⬘-4 on the Mollier chart, Fig. 37. By using a procedure similar to that for heater 1, ⫽ H4⬘ ⫺ ee ⫺ H4) ⫽ 1372.2 ⫺ 0.8(1372.2 ⫺ 1230.0) ⫽ 1258.4 Btu/lb (2927.0 H (H 3⬘ 3⬘ kJ/kg). This is the actual enthalpy of the steam at the inlet to heater 2. Plot this enthalpy on the 200-lb/in2 (abs) (1379.0-kPa) pressure line of the Mollier chart, and find ⫽ 1.613 Btu/(lb ⬚F) [6.8 kJ/(kg ⬚C)], Fig. 37. S4⬘ Using the same procedure with constant-S expansion from we find H5 ⫽ H , 4⬘ 1059.5 Btu/lb (2464.4 kJ/kg) at 16 lb/in2 (abs) (110.3 kPa), the inlet pressure to heater 3. Next find ⫺ ee ⫺ H5) ⫽ 1258.4 ⫺ 0.8(1258.4 ⫺ 1059.5) ⫽ H ⫽ H (H 5⬘ 4⬘ 4⬘ 1099.2 Btu/lb (2556.7 kJ/kg). From the Mollier chart find ⫽ 1.671 Btu/(lb S5⬘ ⬚F) [7.0 kJ/(kg ⬚C)], Fig. 37. Using the same procedure with constant-S expansion from to H6, find H6 ⫽ H5⬘ 898.2 Btu/lb (2089.2 kJ/kg) at 1 inHg absolute (3.4 kPa), the condenser inlet pressure. Then ⫺ ⫺ H6) ⫽ 1099.2 ⫺ 0.8(1099.2 ⫺ 898.2) ⫽ 938.4 H ⫽ H e (H 6⬘ 5⬘ e 5⬘ Btu/lb (2182.7 kJ/kg), the actual enthalpy of the steam at the condenser inlet. Find, on the Mollier chart, the moisture in the turbine exhaust ⫽ 15.1 percent. 2. Determine the overall engine efficiency Overall engine efficiency ee is higher than the engine-section efficiency because there is partial available-energy recovery between sections. Constant-S expansion Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 63. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.61 from the throttle to the 1-inHg absolute (3.4-kPa) exhaust gives H3s, Fig. 37, as 838.3 Btu/lb (1949.4 kJ/kg), assuming that all the steam flows to the condenser. Then, overall ee ⫽ H2 ⫺ /(H2 ⫺ H3S ) ⫽ 1474.5 ⫺ 938.4/1474.5 ⫺ 838.3 ⫽ H6⬘ 0.8425, or 84.25 percent, compared with 0.8 or 80 percent, for individual engine sections. 3. Compute the bleed-steam flow to each feedwater heater For each heater, energy in ⫽ energy out. Also, the heated condensate leaving each heater is a saturated liquid at the heater bleed-steam pressure. To simplify this calculation, assume negligible steam pressure drop between the turbine bleed point and the heater inlet. This assumption is permissible when the distance between the heater and bleed point is small. Determine the pump work by using the chart accompanying the compressed-liquid table in Keenan and Keyes—Thermodynamic Properties of Steam, or the ASME—Steam Tables. For heater 1, energy in ⫽ energy out, or ⫹ H12(1 ⫺ m1) ⫽ H13, where H m 3⬘ 1 m ⫽ bleed-steam flow to the feedwater heater, lb/lb of throttle steam flow. (The subscript refers to the heater under consideration.) Then ⫹ (H11 ⫹ Wp 2)(1 ⫺ H m 3⬘ 1 m1) ⫽ H13, where Wp 2 ⫽ work done by pump 2, Fig. 36, in Btu/lb per pound of throttle flow. Then 1372.2m1 ⫹ (355.4 ⫹ 1.7)(1 ⫺ m1) ⫽ 500.8; m1 ⫽ 0.1416 lb/ lb (0.064 kg/kg) throttle flow; H1 ⫽ H13 ⫹ Wp 1 ⫽ 500.8 ⫹ 4.7 ⫽ 505.5 Btu/lb (1175.8 kJ/kg), where Wp 1 ⫽ work done by pump 1, Fig. 36. For each pump, find the work from the chart accompanying the compressed-liquid table in Keenan and Keyes—Steam Tables by entering the chart at the heater inlet pressure and pro- jecting vertically at constant entropy to the heater outlet pressure, which equals the next heater inlet pressure. Read the enthalpy values at the respective pressures, and subtract the smaller from the larger to obtain the pump work during passage of the feedwater through the pump from the lower to the higher pressure. Thus, Wp 2 ⫽ 1.7 ⫺ 0.0 ⫽ 1.7 Btu/lb (4.0 kJ/kg), from enthalpy values for 200 lb/in2 (abs) (1379.0 kPa) and 750 lb/in2 (abs) (5171.3 kPa), the heater inlet and discharge pressures, respectively. For heater 2, energy in ⫽ energy out, or ⫹ H10(1 ⫺ m1 ⫺ m2) ⫽ H11(1 ⫺ H m 4⬘ 2 m1) ⫹ (H9 ⫹ Wp 3)(1 ⫺ m1 ⫺ m2) ⫽ H11(1 ⫺ m1)1258.4m2 ⫹ (184.4 ⫹ H m 4⬘ 2 0.5)(0.8584 ⫺ m2) ⫽ 355.4(0.8584)m2 ⫽ 0.1365 lb/lb (0.0619 kg/kg) throttle flow. For heater 3, energy in ⫽ energy out, or ⫹ H8(1 ⫺ m1 ⫺ m2 ⫺ m3) ⫽ H m 5⬘ 3 H9(1 ⫺ m1 ⫺ m2) ⫹ (H7 ⫹ Wp 4)(1 ⫺ m1 ⫺ m2 ⫺ m3) ⫽ H9(1 ⫺ m1 ⫺ H m 5⬘ 3 m2)1099.2m3 ⫹ (47.1 ⫹ 0.1)(0.7210 ⫺ m3) ⫽ 184.4(0.7219)m3 ⫽ 0.0942 lb/lb (0.0427 kg/kg) throttle flow. 4. Compute the turbine work output The work output per section W Btu is W1 ⫽ H2 ⫺ ⫽ 1474.5 ⫺ 1372.1 ⫽ 102.3 H3⬘ Btu (107.9 kJ), from the previously computed enthalpy values. Also W2 ⫽ (H ⫺ 3⬘ (1 ⫺ m1) ⫽ (1372.2 ⫺ 1258.4)(1 ⫺ 0.1416) ⫽ 97.7 Btu (103.1 kJ); W3 ⫽ H ) 4⬘ (1 ⫺ m1 ⫺ m2) ⫽ (1258.4 ⫺ 1099.2)(1 ⫺ 0.1416 ⫺ 0.1365) ⫽ 115.0 (H ⫺ H ) 4⬘ 5⬘ Btu (121.3 kJ); W4 ⫽ (1 ⫺ m1 ⫺ m2 ⫺ m3) ⫽ (1099.2 ⫺ 938.4)(1 ⫺ (H ⫺ H ) 5⬘ 6⬘ 0.1416 ⫺ 0.1365 ⫺ 0.0942) ⫽ 100.9 Btu (106.5 kJ). The total work output of the turbine ⫽ We ⫽ ⌺W ⫽ 102.3 ⫹ 97.7 ⫹ 115.0 ⫹ 100.9 ⫽ 415.9 Btu (438.8 kJ). The total Wp ⫽ ⌺Wp ⫽ Wp 1 ⫹ Wp 2 ⫹ Wp 3 ⫹ Wp 4 ⫽ 4.7 ⫹ 1.7 ⫹ 0.5 ⫹ 0.1 ⫽ 7.0 Btu (7.4 kJ). Since the station auxiliaries consume 6 percent of We, the auxiliary consumption ⫽ 0.6(415.9) ⫽ 25.0 Btu (26.4 kJ). Then, net station work w ⫽ 415.9 ⫺ 7.0 ⫺ 25.0 ⫽ 383.9 Btu (405.0 kJ). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 64. 1.62 POWER GENERATION 5. Check the turbine work output The heat added to the cycle Qa Btu/lb ⫽ H2 ⫺ H1 ⫽ 1474.5 ⫺ 505.5 ⫽ 969.0 Btu (1022.3 kJ). The heat rejected from the cycle Qr Btu/lb ⫽ ⫺ H7)(1 ⫺ m1 ⫺ (H6⬘ m2 ⫺ m3) ⫽ (938.4 ⫺ 47.1)(0.6277) ⫽ 559.5 Btu (590.3 kJ). Then We ⫺ Wp ⫽ Qa ⫺ Qr ⫽ 969.0 ⫺ 559.5 ⫽ 409.5 Btu (432.0 kJ). Compare this with We ⫺ Wp computed earlier, or 415.9 ⫺ 7.0 ⫽ 408.9 Btu (431.4 kJ), or a difference of 409.5 ⫺ 408.9 ⫽ 0.6 Btu (0.63 kJ). This is an accurate check; the difference of 0.6 Btu (0.63 kJ) comes from errors in Mollier chart and calculator readings. Assume 408.9 Btu (431.4 kJ) is correct because it is the lower of the two values. 6. Compute the plant and turbine efficiencies Plant energy input ⫽ Qa /eb, where eb ⫽ boiler efficiency. Then plant energy input ⫽ 969.0/0.88 ⫽ 1101.0 Btu (1161.6 kJ). Plant thermal efficiency ⫽ W /(Qa /eb ) ⫽ 383.9 ⫽ 1101.0 ⫽ 0.3486. Turbine thermal efficiency ⫽ We /Qa ⫽ 415.9/ 969.0 ⫽ 0.4292. Plant heat rate ⫽ 3413/0.3486 ⫽ 9970 Btu/kWh (10,329.0 kJ/ kWh), where 3413 ⫽ Btu/kWh. Turbine heat rate ⫽ 3413/0.4292 ⫽ 7950 Btu/ kWh (8387.7 kJ/kWh). Turbine throttle steam rate ⫽ (turbine heat rate)/(H2 ⫺ H1) ⫽ 7950/(1474.5 ⫺ 505.5) ⫽ 8.21 lb/kWh (3.7 kg/kWh). Related Calculations. By using the procedures given, the following values can be computed for any actual steam cycle: engine or turbine efficiency ee ; steam enthalpy at the main-condenser inlet; bleed-steam flow to a feedwater heater; tur- bine or engine work output per section; total turbine or engine work output; station auxiliary power consumption; net station work output; plant energy input; plant thermal efficiency; turbine or engine thermal efficiency; plant heat rate; turbine or engine heat rate; turbine throttle heat rate. To compute any of these values, use the equations given and insert the applicable variables. REHEAT-STEAM CYCLE PERFORMANCE A reheat-steam cycle has a 2000 lb/in2 (abs) (13,790-kPa) throttle pressure at the turbine inlet and a 400-lb/in2 (abs) (2758-kPa) reheat pressure. The throttle and reheat temperature of the steam is 1000⬚F (537.8⬚C); condenser pressure is 1 inHg absolute (3.4 kPa); engine efficiency of the high-pressure and low-pressure turbines is 80 percent. Find the cycle thermal efficiency. Calculation Procedure: 1. Sketch the cycle layout and cycle T-S diagram Figures 38 and 39 show the cycle layout and T-S diagram with each important point numbered. Use a cycle layout and T-S diagram for every calculation of this type because it reduces the possibility of errors. 2. Determine the throttle-steam properties from the steam tables Use the superheated steam tables, entering at 2000 lb/in2 (abs) (13,790 kPa) and 1000⬚F (537.8⬚C) to find throttle-steam properties. Applying the symbols of the T- S diagram in Fig. 39, we get H2 ⫽ 1474.5 Btu/lb (3429.7 kJ/kg); S2 ⫽ 1.5603 Btu/(lb ⬚F) [6.5 kJ/(kg ⬚C)]. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 65. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.63 FIGURE 38 Typical steam reheat cycle. FIGURE 39 Irreversible expansion in reheat cycle. 3. Find the reheat-steam enthalpy Assume a constant-entropy expansion of the steam from 2000 to 400 lb/in2 (13,790 to 2758 kPa). Trace this expansion on a Mollier (H-S) chart, where a constant- entropy process is a vertical line between the initial [2000 lb/in2 (abs) or 13,790 kPa] and reheat [400 lb/in2 (abs) or 2758 kPa] pressures. Read on the Mollier chart H3 ⫽ 1276.8 Btu/lb (2969.8 kJ/kg) at 400 lb/in2 (abs) (2758 kPa). 4. Compute the actual reheat properties The ideal enthalpy drop, throttle to reheat ⫽ H2 ⫺ H3 ⫽ 1474.5 ⫺ 1276.8 ⫽ 197.7 Btu/lb (459.9 kJ/kg). The actual enthalpy drop ⫽ (ideal drop)(turbine efficiency) ⫽ H2 ⫺ ⫽ 197.5(0.8) ⫽ 158.2 Btu/lb (368.0 kJ/kg) ⫽ We 1 ⫽ work output in the H3⬘ high-pressure section of the turbine. Once We 1 is known, can be computed from ⫽ H2 ⫺ We 1 ⫽ 1474.5 ⫺ H H 3⬘ 3⬘ 158.2 ⫽ 1316.3 Btu/lb (3061.7 kJ/kg). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 66. 1.64 POWER GENERATION FIGURE 40 Energy-flow diagram for reheat cycle in Fig. 38. The steam now returns to the boiler and leaves at condition 4, where P4 ⫽ 400 lb/in2 (abs) (2758 kPa); T4 ⫽ 1000⬚F (537.8⬚C); S4 ⫽ 1.7623 Btu/(lb ⬚F) [7.4 kJ/(kg ⬚C)]; H4 ⫽ 1522.4 Btu/lb (3541.1 kJ/kg) from the superheated-steam table. 5. Compute the exhaust-steam properties Use the Mollier chart and an assumed constant-entropy expansion to 1 inHg (3.4 kPa) absolute to determine the ideal exhaust enthalpy, or H5 ⫽ 947.4 Btu/lb (2203.7 kJ/kg). The ideal work of the low-pressure section of the turbine is then H4 ⫺ H5 ⫽ 1522.4 ⫺ 947.4 ⫽ 575.0 Btu/lb (1338 kJ/kg). The actual work output of the low-pressure section of the turbine is We 2 ⫽ H4 ⫺ ⫽ 575.0(0.8) ⫽ 460.8 H5⬘ Btu/lb (1071.1 kJ/kg). Once We 2 is known, can be computed from ⫽ H4 ⫺ We 2 ⫽ 1522.4 ⫺ H H 5⬘ 5⬘ 460.0 ⫽ 1062.4 Btu/lb (2471.1 kJ/kg). The enthalpy of the saturated liquid at the condenser pressure is found in the saturation-pressure steam table at 1 inHg absolute (3.4 kPa) ⫽ H6 ⫽ 47.1 Btu/lb (109.5 kJ/kg). The pump work Wp from the compressed-liquid table diagram in the steam tables is Wp ⫽ 5.5 Btu/lb (12.8 kJ/kg). Then the enthalpy of the water entering the boiler H1 ⫽ H6 ⫹ Wp ⫽ 47.1 ⫹ 5.5 ⫽ 52.6 Btu/lb (122.3 kJ/kg). 6. Compute the cycle thermal efficiency For any reheat cycle, e ⫽ cycle thermal efficiency (H ⫺ H ) ⫹ (H ⫺ H ) ⫺ W 2 3⬘ 4 5⬘ p ⫽ (H ⫺ H ) ⫹ (H ⫺ H ) 2 1 4 3⬘ (1474.5 ⫺ 1316.3) ⫹ (1522.4 ⫺ 1062.4) ⫺ 5.5 ⫽ (1474.5 ⫺ 52.6) ⫹ (1522.4 ⫺ 1316.3) ⫽ 0.3766, or 37.66 percent Figure 40 is an energy-flow diagram for the reheat cycle analyzed here. This diagram shows that the fuel burned in the steam generator to produce energy flow Qa 1 is the largest part of the total energy input. The cold-reheat line carries the major share of energy leaving the high-pressure turbine. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 67. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.65 FIGURE 42 (a) T-S diagram for ideal reheat-regenerative-bleed cycle; (b) T-S diagram for actual cycle. FIGURE 41 Combined reheat and bleed-regenerative cycle. Related Calculations. Reheat-regenerative cycles are used in some large power plants. Figure 41 shows a typical layout for such a cycle having three stages of feedwater heating and one stage of reheating. The heat balance for this cycle is computed as shown above, with the bleed-flow terms m computed by setting up an energy balance around each heater, as in earlier calculation procedures. By using a T-S diagram, Fig. 42, the cycle thermal efficiency is W Q ⫺ Q Q a r r e ⫽ ⫽ ⫽ 1 ⫺ Q Q Q ⫹ Q a a a 1 a 2 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 68. 1.66 POWER GENERATION FIGURE 44 T-S diagram for multiple re- heat stages. FIGURE 43 Energy flow of cycle in Fig. 41. Based on 1 lb (0.5 kg) of working fluid entering the steam generator and turbine throttle, Q ⫽ (1 ⫺ m ⫺ m ⫺ m )(H ⫺ H ) r 1 2 3 7 8 Q ⫽ (H ⫺ H ) a 1 2 1 Q ⫽ (1 ⫺ m )(H ⫺ H ) a 2 1 4 3 Figure 43 shows the energy-flow chart for this cycle. Some high-pressure plants use two stages of reheating, Fig. 44, to raise the cycle efficiency. With two stages of reheating, the maximum number generally used, and values from Fig. 44. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 69. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.67 FIGURE 45 Mollier chart of turbine condition lines. (H ⫺ H ) ⫹ (H ⫺ H ) ⫹ (H ⫺ H ) ⫺ W 2 3 4 5 6 7 p e ⫽ (H ⫺ H ) ⫹ (H ⫺ H ) ⫹ (H ⫺ H ) 2 1 4 3 6 5 MECHANICAL-DRIVE STEAM-TURBINE POWER- OUTPUT ANALYSIS Show the effect of turbine engine efficiency on the condition lines of a turbine having engine efficiencies of 100 (isentropic expansion), 75, 50, 25, and 0 percent. How much of the available energy is converted to useful work for each engine efficiency? Sketch the effect of different steam inlet pressures on the condition line of a single-nozzle turbine at various loads. What is the available energy, Btu/lb of steam, in a noncondensing steam turbine having an inlet pressure of 1000 lb/in2 (abs) (6895 kPa) and an exhaust pressure of 100 lb/in2 (gage) (689.5 kPa)? How much work will this turbine perform if the steam flow rate to it is 1000 lb/s (453.6 kg/s) and the engine efficiency is 40 percent? Calculation Procedure: 1. Sketch the condition lines on the Mollier chart Draw on the Mollier chart for steam initial- and exhaust-pressure lines, Fig. 45, and the initial-temperature line. For an isentropic expansion, the entropy is constant during the expansion, and the engine efficiency ⫽ 100 percent. The expansion or condition line is a vertical trace from h1 on the initial-pressure line to h2, on the exhaust-pressure line. Draw this line as shown in Fig. 45. For zero percent engine efficiency, the other extreme in the efficiency range, h1 ⫽ h2 and the condition line is a horizontal line. Draw this line as shown in Fig. 45. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 70. 1.68 POWER GENERATION FIGURE 46 Turbine condition line shifts as the inlet steam pressure varies. Between 0 and 100 percent efficiency, the condition lines become more nearly vertical as the engine efficiency approaches 100 percent, or an isentropic expansion. Draw the condition lines for 25, 50, and 75 percent efficiency, as shown in Fig. 45. For the isentropic expansion, the available energy ⫽ h1 ⫺ h2s, Btu/lb of steam. This is the energy that an ideal turbine would make available. For actual turbines, the enthalpy at the exhaust pressure h2 ⫽ h1 ⫺ (available energy)(engine efficiency)/100, where available energy ⫽ h1 ⫺ h2s for an ideal turbine working between the same initial and exhaust pressures. Thus, the available energy converted to useful work for any engine efficiency ⫽ (ideal available energy, Btu/lb)(engine efficiency, percent)/100. Using this relation, the available energy at each of the given engine efficiencies is found by substituting the ideal available energy and the actual engine efficiency. 2. Sketch the condition lines for various throttle pressures Draw the throttle- and exhaust-pressure lines on the Mollier chart, Fig. 46. Since the inlet control valve throttles the steam flow as the load on the turbine decreases, the pressure of the steam entering the turbine nozzle is lower at reduced loads. Show this throttling effect by indicating the lower inlet pressure lins, Fig. 46, for the reduced loads. Note that the lowest inlet pressure occurs at the minimum plotted load—25 percent of full load—and the maximum inlet pressure at 125 percent of full load. As the turbine inlet steam pressure decreases, so does the available energy, because the exhaust enthalpy rises with decreasing load. 3. Compute the turbine available energy and power output Use a noncondensing-turbine performance chart, Fig. 47, to determine the available energy. Enter the bottom of the chart at 1000 lb/in2 (abs) (6895 kPa) and project vertically upward until the 100-lb/in2 (gage) (689.5-kPa) exhaust-pressure curve is intersected. At the left, read the available energy as 205 Btu/lb (476.8 kJ/kg) of steam. With the available energy, flow rate, and engine efficiency known, the work output ⫽ (available energy, Btu/lb)(flow rate, lb/s)(engine efficiency/100)/[550 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 71. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.69 FIGURE 47 Available energy in turbine depends on the initial steam state and the exhaust pressure. ft lb/(s hp)]. [Note: 550 ft lb/(s hp) ⫽ 1 N m/(W s).] For this turbine, work output ⫽ (205 Btu/lb)(1000 lb/s)(40/100)/550 ⫽ 149 hp (111.1 kW). Related Calculations. Use the steps given here to analyze single-stage non- condensing mechanical-drive turbines for stationary, portable, or marine applica- tions. Performance curves such as Fig. 47 are available from turbine manufacturers. Single-stage noncondensing turbines are for feed-pump, draft-fan, and auxiliary- generator drive. CONDENSING STEAM-TURBINE POWER-OUTPUT ANALYSIS What is the available energy in steam supplied to a 5000-kW turbine if the inlet steam conditions are 1000 lb/in2 (abs) (6895 kPa) and 800⬚F (426.7⬚C) and the turbine exhausts at 1 inHg absolute (3.4 kPa)? Determine the theoretical and actual heat rate of this turbine if its engine efficiency is 74 percent. What are the full-load output and steam rate of the turbine? Calculated Procedure: 1. Determine the available energy in the steam Enter Fig. 48 at the bottom at 1000-lb/in2 (abs) (6895.0-kPa) inlet pressure, and project vertically upward to the 800⬚F (426.7⬚C) 1-in (3.4-kPa) exhaust-pressure curve. At the left, read the available energy as 545 Btu/lb (1267.7 kJ/kg) of steam. 2. Determine the heat rate of the turbine Enter Fig. 49 at an initial steam temperature of 800⬚F (426.7⬚C), and project ver- tically upward to the 1000-lb/in2 (abs) (6895.0-kPa) 1-in (3.4-kPa) curve. At the left, read the theoretical heat rate as 8400 Btu/kWh (8862.5 kJ/kWh). When the theoretical heat rate is known, the actual heat rate is found from: actual heat rate HR, Btu/kWh ⫽ (theoretical heat rate, Btu/kWh)/(engine effi- ciency). Or, actual HR ⫽ 8400/0.74 ⫽ 11,350 Btu/kWh (11,974.9 kJ/kWh). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 72. 1.70 POWER GENERATION FIGURE 48 Available energy for typical condensing turbines. 3. Compute the full-load and steam rate The energy converted to work, Btu/lb of steam ⫽ (available energy, Btu/lb of steam)(engine efficiency) ⫽ (545)(0.74) ⫽ 403 Btu/lb of steam (937.4 kJ/kg). For any prime mover driving a generator, the full-load output, Btu ⫽ (generator kW rating)(3413 Btu/kWh) ⫽ (5000)(3413) ⫽ 17,060,000 Btu/h (4999.8 kJ/s). The steam flow ⫽ (full-load output, Btu/h)/(work output, Btu/lb) ⫽ 17,060,000/403 ⫽ 42,300 lb/h (19,035 kg/h) of steam. Then the full-load steam rate of the turbine, lb/kWh ⫽ (steam flow, lb/h)/(kW output at full load) ⫽ 42,300/5000 ⫽ 8.46 lb/kWh (3.8 kg/kWh). Related Calculations. Use this general procedure to determine the available energy, theoretical and actual heat rates, and full-load output and steam rate for any stationary, marine, or portable condensing steam turbine operating within the ranges of Figs. 48 and 49. If the actual performance curves are available, use them instead of Figs. 48 and 49. The curves given here are suitable for all preliminary estimates for condensing turbines operating with exhaust pressures of 1 or 3 inHg absolute (3.4 or 10.2 kPa). Many modern turbines operate under these conditions. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 73. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.71 FIGURE 49 Theoretical heat rate for condensing turbines. STEAM-TURBINE REGENERATIVE-CYCLE PERFORMANCE When throttle steam is at 1000 lb/in2 (abs) (6895 kPa) and 800⬚F (426.7⬚C) and the exhaust pressure is 1 inHg (3.4 kPa) absolute, a 5000-kW condensing turbine has an actual heat rate of 11,350 Btu/kWh (11,974.9 kJ/kWh). Three feedwater heaters are added to the cycle, Fig. 50, to heat the feedwater to 70 percent of the maximum possible enthalpy rise. What is the actual heat rate of the turbine? If 10 heaters instead of 3 were used and the water enthalpy were raised to 90 percent of the maximum possible rise in these 10 heaters, would the reduction in the actual heat rate be appreciable? Calculation Procedure: 1. Determine the actual enthalpy rise of the feedwater Enter Fig. 51 at the throttle pressure of 1000 lb/in2 (abs) (6895 kPa), and project vertically upward to the 1-inHg (3.4-kPa) absolute backpressure curve. At the left, read the maximum possible feedwater enthalpy rise as 495 Btu/lb (1151.4 kJ/kg). Since the actual rise is limited to 70 percent of the maximum possible rise by the conditions of the design, the actual enthalpy rise ⫽ (495)(0.70) ⫽ 346.5 Btu/lb (805.9 kJ/kg). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 74. 1.72 POWER GENERATION FIGURE 50 Regenerative feedwater heat- ing. FIGURE 51 Feedwater enthalpy rise. 2. Determine the heat-rate and heater-number correction factors Find the theoretical reduction in straight-condensing (no regenerative heaters) heat rates from Fig. 52. Enter the bottom of Fig. 52 at the inlet steam temperature, 800⬚F (426.7⬚C), and project vertically upward to the 1000-lb/in2 (abs) (6895-kPa) 1-inHg (3.4-kPa) back-pressure curve. At the left, read the reduction in straight-condensing heat rate as 14.8 percent. Next, enter Fig. 52 at the bottom of 70 percent of maximum possible rise in feedwater enthalpy, and project vertically to the three-heater curve. At the left, read the reduction in straight-condensing heat rate for the number of heaters and actual enthalpy rise as 0.71. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 75. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.73 FIGURE 52 Reduction in straight-condensing heat rate obtained by regenerative heating. 3. Apply the heat-rate and heater-number correction factors Full-load regenerative-cycle heat rate, Btu/kWh ⫽ (straight-condensing heat rate, Btu/kWh) [1 ⫺ (heat-rate correction factor)(heater-number correction factor)] ⫽ (13,350)[1 ⫺ (0.148)(0.71)] ⫽ 10,160 Btu/kWh (10,719.4 kJ/kWh). 4. Find and apply the correction factors for the larger number of heaters Enter Fig. 53 at 90 percent of the maximum possible enthalpy rise, and project vertically to the 10-heater curve. At the left, read the heat-rate reduction for the number of heaters and actual enthalpy rise as 0.89. Using the heat-rate correction factor from step 2 and 0.89, found above, we see that the full-load 10-heater regenerative-cycle heat rate ⫽ (11,350)[1 ⫺ (0.148)(0.89)] ⫽ 9850 Btu/kWh (10,392.3 kJ/kWh), by using the same procedure as in step 3. Thus, adding 10 ⫺ 3 ⫽ 7 heaters reduces the heat rate by 10,160 ⫺ 9850 ⫽ 310 Btu/kWh (327.1 kJ/kWh). This is a reduction of 3.05 percent. To determine whether this reduction in heat rate is appreciable, the carrying charges on the extra heaters, piping, and pumps must be compared with the reduc- tion in annual fuel costs resulting from the lower heat rate. If the fuel saving is greater than the carrying charges, the larger number of heaters can usually be jus- tified. In this case, tripling the number of heaters would probably increase the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 76. 1.74 POWER GENERATION FIGURE 53 Maximum possible rise in feedwater enthalpy varies with the number of heaters used. carrying charges to a level exceeding the fuel savings. Therefore, the reduction in heat rate is probably not appreciable. Related Calculations. Use the procedure given here to compute the actual heat rate of steam-turbine regenerative cycles for stationary, marine, and portable in- stallations. Where necessary, use the steps of the previous procedure to compute the actual heat rate of a straight-condensing cycle before applying the present pro- cedure. The performance curves given here are suitable for first approximations in situations where actual performance curves are unavailable. REHEAT-REGENERATIVE STEAM-TURBINE HEAT RATES What are the net and gross heat rates of a 300-kW reheat turbine having an initial steam pressure of 3500 lb/in2 (gage) (24,132.5 kPa) with initial and reheat steam temperatures of 1000⬚F (537.8⬚C) with 1.5 inHg (5.1 kPa) absolute back pressure and six stages of regenerative feedwater heating? Compare this heat rate with that of 3500 lb/in2 (gage) (24,132.5 kPa) 600-mW cross-compound four-flow turbine with 3600/1800 r/min shafts at a 300-mW load. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 77. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.75 FIGURE 54 Full-load heat rates for steam turbines with six feedwater heaters, 1000⬚F/1000⬚F (538⬚C/538⬚C) steam, 1.5-in (38.1-mm) Hg (abs) exhaust pres- sure. Calculation Procedure: 1. Determine the reheat-regenerative heat rate Enter Fig. 54 at 3500-lb/in2 (gage) (24,132.5-kPa) initial steam pressure, and pro- ject vertically to the 300-mW capacity net-heat-rate curve. At the left, read the net heat rate as 7680 Btu/kWh (8102.6 kJ/kWh). On the same vertical line, read the gross heat rate as 7350 Btu/kWh (7754.7 kJ/kWh). The gross heat rate is computed by using the generator-terminal output; the net heat rate is computed after the feedwater-pump energy input is deducted from the generator output. 2. Determine the cross-compound turbine heat rate Enter Fig. 55 at 350 mW at the bottom, and project vertically upward to 1.5-inHg (5.1-kPa) exhaust pressure midway between the 1- and 2-inHg (3.4- and 6.8-kPa) curves. At the left, read the net heat rate as 7880 Btu/kWh (8313.8 kJ/kWh). Thus, the reheat-regenerative unit has a lower net heat rate. Even at full rated load of the cross-compound turbine, its heat rate is higher than the reheat unit. Related Calculations. Use this general procedure for comparing stationary and marine high-pressure steam turbines. The curves given here are typical of those supplied by turbine manufacturers for their turbines. With the price of all commonly used fuels on the rise, nuclear power is increas- ingly being looked at by engineers worldwide. A number of nations—the United Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 78. 1.76 POWER GENERATION FIGURE 55 Heat rate of a cross-compound four-flow steam turbine with 3600/1800-r/min shafts. States, Japan, France, England, and Germany—are currently generating portions of their electricity needs by using nuclear energy. Today’s nuclear plants use fission to generate the steam for their turbines. Fusion is now being seriously investigated as the next source of nuclear power. In June 2005, France was selected as the country in which an experimental fusion reactor would be built. Test results will be provided to participating countries. Fusion reactors produce much less nuclear waste than fission-based reactors. According to an article in The New York Times,* using data from Princeton Plasma Physics Laboratory and the Energy Information Administration, the daily waste from a 1000-MW electric generating plant using 9000 tons of coal per day is 30,000 tons of CO2, 600 tons of SO2, and 80 tons of NO. A nuclear fission plant using 14.7 lb (6.67 kg) of uranium produces 6.6 lb (2.99 kg) of highly radioactive material. The projected nuclear fusion plant using 1 lb (0.454 kg) of deuterium and 1.5 lb (0.68 kg) of tritium produces 4.0 lb (1.8 kg) of helium. A large demonstration project is expected to begin operating in approximately 2030. The commercial fusion reactor is expected in approximately 2050. Thus, while the fusion reactor has great promise, its commercial utilization is expected to take many years to develop. At this writing, the consensus is that both fission and fusion nuclear power will find use during the 21st century. STEAM TURBINE–GAS TURBINE CYCLE ANALYSIS Sketch the cycle layout, T-S diagram, and energy-flow chart for a combined steam turbine–gas turbine cycle having one stage of regenerative feedwater heating and *The New York Times, June 29, 2005. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 79. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.77 FIGURE 56 Combined gas turbine–steam turbine cycle. one stage of economizer feedwater heating. Compute the thermal efficiency and heat rate of the combined cycle. Calculation Procedure: 1. Sketch the cycle layout Figure 56 shows the cycle. Since the gas-turbine exhaust-gas temperature is usually higher than the bleed-steam temperature, the economizer is placed after the regen- erative feedwater heater. The feedwater will be progressively heated to a higher temperature during passage through the regenerative heater and the gas-turbine economizer. The cycle shown here is only one of many possible combinations of a steam plant and a gas turbine. 2. Sketch the T-S diagram Figure 57 shows the T-S diagram for the combined gas turbine–steam turbine cycle. There is irreversible heat transfer QT from the gas-turbine exhaust to the feedwater in the economizer, which helps reduce the required energy input Qa 2. 3. Sketch the energy-flow chart Choose a suitable scale for the energy input, and proportion the energy flow to each of the other portions of the cycle. Use a single line when the flow is too small to plot to scale. Figure 58 shows the energy-flow chart. 4. Determine the thermal efficiency of the cycle Since e ⫽ W/Qa, e ⫽ Qa ⫺ Qr /Qa ⫽ 1 ⫺ [Qr 1 ⫹ Qr 2 /(Qa 1 ⫹ Qa 2)], given the notation in Figs. 56, 57, and 58. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 80. 1.78 POWER GENERATION FIGURE 57 T-S charts for combined gas tur- bine–steam turbine cycle have irreversible heat transfer Q from gas-turbine exhaust to the feed- water. The relative weight of the gas wg to 1 lb (0.5 kg) of water must be computed by taking an energy balance about the economizer. Or, H7 ⫺ H6 ⫽ wg (H4 ⫺ H5). Using the actual values for the enthalpies, solve this equation for wg . With wg known, the other factors in the efficiency computation are Q ⫽ w (H ⫺ H ) r 1 g 5 1 Q ⫽ (1 ⫺ m)(H ⫺ H ) r 2 10 11 Q ⫽ w (H ⫺ H ) a 1 g 3 2 Q ⫽ H ⫺ H a 2 8 7 The bleed-steam flow m is calculated from an energy balance about the feedwater heater. Note that the units for the above equations can be any of those normally used in steam- and gas-turbine analyses. Calculation Procedure: 1. Find the amount of oxygen required for complete combustion of the fuel Eight atoms of carbon in C8 combine with 8 molecules of oxygen, O2, and produce 8 molecules of carbon dioxide, 8CO2. similarly, 9 molecules of hydrogen, H2, in H18 combine with 9 atoms of oxygen, O, or 4.5 molecules of oxygen, to form 9 molecules of water, 9H2O. Thus, 100 percent, or the stoichiometric, air quantity required for complete combustion of a mole of fuel, C8H18, is proportional to 8 ⫹ 12.5 moles of oxygen, O2. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 81. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.79 FIGURE 58 Energy-flow chart of the gas turbine–steam turbine cycle in Fig. 56. 2. Establish the chemical equation for complete combustion with 100 percent air With 100 percent air: C8H18 ⫹ 12.5 O2 ⫹ (3.784 ⫻ 12.5)N2 → 8CO2 ⫹ 9H2O ⫹ 47.3N2, where 3.784 is a derived volumetric ratio of atmospheric nitrogen, (N2), to oxygen, O2, in dry air. The (N2) includes small amounts of inert and inactive gases. See Related Calculations of this procedure. 3. Establish the chemical equation for complete combustion with 400 percent of the stoichiometric air quantity, or 300 percent excess air With 400 percent air: C8H18 ⫹ 50 O2 ⫹ (4 ⫻ 47.3)N2 → 8CO2 ⫹ 9H2O ⫹ 189.2N2 ⫹ (3 ⫻ 12.5)O2. 4. Compute the molecular weights of the components in the combustion process Molecular weight of C8H18 ⫽ [(12 ⫻ 8) ⫹ (1 ⫻ 18)] ⫽ 114; O2 ⫽ 16 ⫻ 2 ⫽ 32; N2 ⫽ 14 ⫻ 2 ⫽ 28; CO2 ⫽ [(12 ⫻ 1) ⫹ (16 ⫻ 2)] ⫽ 44; H2O ⫽ [(1 ⫻ 2) ⫹ (16 ⫻ 1)] ⫽ 18. 5. Compute the relative weights of the reactants and products of the combustion process Relative weight ⫽ moles ⫻ molecular weight. Coefficients of the chemical equation in step 3 represent the number of moles of each component. Hence, for the reac- tants, the relative weights are: C8H18 ⫽ 1 ⫻ 114 ⫽ 114; O2 ⫽ 50 ⫻ 32 ⫽ 1600; N2 ⫽ 189.2 ⫻ 28 ⫽ 5298. Total relative weight of the reactants is 7012. For the products, the relative weights are: CO2 ⫽ 8 ⫻ 44 ⫽ 352; H2O ⫽ 9 ⫻ 18 ⫽ 162; N2 ⫽ 189.2 ⫻ 28 ⫽ 5298; O2 ⫽ 37.5 ⫻ 32 ⫽ 1200. Total relative weight of the products is 7012, also. 6. Compute the enthalpy of the products of the combustion process Enthalpy of the products of combustion, hp ⫽ mp where mp ⫽ number (h ⫺ h ), 1600 77 of moles of the products; ⫽ enthalpy of the products at 1600⬚F (871⬚C); h77 ⫽ h1600 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 82. 1.80 POWER GENERATION enthalpy of the products at 77⬚F (25⬚C). Thus, hp ⫽ (8 ⫹ 9 ⫹ 189.2 ⫹ 37.5)(15,400 ⫺ 3750) ⫽ 2,839,100 Btu [6,259,100 Btu (SI)]. 7. Compute the air supply temperature at the combustion chamber inlet Since the combustion process is adiabatic, the enthalpy of the reactants hr ⫽ hp, where hr ⫽ (relative weight of the fuel ⫻ its heating value) ⫹ [relative weight of the air ⫻ its specific heat ⫻ (air supply temperature ⫺ air source temperature)]. Therefore, hr ⫽ (114 ⫻ 19,100) ⫹ [(1600 ⫹ 5298) ⫻ 0.24 ⫻ (ta ⫺ 77)] ⫹ 2,839,100 Btu [6,259,100 Btu (SI)]. Solving for the air supply temperature, ta ⫽ [(2,839,100 ⫺ 2,177,400/1655.5] ⫹ 77 ⫽ 477⬚F (247⬚C). Related Calculations. This procedure, appropriately modified, may be used to deal with similar questions involving such things as other fuels, different amounts of excess air, and variations in the condition(s) being sought under certain given circumstances. The coefficient, (?) ⫽ 3.784 in step 2, is used to indicate that for each unit of volume of oxygen, O2, 12.5 in this case, there will be 3.784 units of nitrogen, N2. This equates to an approximate composition of air as 20.9 percent oxygen and 79.1 percent ‘‘atmospheric nitrogen,’’ (N2). In turn, this creates a paradox, because page 200 of Principles of Engineering Thermodynamics, by Kiefer, et al., John Wiley Sons, Inc., states air to be 20.99 percent oxygen and 79.01 percent atmospheric nitrogen, where the ratio (N2)/O2 ⫽ (?) ⫽ 79.01/20.99 ⫽ 3.764. Also, page 35 of Applied Energy Conversion, by Skrotski and Vopat, McGraw- Hill, Inc., indicates an assumed air analysis of 79 percent nitrogen and 21 percent oxygen, where (?) ⫽ 3.762. On that basis, a formula is presented for the amount of dry air chemically necessary for complete combustion of a fuel consisting of atoms of carbon, hydrogen, and sulfur, or C, H, and S, respectively. That formula is: Wa ⫽ 11.5C ⫹ 34.5[H ⫺ (0/8)] ⫹ 4.32S, lb air/lb fuel (kg air/kg fuel). The following derivation for the value of (?) should clear up the paradox and show that either 3.784 or 3.78 is a sound assumption which seems to be wrong, but in reality is not. In the above equation for Wa, the carbon hydrogen, or sulfur coefficient, Cx ⫽ (MO2 /DO2)Mx, where MO2 is the molecular weight of oxygen, O2; DO2 is the decimal fraction for the percent, by weight, of oxygen, O2, in dry air containing ‘‘atmospheric nitrogen,’’ (N2), and small amounts of inert and inactive gases: Mx is the formula weight of the combustible element in the fuel, as indicated by its relative amount as a reactant in the combustion equation. The alternate eval- uation of Cx is obtained from stoichiometric chemical equations for burning the combustible elements of the fuel, i.e., C ⫹ O2 ⫹ (?)N2 → CO2 ⫹ (?)N2; 2H2 ⫹ O2 ⫹ (?)N2 → 2H2O ⫹ (?)N2; S ⫹ O2 ⫹ (?)N2 → SO2 ⫹ (?)N2. Evidently, Cx ⫽ [MO2 ⫹ (?) ⫻ MN2)]/Mx, where MN2 is the molecular weight of nitrogen, N2, and the other items are as before. Equating the two expressions, Cx ⫽ [MO2 ⫹ (? ⫻ MN2)]/Mx ⫽ (MO2 /DO2)Mx, reveals that the Mx terms cancel out, indicating that the formula weight(s) of com- bustible components are irrelevant in solving for (?). Then, (?) ⫽ (1 ⫺ DO2)[M O2 /(MN2 ⫻ DO2)]. From the above-mentioned book by Kiefer, et al., DO2 ⫽ 0.23188. From Marks’ Standard Handbook for Mechanical Engineers, McGraw- Hill, Inc., MO2 ⫽ 31.9988 and MN2 ⫽ 28.0134. Thus, (?) ⫽ (1 ⫺ 0.23188)[31.9988/(28.0134 ⫻ 0.23188)] ⫽ 3.7838. This demonstrates that the use of (?) ⫽ 3.784, or 3.78, is justified for combustion equations. By using either of the two evaluation equations for Cx, and with accurate values for Mx, i.e., MC ⫽ 12.0111; MH ⫽ 2 ⫻ 2 ⫻ 1.00797 ⫽ 4.0319; MS ⫽ 32.064, from Marks’ M.E. Handbook, the more precise values for CC, CH, and CS are found out to be 11.489, 34.227, and 4.304, respectively. However, the actual Cx values, 11.5, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 83. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.81 FIGURE 59 Gas turbine flow diagram. 34.5, and 4.32, used in the formula for Wa are both brief for simplicity and rounded up to be on the safe side. GAS TURBINE COMBUSTION CHAMBER INLET AIR TEMPERATURE A gas turbine combustion chamber is well insulated so that heat losses to the atmosphere are negligible. Octane, C8H18, is to be used as the fuel and 400 percent of the stoichiometric air quantity is to be supplied. The air first passes through a regenerative heater and the air supply temperature at the combustion chamber inlet is to be set so that the exit temperature of the combustion gases is 1600⬚F (871⬚C). (See Fig. 59.) Fuel supply temperature is 77⬚F (25⬚C) and its heating value is to be taken as 19,000 Btu/lbm (44,190 kJ/kg) relative to a base of 77⬚F (15⬚C). The air may be treated in calculations as a perfect gas with a constant-pressure specific heat of 0.24 Btu/(lb ⬚F) [1.005 kJ/(kg ⬚C)]. The products of combustion have an enthalpy of 15,400 Btu/lb mol) [33,950 Btu/(kg mol)] at 1600⬚F (871⬚C) and an enthalpy of 3750 Btu/(lb mol) [8270 Btu/(kg mol)] at 77⬚F (24⬚C). De- termine, assuming complete combustion and neglecting dissociation, the required air temperature at the inlet of the combustion chamber. Calculation Procedure: 1. Find the amount of oxygen required for complete combustion of the fuel Eight atoms of carbon in C8 combine with 8 molecules of oxygen, O2, and produce 8 molecules of carbon dioxide, 8CO2. Similarly, 9 molecules of hydrogen, H2, in H18 combine with 9 atoms of oxygen, O, or 4.5 molecules of oxygen, to form 9 molecules of water, 9H2O. Thus, 100 percent, or the stoichiometric, air quantity Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 84. 1.82 POWER GENERATION required for complete combustion of a mole of fuel, C8H18, is proportional to 8 ⫹ 12.5 mol of oxygen, O2. 2. Establish the chemical equation for complete combustion with 100 percent air With 100 percent air: C8H18 ⫹ 12.5O2 ⫹ (3.784 ⫻ 12.5)N2 → 8CO2 ⫹ 9H2O ⫹ 47.3N2, where 3.784 is a derived volumetric ratio of atmospheric nitrogen (N2) to oxygen O2 in dry air. The N2 includes small amounts of inert and inactive gases. See Related Calculations of this procedure. 3. Establish the chemical equation for complete combustion with 400 percent of the stoichiometric air quantity, or 300 percent excess air With 400 percent air: C8H18 ⫹ 50O2 ⫹ (4 ⫻ 47.3)N2 → 8CO2 ⫹ 9H2O ⫹ 189.2N2 ⫹ (3 ⫻ 12.5)O2. 4. Compute the molecular weights of the components in the combustion proc- ess Molecular weight of C8H18 ⫽ [(12 ⫻ 8) ⫹ (1 ⫻ 18)] ⫽ 114; O2 ⫽ 16 ⫻ 2 ⫽ 32; N2 ⫽ 14 ⫻ 2 ⫽ 28; CO2 ⫽ [(12 ⫻ 1) ⫹ (16 ⫻ 2)] ⫽ 44; H2O ⫽ [(1 ⫻ 2) ⫹ (16 ⫻ 1)] ⫽ 18. 5. Compute the relative weights of the reactants and products of the combustion process Relative weight ⫽ moles ⫻ molecular weight. Coefficients of the chemical equation in step 3 represent the number of moles of each component. Hence, for the reac- tants, the relative weights are C8H18 ⫽ 1 ⫻ 114 ⫽ 114; O2 ⫽ 50 ⫻ 32 ⫽ 1600; N2 ⫽ 189.2 ⫻ 28 ⫽ 5298. Total relative weight of the reactants is 7012. For the products, the relative weights are CO2 ⫽ 8 ⫻ 44 ⫽ 352; H2O ⫽ 9 ⫻ 18 ⫽ 162; N2 ⫽ 189.2 ⫻ 28 ⫽ 5298; O2 ⫽ 37.5 ⫻ 32 ⫽ 1200. Total relative weight of the products is 7012 also. 6. Compute the enthalpy of the products of the combustion process Enthalpy of the products of combustion, hp ⫽ mp(h1600 ⫺ h77), where mp ⫽ number of moles of the products; h1600 ⫽ enthalpy of the products at 1600⬚F (871⬚C); h77 ⫽ enthalpy of the products at 77⬚F (25⬚C). Thus, hp ⫽ (8 ⫹ 9 ⫹ 189.2 ⫹ 37.5)(15,400 ⫺ 3750) ⫽ 2,839,100 Btu [6,259,100 Btu (SI)]. 7. Compute the air supply temperature at the combustion chamber inlet Since the combustion process is adiabatic, the enthalpy of the reactants hr ⫽ hp, where hr ⫽ (relative weight of the fuel ⫻ its heating value) ⫹ [relative weight of the air ⫻ its specific heat ⫻ (air supply temperature ⫺ air source temperature)]. Therefore, hr ⫽ (114 ⫻ 19,100) ⫹ [(1600 ⫹ 5298) ⫻ 0.24 ⫻ (ta ⫺ 77)] ⫹ 2,839,100 Btu [6,259,100 Btu (SI)]. Solving for the air supply temperature, ta ⫽ [(2,839,100 ⫺ 2,177,400/1655.5] ⫹ 77 ⫽ 477⬚F (247⬚C). Related Calculations. This procedure, appropriately modified, may be used to deal with similar questions involving such things as other fuels, different amounts of excess air, and variations in the condition(s) being sought under certain given circumstances. The coefficient, (?) ⫽ 3.784 in step 2, is used to indicate that for each unit of volume of oxygen, O2, 12.5 in this case, there will be 3.784 units of nitrogen, N2. This equates to an approximate composition of air as 20.9 percent oxygen and 79.1 percent ‘‘atmospheric nitrogen’’ (N2). In turn, this creates a paradox, because page Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 85. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.83 200 of Principles of Engineering Thermodynamics, by Kiefer et al., John Wiley Sons, Inc., states air to be 20.99 percent oxygen and 79.01 percent atmospheric nitrogen, where the ratio N2 /O2 ⫽ (?) ⫽ 79.01/20.99 ⫽ 3.764. Also, page 35 of Applied Energy Conversion, by Skrotski and Vopat, McGraw- Hill, Inc., indicates an assumed air analysis of 79 percent nitrogen and 21 percent oxygen, where (?) ⫽ 3.762. On that basis, a formula is presented for the amount of dry air chemically necessary for complete combustion of a fuel consisting of atoms of carbon, hydrogen, and sulfur, or C, H, and S, respectively. That formula is Wa ⫽ 11.5C ⫹ 34.5[H ⫺ (0/8)] ⫹ 4.32S, lb air/lb fuel (kg air/kg fuel). The following derivation for the value of (?) should clear up the paradox and show that either 3.784 or 3.78 is a sound assumption which seems to be wrong, but in reality is not. In the above equation for Wa, the carbon, hydrogen, or sulfur coefficient, Cx ⫽ (MO2 /DO2)Mx, where MO2 is the molecular weight of oxygen, O2; DO2 is the decimal fraction for the percent, by weight, of oxygen, O2, in dry air containing ‘‘atmospheric nitrogen,’’ (N2), and small amounts of inert and inactive gases: Mx is the formula weight of the combustible element in the fuel, as indicated by its relative amount as a reactant in the combustion equation. The alternate eval- uation of Cx is obtained from stoichiometric chemical equations for burning the combustible elements of the fuel, i.e., C ⫹ O2 ⫹ (?)N2 → CO2 ⫹ (?)N2; 2H2 ⫹ O2 ⫹ (?)N2 → 2H2O ⫹ (?)N2; S ⫹ O2 ⫹ (?)N2 → SO2 ⫹ (?)N2. Evidently, Cx ⫽ [MO2 ⫹ (? ⫻ MN2)]/Mx, where MN2 is the molecular weight of nitrogen, N2, and the other items are as before. Equating the two expressions, Cx ⫽ [MO2 ⫹ (? ⫻ MN2)]/Mx ⫽ (MO2 /DO2)Mx, reveals that the Mx terms cancel out, indicating that the formula weight(s) of com- bustible components are irrelevant in solving for (?). Then, (?) ⫽ (1 ⫺ DO2)[MO2 /(MN2 ⫻ DO2)]. From the above-mentioned book by Kiefer, et al., DO2 ⫽ 0.23188. From Marks’ Standard Handbook for Mechanical Engineers, McGraw-Hill, Inc., MO2 ⫽ 31.9988 and MN2 ⫽ 28.0134. Thus, (?) ⫽ (1 ⫺ 0.23188)[31.9988/(28.0134 ⫻ 0.23188)] ⫽ 3.7838. This demonstrates that the use of (?) ⫽ 3.784, or 3.78, is justified for combustion equations. By using either of the two evaluation equations for Cx, and with accurate values of Mx, i.e., MC ⫽ 12.0111; MH ⫽ 2 ⫻ 2 ⫻ 1.00797 ⫽ 4.0319; MS ⫽ 32.064, from Marks’ M.E. Handbook, the more precise values for CC, CH, and CS are found out to be 11.489, 34.227, and 4.304, respectively. However, the actual Cx values, 11.5, 34.5, and 4.32, used in the formula for Wa are both brief for simplicity and rounded up to be on the safe side. REGENERATIVE-CYCLE GAS-TURBINE ANALYSIS What is the cycle air rate, lb/kWh, for a regenerative gas turbine having a pressure ratio of 5, an air inlet temperature of 60⬚F (15.6⬚C), a compressor discharge tem- perature of 1500⬚F (815.6⬚C), and performance in accordance with Fig. 60? Deter- mine the cycle thermal efficiency and work ratio. What is the power output of a regenerative gas turbine if the work input to the compressor is 4400 hp (3281.1 kW)? Calculation Procedure: 1. Determine the cycle rate Use Fig. 60, entering at the pressure ratio of 5 in Fig. 60c and projecting to the 1500⬚F (815.6⬚C) curve. At the left, read the cycle air rate as 52 lb/kWh (23.6 kg/kWh). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 86. 1.84 POWER GENERATION FIGURE 60 (a) Schematic of regenerative gas turbine; (b), (c), and (d) gas-turbine performance based on a regenerator effectiveness of 70 percent, compressor and turbine efficiency of 85 percent; air inlet ⫽ 60⬚F (15.6⬚C); no pressure losses. 2. Find the cycle thermal efficiency Enter Fig. 60b at the pressure ratio of 5 and project vertically to the 1500⬚F (815.6⬚C) curve. At left, read the cycle thermal efficiency as 35 percent. Note that this point corresponds to the maximum efficiency obtainable from this cycle. 3. Find the cycle work ratio Enter Fig. 60d at the pressure ratio of 5 and project vertically to the 1500⬚F (815.6⬚C) curve. At the left, read the work ratio as 44 percent. 4. Compute the turbine power output For any gas turbine, the work ratio, percent ⫽ 100wc /wt, where wc ⫽ work input to the turbine, hp; wt ⫽ work output of the turbine, hp. Substituting gives 44 ⫽ 100(4400)/wt ; wt ⫽ 100(4400)/44 ⫽ 10,000 hp (7457.0 kW). Related Calculations. Use this general procedure to analyze gas turbines for power-plant, marine, and portable applications. Where the operating conditions are Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 87. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.85 FIGURE 61 (a) Effect of turbine-inlet on cycle performance; (b) effect of regenerator effective- ness; (c) effect of compressor inlet-air temperature; (d) effect of inlet-air temperature on turbine- cycle capacity. These curves are based on a turbine and compressor efficiency of 85 percent, a regenerator effectiveness of 70 percent, and a 1500⬚F (815.6⬚C) inlet-gas temperature. different from those given here, use the manufacturer’s engineering data for the turbine under consideration. Figure 61 shows the effect of turbine-inlet temperature, regenerator effectiveness, and compressor-inlet-air temperature on the performance of a modern gas turbine. Use these curves to analyze the cycles of gas turbines being considered for a par- ticular application if the operating conditions are close to those plotted. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 88. 1.86 POWER GENERATION FIGURE 62 (a) Turbine steam flow diagram. (b) Temperature-entropy schematic for steam flow. EXTRACTION TURBINE kW OUTPUT An automatic extraction turbine operates with steam at 400 lb/in2 absolute (2760 kPa), 700⬚F (371⬚C) at the throttle; its extraction pressure is 200 lb/in2 (1380 kPa) and it exhausts at 110 lb/in2 absolute (760 kPa). At full load 80,000 lb/h (600 kg/s) is supplied to the throttle and 20,000 lb/h (150 kg/s) is extracted at the bleed point. What is the kW output? Calculation Procedure: 1. Determine steam conditions at the throttle, bleed point, and exhaust Steam flow through the turbine is indicated by ‘‘enter’’ at the throttle, ‘‘extract’’ at the bleed point, and ‘‘exit’’ at the exhaust, as shown in Fig. 62a. The steam process is considered to be at constant entropy, as shown by the vertical isentropic line in Fig. 62b. At the throttle, where the steam enters at the given pressure, p1 ⫽ 400 lb/in2 absolute (2760 kPa) and temperature, t1 ⫽ 700⬚F (371⬚C), steam enthalpy, h1 ⫽ 1362.7 Btu/lb (3167.6 kJ/kg) and its entropy, s1 ⫽ 1.6398, as indicated by Table 3, Vapor of the Steam Tables mentioned under Related Calculations of this procedure. From the Mollier chart, a supplement to the Steam Tables, the following conditions are found along the vertical isentropic line where s1 ⫽ sx ⫽ s2 ⫽ 1.6398 Btu/(lb ⬚F) (6.8655 kJ/kg ⬚C): At the bleed point, where the given extraction pressure, px ⫽ 200 lb/in2 (1380 kPa) and the entropy, sx, is as mentioned above, the enthalpy, hx ⫽ 1284 Btu/lb (2986 kJ/kg) and the temperature tx ⫽ 528⬚F (276⬚C). At the exit, where the given exhaust pressure, p2 ⫽ 110 lb/in2 (760 kPa) and the entropy, s2, is as mentioned above, the enthalpy, h2 ⫽ 1225 Btu/lb (2849 kJ/kg) and the temperature, t2 ⫽ 400⬚F (204⬚C). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 89. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.87 2. Compute the total available energy to the turbine Between the throttle and the bleed point the available energy to the turbine, AE1 ⫽ Q1(h1 ⫺ hx ), where the full load rate of steam flow, Q1 ⫽ 80,000 lb/h (600 kg/s); other values are as before. Hence, AE1 ⫽ 80,000 ⫻ (1362.7 ⫺ 1284) ⫽ 6.296 ⫻ 106 Btu/h (1845 kJ/s). Between the bleed point and the exhaust the available energy to the turbine, AE2 ⫽ (Q1 ⫺ Q2)(hx ⫺ h2), where the extraction flow rate, Qx ⫽ 20,000 lbm /h (150 kg/s); other values as before. Then, AE2 ⫽ (80,000 ⫺ 20,000)(1284 ⫺ 1225) ⫽ 3.54 ⫻ 106 Btu/h (1037 kJ/s). Total available energy to the turbine, AE ⫽ AE1 ⫹ AE2 ⫽ 6.296 ⫻ 106 ⫹ (3.54 ⫻ 106 ) ⫽ 9.836 ⫻ 106 Btu/h (172.8 ⫻ 103 kJ/s). 3. Compute the turbine’s kW output The power available to the turbine to develop power at the shaft, in kilowatts, kW ⫽ AE /(Btu/kW h) ⫽ 9.836 ⫻ 106 /3412.7 ⫽ 2880 kW. However, the actual power developed at the shaft, kWa ⫽ kW ⫻ e, where e is the mechanical efficiency of the turbine. Thus, for an efficiency, e ⫽ 0.90, then kWa ⫽ 2880 ⫻ 0.90 ⫽ 2590 kW (2590 kJ/s). Related Calculations. The Steam Tables appear in Thermodynamic Properties of Water Including Vapor, Liquid, and Solid Phases, 1969, Keenan, et al., John Wiley Sons, Inc. Use later versions of such tables whenever available, as nec- essary. Steam Properties and Processes STEAM MOLLIER DIAGRAM AND STEAM TABLE USE (1) Determine from the Mollier diagram for steam (a) the enthalpy of 100 lb/in2 (abs) (689.5-kPa) saturated steam, (b) the enthalpy of 10-lb/in2 (abs) (68.9-kPa) steam containing 40 percent moisture, (c) the enthalpy of 100-lb/in2 (abs) (689.5- kPa) steam at 600⬚F (315.6⬚C). (2) Determine from the steam tables (a) the en- thalpy, specific volume, and entropy of steam at 145.3 lb/in2 (gage) (1001.8 kPa); (b) the enthalpy and specific volume of superheated steam at 1100 lb/in2 (abs) (7584.2 kPa) and 600⬚F (315.6⬚C); (c) the enthalpy and specific volume of high- pressure steam at 7500 lb/in2 (abs) (51,710.7 kPa) and 1200⬚F (648.9⬚C); (d) the enthalpy, specific volume, and entropy of 10-lb/in2 (abs) (68.9-kPa) steam contain- ing 40 percent moisture. Calculation Procedure: 1. Use the pressure and saturation (or moisture) lines to find enthalpy (a) Enter the Mollier diagram by finding the 100-lb/in2 (abs) (689.5-kPa) pressure line, Fig. 63. In the Mollier diagram for steam, the pressure lines slope upward to the right from the lower left-hand corner. For saturated steam, the enthalpy is read at the intersection of the pressure line with the saturation curve ceƒ, Fig. 63. Thus, project along the 100-lb/in2 (abs) (689.5-kPa) pressure curve, Fig. 63, until it intersects the saturation curve, point g. From here project horizontally to Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 90. 1.88 POWER GENERATION FIGURE 63 Simplified Mollier diagram for steam. the left-hand scale of Fig. 63 and read the enthalpy of 100-lb/in2 (abs) (689.5-kPa) saturated steam as 1187 Btu/lb (2761.0 kJ/kg). (The Mollier diagram in Fig. 63 has fewer grid divisions than large-scale diagrams to permit easier location of the major elements of the diagram.) (b) On a Mollier diagram, the enthalpy of wet steam is found at the intersection of the saturation pressure line with the percentage-of-moisture curve corresponding to the amount of moisture in the steam. In a Mollier diagram for steam, the moisture curves slope downward to the right from the saturated liquid line cd, Fig. 63. To find the enthalpy of 10-lb/in2 (abs) (68.9-kPa) steam containing 40 percent moisture, project along the 1-lb/in2 (abs) (68.9-kPa) saturation pressure line until the 40 percent moisture curve is intersected, Fig. 63. From here project horizontally to the left-hand scale and read the enthalpy of 10-lb/in2 (68.9-kPa) wet steam containing 40 percent moisture as 750 Btu/lb (1744.5 kJ/kg). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 91. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.89 2. Find the steam properties from the steam tables (a) Steam tables normally list absolute pressures or temperature in degrees Fahren- heit as one of their arguments. Therefore, when the steam pressure is given in terms of a gage reading, it must be converted to an absolute pressure before the table can be entered. To convert gage pressure to absolute pressure, add 14.7 to the gage pressure, or pa ⫽ pg ⫹ 14.7. In this instance, pa ⫽ 145.3 ⫹ 14.7 ⫽ 160.0 lb/in2 (abs) (1103.2 kPa). Once the absolute pressure is known, enter the saturation pres- sure table of the steam table at this value, and project horizontally to the desired values. For 160-lb/in2 (abs) (1103.2-kPa) steam, using the ASME or Keenan and Keyes—Thermodynamic Properties of Steam, we see that the enthalpy of evapo- ration ⫽ 859.2 Btu/lb (1998.5 kJ/kg), and the enthalpy of saturated vapor hg ⫽ hƒg 1195.1 Btu/lb (2779.8 kJ/kg), read from the respective columns of the steam tables. The specific volume vg of the saturated vapor of 160-lb/in2 (abs) (1103.2-kPa) steam is, from the tables, 2.834 ft3 /lb (0.18 m3 /kg), and the entropy sg is 1.5640 Btu/(lb ⬚F) [6.55 kJ/(kg ⬚C)]. (c) Every steam table contains a separate tabulation of properties of superheated steam. To enter the superheated steam table, two arguments are needed—the ab- solute pressure and the temperature of the steam. To determine the properties of 1100-lb/in2 (abs) (7584.5-kPa) 600⬚F (315.6⬚C) steam, enter the superheated steam table at the given absolute pressure and project horizontally from this absolute pressure [1100 lb/in2 (abs) or 7584.5 kPa] to the column corresponding to the superheated temperature (600⬚F or 315.6⬚C) to read the enthalpy of the superheated vapor as h ⫽ 1236.7 Btu/lb (2876.6 kJ/kg) and the specific volume of the super- heated vapor v ⫽ 0.4532 ft3 /lb (0.03 m3 /kg). (c) For high-pressure steam use the ASME—Steam Table, entering it in the same manner as the superheated steam table. Thus, for 7500-lb/in2 (abs) (51,712.5 kPa) 1200⬚F (648.9⬚C) steam, the enthalpy of the superheated vapor is 1474.9 Btu/lb (3430.6 kJ/kg), and the specific volume of the superheated vapor is 0.1060 ft3 /lb (0.0066 m3 /kg). (d) To determine the enthalpy, specific volume, and the entropy of wet steam having y percent moisture by using steam tables instead of the Mollier diagram, apply these relations: h ⫽ hg ⫺ 100; v ⫽ vg ⫺ /100; s ⫽ sg ⫺ /100, yh / yv ys ƒg ƒg ƒg where y ⫽ percentage of moisture expressed as a whole number. For 10-lb/in2 (abs) (68.9-kPa) steam containing 40 percent moisture, obtain the needed values—hg, vg, sg, and —from the saturation-pressure steam table and substitute in h , v , s ƒg ƒg ƒg the above relations. Thus, Note that Keenan and Keyes, in Thermodynamic Properties of Steam, do not tabulate Therefore, this value must be obtained by subtraction of the tabulated v . ƒg values, or ⫽ vg ⫺ The value thus obtained is used in the relation for the v v . v ƒg ƒ ƒg volume of the wet steam. For 10-lb/in2 (abs) (68.9-kPa) steam containing 40 per- cent moisture, vg ⫽ 38.42 ft3 /lb (2.398 m3 /kg) and ⫽ 0.017 ft3 /lb (0.0011 m3 / vƒ kg). Then ⫽ 38.42 ⫺ 0.017 ⫽ 28.403 ft3 /lb (1.773 m3 /kg). vƒg Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 92. 1.90 POWER GENERATION In some instances, the quality of steam may be given instead of its moisture content in percentage. The quality of steam is the percentage of vapor in the mix- ture. In the above calculation, the quality of the steam is 60 percent because 40 percent is moisture. Thus, quality ⫽ 1 ⫺ m, where m ⫽ percentage of moisture, expressed as a decimal. INTERPOLATION OF STEAM TABLE VALUES (1) Determine the enthalpy, specific volume, entropy, and temperature of saturated steam at 151 lb/in2 (abs) (1041.1 kPa). (2) Determine the enthalpy, specific volume, entropy, and pressure of saturated steam at 261⬚F (127.2⬚C). (3) Find the pressure of steam at 1000⬚F (537.8⬚C) if its specific volume is 2.6150 ft3 /lb (0.16 m3 /kg). (4) Calculate the enthalpy, specific volume, and entropy of 300-lb/in2 (abs) (2068.5- kPa) steam at 567.22⬚F (297.3⬚C). Calculation Procedure: 1. Use the saturation-pressure table Study of the saturation-pressure table shows that there is no pressure value for 151 lb/in2 (abs) (1041.1 kPa) listed. So it will be necessary to interpolate between the next higher and next lower tabulated pressure values. In this instance, these values are 152 and 150 lb/in2 (abs) (1048.0 and 1034.3 kPa), respectively. The pressure for which properties are being found [151 lb/in2 (abs) or 1041.1 kPa] is called the intermediate pressure. At 152 lb/in2 (abs) (1048.0 kPa), hg ⫽ 1194.3 Btu/lb (2777.5 kJ/kg); vg ⫽ 2.977 ft3 /lb (0.19 m3 /kg); sg ⫽ 1.5683 Btu/(lb ⬚F) [6.67 kJ/(kg ⬚C)/ t ⫽ 359.46⬚F (181.9⬚C). At 150 lb/in2 (abs) (1034.3 kPa), hg ⫽ 1194.1 Btu/lb (2777.5 kJ/kg); vg ⫽ 3.015 ft3 /lb (0.19 m3 /kg); sg ⫽ 1.5694 Btu/(lb ⬚F) [6.57 kJ/(kg ⬚C); t ⫽ 358.42⬚F (181.3⬚C). For the enthalpy, note that as the pressure increases, so does hg . Therefore, the enthalpy at 151 lb/in2 (abs) (1041.1 kPa), the intermediate pressure, will equal the enthalpy at 150 lb/in2 (abs) (1034.3 kPa) (the lower pressure used in the interpo- lation) plus the proportional change (difference between the intermediate pressure and the lower pressure) for a 1-lb/in2 (abs) (6.9-kPa) pressure increase. Or, at any higher pressure, hgi ⫽ hgl ⫹ [(pi ⫺ pl )/(ph ⫺ pl )](hh ⫺ hl ), where hgi ⫽ enthalpy at the intermediate pressure; hgl ⫽ enthalpy at the lower pressure used in the in- terpolation; hh ⫽ enthalpy at the higher pressure used in the interpolation; pi ⫽ intermediate pressure; ph and pl ⫽ higher and lower pressures, respectively, used in the interpolation. Thus, from the enthalpy values obtained from the steam table for 150 and 152 lb/in2 (abs) (1034.3 and 1048.0 kPa), hgi ⫽ 1194.1 ⫹ [(151 ⫺ 150)/(152 ⫺ 150)](1194.3 ⫺ 1194.1) ⫽ 1194.2 Btu/lb (2777.7 kJ/kg) at 151 lb/ in2 (abs) (1041.1 kPa) saturated. Next study the steam table to determine the direction of change of specific volume between the lower and higher pressures. This study shows that the specific volume decreases as the pressure increases. Therefore, the specific volume at 151 lb/in2 (abs) (1041.1 kPa) (the intermediate pressure) will equal the specific volume at 150 lb/in2 (abs) (1034.3 kPa) (the lower pressure used in the interpolation) minus the proportional change (difference between the intermediate pressure and the lower interpolating pressure) for a 1-lb/in2 (abs) pressure increase. Or, at any pressure, vgi ⫽ vgl ⫺ [(pi ⫺ pl )/(ph ⫺ pl )](vl ⫺ vh ), where the subscripts are the same as Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 93. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.91 above and v ⫽ specific volume at the respective pressure. With the volume values obtained from steam tables for 150 and 152 lb/in2 (abs) (1034.3 and 1048.0 kPa), vgi ⫽ 3.015 ⫺ [(151 ⫺ 150)/(152 ⫺ 150)](3.015 ⫺ 2.977) ⫽ 2.996 ft3 /lb (0.19 m3 /kg) and 151 lb/in2 (abs) (1041.1 kPa) saturated. Study of the steam table for the direction of entropy change shows that entropy, like specific volume, decreases as the pressure increases. Therefore, the entropy at 151 lb/in2 (abs) (1041.1 kPa) (the intermediate pressure) will equal the entropy at 150 lb/in2 (abs) (1034.3 kPa) (the lower pressure used in the interpolation) minus the proportional change (difference between the intermediate pressure and the lower interpolating pressure) for a 1-lb/in2 (abs) (6.9-kPa) pressure increase. Or, at any higher pressure, sgi ⫽ sgl ⫺ [(pi ⫺ pl )/(ph ⫺ pl )](sl ⫺ sh ) ⫽ 1.5164 ⫺ [(151 ⫺ 150)/(152 ⫺ 150)](1.5694 ⫺ 1.5683) ⫽ 1.56885 Btu/(lb ⬚F) [6.6 kJ/(kg ⬚C)] at 151 lb/in2 (abs) (1041.1 kPa) saturated. Study of the steam table for the direction of temperature change shows that the saturation temperature, like enthalpy, increases as the pressure increases. Therefore, the temperature at 151 lb/in2 (abs) (1041.1 kPa) (the intermediate pressure) will equal the temperature at 150 lb/in2 (abs) (1034.3 kPa) (the lower pressure used in the interpolation) plus the proportional change (difference between the intermediate pressure and the lower interpolating pressure) for a 1-lb/in2 (abs) (6.9-kPa) increase. Or, at any higher pressure, tgi ⫽ tgl ⫹ [(pi ⫺ pl )/(ph ⫺ pl )](th ⫺ tl ) ⫽ 358.42 ⫹ [(151 ⫺ 150)/(152 ⫺ 150)](359.46 ⫺ 358.42) ⫽ 358.94⬚F (181.6⬚C) at 151 lb/in2 (abs) (1041.1 kPa) saturated. 2. Use the saturation-temperature steam table Study of the saturation-temperature table shows that there is no temperature value of 261⬚F (127.2⬚C) listed. Therefore, it will be necessary to interpolate between the next higher and next lower tabulated values. In this instance these values are 262 and 260⬚F (127.8 and 126.7⬚C), respectively. The temperature for which properties are being found (261⬚F or 127.2⬚C) is called the intermediate temperature. For enthalpy, note that as the temperature increases, so does hg . Therefore, the enthalpy at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the enthalpy at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) plus the pro- portional change (difference between the intermediate temperature and the lower temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature, hgi ⫽ hgl ⫹ [(ti ⫺ tl )/(th ⫺ tl )](hh ⫺ hl ), where hgl ⫽ enthalpy at the lower tem- perature used in the interpolation; hh ⫽ enthalpy at the higher temperature used in the interpolation; ti ⫽ intermediate temperature; th and tl ⫽ higher and lower tem- peratures, respectively, used in the interpolation. Thus, from the enthalpy values obtained from the steam table for 260 and 262⬚F (126.7 and 127.8⬚C), hgi ⫽ 1167.3 ⫹ [(261 ⫺ 260)/(262 ⫺ 260)](1168.0 ⫺ 1167.3) ⫽ 1167.65 Btu/lb (2716.0 kJ/kg) at 260⬚F (127.2⬚C) saturated. Next, study the steam table to determine the direction of change of specific volume between the lower and higher temperatures. This study shows that the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 94. 1.92 POWER GENERATION specific volume decreases as the pressure increases. Therefore, the specific volume at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the specific volume at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) minus the pro- portional change (difference between the intermediate temperature and the lower interpolating temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature, vgi ⫽ vgl ⫺ [(ti ⫺ tl )/(th ⫺ tl )](vl ⫺ vh ) ⫽ 11.763 ⫺ [(261 ⫺ 260)/ (262 ⫺ 260)](11.763 ⫺ 11.396) ⫽ 11.5795 ft3 /lb (0.7 m3 /kg) at 261⬚F (127.2⬚C) saturated. Study of the steam table for the direction of entropy change shows that entropy, like specific volume, decreases as the temperature increases. Therefore, the entropy at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the entropy at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) minus the proportional change (difference between the intermediate temperature and the lower temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature, sgi ⫽ sgl ⫺ [(ti ⫺ tl )/(hh ⫺ tl )](sl ⫺ sh ) ⫽ 1.6860 ⫺ [(261 ⫺ 260)/(262 ⫺ 260)](1.6860 ⫺ 1.6833) ⫽ 1.68465 Btu/(lb ⬚F) [7.1 kJ/(kg ⬚C)] at 261⬚F (127.2⬚C). Study of the steam table for the direction of pressure change shows that the saturation pressure, like enthalpy, increases as the temperature increases. Therefore, the pressure at 261⬚F (127.2⬚C) (the intermediate temperature) will equal the pres- sure at 260⬚F (126.7⬚C) (the lower temperature used in the interpolation) plus the proportional change (difference between the intermediate temperature and the lower interpolating temperature) for a 1⬚F (0.6⬚C) temperature increase. Or, at any higher temperature, pgi ⫽ pgl ⫹ [(ti ⫺ tl )/(th ⫺ tl )](ph ⫺ pl ) ⫽ 35.429 ⫹ [(261 ⫺ 260)(262 ⫺ 260)](36.646 ⫺ 35.429) ⫽ 36.0375 lb/in2 (abs) (248.5 kPa) at 261⬚F (127.2⬚C) saturated. 3. Use the superheated steam table Choose the superheated steam table for steam at 1000⬚F (537.9⬚C) and 2.6150 ft3 /lb (0.16 m3 /kg) because the highest temperature at which saturated steam can exist is 705.4⬚F (374.1⬚C). This is also the highest temperature tabulated in some saturated-temperature tables. Therefore, the steam is superheated when at a tem- perature of 1000⬚F (537.9⬚C). Look down the 1000⬚F (537.9⬚C) columns in the superheated steam table until a specific volume value of 2.6150 (0.16) is found. This occurs between 325 lb/in2 (abs) (2240.9 kPa, v ⫽ 2.636 or 0.16) and 330 lb/in2 (abs) (2275.4 kPa, v ⫽ 2.596 or 0.16). Since there is no volume value exactly equal to 2.6150 tabulated, it will be necessary to interpolate. List the values from the steam table thus: Note that as the pressure rises, at constant temperature, the volume decreases. Therefore, the intermediate (or unknown) pressure is found by subtracting from the higher interpolating pressure [330 lb/in2 (abs) or 2275.4 kPa in this instance] the product of the proportional change in the specific volume and the difference in the pressures used for the interpolation. Or, pgi ⫽ ph ⫺ [vi ⫺ vh )/(vl ⫺ vh )](ph ⫺ pl ), where the subscripts h, l, and i refer to the high, low, and intermediate (or unknown) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 95. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.93 pressures, respectively. In this instance, pgi ⫽ 330 ⫺ [(2.615 ⫺ 2.596)/(2.636 ⫺ 2.596)](330 ⫺ 325) ⫽ 327.62 lb/in2 (abs) (2258.9 kPa) at 1000⬚F (537.9 kPa) and a specific volume of 2.6150 ft3 /lb (0.16 m3 /kg). 4. Use the superheated steam table When a steam pressure and temperature are given, determine, before performing any interpolation, the state of the steam. Do this by entering the saturation-pressure table at the given pressure and noting the saturation temperature. If the given tem- perature exceeds the saturation temperature, the steam is superheated. In this in- stance, the saturation-pressure table shows that at 300 lb/in2 (abs) (2068.5 kPa) the saturation temperature is 417.33⬚F (214.1⬚C). Since the given temperature of the steam is 567.22⬚F (297.3⬚C), the steam is superheated because its actual temperature is greater than the saturation temperature. Enter the superheated steam table at 300 lb/in2 (abs) (2068.5 kPa), and find the next temperature lower than 567.22⬚F (297.3⬚C); this is 560⬚F (293.3⬚C). Also find the next higher temperature; this is 580⬚F (304.4⬚C). Tabulate the enthalpy, specific volume, and entropy for each temperature thus: Use the same procedures for each property—enthalpy, specific volume, and entropy—as given in step 2 above; but change the sign between the lower volume and entropy and the proportional factor (temperature in this instance), because for superheated steam the volume and entropy increase as the steam temperature in- creases. Thus Note: Also observe the direction of change of a property before interpolating. Use a plus or minus sign between the higher interpolating value and the proportional change depending on whether the tabulated value increases (⫹) or decreases (⫺). CONSTANT-PRESSURE STEAM PROCESS Three pounds of wet steam, containing 15 percent moisture and initially at a pres- sure of 400 lb/in2 (abs) (2758.0 kPa), expands at constant pressure (P ⫽ C) to 600⬚F (315.6⬚C). Determine the initial temperature T1, enthalpy H1, internal energy E1, volume V1, entropy S1, final entropy H2, internal energy E2, volume V2, entropy Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 96. 1.94 POWER GENERATION FIGURE 64 Constant-pressure process. S2, heat added to the steam Q1, work output W2, change in initial energy ⌬E, change in specific volume ⌬V, change in entropy ⌬S. Calculation Procedure: 1. Determine the initial steam temperature from the steam tables Enter the saturation-pressure table at 400 lb/in2 (abs) (2758.0 kPa), and read the saturation temperature as 444.59⬚F (229.2⬚C). 2. Correct the saturation values for the moisture of the steam in the initial state Sketch the process on a pressure-volume (P-V), Mollier (H-S), or temperature en- tropy (T-S) diagram, Fig. 64. In state 1, y ⫽ moisture content ⫽ 15 percent. Using the appropriate values from the saturation-pressure steam table for 40 lb/in2 (abs) (2758.0 kPa), correct them for a moisture content of 15 percent: 3. Determine the steam properties in the final state Since this is a constant-pressure process, the pressure in state 2 is 400 lb/in2 (abs) (2758.0 kPa), the same as state 1. The final temperature is given as 600⬚F (315.6⬚C). This is greater than the saturation temperature of 444.59⬚F (229.2⬚C). Hence, the steam is superheated when in state 2. Use the superheated steam tables, entering at 400 lb/in2 (abs) (2758.8 kPa) and 600⬚F (315.6⬚C). At this condition, H2 ⫽ 1306.9 Btu/lb (3039.8 kJ/kg); V2 ⫽ 1.477 ft3 /lb (0.09 m3 /kg). Then E2 ⫽ h2g ⫺ P2V2 / J ⫽ 1306.9 ⫺ 400(144)(1.477)/778 ⫽ 1197.5 Btu/lb (2785.4 kJ/kg). In this equa- tion, the constant 144 converts pounds per square inch to pounds per square foot, absolute, and J ⫽ mechanical equivalent of heat ⫽ 778 ft lb/Btu (1 N m/J). From the steam tables, S2 ⫽ 1.5894 Btu/(lb ⬚F) [6.7 kJ/(kg ⬚C)]. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 97. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.95 4. Compute the process inputs, outputs, and changes W2 ⫽ (P1 /J)(V2 ⫺ V1)m ⫽ [400(144)/778](1.4770 ⫺ 0.9900)(3) ⫽ 108.1 Btu (114.1 kJ). In this equation, m ⫽ weight of steam used in the process ⫽ 3 lb (1.4 kg). Then 5. Check the computations The work output W2 should equal the change in internal energy plus the heat input, or W2 ⫽ E1 ⫺ E2 ⫹ Q1 ⫽ ⫺550.2 ⫹ 658.5 ⫽ 108.3 Btu (114.3 kJ). This value very nearly equals the computed value of W2 ⫽ 108.1 Btu (114.1 kJ) and is close enough for all normal engineering computations. The difference can be traced to calculator input errors. In computing the work output, the internal-energy change has a negative sign because there is a decrease in E during the process. Related Calculations. Use this procedure for all constant-pressure steam pro- cesses. CONSTANT-VOLUME STEAM PROCESS Five pounds (2.3 kg) of wet steam initially at 120 lb/in2 (abs) (827.4 kPa) with 30 percent moisture is heated at constant volume (V ⫽ C) to a final temperature of 1000⬚F (537.8⬚C). Determine the initial temperature T1, enthalpy H1, internal energy E1, volume V1, final pressure P2, enthalpy H2, internal energy E2, volume V2, heat added Q1, work output W, change in internal energy ⌬E, volume ⌬V, and entropy ⌬S. Calculation Procedure: 1. Determine the initial steam temperature from the steam tables Enter the saturation-pressure table at 120 lb/in2 (abs) (827.4 kPa), the initial pres- sure, and read the saturation temperature T1 ⫽ 341.25⬚F (171.8⬚C). 2. Correct the saturation values for the moisture in the steam in the initial state Sketch the process on P-V, H-S, or T-S diagrams, Fig. 65. Using the appropriate values from the saturation-pressure table for 120 lb/in2 (abs) (827.4 kPa), correct them for a moisture content of 30 percent: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 98. 1.96 POWER GENERATION FIGURE 65 Constant-volume process. 3. Determine the steam volume in the final state We are given T2 ⫽ 1000⬚F (537.8⬚C). Since this is a constant-volume process, V2 ⫽ V1 ⫽ 2.6150 ft3 /lb (0.16 m3 /kg). The total volume of the vapor equals the product of the specific volume and the number of pounds of vapor used in the process, or total volume ⫽ 2.6150(5) ⫽ 13.075 ft3 (0.37 m3 ). 4. Determine the final steam pressure The final steam temperature (1000⬚F or 537.8⬚C) and the final steam volume (2.6150 ft3 /lb or 0.16 m3 /kg) are known. To determine the final steam pressure, find in the steam tables the state corresponding to the above temperature and spe- cific volume. Since a temperature of 1000⬚F (537.8⬚C) is higher than any saturation temperature (705.4⬚F or 374.1⬚C is the highest saturation temperature for saturated steam), the steam in state 2 must be superheated. Therefore, the superheated steam tables must be used to determine P2. Enter the 1000⬚F (537.8⬚C) column in the steam table, and look for a super- heated-vapor specific volume of 2.6150 ft3 /lb (0.16 m3 /kg). At a pressure of 325 lb/in2 (abs) (2240.9 kPa), and at a pressure of 330 lb/in2 (abs) (227.4 kPa) Thus, 2.6150 lies between 325 and 330 lb/in2 (abs) (2240.9 and 2275.4 kPa). To determine the pressure corresponding to the final volume, it is necessary to inter- polate between the specific-volume values, or P2 ⫽ 330 ⫺ [(2.615 ⫺ 2.596)/ (2.636 ⫺ 2.596)](330 ⫺ 325) ⫽ 327.62 lb/in2 (abs) (2258.9 kPa). In this equation, the volume values correspond to the upper [330 lb/in2 (abs) or 2275.4 kPa], lower [325 lb/in2 (abs) or 2240.9 kPa], and unknown pressures. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 99. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.97 5. Determine the final enthalpy, entropy, and internal energy The final enthalpy can be interpolated in the same manner, using the enthalpy at each volume instead of the pressure. Thus H2 ⫽ 1524.5 ⫺ [(2.615 ⫺ 2.596)/ (2.636 ⫺ 2.596)](1524.5 ⫺ 1524.4) ⫽ 1524.45 Btu/lb (3545.8 kJ/kg). Since the difference in enthalpy between the two pressures is only 0.1 Btu/lb (0.23 kJ/kg) (⫽ 1524.5 ⫺ 1524.4), the enthalpy at 327.62 lb/in2 (abs) could have been assumed equal to the enthalpy at the lower pressure [325 lb/in2 (abs) or 2240.9 kPa], or 1524.4 Btu/lb (3545.8 kJ/kg), and the error would have been only 0.05 Btu/lb (0.12 kJ/kg), which is negligible. However, where the enthalpy values vary by more than 1.0 Btu/lb (2.3 kJ/kg), interpolate as shown, if accurate results are desired. Find S2 by interpolating between pressures, or 6. Compute the changes resulting from the process Here Q1 ⫽ (E2 ⫺ E1)m ⫽ (1365.9 ⫺ 868.9)(5) ⫽ 2485 Btu (2621.8 kJ); ⌬S ⫽ (S2 ⫺ S1)m ⫽ (1.7854 ⫺ 1.2589)(5) ⫽ 2.6325 Btu/⬚F (5.0 kJ/⬚C). By definition, W ⫽ 0; ⌬V ⫽ 0; ⌬E ⫽ Q1. Note that the curvatures of the constant- volume line on the T-S chart, Fig. 65, are different from the constant-pressure line, Fig. 64. Adding heat Q1 to a constant-volume process affects only the internal energy. The total entropy change must take into account the total steam mass m ⫽ 5 lb (2.3 kg). Related Calculations. Use this general procedure for all constant-volume steam processes. CONSTANT-TEMPERATURE STEAM PROCESS Six pounds (2.7 kg) of wet steam initially at 1200 lb/in2 (abs) (8274.0 kPa) and 50 percent moisture expands at constant temperature (T ⫽ C) to 300 lb/in2 (abs) (2068.5 kPa). Determine the initial temperature T1, enthalpy H1, internal energy E1, specific volume V1, entropy S1, final temperature T2, enthalpy H2, internal energy E2, volume V2, entropy S2, heat added Q1, work output W2, change in internal energy ⌬E, volume ⌬V, and entropy ⌬S. Calculation Procedure: 1. Determine the initial steam temperature from the steam tables Enter the saturation-pressure table at 1200 lb/in2 (abs) (8274.0 kPa), and read the saturation temperature T1 ⫽ 567.22⬚F (297.3⬚C). 2. Correct the saturation values for the moisture in the steam in the initial state Sketch the process on P-V, H-S, or T-S diagrams, Fig. 66. Using the appropriate values from the saturation-pressure table for 1200 lb/in2 (abs) (8274.0 kPa), correct them for the moisture content of 50 percent: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 100. 1.98 POWER GENERATION FIGURE 66 Constant-temperature process. 3. Determine the steam properties in the final state Since this is a constant-temperature process, T2 ⫽ T1 ⫽ 567.22⬚F (297.3⬚C); P2 ⫽ 300 lb/in2 (abs) (2068.5 kPa), given. The saturation temperature of 300 lb/in2 (abs) (2068.5 kPa) is 417.33⬚F (214.1⬚C). Therefore, the steam is superheated in the final state because 567.22⬚F (297.3⬚C) ⬎ 417.33⬚F (214.1⬚C), the saturation temperature. To determine the final enthalpy, entropy, and specific volume, it is necessary to interpolate between the known final temperature and the nearest tabulated temper- atures greater and less than the final temperature. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 101. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.99 4. Compute the process changes Here Q1 ⫽ T(S2 ⫺ S1)m, where T1 ⫽ absolute initial temperature, ⬚R. So Q1 ⫽ (567.22 ⫹ 460)(1.6093 ⫺ 1.0689)(6) ⫽ 3330 Btu (3513.3 kJ). Then Related Calculations. Use this procedure for any constant-temperature steam process. CONSTANT-ENTROPY STEAM PROCESS Ten pounds (4.5 kg) of steam expands under two conditions—nonflow and steady flow—at constant entropy (S ⫽ C) from an initial pressure of 2000 lb/in2 (abs) (13,790.0 kPa) and a temperature of 800⬚F (426.7⬚C) to a final pressure of 2 lb/ in2 (abs) (13.8 kPa). In the steady-flow process, assume that the initial kinetic energy Ek 1 ⫽ the final kinetic energy Ek 2. Determine the initial enthalpy H1, internal energy E1, volume V1, entropy S1, final temperature T2, percentage of moisture y, enthalpy H2, internal energy E2, volume V2, entropy S2, change in internal energy ⌬E, enthalpy ⌬H, entropy ⌬S, volume ⌬V, heat added Q1, and work output W2. Calculation Procedure: 1. Determine the initial enthalpy, volume, and entropy from the steam tables Enter the superheated-vapor table at 2000 lb/in2 (abs) (13,790.0 kPa) and 800⬚F (427.6⬚C), and read H1 ⫽ 1335.5 Btu/lb (3106.4 kJ/kg); V1 ⫽ 0.3074 ft3 /lb (0.019 m3 /kg); S1 ⫽ 1.4576 Btu/(lb ⬚F) [6.1 kJ/(kg ⬚C)]. 2. Compute the initial energy 3. Determine the vapor properties on the final state Sketch the process on P-V, H-S, or T-S diagrams, Fig. 67. Note that the expanded steam is wet in the final state because the 2-lb/in2 (abs) (13.8-kPa) pressure line is under the saturation curve on the H-S and T-S diagrams. Therefore, the vapor prop- erties in the final state must be corrected for the moisture content. Read, from the saturation-pressure steam table, the liquid and vapor properties at 2 lb/in2 (abs) (13.8 kPa). Tabulate these properties thus: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 102. 1.100 POWER GENERATION FIGURE 67 Constant-entropy process. Since this is a constant-entropy process, S2 ⫽ S1 ⫽ sg ⫺ Solve for y2, the y s . 2 ƒg percentage of moisture in the final state. Or, y2 ⫽ (sg ⫺ S1)/ ⫽ (1.9200 ⫺ sƒg 1.4576)/1.7451 ⫽ 0.265, or, 26.5 percent. Then 4. Compute the changes resulting from the process The total change in properties is for 10 lb (4.5 kg) of steam, the quantity used in this process. Thus, So Q1 ⫽ 0 Btu. (By definition, there is no transfer of heat in a constant-entropy process.) Nonflow W2 ⫽ ⌬E ⫽ 4236 Btu (4469.2 kJ). Steady flow W2 ⫽ ⌬H ⫽ 4902 Btu (5171.9 kJ). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 103. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.101 FIGURE 68 Irreversible adiabatic process. Note: In a constant-entropy process, the nonflow work depends on the change in internal energy. The steady-flow work depends on the change in enthalpy and is larger than the nonflow work by the amount of the change in the flow work. IRREVERSIBLE ADIABATIC EXPANSION OF STEAM Ten pounds (4.5 kg) of steam undergoes a steady-flow expansion from an initial pressure of 2000 lb/in2 (abs) (13,790.0 kPa) and a temperature 800⬚F (426.7⬚C) to a final pressure of 2 lb/in2 (abs) (13.9 kPa) at an expansion efficiency of 75 percent. In this steady flow, assume Ek 1 ⫽ Ek 2. Determine ⌬E, ⌬H, ⌬S, ⌬V, Q, and W2. Calculation Procedure: 1. Determine the initial vapor properties from the steam tables Enter the superheated-vapor tables at 2000 lb/in2 (abs) (13,790.0 kPa) and 800⬚F (426.7⬚C), and read H1 ⫽ 1335.5 Btu/lb (3106.4 kJ/kg); V1 ⫽ 0.3074 ft3 /lb (0.019 m3 /kg); E1 ⫽ 1221.6 Btu/lb (2840.7 kJ/kg); S1 ⫽ 1.4576 Btu/(lb ⬚F) [6.1 kJ/(kg ⬚C)]. 2. Determine the vapor properties in the final state Sketch the process on P-V, H-S, or T-S diagram, Fig. 68. Note that the expanded steam is wet in the final state because the 2-lb/in2 (abs) (13.9-kPa) pressure line is under the saturation curve on the H-S and T-S diagram. Therefore, the vapor prop- erties in the final state must be corrected for the moisture content. However, the actual final enthalpy cannot be determined until after the expansion efficiency [H1 ⫺ H2(H1 ⫺ H2s )] is evaluated. To determine the final enthalpy H2, another enthalpy H2s must be computed by assuming a constant-entropy expansion to 2 lb/in2 (abs) (13.8 kPa) and a temper- ature of 126.08⬚F (52.3⬚C). Enthalpy H2s will then correspond to a constant-entropy expansion into the wet region, and the percentage of moisture will correspond to Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 104. 1.102 POWER GENERATION the final state. This percentage is determined by finding the ratio of sg ⫺ S1 to or y2s ⫽ sg ⫺ ⫽ 1.9200 ⫺ 1.4576/1.7451 ⫽ 0.265, where sg and are s , S /s s ƒg 1 ƒg ƒg entropies at 2 lb/in2 (abs) (13.8 kPa). Then H2s ⫽ hg ⫺ ⫽ 1116.2 ⫺ y h 2s ƒg 0.265(1022.2) ⫽845.3 Btu/lb (1966.2 kJ/kg). In this relation, hg and are en- hƒg thalpies at 2 lb/in2 (abs) (13.8 kPa). The expansion efficiency, given as 0.75, is H1 ⫺ H2 /(H1 ⫺ H2s ) ⫽ actual work/ideal work ⫽ 0.75 ⫽ 1335.5 ⫺ H2 /(1335.5 ⫺ 845.3). Solve for H2 ⫽ 967.9 Btu/lb (2251.3 kJ/kg). Next, read from the saturation-pressure steam table the liquid and vapor prop- erties at 2 lb/in2 (abs) (13.8 kPa). Tabulate these properties thus: Since the actual final enthalpy H2 is different from H2s, the final actual moisture y2 must be computed by using H2. Or, y2 ⫽ hg ⫺ H2 / ⫽ 1116.1 ⫺ 967.9/ hƒg 1022.2 ⫽ 0.1451. Then 3. Compute the changes resulting from the process The total change in properties is for 10 lb (4.5 kg) of steam, the quantity used in this process. Thus So Q ⫽ 0; by definition, W2 ⫽ ⌬H ⫽ 3676 Btu (3878.4 kJ) for the steady-flow process. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 105. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.103 FIGURE 69 Irreversible adiabatic com- pression process. IRREVERSIBLE ADIABATIC STEAM COMPRESSION Two pounds (0.9 kg) of saturated steam at 120 lb/in2 (abs) (827.4 kPa) with 80 percent quality undergoes nonflow adiabatic compression to a final pressure of 1700 lb/in2 (abs) (11,721.5 kPa) at 75 percent compression efficiency. Determine the final steam temperature T2, change in internal energy ⌬E, change in entropy ⌬S, work input W, and heat input Q. Calculation Procedure: 1. Determine the vapor properties in the initial state From the saturation-pressure steam tables, T1 ⫽ 341.25⬚F (171.8⬚C) at a pressure of 120 lb/in2 (abs) (827.4 kPa) saturated. With x1 ⫽ 0.8, E1 ⫽ ⫽ u ⫹ x u ƒ 1 ƒg 312.05 ⫹ 0.8(795.6) ⫽ 948.5 Btu/lb (2206.5 kJ/kg), from internal-energy values from the steam tables. The initial entropy is S1 ⫽ ⫽ 0.4916 ⫹ s ⫹ x s ƒ 1 ƒg 0.8(1.0962) ⫽ 1.3686 Btu/(lb ⬚F) [5.73 kJ/(kg ⬚C)]. 2. Determine the vapor properties in the final state Sketch a T-S diagram of the process, Fig. 69. Assume a constant-entropy compres- sion from the initial to the final state. Then S2s ⫽ S1 ⫽ 1.3686 Btu/(lb ⬚F) [5.7 kJ/(kg ⬚C)]. The final pressure, 1700 lb/in2 (abs) (11,721.5 kPa), is known, as is the final entropy, 1.3686 Btu/(lb ⬚F) [5.7 kJ/(kg ⬚C)] with constant-entropy expansion. The T-S diagram (Fig. 69) shows that the steam is superheated in the final state. Enter the superheated steam table at 1700 lb/in2 (abs) (11,721.5 kPa), project across to an entropy of 1.3686, and read the final steam temperature as 650⬚F (343.3⬚C). (In most cases, the final entropy would not exactly equal a tabulated value, and it would be necessary to interpolate between tabulated entropy values to determine the in- termediate pressure value.) From the same table, at 1700 lb/in2 (abs) (11.721.5 kPa) and 650⬚F (343.3⬚C), H2s ⫽ 1214.4 Btu/lb (2827.4 kJ/kg); V2s ⫽ 0.2755 ft3 /lb (0.017 m3 /lb). Then Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 106. 1.104 POWER GENERATION E2s ⫽ H2s ⫺ P2V2s /J ⫽ 1214.4 ⫺ 1700(144)(0.2755)/788 ⫽ 1127.8 Btu/lb (2623.3 kJ/kg). Since E1 and E2s are known, the ideal work W can be computed. Or, W ⫽ E2s ⫺ E1 ⫽ 1127.8 ⫺ 948.5 ⫽ 179.3 Btu/lb (417.1 kJ/kg). 3. Compute the vapor properties of the actual compression Since the compression efficiency is known, the actual final internal energy can be found from compression efficiency ⫽ ideal W /actual W ⫽ E2s ⫺ E1 /(E2 ⫺ E1), or 0.75 ⫽ 1127.8 ⫺ 948.5/(E2 ⫺ 948.5); E2 ⫽ 1187.6 Btu/lb (2762.4 kJ/kg). Then E ⫽ (E2 ⫺ E2)m ⫽ (1187.6 ⫺ 948.5)(2) ⫽ 478.2 Btu (504.5 kJ) for 2 lb (0.9 kg) of steam. The actual work input W ⫽ ⌬E ⫽ 478.2 Btu (504.5 kJ). By definition, Q ⫽ 0. Last, the actual final temperature and entropy must be computed. The final actual internal energy E2 ⫽ (1187.6 Btu/lb (2762.4 kJ/kg) is known. Also, the T-S dia- gram shows that the steam is superheated. However, the superheated steam tables do not list the internal energy of the steam. Therefore, it is necessary to assume a final temperature for the steam and then compute its internal energy. The computed value is compared with the known internal energy, and the next assumption is adjusted as necessary. Thus, assume a final temperature of 720⬚F (382.2⬚C). This assumption is higher than the ideal final temperature of 650⬚F (343.3⬚C) because the T-S diagram shows that the actual final temperature is higher than the ideal final temperature. Using values from the superheated steam table for 1700 lb/in2 (abs) (11,721.5 kPa) and 720⬚F (382.2⬚C), we find PV 1700(144)(0.3283) E ⫽ H ⫺ ⫽ 1288.4 ⫺ ⫽ 1185.1 Btu/lb (2756.5 kJ/kg) J 778 This value is less than the actual internal energy of 1187.6 Btu/lb (2762.4 kJ/ kg). Therefore, the actual temperature must be higher than 720⬚F (382.2⬚C), since the internal energy increases with temperature. To obtain a higher value for the internal energy to permit interpolation between the lower, actual, and higher values, assume a higher final temperature—in this case, the next temperature listed in the steam table, or 740⬚F (393.3⬚C). Then, for 1700 lb/in2 (abs) (11,721.5 kPa) and 740⬚F (393.3⬚C), 1700(144)(0.3410) E ⫽ 1305.8 ⫺ ⫽ 1198.5 Btu/lb (2757.7 kJ/kg) 778 This value is greater than the actual internal energy of 1187.6 Btu/lb (2762.4 kJ/kg). Therefore, the actual final temperature of the steam lies somewhere between 720 and 740⬚F (382.2 and 393.3⬚C). Interpolate between the known internal ener- gies to determine the final steam temperature and final entropy. Or, 1178.6 ⫺ 1185.1 T ⫽ 720 ⫹ (740 ⫺ 720) ⫽ 723.7⬚F (384.3⬚C) 2 1198.5 ⫺ 1185.1 1187.6 ⫺ 1185.1 S ⫽ 1.4333 ⫹ (1.4480 ⫺ 1.4333) 1 1198.5 ⫺ 1185.1 ⫽ 1.4360 Btu/(lb ⬚F) [6.0 kJ/(kg ⬚C)] ⌬S ⫽ (S ⫺ S )m ⫽ (1.4360 ⫺ 1.3686)(2) ⫽ 0.1348 Btu/⬚F (0.26 kJ/⬚C) 2 1 Note that the final actual steam temperature is 73.7⬚F (40.9⬚C) higher than that (650⬚F or 343.3⬚C) for the ideal compression. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 107. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.105 FIGURE 70 Throttling process for steam. Related Calculations. Use this procedure for any irreversible adiabatic steam process. THROTTLING PROCESSES FOR STEAM AND WATER A throttling process begins at 500 lb/in2 (abs) (3447.5 kPa) and ends at 14.7 lb/ in2 (abs) (101.4 kPa) with (1) steam at 500 lb/in2 (abs) (3447.5 kPa) and 500⬚F (260.0⬚C); (2) steam at 500 lb/in2 (abs) (3447.5 kPa) and 4 percent moisture; (3) steam at 500 lb/in2 (abs) (3447.5 kPa) with 50 percent moisture; and (4) saturated water at 500 lb/in2 (abs) (3447.5 kPa). Determine the final enthalpy H2, temperature T2, and moisture content y2 for each process. Calculation Procedure: 1. Compute the final-state conditions of the superheated steam From the superheated steam table for 500 lb/in2 (abs) (3447.5 kPa) and 500⬚F (260.0⬚C), H1 ⫽ 1231.3 Btu/lb (2864.0 kJ/kg). By definition of a throttling process, H1 ⫽ H2 ⫽ 1231.3 Btu/lb (2864.0 kJ/kg). Sketch the T-S diagram for a throttling process, Fig. 70. To determine the final temperature, enter the superheated steam table at 14.7 lb/in2 (abs) (101.4 kPa), the final pressure, and project across to an enthalpy value equal to or less than known enthalpy, 1231.3 Btu/lb (2864.0 kJ/kg). (The super- heated steam table is used because the T-S diagram, Fig. 70, shows that the steam is superheated in the final state.) At 14.7 lb/in2 (abs) (101.4 kPa) there is no tab- ulated enthalpy value that exactly equals 1231.3 Btu/lb (2864.0 kJ/kg). The next lower value is 1230 Btu/lb (2861.0 kJ/kg) at T ⫽ 380⬚F (193.3⬚C). The next higher value at 14.7 lb/in2 (abs)(101.4 kPa) is 1239.9 Btu/lb (2884.0 kJ/kg) at T ⫽ 400⬚F (204.4⬚C). Interpolate between these enthalpy values to find the final steam tem- perature: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 108. 1.106 POWER GENERATION 1231.3 ⫺ 1230.5 T ⫽ 380 ⫹ (400 ⫺ 380) ⫽ 381.7⬚F (194.3⬚C) 2 1239.9 ⫺ 1230.5 The steam does not contain any moisture in the final state because it is superheated. 2. Compute the final-state conditions of the slightly wet steam Determine the enthalpy of 500-lb/in2 (abs) (3447.5-kPa) saturated steam from the saturation-pressure steam table: h ⫽ 1204.4 Btu/lb (2801.4 kJ/kg) h ⫽ 755.0 Btu/lb (1756.1 kJ/kg) g ƒg Correct the enthalpy for moisture: H ⫽ h ⫺ y h ⫽ 1204.4 ⫺ 0.04(755.0) ⫽ 1174.2 Btu/lb (2731.2 kJ/kg) 1 g 1 ƒg Then, by definition, H2 ⫽ H1 ⫽ 1174.2 Btu/lb (2731.2 kJ/kg). Determine the final condition of the throttled steam (wet, saturated, or super- heated) by studying the T-S diagram. If a diagram were not drawn, you would enter the saturation-pressure steam table at 14.7 lb/in2 (abs) (101.4 kPa), the final pres- sure, and check the tabulated hg . If the tabulated hg were greater than H1, the throttled steam would be superheated. If the tabulated hg were less than H1, the throttled steam would be saturated. Examination of the saturation-pressure steam table shows that the throttled steam is superheated because H1 ⬎ hg . Next, enter the superheated steam table to find an enthalpy value of H1 at 14.7 lb/in2 (abs) (101.4 kPa). There is no value equal to 1174.2 Btu/lb (2731.2 kJ/kg). The next lower value is 1173.8 Btu/lb (2730.3 kJ/kg) at T ⫽ 260⬚F (126.7⬚C). The next higher value at 14.7 lb/in2 (abs) (101.4 kPa) is 1183.3 Btu/lb (2752.4 kJ/kg) at T ⫽ 280⬚F (137.8⬚C). Interpolate between these enthalpy values to find the final steam temperature: 1174.2 ⫺ 1173.8 T ⫽ 260 ⫹ (280 ⫺ 260) ⫽ 260.8⬚F (127.1⬚C) 2 1183.3 ⫺ 1173.8 This is higher than the temperature of saturated steam at 14.7 lb/in2 (abs) (101.4 kPa)—212⬚F (100⬚C)— giving further proof that the throttled steam is superheated. The throttled steam, therefore, does not contain any moisture. 3. Compute the final-state conditions of the very wet steam Determine the enthalpy of 500-lb/in2 (abs) (3447.5-kPa) saturated steam from the saturation-pressure steam table. Or, hg ⫽ 1204.4 Btu/lb (2801.4 kJ/kg); ⫽ 755.0 hƒg Btu/lb (1756.1 kJ/kg). Correct the enthalpy for moisture: H ⫽ H ⫽ h ⫺ y h ⫽ 1204.4 ⫺ 0.5(755.0) ⫽ 826.9 Btu/lb (1923.4 kJ/kg) 1 2 g 1 ƒg Then, by definition, H2 ⫽ H1 ⫽ 826.9 Btu/lb (1923.4 kJ/kg). Compare the final enthalpy, H2 ⫽ 826.9 Btu/lb (1923.4 kJ/kg), with the enthalpy of saturated steam at 14.7 lb/in2 (abs) (101.4 kPa), or 1150.4 Btu/lb (2675.8 kJ/ kg). Since the final enthalpy is less than the enthalpy of saturated steam at the same pressure, the throttled steam is wet. Since H1 ⫽ hg ⫺ y2 ⫽ (hg ⫺ H1)/ y h , h . 2 ƒg ƒg With a final pressure of 14.7 lb/in2 (abs) (101.4 kPa), use hg and values at this hƒg pressure. Or, 1150.4 ⫺ 826.9 y ⫽ ⫽ 0.3335, or 33.35% 2 970.3 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 109. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.107 FIGURE 71 Reversible heating process. The final temperature of the steam T2 is the same as the saturation temperature at the final pressure of 14.7 lb/in2 (abs) (101.4 kPa), or T2 ⫽ 212⬚F (100⬚C). 4. Compute the final-state conditions of saturated water Determine the enthalpy of 500-lb/in2 (abs) (3447.5-kPa) saturated water from the saturation-pressure steam table at 500 lb/in2 (abs) (3447.5 kPa); H1 ⫽ ⫽ 449.4 hƒ Btu/lb (1045.3 kJ/kg) ⫽ H2, by definition. The T-S diagram, Fig. 70, shows that the throttled water contains some steam vapor. Or, comparing the final enthalpy of 449.4 Btu/lb (1045.3 kJ/kg) with the enthalpy of saturated liquid at the final pres- sure, 14.7 lb/in2 (abs) (101.4 kPa), 180.07 Btu/lb (418.8 kJ/kg), shows that the liquid contains some vapor in the final state because its enthalpy is greater. Since H1 ⫽ H2 ⫽ hg ⫺ y2 ⫽ (hg ⫺ H1)/ . Using enthalpies at 14.7 lb/ y h , h 2 ƒg ƒg in2 (abs) (101.4 kPa) of hg ⫽ 1150.4 Btu/lb (2675.8 kJ/kg) and ⫽ 970.3 Btu/ hƒg lb (2256.9 kJ/kg) from the saturation-pressure steam table, we get y2 ⫽ 1150.4 ⫺ 449.4/970.3 ⫽ 0.723. The final temperature of the steam is the same as the satu- ration temperature at the final pressure of 14.7 lb/in2 (abs) (101.4 kPa), or T2 ⫽ 212⬚F (100⬚C). Note: Calculation 2 shows that when you start with slightly wet steam, it can be throttled (expanded) through a large enough pressure range to produce super- heated steam. This procedure is often used in a throttling calorimeter to determine the initial quality of the steam in a pipe. When very wet steam is throttled, calcu- lation 3, the net effect may be to produce drier steam at a lower pressure. Throttling saturated water, calculation 4, can produce partial or complete flashing of the water to steam. All these processes find many applications in power-generation and pro- cess-steam plants. REVERSIBLE HEATING PROCESS FOR STEAM Subcooled water at 1500 lb/in2 (abs) (10,342.5 kPa) and 140⬚F (60.0⬚C), state 1, Fig. 71, is heated at constant pressure to state 4, superheated steam at 1500 lb/in2 (abs) (10,342.5 kPa) and 1000⬚F (537.8⬚C). Find the heat added (1) to raise the compressed liquid to saturation temperature, (2) to vaporize the saturated liquid to Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 110. 1.108 POWER GENERATION saturated steam, (3) to superheat the steam to 1000⬚F (537.8⬚C), and (4) Q1, ⌬V, and ⌬S from state 1 to state 4. Calculation Procedure: 1. Sketch the T-S diagram for this process Figure 71 is typical of a steam boiler and superheater. Feedwater fed to a boiler is usually subcooled liquid. If the feedwater pressure is relatively high, subcooling must be taken into account, if accurate results are desired. Some authorities rec- ommend that at pressures below 400 lb/in2 (abs) (2758.0 kPa) subcooling be ig- nored and values from the saturated-steam table be used. This means that the en- thalpies and other properties listed in the steam table corresponding to the actual water temperature are sufficiently accurate. But above 400 lb/in2 (abs) 2758.0 kPa), the compressed-liquid table should be used. 2. Determine the initial properties of the liquid In the saturation-temperature steam table read, at 140⬚F (60.0⬚C), ⫽ 107.89 hƒ Btu/lb (251.0 kJ/kg); ⫽ 2.889 lb/in2 (19.9 kPa); ⫽ 0.01629 ft3 /lb (0.0010 p v ƒ ƒ m3 /kg); ⫽ 0.1984 Btu/(lb ⬚F) [0.83 kJ/(kg ⬚C)]. sƒ Next, the enthalpy, volume, and entropy of the water at 1500 lb/in2 (abs) (10,342.5 kPa) and 140⬚F (60.0⬚C) must be found. Since the water is at a much higher pressure than that corresponding to its temperature [1500 versus 2.889 lb/ in2 (abs)], the compressed-liquid portion of the steam table must be used. This table shows that three desired properties are plotted for 32, 100, and 200⬚F (0.0, 37.8, and 93.3⬚C) and higher temperatures. However, 140⬚F (60.0⬚C) is not included. Therefore, it is necessary to interpolate between 100 and 200⬚F (37.8 and 93.3⬚C). Thus, at 1500 lb/in2 (abs) (10,342.5 kPa) in the compressed-liquid table: Each property is interpolated in the following way: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 111. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.109 These interpolated values must now be used to correct the saturation data at 140⬚F (60.0⬚C) to the actual subcooled state 1 properties. Thus, at 1500 lb/in2 (abs) (10,342.5 kPa) and 140⬚F (60.0⬚C). DETERMINING STEAM ENTHALPY AND QUALITY USING THE STEAM TABLES What is the enthalpy of 200-lb/in2 absolute (1378 kPa) wet steam having a 70 percent quality? Determine the quality of 160-lb/in2 absolute (1102.4 kPa) wet steam if its enthalpy is 900 Btu/lb (2093.4 kJ/kg). Use the steam tables to deter- mine the needed values. Calculation Procedure: 1. Compute the enthalpy of the wet steam The enthalpy of wet steam is a function of its quality, or dryness fraction. Thus, 70 percent quality steam will be 70 percent dry and 30 percent wet. In equa- tion form, the enthalpy of wet steam, Btu/lb (kJ/kg), h ⫽ Xhg ⫹ (1 ⫺ X) , where hƒ X ⫽ steam quality or dryness fraction expressed as a decimal; hg ⫽ enthalpy of satu- rated steam vapor, Btu/lb (kJ/kg); ⫽ enthalpy of saturated water fluid, Btu/lb hƒ (kJ/kg). Substituting in this equation using steam-table values, h ⫽ 0.70(1187.2) ⫹ (1 ⫺ 0.70)298.4 ⫽ 920.56 Btu/lb (2141.22 kJ/kg). Note that the enthalpy of wet steam is not a simple product of the dryness factor (i.e. quality) and the enthalpy of the saturated steam. Instead, the enthalpy of the saturated liquid at the saturation pressure must also be included, adjusted for the quality of the steam. 2. Determine the quality of the steam Knowing the absolute pressure and enthalpy of steam we can determine its quality from, X ⫽ )/ ⫺ , where the symbols are defined as given above. (h ⫺ h (h h ) ƒ g ƒ Substituting, using steam-table values, X ⫽ (900 ⫺ 335.93)/(1195.1 ⫺ 335.93) ⫽ 0.6565; say 65.7 percent. Related Calculations. Wet steam is a fact of life in industrial plants of every type. When steam is wet it means that a larger number of pounds (kg) of steam are needed to perform a needed task. Thus, if a process requires 1000 lb (454 kg) of saturated dry steam, it will need 1050 lb (476.7 kg) of steam having a 95 percent quality, i.e. 5 percent more. With the 70 percent quality steam mentioned earlier, the same process would require 1300 lb (590.2 kg) of saturated dry steam. So it is easy to see why it is important to deliver dry saturated steam to a process because the overall steam consumption is reduced. In today’s energy-conscious engineering world, wet steam is an undesirable com- modity unless the wetness results from doing useful work. Where steam wetness occurs because of poor pipe or equipment insulation, shoddy piping layout, or other engineering or installation errors, energy is being wasted. As the above examples show, steam consumption, and generation cost, can rise as much as 30 percent because of wet steam. Every plant designer should keep wetness in mind when designing industrial steam plants of any type serving chemical, steel, textile, marine, automotive, air- craft, etc. industries. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 112. 1.110 POWER GENERATION The procedure given here can be used in any application where steam is a process fluid, and this procedure can also be used when measuring steam quality using a calorimeter of any type. MAXIMIZING COGENERATION ELECTRIC-POWER AND PROCESS-STEAM OUTPUT An industrial cogeneration plant is being designed for a process requiring electric power and steam for operating the manufacturing equipment in the plant. In the design analysis a steam pressure of 650 lb/in2 (gage) (4478.5 kPa) saturated is being considered for a steam turbine to generate the needed electricity, with the exhaust steam being used in the process sections of the plant at 150 lb/in2 (gage) (1033.5 kPa). If greater process efficiencies can be obtained in manufacturing with a higher exhaust temperature, compare the effect of greater turbine inlet pressure and temperature on the resulting exhaust steam temperature and heat content. Use typical pressure and temperature levels met in industrial steam-turbine applications. Calculation Procedure: 1. Assemble data on typical industrial steam-turbine inlet conditions Using data obtained from steam-turbine manufacturers, list the typical inlet pres- sures and temperatures used today in a tabulation such as that in Fig. 72. This listing shows typical pressure ranges from 650 lb/in2 (gage) (4478.5 kPa) saturated to 1200 lb/in2 (gage) (8268 kPa) at temperatures from saturated, 498⬚F (259⬚C), to 950⬚F (510⬚C), at 1200 lb/in2 (gage). 2. Plot the turbine expansion process for each pressure being considered Using an H-s diagram, Fig. 72, plot the expansion from the turbine inlet to exhaust at 150 lb/in2 (gage) (1033.5 kPa). Read the temperature and the enthalpy at the exhaust and tabulate each value as in the figure. 3. Compute the percent increase in the exhaust enthalpy Using data from the H-s chart, we see that with an inlet pressure of 650 lb/in2 (gage) saturated (4487.5 kPa), the exhaust steam has 8 percent moisture and an enthalpy change of 70 Btu/lb (163.1 kJ/kg) with an exhaust temperature of 360⬚F (182⬚C). Raising the turbine inlet steam temperature to 750⬚F (398.9⬚C) increases the enthalpy change during expansion to 1380 ⫺ 1264 ⫽ 116 Btu/lb (270.28 kJ/kg). This is an increase of 116 ⫺ 70 ⫽ 46 Btu/lb (107.2 kJ/kg) over the saturated- steam inlet enthalpy change during expansion from inlet to exhaust. Then, the percent increase in enthalpy change for these two inlet conditions is 100(116 ⫺ 70)/70 ⫽ 65.7 percent; say 66 percent, as tabulated. Continuing these calculations and tabulating the results shows that—for the pres- sures and temperatures considered—an increase of up to 148 percent in the enthalpy change can be obtained. Likewise, an exhaust superheat up to 320⬚F (160⬚C) can be obtained. Depending on the process served, a suitable steam pressure and tem- perature can be chosen for the turbine inlet conditions to maximize the efficiency of the process(es) served by this cogeneration plant. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 113. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.111 FIGURE 72 H-s plot of expansion in a steam turbine. (Power.) Related Calculations. Use this general approach for steam cogeneration plant design for industries in any of these fields—chemical, petroleum, textile, food, tobacco, agriculture, manufacturing, automobile, etc. While the cogeneration plant considered here exhausts directly to the process mains, condensing turbines can also be used. With such machines, process steam is extracted at a suitable point in the cycle to provide the needed pressure and temperature. Condensing steam turbines with steam extraction for process needs offer a sig- nificant increase in electric-power production. Further, they reduce the effects of seasonal price fluctuations for power sales, and potentially lower life-cycle costs. Capital cost of such machines, however, is higher. Turbine suppliers are now also offering efficient, high-speed geared turbo- generators for cogeneration. One such supplier states that the incremental payback for a geared unit can be extremely high in cogeneration applications. Much of the data in this procedure came from Power magazine. SI values were added by the editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 114. 1.112 POWER GENERATION Economic Analyses of Alternative Energy Sources CHOICE OF MOST ECONOMIC ENERGY SOURCE USING THE TOTAL-ANNUAL-COST METHOD A penal institution needs 50,000 MWh of electric energy per year with a maximum demand of 10 MW. This can be purchased from the local utility for $480,000 annually. As an alternate scheme, the penal institution is considering installing a 10-MW steam-turbine plant using equipment purchased at reduced prices. Three different plants are being proposed, as shown in Table 4. The plant will run 24 h/ day, with inmate operators working an 8-h day, 5 days per week. The average annual salary for the inmate operators, which includes their prison costs, is $5200. Maintenance costs are estimated at 90 cents per ton of coal burned for all alternative plants. From past experience, the designers found that installed costs for penal institutions exceed estimates by 20 percent; so this allowance is made as a contin- gency. General operating supplies are estimated as $10,000 annually for all plants being considered. The useful life of each plant is taken as 15 years; the cost of money is 6 percent. Real estate taxes on the real estate for the plant are estimated to be 4 percent; various operating taxes add 1 percent to annual operating costs. Annual insurance costs are 0.2 percent of all equipment costs. Coal fuel costs $6.50 per short ton and has a higher heating value (HHV) of 14,200 Btu/lb (33,086 kJ/ kg). Determine which scheme is the economic one, using the total-annual-cost method, assuming equal reliability. Calculation Procedure: 1. Determine the required boiler capacity for each proposed plant Use the relation: Required boiler capacity, lb/h (kg/h) ⫽ (plant heat rate, lb/kWh [kg/kWh])(plant capacity, kW). Find the full-load steam rate and plant capacity in Table 4. Then the required boiler capacity, C, for each plant is Plan A: C ⫽ 11.8(10,000) ⫽ 118,000, say 120,000 lb/h (54,480 kg/h) Plan B: C ⫽ 10.5(10,000) ⫽ 105,000, say 110,000 lb/h (49,940 kg/h) Plan C: C ⫽ 8.5(10,000) ⫽ 85,000, say 90,000 lb/h (40,860 kg/h) The boiler capacity was rounded off to the next-higher standard boiler rating for each plan. This permits purchase of a standardized boiler while providing a nominal amount of extra capacity. Do not, in general, round off to the next-lower capacity because this can result in choosing a boiler with too small a capacity for the full- load output of the turbine. 2. Compute the cost of the boiler and related equipment As noted earlier, this is a plant for a penal institution, and the equipment is being obtained at reduced cost. However, the procedure for computing the cost of the boiler and its related equipment is the same, regardless of the equipment cost and the type of use of the equipment. Hence, use the relation: Installed boiler cost, $ ⫽ (required boiler capacity, lb/h [kg/h])(boiler cost, $/lb h capacity). Using the cost data in Table 4, installed boiler cost is Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 115. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.113 TABLE 4 Data for Three Plant Options Plan A Plan B Plan C Throttle steam conditions: Pressure, psig. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 400 900 Temperature, ⬚F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550 700 850 Station steam rate (full load), lb/kWh. . . . . . . . . . . . . . . 11.8 10.5 8.5 Average station heat rate, Btu/kWh. . . . . . . . . . . . . . . . . 16,000 13,500 12,000 Unit installation costs: Steam generator and auxiliaries, dollars per lb h capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.08 4.73 6.45 Steam turbine and auxiliaries, dollars per kW capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38.00 42.00 51.00 Electrical equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . $200,000 $200,000 $200,000 Structures and miscellaneous. . . . . . . . . . . . . . . . . . . . . $200,000 $200,000 $200,000 Plant operators per shift . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4 4 TABLE 5 Plant Equipment Costs Investment Plan A Plan B Plan C Boiler equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . $490,000 $520,000 $580,000 Turbine equipment. . . . . . . . . . . . . . . . . . . . . . . . . . 380,000 420,000 510,000 Electrical equipment . . . . . . . . . . . . . . . . . . . . . . . . 200,000 200,000 200,000 Structures and miscellaneous. . . . . . . . . . . . . . . . . 200,000 200,000 200,000 Total estimated items . . . . . . . . . . . . . . . . . . . . . $1,270,000 $1,340,000 $1,490,000 Contingencies at 20 percent. . . . . . . . . . . . . . . . . . 250,000 270,000 300,000 Total investment . . . . . . . . . . . . . . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000 Plan A: 120,000($4.08) ⫽ $490,000 Plan B: 110,000($4.73) ⫽ $520,000 Plan C: 90,000($6.45) ⫽ $580,000 3. Compute the cost of the turbine and auxiliaries Use the same procedure as in step 2, except that the plant capacity and turbine costs are substituted in the cost relation. Using the cost tabulated for each plan in Table 4, the installed turbine cost is Plan A: (10,000 kW)($38) ⫽ $380,000 Plan B: (10,000 kW)($42) ⫽ $420,000 Plan C: (10,000 kW)($51) ⫽ $510,000 4. Find the total investment required Tabulate the costs for each plan, as shown in Table 5. Note that the electrical equipment and structures costs are the same for all plans. Once the total estimated cost for each plan is found, apply the 20 percent contingency allowance to find the total investment required. Summarize the cost for each plan, as shown in Table 5. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 116. 1.114 POWER GENERATION 5. Determine the depreciation and fixed-charge rates The depreciation rate ⫽ i/[(1 ⫹ i)n ⫺ 1], where i ⫽ earnings rate or interest that the investment would yield (6 percent for this plant); n ⫽ expected life of the capital equipment, years. Or, depreciation rate ⫽ 0.06/[(1.06)15 ⫺ 1] ⫽ 0.0431. The fixed-charge rate ⫽ 兺 (interest rate, depreciation rate, tax rate, and insurance rate). Or, fixed-charge rate ⫽ 0.06 ⫹ 0.0431 ⫹ 0.04 ⫹ 0.002 ⫽ 0.1451, or 14.51%. 6. Find the unit and total fuel costs Unit fuel cost, $/ton(10)6 /2000 (HHV) ⫽ $6.50(10)6 /2000(14,200) ⫽ 22.9 cents per 106 Btu. Then total annual fuel cost ⫽ (unit fuel cost, cents/106 Btu)(annual kWh energy consumption)(average plant heat rate, Btu/kWh). For each plan, total annual fuel cost is Plan A: 0.229(50)(16,000) ⫽ $183,200 Plan B: 0.229(50)(13,500) ⫽ $154,575 Plan C: 0.229(50)(12,000) ⫽ $137,400 7. Compute the annual operating and maintenance costs There are 21 shifts per week (⫽ 3 shifts per day ⫻ 7 days per week). Any operator works 5 shifts per week (⫽ 1 shift per day ⫻ 5 working days per week). Then each position requires 21/5 ⫽ 4.2 operators to keep the post continuously staffed. Assume that the operators are interchangeable in their posts. Then all plans will need (4 operators per shift)(4.2 people per position) ⫽ 16.8, or 17, operators. Since each operator earns prison wages and keep costs of $5200 annually, the annual cost of operating labor ⫽ (17 operators)($5200) ⫽ $88,400, say, $88,000 per year, av- erage, over the life of the plant. Average annual maintenance cost ⫽ (tons of coal burned per year)(maintenance cost per ton of coal) ⫽ (annual total fuel cost/cost per ton of coal)(maintenance cost per ton of coal). For each plan, average annual maintenance cost is $183,200 Plan A: (0.90) ⫽ $25,366 冉 冊 $6.50 $154,575 Plan B: (0.90) ⫽ $21,403 冉 冊 $6.50 $137,400 Plan C: (0.90) ⫽ $19,025 冉 冊 $6.50 8. Calculate the total annual operating cost and the total annual cost Tabulate each operating cost as shown in Table 6. Then add the investment charges to obtain the total annual cost. This gives the results shown at the bottom of Ta- ble 6. 9. Analyze the costs and choose the best alternative plan For the given conditions of fixed-charge rate (14.51 percent) and fuel costs, plan B incurs the least annual cost of the three plans. However, since the utility can supply electric power for less than any of these private plants, the utility supply would be the logical choice. In certain circumstances, even if the utility service cost $520,000, it might be selected in preference to the private plant plan B. If no utility service were available, plan B would undoubtedly be chosen for this instal- lation. Related Calculations. Use this procedure to determine the most economic en- ergy source for any type of power plant—steam, diesel, gas-turbine, windmill— Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 117. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.115 TABLE 6 Annual Costs for Each Plant Annual operating costs Plan A Plan B Plan C Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $183,000 $155,000 $138,000 Operating labor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88,000 88,000 88,000 Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25,000 21,000 19,000 Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10,000 10,000 10,000 Subtotal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $306,000 $274,000 $255,000 Operating taxes at 1 percent . . . . . . . . . . . . . . . . . . . . . . . 3,000 3,000 3,000 Total annual operating costs . . . . . . . . . . . . . . . . . . . . . $309,000 $277,000 $258,000 Total annual costs Plan A Plan B Plan C Utility service Investment charges at 14.5 percent. . . . . . $221,000 $233,000 $260,000 Operating costs . . . . . . . . . . . . . . . . . . . . . . 309,000 277,000 258,000 Total annual costs . . . . . . . . . . . . . . . . . . $530,000 $510,000 $518,000 $480,000 serving any type of load—utility, industrial, commercial, institutional, etc. This procedure is the work of B. G. A. Skrotzki and William Vopat. SEVEN COMPARISON METHODS FOR ENERGY-SOURCE CHOICE Using the data in the previous calculation procedure, study the following: (1) in- fluence of fixed-charge rate, (2) influence of annual operating ocsts, (3) rate of return to be realized, (4) return available for fixed charges, (5) justifiable investment, (6) present worth, and (7) capitalized cost. The variables that must be considered are stated in each calculation procedure. Calculation Procedure: 1. Compare the total annual costs by using the fixed-charge rate Assume the fixed-charge rate is 7 percent instead of the 14.51 percent in the pre- vious calculation procedure. Compute investment charges at 7 percent and the total annual operating costs as in step 8 of the previous procedure. Total annual operating costs are then Total annual costs Plan A Plan B Plan C Investment charges at 7 percent . . . . . . . . . . . . $106,000 $113,000 $125,000 Operating costs . . . . . . . . . . . . . . . . . . . . . . . . . . 309,000 277,000 258,000 Total annual cost . . . . . . . . . . . . . . . . . . . . . . $415,000 $390,000 $383,000 Reduction in the fixed-charge rate could result from a smaller cost of money, or a longer life span, or both. As the tabulation above shows, plan C is now the optimum Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 118. 1.116 POWER GENERATION plan because it is considerably less costly than that of the utility service. From this result an important principle is clear: As the fixed-charge rate decreases, the alter- natives involving higher capital investment are favored as optimum choices. This means that as the cost of money use decreases, or the life span of the investment increases, more money can be invested to secure greater overall economy. This principle holds only when the annual operating cost of each alternative remains unchanged. 2. Analyze the effect of increased fuel costs In thermal stations, fuel prices control the general magnitude of the annual operating costs. Thus, the fuel purchase price will have a controlling effect on the economic conclusions drawn from financial comparisons. Assume that the fuel cost doubles to $13 per short ton. Compute the fuel costs as in step 6 of the previous calculation procedure. Tabulate the total annual costs as follows: Total annual costs Plan A Plan B Plan C Investment charges at 14.5 percent . . . . . . . . . . . . $221,000 $233,000 $260,000 Operating costs with $13 coal . . . . . . . . . . . . . . . . 492,000 432,000 396,000 Total annual cost . . . . . . . . . . . . . . . . . . . . . . . . . $713,000 $665,000 $656,000 For this condition, the plan requiring the highest investment again becomes the optimum one. From this a corollary principle may be deduced: As the cost of fuel increases, plans involving higher capital investment are favored as optimum. Thus, as the cost of fuel increases, more money may be invested to gain increased effi- ciency. This holds only when the fixed-charge rate is common to all plans and remains constant. 3. Determine the optimum investment based on the earned rate of return Assume the same data as previously, except that the utility service costs $530,000 annually. This price could be obtained by getting a direct quote from the utility. Compute all the items of annual cost except interest and depreciation. Then the difference between the utility service cost of $530,000 and the computed annual costs represents a saving available for the annual interest on the investment and its amortization. The sum of the differences, $A, and the investment from step 4 of the previous calculation procedure will be the principal, $P, for each plan. Solve for the interest rate, I, using a calculator, or trial and error, using A/P ⫽ i/[1 ⫺ (1 ⫹ i)⫺n ]. Tabulate the results thus: Plan A Plan B Plan C Utility service Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000 Annual taxes on capital . . . . . . . . . . . . 61,000 64,000 72,000 Annual insurance costs . . . . . . . . . . . . . 3,000 3,000 4,000 Annual costs excluding interest and depreciation . . . . . . . . . . . . . . . . . . . . $373,000 $344,000 $334,000 $530,000 Difference available for annuity A . . . $157,000 $186,000 $196,000 Base Investment P . . . . . . . . . . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000 Ratio A/P. . . . . . . . . . . . . . . . . . . . . . . . 0.1033 0.1155 0.1094 Rate of return i, percent . . . . . . . . . . . . 6.4 7.8 7.0 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 119. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.117 Note that all the plans yield a greater rate of return than the minimum interest rate of 6 percent. This means it will be desirable to build one of these plants rather than purchase energy at a cost of $530,000 per year from the utility. While plan B yields the maximum rate of return on the investment (7.8 percent), an attractive rate of return can be earned by the higher investment of plan C. The limit of investment is determined by that value where the last additional dollar invested earns at least the minimum rate of return desired—in this case 6 percent. This method investigates the rate of return on increment investments. Use the same procedure as above, but find the return earned on the differences of in- vestment for the various plans. Solve for the rate of return by using a calculator or trial and error, and tabulate the results thus: Difference of plans B over A C over B Saving in annual costs, excluding interest and depreciation, ⌬A . . $29,000 $10,000 Additional capital investment, ⌬P. . . . . . . . . . . . . . . . . . . . . . . . . . . . $90,000 $180,000 Ratio ⌬A/⌬P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3222 0.0556 Rate of return on increment investment,a percent. . . . . . . . . . . . . . . 31.7 Negative a By trial solution for i from ⌬A A i ⫽ ⫽ ⫺n ⌬P P 1 ⫺ (1 ⫹ i) Note that while plan A showed a satisfactory rate of return of 6.4 percent, the additional investment of $90,000 for plan B earns the high rate of return of 31.7 percent, which raises the overall rate of return of plan B to 7.8 percent. The ad- ditional investment of $180,000 for plan C over plan B does not earn enough to amortize the extra cost, as indicated by the negative interest rate. This negative return reduces the overall return of plan C to 7.0 percent. Hence, plan B is the optimum scheme for the given conditions. The same conclusion was reached by the total-annual-cost method in the previous calculation procedure. Recognize the importance of thoroughly investigating all possibilities in eco- nomic studies by considering the following situation. If plan B had been overlooked, the return of the additional investment for plan C over plan A would have been as follows: Saving in annual costs, excluding interest and depreciation, ⌬A. . . . . . . . . . . . . . . . . . $39,000 Additional capital investment ⌬P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $270,000 Ratio ⌬A/⌬P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1445 Rate of return on increment investment, percent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7 Thus, the rate of return on the increment investment would be reduced from 31 to 11 percent. 4. Compute the return available for fixed charges This method is popular because it eliminates the trial and error or interpolation required in the rate-of-return method. The fixed-charge rate is easily calculated from the desired rate of return and life of the project, including the tax and insurance premium rates. From the proposals studied, determine the savings as a percentage of the corresponding investments that are available to meet the fixed charges. Com- pute and tabulate the investment savings, increment investment, and their ratio; tabulate them thus: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 120. 1.118 POWER GENERATION Plan A Plan B Plan C Utility service Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000 $530,000 Savings available for investment charges . . . . . . . . . . . . . . . . . . . . . . . . $221,000 $253,000 $272,000 Base Investment. . . . . . . . . . . . . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000 0 Ratio of savings to investment,a percent . . . . . . . . . . . . . . . . . . . . . . . . 14.6 15.7 15.2 Increment savings . . . . . . . . . . . . . . . . . $32,000 $19,000 Increment investment . . . . . . . . . . . . . . $90,000 $180,000 Ratio of increment savings to increment investment, percenta . . . . 35.6 10.6 a Rates comparable to fixed-charge rate ⫽ 14.51 percent. Again, Plan B is the optimum one. This is the same conclusion as drawn from the previous methods of comparison. 5. Determine the maximum justifiable investment In this method, the savings available for investment charges are divided by the fixed-charge rate to arrive at a justifiable maximum investment. Compare the jus- tifiable maximum investment with the actual investment required for the given plan. If the actual investment is equal to, or less than, the justifiable investment, then the plan may prove satisfactory. Compute the actual increment investment and tabulate thus: Plan A Plan B Plan C Utility service Annual operating costs . . . . . . . . . . . . . . $309,000 $277,000 $258,000 $530,000 Savings available for investment charges. . . . . . . . . . . . . . . . . . . . . . . . . . $220,000 $253,000 $272,000 Base Fixed-charge rate, percent . . . . . . . . . . . . 14.5 14.5 14.5 Justifiable investment . . . . . . . . . . . . . . . . $1,525,000 $1,745,000 $1,877,000 Active investment needed . . . . . . . . . . . . $1,520,000 $1,610,000 $1,790,000 Increment savings. . . . . . . . . . . . . . . . . . . $32,000 $19,000 Fixed-charge rate, percent . . . . . . . . . . . . 14.5 14.5 Justifiable added investment . . . . . . . . . . $221,000 $131,000 Actual increment investment needed . . . $90,000 $180,000 Actual investments for the three plans, A, B, and C, are less than the maximum justifiable investments and hence appear to be satisfactory. Again, plan B is the optimum choice. 6. Compute the total present worth of each alternative The plan having the lowest present worth involves a minimum overall expenditure during the life of the investment and therefore is the desirable, or optimum, plan. Compute the annual taxes on capital and the annual insurance cost. Determine the present-worth factor, PWF, from the relation in step 3 above for 6 percent interest and a 15-year life. Then PWF ⫽ 0.103. Solve for the present worth of the costs excluding interest and depreciation and add the investment, $P, as in Wp ⫽ [P ⫹ (t ⫹ j)P ⫹ Co]/PWF, where Wp ⫽ total present worth of a series of annual expen- ditures, t ⫽ tax rate on capital equipment, j ⫽ insurance rate, Co ⫽ annual operating expenses. Tabulate the results thus: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 121. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.119 Plan A Plan B Plan C Utility service Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000 $530,000 Annual taxes on capital . . . . . . . . . . . . 61,000 64,000 72,000 Annual insurance costs . . . . . . . . . . . . . 3,000 3,000 4,000 Annual costs excluding interest and depreciation . . . . . . . . . . . . . . . . . . . . $373,000 $344,000 $334,000 $530,000 Present-worth factor . . . . . . . . . . . . . . . 0.103 0.103 0.103 0.103 Present worth of costs excluding interest and depreciation. . . . . . . . . . $3,620,000 $3,340,000 $3,240,000 $5,150,000 Investment P 1,520,000 1,610,000 1,790,000 0 Total present worth Wp . . . . . . . . . . . . . $5,140,000 $4,950,000 $5,030,000 $5,150,000 Plan B, having the minimum present worth, entails the least total expenditures over the life of 15 years and is therefore the optimum one, as previously. 7. Determine the total capitalized cost of each alternative Use the relation Wc for the total capitalized cost. i ⫹ t ⫹ j P ⫹ C 冋 册 o n (1 ⫹ i) ⫺ 1 W ⫽ P ⫹ c i Compute the total capitalized cost for each alternative and tabulate thus: Plan A Plan B Plan C Utility service Annual operating costs . . . . . . . . . . . . . $309,000 $277,000 $258,000 Annual depreciation costs . . . . . . . . . . 66,000 70,000 77,000 Annual taxes on capital . . . . . . . . . . . . 61,000 64,000 72,000 Annual insurance costs . . . . . . . . . . . . . 3,000 3,000 4,000 Annual costs excluding interest. . . . . . $439,000 $414,000 $411,000 $530,000 Interest rate i . . . . . . . . . . . . . . . . . . . . . 0.06 0.06 0.06 0.06 Capitalized costs of annual costs excluding interest . . . . . . . . . . . . . . . $7,310,000 $6,900,000 $6,850,000 $8,833,000 Investment P 1,520,000 1,610,000 1,790,000 0 Total capitalized cost Wc . . . . . . . . . . . $8,837,000 $8,517,000 $8,640,000 $8,833,000 Plan B, having the minimum capitalized cost, entails the smallest annual costs and is therefore the optimum alternative, as in all previous analyses. Related Calculations. Use any of, or all, the given comparison methods for any type of energy source—steam, diesel, gas-turbine, windmill, etc.—serving any type of load—utility, industrial, commercial, institutional, marine, etc. SELECTION OF PRIME MOVER BASED ON ANNUAL COST ANALYSES An electrical load in a penal institution has the following annual duration charac- teristics: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 122. 1.120 POWER GENERATION Kilowatts Hours 5000 200 4000 4000 2000 2000 1000 1000 500 1560 Two plants, a steam-turbine plant and a diesel-engine plant, are being considered. For the steam plant, coal at $4.50 per short ton with a heating value of 14,000 Btu/ lb (32,620 kJ/kg) is available. For the diesel plant, oil at $2.10/bbl containing 358 lb (162.5 kg) with a heating value of 18,500 Btu/lb (43,105 kJ/kg) is available. The performance characteristics of the plants are as follows: Steam: I ⫽ 106 (6 ⫹ 8L ⫹ 0.111L3 ); diesel: I ⫽ 106 (9 ⫹ 4L ⫹ [0.52L2 ⫺ 0.016L3 ]). The steam plant requires three more operators at a cost of $4000 per person than does the diesel plant. The fixed-charge rate is 12 percent for each plant. Which plant should be selected if the steam plant costs $230/kW installed? No reserve capacity is required. Use the total annual costs as the basis of comparison of the plants. Calculation Procedure: 1. Compute the unit fuel cost for each prime mover The steam-plant unit fuel cost, cents per million Btu ⫽ 106 (fuel cost, cents/lb)/ (lb/ton)(heating value, Btu/lb) ⫽ 106 (450)/(2000)(14,000) ⫽ 16.07 cents per 106 /Btu. The diesel unit fuel cost, cents per million Btu ⫽ 31.71 cents per 106 , using the same method of calculation. 2. Find the total energy input for each plant Use the input-output characteristic for each plant, and compute the total energy for each load tabulated above. Thus, for the steam plant and the load of 5000 kW ⫽ 5 MW, I ⫽ 106 (6 ⫹ 8 ⫻ 5 ⫹ 0.11 ⫻ 125) ⫽ 59.9 ⫻ 106 Btu/h (17.55 MW). Tabulate the computed input for each plant for each load on a per-hour basis. Multiply the per-hour input by the number of hours at this load to obtain the total input at each load. Thus, for 200 h at 5 MW, total input ⫽ (200 h)(59.9 ⫻ 106 Btu/ h) ⫽ 11.98 ⫻ 109 Btu (12.58 ⫻ 109 kJ). Tabulate these values: Calculation of Fuel Requirements MW h MW Steam plant Input rate, 106 Btu/h Total input, 109 Btu Diesel plant Input rate, 106 Btu/h Total input, 109 Btu 5 200 1,000 59.90 11.98 40.00 8.00 4 4,000 16,000 45.12 180.48 32.30 129.20 2 2,000 4,000 22.89 45.78 18.95 37.90 1 1,000 1,000 14.11 14.11 13.50 13.50 1 ⁄2 1,560 780 10.01 15.62 11.13 17.36 Total . . . 8,760 22,780 . . . . . . . . . 267.97 . . . . . . . . . 205.96 Average heat rate, Btu/kWh. . . . . . . . . . . 11,800 . . . . . . . . . 9050 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 123. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.121 Find the heat rate, HR, for each plant by dividing the tabulated total heat input by the total-load energy, MWh. For the steam plant, HR ⫽ 11,800 Btu/kWh (11,210 kL/kWh). Tabulate the values for each plant. 3. Calculate the total annual cost for each plant The total annual cost is Co ⫽ Cc ⫹ Cf ⫹ Cl, where the subscripts are c ⫽ capital- cost write off, f ⫽ fuel cost, and l ⫽ labor cost. For the steam plant, Cc ⫽ ($230)(5000)(0.12) ⫽ $318,000. And Cf ⫽ (0.1607)(267,970 ⫻ 109 ) ⫽ $43,063; Cl ⫽ 3($4000) ⫽ $12,000. Then Co ⫽ $193,063 per year. For the diesel plant, Co ⫽ $197,310 per year. 4. Choose the most economical prime mover Since the steam plant has a lower total annual cost, choose it in preference to the diesel plant, despite the higher first cost and greater relative labor cost of the steam plant. These costs are outweighed by the lower fuel cost of the steam plant. 5. Evaluate the effect of relative fuel prices Since fuel prices vary, particularly the price of oil, total-annual-cost comparisons depend on the relative fuel prices at the time of the study. To give a complete picture of the importance of fuel prices in determining plant economics, the cost comparison can be set up to determine the relationship of fuel prices for given conditions of labor costs, thermal efficiency, and loan conditions. From the foregoing, the plants are equally desirable when their annual costs are equal, Co,steam ⫽ Co,diesel. Or, (Cc ⫹ Cf)steam ⫽ (Cc ⫹ Cf)diesel. Substituting previously computed values, ($138,000) ⫹ [(267,970 ⫻ 106 ⫻ S)/(2000 ⫻ 14,000)] ⫹ $12,000 ⫽ ($132,000) ⫹ [(205,960 ⫻ 106 ⫻ D)/(358 ⫻ 18,500)], where S ⫽ price of coal, $/ton; D ⫽ price of oil, $/bbl. Solving gives D ⫽ 0.578 ⫹ 0.308S. Thus, for coal at $4.50/ton, oil would have to be D ⫽ 0.578 ⫹ 0.308(4.50) ⫽ $1.96/bbl to produce equal fuel costs. With oil at $2.10/bbl, coal would have to be $4.94/ton for equal fuel costs. Related Calculations. Use this annual-cost procedure to compare any type of prime mover—steam, diesel, natural-gas, gas-turbine, windmill—serving any type of load—utility, industrial, institutional, or marine. Note that the relative current prices of fuel do not change the steps or methods given here. Hence, use the same steps, inserting today’s fuel prices. This procedure is the work of B. G. A. Skrotzki and William Vopat, as reported in their book Power Station Engineering and Econ- omy, McGraw-Hill. DETERMINING IF A PRIME MOVER SHOULD BE OVERHAULED A 20-MW steam turbine has been in use for 4 years and generates 105,000,000 kWh annually. The first year this energy was produced at an average heat rate of 15,000 Btu/kWh (14,250 kJ/kWh). Because of blade erosion the average heat rate increases at the rate of 0.05 percent per 1000 h of operation. This increase is expected to continue for the life of the machine despite adequate routine mainte- nance. To reblade the turbine and restore its original efficiency will cost $35,000. The total life of the turbine is 15 years, and it will be required to produce the same amount of energy annually in the future. If fuel costs 20 cents per million Btu, a Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 124. 1.122 POWER GENERATION minimum of 6 percent return is required on an investment, and the turbine operates 6000 h/yr, determine when, and if, the turbine should be rebladed. The rebladed turbine will deteriorate in efficiency at the same rate as at present, s. Calculation Procedure: 1. Calculate the annual fuel-cost increase after the first year During the first year, when the heat rate, HR, was 15,000 Btu/h (14,250 kJ/kWh), fuel cost ⫽ (total output, kWh)(HR, Btu/kWh)(fuel costs, cents per 106 Btu) ⫽ (105,000,000)(15,000)(0.2/106 ) ⫽ $315,000. The increase in fuel cost per year, resulting from blade corrosion, is (annual fuel cost, $)(hours of operation per year)(HR increase, percent per 1000 h) ⫽ ($315,000)(6000)(0.0005/1000) ⫽ $945. 2. Compute, and tabulate, the fuel-cost increase Prepare a tabulation such as that below listing the turbine age in years and the increase in fuel cost resulting from blade erosion. Thus, there is no increase in fuel cost during the first year. During the second year the fuel cost increases by $945, as computed in step 1 above. During the third year the fuel cost increases by 2($945) ⫽ $1890. Compute the subsequent annual fuel saving when the turbine is rebladed in a given year, and enter the results in the second column below. In the third column enter the life of the reblading investment ⫽ 15 ⫺ year in which the turbine was rebladed. Turbine Reblading Evaluation Turbine age n, yr Subsequent annual saving n when rebladed in given year Life of reblading investment m, yr 0.06 ⫺m 1 ⫺ 1.06 Annual interest and amortization of $35,000 invested in year n, i ⫽ 0.06 Rate of return for subsequent annual saving referred to $35,000 investment, percent 1 $ 0 14 0.1076 $ 3,760 Negative 2 945 13 0.1130 3,960 Negative 3 1,890 12 0.1193 4,180 Negative 4 2,835 11 0.1268 4,440 Negative 5 3,780 10 0.1359 4,750 1.5 6 4,725 9 0.1470 5,150 4.1 7 5,670 8 0.1610 5,640 6.2 8 6,615 7 0.1791 6,270 7.5 9 7,560 6 0.2034 7,120 8.0 10 8,505 5 0.2374 8,300 6.9 11 9,450 4 0.2886 10,100 3.2 12 10,395 3 0.3741 13,100 Negative 13 11,340 2 0.5454 19,100 Negative 14 12,285 1 1.06 37,100 Negative 15 13,230 0 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 125. MODERN POWER-PLANT CYCLES AND EQUIPMENT 1.123 3. Determine the reblading annual interest and amortization rates The factor for computing the annual interest and amortization is i/(1 ⫺ [1 ⫹ I]⫺m ), where I ⫽ desired rate of return; m ⫽ life of reblading investment, years. Thus, with a reblading investment life of 4 years and n ⫽ 11 years, factor ⫽ 0.2886, after substituting the above equation. To find the annual interest and amortization on the $35,000 reblading investment, take the product of the factor and the reblading cost, or (0.2886)($35,000) ⫽ $10,101. The rate of return on the $35,000 investment is found by setting up the ratio of annual savings, A, to the investment, P, and solving by calculator, or trial and error, the relation A/P ⫽ i/(1 ⫺ [1 ⫹ i]⫺n ) for I, the rate of return. For n ⫽ 9 years, I ⫽ 8.0 percent. Enter this value in the table above. Computing the other rates of return in the same manner shows that the maximum rate of return occurs when reblading is done in the ninth year. The 8 percent return exceeds the minimum desired 6 percent rate of return. Related Calculations. Use this procedure to compute the timing of and return on investment for any expenditure which improves the efficiency of any type of prime mover—steam turbine, diesel engine, gasoline engine, gas turbine, hydraulic turbine, windmill, etc., in a stationary, portable or marine installation. This proce- dure is the work of B. G. A. Skrotzki and William Vopat. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 126. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. MODERN POWER-PLANT CYCLES AND EQUIPMENT
- 127. 2.1 SECTION 2 STEAM CONDENSING SYSTEMS AND AUXILIARIES Design of Condenser Circulating-Water Systems for Power Plants 2.1 Designing Cathodic-Protection Systems for Power-Plant Condensers 2.7 Steam-Condenser Performance Analysis 2.12 Steam-Condenser Air Leakage 2.16 Steam-Condenser Selection 2.17 Air-Ejector Analysis and Selection 2.18 Surface-Condenser Circulating-Water Pressure Loss 2.20 Surface-Condenser Weight Analysis 2.22 Weight of Air in Steam-Plant Surface Condenser 2.23 Barometric-Condenser Analysis and Selection 2.24 Cooling-Pond Size for a Known Heat Load 2.26 DESIGN OF CONDENSER CIRCULATING-WATER SYSTEMS FOR POWER PLANTS Design a condenser circulating-water system for a turbine-generator steam station located on a river bank. Show how to choose a suitable piping system and cooling arrangement. Determine the number of circulating-water pumps and their capacities to use. Plot an operating-point diagram for the various load conditions in the plant. Choose a suitable intake screen arrangement for the installations. Calculation Procedure: 1. Choose the type of circulating-water system to use There are two basic types of circulating-water systems used in steam power plants today—the once-through systems, Fig. 1a, and the recirculating-water system, Fig. 1b. Each has advantages and disadvantages. In the once-through system, the condenser circulating water is drawn from a nearby river or sea, pumped by circulating-water pumps at the intake structure through a pipeline to the condenser. Exiting the condenser, the water returns to the river or sea. Advantages of a once-through system include: (a) simple piping ar- rangement; (b) lower cost where the piping runs are short; (c) simplicity of operation—the cooling water enters, then leaves the system. Disadvantages of once- through systems include: (a) possibility of thermal pollution—i.e., temperature in- crease of the river or sea into which the warm cooling water is discharged; (b) loss of cooling capacity in the event of river or sea level decrease during droughts; (c) trash accumulation at the inlet, reducing water flow, during periods of river or sea pollution by external sources. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 128. 2.2 POWER GENERATION River flow River flow FIGURE 1 a. Once-through circulating-water system discharges warm water from the condenser directly to river or sea. Fig. 1b. Recirculating- water system reuses water after it passes through cooling tower and sta- tionary screen. (Power.) Recirculating systems use small amounts of water from the river or sea, once the system has been charged with water. Condenser circulating water is reused in this system after passing through one or more cooling towers. Thus, the only water taken from the river or sea is that needed for makeup of evaporation and splash losses in the cooling tower. The only water discharged to the river or sea is the cooling-tower blowdown. Advantages of the recirculating-water system include: (a) low water usage from the river or sea; (b) little or no thermal pollution of the supply water source because the cooling-tower blowdown is minimal; (c) remote chance of the need for service reductions during drought seasons. Disadvantages of recirculating systems include: (a) possible higher cost of the cooling tower(s) compared to the discharge piping in the once-through system; (b) greater operating Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 129. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.3 complexity of the cooling tower(s), their fans, motors, pumps, etc.; (c) increased maintenance requirements of the cooling towers and their auxiliaries. The final choice of the type of cooling system to use is based on an economic study which factors in the reliability of the system along with its cost. For the purposes of this procedure, we will assume that a once-through system with an intake length of 4500 ft (1372 m) and a discharge length of 4800 ft (1463 m) is chosen. The supply water level (a river in this case) can vary between ⫹5 ft (1.5 m) and ⫹45 ft (13.7 m). 2. Plot the operating-point diagram for the pumping system The maximum cooling-water flow rate required, based on full-load steam flow through the turbine-generator, is 314,000 gpm (19,813 L/s). Intermediate flow rates of 283,000 gpm (17,857 L/s) and 206,000 gpm (12,999 L/s) for partial loads are also required. To provide for safe 24-hour, 7-day-per-week operation of a circulating-water system, plant designers choose a minimum of two water pumps. As further safety step, a third pump is usually also chosen. That will be done for this plant. Obtaining the pump characteristic curve from the pump manufacturer, we plot the operating-point diagram, Fig. 2, for one-pump, two-pump, and three-pump op- eration against the system characteristic curve for river (weir) levels of ⫹5 ft (1.5 m) and ⫹45 ft (13.7 m). We also plot on the operating-point diagram the seal-well weir curve. The operating-point diagram is a valuable tool for both plant designers and operators because it shows the correct operating range of the circulating-water pumps. Proper use of the diagram can extend pump reliability and operating life. 3. Construct the energy-gradient curves for the circulating-water system Using the head and flow data already calculated and assembled, plot the energy- gradient curve, Fig. 3, for several heads and flow rates. The energy-gradient curve, like the operating-point diagram, is valuable to both design engineers and plant operators. Practical experience with a number of actual circulating-water installa- tions shows that early, and excessive, circulating-pump wear can be traced to the absence of an operating-point diagram and an energy-gradient curve, or to the lack of use of both these important plots by plant operating personnel. In the once-through circulating-water system being considered here, the total conduit (pipe) length is 4500 ⫹ 4800 ⫽ 9300 ft (2835 m), or 1.76 mi (2.9 km). This conduit length is not unusual—some plants may have double this length of run. Such lengths, however, are much longer than those met in routine interior plant design where 100 ft (30.5 m) are the norm for ‘‘long’’ pipe runs. Because of the extremely long piping runs that might be met in circulating-water system design, the engineer must exercise extreme caution during system design—checking and double-checking all design assumptions and calculations. 4. Analyze the pump operating points Using the operating-point diagram and the energy-gradient curves, plot the inter- section of the system curves for each intake water level vs. the characteristic curves for the number of pumps operating, Fig. 3. Thus, we see that with one pump operating, the circulating-water flow is 120,000 gpm (7572 L/s) at 48.2 ft (14.7 m) total dynamic head. With a weir level of ⫹5 ft (1.5 m), and two pumps operating, the flow is 206,000 gpm (12,999 L/s) at 79 ft (24.1 m) total dynamic head. When three pumps are Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 130. 2.4 POWER GENERATION FIGURE 2 Operating-point diagram shows the correct operating range of the circulating-water pumps. (Power.) used at the 5-ft (1.5 m) level, the flow is 225,000 gpm (14,198 L/s) at 79 ft (24.1 m) total dynamic head. Using the sets of curves mentioned here you can easily get a complete picture of the design and operating challenges faced in this, and similar, plants. The various aspects of this are discussed under Related Calculations, below. 5. Choose the type of intake structure and trash rack to use Every intake structure must provide room for the following components: (a) cir- culating-water or makeup-water pumps; (b) trash racks; (c) trash-removal screens—either fixed or traveling; (c) crane for handling pump removal or instal- lation; (d) screen wash pump; (e) access ladders and platforms. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 131. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.5 FIGURE 3 Energy-gradient diagram shows the actual system pressure values and is valuable in system design and operation. (Power.) A typical intake structure having these components is shown in Fig. 4. This structure will be chosen for this installation because it meets the requirements of the design. Trash-rack problems are among the most common in circulating-water systems and often involve unmanageable weed entanglements, rather than general debris. The type of trash rack and rack-cleaning facilities used almost exclusively in the United States and many international plants, is shown in Fig. 4. Usually, the trash rack is inclined and bars are spaced at about 3-in (76.2-mm). The trash rake may be mechanical or manual. The two usual rake designs are the unguided rake, which rides on the trash bars, and the guided rake, which runs in guides on the two sides of the water channel. If the trash bars are vertical, the guided rake is almost a necessity to keep the rake on the bars. But neither solves all the problems. If seaweed or grass loads are particularly severe, alternative trash rakes, such as the catenary or other moving-belt rakes, should be considered. These are rarely put into original domestic installations. There are many other alternative types of trash racks and rakes in use throughout the world that are successful in handling heavy Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 132. 2.6 POWER GENERATION FIGURE 4 Intake structure has trash rack, traveling screen, pumps, and crane for dependable operation of the circulating-water system. (Power.) loads. Log booms, skimmer walls, channel modifications, and specialized raking equipment can sometimes alleviate raking problems. Traveling screens follow the trash racks. These usually are of the vertical flow- through type. European practice uses alternative screens, such as center-flow, dual- flow, and drum screens. Traveling screens may be one- or two-speed. Most two- speed screens operate in the range of 3 to 12 fpm (0.9 to 3.7 m/min) but speeds as high as 30 fpm (9.1 m/min) have been used. Wear is much greater at higher speeds. Depending on the type of piping used in the circulating-water system—concrete or steel—some form of cathodic protection may be needed, in addition to the trash racks and rakes. Cathodic protection is needed primarily when steel pipe is used for the circulating water system. Concrete pipe does not, in general, require such protection. Since the piping in once-through systems can be 10 to 12 ft ( 3 to 3.7 m) in diameter, use of the cathodic protection is an important step in protecting an expensive investment. Cathodic protection methods are discussed elsewhere in this handbook. Related Calculations. Designing a condenser circulating-water system can be a complex task when the water supply is undependable. With a fixed-level supply, the design procedure is simpler. The above procedure covers the main steps in such designs. Head loss, pipe size, and other considerations are covered in detail in separate procedures given elsewhere in this handbook. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 133. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.7 Construction of the operating-point diagram and the energy-gradient chart are important steps in the system design. Further, these two plots are valuable to op- erating personnel because they give the design assumptions for the system. When pressures or flow rates change, the operator will know that the system requires inspection to pinpoint the cause of the change. The design procedure given here can be used for other circulating-water ap- plications, such as those for refrigeration condensers, air-conditioning systems, internal-combustion-engine plants, etc. Data given here are the work of R. T. Richards, Burns Roe Inc., as reported in Power magazine. SI values were added by the handbook editor. DESIGNING CATHODIC-PROTECTION SYSTEMS FOR POWER-PLANT CONDENSERS Design a cathodic-protection system for an uncoated 10,000-tube steam condenser having an exposed waterbox/tubesheet surface area of 1000 ft2 (92.9 m2 ). Deter- mine the protective current needed for this condenser if the design current density is 0.2 amp/ft2 (2.15 amp/m2 ) and 95 percent effective surface coverage will be maintained. How many anodes of magnesium, zinc, and aluminum would be needed in seawater to supply 50 amp for protection? Compare the number of anodes that would be needed in fresh water to supply 50 amp for protection. Calculation Procedure: 1. Determine the required protective current needed Cathodic protection of steam condensers is most often used to reduce galvanic corrosion of ferrous waterboxes coupled to copper-alloy tubesheets and tubes. Sys- tems are also used to mitigate attack of both iron-based waterboxes and copper- alloy tubesheets in condensers tubed with titanium or stainless steel. Cathodic protection is achieved by forcing an electrolytic direct current to flow to the structure to be protected. The name is derived from the fact that the protected structure is forced to be the cathode in a controlled electrolytic circuit. There are two ways this current may be generated: (1) Either an external direct- current power source can be used, as in an impressed-current system, Fig. 5a, or (2) a piece of a more eletronegative metal can be electrically coupled to the struc- ture, as in a sacrificial anode system, Fig. 5b. The first step in the design of a cathodic-protection system is to estimate the current requirement. The usual procedure is to calculate the exposed waterbox and tubesheet area, and then compute the total current needed by assuming a current density. In practice, current needs are often estimated by applying a test current to the structure and measuring the change in structure potential. Table 1 lists actual current densities used by utilities to protect condensers made of several different combinations of metals. The values given were taken from a survey prepared for the Electric Power Research Institute ‘‘Current Cathodic Pro- tection Practice in Steam Surface Condensers,’’ CS-2961, Project 1689-3, on which this procedure and its source are based. With a design current density of 0.2 amp/ft2 (2.15 amp/m2 ), the total protective current need ⫽ 0.2 (1000) ⫽ 200 amp. With the 95 percent effective surface cov- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 134. 2.8 POWER GENERATION Power Supply Auxiliary anode Protected structure (cathode) Sacrificial anode Protected structure (a) (b) FIGURE 5 a. Impressed-current cathodic protection system uses exter- nal source to provide protective current. Fig. 5b. Sacrificial-anode cathodic protection uses piece of metal more electronegative than the structure for protection. (Power.) erage, 5 percent of the surface will be exposed through coating faults. Hence, the required protective current will be 0.05(200) ⫽ 10 amp. Clearly, gross miscalcu- lations are possible if the effectiveness of the coating is incorrectly estimated. The value of 0.2 amp/ft2 (2.15 amp/m2 ) is taken from the table mentioned above. Another problem in estimating protective-current requirements occurs when con- densers are tubed with noble alloy tubing such as stainless steel or titanium. In this case, a significant length of tubing (up to 20 ft—6.1 m) may be involved in the galvanic action, depending on the water salinity, temperature, and the tube material. This length dictates the anode/cathode area ratio and, thus, the rate of galvanic corrosion. Protective-current needs for this type of condenser can be unusually high. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 135. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.9 TABLE 1 Current Densities Used for Various Condenser Materials* Condenser materials Waterbox Tubesheet Tubes Design current density amp/ft2 amp/m2 Average water salinity ppm Carbon steel Aluminum bronze 90-10 Cu Ni 0.05 0.54 1000 Cast iron Muntz AL-6X stainless steel 0.1 1.08 35,000 Epoxy-coated carbon steel Epoxy-coated copper-nickel Titanium 0.07 0.75 35,000 Carbon steel Muntz Aluminum brass 0.06 0.65 1000 Carbon steel Muntz 90-10 Cu Ni 0.06 0.65 1000 Carbon steel Muntz Aluminum brass 0.2 2.2 30,000 *Power TABLE 2 Current Output that can be Expected from Typical Sacrificial Anodes Materials* Current range seawater, amp Current range fresh water, amp Magnesium 1.4–2.3 0.014–0.023 Zinc 0.5–0.8 0.005–0.008 Aluminum 0.5–0.8 0.005–0.008 *Power 2. Select the type of protective system to use Protective-current needs generally determine whether an impressed-current or sac- rificial-anode system should be used. For a surface condenser, the sacrificial-anode system generally become impractical at current levels over 50 amp. For a sacrificial-type system, the current output can be estimated by determining the effective voltage and the resistance between anode and structure. The effective voltage between anode and structure is defined as the anode-to-structure open- circuit voltage minus the back-emf associated with polarization at both anode and structure. This voltage depends primarily on the choice of materials, as shown in Table 2. Resistance of the metallic path is usually negligible for an uncoated structure and the electrolytic resistance is dominant. For a coated structure, this resistance may become significant. The maximum achievable current output can be estimated by considering the case of an uncoated structure. 3. Determine the number of anodes needed for various sacrificial materials Table 2 gives a range of current outputs estimated for different sacrificial materials with an anode of the dimensions shown in Fig. 6. Thus, for any sacrificial material, number of anodes needed ⫽ (required protective-current output, amp)/(current out- put for the specific sacrificial material, amp). Since the condenser being considered here is cooled by seawater, we will use the values in the first column in Table 1. For magnesium, number of anodes required Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 136. 2.10 POWER GENERATION 6 IN. (152 CM) 10 IN. (254 CM) 1 1/4 IN. (32 CM) FIGURE 6 Typical sacrificial anode consists of a flat slab of the consumable metal into which fastening straps are cast. (Power.) ⫽ 50/2.3 ⫽ 21.739; say 22 anodes. For zinc, number of anodes required ⫽ 50/ 0.8 ⫽ 62.5; say 63 anodes. For aluminum, number of anodes required ⫽ 50/0.8 ⫽ 62.5; say 63. From a practical standpoint, 63 sacrificial anodes is an excessive number to install in most condenser waterboxes. The respective service of these anodes at 50 amp are about three months for magnesium, six months for zinc and aluminum. This short service further reduces the practicality of sacrificial anodes at high protective current levels. However, in fresh water, the current output is lower and is limited by the higher resistance of the water. Corresponding service lives are 5 to 10 years for magne- sium, and 40 to 60 years for zinc and aluminum. Protective coating further reduces the effective wetted surface area and lowers the required protective current at the same time as it limits the current output of the anodes. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 137. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.11 FIGURE 7 Bayonet-type impressed-current anode is located for optimum current throw onto the condenser tubesheet. (Power.) 4. Choose the type of anode material to use Several different factors affect the choice of anode material in both sacrificial and impressed-current systems. Choice of a sacrificial-anode material is largely deter- mined by the current density needed, but the efficiency of the material is also important. In an anode material that is 50 percent efficient, half the material dete- riorates without providing any useful current. Typical electrochemical efficiencies are: magnesium, 40–50 percent; zinc, 90 percent; aluminum, 80 percent. Here are brief features of several important anode materials: Magnesium anodes provide a high driving voltage, but are not as efficient as zinc or aluminum. Zinc anodes are excellent as sacrificial material; at temperatures above 140⬚F (60⬚C), zinc may passivate, providing almost no protective current. Aluminum anodes are not widely used to protect surface condensers because of performance problems. Steel anodes are used in a few power plants to protect copper-alloy tubesheets, but they are less efficient than traditional materials. Impressed-current systems, Fig. 5a, use anodes of platinized alloy, lead alloy, or iron alloy. Platinized- and lead-alloy anodes are favored in seawater, while iron- alloy anodes are favored in low-salinity water. Platinized- and lead-alloy anodes can be operated at higher current density than those of iron alloy, so fewer anodes are needed in the waterbox. Platinized-titanium anodes can be operated at current densities up to 1000 amp/ft2 (10,764 amp/m2 ) and voltages up to about 8 V in seawater. Such anodes should have a service life of 10 to 20 years, depending on the current density and the platinum plating thickness. Lead-alloy anodes are widely used in seawater applications. These anodes can be operated at current densities as high as 10 to 20 amp/ft2 (107.6 to 215.3 amp/ m2 ) with a life expectancy of more than 10 years. Related Calculations. This procedure outlines the essentials of sizing anodes for protecting steam surface condensers. For more detailed information, refer to the report mentioned in step 1 of this procedure. Data for this procedure were compiled Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 138. 2.12 POWER GENERATION FIGURE 8 Temperatures governing condenser performance. by John Reason and reported in Power magazine, using the report mentioned earlier. SI values were added by the handbook editor. STEAM-CONDENSER PERFORMANCE ANALYSIS (a) Find the required tube surface area for a shell-and-tube type of condenser serv- ing a steam turbine when the quantity of steam condensed S is 25,000 lb/h (3.1 kg/s); condenser back pressure ⫽ 2 inHg absolute (6.8 kPa); steam temperature ts ⫽ 101.1⬚F (38.4⬚C); inlet water temperature t1 ⫽ 80⬚F (26.7⬚C); tube length per pass L ⫽ 14 ft (4.3 m); water velocity V ⫽ 6.5 ft/s (2.0 m/s); number of passes ⫽ 2; tube size and gage: 3 ⁄4-in (1.9 cm), no. 18 BWG; cleanliness factor ⫽ 0.80. (b) Compute the required area and cooling-water flow rate for the same conditions as (a) except that cooling water enters at 85⬚F (29.4⬚C). (c) If the steam flow through the condenser in (a) decreases to 15,000 lb/h (1.9 kg/s), what will be the absolute steam pressure in the condenser shell? Calculation Procedure: 1. Sketch the condenser, showing flow conditions (a) Figure 8 shows the condenser and the flow conditions prevailing. 2. Determine the condenser heat-transfer coefficient Use standard condenser-tube engineering data available from the manufacturer or Heat Exchange Institute. Table 3 and Fig. 9 show typical condenser-tube data used in condenser selection. These data are based on a minimum water velocity of 3 ft /s (0.9 m/s) through the condenser tubes, a minimum absolute pressure of 0.7 inHg (2.4 kPa) in the condenser shell, and a minimum ⌬t terminal temperature difference ts ⫺ t2 of 5⬚F (2.8⬚C). These conditions are typical for power-plant surface con- densers. Enter Fig. 9 at the bottom at the given water velocity, 6.5 ft/s (2.0 m/s), and project vertically upward until the 3 ⁄4-in (1.9-cm) OD tube curve is intersected. From this point, project horizontally to the left to read the heat-transfer coefficient U ⫽ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 139. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.13 TABLE 3 Standard Condenser Tube Data FIGURE 9 Heat-transfer and correction curves for calculating surface-condenser performances. 690 Btu/(ft2 䡠 ⬚F) [14,104.8 kJ/(m2 䡠 ⬚C)] LMTD (log mean temperature difference). Also read from Fig. 9 the temperature correction factor for an inlet-water temper- ature of 80⬚F (26.7⬚C) by entering at the bottom at 80⬚F (26.7⬚C) and projecting vertically upward to the temperature-correction curve. From the intersection with this curve, project to the right to read the correction as 1.04. Correct U for tem- perature and cleanliness by multiplying the value obtained from the chart by the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 140. 2.14 POWER GENERATION correction factors, or U ⫽ 690(1.04)(0.80) ⫽ 574 Btu/(ft2 䡠 h 䡠 ⬚F) [11,733.6 kJ/ (m2 䡠 h 䡠 ⬚C)] LMTD. 3. Compute the tube constant Read from Table 3, for two passes through 3 ⁄4-in (1.9-cm) OD 18 BWG tubes, k ⫽ a constant ⫽ 0.377. Then kL/V ⫽ 0.377(14)/6.5 ⫽ 0.812. 4. Compute the outlet-water temperature The equation for outlet-water temperature is t2 ⫽ ts ⫺ (ts ⫺ t1 /ex ), where x ⫽ (kL /V)(U/500), or x ⫽ 0.812(574/500) ⫽ 0.932. Then ex ⫽ 2.71830.932 ⫽ 2.54. With this value known, t2 ⫽ 101.1 ⫺ (101.1 ⫺ 80/2.54) ⫽ 92.8⬚F (33.8⬚C). Check to see that ⌬t(ts ⫺ t2) is less than the minimum 5⬚F (2.8⬚C) terminal difference. Or, 101.1 ⫺ 92.8 ⫽ 8.3⬚F (4.6⬚C), which is greater than 5⬚F (2.8⬚C). 5. Compute the required tube surface area The required cooling-water flow, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(25,000)/ [500(92.8 ⫺ 80)] ⫽ 3700 gal/min (233.4 L/s). This equation assumes that 950 Btu is to be removed from each pound (2209.7 kJ/kg) of steam condensed. When a different quantity of heat must be removed, use the actual quantity in place of the 950 in this equation. With the tube constant kL/V and cooling-water flow rate known, the required area is computed from A ⫽ (kL/V)(gpm) ⫽ (0.812)(3700) ⫽ 3000 ft2 (278.7 m2 ). Since the value of U was not corrected for condenser loading, it is necessary to check whether such a correction is needed. Condenser loading ⫽ S/A ⫽ 25,000/ 3000 ⫽ 8.33 lb/ft2 (40.7 kg/m2 ). Figure 9 shows that no correction (correction factor ⫽ 1.0) is necessary for loadings greater than 8.0 lb/ft2 (39.1 kg/m2 ). There- fore, the loading for this condenser is satisfactory without correction. This step concludes the general calculation procedure for a surface condenser serving any steam turbine. The next procedure shows the method to follow when a higher cooling-water inlet temperature prevails. 6. Compute the cooling-water outlet temperature (b) Higher cooling water temperature. From Fig. 9 for 85⬚F (29.4⬚C) cooling-water inlet temperature and a 0.80 cleanliness factor, U ⫽ 690(1.06)(0.80) ⫽ 585 Btu/ (ft2 䡠 h 䡠 ⬚F) [3.3 kJ/(m2 䡠 ⬚C 䡠 s)] LMTD. Given data from Table 3, the tube constant kL/V ⫽ 0.377(14)/6.5 ⫽ 0.812. Then x ⫽ (kL/V)(U/500) ⫽ 0.812(585/500) ⫽ 0.950. Using this exponent, we get ex ⫽ 2.81830.950 ⫽ 2.586. The cooling-water outlet temperature is then t2 ⫽ ts ⫺ (ts ⫺ t1 /ex ) ⫽ 101.1 ⫺ (101.1 ⫺ 85)/2.586 ⫽ 94.9⬚F (34.9⬚C). Check to see that ⌬t(ts ⫺ t2) is greater than the minimum 5⬚F (2.8⬚C) terminal temperature difference. Or, 101.1 ⫺ 94.9 ⫽ 6.5⬚F (3.6⬚C), which is greater than 5⬚F (2.8⬚C). 7. Compute the water flow rate, required area, and loading The required cooling-water flow, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(25,000)/ [500(94.9 ⫺ 85)] ⫽ 4800 gal/min (302.8 L/s). With the tube constant kL/V and cooling-water flow rate known, the required area is computed from A ⫽ (kL/V)(gpm) ⫽ 0.812(4800) ⫽ 3900 ft2 (362.3 m2 ). Then loading ⫽ S/A ⫽ 25,000/3900 ⫽ 6.4 lb/ft2 (31.2 kg/m2 ). Since the loading is less than 8 lb/ft2 (39.1 kg/m2 ), refer to Fig. 9 to obtain the loading correction factor. Enter at the bottom at 6.4 lb/ft2 (31.2 kg/m2 ), and project vertically to the loading curve. At the right, read the loading correction factor as Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 141. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.15 0.95. Now the value of U already computed must be corrected, and all dependent quantities recalculated. 8. Recalculate the condenser proportions First, correct U for loading. Or, U ⫽ 585(0.95) ⫽ 555. Then x ⫽ 0.812(555/500) ⫽ 0.90; ex ⫽ 2.71830.90 ⫽ 2.46; t2 ⫽ 101.1 ⫺ (101.1 ⫺ 85/2.46) ⫽ 94.6⬚F (34.8⬚C). Check ⌬t ⫽ ts ⫺ t2 ⫽101.1 ⫺ 94.6 ⫽ 6.5⬚F (3.6⬚C), which is greater than 5⬚F (2.8⬚C). The cooling-water flow rate, gal/min ⫽ 950(25,000)/[500(94.6 ⫺ 85)] ⫽ 4950 gal/min (312.3 L/s). Then A ⫽ 0.812(4950) ⫽ 4020 ft2 (373.5 m2 ), and loading ⫽ 25,000/4020 ⫽ 6.23 lb/ft2 (30.4 kg/m2 ). Check the correction factor for this loading in Fig. 9. The correction factor is 0.94, compared with 0.95 for the first calculation. Since the value of U would be changed only about 1 percent by using the lower factor, the calculations need not be revised further. Where U would change by a larger amount—say 5 percent or more—it would be necessary to repeat the procedure just detailed, applying the new correction factor. Note that the 5⬚F (2.8⬚C) increase in cooling-water temperature (from 80 to 85⬚F or 26.7 to 29.4⬚C) requires an additional 1020 ft2 (94.8 m2 ) of condenser surface and 125 gal/min (7.9 L/s) of cooling-water flow to maintain the same back pres- sure. These increments will vary, depending on the temperature level at which the increase occurs. The effect of reduced steam flow on the steam pressure in the condenser shell will not be computed because the recalculation above is the last step in part (b) of this procedure. (c) Reduced steam flow to condenser. 9. Determine the condenser loading From procedure (a) above, the cooling-water flow ⫽ 3700 gal/min (233.4 L/s); condenser surface A ⫽ 3000 ft2 (278.7 m2 ). Then, with a 15,000-lb/h (1.9-kg/s) steam flow, loading ⫽ S/A ⫽ 15,000/3000 ⫽ 5 lb/ft2 (24.4 kg/m2 ). 10. Compute the heat-transfer coefficient Correct the previous heat-transfer rate U ⫽ 690 Btu/(ft2 䡠 h 䡠 ⬚F) [3.9 kJ/(m2 䡠 ⬚C 䡠 s)] LMTD for temperature, cleanliness, and loading. Or, U ⫽ 690(1.04)(0.80)(0.89) ⫽ 511 Btu/(ft2 䡠 h 䡠 ⬚F) [2.9 kJ/(m2 䡠 ⬚C 䡠 s)] LMTD, given the correction factors from Fig. 9. 11. Compute the final steam temperature As before, x ⫽ (kL/V)(U/500 ) ⫽ (0.377)(14/6.5)(511/500) ⫽ 0.830. Then ⌬t ⫽ t2 ⫺ t1 ⫽ 950S/(500gpm) ⫽ 950(15,000)/[500(3700)] ⫽ 7.7⬚F (4.3⬚C). With t1 ⫽ 80⬚F (26.7⬚C), t2 ⫽ ⌬t ⫹ t1 ⫽ 7.7 ⫹ 80 ⫽ 87.7⬚F (30.9⬚C). Since t2 ⫽ ts ⫺ t1)/e x , ex ⫽ ts ⫺ t1 /(ts ⫺ t2), or 2.71830.830 ⫽ ts ⫺ 80/(ts ⫺ 87.7). Solve for ts; or, ts ⫽ 201.1 ⫺ 80/1.294 ⫽ 93.6⬚F (34.2⬚C). At a saturation temperature of 93.6⬚F (34.2⬚C), the steam table (saturation tem- perature) shows that the steam pressure in the condenser shell is 1.59 inHg (5.4 kPa). Check the ⌬t terminal temperature difference. Or, ⌬t ⫽ ts ⫺ t2 ⫽ 93.6 ⫺ 87.7 ⫽ 5.9⬚F (3.3⬚C). Since the terminal temperature difference is greater than 5⬚F (2.8⬚C), the calculated performance can be realized. Related Calculations. The procedures and data given here can be used to com- pute the required cooling-water flow, cooling-water temperature rise, quantity of steam condensed by a given cooling-water flow rate and temperature rise, required Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 142. 2.16 POWER GENERATION condenser surface area, tube length per pass, water velocity, steam temperature in condenser, cleanliness factor, and heat-transfer rate. Whereas Fig. 9 is suitable for all usual condenser calculations for the ranges given, check the Heat Exchange Institute for any new curves that might have been made available before you make the final selection of very large condensers (more than 100,000 lb/h or 12.6 kg/s of steam flow). Note: The design water temperature used for condensers is either the average summer water temperature or the average annual water temperature, depending on which is higher. The design steam load is the maximum steam flow expected at the full-load rating of the turbine or engine. Usual shell-and-tube condensers have tubes that vary in length from about 8 ft (2.4 m) in the smallest sizes to about 40 ft (12.2 m) or more in the largest sizes. Each square foot of tube surface will condense 7 to 20 lb/h (0.88 to 2.5 g/s) of steam with a cooling-water circulating rate of 0.1 to 0.25 gal/(lb 䡠 min) [0.014 to 0.035 L/(kg 䡠 s)] of steam condensed. The method presented here is the work of Glenn C. Boyer. STEAM-CONDENSER AIR LEAKAGE The air leakage into a condenser is estimated to be 12 ft3 /min (0.34 m3 /min) of 70⬚F (21⬚C) air at 14.7 lb/in2 (101 kPa). At the air outlet connection on the con- denser, the temperature is 84⬚F (29⬚C) and the total (mixture) pressure is 1.80 inHg absolute (6.1 kPa). Determine the quantity of steam, lbm /h (kg/h), lost from the condenser. Calculation Procedure: 1. Compute the mass rate of flow per hour of the air leakage The mass rate of flow per hour of the estimated dry air leakage into the condenser, wa ⫽ pV/RaT, where the air pressure, p ⫽ 14.7 ⫻ 144 lbƒ /ft2 (101 kPa); volumetric flow rate, V ⫽ 12 ⫻ 60 ⫽ 720 ft3 /h (20.4 m3 /h); gas constant for air, Ra ⫽ 53.34 ft 䡠 lb/(lb 䡠 ⬚R) [287(m 䡠 N/kg 䡠 K)]; air temperature, T ⫽ 70 ⫹ 460 ⫽ 530⬚R (294 K). Then, wa ⫽ (14.7 ⫻ 144)(720)/(53.34 ⫻ 530) ⫽ 53.9 lb/h (24.4 kg/h). 2. Determine the partial pressure of the air in the mixture The partial pressure of the air in the mixture of air and steam, pa ⫽ pm ⫺ pv, where the mixture pressure, pm ⫽ 1.80 ⫻ 0.491 ⫽ 0.884 lb/in2 (6.09 kPa); partial vapor pressure, pv ⫽ 0.577 lb/in2 (3.98 kPa), as found in the Steam Tables mentioned under Related Calculations of this procedure. Then, pa ⫽ 0.884 ⫺ 0.577 ⫽ 0.307 lb/in2 (2.1 kPa). 3. Compute the humidity ratio of the mixture The humidity ratio of the mixture, wv ⫽ Rapv /(Rvpa), where the gas constant for steam vapor, Rv ⫽ 85.8 ft 䡠 lb/(lbm 䡠 ⬚R) [462(J/kg 䡠 K)], as found in a reference mentioned under Related Calculations of this procedure. Then, wv ⫽ 53.34 ⫻ 0.577/(85.8 ⫻ 0.307) ⫽ 1.17 lb vapor/lb dry air (0.53 kg/kg). 4. Compute the rate of steam lost from the condenser Steam is lost from the condenser at the rate of wh ⫽ wv ⫻ wa ⫽ 1.17 ⫻ 53.9 ⫽ 63.1 lb/h (28.6 kg/h). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 143. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.17 TABLE 4 Typical Design Conditions for Steam Condensers Related Calculations. The partial vapor pressure in step 2 was found at 84⬚F (29⬚C) under Table 1, Saturation: Temperatures of Thermodynamic Properties of Water Including Vapor, Liquid, and Solid Phases, 1969, Keenan, et al., John Wiley Sons, Inc. Use the later versions of such tables whenever available, as necessary. The gas constant for water vapor in step 3 was obtained from Principals of Engi- neering Thermodynamics, 2d edition, by Kiefer, et al., John Wiley Sons, Inc. STEAM-CONDENSER SELECTION Select a condenser for a steam turbine exhausting 150,000 lb/h (18.9 kg/s) of steam at 2 inHg absolute (6.8 kPa) with a cooling-water inlet temperature of 75⬚F (23.9⬚C). Assume a 0.85 condition factor, 7 ⁄8-in (2.2-cm) no. 18 BWG tubes, and an 8-ft/s (2.4-m/s) water velocity. The water supply is restricted. Obtain condenser constants from the Heat Exchange Institute, Steam Surface Condenser Standards. Calculation Procedure: 1. Select the ts ⫺ t1 temperature difference Table 4 shows customary design conditions for steam condensers. With an inlet- water temperature at 75⬚F (23.9⬚C) and an exhaust steam pressure of 2.0 inHg absolute (6.8 kPa), the customary temperature difference ts ⫺ t1 ⫽ 26.1⬚F (14.5⬚C). With a sufficient water supply and a siphonic circuitry, (t2 ⫺ t1)/(ts ⫺ t1) is usually between 0.5 and 0.55. For a restricted water supply or high frictional resistance and static head, the value of this factor ranges from 0.55 to 0.75. 2. Compute the LMTD across the condenser With 75⬚F (23.9⬚C) inlet water, ts ⫺ t1 ⫽ 101.14 ⫺ 75 ⫽ 26.14⬚F (14.5⬚C), given the steam temperature in the saturation-pressure table. Once ts ⫺ t1 is known, it is necessary to assume a value for the ratio (t2 ⫺ t1)/(ts ⫺ t1). As a trial, assume 0.60, since the water supply is restricted. Then (t2 ⫺ t1)/(ts ⫺ t1), ⫽ 0.60 ⫽ (t2 ⫺ t1)/ 26.14. Solving, we get t2 ⫺ t1 ⫽ 15.68⬚F (8.7⬚C). The difference between the steam temperature ts and the outlet temperature t2 is then ts ⫺ t2 ⫽ 26.14 ⫺ 15.68 ⫽ 10.46⬚F (5.8⬚C). Checking, we find t2 ⫽ t1 ⫹ (t2 ⫺ t1) ⫽ 75 ⫹ 15.68 ⫽ 90.68⬚F (50.38⬚C); ts ⫺ t2 ⫽ 101.14 ⫺ 90.68 ⫽ 10.46⬚F (5.8⬚C). This value is greater than the required minimum value of 5⬚F (2.8⬚C) for ts ⫺ t2. The assumed ratio 0.60 is therefore satisfactory. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 144. 2.18 POWER GENERATION Were ts ⫺ t2 less than 5⬚F (2.8⬚C), another ratio value would be assumed and the difference computed again. You would continue doing this until a value of ts ⫺ t2 greater than 5⬚F (2.8⬚C) were obtained. Then LMTD ⫽ (t2 ⫺ t1)/ln[(ts ⫺ t1)/(ts ⫺ t2)]; LMTD ⫽ 15.68/ln (26.1/10.46) ⫽ 17.18⬚F (9.5⬚C). 3. Determine the heat-transfer coefficient From the Heat Exchange Institute or manufacturer’s data U is 740 Btu/(ft2 䡠 h 䡠 ⬚F) [4.2 kJ/(m2 䡠 ⬚C 䡠 s)] LMTD for a water velocity of 8 ft/s (2.4 m/s). If these data are not available, Fig. 9 can be used with complete safety for all preliminary se- lections. Now U must be corrected for the inlet-water temperature, 75⬚F (23.9⬚C), and the condition factor, 0.85, which is a term used in place of the correction factor by some authorities. From Fig. 9, the correction for 75⬚F (23.9⬚C) inlet water ⫽ 1.04. Then actual U⫽ 740(1.04)(0.85) ⫽ 655 Btu/(ft2 䡠 h 䡠 ⬚F) [3.7 kJ/(m2 䡠 ⬚C 䡠 s)] LMTD. 4. Compute the steam condensation rate The heat-transfer rate per square foot of condenser surface with a 17.18⬚F (9.5⬚C) LMTD is U(LMTD) ⫽ 655(17.18) ⫽ 11,252.9 Btu/(ft2 䡠 h) [35.5 kJ/(m2 䡠 s)]. Condensers serving steam turbines are assumed, for design purposes, to remove 950 Btu/lb (2209.7 kJ/kg) of steam condensed. Therefore, the steam condensation rate for any condenser is [Btu/(ft2 䡠 h)]/950, or 1252.9/950 ⫽ 11.25 lb/(ft2 䡠 h) [15.3 g/(m2 䡠 s)]. 5. Compute the required surface area and water flow The required surface area ⫽ steam flow (lb/h)/[condensation rate, lb/(ft2 䡠 h)], or with a 150,000-lb/h (18.9-kg/s) flow, 150,000/11.25 ⫽ 13,320 ft2 (1237.4 m2 ). The water flow rate, gal/min ⫽ 950S/[500(t2 ⫺ t1)] ⫽ 950(150,000)/ [500(15.68)] ⫽ 18,200 gal/min (1148.1 L/s). Related Calculations. See the previous calculation procedure for steps in de- termining the water-pressure loss through a surface condenser. To choose a surface condenser for a steam engine, use the same procedures as given above, except that the heat removed from the exhaust steam is 1000 Btu/lb (2326.9 kJ/kg). Use a condition (cleanliness) factor of 0.65 for steam engines be- cause the oil in the exhaust steam fouls the condenser tubes, reducing the rate of heat transfer. The condition (cleanliness) factor for steam turbines is usually as- sumed to be 0.8 to 0.9 for relatively clean, oil-free cooling water. At loads greater than 50 percent of the design load, ts ⫺ t1 follows a straight- line relationship. Thus, in the above condenser, ts ⫺ t1 ⫽ 26.14⬚F (14.5⬚C) at the full load of 150,000 lb/h (18.9 kg/s). If the load falls to 60 percent (90,000 lb/h or 11.3 kg/s), then ts ⫺ t1 ⫽ 26.14(0.60) ⫽ 15.7⬚F (8.7⬚C). At 120 percent load (180,00 lb/h or 22.7 kg/s), ts ⫺ t1 ⫽ 26.14(1.20) ⫽ 31.4⬚F (17.4⬚C). This straight- line law is valid with constant inlet-water temperature and cooling-water flow rate. It is useful in analyzing condenser operating conditions at other than full load. Single- or multiple-pass surface condensers may be used in power services. When a liberal supply of water is available, the single-pass condenser is often chosen. With a limited water supply, a two-pass condenser is often chosen. AIR-EJECTOR ANALYSIS AND SELECTION Choose a steam-jet air ejector for a condenser serving a 250,000-lb/h (31.5-kg/s) steam turbine exhausting at 2 inHg absolute (6.8 kPa). Determine the number of Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 145. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.19 stages to use, the approximate steam consumption and the quantity of air and vapor mixture the ejector will handle. Calculation Procedure: 1. Select the number of stages for the ejector Use Fig. 10 as a preliminary guide to the number of stages required in the ejector. Enter at 2-inHg absolute (6.8-kPa) condenser pressure, and project horizontally to the stage area. This shows that a two-stage ejector will probably be satisfactory. Check the number of stages above against the probable overload range of the prime mover by using Fig. 11. Enter at 2-inHg absolute (6.8-kPa) condenser pres- sure, and project to the two-stage curve. This curve shows that a two-stage ejector can readily handle a 25-percent overload of the prime mover. Also, the two-stage curve shows that this ejector could handle up to 50 percent overload with an in- crease in the condenser absolute pressure of only 0.4 inHg (1.4 kPa). This is shown by the pressure, 2.4 inHg absolute (8.1 kPa), at which the two-stage curve crosses the 150 percent overload ordinate (Fig. 11). 2. Determine the ejector operating conditions Use the Heat Exchange Institute or manufacturer’s data. Table 5 excerpts data from the Heat Exchange Institute for condensers in the range considered in this proce- dure. Study of Table 5 shows that a two-stage condensing ejector unit serving a 250,000-lb/h (31.5-kg/s) steam turbine will require 450 lb/h (56.7 g/s) of 300- lb/in2 (gage) (2068.5-kPa) steam. Also, the ejector will handle 7.5 ft3 /min (0.2 m3 /min) of free, dry air, or 33.75 lb/h (4.5 g/s) of air. It will remove up to 112.5 lb/h (14.2 g/s) of an air-vapor mixture. The actual air leakage into a condenser varies with the absolute pressure in the condenser, the tightness of the joints, and the conditions of the tubes. Some au- thorities cite a maximum leakage of about 250-lb/h (31.5-g/s) steam flow. At 400,000 lb/h (50.4 kg/s), the leakage is 160 lb/h (20.2 g/s); at 250,000 lb/h (31.5 kg/s), it is 130 lb/h (16.4 g/s) of air-vapor mixture. A condenser in good condition will usually have less leakage. For an installation in which the manufacturer supplies data on the probable air leakage, use a psychrometric chart to determine the weight of water vapor contained in the air. Thus, at 2 inHg absolute (6.8 kPa) and 80⬚F (26.7⬚C), each pound of air will carry with it 0.68 lb (0.68 kg/kg) of water vapor. In a surface condenser into which 20 lb. (9.1 kg) of air leaks, the ejector must handle 20 ⫹ 20(0.68) ⫽ 33.6 lb/h (4.2 g/s) of air-vapor mixture. Table 5 shows that this ejector can readily handle this quantity of air-vapor mixture. Related Calculations. When you choose an air ejector for steam-engine ser- vice, double the Heat Exchange Institute steam-consumption estimates. For most low-pressure power-plant service, a two-stage ejector with inter- and after con- densers is satisfactory, although some steam engines operating at higher absolute exhaust pressures require only a single-stage ejector. Twin-element ejectors have two sets of stages; one set serves as a spare and may also be used for capacity regulation in stationary and marine service. The capacity of an ejector is constant for a given steam pressure and suction pressure. Raising the steam pressure will not increase the ejector capacity. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 146. 2.20 POWER GENERATION FIGURE 10 Steam-ejector capacity-range chart. SURFACE-CONDENSER CIRCULATING-WATER PRESSURE LOSS Determine the circulating-water pressure loss in a two-pass condenser having 12,000 ft2 (1114.8 m2 ) of condensing surface, a circulating-water flow rate of 10,000 gal/min (630.8 L/s), 3 ⁄4-in (1.9-cm) no. 16 BWG tubes, a water flow rate of 7 ft/ s (2.1 m/s), external friction of 20 ft of water (59.8 kPa), and a 10-ft-of-water (29.9-kPa) siphonic effect on the circulating-water discharge. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 147. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.21 FIGURE 11 Steam-jet ejector characteristics. TABLE 5 Air-Ejector Capacities for Surface Condensers for Steam Turbines* Calculation Procedure: 1. Determine the water flow rate per tube Use a tabulation of condenser-tube engineering data available from the manufacturer or the Heat Exchange Institute, or complete the water flow rate from the physical dimensions of the tube thus: 3 ⁄4-in (1.9 cm) no. 16 BWG tube ID ⫽ 0.620 in (1.6 cm) from a tabulation of condenser-tube data, such as Table 3. Assume a water velocity of 1 ft/s (0.3 m/s). Then a 1-ft (0.3-m) length of the tube will contain (12)(0.620)2 /4 ⫽ 3.62 in3 (59.3 cm3 ) of water. This quantity of water will flow through the tube for each foot of length per second of water velocity [194.6 cm3 / (m 䡠 s)]. The flow per minute will be 3.62 (60 s/min) ⫽ 217.2 in3 /min (3559.3 cm3 /min). Since 1 U.S. gal ⫽ 231 in3 (3.8 L), the gal/min flow at a 1 ft/s (0.3 m/s) velocity ⫽ 217.2/231 ⫽ 0.94 gal/min (0.059 L/s). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 148. 2.22 POWER GENERATION With an actual velocity of 7 ft/s (2.1 m/s), the water flow rate per tube is 7(0.94) ⫽ 6.58 gal/min (0.42 L/s). 2. Determine the number of tubes and length of water travel Since the water flow rate through the condenser is 10,000 gal/min (630.8 L/s) and each tube conveys 6.58 gal/min (0.42 L/s), the number of tubes ⫽ 10,000/6.58 ⫽ 1520 tubes per pass. Next, the total length of water travel for a condenser having A ft2 of condensing surface is computed from A(number of tubes)(outside area per linear foot, ft2 ). The outside area of each tube can be obtained from a table of tube properties, such as Table 3; or computed from (OD, in)()(12)/144, or (0.75)()(12)/144 ⫽ 0.196 ft2 /lin ft (0.06 m2 /m). Then, total length of travel ⫽ 12,000/[(1520)(0.196)] ⫽ 40.2 ft (12.3 m). Since the condenser has two passes, the length of tube per pass ⫽ 40.2/2 ⫽ 20.1 ft (6.1 m). Since each pass has an equal number of tubes and there are two passes, the total number of tubes in the condenser ⫽ 2 passes (1520 tubes per pass) ⫽ 3040 tubes. 3. Compute the friction loss in the system Use the Heat Exchange Institute or manufacturer’s curves to find the friction loss per foot of condenser tube. At 7 ft/s (2.1 m/s), the Heat Exchange Institute curve shows the head loss is 0.4 ft of head per foot (3.9 kPa/m) of travel for 3 ⁄4-in (1.9- cm) no. 16 BWG tubes. With a total length of 40.2 ft (12.3 m), the tube head loss is 0.4(40.2) ⫽ 16.1 ft (48.1 kPa). Use the Heat Exchange Institute or manufacturer’s curves to find the head loss through the condenser waterboxes. From the first reference, for a velocity of 7 ft/ s (2.1 m/s), head loss ⫽ 1.4 ft (4.2 kPa) of water for a single-pass condenser. Since this is a two-pass condenser, the total waterbox head loss ⫽ 2(1.4) ⫽ 2.8 ft (8.4 kPa). The total condenser friction loss is then the sum of the tube and waterbox losses, or 16.1 ⫹ 2.8 ⫽ 18.9 ft (56.5 kPa) of water. With an external friction loss of 20 ft (59.8 kPa) in the circulating-water piping, the total loss in the system, without siphonic assistance, is 18.9 ⫹ 20 ⫽ 38.9 ft (116.3 kPa). Since there is 10 ft (29.9 kPa) of siphonic assistance, the total friction loss in the system with siphonic as- sistance is 38.9 ⫺ 10 ⫽ 28.9 ft (86.3 kPa). In choosing a pump to serve this system, the frictional resistance of 28.9 ft (86.3 kPa) would be rounded to 30 ft (89.7 kPa), and any factor of safety added to this value of head loss. Note: The most economical cooling-water velocity in condenser tubes is 6 to 7 ft/s (1.8 to 2.1 m/s); a velocity greater than 8 ft/s (2.4 m/s) should not be used, unless warranted by special conditions. SURFACE-CONDENSER WEIGHT ANALYSIS A turbine exhaust nozzle can support a weight of 100,000 lb (444,822.2 N). De- termine what portion of the total weight of a surface condenser must be supported by the foundation if the weight of the condenser is 275,000 lb (1,223,261.1 N), the tubes and waterboxes have a capacity of 8000 gal (30,280.0 L), and the steam space has a capacity of 30,000 gal (113,550.0 L) of water. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 149. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.23 Calculation Procedure: 1. Compute the maximum weight of the condenser The maximum weight on a condenser foundation occurs when the shell, tubes, and waterboxes are full of water. This condition could prevail during accidental flooding of the steam space or during tests for tube leaks when the steam space is purpose- fully flooded. In either circumstance, the condenser foundation and spring supports, if used, must be able to carry the load imposed on them. To compute this load, find the sum of the individual weights: 2. Compute the foundation load The turbine nozzle can support 100,000 lb (444,822.2 N). Therefore, the foundation must support 591,540 ⫺ 100,000 ⫽ 491,540 lb (2,186,479.0 N). For foundation design purposes this would be rounded to 495,000 lb (2,201,869.9 N). Related Calculations. When you design a condenser foundation, do the fol- lowing: (1) Leave enough room at one end to permit withdrawal of faulty tubes and insertion of new tubes. Since some tubes may exceed 40 ft (12.3 m) in length, careful planning is needed to provide sufficient installation space. During the design of a power plant, a template representing the tube length is useful for checking the tube clearance on a scale plan and side view of the condenser installation. When there is insufficient room for tube removal with one shape of condenser, try another with shorter tubes. (2) Provide enough headroom under the condenser to produce the required sub- mergence on the condensate-pump impeller. Most condensate pumps require at least 3-ft (0.9 m) submergence. If necessary, the condensate pump can be installed in a pit under the condenser, but this should be avoided if possible. WEIGHT OF AIR IN STEAM-PLANT SURFACE CONDENSER The vacuum in a surface condenser is 28-in (71.12-cm) Hg referred to a 30-in (76.2-cm) barometer. The temperature in the condenser is 80⬚F (26.7⬚C). What is the percent by weight of the air in the condenser? Calculation Procedure: 1. Find the absolute pressure in the condenser From the steam tables at 80⬚F (26.7⬚C), 1 in (2.54 cm) Hg exerts a pressure of 0.4875 psi (3.36 kPa). 2. Determine the weight of water per lb (kg) of dry air in the condenser In a condenser, the steam (water vapor) is condensing in contact with the tubes and may be taken as saturated. At 80⬚F (26.7⬚C), the absolute pressure of saturated Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 150. 2.24 POWER GENERATION steam is 0.5067 psia (3.39 kPa), from the steam tables. In the condensing condition, there are 0.622 lb (0.28 kg) of water per pound (kg) of dry air. Since the water content of the air is a function of the partial pressures, (0.622) (0.5067)/[(2 ⫻ 0.5067)] ⫽ 0.673 lb of water per lb of dry air (0.305 kg). 3. Compute the percent of air by weight Use the relation, percent of air by weight ⫽ (100)(1)/(1 ⫹ 0.672) ⫽ (100)(1)/(1 ⫹ 0.672) ⫽ 59.8 percent by weight of air. Related Calculations. Use this general procedure for analyzing the air in sur- face condensers serving steam turbines of all types. BAROMETRIC-CONDENSER ANALYSIS AND SELECTION Select a countercurrent barometric condenser to serve a steam turbine exhausting 25,000 lb/h (3.1 kg/s) of steam at 5 inHg absolute (16.9 kPa). Determine the quantity of cooling water required if the water inlet temperature is 50⬚F (10.0⬚C). What is the required dry-air capacity of the ejector? What is the required pump head if the static head is 40 ft (119.6 kPa) and the pipe friction is 15 ft of water (44.8 kPa)? Calculation Procedure: 1. Find the steam properties from the steam tables At 5 inHg absolute (16.9 kPa), hg ⫽ 1119.4 Btu/lb (2603.7 kJ/kg), from the sat- uration-pressure table. If the condensing water were to condense the steam without subcooling the condensate, the final temperature of the condensate, from the steam tables, would be 133.76⬚F (56.5⬚C), corresponding to the saturation temperature. However, subcooling almost always occurs, and the usual practice in selecting a countercurrent barometric condenser is to assume the final condensate temperature tc will be 5⬚F (2.8⬚C) below the saturation temperature corresponding to the absolute pressure in the condenser. Given a 5⬚F (2.8⬚C) difference, tc ⫽ 133.76 ⫺ 5 ⫽ 128.76⬚F (53.7⬚C). Interpolating in the saturation-temperature steam table, we find the enthalpy of the condensate hƒ at 128.76⬚F (53.7⬚C) is 96.6 Btu/lb (224.8 kJ/ kg). 2. Compute the quantity of condensing water required In any countercurrent barometric condenser, the quantity of cooling water Q lb/h required is Q ⫽ W(hg ⫺ ht)/(tc ⫺ t1), where W ⫽ weight of steam condensed, lb/ h; t1 ⫽ cooling-water inlet temperature, ⬚F. Then Q ⫽ 25,000(1119.4 ⫺ 96.66)/ (128.76 ⫺ 50) ⫽ 325,000 lb/h (40.9 kg/s). By converting to gallons per minute, Q ⫽ 325,000/500 ⫽ 650 gal/min (41.0 L/s). 3. Determine the required ejector dry-air capacity Use the Heat Exchanger Institute or a manufacturer’s tabulation of free, dry-air leakage and the allowance for air in the cooling water to determine the required dry-air capacity. Thus, from Table 6, the free, dry-air leakage for a barometric condenser serving a turbine is 3.0 ft3 /min (0.08 m3 /min) of air and vapor. The Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 151. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.25 TABLE 6 Free, Dry-Air Leakage [ft3 /min (m3 /s) at 70⬚F or 21.1⬚C air and vapor mixture, 71 ⁄2⬚ below vacuum temperature or 4.2⬚ for Celsius] FIGURE 12 Allowance for air in condenser injection water. allowance for air in the 50⬚F (10.0⬚C) cooling water is 3.3 ft3 /min (0.09 m3 /min) of air at 70⬚F (21.1⬚C) per 1000 gal/min (63.1 L/s) of cooling water, Fig 12. The total dry-air leakage is the sum, or 3.0 ⫹ 3.3 ⫽ 6.3 ft3 /min (0.18 m3 /min). Thus, the ejector must be capable of handling at least 6.3 ft3 /min (0.18 m3 /min) of dry air to serve this barometric condenser at its rated load of 25,000 lb/h (3.1 kg/s) of steam. Where the condenser will operate at a lower vacuum (i.e., a higher absolute pressure), overloads up to 50 percent may be met. To provide adequate dry-air handling capacity at this overload with the same cooling-water inlet temperature, find the free, dry-air leakage at the higher condensing rate from Table 6 and add Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 152. 2.26 POWER GENERATION this to the previously found allowance for air in the cooling water. Or, 4.5 ⫹ 3.3 ⫽ 7.8 ft3 /min (0.22 m3 /min). An ejector capable of handling up to 10 ft3 /min (0.32 m3 /min) would be a wise choice for this countercurrent barometric condenser. 4. Determine the pump head required Since a countercurrent barometric condenser operates at pressures below atmos- pheric, it assists the cooling-water pump by ‘‘sucking’’ the water into the condenser. The maximum assist that can be assumed is 0.75V, where V ⫽ design vacuum, inHg. In this condenser with a 26-in (88.0-kPa) vacuum, the maximum assist is 0.75(26) ⫽ 19.5 inHg (66.0 kPa). Converting to feet of water, using 1.0 inHg ⫽ 1.134 ft (3.4 kPa) of water, we find 19.5(1.134) ⫽ 22.1 ft (66.1 kPa) of water. The total head on the pump is then the sum of the static and friction heads less 0.75V, expressed in feet of water. Or, the total head on the pump ⫽ 40 ⫹ 15 ⫺ 22.1 ⫽ 32.9 ft (98.4 kPa). A pump with a total head of at least 35 ft (104.6 kPa) of water would be chosen for this condenser. Where corrosion or partial clogging of the piping is expected, a pump with a total head of 50 ft (149.4 kPa) would probably be chosen to ensure sufficient head even though the piping is partially clogged. Related Calculations. (1) When a condenser serving a steam engine is being chosen, use the appropriate dry-air leakage value from Table 6. (2) For ejector-jet barometric condensers, assume the final condensate temperature tc as 10 to 20⬚F (5.6 to 11.1⬚C) below the saturation temperature corresponding to the absolute pres- sure in the condenser. This type of condenser does not use an ejector, but it requires 25 to 50 percent more cooling water than the countercurrent barometric condenser for the same vacuum. (3) The total pump head for an ejector-jet barometric con- denser is the sum of the static and friction heads plus 10 ft (29.9 kPa). The addi- tional positive head is required to overcome the pressure loss in spray nozzles. COOLING-POND SIZE FOR A KNOWN HEAT LOAD How many spray nozzles and what surface area are needed to cool 10,000 gal/min (630.8 L/s) of water from 120 to 90⬚F (48.9 to 32.2⬚C) in a spray-type cooling pond if the average wet-bulb temperature is 650⬚F (15.6⬚C)? What would the ap- proximate dimensions of the cooling pond be? Determine the total pumping head if the static head is 10 ft (29.9 kPa), the pipe friction is 35 ft of water (104.6 kPa), and the nozzle pressure is 8 lb/in2 (55.2 kPa). Calculation Procedure: 1. Compute the number of nozzles required Assume a water flow of 50 gal/min (3.2 L/s) per nozzle; this is a typical flow rate for usual cooling-pond nozzles. Then the number of nozzles required ⫽ (10,000 gal/min)/(50 gal/min per nozzle) ⫽ 200 nozzles. If 6 nozzles are used in each spray group, a series of crossed arms, with each arm containing one or more noz- zles, then 200 nozzles/6 nozzles per spray group ⫽ 331 ⁄3 spray groups will be needed. Since a partial spray group is seldom used, 34 spray groups would be chosen. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 153. STEAM CONDENSING SYSTEMS AND AUXILIARIES 2.27 FIGURE 13 Spray-pond nozzle and piping layout. 2. Determine the surface area required Usual design practice is to provide 1 ft2 (0.09 m2 ) of pond area per 250 lb (113.4 kg) of water cooled for water quantities exceeding 1000 gal/min (63.1 L/s). Thus, in this pond, the weight of water cooled ⫽ (10,000 gal/min)(8.33 lb/gal)(60 min/ h) ⫽ 4,998,000, say 5,000,000 lb/h (630.0 kg/s). Then, the area required, given 1 ft2 of pond area per 250 lb of water (0.82 m2 per 1000 kg) cooled ⫽ 5,000,000/ 250 ⫽ 20,000 ft2 (1858.0 m2 ). As a cross-check, use another commonly accepted area value: 125 Btu/(ft2 䡠 ⬚F) [2555.2 kJ/(m2 䡠 ⬚C)] is the difference between the air wet-bulb temperature and the warm entering-water temperature. This is the equivalent of (120 ⫺ 60)(125) ⫽ 7500 Btu/ft2 (85,174 kJ/m2 ) in this spray pond, because the air wet-bulb temperature is 60⬚F (15.6⬚C) and the warm-water temperature is 120⬚F (48.9⬚C). The heat removed from the water is (lb/h of water)(temperature decrease, ⬚F)(specific heat of water) ⫽ (5,000,000)(120 ⫺ 90)(1.0) ⫽ 150,000,000 Btu/h (43,960.7 kW). Then, area required ⫽ (heat removed, Btu/h)/(heat removal, Btu/ft2 ) ⫽ 150,000,000/7500 ⫽ 20,000 ft2 (1858.0 m2 ). This checks the previously obtained area value. 3. Determine the spray-pond dimensions Spray groups on the same header or pipe main are usually arranged on about 12- ft (3.7-m) centers with the headers or pipe mains spaced on about 25-ft (7.6-m) centers, Fig. 13. Assume that 34 spray groups are used, instead of the required 331 ⁄3, to provide an equal number of groups in two headers and a small extra capacity. Sketch the spray pond and headers, Fig. 13. This shows that the length of each header will be about 204 ft (62.2 m) because there are seventeen 12-ft (3.7-m) spaces between spray groups in each header. Allowing 3 ft (0.9 m) at each end of a header for fittings and clean-outs gives an overall header length of 210 ft (64.0 m). The distance between headers is 25 ft (7.6 m). Allow 25 ft (7.6 m) between the outer sprays and the edge of the pond. This gives an overall width of 85 ft (25.9 m) for the pond, if we assume the width of each arm in a spray group is 10 ft (3.0 m). The overall length will then be 210 ⫹ 25 ⫹ 25 ⫽ 260 ft (79.2 m). A Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 154. 2.28 POWER GENERATION cold well for the pump suction and suitable valving for control of the incoming water must be provided, as shown in Fig 13. The water depth in the pond should be 2 to 3 ft (0.6 to 0.9 m). 4. Compute the total pumping head The total head, ft of water ⫽ static head ⫹ friction head ⫹ required nozzle head ⫽ 10 ⫹ 35 ⫹ 80(0.434) ⫽ 48.5 ft (145.0 kPa) of water. A pump having a total head of at least 50 ft (15.2 m) of water would be chosen for this spray pond. If future expansion of the pond is anticipated, compute the probable total head re- quired at a future date, and choose a pump to deliver that head. Until the pond is expanded, the pump would operate with a throttled discharge. Normal nozzle inlet pressures range from about 6 to 10 lb/in2 (41.4 to 69.0 kPa). Higher pressures should not be used, because there will be excessive spray loss and rapid wear of the nozzles. Related Calculations. Unsprayed cooling ponds cool 4 to 6 lb (1.8 to 2.7 kg) of water from 100 to 70⬚F/ft2 (598.0 to 418.6⬚C/m2 ) of water surface. An alter- native design rule is to assume that the pond will dissipate 3.5 Btu/ft2 䡠 h) (11.0 W/m2 ) water surface per degree difference between the wet-bulb temperature of the air and the entering warm water. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM CONDENSING SYSTEMS AND AUXILIARIES
- 155. 3.1 SECTION 3 COMBUSTION Combustion Calculations Using the Million BTU (1.055 MJ) Method 3.1 Savings Produced by Preheating Combustion Air 3.4 Combustion of Coal Fuel in a Furnace 3.5 Percent Excess Air While Burning Coke 3.8 Combustion of Fuel Oil in a Furnace 3.9 Combustion of Natural Gas in a Furnace 3.11 Combustion of Wood Fuel in a Furnace 3.17 Molal Method of Combustion Analysis 3.19 Final Combustion Products Temperature Estimate 3.22 COMBUSTION CALCULATIONS USING THE MILLION BTU (1.055MJ) METHOD The energy absorbed by a steam boiler fired by natural gas is 100-million Btu/hr (29.3 MW). Boiler efficiency on a higher heating value (HHV) basis is 83 percent. If 15 percent excess air is used, determine the total air and flue-gas quantities produced. The approximate HHV of the natural gas is 23,000 Btu/lb (53,590 kJ/ kg). Ambient air temperature is 80⬚F (26.7⬚C) and relative humidity is 65 percent. How can quick estimates be made of air and flue-gas quantities in boiler operations when the fuel analysis is not known? Calculation Procedure: 1. Determine the energy input to the boiler The million Btu (1.055MJ) method combustion calculations is a quick way of estimating air and flue-gas quantities generated in boiler and heater operations when the ultimate fuel analysis is not available and all the engineer is interested in is good estimates. Air and flue-gas quantities determined may be used to calculate the size of fans, ducts, stacks, etc. It can be shown through comprehensive calculations that each fuel such as coal, oil, natural gas, bagasse, blast-furnace gas, etc. requires a certain amount of dry stochiometric air per million Btu (1.055MJ) fired on an HHV basis and that this quantity does not vary much with the fuel analysis. The listing below gives the dry air required per million Btu (1.055MJ) of fuel fired on an HHV basis for various fuels. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 156. 3.2 POWER GENERATION Combustion Constants for Fuels Fuel Constant, lb dry air per million Btu (kg/MW) Blast furnace gas 575 (890.95) Bagasse 650 (1007.2) Carbon monoxide gas 670 (1038.2) Refinery and oil gas 720 (1115.6) Natural gas 730 (1131.1) Furnace oil and lignite 745–750 (1154.4–1162.1) Bituminous coals 760 (1177.6) Anthracite coal 780 (1208.6) Coke 800 (1239.5) To determine the energy input to the boiler, use the relation Qf ⫽ (Qs)/Eh, where energy input by the fuel, Btu/hr (W); Qs ⫽ energy absorbed by the steam in the boiler, Btu/Hr (W); Qs ⫽ energy absorbed by the steam, Btu/hr (W); Eh ⫽ effi- ciency of the boiler on an HHV basis. Substituting for this boiler, Qf ⫽ 100/ 0.83 ⫽ 120.48 million Btu/hr on an HHV basis (35.16 MW). 2. Estimate the quantity of dry air required by this boiler The total air required Ta ⫽ (Qf)(Fuel constant from list above). For natural gas, Ta ⫽ (120.48)(730) ⫽ 87,950 lb/hr (39,929 kg/hr). With 15 percent excess air, total air required ⫽ (1.15)(87,950) ⫽ 101,142.5 lb/hr (45,918.7 kg/hr). 3. Compute the quantity of wet air required Air has some moisture because of its relative humidity. Estimate the amount of moisture in dry air in M lb/lb (kg/kg) from, M ⫽ 0.622 (pw)/(14.7 ⫺ pw), where 0.622 is the ratio of the molecular weights of water vapor and dry air; pw ⫽ partial pressure of water vapor in the air, psia (kPa) ⫽ saturated vapor pressure (SVP) ⫻ relative humidity expressed as a decimal; 14.7 ⫽ atmospheric pressure of air at sea level (101.3 kPa). From the steam tables, at 80 F (26.7 C), SVP ⫽ 0.5069 psia (3.49 kPa). Substituting, M ⫽ 0.622 (0.5069 ⫻ 0.65)/(14.7 ⫺ [0.5069 ⫻ 0.65]) ⫽ 0.01425 lb of moisture/lb of dry air (0.01425 kg/kg). The total flow rate of the wet air then ⫽ 1.0142 (101,142.5) ⫽ 102,578.7 lb/hr (46,570.7 kg/hr). To convert to a volumetric-flow basis, recall that the density of air at 80⬚F (26.7⬚C) and 14.7 psia (101.3 kPa) ⫽ 39/(480 ⫹ 80) ⫽ 0.0722 lb/cu ft (1.155 kg/cu m). In this relation, 39 ⫽ a constant and the temperature of the air is converted to degrees Rankine. Hence, the volumetric flow ⫽ 102,578.7/(60 min/hr)(0.0722) ⫽ 23,679.3 actual cfm (670.1 cm m/min). 4. Estimate the rate of fuel firing and flue-gas produced The rate of fuel firing ⫽ Qf /HHV ⫽ (120.48 ⫻ 106 )/23,000 ⫽ 5238 lb/hr (2378 kg/hr). Hence, the total flue gas produced ⫽ 5238 ⫹ 102,578 ⫽ 107,816 lb/hr (48,948 kg/hr). If the temperature of the flue gas is 400⬚F (204.4⬚C) (a typical value for a natural- gas fired boiler), then the density, as in Step 3 is: 39/(400 ⫹ 460) ⫽ 0.04535 lb/ cu ft (0.7256 kg/cu m). Hence, the volumetric flow ⫽ (107,816)/(60 min/hr ⫻ 0.04535) ⫽ 39,623.7 actual cfm (1121.3 cu m/min). Related Calculations. Detailed combustion calculations based on actual fuel gas analysis can be performed to verify the constants given in the list above. For example, let us say that the natural-gas analysis was: Methane ⫽ 83.4 percent; Ethane ⫽ 15.8 percent; Nitrogen ⫽ 0.8 percent by volume. First convert the analysis to a percent weight basis: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 157. COMBUSTION 3.3 Fuel Percent volume MW Col. 2 ⫻ Col. 3 Percent weight Methane 83.4 16 1334.4 72.89 Ethane 15.8 30 474 25.89 Nitrogen 0.8 28 22.4 1.22 Note that the percent weight in the above list is calculated after obtaining the sum under Column 2 ⫻ Column 3. Thus, the percent methane ⫽ (1334.4)/(1334.4 ⫹ 474 ⫹ 22.4) ⫽ 72.89 percent. From a standard reference, such as Ganapathy, Steam Plant Calculations Man- ual, Marcel Dekker, Inc., find the combustion constants, K, for various fuels and use them thus: For the air required for combustion, Ac ⫽ (K for methane)(percent by weight methane from above list) ⫹ (K for ethane)(percent by weight ethane); or Ac ⫽ (17.265)(0.7289) ⫹ (16.119)(0.2589) ⫽ 16.76 lb/lb (16.76 kg/kg). Next, compute the higher heating value of the fuel (HHV) using the air constants from the same reference mentioned above. Or HHV ⫽ (heat of combustion for methane)(percent by weight methane) ⫹ (heat of combustion of ethane)(percent by weight ethane) ⫽ (23,879)(0.7289) ⫹ (22,320)(0.2589) ⫽ 23,184 Btu/lb (54,018.7 kJ/kg). Then, the amount of fuel equivalent to 1,000,000 Btu (1,055,000 kJ) ⫽ (1,000,000)/23,184 ⫽ 43.1 lb (19.56 kg), which requires, as computed above, (43.1)(16.76) ⫽ 722.3 lb dry air (327.9 kg), which agrees closely with the value given in Step 1, above. Similarly, if the fuel were 100 percent methane, using the steps given above, and suitable constants from the same reference work, the air required for combus- tion is 17.265 lb/lb (7.838 kg/kg) of fuel. HHV ⫽ 23,879 Btu/lb (55,638 kJ/kg). Hence, the fuel in 1,000,000 Btu (1,055,000 kJ) ⫽ (1,000,000)/(23,879) ⫽ 41.88 lb (19.01 kg). Then, the dry air per million Btu (1.055 kg) fired ⫽ (17.265) (41.88) ⫽ 723 lb (328.3 kg). Likewise, for propane, using the same procedure, 1 lb (0.454 kg) requires 15.703 lb (7.129 kg) air and 1 million Btu (1,055,000 kJ) has (1,000,000)/21,661 ⫽ 46.17 lb (20.95 kg) fuel. Then, 1 million Btu (1,055,000 kJ) requires (15.703)(46.17) ⫽ 725 lb (329.2 kg) air. This general approach can be used for various fuel oils and solid fuels—coal, coke, etc. Good estimates of excess air used in combustion processes may be obtained if the oxygen and nitrogen in dry flue gases are measured. Knowledge of excess air amounts helps in performing detailed combustion and boiler efficiency calculations. Percent excess air, EA ⫽ 100(O2–CO2)/[0.264 ⫻ N2–(O2–CO/2)], where O2 ⫽ oxygen in the dry flue gas, percent volume; CO ⫽ percent volume carbon mon- oxide; N2 ⫽ percent volume nitrogen. You can also estimate excess air from oxygen readings. Use the relation, EA ⫽ (constant from list below)((O2)/(21–O2). Constants for Excess Air Calculations Fuel Constant Carbon 100 Hydrogen 80 Carbon monoxide 121 Sulfur 100 Methane 90 Oil 94.5 Coal 97 Blast furnace gas 223 Coke oven gas 89.3 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 158. 3.4 POWER GENERATION If the percent volume of oxygen measured is 3 on a dry basis in a natural-gas (methane) fired boiler, the excess air, EA ⫽ (90)[3/(21–3)] ⫽ 15 percent. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries. SAVINGS PRODUCED BY PREHEATING COMBUSTION AIR A 20,000 sq ft (1858 sq m) building has a calculated total seasonal heating load of 2,534,440 MBH (thousand Btu) (2674 MJ). The stack temperature is 600⬚F (316⬚C) and the boiler efficiency is calculated to be 75 percent. Fuel oil burned has a higher heating value of 140,000 Btu/gal (39,018 MJ/L). A preheater can be purchased and installed to reduce the breeching discharge combustion air temper- ature by 250⬚F (139⬚C) to 350⬚F (177⬚C) and provide the burner with preheated air. How much fuel oil will be saved? What will be the monetary saving if fuel oil is priced at 80 cents per gallon? Calculation Procedure: 1. Compute the total combustion air required by this boiler A general rule used by design engineers is that 1 cu ft (0.0283 cu m) of combustion air is required for each 100 Btu (105.5 J) released during combustion. To compute the combustion air required, use the relation CA ⫽ H/100 ⫻ Boiler efficiency, expressed as a decimal, where CA ⫽ annual volume of combustion air, cu ft (cu m); H ⫽ total seasonal heating load, Btu/yr (kJ/yr). Substituting for this boiler, CA ⫽ (2,534,400)(1000)/100 ⫻ 0.75 ⫽ 33,792,533 cu ft/yr (956.329 cu m). 2. Calculate the annual energy savings The energy savings, ES ⫽ (stack temperature reduction, deg F)(cu ft air per yr)(0.018), where the constant 0.018 is the specific heat of air. Substituting, ES ⫽ (250)(33,792,533)(0.018) ⫽ 152,066,399 Btu/yr (160,430 kJ/yr). With a boiler efficiency of 75 percent, each gallon of oil releases 0.75 ⫻ 140,000 Btu/gal ⫽ 105,000 Btu (110.8 jk). Hence, the fuel saved, FS ⫽ ES/usuable heat in fuel, Btu/gal. Or, FS ⫽ 152,066,399/105,000 ⫽ 1448.3 gal/yr (5.48 cu m/yr). With fuel oil at $1.10 per gallon, the monetary savings will be $1.10 (1448.3) ⫽ $1593.13. If the preheater cost $6000, the simple payoff time would be $6000/ 1593.13 ⫽ 3.77 years. Related Calculations. Use this procedure to determine the potential savings for burning any type of fuel—coal, oil, natural gas, landfill gas, catalytic cracker offgas, hydrogen purge gas, bagesse, sugar cane, etc. Other rules of thumb used by designers to estimate the amount of combustion air required for various fuels are: 10 cu ft of air (0.283 cu m) per 1 cu ft (0.0283 cu m) of natural gas; 1300 cu ft of air (36.8 cu m) per gal (0.003785 cu m) of No. 2 fuel oil; 1450 cu ft of air (41 cu m) per gal of No. 5 fuel oil; 1500 cu ft of air (42.5 cu m) per gal of No. 6 fuel oil. These values agree with that used in the above computation—i.e. 100 cu ft per 100 Btu of 140,000 Btu per gal oil ⫽ 140,000/100 ⫽ 1400 cu ft per gal (39.6 cu m/0.003785 cu m). This procedure is the work of Jerome F. Mueller, P.E. of Mueller Engineering Corp. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 159. COMBUSTION 3.5 COMBUSTION OF COAL FUEL IN A FURNACE A coal has the following ultimate analysis (or percent by weight): C ⫽ 0.8339; H2 ⫽ 0.0456; O2 ⫽ 0.0505; N2 ⫽ 0.0103; S ⫽ 0.0064; ash ⫽ 0.0533; total ⫽ 1.000 lb (0.45 kg). This coal is burned in a steam-boiler furnace. Determine the weight of air required for theoretically perfect combustion, the weight of gas formed per pound (kilogram) of coal burned, and the volume of flue gas, at the boiler exit temperature of 600⬚F (316⬚C) per pound (kilogram) of coal burned; air required with 20 percent excess air, and the volume of gas formed with this excess; the CO2 percentage in the flue gas on a dry and wet basis. Calculation Procedure: 1. Compute the weight of oxygen required per pound of coal To find the weight of oxygen required for theoretically perfect combustion of coal, set up the following tabulation, based on the ultimate analysis of the coal: Note that of the total oxygen needed for combustion, 0.0505 lb (0.023 kg), is furnished by the fuel itself and is assumed to reduce the total external oxygen required by the amount of oxygen present in the fuel. The molecular-weight ratio is obtained from the equation for the chemical reaction of the element with oxygen in combustion. Thus, for carbon C ⫹ O2 → CO2, or 12 ⫹ 32 ⫽ 44, where 12 and 32 are the molecular weights of C and O2, respectively. 2. Compute the weight of air required for perfect combustion Air at sea level is a mechanical mixture of various gases, principally 23.2 percent oxygen and 76.8 percent nitrogen by weight. The nitrogen associated with the 2.5444 lb (1.154 kg) of oxygen required per pound (kilogram) of coal burned in this furnace is the product of the ratio of the nitrogen and oxygen weights in the air and 2.5444, or (2.5444)(0.768/0.232) ⫽ 8.4228 lb (3.820 kg). Then the weight of air required for perfect combustion of 1 lb (0.45 kg) of coal ⫽ sum of nitrogen and oxygen required ⫽ 8.4228 ⫹ 2.5444 ⫽ 10.9672 lb (4.975 kg) of air per pound (kilogram) of coal burned. 3. Compute the weight of the products of combustion Find the products of combustion by addition: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 160. 3.6 POWER GENERATION 4. Convert the flue-gas weight to volume Use Avogadro’s law, which states that under the same conditions of pressure and temperature, 1 mol (the molecular weight of a gas expressed in lb) of any gas will occupy the same volume. At 14.7 lb/in2 (abs) (101.3 kPa) and 32 ⬚F (0⬚C), 1 mol of any gas occupies 359 ft3 (10.2 m3 ). The volume per pound of any gas at these conditions can be found by dividing 359 by the molecular weight of the gas and correcting for the gas temperature by multiplying the volume by the ratio of the absolute flue-gas temperature and the atmospheric temperature. To change the weight analysis (step 3) of the products of combustion to volumetric analysis, set up the calculation thus: In this calculation, the temperature correction factor 2.15 ⫽ absolute flue-gas tem- perature, ⬚R/absolute atmospheric temperature, ⬚R ⫽ (600 ⫹ 460)/(32 ⫹ 460). The total weight of N2 in the flue gas is the sum of the N2 in the combustion air and the fuel, or 8.4228 ⫹ 0.0103 ⫽ 8.4331 lb (3.8252 kg). The value is used in com- puting the flue-gas volume. 5. Compute the CO2 content of the flue gas The volume of CO2 in the products of combustion at 600⬚F (316⬚C) is 53.6 ft3 (1.158 m3 ), as computed in step 4; and the total volume of the combustion products is 303.85 ft3 (8.604 m3 ). Therefore, the percent CO2 on a wet basis (i.e., including the moisture in the combustion products) ⫽ ft3 CO2 /total ft3 ⫽ 53.6/303.85 ⫽ 0.1764, or 17.64 percent. The percent CO2 on a dry, or Orsat, basis is found in the same manner, except that the weight of H2O in the products of combustion, 17.6 lb (7.83 kg) from step 4, is subtracted from the total gas weight. Or, percent CO2, dry, or Orsat basis ⫽ (53.6)/(303.85 ⫺ 17.6) ⫽ 0.1872, or 18.72 percent. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 161. COMBUSTION 3.7 6. Compute the air required with the stated excess flow With 20 percent excess air, the air flow required ⫽ (0.20 ⫹ 1.00)(air flow with no excess) ⫽ 1.20 (10.9672) ⫽ 13.1606 lb (5.970 kg) of air per pound (kilogram) of coal burned. The air flow with no excess is obtained from step 2. 7. Compute the weight of the products of combustion The excess air passes through the furnace without taking part in the combustion and increases the weight of the products of combustion per pound (kilogram) of coal burned. Therefore, the weight of the products of combustion is the sum of the weight of the combustion products without the excess air and the product of (per- cent excess air)(air for perfect combustion, lb); or, given the weights from steps 3 and 2, respectively, ⫽ 11.9139 ⫹ (0.20)(10.9672) ⫽ 14.1073 lb (6.399 kg) of gas per pound (kilogram) of coal burned with 20 percent excess air. 8. Compute the volume of the combustion products and the percent CO2 The volume of the excess air in the products of combustion is obtained by con- verting from the weight analysis to the volumetric analysis and correcting for tem- perature as in step 4, using the air weight from step 2 for perfect combustion and the excess-air percentage, or (10.9672)(0.20)(359/28.95)(2.15) ⫽ 58.5 ft3 (1.656 m3 ). In this calculation the value 28.95 is the molecular weight of air. The total volume of the products of combustion is the sum of the column for perfect com- bustion, step 4, and the excess-air volume, above, or 303.85 ⫹ 58.5 ⫽ 362.35 ft3 (10.261 m3 ). By using the procedure in step 5, the percent CO2, wet basis ⫽ 53.6/362.35 ⫽ 14.8 percent. The percent CO2, dry basis ⫽ 53.8/(362.35 ⫺ 17.6) ⫽ 15.6 percent. Related Calculations. Use the method given here when making combustion calculations for any type of coal—bituminous, semibituminous, lignite, anthracite, cannel, or cooking—from any coal field in the world used in any type of furnace—boiler, heater, process, or waste-heat. When the air used for combustion contains moisture, as is usually true, this moisture is added to the combustion- formed moisture appearing in the products of combustion. Thus, for 80⬚F (26.7⬚C) air of 60 percent relative humidity, the moisture content is 0.013 lb/lb (0.006 kg/ kg) of dry air. This amount appears in the products of combustion for each pound of air used and is a commonly assumed standard in combustion calculations. Fossil-fuel-fired power plants release sulfur emissions to the atmosphere. In turn, this produces sulfates, which are the key ingredient in acid rain. The federal Clean Air Act regulates sulfur dioxide emissions from power plants. Electric utilities which burn high-sulfur coal are thought to produce some 35 percent of atmospheric emissions of sulfur dioxide in the United States. Sulfur dioxide emissions by power plants have declined some 30 percent since passage of the Clean Air Act in 1970, and a notable decline in acid rain has been noted at a number of test sites. In 1990 the Acid Rain Control Program was created by amendments to the Clean Air Act. This program further reduces the allowable sulfur dioxide emissions from power plants, steel mills, and other industrial facil- ities. The same act requires reduction in nitrogen oxide emissions from power plants and industrial facilities, so designers must keep this requirement in mind when designing new and replacement facilities of all types which use fossil fuels. Coal usage in steam plants is increasing throughout the world. An excellent example of this is the New England Electric System (NEES). This utility has been converting boiler units from oil to coal firing. Their conversions have saved cus- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 162. 3.8 POWER GENERATION FIGURE 1 Energy Independence transports coal to central stations. (Power.) tomers more than $60-million annually by displacing about 14-million bbl (2.2 million cu m) of oil per year. To reduce costs, the company built the first coal-fired collier, Fig. 1, in more than 50 years in the United States, and assumed responsibility for coal transpor- tation to its stations, cutting operating costs by more than $2-million per year. The collier makes economic sense because the utility stations in the system are not accessible by rail. This ship, the Energy Independence, has been an economic suc- cess for the utility. Measuring 665 ft (203 m) long by 95 ft (29 m) wide by 56 ft (17 m) deep with a 34-ft (10-m) draft, the vessel discharges a typical 40,000-ton load in 12 hours. Data in these two paragraphs and Fig. 1 are from Power magazine. PERCENT EXCESS AIR WHILE BURNING COAL A certain coal has the following composition by weight percentages: carbon 75.09, nitrogen 1.56, ash 3.38, hydrogen, 5.72, oxygen 13.82, sulfur 0.43. When burned in an actual furnace, measurements showed that there was 8.93 percent combustible in the ash pit refuse and the following Orsat analysis in percentages was obtained: carbon dioxide 14.2, oxygen 4.7, carbon monoxide 0.3. If it can be assumed that there was no combustible in the flue gas other that the carbon monoxide reported, calculate the percentage of excess air used. Calculation Procedure: 1. Compute the amount of theoretical air required per lbm (kg) of coal Theoretical air required per pound (kilogram) of coal, wta ⫽ 11.5C⬘ ⫹ 34.5[H⬘2 ⫺ O⬘2 /8)] ⫹ 4.32S⬘, where C⬘, H⬘2, O⬘2, and S⬘ represent the percentages by weight, expressed as decimal fractions, of carbon, hydrogen, oxygen, and sulfur, respec- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 163. COMBUSTION 3.9 tively. Thus, wta ⫽ 11.5(0.7509) ⫹ 34.5[0.0572 ⫺ (0.1382/8)] ⫹ 4.32(0.0043) ⫽ 10.03 lb (4.55 kg) of air per lb (kg) of coal. The ash and nitrogen are inert and do not burn. 2. Compute the correction factor for combustible in the ash The correction factor for combustible in the ash, C1 ⫽ (wfCf ⫺ wrCr)/(wf ⫻ 100), where the amount of fuel, wf ⫽ 1 lb (0.45 kg) of coal; percent by weight, expressed as a decimal fraction, of carbon in the coal, Cf ⫽ 75.09; percent by weight of the ash and refuse in the coal, wr ⫽ 0.0338; percent by weight of combustible in the ash, Cr ⫽ 8.93. Hence, C1 ⫽ [(1 ⫻ 75.09) ⫺ (0.0338 ⫻ 8.93)]/(1 ⫻ 100) ⫽ 0.748. 3. Compute the amount of dry flue gas produced per lb (kg) of coal The lb (kg) of dry flue gas per lb (kg) of coal, wdg ⫽ C1(4CO2 ⫹ O2 ⫹ 704)/ [3(CO2 ⫹ CO)], where the Orsat analysis percentages are for carbon dioxide, CO2 ⫽ 14.2; oxygen, O2 ⫽ 4.7; carbon monoxide, CO ⫽ 0.3. Hence, wdg ⫽ 0.748 ⫻ [(4 ⫻ 14.2) ⫹ 4.7 ⫹ 704)]/[3(14.2 ⫹ 0.3)] ⫽ 13.16 lb/lb (5.97 kg/kg). 4. Compute the amount of dry air supplied per lb (kg) of coal The lb (kg) of dry air supplied per lb (kg) of coal, wda ⫽ wdg ⫺ C1 ⫹ 8[H⬘2 ⫺ (O⬘2 /8)] ⫺ (N⬘2 /N), where the percentage by weight of nitrogen in the fuel, N⬘2 ⫽ 1.56, and ‘‘atmospheric nitrogen’’ in the supply air, N2 ⫽ 0.768; other values are as given or calculated. Then, wda ⫽ 13.16 ⫺ 0.748 ⫹ 8[0.0572 ⫺ (0.1382/8)] ⫺ (0.0156/0.768) ⫽ 12.65 lb/lb (5.74 kg/kg). 5. Compute the percent of excess air used Percent excess air ⫽ (wda ⫺ wta)/wta ⫽ (12.65 ⫺ 10.03)/10.03 ⫽ 0.261, or 26.1 percent. Related Calculations. The percentage by weight of nitrogen in ‘‘atmospheric air’’ in step 4 appears in Principles of Engineering Thermodynamics, 2nd edition, by Kiefer et al., John Wiley Sons, Inc. COMBUSTION OF FUEL OIL IN A FURNACE A fuel oil has the following ultimate analysis: C ⫽ 0.8543; H2 ⫽ 0.1131; O2 ⫽ 0.0270; N2 ⫽ 0.0022; S ⫽ 0.0034; total ⫽ 1.0000. This fuel oil is burned in a steam-boiler furnace. Determine the weight of air required for theoretically perfect combustion, the weight of gas formed per pound (kilogram) of oil burned, and the volume of flue gas, at the boiler exit temperature of 600⬚F (316⬚C), per pound (kilogram) of oil burned; the air required with 20 percent excess air, and the volume of gas formed with this excess; the CO2 percentage in the flue gas on a dry and wet basis. Calculation Procedure: 1. Compute the weight of oxygen required per pound (kilogram) of oil The same general steps as given in the previous calculation procedure will be followed. Consult that procedure for a complete explanation of each step. Using the molecular weight of each element, we find Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 164. 3.10 POWER GENERATION 2. Compute the weight of air required for perfect combustion The weight of nitrogen associated with the required oxygen ⫽ (3.1593)(0.768/ 0.232) ⫽ 10.458 lb (4.706 kg). The weight of air required ⫽ 10.4583 ⫹ 3.1593 ⫽ 13.6176 lb/lb (6.128 kg/kg) of oil burned. 3. Compute the weight of the products of combustion As before, 4. Convert the flue-gas weight to volume As before, In this calculation, the temperature correction factor 2.15 ⫽ absolute flue-gas temperature, ⬚R/absolute atmospheric temperature, ⬚R ⫽ (600 ⫹ 460)/(32 ⫹ 460). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 165. COMBUSTION 3.11 The total weight of N2 in the flue gas is the sum of the N2 in the combustion air and the fuel, or 10.4580 ⫹ 0.0022 ⫽ 10.4602 lb (4.707 kg). 5. Compute the CO2 content of the flue gas CO2, wet basis, ⫽ 55.0/387.82 ⫽ 0.142, or 14.2 percent. CO2, dry basis, ⫽ 55.0/ (387.2 ⫺ 43.5) ⫽ 0.160, or 16.0 percent. 6. Compute the air required with stated excess flow The pounds (kilograms) of air per pound (kilogram) of oil with 20 percent excess air ⫽ (1.20)(13.6176) ⫽ 16.3411 lb (7.353 kg) of air per pound (kilogram) of oil burned. 7. Compute the weight of the products of combustion The weight of the products of combustion ⫽ product weight for perfect combustion, lb ⫹ (percent excess air)(air for perfect combustion, lb) ⫽ 14.6173 ⫹ (0.20)(13.6176) ⫽ 17.3408 lb (7.803 kilogram) of flue gas per pound (kilogram) of oil burned with 20 percent excess air. 8. Compute the volume of the combustion products and the percent CO2 The volume of excess air in the products of combustion is found by converting from the weight to the volumetric analysis and correcting for temperature as in step 4, using the air weight from step 2 for perfect combustion and the excess-air per- centage, or (13.6176)(0.20)(359/28.95)(2.15) ⫽ 72.7 ft3 (2.058 m3 ). Add this to the volume of the products of combustion found in step 4, or 387.82 ⫹ 72.70 ⫽ 460.52 ft3 (13.037 m3 ). By using the procedure in step 5, the percent CO2, wet basis ⫽ 55.0/460.52 ⫽ 0.1192, or 11.92 percent. The percent CO2, dry basis ⫽ 55.0/(460.52 ⫺ 43.5) ⫽ 0.1318, or 13.18 percent. Related Calculations. Use the method given here when making combustion calculations for any type of fuel oil—paraffin-base, asphalt-base, Bunker C, no. 2, 3, 4, or 5—from any source, domestic or foreign, in any type of furnace—boiler, heater, process, or waste-heat. When the air used for combustion contains moisture, as is usually true, this moisture is added to the combustion-formed moisture ap- pearing in the products of combustion. Thus, for 80⬚F (26.7⬚C) air of 60 percent relative humidity, the moisture content is 0.013 lb/lb (0.006 kg/kg) of dry air. This amount appears in the products of combustion for each pound (kilogram) of air used and is a commonly assumed standard in combustion calculations. COMBUSTION OF NATURAL GAS IN A FURNACE A natural gas has the following volumetric analysis at 60⬚F (15.5⬚C): CO2 ⫽ 0.004; CH4 ⫽ 0.921; C2H6 ⫽ 0.041; N2 ⫽ 0.034; total ⫽ 1.000. This natural gas is burned in a steam-boiler furnace. Determine the weight of air required for theoretically perfect combustion, the weight of gas formed per pound of natural gas burned, and the volume of the flue gas, at the boiler exit temperature of 650⬚F (343⬚C), per pound (kilogram) of natural gas burned; air required with 20 percent excess air, and the volume of gas formed with this excess; CO2 percentage in the flue gas on a dry and wet basis. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 166. 3.12 POWER GENERATION Calculation Procedure: 1. Compute the weight of oxygen required per pound of gas The same general steps as given in the previous calculation procedures will be followed, except that they will be altered to make allowances for the differences between natural gas and coal. The composition of the gas is given on a volumetric basis, which is the usual way of expressing a fuel-gas analysis. To use the volumetric-analysis data in com- bustion calculations, they must be converted to a weight basis. This is done by dividing the weight of each component by the total weight of the gas. A volume of 1 ft3 (1 m3 ) of the gas is used for this computation. Find the weight of each component and the total weight of 1 ft3 (1 m3 ) as follows, using the properties of the combustion elements and compounds given in Table 1: The sum of the weight percentages ⫽ 1.03 ⫹ 86.25 ⫹ 7.18 ⫹ 5.54 ⫽ 100.00. This sum checks the accuracy of the weight calculation, because the sum of the weights of the component parts should equal 100 percent. Next, find the oxygen required for combustion. Since both the CO2 and N2 are inert, they do not take part in the combustion; they pass through the furnace un- changed. Using the molecular weights of the remaining components in the gas and the weight percentages, we have In this calculation, the molecular-weight ratio is obtained from the equation for the combustion chemical reaction, or CH4 ⫹ 2O2 ⫽ CO2 ⫹ 2H2O, that is, 16 ⫹ 64 ⫽ 44 ⫹ 36, and C2H6 ⫹ 7 ⁄2O2 ⫽ 2CO2 ⫹ 3H2O, that is 30 ⫹ 112 ⫽ 88 ⫹ 54. See Table 2 from these and other useful chemical reactions in combustion. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 167. 3.13 TABLE 1 Properties of Combustion Elements* Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 168. 3.14 TABLE 2 Chemical Reactions Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 169. COMBUSTION 3.15 2. Compute the weight of air required for perfect combustion The weight of nitrogen associated with the required oxygen ⫽ (3.742)(0.768/0.232) ⫽ 12.39 lb (5.576 kg). The weight of air required ⫽ 12.39 ⫹ 3.742 ⫽ 16.132 lb/ lb (7.259 kg/kg) of gas burned. 3. Compute the weight or the products of combustion 4. Convert the flue-gas weight to volume The products of complete combustion of any fuel that does not contain sulfur are CO2, H2O, and N2. Using the combustion equation in step 1, compute the products of combustion thus: CH4 ⫹ 2O2 ⫽ CO2 ⫹ H2O; 16 ⫹ 64 ⫽ 44 ⫹ 36; or the CH4 burns to CO2 in the ratio of 1 part CH4 to 44/16 parts CO2. Since, from step 1, there is 0.03896 lb CH4 per ft3 (0.624 kg/m3 ) of natural gas, this forms (0.03896)(44/16) ⫽ 0.1069 lb (0.048 kg) of CO2. Likewise, for C2H6, (0.003247)(88/30) ⫽ 0.00952 lb (0.004 kg). The total CO2 in the combustion prod- ucts ⫽ 0.00464 ⫹ 0.1069 ⫹ 0.00952 ⫽ 0.11688 lb (0.053 kg), where the first quantity is the CO2 in the fuel. Using a similar procedure for the H2O formed in the products of combustion by CH4, we find (0.03896)(36/16) ⫽ 0.0875 lb (0.039 kg). For C2H6, (0.003247)(54/ 30) ⫽ 0.005816 lb (0.003 kg). The total H2O in the combustion products ⫽ 0.0875 ⫹ 0.005816 ⫽ 0.093316 lb (0.042 kg). Step 2 shows that 12.39 lb (5.58 kg) of N2 is required per lb (kg) of fuel. Since 1 ft3 (0.028 m3 ) of the fuel weights 0.04517 lb (0.02 kg), the volume of gas which weighs 1 lb (2.2 kg) is 1/0.04517 ⫽ 22.1 ft3 (0.626 m3 ). Therefore, the weight of N2 per ft3 of fuel burned ⫽ 12.39/22.1 ⫽ 0.560 lb (0.252 kg). This, plus the weight of N2 in the fuel, step 1, is 0.560 ⫹ 0.0025 ⫽ 0.5625 lb (0.253 kg) of N2 in the products of combustion. Next, find the total weight of the products of combustion by taking the sum of the CO2, H2O, and N2 weights, or 0.11688 ⫹ 0.09332 ⫹ 0.5625 ⫽ 0.7727 lb (0.35 kg). Now convert each weight to ft3 at 650⬚F (343⬚C), the temperature of the com- bustion products, or: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 170. 3.16 POWER GENERATION In this calculation, the value of 379 is used in the molecular-weight ratio because at 60⬚F (15.6⬚C) and 14.7 lb/in2 (abs) (101.3 kPa), the volume of 1 lb (0.45 kg) of any gas ⫽ 379/gas molecular weight. The fuel gas used is initially at 60⬚F (15.6⬚C) and 14.7 lb/in2 (abs) (101.3 kPa). The ratio 2.255 ⫽ (650 ⫹ 460)/(32 ⫹ 460). 5. Compute the CO2 content of the flue gas CO2, wet basis ⫽ 2.265/23.88 ⫽ 0.947, or 9.47 percent. CO2 dry basis ⫽ 2.265/ (23.88 ⫺ 4.425) ⫽ 0.1164, or 11.64 percent. 6. Compute the air required with the stated excess flow With 20 percent excess air, (1.20)(16.132) ⫽ 19.3584 lb of air per lb (8.71 kg/kg) of natural gas, or 19.3584/22.1 ⫽ 0.875 lb of air per ft3 (13.9 kg/m3 ) of natural gas. See step 4 for an explanation of the value 22.1. 7. Compute the weight of the products of combustion Weight of the products of combustion ⫽ product weight for perfect combustion, lb ⫹ (percent excess air) (air for perfect combustion, lb) ⫽ 16.80 ⫹ (0.20)(16.132) ⫽ 20.03 lb (9.01 kg). 8. Compute the volume of the combustion products and the percent CO2 The volume of excess air in the products of combustion is found by converting from the weight to the volumetric analysis and correcting for temperature as in step 4, using the air weight from step 2 for perfect combustion and the excess-air per- centage, or (16.132/22.1)(0.20)(379/28.95)(2.255) ⫽ 4.31 ft3 (0.122 m3 ). Add this to the volume of the products of combustion found in step 4, or 23.88 ⫹ 4.31 ⫽ 28.19 ft3 (0.798 m3 ). By the procedure in step 5, the percent CO2, wet basis ⫽ 2.265/28.19 ⫽ 0.0804, or 8.04 percent. The percent CO2, dry basis ⫽ 2.265/(28.19 ⫺ 4.425) ⫽ 0.0953, or 9.53 percent. Related Calculations. Use the method given here when making combustion calculations for any type of gas used as a fuel—natural gas, blast-furnace gas, coke- oven gas, producer gas, water gas, sewer gas—from any source, domestic or for- eign, in any type of furnace—boiler, heater, process, or waste-heat. When the air used for combustion contains moisture, as is usually true, this moisture is added to the combustion-formed moisture appearing in the products of combustion. Thus, for 80⬚F (26.7⬚C) air of 60 percent relative humidity, the moisture content is 0.013 lb/lb (0.006 kg/kg) of dry air. This amount appears in the products of combustion for each pound of air used and is a commonly assumed standard in combustion calculations. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 171. COMBUSTION 3.17 COMBUSTION OF WOOD FUEL IN A FURNACE The weight analysis of a yellow-pine wood fuel is: C ⫽ 0.490; H2 ⫽ 0.074; O2 ⫽ 0.406; N2 ⫽ 0.030. Determine the weight of oxygen and air required with perfect combustion and with 20 percent excess air. Find the weight and volume of the products of combustion under the same conditions, and the wet and dry CO2. The flue-gas temperature is 600⬚F (316⬚C). The air supplied for combustion has a mois- ture content of 0.013 lb/lb (0.006 kg/kg) of dry air. Calculation Procedure: 1. Compute the weight of oxygen required per pound of wood The same general steps as given in earlier calculation procedures will be followed; consult them for a complete explanation of each step. Using the molecular weight of each element, we have 2. Compute the weight of air required for complete combustion The weight of nitrogen associated with the required oxygen ⫽ (1.493)(0.768/0.232) ⫽ 4.95 lb (2.228 kg). The weight of air required ⫽ 4.95 ⫹ 1.493 ⫽ 6.443 lb/lb (2.899 kg/kg) of wood burned, if the air is dry. But the air contains 0.013 lb of moisture per lb (0.006 kg/kg) of air. Hence, the total weight of the air ⫽ 6.443 ⫹ (0.013)(6.443) ⫽ 6.527 lb (2.937 kg). 3. Compute the weight of the products of combustion Use the following relation: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 172. 3.18 POWER GENERATION 4. Convert the flue-gas weight to volume Use, as before, the following tabulation: In this calculation the temperature correction factor 2.15 ⫽ (absolute flue-gas tem- perature, ⬚R)/(absolute atmospheric temperature, ⬚R) ⫽ (600 ⫹ 460)/(32 ⫹ 460). The total weight of N2 is the sum of the N2 in the combustion air and the fuel. 5. Compute the CO2 content of the flue gas The CO2, wet basis ⫽ 31.5/233.2 ⫽ 0.135, or 13.5 percent. The CO2, dry basis ⫽ 31.5/(233.2 ⫺ 28.6 ⫺ 35.9) ⫽ 0.187, or 18.7 percent. 6. Compute the air required with the stated excess flow With 20 percent excess air, (1.20)(6.527) ⫽ 7.832 lb (3.524 kg) of air per lb (kg) of wood burned. 7. Compute the weight of the products of combustion The weight of the products of combustion ⫽ product weight for perfect combustion, lb ⫹ (percent excess air)(air for perfect combustion, lb) ⫽ 8.280 ⫹ (0.20)(6.527) ⫽ 9.585 lb (4.313 kg) of flue gas per lb (kg) of wood burned with 20 percent excess air. 8. Compute the volume of the combustion products and the percent CO2 The volume of the excess air in the products of combustion is found by converting from the weight to the volumetric analysis and correcting for temperature as in step 4, using the air weight from step 2 for perfect combustion and the excess-air per- centage, or (6.527)(0.20)(359/28.95)(2.15) ⫽ 34.8 ft3 (0.985 m3 ). Add this to the volume of the products of combustion found in step 4, or 233.2 ⫹ 34.8 ⫽ 268.0 ft3 (7.587 m3 ). By using the procedure in step 5, the percent CO2, wet basis ⫽ 31.5/268 ⫽ 0.1174, or 11.74 percent. The percent CO2, dry basis ⫽ 31.5/(268 ⫺ 28.6 ⫺ 35.9 ⫺ 0.20 ⫻ 0.837) ⫽ 0.155, or 15.5 percent. In the dry-basis calculation, the factor (0.20)(0.837) is the outside moisture in the excess air. Related Calculations. Use the method given here when making combustion calculations for any type of wood or woodlike fuel—spruce, cypress, maple, oak, sawdust, wood shavings, tanbark, bagesse, peat, charcoal, redwood, hemlock, fir, ash, birch, cottonwood, elm, hickory, walnut, chopped trimmings, hogged fuel, straw, corn, cottonseed hulls, city refuse—in any type of furnace—boiler, heating, process, or waste-heat. Most of these fuels contain a small amount of ash—usually less than 1 percent. This was ignored in this calculation procedure because it does not take part in the combustion. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 173. COMBUSTION 3.19 Industry is making greater use of discarded process waste to generate electricity and steam by burning the waste in a steam boiler. An excellent example is that of Agrilectric Power Partners Ltd., Lake Charles, LA. This plant burns rice hulls from its own process and buys other producers’ surplus rice hulls for continuous opera- tion. Their plant is reported as the first small-power-production facility to operate on rice hulls. By burning the waste rice hulls, Agrilectric is confronting, and solving, an en- vironmental nuisance often associated with rice processing. When rice hulls are disposed of by being spread on land adjacent to the mill, they often smolder, cre- ating continuous, uncontrolled burning. Installation of its rice-hull burning, electric- generating plant has helped Agrilectric avoid the costs associated with landfilling and disposal, as well as potential environmental problems. The boiler supplies steam for a turbine-generator with an output ranging from 11.2 to 11.8 MW. Excess power that cannot be used in the plant is sold to the local utility at a negotiated price. Thus, the combustion of an industrial waste is produc- ing useful power while eliminating the environmental impact of the waste. The advent of PURPA (Public Utility Regulatory Policies Act) requiring local utilities to purchase power from such plants has been a major factor in the design, devel- opment, and construction of many plants by food processors to utilize waste ma- terials for combustion and power production. MOLAL METHOD OF COMBUSTION ANALYSIS A coal fuel has this ultimate analysis: C ⫽ 0.8339; H2 ⫽ 0.0456; O2 ⫽ 0.0505; N2 ⫽ 0.0103; S ⫽ 0.0064; ash ⫽ 0.0533; total ⫽ 1.000. This coal is completely burned in a boiler furnace. Using the molal method, determine the weight of air required per lb (kg) of coal with complete combustion. How much air is needed with 25 percent excess air? What is the weight of the combustion products with 25 percent excess air? The combustion air contains 0.013 lb of moisture per lb (0.006 kg/kg) of air. Calculation Procedure: 1. Convert the ultimate analysis to moles A mole of any substance is an amount of the substance having a weight equal to the molecular weight of the substance. Thus, 1 mol of carbon is 12 lb (5.4 kg) of carbon, because the molecular weight of carbon is 12. To convert an ultimate anal- ysis of a fuel to moles, assume that 100 lb (45 kg) of the fuel is being considered. Set up a tabulation thus: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 174. 3.20 POWER GENERATION 2. Compute the mols of oxygen for complete combustion From Table 2, the burning of carbon to carbon dioxide requires 1 mol of carbon and 1 mol of oxygen, yielding 1 mol of CO2. Using the molal equations in Table 2 for the other elements in the fuel, set up a tabulation thus, entering the product of columns 2 and 3 in column 4: 3. Compute the moles of air for complete combustion Set up a similar tabulation for air, thus: In this tabulation, the factors in column 3 are constants used for computing the total moles of air required for complete combustion of each of the fuel elements listed. These factors are given in the Babcock Wilcox Company—Steam: Its Generation and Use and similar treatises on fuels and their combustion. A tabu- lation of these factors is given in Table 3. An alternative, and simpler, way of computing the moles of air required is to convert the required O2 to the corresponding N2 and find the sum of the O2 and N2. Or, 376O2 ⫽ N2; N2 ⫹ O2 ⫽ moles of air required. The factor 3.76 converts the required O2 to the corresponding N2. These two relations were used to convert the 0.158 mol of O2 in the above tabulation to moles of air. Using the same relations and the moles of O2 required from step 2, we get (3.76)(7.942) ⫽ 29.861 mol of N2. Then 29.861 ⫹ 7.942 ⫽ 37.803 mol of air, which agrees closely with the 37.823 mol computed in the tabulation. The differ- ence of 0.02 mol is traceable to roundings. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 175. COMBUSTION 3.21 TABLE 3 Molal Conversion Factors 4. Compute the air required with the stated excess air With 25 percent excess, the air required for combustion ⫽ (125/100)(37.823) ⫽ 47.24 mol. 5. Compute the mols of combustion products Using data from Table 3, and recalling that the products of combustion of a sulfur- containing fuel are CO2, H2O, and SO2, and that N2 and excess O2 pass through the furnace, set up a tabulation thus: In this calculation, the total moles of CO2 is obtained from step 2. The moles of H2 in 100 lb (45 kg) of the fuel, 2.280, is assumed to form H2O. In addition, the air from step 4, 47.24 mol, contains 0.013 lb of moisture per lb (0.006 kg/kg) of air. This moisture is converted to moles by dividing the molecular weight of air, 28.95, by the molecular weight of water, 18, and multiplying the result by the moisture content of the air, or (28.95/18)(0.013) ⫽ 0.0209, say 0.021 mol of water per mol of air. The product of this and the moles of air gives the total moles of Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 176. 3.22 POWER GENERATION moisture (water) in the combustion products per 100 lb (45 kg) of fuel fired. To this is added the moles of O2, 0.158, per 100 lb (45 kg) of fuel, because this oxygen is assumed to unite with hydrogen in the air to form water. The nitrogen in the products of combustion is that portion of the moles of air required, 47.24 mol from step 4, times the proportion of N2 in the air, or 0.79. The excess O2 passes through the furnace and adds to the combustion products and is computed as shown in the tabulation. Subtracting the total moisture, 3430 mol, from the total (or wet) com- bustion products gives the moles of dry combustion products. Related Calculations. Use this method for molal combustion calculations for all types of fuels—solid, liquid, and gaseous—burned in any type of furnance—boiler, heater, process, or waste-heat. Select the correct factors from Table 3. FINAL COMBUSTION PRODUCTS TEMPERATURE ESTIMATE Pure carbon is burned to carbon dioxide at constant pressure in an insulated cham- ber. An excess air quantity of 20 percent is used and the carbon and the air are both initially at 77⬚F (25⬚C). Assume that the reaction goes to completion and that there is no dissociation. Calculate the final product’s temperature using the follow- ing constants: Heating value of carbon, 14,087 Btu/lb (32.74 ⫻ 103 kJ/kg); con- stant-pressure specific heat of oxygen, nitrogen, and carbon dioxide are 0.240 Btu /lbm (0.558 kJ/kg), 0.285 Btu/lbm (0.662 kJ/kg), and 0.300 Btu/lb (0.697 kJ/kg), respectively. Calculation Procedure: 1. Establish the chemical equation for complete combustion with 100 percent air With 100 percent air: C ⫹ O2 ⫹ 3.78N2 → CO2 ⫹ 3.78N2, where approximate molecular weights are: for carbon, MC ⫽ 12; oxygen, MO2 ⫽ 32; nitrogen, MN2 ⫽ 28; carbon dioxide, MCO2 ⫽ 44. See the Related Calculations of this procedure for a general description of the 3.78 coefficient for N2. 2. Establish the chemical equation for complete combustion with 20 percent excess air With 20 percent excess air: C ⫹ 1.2 O2 ⫹ (1.2 ⫻ 3.78)N2 → CO2 ⫹ 0.2 O2 ⫹ (1.2 ⫻ 3.78)N2. 3. Compute the relative weights of the reactants and products of the combustion process Relative weight ⫽ moles ⫻ molecular weight. Coefficients of the chemical equation in step 2 represent the number of moles of each component. Hence, for the reac- tants, the relative weights are: for C ⫽ 1 ⫻ MC ⫽ 1 ⫻ 12 ⫽ 12; O2 ⫽ 1.2 ⫻ MO2 ⫽ 1.2 ⫻ 32 ⫽ 38.4; N2 ⫽ (1.2 ⫻ 3.78)MN2 ⫽ (1.2 ⫻ 3.78 ⫻ 28) ⫽ 127. For the products, relative weights are: for CO2 ⫽ 1 ⫻ MCO2 ⫽ 1 ⫻ 44 ⫽ 44; O2 ⫽ 0.2 ⫻ MO2 ⫽ 0.2 ⫻ 32 ⫽ 6.4; N2 ⫽ 127, unchanged. It should be noted that the total relative weight of the reactants equal that of the products at 177.4. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 177. COMBUSTION 3.23 4. Compute the relative weights of the products of combustion on the basis of a per unit relative weight of carbon Since the relative weight of carbon, C ⫽ 12 in step 3; hence, on the basis of a per unit relative weight of carbon, the corresponding relative weights of the products are: for carbon dioxide, wCO2 ⫽ MCO2 /12 ⫽ 44/12 ⫽ 3.667; oxygen, wO2 ⫽ MO2 /12 ⫽ 6.4/12 ⫽ 0.533; nitrogen, wN2 ⫽ MN2 /12 ⫽ 127/12 ⫽ 10.58. 5. Compute the final product’s temperature Since the combustion chamber is insulated, the combustion process is considered adiabatic. Hence, on the basis of a per unit mass of carbon, the heating value (HV) of the carbon ⫽ the corresponding heat content of the products. Thus, relative to a temperature base of 77⬚F (25⬚C), 1 ⫻ HVC ⫽ [(wCO2 ⫻ cpCO2) ⫹ (wO2 ⫻ cpO2) ⫹ (wH2 ⫹ cpN2)](t2 ⫺ 77), where the heating value of carbon, HVC ⫽ 14,087 Btu/lbm (32.74 ⫻ 103 kJ/kg); the constant-pressure specific heat of carbon dioxide, oxygen, and nitrogen are cpCO2 ⫽ 0.300 Btu/lb (0.697 kJ/kg), cpO2 ⫽ 0.240 Btu /lb (0.558 kJ/kg), and cpN2 ⫽ 0.285 Btu/lb (0.662 kJ/kg), respectively; final prod- uct temperature is t2; other values as before. Then, 1 ⫻ 14,087 ⫽ [(3.667 ⫻ 0.30) ⫹ (0.533 ⫻ 0.24) ⫹ (10.58 ⫻ 0.285)(t2 ⫺ 77)]. Solving, t2 ⫽ 3320 ⫹ 77 ⫽ 3397⬚F (1869⬚C). Related Calculations. In the above procedure it is assumed that the carbon is burned in dry air. Also, the nitrogen coefficient of 3.78 used in the chemical equa- tion in step 1 is based on a theoretical composition of dry air as 79.1 percent nitrogen and 20.9 percent oxygen by volume, so that 79.1/20.9 ⫽ 3.78. For a more detailed description of this coefficient see the Related Calculations under the pro- cedure for ‘‘Gas Turbine Combustion Chamber Inlet Air Temperature’’ elsewhere in this handbook. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 178. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. COMBUSTION
- 179. 4.1 SECTION 4 STEAM GENERATION EQUIPMENT AND AUXILIARIES Determining Equipment Loading for Generating Steam Efficiently 4.2 Steam Conditions with Two Boilers Supplying the Same Line 4.6 Generating Saturated Steam by Desuperheating Superheated Steam 4.7 Determining Furnace-Wall Heat Loss 4.8 Converting Power-Generation Pollutants from Mass to Volumetric Units 4.10 Steam Boiler Heat Balance Determination 4.11 Steam Boiler, Economizer, and Air- Heater Efficiency 4.14 Fire-Tube Boiler Analysis and Selection 4.16 Safety-Valve Steam-Flow Capacity 4.18 Safety-Valve Selection for a Watertube Steam Boiler 4.19 Steam-Quality Determination with a Throttling Calorimeter 4.24 Steam Pressure Drop in a Boiler Superheater 4.25 Selection of a Steam Boiler for a Given Load 4.26 Selecting Boiler Forced- and Induced- Draft Fans 4.30 Power-Plant Fan Selection from Capacity Tables 4.33 Fan Analysis at Varying RPM, Pressure, and Air or Gas Capacity 4.35 Boiler Forced-Draft Fan Horsepower Determination 4.37 Effect of Boiler Relocation on Draft Fan Performance 4.38 Analysis of Boiler Air Ducts and Gas Uptakes 4.38 Determination of the Most Economical Fan Control 4.44 Smokestack Height and Diameter Determination 4.46 Power-Plant Coal-Dryer Analysis 4.48 Coal Storage Capacity of Piles and Bunkers 4.50 Properties of a Mixture of Gases 4.51 Steam Injection in Air Supply 4.52 Boiler Air-Heater Analysis and Selection 4.53 Evaluation of Boiler Blowdown, Deaeration, Steam and Water Quality 4.55 Heat-Rate Improvement Using Turbine- Driven Boiler Fans 4.56 Boiler Fuel Conversion from Oil or Gas to Coal 4.60 Energy Savings from Reduced Boiler Scale 4.64 Ground Area and Unloading Capacity Required for Coal Burning 4.66 Heat Recovery from Boiler Blowdown Systems 4.67 Boiler Blowdown Percentage 4.69 Sizing Flash Tanks to Conserve Energy 4.70 Flash Tank Output 4.71 Determining Waste-Heat Boiler Fuel Savings 4.74 Figuring Flue-Gas Reynolds Number by Shortcuts 4.75 Determining the Feasibility of Flue-Gas Recirculation for No Control in Packaged Boilers 4.77 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 180. 4.2 POWER GENERATION DETERMINING EQUIPMENT LOADING FOR GENERATING STEAM EFFICIENTLY A plant has a steam generator capable of delivering up to 1000,000 lb/h (45,400 kg/h) of saturated steam at 400 lb/in2 (gage) (2756 kPa). The plant also has an HRSG capable of generating up to 1000,000 lb/h (45,400 kg/h) of steam in the fired mode at the same pressure. How should each steam generator be loaded to generate a given quantity of steam most efficiently? Calculation Procedure: 1. Develop the HRSG characteristics In cogeneration and combined-cycle steam plants (gas turbine plus other prime movers), the main objective of supervising engineers is to generate a needed quan- tity of steam efficiently. Since there may be both HRSGs and steam boilers in the plant, the key to efficient operation is an understanding of the performance char- acteristics of each piece of equipment as a function of load. In this plant, the HRSG generates saturated steam at 400 lb/in2 (gage) (2756 kPa) from the exhaust of a gas turbine. It can be supplementary-fired to generate additional steam. Using the HRSG simulation approach given in another calculation procedure in this handbook, the HRSG performance at different steam flow rates should be developed. This may be done manually or by using the HRSG software developed by the author. 2. Select the gas/steam temperature profile in the design mode Using a pinch point of 15⬚F (8.33⬚C) and approach point of 17⬚F (9.44⬚C), a tem- perature profile is developed as discussed in the procedure for HRSG simulation. The HRSG exit gas temperature is 319⬚F (159.4⬚C) while generating 25,000 lb/h (11,350 kg/h) of steam at 400 lb/in2 (gage) (2756 kPa) using 230⬚F (110⬚C) feed- water. 3. Prepare the gas/steam temperature profile in the fired mode A simple approach is to use the fact that supplementary firing is 100 percent effi- cient, as discussed in the procedure on HRSG simulation. All the fuel energy goes into generating steam in single-pressure HRSGs. Compute the duty of the HRSG—i.e., the energy absorbed by the steam—in the unfired mode, which is 25.4 MM Btu/h (7.44 MW). The energy required to gen- erate 50,000 lb/h (22,700 kg/h) of steam is 50.8 MM Btu/h (14.88 MW). Hence, the additional fuel required ⫽ 50.8 ⫺ 25.4 ⫽ 25.4 MM Btu/h (7.44 MW). If a manual or computer simulation is done on the HRSG, fuel consumption will be seen to be 24.5 MM Btu/h (7.18 MW) on a Lower Heating Value (LHV) basis. Similarly, the performance at other steam flows is also computed and summarized in Table 1. Note that the exit gas temperature decreases as the steam flow increases. This aspect of an HRSG is discussed in the simulation procedure elsewhere in this handbook. 4. Develop the steam-generator characteristics Develop the performance of the steam generator at various loads. Steam-generator suppliers will gladly provide this information in great detail, including plots and tabulations of the boiler’s performance. As shown in Table 2, the exit-gas temper- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 181. 4.3 TABLE 1 Performance of HRSG Load, % 25 50 75 100 Steam generation, lb/h (kg/h) 25,000 (11,350) 50,000 (22,700) 75,000 (34,050) 100,000 (45,400) Duty, MM Btu/h (MW) 25.4 (7.4) 50.8 (14.9) 76.3 (22.4) 101.6 (29.8) Exhaust gas flow, lb/h (kg/h) 152,000 (69,008) 153,140 (69,526) 154,330 (70,066) 155,570 (70,629) Exit gas temperature, ⬚F (⬚C) 319 (159) 285 (141) 273 (134) 269 (132) Fuel fired, MM Btu/h LHV basis (MW) 0 (0) 24.50 (7.2) 50.00 (14.7) 76.50 (22.4) ASME PTC 4.4 efficiency, % 70.80 83.79 88.0 89.53 Boiler pressure ⫽ 400 lb/in2 (gage) (2756 kPa); feedwater temperature ⫽ 230⬚F (110⬚C); blowdown ⫽ 5 percent. Fuel used: natural gas; percent volume C1 ⫽ 97; C2 ⫽ 2; C3 ⫽ 1; HHV ⫽ 1044 Btu/ft3 (38.9 MJ/m3 ); LHV ⫽ 942 Btu/ ft3 (35.1 MJ/m3 ). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 182. 4.4 TABLE 2 Performance of Steam Generator Load, % 25 50 75 100 Steam generation, lb/h (kg/h) 25,000 (11,350) 50,000 (22,700) 75,000 (34,050) 100,000 (45,400) Duty, MM Btu/h (MW) 25.4 (7.4) 50.8 (14.9) 76.3 (22.4) 101.6 (29.8) Excess air, % 30 10 10 10 Flue gas, lb/h (kg/h) 30,140 (13,684) 50,600 (22,972) 76,150 (34,572) 101,750 (46,195) Exit gas temperature, ⬚F (⬚C) 265 (129) 280 (138) 300 (149) 320 (160) Heat losses, % —Dry gas loss 3.93 3.56 3.91 4.27 —Air moisture loss 0.10 0.09 0.10 0.11 —Fuel moisture loss 10.43 10.49 10.58 10.66 —Radiation loss 2.00 1.00 0.70 0.50 Efficiency, % —Higher Heating Value basis 83.54 84.86 84.70 84.46 —Lower Heating Value basis 92.58 94.05 93.87 93.60 Fuel fired, MM Btu/h LHV basis (MW) 27.50 (8.06) 54.00 (15.8) 81.30 (23.8) 108.60 (31.8) Boiler pressure ⫽ 400 lb/in2 (gage) (2756 kPa); feedwater temperature ⫽ 230⬚F (110⬚C); blowdown ⫽ 5 percent. Fuel used: natural gas; percent volume C1 ⫽ 97; C2 ⫽ 2; C3 ⫽ 1; HHV ⫽ 1044 Btu/ft3 (38.9 MJ/m3 ); LHV ⫽ 942 Btu/ ft3 (35.1 MJ/m3 ). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 183. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.5 TABLE 3 Fuel Consumption at Various Steam Loads Total steam HRSG steam SG steam HRSG fuel Sg fuel Total fuel lb/h lb/h lb/h MM Btu/h MM Btu/h MM Btu/h 150,000 50,000 100,000 24.50 108.60 133.10 150,000 75,000 75,000 50.00 81.30 131.30 150,000 100,000 50,000 76.50 54.00 130.50 100,000 0 100,000 0 108.60 108.60 100,000 25,000 75,000 0 81.30 81.30 100,000 50,000 50,000 24.50 54.00 78.50 100,000 75,000 25,000 50.00 27.50 77.50 100,000 100,000 0 75.60 0 76.50 50,000 0 50,000 0 54.00 54.00 50,000 25,000 25,000 0 27.50 27.50 50,000 50,000 0 24.50 0 24.50 kg/h kg/h SI Units kg/h MW MW MW 68,100 22,700 45,400 7.2 31.8 38.9 68,100 34,050 34,050 14.7 23.8 38.5 68,100 45,400 45,400 22.4 15.8 38.2 45,400 0 45,400 0 31.8 31.8 45,400 11,350 34,050 0 23.8 23.8 45,400 22,700 22,700 7.2 15.8 23.0 45,400 34,050 11,350 14.7 8.1 22.7 45,400 45,400 0 22.4 0 22.4 22,700 0 22,700 0 15.8 15.8 22,700 11,350 11,350 0 8.1 8.1 22,700 11,350 0 7.2 0 7.2 ature decreases as the load on the steam generator declines. This is because the ratio of gas/steam is maintained at nearly unity, unlike in an HRSG where the gas flow remains constant and steam flow alone is varied. Further, the radiation losses in a steam boiler increase at lower duty, while the exit-gas losses decrease. However, the boiler’s efficiency falls within a narrow range. Table 3 also shows the steam generator’s fuel consumption at various loads. 5. Calculate steam vs. fuel data for combined operation of the equipment The next step is to develop, for combined operation of the HRSG and steam gen- erator, a steam flow vs. fuel table such as that in Table 3. For example, 150,000 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 184. 4.6 POWER GENERATION lb/h (68,100 kg/h) of steam could be generated in several ways—50,000 lb/h (22,700 kg/h) in the HRSG and 100,000 lb/h (45,400 kg/h) in the steam generator. Or each could generate 75,000 lb/h (34,050 kg/h); or 100,000 lb/h (45,400 kg/h) in the HRSG and the remainder in the steam generator. The table shows that max- imizing the HRSG output first is the most efficient way of generating steam because no fuel is required to generate up to 25,000 lb/h (11,350 kg/h) of steam. However, this may not always be possible because of the plant operating mode, equipment availability, steam temperature requirements, etc. Note also that the gas pressure drop in an HRSG does not vary significantly with load as the gas mass flow remains nearly constant. The gas pressure drop increases slightly as the firing temperature increases. On the other hand, the steam generator fan power consumption vs. load increases more in proportion to load. It is also seen that at higher steam capacities the difference in fuel consumption between the various modes of operation is small. At 150,000 lb/h (68,100 kg/h), the difference is about 3 MM Btu/h (0.88 MW), while at 100,000 lb/h (45,400 kg/h), the difference is 30 MM Btu/h (8.79 MW). This difference should also be kept in mind while developing an operational strategy. If a superheater is used, the performance of the superheater would have to be analyzed. Steam generators can generally maintain the steam temperature from 40 to 100 percent load, while in HRSGs the range is much larger as the steam tem- perature increases with firing temperature and can be controlled. Related Calculations. Developing the performance characteristics of each piece of equipment as a function of load is the key to determining the mode of operation and loading of each type of steam producer. For best results, develop a performance curve for the steam generator, including all operating costs such as fan power consumption, pump power consumption, and gas-turbine power output as a function of load. This gives more insight into the total costs in addition to fuel cost, which is the major cost. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. The HRSG software mentioned in this procedure is available from Mr. Ganapathy. STEAM CONDITIONS WITH TWO BOILERS SUPPLYING THE SAME STEAM LINE Two closely adjacent steam boilers discharge equal amounts of steam into the same short steam main. Steam from boiler No. 1 is at 200 lb/in2 (1378 kPa) and 420⬚F (215.6⬚C) while steam from boiler No. 2 is at 200 lb/in2 (1378 kPa) and 95 percent quality. (a) What is the equilibrium condition after missing of the steam? (b) What is the loss of entropy by the higher temperature steam? Assume negligible pressure drop in the short steam main connecting the boilers. Calculation Procedure: 1. Determine the enthalpy of the mixed steam Use the T-S diagram, Fig. 1, to plot the condition of the mixed steam. Then, since equal amounts of steam are mixed, the final enthalpy, H3 ⫽ (H1 ⫹ H2)/2. Substi- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 185. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.7 800˚F (471˚C) 841.8˚F (449.8˚C) 200 psia (1378 kPa) FIGURE 1 T-S plot of conditions with two boilers on line. tuting, using date from the steam tables and Mollier chart, H3 ⫽ (1225 ⫹ 1164)/ 2 ⫽ 1194.5 Btu/lb (2783.2 kJ/kg). 2. Find the quality of the mixed steam Entering the steam tables at 200 lb/in2 (1378 kPa), find the enthalpy of the liquid as 355.4 Btu/lb (828.1 kJ/kg) and the enthalpy of vaporization as 843.3 Btu/lb (1964.9 kJ/kg). Then, using the equation for wet steam with the known enthalpy of the mixture from Step 1, 1194.5 ⫽ Hƒ ⫹ x3 (Hƒg) ⫽ 355.4 ⫹ x3 (843.3); x3 ⫽ 0.995, or 99.5 percent quality. 3. Find the entropy loss by the higher pressure steam The entropy loss by the higher-temperature steam, referring to the Mollier chart plot, is S1 ⫺ S2 ⫽ 1.575 ⫺ 1.541 ⫽ 0.034 entropy units. The lower-temperature steam gains S3 ⫺ S2 ⫽ 1.541 ⫺ 1.506 ⫽ 0.035 units of entropy. Related Calculations. Use this general approach for any mixing of steam flows. Where different quantities of steam are being mixed, use the proportion of each quantity to the total in computing the enthalpy, quality, and entropy of the mixture. GENERATING SATURATED STEAM BY DESUPERHEATING SUPERHEATED STEAM Superheated steam generated at 1350 lb/in2 (abs) (9301.5 kPa) and 950⬚F (510⬚C) is to be used in a process as saturated steam at 1000 lb/in2 (abs) (6890 kPa). If the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 186. 4.8 POWER GENERATION superheated steam is desuperheated continuously by injecting water at 500⬚F (260⬚C), how many pounds (kg) of saturated steam will be produced per pound (kg) of superheated steam? Calculation Procedure: 1. Using the steam tables, determine the steam and water properties Rounding off the enthalpy and temperature values we find that: Enthalpy of the superheated steam at 1350 lb/in2 (abs) (9301.5 kPa) and 950⬚F (510⬚C) ⫽ H1 ⫽ 1465 Btu/lb (3413.5 kJ/kg); Enthalpy of saturated steam at 1000 lb/in2 (abs) (6890 kPa) ⫽ H2 ⫽ 1191 Btu/lb (2775 kJ/kg); Enthalpy of water at 500⬚F (260⬚C) ⫽ (500 ⫺ 32) ⫽ H3 ⫽ 488 Btu/lb (1137 kJ/kg). 2. Set up a heat-balance equation and solve it L X ⫽ lb (kg) of water at 500⬚F (260⬚C) required to desuperheat the superheated steam. Then, using the symbols given above, H1 ⫹ X(H3) ⫽ (1 ⫹ X)H2. Solving for X ⫽ (H1 ⫺ H2)/(H2 ⫺ H3) ⫽ (1465 ⫺ 1191)/(1191 ⫺ 488) ⫽ 0.39. Then, 1.0 ⫹ 0.39 ⫽ 1.39 lb (0.63 kg) of saturated steam produced per lb (kg) of super- heated steam. Thus, if the process used 1000 lb (454 kg) of saturated steam at 1000 lb/in2 (abs) (689 kPa), the amount of superheated steam needed to produce this saturated steam would be 1000/1.39 ⫽ 719.4 lb (326.6 kg). Related Calculations. Desuperheating superheated steam for process and other use is popular because it can save purchase and installation of a separate steam generator for the lower pressure steam. While there is a small loss of energy in desuperheating (from heat losses in the piping and desuperheater), this loss is small compared to the savings made. That’s why you’ll find desuperheating being used in central stations, industrial, commercial and marine plants throughout the world. DETERMINING FURNACE-WALL HEAT LOSS A furnace wall consists of 9-in (22.9-cm) thick fire brick, 4.5-in (11.4-cm) Sil-O- Cel brick, 4-in (10.2-cm) red brick, and 0.25-in (0.64-cm) transite board. The ther- mal conductivity, k, values, Btu/(ft2 )(⬚F)(ft) [kJ/(m2 )(⬚C)(m)] are as follows: 0.82 at 1800⬚F (982⬚C) for fire brick; 0.125 at 1800⬚F (982⬚C) for Sil-O-Cel; 0.52 at 500⬚F (260⬚C) for transite. A temperature of 1800⬚F (982⬚C) exists on the inside wall of the furnace and 200⬚F (93.3⬚C) on the outside wall. Determine the heat loss per hour through each 10 ft2 (0.929 m2 ) of furnace wall. What is the temperature of the wall at the joint between the fire brick and Sil-O-Cel? Calculation Procedure: 1. Find the heat loss through a unit area of the furnace wall Use the relation Q ⫽ ⌬t/R, where Q ⫽ heat transferred, Btu/h (W); ⌬t ⫽ temper- ature difference between the inside of the furnace wall and the outside, ⬚F (⬚C); R ⫽ resistance of the wall to heat flow ⫽ L/(kXA), where L ⫽ length of path Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 187. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.9 through which the heat flow, ft (m); k ⫽ thermal conductivity, as defined above; A ⫽ area of path of heat flow, ft2 (m2 ). Where there is more than one resistance to heat flow, add them to get the total resistance. Substituting, the above values for this furnace wall, remembering that there are three resistances in series and solving for the heat flow through one square ft (0.0.0929 m2 ), Q ⫽ (1800 ⫺ 200)/{[(1/0.82)(9/12)] ⫹ [(1/0.125)(4.5/12)] ⫹ [(1 /052)(4/12)] ⫹ [(1/0.23)(0.25/12)]} ⫽ 344 Btu/h ft2 (1083.6 W/m2 ), or 10 (344) ⫽ 34400 Btu/h for 10 ft2 (10,836 W/10 m2 ). 2. Compute the temperature within the wall at the stated joint Use the relation, (⌬t)/(⌬t1) ⫽ (R/R1), where ⌬t ⫽ temperature difference across the wall, ⬚F (⬚C); ⌬t1 ⫽ temperature at the joint being considered, ⬚F (⬚C); R ⫽ total resistance of the wall; R1 ⫽ resistance of the first portion of the wall between the inside and the joint in question. Substituting, (1800 ⫺ 200)/(⌬t1) ⫽ 4.646/0.915); ⌬t1 ⫽ 315⬚F (157.2⬚C). Then the interface temperature at the between the fire brick and the Sil-O-Cel is 1800 ⫺ 315 ⫽ 1485⬚F (807.2⬚C) Related Calculations. The coefficient of thermal conductivity given here, Btu/(ft2 )(⬚F)(ft) is sometimes expressed in terms of per inch of thickness, instead of per foot. Either way, the conversion is simple. In SI units, this coefficient is expressed in kJ/(m2 )(⬚C)(m), or cm2 and cm. The exterior temperature of a furnace wall is an important considered in boiler and process unit design from a human safety standpoint. Excess exterior tempera- tures can cause injury to plant workers. Further, the higher the exterior temperature of a furnace wall, the larger the heat loss from the fired vessel. Therefore, both safety and energy conservation considerations are important in furnace design. Typical interior furnace temperatures encountered in modern steam boilers range from 2400⬚F (1316⬚C) near the fuel burners to 1600⬚F (871⬚C) in the superheater interior. With today’s emphasis on congeneration and energy conservation, many different fuels are being burned in boilers. Thus, a plant in Louisiana burns rice to generate electricity while disposing of a process waste material. Rice hulls, which comprise 20 percent of harvested rice, are normally processed in a hammermill to increase their bulk from about 11 lb/ft3 (176 kg/m3 ) to 20 lb/ft3 (320 kg/m3 ). Then they are spread or piled on land adjacent to the rice mill. The hulls often smolder in the fields, like mine tailings from coal production. Con- tinuous, uncontrolled burning may result, creating an environmental hazard and problem. Burning rice hulls in a boiler furnace may create unexpected temperatures both inside and outside the furnace. Hence, it is important that the designer be able to analyze both the interior and exterior furnace temperatures using the procedure given here. Another modern application of waste usage for power generation is the burning of sludge in a heat-recovery boiler to generate electricity. Sludge from a wastewater plant is burned in a combustor to generate steam for a turbogenerator. Not only are fuel requirements for the boiler reduced, there is also significant savings of fuel used to incinerate the sludge in earlier plants. Again, the furnace temperature is an important element in designing such plants. The data present in these comments on new fuels for boilers is from Power magazine. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 188. 4.10 POWER GENERATION CONVERTING POWER-GENERATION POLLUTANTS FROM MASS TO VOLUMETRIC UNITS In the power-generation industry, emission levels of pollutants such as CO and NOx are often specified in mass units such as pounds per million Btu (kg per 1.055 MJ) and volumetric units such as ppm (parts per million) volume. Show how to relate these two measures for a gaseous fuel having this analysis: Methane ⫽ 97 percent; Ethane ⫽ 2 percent; Propane ⫽ 1 percent by volume, and excess air ⫽ 10 percent. Ambient air temperature during combustion ⫽ 80⬚F (26.7⬚C) and relative humid- ity ⫽ 60 percent; fuel higher heating value, HHV ⫽ 23,759 Btu/lb (55,358 kJ/ kg); 100 moles of fuel gas is the basis of the flue gas analysis. Calculation Procedure: 1. Find the theoretical dry air required, and the moisture in the actual air The theoretical dry air requirements, in M moles, can be computed from the sum of (ft3 of air per ft3 of combustible gas)(percent of combustible in the fuel) using data from Ganapathy, Steam Plant Calculations Manual, Marcel Dekker, Inc. thus: M ⫽ (9.528 ⫻ 97) ⫹ (16.675 ⫻ 2) ⫹ (23.821 ⫻ 1) ⫽ 981.4 moles. Then, with 10 percent excess air, excess air, EA ⫽ 0.1(981.4) ⫽ 98.1 moles. The excess oxygen, O2 ⫽ (98.1 moles)(0.21) ⫽ 20.6 moles, where 0.21 ⫽ moles of oxygen in 1 mole of air. The nitrogen, N2, produced by combustion ⫽ (1.1 for excess air)(981.4 moles)(0.79 moles of nitrogen in 1 mole of air) ⫽ 852.8 moles; round to 853 moles for additional calculations. The moisture in the air ⫽ (981.4 ⫹ 98.1)(29 ⫻ 0.0142/18) ⫽ 24.69, say 24.7 moles. In this computation the values 29 and 18 are the molecular weights of dry air and water vapor, respectively, while 0.0142 is the lb (0.0064 kg) moisture per lb of dry air as shown in the previous procedure. 2. Compute the flue gas analysis for the combustion Using the given data, CO2 ⫽ (1 ⫻ 97) ⫹ (2 ⫻ 2) ⫹ (3 ⫻ 1) ⫽ 104 moles. For H2O ⫽ (2 ⫻ 97) ⫹ (3 ⫻ 2) ⫹ (4 ⫻ 1) ⫹ 24.7 ⫽ 228.7 moles. From step 1, N2 ⫽ 853 moles; O2 ⫽ 20.6 moles. Now, the total moles ⫽ 104 ⫹ 228.7 ⫹ 853 ⫹ 20.6 ⫽ 1206.3 moles. The percent volume of CO2 ⫽ (104/1206.3)(100) ⫽ 8.6; the percent H2O ⫽ (228.7/ 1206.1)(100) ⫽ 18.96; the percent N2 ⫽ (853/1206.3)(100) ⫽ 70.7; the percent O2 ⫽ (20.6/1206.3)(100) ⫽ 1.71. 3. Find the amount of flue gas produced per million Btu (1.055 MJ) To relate the pounds per million Btu (1.055 MJ) of NOx or CO produced to ppmv, we must know the amount of flue gas produced per million Btu (1.055 MJ). From step 2, the molecular weight of the flue gases ⫽ [(8.68 ⫻ 44) ⫹ (18.96 ⫻ 18) ⫹ (70.7 ⫻ 28) ⫹ (1.71 ⫻ 32)]/100 ⫽ 27.57. The molecular weight of the fuel ⫽ [(97 ⫻ 16) ⫹ (2 ⫻ 30) ⫹ (1 ⫻ 44)]/100 ⫽ 16.56. Now the ratio of flue gases/fuel ⫽ (1206.3 ⫻ 27.57)/(100 ⫻ 16.56) ⫽ 20.08 lb flue gas/lb fuel (9.12 kg/kg). Hence, 1 million Btu fired produces (1,000,000)/ 23,789 ⫽ 42 lb (19.1 kg) fuel ⫽ (42)(20.08) ⫽ 844 lb (383 kg) wet flue gases. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 189. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.11 4. Calculate ppm values for the gases Let 1 million Btu fired generate N lb (kg) of NOx. For emission calculations, NOx is considered to have a molecular weight of 46. Also, the reference for NOx or CO regulations is 3 percent dry oxygen by volume for steam generators. Hence, we have the relation, NN ⫽ 106 (Yx)(N/46)(MWƒg)[(21 ⫺ 3)/(21 ⫺ O2XY)], where VN ⫽ ppm dry NOx; Y ⫽ 100/(100 ⫺ percent H2O), where percent H2O is the percent volume water vapor in the flue gases; N ⫽ lb (kg) of NOx per million Btu (1.055 MJ) fired on an HHV basis, MWƒg ⫽ molecular weight of wet flue gases; Wgm ⫽ amount of wet flue gas produced per million Btu (1.055 MJ) fired. Substituting in the above relation, VN ⫽ 106 (Nx)[100/(100 ⫺ 18.96)] (27.57/ 844)(21 ⫺ 3)/[(21 ⫺ 1.71)(100/(100 ⫺ 18.96)] ⫽ 832N. Similarly, Vc ⫽ ppmv CO2 generated per million Btu (1.055 MJ) fired ⫽ 1367⬚C, where C ⫽ lb (kg) of CO generated per million Btu (1.055 MJ) and Vc ⫽ amount in ppmvd (dry). The effect of excess air on these calculations is not at all significant. One may perform these calculations at 30 percent excess air and still show that VN ⫽ 832N and Vc ⫽ 1367 for natural gas. Related Calculations. These calculations for oil fuels also to show that VN ⫽ 783N and Vc ⫽ 1286C. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. STEAM BOILER HEAT BALANCE DETERMINATION A steam generator having a maximum rated capacity of 60,000 lb/h (27,000 kg/ h) is operating at 45,340 lb/h (20,403 kg/h), delivering 125-lb/in2 (gage) 400⬚F (862-kPa, 204⬚C) steam with a feedwater temperature of 181⬚F (82.8⬚C). At this generating rate, the boiler requires 4370 lb/h (1967 kg/h) of West Virginia bitu- minous coal having a heating value of 13,850 Btu/lb (32,215 kJ/kg) on a dry basis. The ultimate fuel analysis is: C ⫽ 0.7757; H2 ⫽ 0.0507; O2 ⫽ 0.0519; N2 ⫽ 0.0120; S ⫽ 0.0270; ash ⫽ 0.0827; total ⫽ 1.0000. The coal contains 1.61 percent moisture. The boiler-room intake air and the fuel temperature ⫽ 79⬚F (26.1⬚C) dry bulb, 71⬚F (21.7⬚C) wet bulb. The flue-gas temperature is 500⬚F (260⬚C), and the analysis of the flue gas shows these percentages: CO2 ⫽ 12.8; CO ⫽ 0.4; O2 ⫽ 6.1; N2 ⫽ 80.7; total ⫽ 100.0. Measured ash and refuse ⫽ 9.42 percent of dry coal; combus- tible in ash and refuse ⫽ 32.3 percent. Compute a heat balance for this boiler based on these test data. The boiler has four water-cooled furnace walls. Calculation Procedure: 1. Determine the heat input to the boiler In a boiler heat balance the input is usually stated in Btu per pound of fuel as fired. Therefore, input ⫽ heating value of fuel ⫽ 13,850 Btu/lb (32,215 kJ/kg). 2. Compute the output of the boiler The output of any boiler ⫽ Btu/lb (kJ/kg) of fuel ⫹ the losses. In this step the first portion of the output, Btu/lb (kJ/kg) of fuel will be computed. The losses will be computed in step 3. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 190. 4.12 POWER GENERATION First find Ws, lb of steam produced per lb of fuel fired. Since 45,340 lb/h (20,403 kg/h) of steam is produced when 4370 lb/h (1967 kg/h) of fuel is fired, Ws ⫽ 45,340/4370 ⫽ 10.34 lb of steam per lb (4.65 kg/kg) of fuel. Once Ws is known, the output h1 Btu/lb of fuel can be found from h1 ⫽ Ws(hs ⫺ hw), where hs ⫽ enthalpy of steam leaving the superheater, or boiler if a super- heater is not used; hw ⫽ enthalpy of feedwater, Btu/lb. For this boiler with steam at 125 lb/in2 (gage) [⫽ 139.7 lb/in2 (abs)] and 400⬚F (930 kPa, 204⬚C), hs ⫽ 1221.2 Btu/lb (2841 kJ/kg), and hw ⫽ 180.92 Btu/lb (420.8 kJ/kg), from the steam tables. Then h1 ⫽ 10.34(1221.2 ⫺ 180.92) ⫽ 10,766.5 Btu/lb (25,043 kJ/kg) of coal. 3. Compute the dry flue-gas loss For any boiler, the dry flue-gas loss h2 Btu/lb (kJ/kg) of fuel is given by h2 ⫽ 0.24Wg ⫻ (Tg ⫺ Ta), where Wg ⫽ lb of dry flue gas per lb of fuel; Tg ⫽ flue-gas exit temperature,⬚F; Ta ⫽ intake-air temperature,⬚F. Before Wg can be found, however, it must be determined whether any excess air is passing through the boiler. Compute the excess air, if any, from excess air, percent ⫽ 100 (O2 ⫺ 1 ⁄2CO)/[0.264N2 ⫺ (O2 ⫺ 1 ⁄2CO)], where the symbols refer to the elements in the flue-gas analysis. Substituting values from the flue-gas anal- ysis gives excess air ⫽ 100(6.1 ⫺ 0.2)/[0.264 ⫻ 80.7 ⫺ (6.1 ⫺ 0.2)] ⫽ 38.4 percent. Using the method given in earlier calculation procedures, find the air required for complete combustion as 10.557 lb/lb (4.571 kg/kg) of coal. With 38.4 percent excess air, the additional air required ⫽ (10.557)(0.384) ⫽ 4.053 lb/lb (1.82 kg/ kg) of fuel. From the same computation in which the air required for complete combustion was determined, the lb of dry flue gas per lb of fuel ⫽ 11.018 (4.958 kg/kg). Then, the total flue gas at 38.4 percent excess air ⫽ 11.018 ⫹ 4.053 ⫽ 15.071 lb/lb (6.782 kg/kg) of fuel. With a flue-gas temperature of 500⬚F (260⬚C), and an intake-air temperature of 79⬚F (26.1⬚C), h2 ⫽ 0.24(15.071)(500 ⫺ 70) ⫽ 1524 Btu/lb (3545 kJ/kg) of fuel. 4. Compute the loss due to evaporation of hydrogen-formed water Hydrogen in the fuel is burned in forming H2O. This water is evaporated by heat in the fuel, and less heat is available for producing steam. This loss is h3 Btu/lb of fuel ⫽ 9H(1089 ⫺ Tƒ ⫹ 0.46Tg), where H ⫽ percent H2 in the fuel ⫼ 100; Tƒ ⫽ temperature of fuel before combustion,⬚F; other symbols as before. For this fuel with 5.07 percent H2, h3 ⫽ 9(5.07/100)(1089 ⫺ 79 ⫹ 0.46 ⫻ 500) ⫽ 565.8 Btu/lb (1316 kJ/kg) of fuel. 5. Compute the loss from evaporation of fuel moisture This loss is h4 Btu/lb of fuel ⫽ Wmƒ(1089 ⫺ Tƒ ⫹ 0.46Tg), where Wmƒ ⫽ lb of moisture per lb of fuel; other symbols as before. Since the fuel contains 1.61 percent moisture, in terms of dry coal this is (1.61)/(100 ⫺ 1.61) ⫽ 0.0164, or 1.64 percent. Then h4 ⫽ (1.64/100)(1089 ⫺ 79 ⫹ 0.46 ⫻ 500) ⫽ 20.34 Btu/lb (47.3 kJ/kg) of fuel. 6. Compute the loss from moisture in the air This loss is h5 Btu/lb of fuel ⫽ 0.46Wma(Tg ⫺ Ta), Wma ⫽ (lb of water per lb of dry air)(lb air supplied per lb fuel). From a psychrometric chart, the weight of moisture per lb of air at a 79⬚F (26.1⬚C) dry-bulb and 71⬚F (21.7⬚C) wet-bulb temperature is 0.014 (0.006 kg). The combustion calculation, step 3, shows that the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 191. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.13 total air required with 38.4 percent excess air ⫽ 10.557 ⫹ 4.053 ⫽ 14.61 lb of air per lb (6.575 kg/kg) of fuel. Then, Wma ⫽ (0.014)(14.61) ⫽ 0.2045 lb of moisture per lb (0.092 kg/kg) of air. And h5 ⫽ (0.46)(0.2045)(500 ⫺ 79) ⫽ 39.6 Btu/lb (92.1 kJ/kg) of fuel. 7. Compute the loss from incomplete combustion of C to CO2 in the stack This loss is h6 Btu/lb of fuel ⫽ [CO/CO ⫹ CO2)](C)(10.190), where CO and CO2 are the percent by volume of these compounds in the flue gas by Orsat analysis; C ⫽ lb carbon per lb of coal. With the given flue-gas analysis and the coal ultimate analysis, h6 ⫽ 0.4/(0.4 ⫹ 12.8)[(77.57)/(100)](10.190) ⫽ 239.5 Btu/lb (557 kJ/kg) of fuel. 8. Compute the loss due to unconsumed carbon in the refuse This loss is h7 Btu/lb of fuel ⫽ Wc(14,150), where Wc ⫽ lb of unconsumed carbon in refuse per lb of fuel fired. With an ash and refuse of 9.42 percent of the dry coal and combustible in the ash and refuse of 32.3 percent, h7 ⫽ (9.42/100) (32.3/100)(14,150) ⫽ 430.2 Btu/lb (1006 kJ/kg) of fuel. 9. Find the radiation loss in the boiler furnace Use the American Boiler and Affiliated Industries (ABAI) chart, or the manufac- turer’s engineering data to approximate the radiation loss in the boiler. Either source will show that the radiation loss is 1.09 percent of the gross heat input. Since the gross heat input is 13,850 Btu/lb (32,215 kJ/kg) of fuel, the radiation loss ⫽ (13,850)(1.09/100) ⫽ 151.0 Btu/lb (351.2 kJ/kg) of fuel. 10. Summarize the losses; find the unaccounted-for loss Set up a tabulation thus, entering the various losses computed earlier. The unaccounted-for loss is found by summing all the other losses, 3 through 9, and subtracting from 100.00. Related Calculations. Use this method to compute the heat balance for any type of boiler—watertube or firetube—in any kind of service—power, process, or heating—using any kind of fuel—coal, oil, gas, wood, or refuse. Note that step 3 shows how to compute excess air from an Orsat flue-gas analysis. More stringent environmental laws are requiring larger investments in steam- boiler pollution-control equipment throughout the world. To control sulfur emis- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 192. 4.14 POWER GENERATION sions, expensive scrubbers are required on large boilers. Without such scrubbers the sulfur emissions can lead to acid rain, smog, and reduced visibility in the area of the plant and downwind from it. With the increased number of free-trade agreements between adjacent countries, cross-border pollution is receiving greater attention. The reason for this increased attention is because not all countries have the same environmental control re- quirements. When a country with less stringent requirements pollutes an adjacent country having more stringent pollution regulations, both political and regulatory problems can arise. For example, two adjacent countries are currently discussing pollution problems of a cross-border type. One country’s standard for particulate emissions is 10 times weaker than the adjacent country’s, while its sulfur dioxide limit is 8 times weaker. With such a wide divergence in pollution requirements, cross-border flows of pol- lutants can be especially vexing. All boiler-plant designers must keep up to date on the latest pollution regula- tions. Today there are some 90,000 environmental regulations at the federal, state, and local levels, and more than 40 percent of these regulations will change during the next 12 months. To stay in compliance with such a large number of regulations requires constant attention to those regulations applicable to boiler plants. STEAM BOILER, ECONOMIZER, AND AIR-HEATER EFFICIENCY Determine the overall efficiency of a steam boiler generating 56,00 lb/h (7.1 kg/s) of 600 lb/in2 (abs) (4137.0 kPa) 800⬚F (426.7⬚C) steam. The boiler is con- tinuously blown down at the rate of 2500 lb/h (0.31 kg/s). Feedwater enters the economizer at 300⬚F (148.9⬚C). The furnace burns 5958 lb/h (0.75 kg/s) of 13,100- Btu/lb (30,470.6-kJ/kg), HHV (higher heating value) coal having an ultimate anal- ysis of 68.5 percent C, 5 percent H2, 8.9 percent O2, 1.2 percent N2, 3.2 percent S, 8.7 percent ash, and 4.5 percent moisture. Air enters the boiler at 63⬚F (17.2⬚C) dry-bulb and 56⬚F (13.3⬚C) wet-bulb temperature, with 56 gr of vapor per lb (123.5 gr/kg) of dry air. Carbon in the fuel refuse is 7 percent, refuse is 0.093 lb/lb (0.2 kg/kg) of fuel. Feedwater leaves the economizer at 370⬚F (187.8⬚C). Flue gas enters the economizer at 850⬚F (454.4⬚C) and has an analysis of 15.8 percent CO2, 2.8 percent O2, and 81.4 percent N2. Air enters the air heater at 63⬚F (17.2⬚C) with 56 gr/lb (123.5 gr/kg) of dry air; air leaves the heater at 480⬚F (248.9⬚C). Gas enters the air heater at 570⬚F (298.9⬚C), and 14 percent of the air to the furnace comes from the mill fan. Determine the steam generator overall efficiency, economizer efficiency, and air-heater efficiency. Figure 2 shows the steam generator and the flow factors that must be considered. Calculation Procedure: 1. Determine the boiler output The boiler output ⫽ S(hg ⫺ hƒ1) ⫹ Sr(hg3 ⫺ hg2) ⫹ B(hƒ3 ⫺ hƒ1), where S ⫽ steam generated, lb/h; hg ⫽ enthalpy of the generated steam, Btu/lb; hƒ1 ⫽ enthalpy of inlet feedwater; Sr ⫽ reheated steam flow, lb/h (if any); hg3 ⫽ outlet enthalpy of reheated steam; hg2 ⫽ inlet enthalpy of reheated steam; B ⫽ blowoff, lb/h; hƒ3 ⫽ blowoff enthalpy, where all enthalpies are in Btu/lb. Using the appropriate steam Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 193. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.15 FIGURE 2 Points in a steam generator where temperatures and enthalpies are measured in determining the boiler efficiency. table and deleting the reheat factor because there is no reheat, we get boiler output ⫽ 56,000(1407.7 ⫺ 269.6) ⫹ 2500(471.6 ⫺ 269.6) ⫽ 64,238,600 Btu/h (18,826.5 kW). 2. Compute the heat input to the boiler The boiler input ⫽ FH, where F ⫽ fuel input, lb/h (as fired); H ⫽ higher heating value, Btu/lb (as fired). Or, boiler input ⫽ 5958(13,100) ⫽ 78,049,800 Btu/h (22,874.1 kW). 3. Compute the boiler efficiency The boiler efficiency ⫽ (output, Btu/h)/(input, Btu/h) ⫽ 64,238,600/78,049,800 ⫽ 0.822, or 82.2 percent. 4. Determine the heat absorbed by the economizer The heat absorbed by the economizer, Btu/h ⫽ ww(hƒ2 ⫺ hƒ1), where ww ⫽ feed- water flow, lb/h; hƒ1 and hƒ2 ⫽ enthalpies of feedwater leaving and entering the economizer, respectively, Btu/lb. For this economizer, with the feedwater leaving the economizer at 370⬚F (187.8⬚C) and entering at 300⬚F (148.9⬚C), heat absorbed ⫽ (56,000 ⫹ 2500)(342.79 ⫺ 269.59) ⫽ 4.283,000 Btu/h (1255.2 kW). Note that the total feedwater flow ww is the sum of the steam generated and the continuous blowdown rate. 5. Compute the heat available to the economizer The heat available to the economizer, Btu/h ⫽ HgF, where Hg ⫽ heat available in flue gas, Btu/lb of fuel ⫽ heat available in dry gas ⫹ heat available in flue-gas vapor, Btu/lb of fuel ⫽ (t;3 ⫺ tƒ1)(0.24G) ⫹ (t3 ⫺ tƒ1)(0.46){Mƒ ⫹ 8.94H2 ⫹ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 194. 4.16 POWER GENERATION Ma[G ⫺ Cb ⫺ N2 ⫺ 7.94(H2 ⫺ O2 /8)]}, where G ⫽ {[11CO2 ⫹ 8O2 ⫹ 7(N2 ⫹ CO)]/[3(CO2 ⫹ CO)]}(Cb ⫹ S/2.67) ⫹ S/1.60; Mƒ ⫽ lb of moisture per lb fuel burned; Ma ⫽ lb of moisture per lb of dry air to furnace; Cb ⫽ lb of carbon burned per lb of fuel burned ⫽ C ⫽ RCr; Cr ⫽ lb of combustible per lb of refuse; R ⫽ lb of refuse per lb of fuel; H2, N2, C, O2, S ⫽ lb of each element per lb of fuel, as fired; CO2, CO, O2, N2 ⫽ percentage parts of volumetric analysis of dry combustion gas entering the economizer. Substituting gives Cb ⫽ 0.685 ⫺ (0.093)(0.07) ⫽ 0.678 lb/lb (0.678 kg/kg) fuel; G ⫽ [11(0.158) ⫹ 8(0.028) ⫹ 7(0.814)]/[3(0.158)] ⫻ (0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60; G ⫽ 11.18 lb/lb (11.18 kg/kg) fuel. Hg ⫽ (800 ⫺ 300)(0.24) ⫻ (11.18) ⫹ (800 ⫺ 300)(0.46){0.045 ⫹ (8.9)(0.05) ⫹ 56/ 7000[11.18 ⫺ 0.678 ⫺ 0.012 ⫺ 7.94 ⫻ (0.05 ⫺ 0.089/8)]}; Hg ⫽ 1473 Btu/lb (3426.2 kJ/kg) fuel. Heat available ⫽ HgF ⫽ (1473)(5958) ⫽ 8,770,000 Btu/h (2570.2 kW). 6. Compute the economizer efficiency The economizer efficiency ⫽ (heat absorbed, Btu/h)/(heat available, Btu/h) ⫽ 4,283,000/8,770,000 ⫽ 0.488, or 48.8 percent. 7. Compute the heat absorbed by air heater The heat absorbed by the air heater, Btu/lb of fuel, ⫽ Ah(t2 ⫺ t1)(0.24 ⫹ 0.46Ma), where Ah ⫽ air flow through heater, lb/lb fuel ⫽ A ⫺ Am; A ⫽ total air to furnace, lb/lb fuel ⫽ G ⫺ Cb ⫺ N2 ⫺ 7.94(H2 ⫺ O2 /8); G ⫽ similar to economizer but based on gas at the furnace exit; Am ⫽ external air supplied by the mill fan or other source, lb/lb of fuel. Substituting shows G ⫽ [11(0.16) ⫹ 8(0.26) ⫹ 7(0.184)]/ [3(0.16)](0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60; G ⫽ 11.03 lb/lb (11.03 kg/kg) fuel; A ⫽ 11.03 ⫺ 0.69 ⫺ 0.012 ⫺ 7.94(0.05 ⫺ 0.089/8); A ⫽ 10.02 lb/lb (10.02 kg/ kg) fuel. Heat absorbed ⫽ (1 ⫺ 0.15)(10.02)(480 ⫺ 63)(0.24 ⫹ 56/7000) ⫽ 865.5 Btu/lb (2013.2 kJ/kg fuel. 8. Compute the heat available to the air heater The heat available to the air heater, Btu/h ⫽ (t5 ⫺ t1)0.24G ⫹ (t5 ⫺ t1)0.46(Mƒ ⫹ 8.94H2 ⫹ Ma A). In this relation, all symbols are the same as for the economizer except that G and A are based on the gas entering the heater. Substituting gives G ⫽ [11(0.15) ⫹ 8(0.036) ⫹ 7(0.814)]/[3(0.15)](0.678 ⫹ 0.032/2.67) ⫹ 0.032/1.60; G ⫽ 11.72 lb/lb (11.72 kg/kg) fuel. And A ⫽ 11.72 ⫺ 0.69 ⫺ 0.012 ⫺ 7.94(0.05 ⫺ 0.089/8) ⫽ 10.71 lb/lb (10.71 kg/kg) fuel. Heat available ⫽ (570 ⫺ 3)(0.24)(11.72) ⫹ (570 ⫺ 63)(0.46)[0.045 ⫹ 8.94(0.05) ⫹ 56/7000(10.71)] ⫽ 1561 Btu/lb (3630.9 kJ/kg). 9. Compute the air-heater efficiency The air-heater efficiency ⫽ (heat absorbed, Btu/lb fuel)/(heat available, Btu/lb fuel) ⫽ 865.5/1561 ⫽ 0.554, or 55.4 percent. Related Calculations. The above procedure is valid for all types of steam gen- erators, regardless of the kind of fuel used. Where oil or gas is the fuel, alter the combustion calculations to reflect the differences between the fuels. Further, this procedure is also valid for marine and portable boilers. FIRE-TUBE BOILER ANALYSIS AND SELECTION Determine the heating surface in an 84-in (213.4-cm) diameter fire-tube boiler 18 ft (5.5 m) long having 84 tubes of 4-in (10.2-cm) ID if 25 percent of the upper Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 195. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.17 shell ends are heat-insulated. How much steam is generated if the boiler evaporates 34.5 lb/h of water per 12 ft2 [3.9 g/(m2 䡠 s)] of heating surface? How much heat is added by the boiler if it operates at 200 lb/in2 (abs) (1379.0 kPa) with 200⬚F (93.3⬚C) feedwater? What is the factor of evaporation for this boiler? How much hp is developed by the boiler if 7,000,000 Btu/h (2051.4 kW) is delivered to the water? Calculation Procedure: 1. Compute the shell area exposed to furnace gas Shell area ⫽ DL(1 ⫺ 0.25), where D ⫽ boiler diameter, ft; L ⫽ shell length, ft; 1 ⫺ 0.25 is the portion of the shell in contact with the furnace gas. Then shell area ⫽ (84/12)(18)(0.75) ⫽ 297 ft2 (27.0 m2 ). 2. Compute the tube area exposed to furnace gas Tube area ⫽ dLN, where ⫽ tube ID, ft; L ⫽ tube length, ft; N ⫽ number of tubes in boiler. Substituting gives tube area ⫽ (4/12)(18)(84) ⫽ 1583 ft2 (147.1 m2 ). 3. Compute the head area exposed to furnace gas The area exposed to furnace gas is twice (since there are two heads) the exposed head area minus twice the area occupied by the tubes. The exposed head area is (total area)(1 ⫺ portion covered by insulation, expressed as a decimal). Substituting, we get 2D2 /4 ⫺ (2)(84)d2 /4 ⫽ 2/4(84/12)2 (0.75) ⫺ (2)(84)(4/12)2 /4 ⫽ head area ⫽ 43.1 ft2 (4.0 m2 ). 4. Find the total heating surface The total heating surface of any fire-tube boiler is the sum of the shell, tube, and head areas, or 297.0 ⫹ 1583 ⫹ 43.1 ⫽ 1923 ft2 (178.7 m2 ), total heating surface. 5. Compute the quantity of steam generated Since the boiler evaporates 34.5 lb/h of water per 12 ft2 [3.9 g/(m2 䡠 s)] of heating surface, the quantity of steam generated ⫽ 34.5 (total heating surface, ft2 )/12 ⫽ 34.5(1923.1)/12 ⫽ 5200 lb/h (0.66 kg/s). Note: Evaporation of 34.5 lb/h (0.0043 kg/s) from and at 212⬚F (100.0⬚C) is the definition of the now-discarded term boiler hp. However, this term is still met in some engineering examinations and is used by some manufacturers when com- paring the performance of boilers. A term used in lieu of boiler horsepower, with the same definition, is equivalent evaporation. Both terms are falling into disuse, but they are included here because they still find some use today. 6. Determine the heat added by the boiler Heat added, Btu/lb of steam ⫽ hg ⫺ hƒ1; from steam table values 1198.4 ⫺ 167.99 ⫽ 1030.41 Btu/lb (2396.7 kJ/kg). An alternative way of computing heat added is hg ⫺ (feedwater temperature,⬚F, ⫺ 32), where 32 is the freezing temper- ature of water on the Fahrenheit scale. By this method, heat added ⫽ 1198.4 ⫺ (200 ⫺ 32) ⫽ 1030.4 Btu/lb (2396.7 kJ/kg). Thus, both methods give the same results in this case. In general, however, use of steam table values is preferred. 7. Compute the factor of evaporation The factor of evaporation is used to convert from the actual to the equivalent evap- oration, defined earlier. Or, factor of evaporation ⫽ (heat added by boiler, Btu/lb)/970.3, where 970.3 Btu/lb (2256.9 kJ/kg) is the heat added to develop 1 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 196. 4.18 POWER GENERATION boiler hp (bhp) (0.75 kW). Thus, the factor of evaporation for this boiler ⫽ 1030.4/970.3 ⫽ 1.066. 8. Compute the boiler hp output Boiler hp ⫽ (actual evaporation, lb/h) (factor of evaporation)/34.5. In this relation, the actual evaporation must be computed first. Since the furnace delivers 7,000,000 Btu/h (2051.5 kW) to the boiler water and the water absorbs 1030.4 Btu/lb (2396.7 kJ/kg) to produce 200-lb/in2 (abs) (1379.0-kPa) steam with 200⬚F (93.3⬚C) feed- water, the steam generated, lb/h ⫽ (total heat delivered, Btu/h)/(heat absorbed, Btu/lb) ⫽ 7,000,000/1030.4 ⫽ 6670 lb/h (0.85 kg/s). Then boiler hp ⫽ (6760)(1.066)/34.5 ⫽ 209 hp (155.9 kW). The rated hp output of horizontal fire-tube boilers with separate supporting walls is based on 12 ft2 (1.1 m2 ) of heating surface per boiler hp. Thus, the rated hp of the boiler ⫽ 1923.1/12 ⫽ 160 hp (119.3 kW). When producing 209 hp (155.9 kW), the boiler is operating at 209/160, or 1,305 times its normal rating, or (100)(1.305) ⫽ 130.5 percent of normal rating. Note: Today most boiler manufacturers rate their boilers in terms of pounds per hour of steam generated at a stated pressure. Use this measure of boiler output whenever possible. Inclusion of the term boiler hp in this handbook does not in- dicate that the editor favors or recommends its use. Instead, the term was included to make the handbook as helpful as possible to users who might encounter the term in their work. SAFETY-VALVE STEAM-FLOW CAPACITY How much saturated steam at 150 lb/in2 (abs) (1034.3 kPa) can a 2.5-in (6.4-cm) diameter safety valve having a 0.25-in (0.6-cm) lift pass if the discharge coefficient of the valve cd is 0.75? What is the capacity of the same valve if the steam is superheated 100⬚F (55.6⬚C) above its saturation temperature? Calculation Procedure: 1. Determine the area of the valve annulus Annulus area, in2 ⫽ A ⫽ DL, where D ⫽ valve diameter, in; L ⫽ valve lift, in. Annulus area ⫽ (2.5)(0.25) ⫽ 1.966 in2 (12.7 cm2 ). 2. Compute the ideal flow for this safety valve Ideal flow Fi lb/s for any safety valve handling saturated steam is Fi ⫽ A/60, 0.97 ps where ps ⫽ saturated-steam pressure, lb/in2 (abs). For this valve, Fi ⫽ (150)0.97 (1.966)/60 ⫽ 4.24 lb/s (1.9 kg/s). 3. Compute the actual flow through the valve Actual flow Fa ⫽ Fi cd ⫽ (4.24)(0.75) ⫽ 3.18 lb/s (1.4 kg/s) ⫽ (3.18)(3600 s/h) ⫽ 11,448 lb/h (1.44 kg/s). 4. Determine the superheated-steam flow rate The ideal superheated-steam flow Fis lb/s is Fis ⫽ A/[60(1 ⫹ 0.0065ts)], where 0.97 ps ts ⫽ superheated temperature, above saturation temperature,⬚F. The Fis ⫽ (150)0.97 (1.966)/[60(1 ⫹ 0.0065 ⫻ 100)] ⫽ 3.96 lb/s (1.8 kg/s). The actual flow Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 197. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.19 is Fas ⫽ Fiscd ⫽ (3.96)(0.75) ⫽ 2.97 lb/s (1.4 kg/s) ⫽ (2.97)(3600) ⫽ 10,700 lb/ h (1.4 kg/s). Related Calculations. Use this procedure for safety valves serving any type of stationary or marine boiler. SAFETY-VALVE SELECTION FOR A WATERTUBE STEAM BOILER Select a safety valve for a watertube steam boiler having a maximum rating of 100,000 lb/h (12.6 kg/s) at 800 lb/in2 (abs) (5516.0 kPa) and 900⬚F (482.2⬚C). Determine the valve diameter, size of boiler connection for the valve, opening pressure, closing pressure, type of connection, and valve material. The boiler is oil- fired and has a total heating surface of 9200 ft2 (854.7 m2 ) of which 1000 ft2 (92.9 m2 ) is in waterwall surface. Use the ASME Boiler and Pressure Vessel Code rules when selecting the valve. Sketch the escape-pipe arrangement for the safety valve. Calculation Procedure: 1. Determine the minimum valve relieving capacity Refer to the latest edition of the Code for the relieving-capacity rules. Recent edi- tions of the Code require that the safety valve have a minimum relieving capacity based on the pounds of steam generated per hour per square foot of boiler heating surface and waterwall heating surface. In the edition of the Code used in preparing this handbook, the relieving requirement for oil-fired boilers was 10 lb/(ft2 䡠 h) of steam [13.6 g/(m2 䡠 s)] of boiler heating surface, and 16 lb/(ft2 䡠 h) of steam [21.9 g/(m2 䡠 s)] of waterwall surface. Thus, the minimum safety-valve relieving capacity for this boiler, based on total heating surface, would be (8200)(10) ⫹ (1000)(16) ⫽ 92,000 lb/h (11.6 kg/s). In this equation, 1000 ft2 (92.9 m2 ) of waterwall surface is deducted from the total heating surface of 9200 ft2 (854.7 m2 ) to obtain the boiler heating surface of 8200 ft2 (761.8 m2 ). The minimum relieving capacity based on total heating surface is 92,000 lb/h (11.6 kg/s); the maximum rated capacity of the boiler is 100,000 lb/h (12.6 kg/s). Since the Code also requires that ‘‘the safety valve or valves will discharge all the steam that can be generated by the boiler,’’ the minimum relieving capacity must be 100,000 lb/h (12.6 kg/s), because this is the maximum capacity of the boiler and it exceeds the valve capacity based on the heating-surface calculation. If the valve capacity based on the heating-surface steam generation were larger than the stated maximum capacity of the boiler, the Code heating-surface valve capacity would be used in safety-valve selection. 2. Determine the number of safety valves needed Study the latest edition of the Code to determine the requirements for the number of safety valves. The edition of the Code used here requires that ‘‘each boiler shall have at least one safety valve and if it [the boiler] has more than 500 ft2 (46.5 m2 ) of water heating surface, it shall have two or more safety valves.’’ Thus, at least two safety valves are needed for this boiler. The Code further specifies, in the edition used, that ‘‘when two or more safety valves are used on a boiler, they may be mounted either separately or as twin valves made by placing individual valves on Y bases or duplex valves having two valves in the same body casing. Twin Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 198. 4.20 POWER GENERATION valves made by placing individual valves on Y bases, or duplex valves having two valves in the same body, shall be of equal sizes.’’ Also, ‘‘when not more than two valves of different sizes are mounted singly, the relieving capacity of the smaller valve shall not be less than 50 percent of that of the larger valve.’’ Assume that two equal-size valves mounted on a Y base will be used on the steam drum of this boiler. Two or more equal-size valves are usually chosen for the steam drum of a watertube boiler. Since this boiler handles superheated steam, check the Code requirements re- garding superheaters. The Code states that ‘‘every attached superheater shall have one or more safety valves near the outlet.’’ Also, ‘‘the discharge capacity of the safety valve, or valves, on an attached superheater may be included in determining the number and size of the safety valves for the boiler, provided there are no intervening valves between the superheater safety valve and the boiler, and provided the discharge capacity of the safety valve, or valves, on the boiler, as distinct from the superheater, is at least 75 percent of the aggregate valve capacity required.’’ Since the safety valves used must handle 100,000 lb/h (12.6 kg/s), and one or more superheater safety valves are required by the Code, assume that the two steam- drum valves will handle, in accordance with the above requirement, 80,000 lb/h (10.1 kg/s). Assume that one superheater safety valve will be used. Its capacity must then be at least 100,000 ⫺ 80,000 ⫽ 20,000 lb/h (2.5 kg/s). (Use a few superheater safety valves as possible, because this simplifies the installation and reduces cost.) With this arrangement, each steam-drum valve must handle 80,000/ 2 ⫽ 40,000 lb/h (5.0 kg/s) of steam, since there are two safety valves on the steam drum. 3. Determine the valve pressure settings Consult the Code. It requires that ‘‘one or more safety valves on the boiler proper shall be set at or below the maximum allowable working pressure.’’ For modern boilers, the maximum allowable working pressure is usually 1.5, or more, times the rated operating pressure in the lower [under 1000 lb/in2 (abs) or 6895.0 kPa] pressure ranges. To prevent unnecessary operation of the safety valve and to reduce steam losses, the lowest safety-valve setting is usually about 5 percent higher than the boiler operating pressure. For this boiler, the lowest pressure setting would be 800 ⫹ 800(0.05) ⫽ 840 lb/in2 (abs) (5791.8 kPa). Round this to 850 lb/in2 (abs) (5860.8 kPa, or 6.25 percent) for ease of selection from the usual safety-valve rating tables. The usual safety-valve pressure setting is between 5 and 10 percent higher than the rated operating pressure of the boiler. Boilers fitted with superheaters usually have the superheater safety valve set at a lower pressure than the steam-drum safety valve. This arrangement ensures that the superheater safety valve opens first when overpressure occurs. This provides steam flow through the superheater tubes at all times, preventing tube burnout. Therefore, the superheater safety valve in this boiler will be set to open at 850 lb /in2 (abs) (5860.8 kPa), the lowest opening pressure for the safety valves chosen. The steam-drum safety valves will be set to open at a higher pressure. As decided earlier, the superheater safety valve will have a capacity of 20,000 lb/h (2.5 kg/s). Between the steam drum and the superheater safety valve, there is a pressure loss that varies from one boiler to another. The boiler manufacturer supplies a performance chart showing the drum outlet pressure for various percentages of the maximum continuous steaming capacity of the boiler. This chart also shows the superheater outlet pressure for the same capacities. The difference between the drum and superheater outlet pressure for any given load is the superheater pressure loss. Obtain this pressure loss from the performance chart. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 199. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.21 Assume, for this boiler, that the superheater pressure loss, plus any pressure losses in the nonreturn valve and dry pipe, at maximum rating, is 60 lb/in2 (abs) (413.7 kPa). The steam-drum operating pressure will then be superheater outlet pressure ⫹ superheater pressure loss ⫽ 800 ⫹ 60 ⫽ 860 lb/in2 (abs) (5929.7 kPa). As with the superheater safety valve, the steam-drum safety valve is usually set to open at about 5 percent above the drum operating pressure at maximum steam output. For this boiler then, the drum safety-valve set pressure ⫽ 860 ⫹ 860(0.05) ⫽ 903 lb/in2 (abs) (6226.2 kPa). Round this to 900 lb/in2 (abs) (6205.5 kPa) to simplify valve selection. Some designers add the drum safety-valve blowdown or blowback pressure (dif- ference between the valve opening and closing pressures, lb/in2 ) to the total ob- tained above to find the drum operating pressure. However, the 5 percent allowance used above is sufficient to allow for the blowdown in boilers operating at less than 1000 lb/in2 (abs) (6895.0 kPa). At pressures of 1000 lb/in2 (abs) (6895.0 kPa) and higher, add the drum safety-valve blowdown and the 5 percent allowance to the superheater outlet pressure and pressure loss to find the drum pressure. 4. Determine the required valve orifice discharge area Refer to a safety-valve manufacturer’s engineering data listing valve capacities at various working pressures. For the two steam-drum valves, enter the table at 900 lb/in2 (abs) (6205.5 kPa), and project horizontally until a capacity of 40,000 lb/h (5.0 kg/s), or more, is intersected. Here is an excerpt from a typical manufacturer’s capacity table for safety valves handling saturated steam: Thus, at 900 lb/in2 (abs) (6205.5 kPa) a valve with an orifice area of 0.944 in2 (6.4 cm2 ) will have a capacity of 42,200 lb/h (5.3 kg/s) of saturated steam. This is 5.5 percent greater than the required capacity of 40,000 lb/h (5.0 kg/s) for each steam- drum valve. However, the usual selection cannot be made at exactly the desired capacity. Provided that the valve chosen has a greater steam relieving capacity than required, there is no danger of overpressure in the steam drum. Be careful to note that safety valves for saturated steam are chosen for the steam drum because su- perheating of the steam does not occur in the steam drum. The superheater safety valve must handle 20,000 lb/h (2.5 kg/s) of 850 lb/in2 (abs) (5860.8-kPa) steam at 900⬚F (482.2⬚C). Safety valves handling superheated steam have a smaller capacity than when handling saturated steam. To obtain the capacity of a safety valve handling superheated steam, the saturated steam capacity is multiplied by a correction factor that is less than 1.00. An alternative procedure is to divide the required superheated-steam capacity by the same correction factor to obtain the saturated-steam capacity of the valve. The latter procedure will be used here because it is more direct. Obtain the correction factor from the safety-valve manufacturer’s engineering data by entering at the steam pressure and projecting to the steam temperature, as show below. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 200. 4.22 POWER GENERATION Thus, at 850 lb/in2 (abs) (5860.8 kPa) and 900⬚F (482.2⬚C), the correction factor is 0.80. The required saturated steam capacity then is 20,000/0.80 ⫽ 25,000 lb/h (3.1 kg/s). Refer to the manufacturer’s saturated-steam capacity table as before, and at 850 lb/in2 (abs) (5860.8 kPa) find the closest capacity as 31,500 lb/h (4.0 kg/s) for a 0.785-in2 (5.1-cm2 ) orifice. As with the steam-drum valves, the actual capacity of the safety valve is somewhat greater than the required capacity. In general, it is difficult to find a valve with exactly the required steam relieving capacity. 5. Determine the valve nominal size and construction details Turn to the data section of the safety-valve engineering manual to find the valve construction features. For the steam-drum valves having 0.994-in2 (6.4-cm2 ) orifice areas, the engineering data show, for 900-lb/in2 (abs) (6205.5-kPa) service, each valve is 11 ⁄2-in (3.8-cm) unit rated for temperatures up to 1050⬚F (565.6⬚C). The inlet is 900-lb/in2 (6205.5-kPa) 11 ⁄2-in (3.8-cm) flanged connection, and the outlet is a 150-lb/in2 (1034.3-kPa) 3-in (7.6-cm) flanged connection. Materials used in the valve include: body, cast carbon steel; disk seat, stainless steel AISI 321. The overall height is 277 ⁄8 in (70.8 cm); dismantled height is 323 ⁄4 in (83.2 cm). Similar data for the superheated steam valve show, for a maximum pressure of 900 lb/in2 (abs) (6205.5 kPa), that it is a 11 ⁄2-in (3.8-cm) unit rated for temperatures up to 1000⬚F (537.8⬚C). The inlet is a 900-lb/in2 (6205.5-kPa) 11 ⁄2-in (3.8-cm) flanged connection, and the outlet is a 150-lb/in2 (1034.3-kPa) 3-in (7.6-cm) flanged connection. Materials used in the valve include: body, cast alloy steel, ASTM 217-WC6; spindle, stainless steel; spring, alloy steel; disk seat, stainless steel. Overall height is 213 ⁄8 in (54.3 cm); dismantled height is 251 ⁄4 in (64.1 cm). Checking the Code shows that ‘‘every safety valve used on a superheater discharg- ing superheated steam at a temperature over 450⬚F (232.2⬚C) shall have a casing, including the base, body, bonnet and spindle, of steel, steel alloy, or equivalent heat-resisting material. The valve shall have a flanged inlet connection.’’ Thus, the superheater valve selected is satisfactory. 6. Compute the steam-drum connection size The Code requires that ‘‘when a boiler is fitted with two or more safety valves on one connection, this connection to the boiler shall have a cross-sectional area not less than the combined areas of inlet connections of all safety valves with which it connects.’’ The inlet area for each valve ⫽ D2 /4 ⫽ (1.5)2 /4 ⫽ 1.77 in2 (11.4 cm2 ). For two valves, the total inlet area ⫽ 2(1.77) ⫽ 3.54 in2 (22.8 cm2 ). The required minimum diameter of the boiler connection is d ⫽ 2(A/)0.5 , where A ⫽ inlet area. Or, d ⫽ 2(3.54/)0.5 ⫽ 2.12 in (5.4 cm). Select a 21 ⁄2 ⫻ 11 ⁄2 ⫻ 11 ⁄2 in (6.4 ⫻ 3.8 ⫻ 3.8 cm). Y for the two steam-drum valves and a 21 ⁄2-in (6.4-cm) steam-drum outlet connection. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 201. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.23 FIGURE 3 Typical boiler safety-valve discharge elbow and drip-pan connection. (Industrial Valve and Instrument Division of Dresser Industries Inc.) 7. Compute the safety-valve closing pressure The Code requires safety valves to ‘‘close after blowing down not more than 4 percent of the set pressure.’’ For the steam-drum valves the closing pressure will be 900 ⫺ (900)(0.04) ⫽ 865 lb/in2 (abs) (5964.2 kPa). The superheater safety valve will close at 850 ⫺ (850)(0.04) ⫽ 816 lb/in2 (abs) (5626.3 kPa). 8. Sketch the discharge elbow and drip pan Figure 3 shows a typical discharge elbow and drip-pan connection. Fit all boiler safety valves with escape pipes to carry the steam out of the building and away from personnel. Extend the escape pipe to at least 6 ft (1.8 m) above the roof of the building. Use an escape pipe having a diameter equal to the valve outlet size. When the escape pipe is more than 12 ft (3.7 m) long, some authorities recommend increasing the escape-pipe diameter by 1 ⁄2 in (1.3 cm) for each additional 12-ft (3.7- m) length. Excessive escape-pipe length without an increase in diameter can cause a backpressure on the safety valve because of flow friction. The safety valve may then chatter excessively. Support the escape pipe independently of the safety valve. Fit a drain to the valve body and rip pan as shown in Fig. 3. This prevents freezing of the condensate Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 202. 4.24 POWER GENERATION and also eliminates the possibility of condensate in the escape pipe raising the valve opening pressure. When a muffler is fitted to the escape pipe, the inlet diameter of the muffler should be the same as, or larger than, the escape-pipe diameter. The outlet area should be greater than the inlet area of the muffler. Related Calculations. Compute the safety-valve size for fire-tube boilers in the same way as described above, except that the Code gives a tabulation of the required area for safety-valve boiler connections based on boiler operating pressure and heating surface. Thus, with an operating pressure of 200 lb/in2 (gage) (1379.0 kPa) and 1800 ft2 (167.2 m2 ) of heating surface, the Code table shows that the safety- valve connection should have an area of at least 9.148 in2 (59.0 cm2 ). A 31 ⁄2-in (8.9-cm) connection would provide this area; or two smaller connections could be used provided that the sum of their areas exceeded 9.148 in2 (59.0 cm2 ) Note: Be sure to select safety valves approved for use under the Code or local low governing boilers in the area in which the boiler will be used. Choice of an unapproved valve can lead to its rejection by the bureau or other agency controlling boiler installation and operation. STEAM-QUALITY DETERMINATION WITH A THROTTLING CALORIMETER Steam leaves an industrial boiler at 120 lb/in2 (abs) (827.4 kPa) and 341.25⬚F (171.8⬚C). A portion of the steam is passed through a throttling calorimeter and is exhausted to the atmosphere when the barometric pressure is 14.7 lb/in2 (abs) (101.4 kPa). How much moisture does the steam leaving the boiler contain if the temperature of the steam at the calorimeter is 240⬚F (115.6⬚C)? Calculation Procedure: 1. Plot the throttling process on the Mollier diagram Begin with the endpoint, 14.7 lb/in2 (abs) (101.4 kPa) and 240⬚F (115.6⬚C). Plot this point on the Mollier diagram as point A, Fig 4. Note that this point is in the superheat region of the Mollier diagram, because steam at 14.7 lb/in2 (abs) (101.4 kPa) has a temperature of 212⬚F (100.0⬚C), whereas the steam in this calorimeter has a temperature of 240⬚F (115.6⬚C). The enthalpy of the calorimeter steam is, from the Mollier diagram, 1164 Btu/lb (2707.5 kJ/kg). 2. Trace the throttling process on the Mollier diagram In a throttling process, the steam expands at constant enthalpy. Draw a straight, horizontal line from point A to the left on the Mollier diagram until the 120-lb/in2 (abs) (827.4-kPa) pressure curve is intersected, point B, Fig. 4. Read the moisture content of the steam as 3 percent where the 1164-Btu/lb (2707.5-kJ/kg) horizontal trace AB, the 120-lb/in2 (abs) (827.4-kPa) pressure line, and the 3 percent moisture line intersect. Related Calculations. A throttling calorimeter must produce superheated steam at the existing atmospheric pressure if the moisture content of the supply steam is to be found. Where the throttling calorimeter cannot produce superheated steam at atmospheric pressure, connect the calorimeter outlet to an area at a pressure less than atmospheric. Expand the steam from the source, and read the temperature at the calorimeter. If the steam temperature is greater than that corresponding to the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 203. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.25 FIGURE 4 Mollier-diagram plot of a throttling-calorimeter process. absolute pressure of the vacuum area—for example, a temperature greater than 133.76⬚F (56.5⬚C) in an area of 5 inHg (16.9 kPa) absolute pressure—follow the same procedure as given above. Point A would then be in the below-atmospheric area of the Mollier diagram. Trace to the left to the origin pressure, and read the moisture content as before. STEAM PRESSURE DROP IN A BOILER SUPERHEATER What is the pressure loss in a boiler superheater handling ws ⫽ 200,000 lb/h (25.2 kg/s) of saturated steam at 500 lb/in2 (abs) (3447.5 kPa) if the desired outlet temperature is 750⬚F (398.9⬚C)? The steam free-flow area through the superheater tubes As ft2 is 0.500, friction factor ƒ is 0.025, tube ID is 2.125 in (5.4 cm), developed length l of a tube in one circuit is 150 in (381.0 cm), and the tube bend factor Bƒ is 12.0. Calculation Procedure: 1. Determine the initial conditions of the steam To compute the pressure loss in a superheater, the initial specific volume of the steam vg and the mass-flow ratio ws /As must be known. From the steam table, vg ⫽ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 204. 4.26 POWER GENERATION 0.9278 ft3 /lb (0.058 m3 /kg) at 500 lb/in2 (abs) (3447.5 kPa) saturated. The mass- flow ratio ws /As ⫽ 200,000/0.500 ⫽ 400,000. 2. Compute the superheater entrance and exit pressure loss Entrance and exit pressure loss pE lb/in2 ⫽ vƒ /8(0.00001ws /As) ⫽ 0.9278/ 8[(0.00001) ⫻ (400,000)]2 ⫽ 1.856 lb/in2 (12.8 kPa). 3. Compute the pressure loss in the straight tubes Straight-tube pressure loss ps lb/in2 ⫽ vƒƒ/ID(0.00001ws /As)2 ⫽ 0.9278(150) ⫻ (0.025)/2.125[(0.00001)(400,000)]2 ⫽ 26.2 lb/in2 (abs) (180.6 kPa). 4. Compute the pressure loss in the superheater bends Bend pressure loss pb ⫽ 0.0833Bƒ(0.00001ws /As)2 ⫽ 0.0833(12.0)[(0.00001) ⫻ (400,000)]2 ⫽ 16.0 lb/in2 (110.3 kPa). 5. Compute the total pressure loss The total pressure loss in any superheater is the sum of the entrance, straight-tube, bend, and exit-pressure losses. These losses were computed in steps 2, 3, and 4 above. Therefore, total pressure loss pt ⫽ 1.856 ⫹ 26.2 ⫹ 16.0 ⫽ 44.056 lb/in2 (303.8 kPa). Note: Data for superheater pressure-loss calculations are best obtained from the boiler manufacturer. Several manufacturers have useful publications discussing su- perheater pressure losses. These are listed in the references at the beginning of this section. SELECTION OF A STEAM BOILER FOR A GIVEN LOAD Choose a steam boiler, or boilers, to deliver up to 250,000 lb/h (31.5 kg/s) of superheated steam at 800 lb/in2 (abs) (5516 kPa) and 900⬚F (482.2⬚C). Determine the type or types of boilers to use, the capacity, type of firing, feedwater-quality requirements, and best fuel if coal, oil, and gas are all available. The normal con- tinuous steam requirement is 200,000 lb/h (25.2 kg/s). Calculation Procedure: 1. Select type of steam generator Use Fig. 5 as a guide to the usual types of steam generators chosen for various capacities and different pressure and temperature conditions. Enter Fig. 5 at the left at 800 lb/in2 (abs) (5516 kPa), and project horizontally to the right, along AB, until the 250,000-lb/h (31.5-kg/s) capacity ordinate BC is intersected. At B, the oper- ating point of this boiler, Fig. 5 shows that a watertube boiler should be used. Boiler units presently available can deliver steam at the desired temperature of 900⬚F (482.2⬚C). The required capacity of 250,000 lb/h (31.5 kg/s) is beyond the range of packaged watertube boilers—defined by the American Boiler Manufac- turer Association as ‘‘a boiler equipped and shipped complete with fuel-burning equipment, mechanical-draft equipment, automatic controls, and accessories.’’ Shop-assembled boilers are larger units, where all assembly is handled in the builder’s plant but with some leeway in the selection of controls and auxiliaries. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 205. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.27 FIGURE 5 Typical pressure and capacity relationships from steam generators. (Power.) The current maximum capacity of shop-assembled boilers is about 100,000 lb/h (12.6 kg/s). Thus, a standard-design, larger-capacity boiler is required. Study manufacturers’ engineering data to determine which types of watertube boilers are available for the required capacity, pressure, and temperature. This study reveals that, for this installation, a standard, field-assembled, welded-steel-cased, bent-tube, single-steam-drum boiler with a completely water-cooled furnace would be suitable. This type of boiler is usually fitted with an air heater, and an economizer might also be used. The induced- and forced-draft fans are not integral with the boiler. Capacities of this type of boiler usually available range from 50,000 to 350,000 lb/h (6.3 to 44.1 kg/s); pressure from 160 to 1050 lb/in2 (1103.2 to 7239.8 kPa); steam temperature from saturation to 950⬚F (510.0⬚C); fuels—pulverized coal, oil, gas, or a combination; controls—manual to completely automatic; efficiency—to 90 percent. 2. Determine the number of boilers required The normal continuous steam requirement is 200,000 lb/h (25.2 kg/s). If a 250,000-lb/h (31.5-kg/s) boiler were chosen to meet the maximum required output, the boiler would normally operate at 2000,000/250,000, or 80 percent capacity. Obtain the performance chart, Fig. 6, from the manufacturer and study it. This chart shows that at 80 percent load, the boiler efficiency is about equal to that at 100 percent load. Thus, there will not be any significant efficiency loss when the unit is operated at its normal continuous output. The total losses in the boiler are lower at 80 percent load than at full (100 percent) load. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 206. 4.28 POWER GENERATION FIGURE 6 Typical watertube steam-generator losses and efficiency. Since there is not a large efficiency decrease at the normal continuous load, and since there are not other factors that require or make more than one boiler desirable, a single boiler unit would be most suitable for this installation. One boiler is more desirable than two or more because installation of a single unit is simpler and maintenance costs are lower. However, where the load fluctuates widely and two or more boilers could best serve the steam demand, the savings in installation and maintenance costs would be insignificant compared with the extra cost of operating a relatively large boiler installed in place of two or more smaller boilers. Therefore, each installation must be carefully analyzed and a decision made on the basis of the existing conditions. 3. Determine the required boiler capacity The stated steam load is 250,000 lb/h (31.5 kg/s) at maximum demand. Study the installation to determine whether the steam demand will increase in the future. Try to determine the rate of increase in the steam demand; for example, installation of several steam-using process units each year during the next few years will increase the steam demand by a predictable amount every year. By using these data, the rate of growth and total steam demand can be estimated for each year. Where the growth will exceed the allowable overload capacity of the boiler—which can vary from 0 to 50 percent of the full-load rating, depending on the type of unit chosen—consider installing a larger-capacity boiler now to meet future load growth. Where the future load is unpredictable or where no load growth is anticipated, a unit sized to meet today’s load would be satisfactory. If this situation existed in this plant, a 250,000- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 207. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.29 lb/h unit (31.5-kg/s) would be chosen for the load. Any small temporary overloads could be handled by operating the boiler at a higher output for short periods. Alternatively, assume that a load of 25,000 lb/h (3.1 kg/s) will be added to the maximum demand on this boiler each year for the next 5 years. This means that in 5 years the maximum demand will be 250,000 ⫹ 25,000(5) ⫽ 375,000 lb/h (47.2 kg/s). This is an overload of (375,000 ⫺ 250,000)/250,000 ⫽ 0.50, or 50 percent. It is unlikely that the boiler could carry a continuous overload of 50 per- cent. Therefore it might be wise to install a 375,000-lb/h (47.2-kg/s) boiler to meet present and future demands. Base this decision on the accuracy of the future- demand prediction and the economic advantages or disadvantages of investing more money now for a demand that will not occur until some future date. Refer to the section on engineering economics for procedures to follow in economics calcula- tions of this type. Thus, with no increase in the future load, a 250,000-lb/h (31.5-kg/s) unit would be chosen. With the load increase specified, a 375,000-lb/h (47.2-kg/s) unit would be the choice, if there were no major economic disadvantages. 4. Choose the type of fuel to use Watertube boilers of the type being considered will economically burn the three fuels available—coal, oil, or gas—either singly or in combination. In the design considered here, the furnace watercooled surfaces and boiler surfaces are integral parts of each other. For this reason the boiler is well suited for pulverized-coal firing in the 50,000- to 300,000-lb/h (6.3- to 37.8-kg/s) capacity range. Thus, if a 250,000-lb/h (31.5-kg/s) unit were chosen, it could be fired by pulverized coal. With a larger unit of 375,000 lb/h (47.2 kg/s), pulverized coal, oil, or gas firing might be used. Use an economic comparison to determine which fuel would give the lowest overall operating coast for the life of the boiler. 5. Determine the feedwater-quality requirements Watertube boilers of all types require careful control of feedwater quality to prevent scale and sludge deposits in tubes and drums. Corrosion of the interior boiler sur- faces must be controlled. Where all condensate is returned to the boiler, the makeup water must be treated to prevent the conditions just cited. Therefore, a comprehen- sive water-treating system must be planned for, particularly if the raw-water supply is poor. 6. Estimate the boiler space requirements The space occupied by steam-generating units is an important consideration in plants in municipal areas and where power-plant buildings are presently crowded by existing equipment. The manufacturer’s engineering data for this boiler show that for pulverized-coal firing, the hopper-type furnace bottom is best. The data also show that the smallest boiler with a hopper bottom occupies a space 21 ft (6.4 m) wide, 31 ft (9.4 m) high, and 14 ft (4.3 m) front to rear. The largest boiler occupies a space 21 ft (6.4 m) wide, 55 ft (16.8 m) high, and 36 ft (11.0 m) front to rear. Check these dimensions against the available space to determine whether the chosen boiler can be installed without major structural changes. The steel walls permit outdoor or indoor installation with top or bottom support of the boiler op- tional in either method of installation. Related Calculations. Use this general procedure to select boilers for indus- trial, central-station, process, and marine applications. Where a boiler is to burn hazardous industrial waste as a fuel, the designer must carefully observe two waste laws: the 1980 Superfund law and the 1976 Resource Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 208. 4.30 POWER GENERATION Conservation and Recovery Act. These laws regulate the firing of hazardous wastes in boilers to control air pollution and explosion dangers. Since hazardous wastes from industrial operations can vary in composition, it is important that the designer know what variables might be met during actual firing. Without correct analysis of the wastes, air pollution can become a severe problem in the plant locale. The Environmental Protection Agency (EPA) and state regulatory agencies should be carefully consulted before any final design decisions are made for a new and expanded boiler plants. While the firing of hazardous wastes can be a conven- ient way to dispose of them, the potential impact on the environment must be considered before any design is finalized. SELECTING BOILER FORCED- AND INDUCED-DRAFT FANS Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) of air for combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet is 75⬚F (23.9⬚C) and the average temperature of the combustion gas leaving the air heater is 350⬚F (176.7⬚C) with an ambient barometric pressure of 29.9 inHg (101.0 kPa). Pressure losses on the air-inlet side are as follows, in inH2O: air heater, 1.5 (0.37 kPa); air-supply ducts, 0.75 (0.19 kPa); boiler windbox, 1.75 (0.44 kPa); burners, 1.25 (0.31 kPa). Draft losses in the boiler and related equipment are as follows, in inH2O: furnace pressure, 0.20 (0.05 kPa); boiler, 3.0 (0.75 kPa); superheater 1.0 (0.25 kPa); economizer, 1.50 (0.37 kPa); air heater, 2.00 (0.50 kPa); uptake ducts and dampers, 1.25 (0.31 kPa). Determine the fan discharge pressure and hp input. The boiler burns 18,000 lb/h (2.3 kg/s) of oil at full load. Calculation Procedure: 1. Compute the quantity of air required for combustion The combustion calculations show that 200,000 lb/h (25.2 kg/s) of air is theoret- ically required for combustion in this boiler. To this theoretical requirement must be added allowances for excess air at the burner and leakage out of the air heater and furnace. Allow 25 percent excess air for this boiler. The exact allowance for a given installation depends on the type of fuel burned. However, a 25 percent excess- air allowance is an average used by power-plant designers for coal, oil, and gas firing. With this allowance, the required excess air ⫽ 200,000(0.25) ⫽ 50,000 lb/h (6.3 kg/s). Air-heater air leakage varies from about 1 to 2 percent of the theoretically re- quired airflow. Using 2 percent, we see the air-heater leakage allowance ⫽ 200,000(0.02) ⫽ 4,000 lb/h (0.5 kg/s). Furnace air leakage ranges from 5 to 10 percent of the theoretically required airflow. With 7.5 percent, the furnace leakage allowance ⫽ 200,000(0.075) ⫽ 15,000 lb/h (1.9 kg/s). The total airflow required is the sum of the theoretical requirement, excess air, and leakage. Or, 200,000 ⫹ 50,000 ⫹ 4000 ⫹ 15,000 ⫽ 269,000 lb/h (33.9 kg/s). The forced-draft fan must supply at least this quantity of air to the boiler. Usual practice is to allow a 10 to 20 percent safety factor for fan capacity to ensure an adequate air supply at all operating conditions. This factor of safety is applied Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 209. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.31 to the total airflow required. Using a 10 percent factor of safety, we see that fan capacity ⫽ 269,000 ⫹ 269,000(0.1) ⫽ 295,900 lb/h (37.3 kg/s). Round this to 269,000-lb/h (37.3 kg/s) fan capacity. 2. Express the required airflow in cubic feet per minute Convert the required flow in pounds per hour to cubic feet per minute. To do this, apply a factor of safety to the ambient air temperature to ensure an adequate air supply during times of high ambient temperature. At such times, the density of the air is lower, and the fan discharges less air to the boiler. The usual practice is to apply a factor of safety of 20 to 25 percent to the known ambient air temperature. Using 20 percent, we see the ambient temperature for fan selection ⫽ 75 ⫹ 75(0.20) ⫽ 90⬚F (32.2⬚C). The density of air at 90⬚F (32.2⬚C) is 0.0717 lb/ft3 (1.15 kg/m3 ), found in Baumeister and Marks—Standard Handbook for Mechanical En- gineers. Converting gives ft3 /min ⫹ (lb/h)/(60 lb/ft3 ) ⫽ 296,000/60(0.0717) ⫽ 69,400 ft3 /min (32.8 m3 /s). This is the minimum capacity the forced-draft fan may have. 3. Determine the forced-draft discharge pressure The total resistance between the forced-draft fan outlet and furnace is the sum of the losses in the air heater, air-supply ducts, boiler windbox, and burners. For this boiler, the total resistance, inH2O ⫽ 1.5 ⫹ 0.75 ⫹ 1.75 ⫹ 1.25 ⫽ 5.25 inH2O (1.3 kPa). Apply a 15 to 30 percent factor of safety to the required discharge pressure to ensure adequate airflow at all times. Or, fan discharge pressure, with a 20 percent factor of safety ⫽ 5.25 ⫹ 5.25(0.20) ⫽ 6.30 inH2O (1.6 kPa). The fan must there- fore deliver at least 69,400 ft3 /min (32.8 m3 /s) at 6.30 inH2O (1.6 kPa). 4. Compute the power required to drive the forced-draft fan The air hp for any fan ⫽ 0.0001753Hƒ ⫽ total head developed by fan, inH2O; C ⫽ airflow, ft3 /min. For this fan, air hp ⫽ 0.0001753(6.3)(69,400) ⫽ 76.5 hp (57.0 kW). Assume or obtain the fan and fan-driver efficiencies at the rated capacity (69,400 ft3 /min, or 32.8 m3 /s) and pressure (6.30 inH2O, or 1.6 kPa). With a fan efficiency of 75 percent and assuming the fan is driven by an electric motor having an efficiency of 90 percent, we find the overall efficiency of the fan-motor com- bination is (0.75)(0.90) ⫽ 0.675, or 67.5 percent. Then the motor horsepower re- quired ⫽ air hp/overall efficiency ⫽ 76.5/0.675 ⫽ 113.2 hp (84.4 kW). A 125-hp (93.2-kW) motor would be chosen because it is the nearest, next larger unit readily available. Usual practice is to choose a larger driver capacity when the computed capacity is lower than a standard capacity. The next larger standard capacity is generally chosen, except for extremely large fans where a special motor may be ordered. 5. Compute the quantity of flue gas handled The quantity of gas reaching the induced-draft fan is the sum of the actual air required for combustion from step 1, air leakage in the boiler and furnace, and the weight of fuel burned. With an air leakage of 10 percent in the boiler and furnace (this is a typical leakage factor applied in practice), the gas flow is as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 210. 4.32 POWER GENERATION Determine from combustion calculations for the boiler the density of the flue gas. Assume that the combustion calculations for this boiler show that the flue-gas density is 0.045 lb/ft3 (0.72 kg/m3 ) at the exit-gas temperature. To determine the exit-gas temperature, apply a 10 percent factor of safety to the given exit temper- ature, 350⬚F (176.6⬚C). Hence, exit-gas temperature ⫽ 350 ⫹ 350(0.10) ⫽ 385⬚F (196.1⬚C). Then flue-gas flow, ft3 /min ⫽ (flue-gas flow, lb/h)/(60)(flue-gas density, lb/ft3 ) ⫽ 343,600/[(60)(0.045)] ⫽ 127,000 ft3 /min (59.9 m3 /s). Apply a 10 to 25 percent factor of safety to the flue-gas quantity to allow for increased gas flow. With a 20 percent factor of safety, the actual flue-gas flow the fan must handle ⫽ 127,000 ⫹ 127,000(0.20) ⫽ 152,400 ft3 /min (71.8 m3 /s), say 152,500 ft3 /min (71.9 m3 /s) for fan-selection purposes. 6. Compute the induced-draft fan discharge pressure Find the sum of the draft losses from the burner outlet to the induced-draft fan inlet. These losses are as follows for this boiler: Allow a 10 to 25 percent factor of safety to ensure adequate pressure during all boiler loads and furnace conditions. With a 20 percent factor of safety for this fan, the total actual pressure loss ⫽ 8.95 ⫹ 8.95(0.20) ⫽ 10.74 inH2O (2.7 kPa). Round this to 11.0 inH2O (2.7 kPa) for fan-selection purposes. 7. Compute the power required to drive the induced-draft fan As with the forced-draft fan, air hp ⫽ 0.0001753HƒC ⫽ 0.0001753(11.0) ⫻ (127,000) ⫽ 245 hp (182.7 kW). If the combined efficiency of the fan and its driver, assumed to be an electric motor, is 68 percent, the motor hp required ⫽ 245/0.68 ⫽ 360.5 hp (268.8 kW). A 375-hp (279.6-kW) motor would be chosen for the fan driver. 8. Choose the fans from a manufacturer’s engineering data Use the next calculation procedure to select the fans from the engineering data of an acceptable manufacturer. For larger boiler units, the forced-draft fan is usually a backward-curved blade centrifugal-type unit. Where two fans are chosen to op- erate in parallel, the pressure curve of each fan should decrease at the same rate near shutoff so that the fans divide the load equally. Be certain that forced-draft fans are heavy-duty units designed for continuous operations with well-balanced rotors. Choose high-efficiency units with self-limiting power characteristics to pre- vent overloading the driving motor. Airflow is usually controlled by dampers on the fan discharge. Induced-draft fans handle hot, dusty combustion products. For this reason, ex- treme care must be taken to choose units specifically designed for induced-draft service. The usual choice for large boilers is a centrifugal-type unit with forward- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 211. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.33 TABLE 4 Fan Correction Factors or backward-curved, or flat blades, depending on the type of gas handled. Flat blades are popular when the flue gas contains large quantities of dust. Fan bearings are generally water-cooled. Related Calculations. Use the procedure given above for the selection of draft fans for all types of boilers—fire-tube, packaged, portable, marine, and stationary. Obtain draft losses from the boiler manufacturer. Compute duct pressure losses by using the methods given in later procedures in this handbook. POWER-PLANT FAN SELECTION FROM CAPACITY TABLES Choose a forced-draft fan to handle 69,400 ft3 /min (32.8 m3 /s) of 90⬚F (32.2⬚C) air at 6.30-inH2O (1.6-kPa) static pressure and an induced-draft fan to handle 152,500 ft3 /min (72.0 m3 /s) of 385⬚F (196.1⬚C) gas at 11.0-inH2O (2.7-kPa) static pressure. The boiler that these fans serve is installed at an elevation of 5000 ft. (1524 m) above sea level. Use commercially available capacity tables for making the fan choice. The flue-gas density is 0.045 lb/ft3 (0.72 kg/m3 ) at 385⬚F (196.1⬚C). Calculation Procedure: 1. Compute the correction factors for the forced-draft fan Commercial fan-capacity tables are based on fans handling standard air at 70⬚F (21.1⬚C) at a barometric pressure of 29.92 inHg (101.0 kPa) and having a density of 0.075 lb/ft3 (1.2 kg/m3 ). Where different conditions exist, the fan flow rate must be corrected for temperature and altitude. Obtain the engineering data for commercially available forced-draft fans, and turn to the temperature and altitude correction-factor tables. Pick the appropriate correction factors from these tables for the prevailing temperature and altitude of the installation. Thus, in Table 4, select the correction factors for 90⬚F (32.2⬚C) air and 5000-ft (1524.0-m) altitude. These correction factors are CT ⫽ 1.018 for 90⬚F (32.2⬚C) air and CA ⫽ 1.095 for 5000-ft (1524.0-m) altitude. Find the composite correction factor (CCF) by taking the product of the tem- perature and altitude correction factors. Or, CCF ⫽ (1.018)(1.095) ⫽ 1.1147. Now divide the given cubic feet per minute (cfm) by the correction factor to find the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 212. 4.34 POWER GENERATION TABLE 5 Typical Fan Capacities capacity-table ft3 /min. Or, capacity-table ft3 /min ⫽ 69,400/1.147 ⫽ 62,250 ft3 / min (29.4 m3 /s). 2. Choose the fan size from the capacity table Turn to the fan-capacity table in the engineering data, and look for a fan delivering 62,250 ft3 /min (29.4 m3 /s) at 6.3-inH2O (1.6-kPa) static pressure. Inspection of the table shows that the capacities are tabulated for 6.0- and 6.5-inH2O (1.5- and 1.6- kPa) static pressure. There is no tabulation for 6.3-inH2O (1.57-kPa) static pressure. Enter the table at the nearest capacity to that required, 62,250 ft3 /min (29.4 m3 /s), as shown in Table 5. This table, excerpted with permission from the Amer- ican Standard Inc. engineering data, shows that the nearest capacity of this particular type of fan is 62,595 ft3 /min (29.5 m3 /s). The difference, or 62,595 ⫺ 62,250 ⫽ 345 ft3 /min (0.16 m3 /s), is only 345/62,250 ⫽ 0.0055, or 0.55 percent. This is a negligible difference, and the 62,595-ft3 /min (29.5-m3 /s) fan is well suited for its intended use. The extra static pressure of 6.5 ⫺ 6.3 ⫽ 0.2 inH2O (0.05 kPa) is desirable in a forced-draft fan because furnace or duct resistance may increase during the life of the boiler. Also, the extra static pressure is so small that it will not markedly increase the fan power consumption. 3. Compute the fan speed and power input Multiply the capacity-table rpm and brake hp (bhp) by the composite factor to determine the actual rpm and bhp. Thus, with data from Table 5, the actual rpm ⫽ (1096)(1.1147) ⫽ 1221.7 r/min. Actual bhp ⫽ (99.08)(1.1147) ⫽ 110.5 bhp (82.4 kW). This is the hp input required to drive the fan and is close to the 113.2 hp (84.4 kW) computed in the previous calculation procedure. The actual motor hp would be the same in each case because a standard-size motor would be chosen. The difference of 113.2 ⫺ 110.5 ⫽ 2.7 hp (2.0 kW) results from the assumed efficiencies that depart from the actual values. Also, a sea-level attitude was as- sumed in the previous calculation procedure. However, the two methods used show how accurately fan capacity and hp input can be estimated by judicious evaluation of variables. 4. Compute the correction factors for the induced-draft fan The flue-gas density is 0.045 lb/ft3 (0.72 kg/m3 ) at 385⬚F (196.1⬚C). Interpolate in the temperature correction-factor table because a value of 385⬚F (196.1⬚C) is not tabulated. Find the correction factor for 285⬚F (196.1⬚C) thus: [(Actual temperature ⫺ lower temperature)/(higher temperature ⫺ lower temperature)] ⫻ (higher tem- perature correction factor ⫺ lower temperature correction factor) ⫹ lower temper- ature correction factor. Or, [(385 ⫺ 375)/(400 ⫺ 375)](1.273 ⫺ 1.255) ⫹ 1.255 ⫽ 1.262. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 213. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.35 The altitude correction factor is 1.095 for an elevation of 5000 ft (1524.0 m), as shown in Table 4. As for the forced-draft fan, CCF ⫽ CT CA ⫽ (1.262)(1.095) ⫽ 1.3819. Use the CCF to find the capacity-table ft3 /min in the same manner as for the forced-draft fan. Or, capacity-table ft3 /min ⫽ (given ft3 /min)/CCF ⫽ 152,500/1.3819 ⫽ 110,355 ft3 /min (52.1 m3 /s). 5. Choose the fan size from the capacity table Check the capacity table to be sure that it lists fans suitable for induced-draft (elevated-temperature) service. Turn to the 11-inH2O (2.7-kPa) static-pressure ca- pacity table, and find a capacity equal to 110,355 ft3 /min (52.1 m3 /s). In the en- gineering data used for this fan, the nearest capacity at 11-inH2O (2.7-kPa) static pressure is 110,467 ft3 /min (52.1 m3 /s), with an outlet velocity of 4400 ft/min (22.4 m/s), an outlet velocity pressure of 1.210 inH2O (0.30 kPa), a speed of 1222 r/min, and an input hp of 255.5 bhp (190.5 kW). The tabulation of these quantities is of the same form as that given for the forced-draft fan, step 2. The selected capacity of 110,467 ft3 /min (52.1 m3 /s) is entirely satisfactory because it is only 110,467 ⫺ 110,355/110,355 ⫽ 0.00101, to 0.1 percent, higher than the desired capacity. 6. Compute the fan speed and power input Multiply the capacity-table rpm and brake hp by the CCF to determine the actual rpm and brake hp. Thus, the actual rpm ⫽ (1222)(1.3819) ⫽ 1690 r/min. Actual brake hp ⫽ (255.5)(1.3819) ⫽ 353.5 bhp (263.6 kW). This is the hp input required to drive the fan and is close to the 360.5 hp (268.8 kW) computed in the previous calculation procedure. The actual motor horsepower would be the same in each case because a standard-size motor would be chosen. The difference in hp of 360.5 ⫺ 353.5 ⫽ 7.0 hp (5.2 kW) results from the same factors discussed in step 3. Note: The static pressure is normally used in most fan-selection procedures be- cause this pressure value is used in computing pressure and draft losses in boilers, economizers, air heaters, and ducts. In any fan system, the total air pressure ⫽ static pressure ⫹ velocity pressure. However, the velocity pressure at the fan discharge is not considered in draft calculations unless there are factors requiring its evalua- tion. These requirements are generally related to pressure losses in the fan-control devices. Related Calculations. Use the fan-capacity table to obtain these additional de- tails of the fan: outlet inside dimensions (length and width), fan-wheel diameter and circumference, fan maximum bhp, inlet area, fan-wheel peripheral velocity, NAFM fan class, and fan arrangement. Use the engineering data containing the fan- capacity table to find the fan dimensions, rotation and discharge designations, ship- ping weight, and, for some manufacturers, prices. FAN ANALYSIS AT VARYING RPM, PRESSURE, AND AIR OR GAS CAPACITY A fan delivers 12,000 ft3 /min (339.6 m3 /min) at a static pressure of 1 in (0.39 cm) WG at 70⬚F (21.1⬚C) when operating at 400 r/min; required power input is 4 hp (2.98 kW). (a) If in the same installation, 15,000 ft3 /min (424.5 m3 /min) are re- quired, what will be the new fan speed, static pressure, and power input? (b) If the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 214. 4.36 POWER GENERATION air temperature is increased to 200⬚F (93.3⬚C) and the fan speed remains at 400 r /min, what will be the new static pressure and power input with a flow rate of 12,000 ft3 /min (339.5 m3 /min)? (c) If the speed of the fan is increased to deliver 1 in (0.39 cm) WG at 200⬚F (93.3⬚C), what will be the new speed, capacity, and power input? (d) If the speed of the fan is increased so as to deliver the same weight of air at 200⬚F (93.3⬚C) as at 70⬚F (21.1⬚C), what will be the new speed, capacity, static pressure, and power input? Calculation Procedure: 1. Determine the fan speed, static pressure, and power input at the higher flow rate (a) Use the fan laws to determine the required unknowns. The first fan law states: Air or gas capacity varies directly as the fan speed. Thus, the new speed with higher capacity ⫽ 400(15,000/12,000) ⫽ 500 r/min. Hence, the fan speed must be increased by 25 percent, i.e., 100 r/min—to have the fan handle 25 percent more air. This verifies the first fan law that capacity varies directly as fan speed. Use the second fan law to determine the new static pressure. This law states: Fan pressure (static, velocity, and total) varies as the square of the fan speed. Thus, the new static pressure with the larger flow rate ⫽ 1(500/400)2 ⫽ 1.5625 in (3.97 cm). Find the new required power input at the higher flow rate and higher discharge pressure by using the third fan law, which states: Power demand of a fan varies as the cube of the fan speed. Hence, the new power ⫽ 4(500/400)3 ⫽ 7.8125 hp (5.82 kW). 2. Determine the new static pressure and power (b) When the density of air or gas handled by a fan changes, three other fan laws apply. The first of these laws is: At constant fan speed; i.e., rpm, and capacity; i.e., cfm (m3 /min), the pressure developed and required power input vary directly as the air or gas density. For the conditions given here the air density at 70⬚F (21.1⬚C) is 0.075 lb/ft3 (1.2 kg/m3 ); at 200⬚F (93.3⬚C) the air density is 0.06018 lb/ft3 (0.963 kg/m3 ). Then, new static pressure ⫽ 1.0(0.06018/0.075) ⫽ 0.80 in (2.04 cm). The new power is found from 4(0.06018/0.075) ⫽ 3.21 hp (2.39 kW). 3. Find speed, capacity, and power input at the new pressure (c) We now have a constant-pressure output. Under these conditions, with a varying air or gas density, the fan law states that: At constant pressure the speed, capacity, and power vary inversely as the square root of the fluid density. Thus, new speed ⫽ 400 (0.075/0.06018)0.5 ⫽ 446.5 r/min. The new capacity at the 1-in (0.39-cm) static pressure ⫽ 12,000(0.075/ 0.06018)0.5 ⫽ 13,396 r/min at 200⬚F (93.3⬚C). The new power ⫽ 4 (0.075/ 0.06018)0.5 ⫽ 4.46 hp (3.33 kW). 4. Compute the new speed, capacity, static pressure, and power at the increased speed (d) The final fan law states: For a constant weight of air or gas, the speed, capacity, and pressure vary inversely as the density, while the hp varies inversely as the square of the density. Using this law, the new speed ⫽ 400(0.075/0.06018) ⫽ 498.5 r/min. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 215. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.37 The new capacity ⫽ 12,000(0.075/0.06018) ⫽ 14,955 ft3 /min (423.5 m3 /min). Likewise, the new static pressure ⫽ 1.0(0.075/0.06018) ⫽ 1.246 in (3.17 cm). Finally, the new power ⫽ 4 (0.075/0.06018)2 ⫽ 6.21 hp (4.63 kW). Related Calculations. The fan laws, as given here, are powerful in the analysis of the speed, capacity, and pressure of any fan handling air or gases. These laws can be used for fans employed in air conditioning, ventilation, forced and induced draft, kitchen and hood exhausts, etc. The fan can be used in stationary, mobile, marine, aircraft, and similar applications. The fan laws apply equally well. BOILER FORCED-DRAFT FAN HORSEPOWER DETERMINATION Find the motor turbine hp needed to provide forced-draft service to a boiler that burns coal at a rate of 10 tons (9080 kg)/h. The boiler requires 59,000 ft3 /min (5481 cum/min) of air under 6 in (15.2 cm) water gage (WG) from the fan which has a mechanical efficiency of 60 percent. The air is delivered at a total pressure of 6 in (15.2-cm) WG by the fan. What would be the effect on the required power to this fan if the total pressure were doubled to 12 in (30.5 cm) WG? If the required air delivery was increased to 75,000 ft3 /min (2123 m3 /min), at 6 in (15.2 cm) WG, what input hp would be required? Calculation Procedure: 1. Find the required power input to the fan Use the relation, fan hp ⫽ ft3 /min(total pressure developed, lb/ft2 )/33,000(fan ef- ficiency). To apply this equation we must convert the water gage pressure to lb/ft2 by (in WG/12)(62.4 lb/ft3 water density). Or (6/12)(62.4) 31.2 lb/ft2 (1.49 kPa). Substituting, hp ⫽ 59,000(31.2)/33,000(0.60) ⫽ 92.96 hp (69.4 kW). Use a 100- hp (75 kW) motor or turbine to drive this induced-draft fan. 2. Determine the required power input at the higher delivery pressure Use the same relation as in Step 1 to find, hp ⫽ 59,000(62.4)/33,000(0.60) ⫽ 185.9 hp (138.7 kW). Thus, the required power input doubles as the developed pressure doubles. The sharp increase in the power input is a graphic example of why the pressure requirements for any type of fan must be carefully analyzed before the final choice is made. Since the cost of a fan does not rise in direct proportion to its delivery pressure, the engineer should apply a factor of safety to the computed power input to take care of possible future overloads. 3. Find the required power input at the higher flow rate Using the same relation, hp ⫽ 118.2 (88.2 kW). Again, the required power input rises as the output from the fan is increased. This further illustrates the strong need to explore the maximum output requirements before making a final equipment choice. Related Calculations. This approach can be used for any fan used in power- plant, HVAC, and similar applications. The key point to observe is the rise in power requirements as the fan pressure of air volume delivered increases. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 216. 4.38 POWER GENERATION EFFECT OF BOILER RELOCATION ON DRAFT FAN PERFORMANCE An acceptance test of a boiler shows that its induced-draft fan handles 600,000 (272,400 kg) lb per hour of flue gas at 290⬚F (143.3⬚C) against total friction of 10 in (25.4 cm) water gage (WG). The boiler is relocated to a higher elevation where the barometric pressure is 24 inHg (60.96 cmHg), as compared to the original 30 inHg (76.2 cmHg) at sea level. If no changes are made to the equipment except adjustments to the fan and with gas weights and temperatures as before, what are the new volume and suction conditions for the fan design and relocation? Calculation Procedure: 1. Determine the new inlet condition for the fan When a fan is required to handle air or gas at conditions other than standard, a correction must be made in the static pressure and hp (kW). Since a fan is essen- tially a constant-volume machine, the ft3 /min (m3 /min) delivered will not change materially if the speed and system configuration do not change, regardless of the air or gas density. The static pressure, however, changes directly with density. Hence, the static pressure must be carefully calculated for specified conditions. For the situation described here, assume a gas molecular weight of 28, a typical value. The density correction factor can be computed from the ratio of the new-location barometric pressure to the first-location barometric pressure, both expressed inHg (cmHg). Or, density correction factor ⫽ 24 in/30 in ⫽ 0.80. There is no temperature correction factor because the air temperature remains the same. Therefore, at the new elevated location of the boiler the intake condition for the fan will be (10 in WG)(0.8 correction factor) ⫽ 8 in (20.3 cm) WG. 2. Compute the new volume condition Use the relation Volume flow ⫽ (lb/h)(molecular weight of gas)(cfm at inlet con- ditions)[(absolute temperature of flue gas)/(‘‘standard’’ air temperature of 60⬚F in absolute terms)][(reduced barometric pressure)/(reduced barometric pressure ⫺ new suction condition)]. Substituting, new volume condition ⫽ (600,000/28)(379)[(460 ⫹ 290)/(60 ⫹ 460)][(24)/(24 ⫺ 8)] ⫽ 17.57 ⫻ 106 ft3 /h (0.497 m3 /h ⫻ 106 ). Related Calculations. With used power-plant equipment becoming more pop- ular throughout the world (see the classified section of any major engineering mag- azine) it is important that the engineer be able to determine the performance of re- used equipment at different locations. ANALYSIS OF BOILER AIR DUCTS AND GAS UPTAKES Three oil-fired boilers are supplied air through the breeching shown in Fig. 7a. Each boiler will burn 13,600 lb/h (1.71 kg/s) of fuel oil at full load. The draft loss Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 217. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.39 FIGURE 7 (a) Boiler intake-air duct; (b) boiler uptake ducts. through each boiler is 8 inH2O (2.0 kPa). Uptakes from the three boilers are con- nected as shown in Fig. 7b. Determine the draft loss through the entire system if a 50-ft (15.2-m) high metal stack is used and the gas temperature at the stack inlet is 400⬚F (204.4⬚C). Calculation Procedure: 1. Determine the airflow through the breeching Compute the airflow required, cubic feet per pound of oil burned, using the methods given in earlier calculation procedures. For this installation, assume that the com- bustion calculation shows that 250 ft3 /lb (15.6 m3 /kg) of oil burned is required. Then the total airflow required ⫽ (number of boilers)(lb/h oil burned per boiler)(ft3 /lb oil)/(60 min/h) ⫽ (3)(13,600)(250)/60 ⫽ 170,000 ft3 /min (80.2 m3 /s). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 218. 4.40 POWER GENERATION 2. Select the dimensions for each length of breeching duct With the airflow rate of 170,000 ft3 /min (80.2 m3 /s) known, the duct area can be determined by assuming an air velocity and computing the duct area. Ad ft2 from Ad ⫽ (airflow rate, ft3 /min)/(air velocity, ft/min). Once the area is known, the duct can be sized to give this area. Thus, if 9 ft2 (0.8 m2 ) is the required duct area, a duct 3 ⫻ 3 ft (0.9 ⫻ 0.9 m) or 2 ⫻ 4.5 ft (0.6 ⫻ 1.4 m) would provide the required area. In the usual power plant, the room available for ducts limits the maximum allowable duct size. So the designer must try to fit a duct of the required area into the available space. This is done by changing the duct height and width until a duct of suitable area fitting the available space is found. If the duct area is reduced below that required, compute the actual air velocity to determine whether it exceeds recommended limits. In this power plant, the space available in the open area between A and C, Fig. 7, is a square 11 ⫻ 11 ft (3.4 ⫻ 3.4 m). By allowing a 3-in (7.6-cm) clearance around the outside of the duct and using a square duct, its dimensions would be 10.5 ⫻ 10.5 ft (3.2 ⫻ 3.2 m), or a cross-sectional area of (10.5)(10.5) ⫽ 110 ft2 (10.2 m2 ), closely. With 170,000 ft3 /min (80.2 m3 /s) flowing through the duct, the air velocity v ft/min ⫽ ft3 /min/Ad ⫽ 170,000/110 ⫽ 1545 ft/min (7.8 m/s). This is a satisfactory air velocity because the usual plant air system velocity is 1200 to 3600 ft/min (6.1 to 18.3 m/s). Between C and D the open area in this power plant is 10 ft 9 in (3.3 m) by 14 ft (4.3 m). Using the same 3-in (7.6-cm) clearance all around the duct, we find the dimensions of the vertical duct CD are 10.25 ⫻ 13 ft (3.1 ⫻ 4.0 m), or a cross- sectional area of 10.25 ⫻ 13 ⫽ 133 ft2 (12.5 m2 ), closely. The air velocity in this section of the duct is v ⫽ 170,000/133 ⫽ 1275 ft/min (6.5 m/s). Since it is desirable to maintain, if possible, a constant velocity in all sections of the duct where space permits, the size of this duct might be changed so it equals that of AB, 10.5 ⫻ 10.5 ft (3.2 ⫻ 3.2 m). However, the installation costs would probably be high because the limited space available would require alteration of the power- plant structure. Also, the velocity is section CD is above the usual minimum value of 1200 ft/min (6.1 m/s). For these reasons, the duct will be installed in the 10.25 ⫻ 13 ft (3.1 ⫻ 4.0 m) size. Between E and F the vertical distance available for installation of the duct is 3.5 ft (1.1 m), and the horizontal distance is 8.5 ft (2.6 m). Using the same 3-in (7.6-cm) clearance as before gives a 3 ⫻ 8 ft (0.9 ⫻ 2.4 m) duct size, or a cross- sectional area of (3)(8) ⫽ 24 ft2 (2.2 m2 ). At E the duct divides into three equal- size branches, one for each boiler, and the same area, 24 ft2 (2.2 m2 ), is available for each branch duct. The flow in any branch duct is then 170,000/3 ⫽ 56,700 ft3 /min (26.8 m3 /s). The velocity in any of the three equal branches is v ⫽ 56,700/24 ⫽ 2360 ft/min (12.0 m/s). When a duct system has two or more equal- size branches, compute the pressure loss in one branch only because the losses in the other branches will be the same. The velocity in branch EF is acceptable be- cause it is within the limits normally used in power-plant practice. At F the air enters a large plenum chamber, and its velocity becomes negligible because of the large flow area. The boiler forced-draft fan intakes are connected to the plenum chamber. Each of the three ducts feeds into the plenum chamber. 3. Compute the pressure loss in each duct section Begin the pressure-loss calculations at the system inlet, point A, and work through each section to the stack outlet. This procedure reduces the possibility of error and Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 219. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.41 permits easy review of the calculations for detection of errors. Assign letters to each point of the duct where a change in section dimensions or directions, or both, occurs. Use these letters: Point A: Assume that 70⬚F (21.1⬚C) air having a density of 0.075 lb/ft3 (1.2 kg/m3 ) enters the system when the ambient barometric pressure is 29.92 inHg (101.3 kPa). Compute the velocity pressure at point A, in inH2O, from pv ⫽ v2 / [3.06(104 )(460 ⫹ t)], where t ⫽ air temperature,⬚F. Since the velocity of the air at A is 1545 ft/min (11.7 m/s), pv ⫽ (1545)2 /[3.06(104 )(530)] ⫽ 0.147 inH2O (0.037 kPa) at 70⬚F (21.1⬚C). The entrance loss at A, where there is a sharp-edged duct, is 0.5pv, or 0.5(0.147) ⫽ 0.0735 inH2O (0.018 kPa). With rounded inlet, the loss in velocity pressure would be negligible. Section AB: There is a pressure loss due to duct friction between A and B, and B and C. Also, there is a bend loss at points B and C. Compute the duct friction first. For any circular duct, the static pressure loss due to friction ps inH2O ⫽ (0.03L/d1.24 )(v/1000)1.84 , where L ⫽ duct length, ft; d ⫽ duct diameter, in. To convert any rectangular or square duct with sides a and b high and wide, respec- tively, to an equivalent round duct of D-ft diameter, use the relation D ⫽ 2ab/ (a ⫹ b). For this duct, d ⫽ 2(10.5)(10.5)/(10.5 ⫹ 10.5) ⫽ 10.5 ft (3.2 m) ⫽ 126 in (320 cm) ⫽ d. Since this duct is 12 ft (3.7 m) long between A and B, ps ⫽ [0.03(12)/1261.24 ](1.545/1000)1.84 ⫽ 0.002 inH2O (0.50 kPa). Point B: The 45⬚ bend at B has, from Baumeister and Marks—Standards Hand- book for Mechanical Engineers, a pressure drop of 60 percent of the velocity head in the duct, for (0.60)(0.147) ⫽ 0.088 inH2O (20.5 Pa) loss. Section BC: Duct friction in the 14-ft (4.3-m) long downcomer BC is ps ⫽ [0.03(14)/1261.24 ](1545/1000)1.84 ⫽ 0.0023 inH2O (0.56 Pa). Point C: The 45⬚ bend at C has a velocity head loss of 60 percent of the velocity pressure. Determine the velocity pressure in this duct in the same manner as for point A, or pv ⫽ (1545)2 /[3.06(104 )(530)] ⫽ 0.147 inH2O (36.1 Pa), since the velocity at points B and C is the same. Then the velocity head loss ⫽ (0.60)(0.147) ⫽ 0.088 inH2O (21.9 Pa). Section CD: The equivalent round-duct diameter is D ⫽ (2)(10.25)(13)/(10.25 ⫹ 13) ⫽ 11.45 ft (3.5 m) ⫽ 137.3 in (348.7 cm). Duct friction is then ps ⫽ [0.03(9)/137.31.24 ](1275/1000)1.84 ⫽ 0.000934 inH2O (0.23 Pa). Velocity pressure in the duct is pv ⫽ (1275)2 /[3.06(104 )(530)] ⫽ 0.100 inH2O (24.9 Pa). Since there is no room for a transition piece—that is, a duct providing a gradual change in flow area between points C and D–the decrease in velocity pressure from 0.147 to 0.100 in (36.6 to 24.9 Pa), or 0.147 ⫺ 0.10 ⫽ 0.047 inH2O (11.7 Pa), is not converted to static pressure and is lost. Point E: The pressure loss in the right-angle bend at E is, from Baumeister and Marks—Standard Handbook for Mechanical Engineers, 1.2 times the velocity head, or (1.2)(0.1) ⫽ 0.12 inH2O (29.9 Pa). Also, since this is a sharp-edged elbow, there is an additional loss of 50 percent of the velocity head, or (0.5)(0.10) ⫽ 0.05 inH2O (12.4 Pa). The velocity pressure at point E is pv ⫽ (2360)2 /[3.06(104 )(530)] ⫽ 0.343 inH2O (85.4 Pa). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 220. 4.42 POWER GENERATION Section EF: The equivalent round-duct diameter is D ⫽ (2)(3)(8)/(3 ⫹ 8) ⫽ 4.36 ft (1.3 m) ⫽ 52.4 in (133.1 cm). Duct friction ps ⫽ [0.03(40)/52.41.24 ](2360/ 1000)1.84 ⫽ 0.0247 inH2O (6.2 Pa). Air entering the large plenum chamber at F loses all its velocity. There is no static-pressure regain; therefore, the velocity-head loss ⫽ 0.348 ⫺ 0.0 ⫽ 0.348 inH2O (86.6 Pa). 4. Compute the losses in the uptake and stack Convert the airflow of 250 ft3 /lb (15.6 m3 /kg) of fuel oil to pounds of air per pound of fuel oil by multiplying by the density, or 250(0.075) ⫽ 18.75 lb of air per pound of oil. The flue gas will contain 18.75 lb of air ⫹ 1 lb of oil per pound of fuel burned, or (18.75 ⫹ 1)/18.75 ⫽ 1.052 times as much gas leaves the boiler as air enters; this can be termed the flue-gas factor. Point G: The quantity of flue gas entering the stack from each boiler (corrected to a 400⬚F or 204.4⬚C outlet temperature) is, in ⬚R (cfm air to furnace)(stack, ⬚R/air,⬚R)(flue-gas factor). Or stack flue-gas flow ⫽ (56,700)[(400 ⫹ 460)/(70 ⫹ 460)](1.052) ⫽ 97,000 ft3 /min (45.8 m3 /s) per boiler. The total duct area available for the uptake leading to the stack is 9 ⫻ 10 ft (2.7 ⫻ 3.0 m) ⫽ 90 ft2 (8.4 m2 ), based on the clearance above the boilers. The flue-gas velocity for three boilers is v ⫽ (3)(97,000)/90 ⫽ 3235 ft/min (16.4 m/s). The velocity pressure in the uptake is p ⫽ (3235)2 /[3.06(104 )(460 ⫹ 400)] ⫽ 0.397 inH2O (98.8 Pa). Point H: The flue-gas flow from all the boilers is divided equally between three ducts. HG, IG, JG, Fig. 7. It is desirable to maintain the same gas velocity in each duct and have this velocity equal to that in the uptake. The same velocity can be obtained in each duct by making each duct one-third the area of the uptake, or 90/3 ⫽ 30 ft2 (2.8 m2 ). Then v ⫽ 97,000/30 ⫽ 3235 ft/min (16.4 m/s) in each duct. Since the velocity in each duct equals the velocity in the uptake, the velocity pressure in each duct equals that in the uptake, or 0.397 inH2O (98.8 Pa). Ducts HG and JG have two 45⬚ bends in them, or the equivalent of one 90⬚ bend. The velocity-pressure loss in a 90⬚ bend is 1.20 times the velocity head in the duct; or, for either HG or JG, (1.20)(0.397) ⫽ 0.476 inH2O (118.5 Pa). Section HG: The equivalent duct diameter for a 30-ft2 (2.8-m2 ) duct is D ⫽ 2(30/)0.5 ⫽ 6.19 ft (1.9 m) ⫽ 74.2 in (188.4 cm). The duct friction in HG, which equals that in JG, is ps ⫽ [0.03(20)/74.21.24 ](530/860)(3235/1000)1.84 ⫽ 0.01536 inH2O (3.8 Pa), if we correct for the flue-gas temperature with the ratio (70 ⫹ 460)/(400 ⫹ 460) ⫽ 530/860. Section GK: The stack joins the uptake at point G. Assume that this installation is designed for a stack-gas area of 500 lb of oil per square foot (2441.2 kg/m2 ) of stack; for three boilers, stack area ⫽ (3)(13,600 lb/h oil)/500 ⫽ 81.5 ft2 (7.6 m2 ). The stack diameter will then be D ⫽ 2(8.15/)0.5 ⫽ 10.18 ft (3.1 m) ⫽ 122 in (309.9 cm). The gas velocity in the stack is v ⫽ (3)(97,000)/81.5 ⫽ 3570 ft/min (18.1 m/ s). The friction in the stack is ps ⫽ [0.03(50)1221.24 ](3570/1000)1.84 (503/860) ⫽ 0.0194 inH2O (4.8 Pa). 5. Compute the total losses in the system Tabulate the individual losses and find the sum as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 221. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.43 The total loss computed here is the minimum static pressure that must be de- veloped by the draft fans or blowers. This total static pressure can be divided between the forced- and induced-draft fans or confined solely to the forced-draft fans in plants not equipped with an induced-draft fan. If only a forced-draft fan is used, its static discharge pressure should be at least 20 percent greater than the losses, or (1.2)(9.3552) ⫽ 11.21 inH2O (2.8 kPa) at a total airflow of 97,000 ft3 / min (45.8 m3 /s). If more than one forced-draft fan were used for each boiler, each fan would have a total static pressure of at least 11.21 inH2O (2.8 kPa) and a capacity of less than 97,000 ft3 /min (45.8 m3 /s). In making the final selection of the fan, the static pressure would be rounded to 12 inH2O (3.0 kPa). Where dampers are used for combustion-air control, include the wide-open re- sistance of the dampers in computing the total losses in the system at full load on the boilers. Damper resistance values can be obtained from the damper manufac- turer. Note that as the damper is closed to reduce the airflow at lower boiler loads, the resistance through the damper is increased. Check the fan head-capacity curve to determine whether the head developed by the fan at lower capacities is sufficient to overcome the greater damper resistance. Since the other losses in the system will decrease with smaller airflow, the fan static pressure is usually adequate. Note: (1) Follow the notational system used here to avoid errors from plus and minus signs applied to atmospheric pressures and draft. Use of the plus and minus signs does not simplify the calculation and can be confusing. (2) A few designers, reasoning that the pressure developed by a fan varies as the square of the air velocity, square the percentage safety-factor increase before multiplying by the static pressure. Thus, in the above forced-draft fan, the static discharge pressure with a 20 percent increase in pressure would be (1.2)2 (9.3552) ⫽ 13.5 inH2O (3.4 kPa). This procedure provides a wider margin of safety, but is not widely used. (3) Large steam-generating units, some ship propulsion plants, and some pack- aged boilers use only forced-draft fans. Induced-draft fans are eliminated because there is a saving in the total fan hp required, there is no air infiltration into the boiler setting, and a slightly higher boiler efficiency can be obtained. (4) The duct system analyzed here is typical of a study-type design where no refinements are used in bends, downcomers, and other parts of the system. This Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 222. 4.44 POWER GENERATION type of system was chosen for the analysis because it shows more clearly the various losses met in a typical duct installation. The system could be improved by using a bellmouthed intake at A, dividing vanes or splitters in the elbows, a tran- sition in the downcomer, and a transition at F. None of these improvements would be expensive, and they would all reduce the static pressure required at the fan discharge. (5) Do not subtract the stack draft from the static pressure the forced- or induced- draft fan must produce. Stack draft can vary considerably, depending on ambient temperature, wind velocity, and wind direction. Therefore, the usual procedure is to ignore any stack draft in fan-selection calculations because this is the safest procedure. Related Calculations. The procedure given here can be used for all types of boilers fitted with air-supply ducts and uptake breechings—heating, power, process, marine, portable, and packaged. DETERMINATION OF THE MOST ECONOMICAL FAN CONTROL Determine the most economical fan control for a forced- or induced-draft fan de- signed to deliver 140,000 ft3 /min (66.1 m3 /s) at 14 inH2O (3.5 kPa) at full load. Plot the power-consumption curve for each type of control device considered. Calculation Procedure: 1 Determine the types of controls to consider There are five types of controls used for forced- and induced-draft fans: (a) a damper in the duct with constant-speed fan drive; (b) two-speed fan drive; (c) inlet vanes or inlet louvres with a constant speed fan drive; (d) multiple-step variable- speed fan drive; and (e) hydraulic or electric coupling with constant-speed drive giving wide control over fan speed. 2. Evaluate each type of fan control Tabulate the selection factors influencing the control decision as follows, using the control letters in step 1: 3. Plot the control characteristics for the fans Draw the fan head-capacity curve for the airflow or gasflow range considered, Fig. 8. This plot shows the maximum capacity of 140,000 ft3 /min (66.1 m3 /s) and required static head of 14 inH2O (3.5 kPa), point P. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 223. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.45 FIGURE 8 Power requirements for a fan fitted with different types of controls. (American Standard Inc.) Plot the power-input curve ABCD for a constant-speed motor or turbine drive with damper control—type a, listed above—after obtaining from the fan manufac- turer, or damper builder, the input power required at various static pressures and capacities. Plotting these values gives curve ABCD. Fan speed is 1200 r/min. Plot the power-input curve GHK for a two-speed drive, type b. This drive might be a motor with additional winding, or it might be a second motor for use at reduced boiler capacities. With either arrangement, the fan speed at lower boiler capacities is 900 r/min. Plot the power-input curve AFED for inlet-vane control on the forced-draft fan or inlet-louvre control on induced-draft fans. The data for plotting this curve can be obtained from the fan manufacturer. Multiple-step variable-speed fan control, type d, is best applied with steam- turbine drives. In a plant with ac auxiliary motor drives, slip-ring motors with damper integration must be used between steps, making the installation expensive. Although dc motor drives would be less costly, few power plants other than marine Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 224. 4.46 POWER GENERATION TABLE 6 Fan Control Comparison propulsion plants have direct current available. And since marine units normally operate at full load 90 percent of the time or more, part-load operating economics are unimportant. If steam-turbine drive will be used for the fans, plot the power- input curve LMD, using data from the fan manufacturer. A hydraulic coupling or electric magnetic coupling, type e, with a constant- speed motor drive would have the power-input curve DEJ. Study of the power-input curves shows that the hydraulic and electric couplings have the smallest power input. Their first cost, however, is usually greater than any other types of power-saving devices. To determine the return on any extra invest- ment in power-saving devices, an economic study including a load-duration analysis of the boiler load must be made. 4. Compare the return on the extra investment Compute and tabulate the total cost of each type of control system. Then determine the extra investment for each of the more costly control systems by subtracting the cost of type a from the cost of each of the other types. With the extra investment known, compute the lifetime savings in power input for each of the more efficient control methods. With the extra investment and savings resulting from it known, compute the percentage return on the extra investment. Tabulate the findings as in Table 6. In Table 6, considering control type c, the extra cost of type c over type b ⫽ $75,000 ⫺ 50,000 ⫽ $25,000. The total power saving of $6500 is computed on the basis of the cost of energy in the plant for the life of the control. The return on the extra investment then ⫽ $6500/$25,000 ⫽ 0.26, or 26 percent. Type e control provides the highest percentage return on the extra investment. It would probably be chosen if the only measure of investment desirability is the return on the extra investment. However, if other criteria are used, such as a minimum rate of return on the extra investment, one of the other control types might be chose. This is easily determined by studying the tabulation in conjunction with the in- vestment requirement. Related Calculations. The procedure used here can be applied to heating, power, marine and portable boilers of all types. Follow the same steps given above, changing the values to suit the existing conditions. Work closely with the fan and drive manufacturer when analyzing drive power input and costs. SMOKESTACK HEIGHT AND DIAMETER DETERMINATION Determine the required height and diameter of a smokestack to produce 1.0-inH2O (0.25-kPa) draft at sea level if the average air temperature is 60⬚F (15.6⬚C); baro- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 225. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.47 metric pressure is 29.92 inHg (101.3 kPa); the boiler flue gas enters the stack at 500⬚F (260.0⬚C); the flue-gas flow rate is 100 lb/s (45.4 kg/s); The flue-gas density is 0.045 lb/ft3 (0.72 kg/m3 ); and the flue-gas velocity is 30 ft/s (9.1 m/s). What diameter and height would be required for this stack if it were located 5000 ft (1524.0 m) above sea level? Calculation Procedure: 1 Compute the required stack height The required stack height Sh ft ⫽ ds /0.256pK, where ds ⫽ stack draft, inH2O; p ⫽ barometric pressure, inHg; K ⫽ 1/Ta ⫺ 1/Tg, where Ta ⫽ air temperature,⬚R; Tg ⫽ average temperature of stack gas,⬚R. In applying this equation, the temperature of the gas at the stack outlet must be known to determine the average temperature of the gas in the stack. Since the outlet temperature cannot be measured until after the stack is in use, an assumed outlet temperature must be used for design calcu- lations. The outlet temperature depends on the inlet temperature, ambient air tem- perature, and materials used in the stack construction. For usual smokestacks, the gas temperature will decrease 100 to 200⬚F (55.6 to 111.1⬚C) between the stack inlet and outlet. Using a 100⬚F (55.6⬚C) gas-temperature decrease for this stack, we get Sh ⫽ (1.0) ⫹ 0.256(29.92)(1/520 ⫺ 1/910) ⫽ 159 ft (48.5 m). Apply a 10 percent factor of safety. Then the stack height ⫽ (159)(1.10) ⫽ 175 ft (53.3 m). 2. Compute the required stack diameter Stack diameter ds ft is found from ds ⫽ 0.278(WgTg /Vdgp)0.5 , where Wg ⫽ flue-gas flow rate in stack, lb/s; V ⫽ flue-gas velocity in stack, ft/s; dg ⫽ flue-gas density, lb/ft3 . For this stack, ds ⫽ 0.278{(100)(910)/[(30)(0.045)(29.92)]}0.5 ⫽ 13.2 ft (4.0 m), or 13 ft 3 in (4 m 4 cm), rounding to the nearest inch diameter. Note: Use this calculation procedure for any stack material—masonary, steel, brick, or plastic. Most boiler and stack manufacturers use charts based on the equa- tions above to determine the economical height and diameter of a stack. Thus, the Babcock Wilcox Company, New York, Inc., also presents four charts for stack sizing, in Steam: Its Generation and Use. Combustion Engineering, Inc., also pre- sents four charts for stack sizing, in Combustion Engineering. The equations used in the present calculation procedure are adequate for a quick, first approximation of stack height and diameter. 3. Compute the required stack height and diameter at 5000-ft (1524.0-m) elevation Fuels require the same amount of oxygen for combustion regardless of the altitude at which they are burned. Therefore, this stack must provide the same draft as at sea level. But as the altitude above sea level increases, more air must be supplied to the fuel to sustain the same combustion rate, because air above sea level contains less oxygen per cubic foot than at sea level. To accommodate the larger air and flue-gas flow rate without an increase in the stack friction loss, the stack diameter must be increased. To determine the required stack height Se ft at an elevation above sea level, multiply the sea-level height Sh by the ratio of the sea-level and elevated height barometric pressures inHg. Since the barometric pressure at 5000 ft (1524.0 m) is 24.89 inHg (84.3 kPa) and the sea-level barometric pressure is 29.92 inHg (101.3 kPa), Se ⫽ (175)(29.92/24.89) ⫽ 210.2 ft (64.1 m). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 226. 4.48 POWER GENERATION The stack diameter de ft at an elevation above sea level will vary as the 0.40 power of the ratio of the sea-level and altitude barometric pressures, or de ds(pe / p)0.4 , where pe ⫽ barometric pressure of altitude, inHg. For this stack, de ⫽ (13.2)(29.92/24.89)0.4 ⫽ 14.2 ft (4.3 m), or 14 ft 3 in (4 m 34 cm). Related Calculations. The procedure given here can be used for heating, power, marine, industrial, and residential smokestacks or chimneys, regardless of the materials used for construction. When designing smokestacks for use at altitudes above sea level, use step 3, or substitute the actual barometric pressure at the elevated location in the height and diameter equations of steps 1 and 2. POWER-PLANT COAL-DRYER ANALYSIS A power-plant coal dryer receives 180 tons/h (163.3 t/h) of wet coal containing 15 percent free moisture. The dryer is arranged to drain 6 percent of the moisture from the coal, and a moisture content of 1 percent is acceptable in the coal delivered to the power plant. Determine the volume and temperature of the drying gas re- quired for the dryer, the total heat, grate area, and combustion-space volume needed. Ambient air temperature during drying is 70⬚F (21.1⬚C). Calculation Procedure: 1. Compute the quantity of moisture to be removed The total moisture in the coal ⫽ 15 percent. Of this, 6 percent is drained and 1 percent can remain in the coal. The amount of moisture to be removed is therefore 15 ⫺ 6 ⫺ 1 ⫽ 8 percent. Since 180 tons (163.3 t) of coal are received per hour, the quantity of moisture to be removed per minute is [180/(60 min/h)](2000 lb/ ton)(0.08) ⫽ 480 lb/min (3.6 kg/s). 2. Compute the airflow required through the dryer Air enters the dryer at 70⬚F (21.1⬚C). Assume that evaporation of the moisture on the coal takes place at 125⬚F (51.7⬚C)—this is about midway in the usual evapo- ration temperature range of 110 to 145⬚F (43.3 to 62.8⬚C). Determine the moisture content of saturated air at each temperature, using the psychrometric chart for air. Thus, for saturated air at 70⬚F (21.1⬚C) dry-bulb temperature, the weight of the moisture it contains is wm lb (kg) of water per pound (kilogram) of dry air ⫽ 0.0159 (0.00721), whereas at 125⬚F (51.7⬚C), wm ⫽ 0.09537 lb of water per pound (0.04326 kg/kg) of dry air. The weight of water removed per pound of air passing through the dryer is the difference between the moisture content at the leaving temperature, 125⬚F (51.7⬚C), and the entering temperature, 70⬚F (21.1⬚C), or 0.09537 ⫺ 0.01590 ⫽ 0.07947 lb of water per pound (0.03605 kg/kg) of dry air. Since air at 70⬚F (21.1⬚C) has a density of 0.075 lb/ft3 (1.2 kg/m3 ), 1/0.075 ⫽ 13.3 ft3 (0.4 m3 ) of air at 70⬚F (21.1⬚C) must be supplied to absorb 0.07947 lb of water per pound (0.03605 kg/kg) of dry air. With 480 lb/min (3.6 kg/s) of water to be evaporated in the dryer, each cubic foot of air will absorb 0.7947/13.3 ⫽ 0.005945 lb (0.095 kg/m3 ), of moisture, and the total airflow must be (480 lb/ min)/(0.005945) ⫽ 80,800 ft3 /min (38.1 m3 /s), given a dryer efficiency of 100 percent. However, the usual dryer efficiency is about 75 percent, not 100 percent. Therefore, the total actual airflow through the dryer should be 80,800/0.75 ⫽ 107,700 ft3 /min (50.8 m3 /s). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 227. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.49 Note: If desired, a table of moist air properties can be used instead of a psy- chrometric chart to determine the moisture content of the air at the dryer inlet and outlet conditions. The moisture content is read in the humidity ratio Ws column. See the ASHRAE—Guide and Data Book for such a tabulation of moist-air prop- erties. 3. Compute the required air temperature Assume that the heating air enters at a temperature t greater than 125⬚F (51.7⬚C). Set up a heat balance such that the heat given up by the air cooling from t to 125⬚F (51.7⬚C) ⫽ the heat required to evaporate the water on the coal ⫹ the heat required to raise the temperature of the coal and water from ambient to the evaporation temperature ⫹ radiation losses. The heat given up by the air, Btu ⫽ (cfm)(density of air, lb/ft3 )[specific heat of air, Btu/lb 䡠 ⬚F)](t ⫺ evaporation temperature,⬚F). The heat required to evaporate the water, Btu ⫽ (weight of water, lb/min)(hƒg at evaporation temperature). The heat required to raise the temperature of the coal and water from ambient to the evaporation temperature, Btu ⫽ (weight of coal, lb/min)(evaporation temperature ⫺ ambient temperature)[specific heat of coal, Btu/(lb 䡠 ⬚F)] ⫹ (weight of water, lb /min)(evaporation temperature ⫺ ambient temperature)[specific heat of water, Btu /(lb 䡠 ⬚F)]. The heat required to make up for radiation losses, Btu ⫽ {(area of dryer insulated surfaces, ft2 )[heat-transfer coefficient, Btu/(ft2 䡠 ⬚F 䡠 h)](t ⫺ ambient tem- perature) ⫹ (area of dryer uninsulated surfaces, ft2 )[heat-transfer coefficient, Btu/ (ft2 䡠 ⬚F 䡠 h)](t ⫺ ambient temperature)}/60. Compute the heat given up by the air, Btu, as (107,700)(0.075)(0.24)(t ⫺ 70), where 0.075 is the air density and 0.24 is the specific heat of air. Compute the heat required to evaporate the water, Btu, as (480)(1022.9), where 1022.9 ⫽ hƒg at 125⬚F (51.7⬚C) from the steam tables. Compute the heat required to raise the temperature of the coal and water from ambient to the evaporation temperature, Btu, as (6000)(t ⫺ 70)(0.30) ⫹ (480)(t ⫺ 70)(1.0), where 0.30 is the specific heat of the coal and 1.0 is the specific heat of water. Compute the heat required to make up the radiation losses, assuming 3000 ft2 (278.7 m2 ) of insulated and 1500 ft2 (139.4 m2 ) of uninsulated surface in the dryer, with coefficients of heat transfer of 0.35 and 3.0 for the insulated and uninsulated surfaces, respectively. Then radiation heat loss, Btu ⫽ (3000)(0.35)(t ⫺ 70) ⫹ (1500)(3.0)(t ⫺ 70). Set up the heat balance thus and solve for t: (107,7000)(0.075)(0.24)(t ⫺ 70) ⫽ (480)(1022.9) ⫹ (6000)(125 ⫺ 70)(0.30) ⫹ (480)(125 ⫺ 70)(1.0) ⫹ [(3000)(0.35)(t ⫺ 70) ⫹ (1500)(3.0)(t ⫺ 70)]/60; so t ⫽ 406⬚F (207.8⬚C). In this heat balance, the factor 60 is divided into the radiation heat loss to convert flow in Btu/h to Btu/ min because all the other expressions are in Btu/min. 4. Determine the total heat required by the dryer Using the equation of step 3 with t ⫽ 406⬚F (207.8⬚C), we find the total heat ⫽ (107,770)(0.075)(0.24)(406 ⫺ 70) ⫽ 651,000 Btu/min, or 60(651,000) ⫽ 39,060,000 Btu/h (11,439.7 kW) 5. Compute the dryer-furnace grate area Assume that heat for the dryer is produced from coal having a lower heating value of 13,000 Btu/lb (30,238 kJ/kg) and that 40 lb/h of coal is burned per square foot [0.05 kg/(m2 䡠 s)] of grate area with a combustion efficiency of 70 percent. The rate of coal firing ⫽ (Btu/min to dryer)/(coal heating value, Btu/ lb)(combustion efficiency) ⫽ 651,000/(13,000)(0.70) ⫽ 71.5 lb/min, or 60(71.5) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 228. 4.50 POWER GENERATION FIGURE 9 (a) Conical coal pile; (b) triangular coal pile. ⫽ 4990 lb/h (0.63 kg/s). Grate area ⫽ 4990/40 ⫽ 124.75 ft2 , say 125 ft2 (11.6 m2 ). 6. Compute the dryer-furnace volume The usual heat-release rates for dryer furnaces are about 50,000 Btu/(h 䡠 ft3 ) (517.5 kW/m3 ) of furnace volume. For this furnace, which burns 4900 lb/h (0.63 kg/s) of 13,000-Btu/lb (30,238-kJ/kg) coal, the total heat released is 4990(13,000) ⫽ 64,870,000 Btu/h (18,998.8 kW). With an allowable heat release of 50,000 Btu/ (h 䡠 ft3 ) (517.1 kW/m3 ), the required furnace volume ⫽ 64,870,000/50,000 ⫽ 1297.4 ft3 , say 1300 ft3 (36.8 m3 ). Related Calculations. The general procedure given here can be used for any air-heated dryer used to dry moist materials. Thus, the procedure is applicable to chemical, soil, and fertilizer drying, as well as coal drying. In each case, the specific heat of the material dried must be used in place of the specific heat of coal given above. COAL STORAGE CAPACITY OF PILES AND BUNKERS Bituminous coal is stored in a 25-ft (7.5-m) high, 68.8-ft (21.0-m) diameter, cir- cular-base conical pile. How many tons of coal does the pile contain if its base angle is 36⬚? How much bituminous coal is contained in a 25-ft (7.5-m) high rectangular pile 100 ft (30.5 m) long if the pile cross section is a triangle having a 36⬚ base angle? Calculation Procedure: 1. Sketch the coal pile Figure 9a and b shows the two coal piles. Indicate the pertinent dimesions—height, the diameter, length, and base angle—on each sketch. 2. Compute the volume of the coal pile Volume of a right circular cone, ft3 ⫽ r2 h/3, where r ⫽ radius, ft; h ⫽ cone height, ft. Volume of a triangular pile ⫽ bal/2, where b ⫽ base length, ft; a ⫽ altitude, ft; l ⫽ length of pile, ft. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 229. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.51 For this conical pile, volume ⫽ (3.4)2 (25)/3 ⫽ 31,000 ft3 (877.8 m3 ). Since 50 lb of bituminous coal occupies about 1 ft3 of volume (800.9 kg/m3 ), the weight of coal in the conical pile ⫽ (31,000 ft3 )(50 lb/ft3 ) ⫽ 1,550,000 lb, or (1,550,000 lb)/(2000 lb/ton) ⫽ 775 tons (703.1 t). For the triangular pile, base length ⫽ 2h/tan 36⬚ ⫽ (2)(25)/0.727 ⫽ 68.8 ft (21.0 m). Then volume ⫽ (68.8)(25)(100)/2 ⫽ 86,000 ft3 (2435.2 m3 ). The weight of bituminous coal in the pile is, as for the conical pile, (86,000)(50) ⫽ 4,300,000 lb, or (4,300,000 lb)/(2000 lb/ton) ⫽ 2150 tons (1950.4 t). Related Calculations. Use this general procedure to compute the weight of coal in piles of all shapes, and in bunkers, silos, bins, and similar storage com- partments. The procedure can be used for other materials also—grain, sand, gravel, coke, etc. Be sure to use the correct density when converting the total storage volume to total weight. Refer to Baumeister and Marks—Standard Handbook for Mechanical Engineers for a comprehensive tabulation of the densities of various materials. PROPERTIES OF A MIXTURE OF GASES A 10-ft3 (0.3-m3 ) tank holds 1 lb (0.5 kg) of hydrogen (H2), 2 lb (0.9 kg) of nitrogen (N2), and 3 lb (1.4 kg) of carbon dioxide (CO2) at 70⬚F (21.1⬚C). Find the specific volume, pressure, specific enthalpy, internal energy, and specific entropy of the individual gases and of the mixture and the mixture density. Use Avogadro’s and Dalton’s laws and Keenan and Kaye—Thermodynamic Properties of Air, Products of Combustion and Component Gases, Krieger, commonly termed the Gas Tables. Calculation Procedure: 1. Compute the specific volume of each gas Using H, N, and C as subscripts for the respective gases, we see that the specific volume of any gas v ft3 /lb ⫽ total volume of tank, ft3 weight of gas in tank, lb. Thus, vH ⫽ 10/1 ⫽ 10 ft3 /lb (0.6 m3 /kg); vN ⫽ 10/2 ⫽ 5 ft3 /lb (0.3 m3 /kg); vC ⫽ 10/3 ⫽ 3.33 ft3 /lb (0.2 m3 /dg). Then the specific volume of the mixture of gases is vt ft3 /lb ⫽ total volume of gas in tank, ft3 /sum of weight of individual gases, lb ⫽ 10/(1 ⫹ 2 ⫹ 3) ⫽ 1.667 ft3 /lb (0.1 m3 /kg). 2. Determine the absolute pressure of each gas Using P ⫽ RTw/vtM, where P ⫽ absolute pressure of the gas, lb/ft2 (abs); R ⫽ universal gas constant ⫽ 1545; T ⫽ absolute temperature of the gas,⬚R ⫽ ⬚F ⫹ 459.9, usually taken as 460; w ⫽ weight of gas in the tank, lb; vt ⫽ total volume of the gas in the tank, ft3 ; M ⫽ molecular weight of gas. Thus, PH ⫽ (1545)(70 ⫹ 460)(1.0)/[(10)(2.0)] ⫽ 40,530 lb/ft2 (abs) (1940.6 kPa); PN ⫽ (1545)(70 ⫹ 460)(2.0)/[(10)(28)] ⫽ 5850 lb/ft2 (abs) (280.1 kPa); PC ⫽ (1545)(70 ⫹ 460)(3.0)/[(10)(44)] ⫽ 5583 lb/ft2 (abs) (267.3 kPa); Pt ⫽ 兺PH, PN, PC ⫽ 40,530 ⫹ 5850 ⫹ 5583 ⫽ 51,963 lb/ft2 (abs) (2488.0 kPa). 3. Determine the specific enthalpy of each gas Refer to the Gas Tables, entering the left-hand column of the table at the absolute temperature, 530⬚F (294 K), for the gas being considered. Opposite the temperature, read the specific enthalpy in the h column. Thus, hH ⫽ 1796.1 Btu/lb (4177.7 kJ/ kg); hN ⫽ 131.4 Btu/lb (305.6 kJ/kg); hC ⫽ 90.17 Btu/lb (209.7 kJ/kg). The total Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 230. 4.52 POWER GENERATION enthalpy of the mixture of the gases is the sum of the products of the weight of each gas and its specific enthalpy, or (1)(1796.1) ⫹ (2)(131.4) ⫹ (3)(90.17) ⫹ 2329.4 Btu (2457.6 kJ) for the 6 lb (2.7 kg) or 10 ft3 (0.28 m3 ) of gas. The specific enthalpy of the mixture is the total enthalpy/gas weight, lb, or 2329.4/(1 ⫹ 2 ⫹ 3) ⫽ 388.2 Btu/lb (903.0 kJ/kg) of gas mixture. 4. Determine the internal energy of each gas Using the Gas Tables as in step 3, we find EH ⫽ 1260.0 Btu/lb (2930.8 kJ/kg); EN ⫽ 93.8 Btu/lb (218.2 kJ/kg); EC ⫽ 66.3 Btu/lb (154.2 kJ/kg). The total energy ⫽ (1)(1260.0) ⫹ (2)(93.8) ⫹ (3)(66.3) ⫽ 1646.5 Btu (1737.2 kJ). The specific enthalpy of the mixture ⫽ 1646.5/(1 ⫹ 2 ⫹ 3) ⫽ 274.4 Btu/lb (638.3 kJ/kg) of gas mixture. 5. Determine the specific entropy of each gas Using the Gas Tables as in step 3, we get SH ⫽ 15.52 Btu/(lb 䡠 ⬚F) [65.0 kJ/(kg 䡠 ⬚C)]; SN ⫽ 1.558 Btu/(lb 䡠 ⬚F) [4.7 kJ/(kg 䡠 ⬚C)]. The entropy of the mixture ⫽ (1)(12.52) ⫹ (2)(1.558) ⫹ (3)(1.114) ⫽ 18.978 Btu/⬚F (34.2 kJ/⬚C). The specific entropy of the mixture ⫽ 18.978/(1 ⫹ 2 ⫹ 3) ⫽ 3.163 Btu/(lb 䡠 ⬚F) [13.2 kJ/ (kg 䡠 ⬚C) of the gas mixture. 6. Compute the density of the mixture For any gas, the total density dt ⫽ sum of the densities of he individual gases. And since density of a gas ⫽ 1/specific volume, dt ⫽ 1/vt ⫽ 1/vH ⫹ 1/vN ⫹ 1/vC ⫽ 1/10 ⫹ 1/5 ⫹ 1/3.33 ⫽ 0.6 lb/ft3 (9.6 kg/m3 ) of mixture. This checks with step 1, where vt ⫽ 1.667 ft3 /lb (0.1 m3 /kg), and is based on the principle that all gases occupy the same volume. Related Calculations. Use this method for any gases stored in any type of container—steel, plastic, rubber, canvas, etc.—under any pressure from less than atmospheric to greater than atmospheric at any temperature. STEAM INJECTION IN AIR SUPPLY In a certain manufacturing process, a mixture of air and steam at a total mixture pressure of 300 lb/in2 absolute (2068 kPa) and 400⬚F (204⬚C) is desired. The rel- ative humidity of the mixture is to be 60 percent. For a required mixture flow rate of 500 lb/h (3.78 kg/s) determine (a) the volume flow rate of dry air in ft3 /min (m3 /s) of free air, where air is understood to be air at 14.7 lb/in2 (101 kPa) and 70⬚F (21⬚C); and (b) the required rate of steam injection in lb/h (kg/s). Calculation Procedure: 1. Determine the partial pressure of the vapor and that of the air From Table 1, Saturation: Temperatures of the Steam Tables mentioned under Re- lated Calculations of this procedure, at 400⬚F (204⬚C) the steam saturation pressure, Pvs ⫽ 247.31 lb/in2 (1705 kPa), by interpolation. Since the vapor pressure is ap- proximately proportional to the grains of moisture in the mixture, the partial pres- sure of vapor in the mixture, Pvp ⫽ Pvs ⫽ 0.6 ⫻ 247.31 ⫽ 148.4 lb/in2 absolute (1023 kPa), where is the relative humidity as a decimal. Then, the partial pressure Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 231. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.53 of the air in the mixture, Pa ⫽ Pm ⫺ Pvs ⫽ 300 ⫺ 148.4 ⫽ 151.6 lb/in2 absolute (1045 kPa), where Pm is the total mixture pressure. 2. Compute the density of air in the mixture The air density, a ⫽ Pa /(RaTa), where Pa ⫽ 151.6 ⫻ 144 ⫽ 21.83 ⫻ 103 lb/ft2 (1045 kPa); the gas constant for air, Ra ⫽ 53.3 ft 䡠 lb/(lb 䡠 ⬚R) [287 J/(kg 䡠 K)]; absolute temperature of the air Ta ⫽ 400 ⫹ 460 ⫽ 860⬚R (478 K), then, a ⫽ 21.83 ⫻ 103 /(53.3 ⫻ 860) ⫽ 0.4762 lb/ft3 (7.63 kg/m3 ). 3. Find the specific volume of the vapor in the mixture From Table 3, Vapor of the Steam Tables, at 148.4 lb/in2 absolute (1023 kPa) and 400⬚F (204⬚C), the specific volume of the vapor, vv ⫽ 3.261 ft3 /lb (0.2036 m3 /kg), by interpolation. 4. Compute the density of the vapor and that of the mixture The density of the vapor, v ⫽ 1/vv ⫽ 1/3.261 ⫽ 0.3066 lb/ft 3 (4.91 kg/m3 ). The density of the mixture, m ⫽ a ⫹ v ⫽ 0.4762 ⫹ 0.3066 ⫽ 0.7828 lb/ft3 (12.54 kg/m3 ). 5. Compute the amount of air in 500 lb/h (3.78 kg/s) of mixture In 500 lb/h (3.78 kg/s) of mixture, wm, the amount of air, wa ⫽ a ⫻ wm /m ⫽ 0.4762 ⫻ 500/0.7828 ⫽ 304 lb/h (2.30 kg/s). 6. Compute the flow rate of dry air (a) The flow rate of dry air at 14.7 lb/in2 (101 kPa) and 70⬚F (21⬚C), Va ⫽ wa ⫻ Ra ⫻ T/P, where the free air temperature, T ⫽ 70 ⫹ 460 ⫽ 530⬚R (294 K); free air pressure, P ⫽ 14.7 ⫻ 144 ⫽ 2.117 ⫻ 103 lb/ft2 (101 kPa); other values as before. Hence, Va ⫽ 304 ⫻ 53.3 ⫻ 530/(2.117 ⫻ 103 ) ⫽ 4060 ft3 /h ⫽ 67.67 ft3 / min (1.92 ⫻ 10⫺3 m3 /s). 7. Compute the rate of steam injection (b) The rate of steam injection, ws ⫽ ws ⫺ wa ⫽ 500 ⫺ 304 ⫽ 196 lb/h (1.48 kg/s). Related Calculations. The Steam Tables appear in Thermodynamic Properties of Water Including Vapor, Liquid, and Solid Phases, 1969, Keenan, et al., John Wiley Sons, Inc. This procedure considers the air and steam as ideal gases which behave in accordance with the Gibbs-Dalton law of gas mixtures having complete homogeneous molecular dispersion and additive pressures. Also, calculations in steps 2 and 6 are based on Boyle’s law and Charles’ law, which relate pressure, volume, and temperature of a gas, or gas mixture. Clear and concise presentations of these and other significant definitions appear in Thermodynamics and Heat Power, 4th edition, by Irving Granet, Regents/Prentice-Hall, Englewood Cliffs, NJ 07632. BOILER AIR-HEATER ANALYSIS AND SELECTION A boiler manufacturer proposes these two alternatives to a prospective purchaser: (a) A steam-generating unit equipped with a small air heater which results in an overall steam-generating efficiency of 83 percent; (b) A similar steam-generating Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 232. 4.54 POWER GENERATION unit equipped with a larger air heater which results in an overall steam-generating unit efficiency of 87 percent. It is anticipated that coal delivered to the furnace will cost $60.00/ton (0.907 metric ton). The boiler is intended to operate 8000 hours per year and is to deliver one million pounds of steam per hour (126 kg/s) with an enthalpy rise of 1200 Btu per pound mass (2.791 ⫻ 106 J/kg). If the total investment charges are 20 percent, what additional cost can be paid for the larger heater? Indicate the Btu content of coal upon which the selection is based. Calculation Procedure: 1. Compute boiler heat output during one year of intended operation Boiler heat output per year of operation, Q ⫽ H ⫻ S ⫻ ⌬h, where the time used per year, H ⫽ 8000 h (2.88 ⫻ 106 s); rate of steam delivery, S ⫽ 106 lbm /h (126 kg/s); enthalpy rise, ⌬h ⫽ 1200 Btu/lbm (2.791 ⫻ 106 J/kg). Then, Q ⫽ 8000 ⫻ 1200 ⫽ 9.6 ⫻ 1012 Btu/year (22.31 ⫻ 1015 J/year). 2. Compute the annual mass of coal input for each Proposal Proposal (a): Annual coal input, Ca ⫽ Q/(B ⫻ Ea), where the assumed Btu content of coal, B ⫽ 13,500 Btu/lbm (31.4 ⫻ 106 J/kg); efficiency of Proposal (a), Ea ⫽ 0.83. Then, Ca ⫽ 9.6 ⫻ 1012 /(13,500 ⫻ 0.83) ⫽ 711 ⫻ 106 /0.83 ⫽ 857 ⫻ 106 lbm /year (389 ⫻ 106 kg/year). Proposal (b): Annual coal input, Cb ⫽ Q/(B ⫻ Eb), where efficiency of Proposal (b), Eb ⫽ 0.87. Then, Cb ⫽ 711 ⫻ 106 /0.87 ⫽ 817 ⫻ 106 lbm /year (371 ⫻ 106 kg/year). 3. Compute the annual cost of coal for each Proposal Proposal (a): Annual cost of coal, Aa ⫽ (Ca /2000) ⫻ Ct, where the cost per ton (0.907 metric ton) of coal, Ct ⫽ $60.00. Then, Aa ⫽ (857 ⫻ 106 /2000) ⫻ 60 ⫽ $25,710,000/year. Proposal (b): Annual cost of coal, Ab ⫽ (Cb /2000)Ct ⫽ (817 ⫻ 106 /2000) ⫻ 60 ⫽ $24,510,000/year. 4. Compute the additional investment that can be made for the larger heater Additional investment, L, for the larger heater can be found by setting proposed annual costs for Proposal (a) equal to those for Proposal (b) in a ‘‘Break Even’’ equation. Thus, (dc ⫻ Ia) ⫹ Om ⫹ Aa ⫽ [dc ⫻ (Ia ⫹ L)] ⫹ Om ⫹ Ab, where the decimal fraction for total investment charges, dc ⫽ 0.20; total investment charges for Proposal (a) ⫽ Ia, and for Proposal (b) ⫽ (Ia ⫹ L); operating and maintenance charges for either Proposal ⫽ Om; other items as before. Then, (0.20 ⫻ Ia) ⫹ Om ⫹ $25,710,000 ⫽ [0.20 ⫻ (Ia ⫹ L)] ⫹ Om ⫹ $24,510,000. This reduces to, L ⫽ ($25,710,000 ⫺ $24,510,000)/0.20 ⫽ $6,000,000, the additional investment. Note: The $60-per-ton price for coal used here was for example purposes only. Since coal prices vary widely with source region, transport distance, and quality, a high price was used to highlight the importance of investment decisions while reflecting the effects of inflation and the possible demand for specialty coal. The procedural steps remain the same, regardless of the dollar, franc, yen, pound, or other monetary unit price of the coal. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 233. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.55 FIGURE 10 Steam plant containing deaerator and boiler blowdown. EVALUATION OF BOILER BLOWDOWN, DEAERATION, STEAM AND WATER QUALITY A boiler generates 50,000 lb/h (22,700 kg/h) of saturated steam at 300 lb/in2 (abs) (2067 kPa), out of which 10,000 lb/h (4540 kg/h) is taken for process and is returned to the deaerator, Fig. 10, as condensate at 180⬚F (82.2⬚C); the remainder is consumed. Makeup water enters the deaerator at 70⬚F (21.1⬚C) and steam is available at 300 lb/in2 (abs) (2067 kPa) for deaeration. The deaerator operates at 25 lb/in2 (abs) (172.3 kPa). The blowdown has total dissolved solids (TDS) of 1500 ppm (parts per million by weight) and makeup has a TDS of 100 ppm. Evaluate the blowdown and deaeration steam quantities. Calculation Procedure: 1. Understand steam quality and steam purity Steam purity refers to the impurities in steam in ppm. A typical value in low- pressure boilers is 1 ppm. Steam quality, by contrast, refers to the moisture in steam. For example, the operator of a boiler plant will maintain a certain concentration of solids in the boiler drum, depending on either the American Boiler Manufacturers Association or the American Society of Mechanical Engineers recommendations, which can be found in publications of these organizations or in Ganapathy—Steam Plant Calculations Manual, Marcel Dekker, Inc. At 500 lb/in2 (abs) (3445 kPa), for instance, if the boiler water concentration is 2500 ppm and steam purity is 0.5 ppm solids, the steam quality is obtained from: Percent moisture in steam ⫽ (steam purity, ppm)/(boiler water concentration, ppm)(100) ⫽ (0.5/2500)(100) ⫽ 0.02 percent; steam quality ⫽ 100.0 ⫺ 0.02 ⫽ 99.98 percent. 2. Set up the deaerator mass and energy balance From the Fig. 10, the mass balance around the deaerator gives, using the data provided: 10,000 ⫹ D ⫹ M ⫽ F ⫽ 50,000 ⫹ B [Eq. (1)], where D ⫽ deaeration steam flow, lb/h (kg/h); B ⫽ blowdown, lb/h (kg/h); M ⫽ makeup flow, lb/h (kg/h); F ⫽ feedwater flow, lb/h (kg/h). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 234. 4.56 POWER GENERATION From the energy balance around the deaerator, 100,000(148) ⫹ 1202.8(D) ⫹ Mx(38) ⫽ 209(F) ⫽ 209(50,000 ⫹ B), Eq. (2), where 148 Btu/lb 344.8 kJ/kg) ⫽ enthalpy of the condensate return; 1202.8 Btu/lb (2802.5 kK/kg) ⫽ enthalpy of saturated steam going into the drum for deaeration; 38 Btu/lb (88.5 kJ/kg) and 209 Btu/lb (486.9 kJ/kg) are the enthalpies of the makeup and feedwater. Enthalpy of the feedwater is computed at the deaerator operating pressure of 25 lb/in2 (abs) (172.3 kPa), corresponding to a saturation temperature of 240⬚F (115.6⬚C). A balance of the solids in the makeup and blowdown gives 100(M) ⫽ 1500(B), Eq. (3). In this relation we neglect the solids in the steam because they are ex- tremely small in comparison to the solids in the makeup and the blowdown. 3. Find the blowdown, makeup, and feedwater flows for the plant Solving the above three equations—(1), (2), and (3), we have, from (1): D ⫹ M ⫽ 40,000 ⫹ B, Eq. 4. Substituting Eq. (3) in Eq. (4), we have D ⫹ 15(B) ⫽ 40,000 ⫹ B; or D ⫹ 14(B) ⫽ 40,000, Eq. 5. Substituting Eq. (5) and (3) in Eq. (2) and solving for B, we have B ⫽ 2375 lb/h (1078.3 kg/h); D ⫽ 6750 lb/h (3064.5 kg/h); M ⫽ 35,625 lb/h (16,173.8 kg/h); F ⫽ 52,375 lb/h (23,778.3 kg/h). If venting losses are considered, the engineer can add 1 percent to 3 percent to deaeration steam D. Related Calculations. In any steam plant, when performing energy balance calculations, it is important to evaluate deaeration steam and blowdown water quan- tities. Interestingly, these are related to feedwater and makeup water quality and steam purity. These variables must be evaluated together and not in isolation. Blowdown water can be flashed in a flash tank and the flash stream returned to the deaerator. This reduced the steam quantity required for deaeration. Another way to improve the performance of a deaerator is to preheat the deaerator makeup water before it enters the deaerator by using the blowdown water. Using methods similar to those in this procedure, you can study the effect of varying the amount of con- densate returned on the amount of deaeration steam required. This procedure is the work of V. Ganapathy, Heat Transfer Specialist, ABCO Industries, Inc. HEAT-RATE IMPROVEMENT USING TURBINE-DRIVEN BOILER FANS What is the net heat-rate improvement and net kilowatt gain in a steam power plant having a main generating unit rated at 870,000 kW at 2.5 in (6.35 cm) HgA, 0 percent makeup with motor-driven fans if turbine-driven fans are substituted? Plant data are as follows: (a) tandem-compound turbine, four-flow, 3600-r/min 33.5 in (85.1 cm) last-stage buckets with 264-ft2 (24.5-m2 ) total last-stage annulus area; (b) steam conditions 3500 lb/in2 (gage) (24,133 kPa), 1000⬚F/1000⬚F (537.8⬚C/ 537.8⬚C); (c) with main-unit valves wide open, overpressure with motor-driven fans, generator output ⫽ 952,000 kW at 2.5 in (6.35 cm) HgA and 0 percent makeup; net heat rate ⫽ 7770 Btu/kWh (8197.4 kJ/kWh); (d) actual fan hp ⫽ 14,000(10,444 W) at valves wide open, overpressure with no flow or head margins; (e) motor efficiency ⫽ 93 percent; transmission efficiency ⫽ 98 percent; inlet-valve efficiency ⫽ 88 percent; total drive efficiency ⫽ 80 percent; difference between the example drive efficiency and base drive efficiency ⫽ 80 ⫺ 76.7 ⫽ 3.3 percent. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 235. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.57 FIGURE 11 Percentage increase in net kilowatts vs. last stage annulus area for 2400-lb/in2 (gage) when turbine-driven fans are used as com- pared to motors. (Combustion.) Calculation Procedure: 1. Determine the percentage increase in net kilowatt output when turbine- driven fans are used Enter Fig. 11 at 264-ft2 (24.5-m2 ) annulus area end and 14,000 required fan horse- power, and read the increase as 3.6 percent. Hence, the net plant output increase ⫽ 34,272 kW (⫽ 0.036 ⫻ 952,000). 2. Compute the net heat improvement From Fig. 12, the net heat rate improvement ⫽ 0.31 percent. Or, 0.0031(7770) ⫽ 24 Btu (25.3 J). 3. Determine the increase in the throttle and reheater steam flow From Fig. 13, the increase in the throttle and reheater flow is 3.1 percent. This is the additional boiler steam flow required for the turbine-driven fan cycle. 4. Compute the net kilowatt gain and the net heat-rate improvement From Fig. 14 the multipliers for the 2.5 in (6.35 cm) HgA backpressure are 0.98 for net kilowatt gain and 0.91 for net heat rate. Hence, net kW gain ⫽ 34,272(0.98) ⫽ 33,587 kW, and net heat rate improvement ⫽ 24 ⫻ 0.91 ⫽ 22.0 Btu (23.0 J). 5. Determine the overall cycle benefits From Fig. 15 the correction for a drive efficiency of 80 percent compared to the base of 76.7 percent is obtained. Enter the curve with 3.3 percent (⫽ 80 ⫺ 76.7) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 236. 4.58 POWER GENERATION FIGURE 12 Percentage decrease in net heat rate vs. last-stage annulus area for 2400-lb/in2 (gage) when turbine-driven fans are used as com- pared to motors. (Combustion.) and read ⫺6.6-Btu (⫺6.96-J) correction on the net heat rate ⫺0.08 percent of generated kilowatts. To determine the overall cycle benefits, add algebraically to the values obtained from step 4, or net kW gain ⫽ 33,587 ⫹ (⫺0.0008 ⫻ 952,000) ⫽ 32,825 kW; net heat-rate improvement ⫽ 22.1 ⫹ (⫺6.6) ⫽ 15.5 Btu (16.4 J). Related Calculations. This calculation procedure can be used for any maxi- mum-loaded main turbine in utility stations serving electric loads in metropolitan or rural areas. A maximum-loaded main turbine is one designed and sized for the maximum allowable steam flow through its last-stage annulus area. Turbine-driven fans have been in operation in some plants for more than 10 years. Next to feed pumps, the boiler fans are the second largest consumer of auxiliary power in utility stations. Current studies indicate that turbine-driven fans can be economic at 700 MW and above, and possibly as low as 500 MW. Although the turbine-driven fan system will have a higher initial capital cost when compared to a motor-driven fan system, the additional cost will be more than offset by the additional net output in kilowatts. In certain cases, economic studies may show that turbine drives for fans may be advantageous in constant-throttle-flow evaluations. As power plants for utility use get larger, fan power required for boilers is increasing. Environmental factors such as use of SO2 removal equipment are also increasing the required fan power. With these increased fan-power requirements, turbine drive will be the more economic arrangement for many large fossil plants. Further, these drives enable the plant designer to obtain a greater output from each unit of fuel input. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 237. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.59 FIGURE 13 Percentage increase in throttle and reheater flows vs. last- stage annulus area for 2400-lb/in2 (gage) when turbine-driven fans are used as compared to motors. (Combustion.) FIGURE 14 Multiplier to net kilowatt and net heat-rate gains to correct for main-unit exhaust pressure higher than 1.5 inHg (38.1 mmHg). (Combustion.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 238. 4.60 POWER GENERATION FIGURE 15 Corrections for differences in motor-drive system efficiency. (Combustion.) This calculation procedure is based on the work of E. L. Williamson, J. C. Black, A. F. Destribats, and W. N. Iuliano, all of Southern Services, Inc., and F. A. Reed, General Electric Company, as reported in Combustion magazine and in a paper presented before the American Power Conference, Chicago. BOILER FUEL CONVERSION FROM OIL OR GAS TO COAL An industrial plant uses three 400,000-lb (50.4-kg/s) boilers fired by oil, a 600- MW generating unit, and two 400-MW units fired by oil. The high cost of oil, and the predictions that its cost will continue to rise in future years, led the plant owners to seek conversion of the boilers to coal firing. Outline the numerical and engi- neering design factors which must be considered in any such conversion. Calculation Procedure: 1. Evaluate the furnace size considerations The most important design consideration for a steam-generating unit is the fuel to be burned. Furnace size, fuel-burning and preparation equipment, heating-surface quantity and placement, heat-recovery equipment, and air-quality control devices are all fuel-dependent. Further, these items vary considerably among units, depend- ing on the kind of fuel being used. Figure 16 shows the difference in furnace size required between a coal-fired design boiler and an oil- or gas-fired design for the same steaming capacity in lb/ h (kg/s). The major differences between coal firing and oil or natural-gas firing result from the solid form of coal prior to burning and the ash in the products of Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 239. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.61 FIGURE 16 Furnace size comparisons. (Combustion.) combustion. Oil produces only small amounts of ash; natural gas produces no ash. Coal must be stored, conveyed, and pulverized before being introduced into a fur- nace. Oil and gas require little preparation. For these reasons, a boiler designed to burn oil as its primary fuel makes a poor conversion candidate for coal firing. 2. Evaluate the coal properties from various sources Table 7 shows coal properties from many parts of the United States. Note that the heating values range from 12,000 Btu/lb (27,960 kJ/kg) to 6800 Btu/lb (15,844 kJ/kg). For a 600-MW unit, the coal firing rates [450,000 to 794,000 lb/h (56.7 or 99.9 kg/s)] to yield comparable heat inputs provide an appreciation of the coal storage yard and handling requirements for the various coals. On an hourly usage ratio alone, the lower-heating-value coal required 1.76 times more fuel to be han- dled. Pulverizer requirements are shown in Table 8 while furnace sizes needed for the various coals are shown in Fig. 17. 3. Evaluate conversion to coal fuel Most gas-fired boilers can readily be converted to oil at reasonable cost. Little or no derating (reduction of steam or electricity output) is normally required. From an industrial or utility view, conversion of oil- or gas-fired boilers not initially designed to fire coal is totally impractical from an economic viewpoint. Further, the output of the boiler would be severely reduced. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 240. 4.62 POWER GENERATION TABLE 7 Coal Properties—Nominal 600-MW Unit* TABLE 8 Pulverizer Requirements—Nominal 600-MW Unit* For example, the overall plant site requirements for a typical station having a pair of 400-MW units designed to fire natural gas could be an area of 624,000 ft2 (57,970 m2 ). This area would be for turbine bays, steam generators, and cooling towers. (With a condenser, the area required would be less.) To accommodate the same facilities for a coal-fired plant with two 400-MW units, the ground area required would be 20 times greater. The additional facilities required include coal storage yard, ash disposal area, gas-cleaning equipment (scrubbers and precipitators), railroad siding, etc. A coal-fired furnace is nominally twice the size of a gas-fired furnace. For some units the coal-fired boiler requires 4 times the volume of a gas-fired unit. Severe deratings of 40 to 70 percent are usually required for oil- and/or gas-fired boilers not originally designed for coal firing when they are switched to coal fuel. Further, such boilers cannot be economically converted to coal unless they were originally designed to be. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 241. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.63 FIGURE 17 Furnace sizes needed for various coals for efficient opera- tion. (Combustion.) FIGURE 18 ROI evaluation of energy-conservation projects. (Chemical En- gineering.) As an example of the derating required, the 400,000-lb/h (50.4-kg/s) units con- sidered here would have to be derated to 265,000 lb/h (33.4 kg/s) if converted to pulverized-coal firing. This is 66 percent of the original rating. If a spreader stoker were used to fire the boiler, the maximum capacity obtainable would be 200,000 lb/h (25.2 kg/s) of steam. Extensive physical alteration of the boiler would also be required. Thus, a spreader stoker would provide only 50 percent of the original steaming capacity. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 242. 4.64 POWER GENERATION Related Calculations. Conversion of boilers from oil and/or gas firing to coal firing requires substantial capital investment, lengthy outage of the unit while al- terations are being made, and derating of the boiler to about half the designed capacity. For these reasons, most engineers do not believe that conversion of oil- and/or gas-fired boilers to coal firing is economically feasible. The types of boilers which are most readily convertible from oil or gas to coal are those which were originally designed to burn coal (termed reconversion). These are units which were mandated to convert to oil in the late 1960s because of en- vironmental legislation. Where the land originally used for coal storage was not sold or used for other purposes, the conversion problem is relatively minor. But if the land was sold or converted to other uses, there could be a difficult problem finding storage space for the coal. Most of these units were designed to burn low-ash, low-moisture, high-heating- value, and high-ash-fusion coals. Fuels of this quality may no longer be available. Hence reconversion to coal firing may require significant downrating of the boiler. Another important aspect of reconversion is the restoration of the coal storage, handling, and pulverizing equipment. This work will probably require considerable attention. Further, pulverizer capacity may not be sufficient, given the lower grade of fuel that would probably have to be burned. This procedure is based on the work of C. L. Richards, Vice-President, Fossil Power Systems Engineering Research Development, C-E Power Systems, Com- bustion Engineering, Inc., as reported in Combustion magazine. To comply with environmental regulations, a number of coal-burning power plants have installed scrubbers ahead of the stack inlet to reduce sulfur dioxide emissions. Estimates show that some 22 tons/h of waste can be generated by scrub- bers installed in the United States alone. This waste contains ash, limestone, and gypsum. Research at Ohio State University is now directed at using scrubber waste to reclaim coal strip mines, fertilize farm soil by enriching it, and to create concretelike building materials. When scrubber waste is used to treat soil from strip mines, the soil’s acidity is reduced to a level where hardy grasses and alfalfa grow well. It is hoped that the barren sites of strip mines can be converted to useful fields using scrubber waste. Grasses grown on such reclaimed sites are safe for animals to eat. Water leached from treated sites meets Environmental Protection Agency standards for agricultural use. Approval by EPA for use of scrubber waste at such sites is being sought. Further experiments are being conducted on using the scrubber waste on acidic farmland. It will be used alone, or in combination with nutrient-rich sewer sludge. The third use for scrubber waste is as a sort of concrete for roads or the floors of feedlots. Productive use of scrubber waste promises better control of the environment, reducing sulfur dioxide while recovering land that yielded the fuel that produced the CO2. ENERGY SAVINGS FROM REDUCED BOILER SCALE A boiler generates 16,700 lb/h (2.1 kg/s) at 100 percent rating with an efficiency of 75 percent. If 1 ⁄32 in (0.79 mm) of ‘‘normal’’ scale is allowed to form on the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 243. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.65 FIGURE 19 Effects of scale on boiler operation. (Chemical En- gineering.) tubes, determine what savings can be made if 144,000-Btu/gal (40.133-MJ/m3 ) fuel oil costs $1 per gallon ($1 per 3.8 L) and the boiler uses 16.74 million Btu/ h (4.9 MW) operating 8000 h/year. Calculation Procedure: 1. Determine the annual energy usage Compute the annual energy usage from (million Btu/h) (hours of operation annually)/efficiency. For this boiler, annual energy usage ⫽ (16.74)(8000)/0.75 ⫽ 178,560 million Btu (188,380 kJ). 2. Find the energy loss caused by scale on the tubes Enter Fig. 19 at the scale thickness, 1 ⁄32 in (0.79 mm), and project vertically upward to the ‘‘normal’’ scale (salts of Ca and Mg) curve. At the left read the energy loss as 2 percent. Hence, the annual energy loss in heat units ⫽ (178,560 million Btu/ year)(0.02) ⫽ 3571 million Btu/year (130.8 kW) 3. Compute the annual savings if the scale is removed If the scale is removed, then the energy lost, computed in step 2, will be saved. Thus, the annual dollar savings after scale removal ⫽ (heat loss in energy units) (fuel price, $/gal)/(fuel heating value, Btu/gal). Or, savings ⫽ (3571 ⫻ 106 )($1.00)/144,000 ⫽ $26,049. Related Calculations. This approach can be used with any type of boiler—waterturbe, firetube, etc. The data are also applicable to tubed water heaters which are directly fired. Note that when the scale is high in iron and silica that the energy loss is much greater. Thus, with scale of the same thickness [1 ⁄32 in (0.79 mm)], the energy loss for scale high in iron and silica is 7 percent, from Fig. 19. Then the annual loss ⫽ 178,560(0.07) ⫽ 12,500 million Btu/year (3.63 MW). Removing the scale and preventing its reformation will save, assuming the same heating value and cost for the fuel oil, (12,500 ⫻ 106 Btu/year) ($1.00)/144,000 ⫽ $86,805 per year. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 244. 4.66 POWER GENERATION While this calculation gives the energy savings from reduced boiler scale, the results also can be used to determine the amount that can be invested in a water- treatment system to prevent scale formation in a boiler, water heater, or other heat exchanger. Thus, the initial investment in treating equipment can at least equal the projected annual savings produced by the removal of scale. This procedure is the work of Walter A. Hendrix and Guillermo H. Hoyos, Engineering Experiment Station, Georgia Institute of Technology, as reported in Chemical Engineering magazine. GROUND AREA AND UNLOADING CAPACITY REQUIRED FOR COAL BURNING An industrial plant is considering switching from oil to coal firing to reduce fuel costs. Determine the ground are required for 60 days’ coal storage if the plant generates 100,000 lb/h (45,360 kg/h) of steam at a 60 percent winter load factor with a steam pressure of 150 lb/in2 (gage) (1034 kPa), average boiler evaporation is 9.47 steam/lb coal (4.3 kg/kg), coal density ⫽ 50 lb/ft3 (800 kg/m3 ), boiler efficiency is 83 percent with an economizer, and the average storage pile height for the coal is 20 ft (6.096 m). Calculation Procedure: 1. Determine the storage area required for the coal The storage area, A ft2 , can be found from A ⫽ 24WFN/EdH, where H ⫽ steam generation rate, lb/h; F ⫽ load factor, expressed as a decimal; N ⫽ number of days storage required; E ⫽ average boiler evaporation rate, lb/h; d ⫽ density of coal, lb/ft3 ; H ⫽ height of coal pile allowed, ft. Substituting yields A ⫽ 24(100,000)(0.6)(60)/[(9.47)(50)(20)] ⫽ 9123 ft2 (847 m2 ). 2. Find the maximum hourly burning rate of the boiler The maximum hourly burning rate in tons per hour is given by B ⫽ W/2000E, where the symbols are as defined earlier. Substituting, we find B ⫽ 100,000/ 2000(9.47) ⫽ 5.28 tons/h (4.79 t/h). With 24-h use in any day, maximum daily use ⫽ 24 ⫻ 5.28 ⫽ 126.7 tons/day (115 t/day). 3. Find the required unloading rate for this plant As a general rule, the unloading rate should be about 9 times the maximum total plant burning rate. Higher labor and demurrage costs justify higher unloading rates and less manual supervision of coal handling. Find the unloading rate in tons per hour from U ⫽ 9W/2000E, where the symbols are as defined earlier. Substituting gives U ⫽ 9(100,000)/2000(8.47) ⫽ 47.5 tons/h (43.1 t/h). Related Calculations. With the price of oil, gas, wood, and waste fuels rising to ever-higher levels, coal is being given serious consideration by industrial, central- station, commercial, and marine plants. Factors which must be included in any study of conversion to (or original use of) coal include coal delivery to the plant, storage before use, and delivery to the boiler. For land installations, coal is usually received in railroad hopper-bottom cars in net capacities ranging between 50 and 100 tons with 50- and 70-tons (45.4- and 63.5-t) capacity cars being most common. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 245. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.67 FIGURE 20 Typical blowdown heat-recovery sys- tem. (Combustion.) Because cars require time for spotting and moving on the railroad siding, coal is actually delivered to storage for only a portion of the unloading time. Thawing of frozen coal and car shaking also tend to reduce the actual delivery. True un- loading rate may be as low as 50 percent of the continuous-flow capacity of the handling system. Hence, the design coal-handling rate of the conveyor system serv- ing the unloading station should be twice the desired unloading rate. So, for the installation considered in this procedure, the conveyor system should be designed to handle 2(47.5) ⫽ 95 tons/h (86.2 t/h). This will ensure that at least six rail cars of 60-ton (54.4-t) average capacity will be emptied in an 8-h shift, or about 360 tons/day (326.7 t/day). With a maximum daily usage of 126.7 tons/day (115 t/day), as computed in step 2 above, the normal handling of coal, from rail car delivery during the day shift, will accumulate about 3 days’ peak use during an 8-h shift. If larger than normal shipments arrive, the conveyor system can be operated more than 8 h/day to reduce demurrage charges. This procedure is the work of E. R. Harris, Department Head, G. F. Connell, and F. Dengiz, all of the Environmental and Energy Systems, Argonaut Realty Division, General Motors Corporation, as reported in Combustion magazine. HEAT RECOVERY FROM BOILER BLOWDOWN SYSTEMS Determine the heat lost per day from sewering the blowdown from a 600-lb/in2 (gage) (4137-kPa) boiler generating 1 million lb/day (18,939.4 kg/h) of steam at 80 percent efficiency. Compare this loss to the saving from heat recovery if the feedwater has 20 cycles of concentration (that is, 5 percent blowdown), ambient makeup water temperature is 70⬚F (21⬚C), flash tank operating pressure is 10 lb/ in2 (gage) (69 kPa) with 28 percent of the blowdown flashed, blowdown heat ex- changer effluent temperature is 120⬚F (49⬚C), fuel cost is $2 per 106 Btu [$2 per (9.5)6 J], and the piping is arranged as shown in Fig. 20. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 246. 4.68 POWER GENERATION Calculation Procedure: 1. Compute the feedwater flow rate The feedwater flow rate, 106 lb/day ⫽ (steam generated, 106 lb/day)/(100 ⫺ blow- down percentage, or 106 /(100 ⫺ 5) ⫽ 1.053 ⫻ 106 lb/day (0/48 ⫻ 106 kg/day). 2. Find the steam-production equivalent of the blowdown flow The steam-production equivalent of the blowdown ⫽ feedwater flow rate ⫺ steam flow rate ⫽ 1.053 ⫺ 1.0 ⫽ 53,000 lb/day (24,090 kg/day). 3. Compute the heat loss per lb of blowdown The heat loss per lb (kg) of blowdown ⫽ saturation temperature of boiler water ⫺ ambient temperature of makeup water. Or, heat loss ⫽ (488 ⫺ 70) ⫽ 418 Btu/lb (973.9 kJ/kg). 4. Find the total heat loss from sewering When the blowdown is piped to a sewer (termed sewering), the heat in the blow- down stream is lost forever. With today’s high cost of all fuels, the impact on plant economics can be significant. Thus, total heat loss from sewering ⫽ (heat loss per lb of blowdown) (blowdown rate, lb/day) ⫽ 418 Btu/lb (53,000 lb/day) ⫽ 22.2 ⫻ 106 Btu/day (23.4 ⫻ 106 J/day). 5. Determine the fuel-cost equivalent of the blowdown The fuel-cost equivalent of the blowdown ⫽ (heat loss per day, 106 Btu)(fuel cost, $ per 106 Btu)/(boiler efficiency, %), or (22.2)(2)/0.8 ⫽ $55.50 per day. 6. Find the blowdown flow to the heat exchanger With 28 percent of the blowdown flashed to steam, this means that 100 ⫺ 28 ⫽ 72 percent of the blowdown is available for use in the heat exchanger. Since the blowdown total flow rate is 53,000 lb/day (24,090 kg/day), the flow rate to the blowdown heat exchanger will be 0.72(53,000) ⫽ 38,160 lb/day (17,345 kg/day). 7. Determine the daily heat loss to the sewer As Fig. 20 shows, the blowdown water which is not flashed, flows through the heat exchanger to heat the incoming makeup water and then is discharged to the sewer. It is the heat in this sewer discharge which is to be computed here. With a heat-exchanger effluent temperature of 120⬚F (49⬚C) and a makeup water temperature of 70⬚F (21⬚C), the heat loss to the sewer is 120 ⫺ 70 ⫽ 50 Btu/lb (116.5 kJ/kg). And since the flow rate to the sewer is 38,160 lb/day (17,345 kg/day), the total heat loss to the sewer is 50(38,160) ⫽ 1.91 ⫻ 106 Btu/day (2.02 ⫻ 106 kJ/day). 8. Compare the two systems in terms of heat recovered The heat recovered ⫺ heat loss by sewering ⫽ heat loss with recovery ⫽ 22.2 ⫻ 106 Btu/day 么 1.91 ⫻ 106 Btu/day ⫽ 20.3 ⫻ 106 Btu/day (21.4 ⫻ 106 J/day). 9. Determine the percentage of the blowdown heat recovered and dollar savings The percentage of heat recovered ⫽ (heat recovered, Btu/day)/(original loss, Btu/day) ⫽ 20.3/22.2)(100) ⫽ 91 percent. Since the cost of the lost heat was $55.50 per day without any heat recovery, the dollar savings will be 91 percent of this, or 0.91($55.50) ⫽ $50.51 per day, or $18,434.33 per year with 365 days of operation. And as fuel costs rise, which they are almost certain to do in future Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 247. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.69 years, the annual saving will increase. Of course, the cost of the blowdown heat- recovery equipment must be offset against this saving. In general, the savings war- rant the added investment for the extra equipment. Related Calculations. This procedure is valid for any type of steam-generating equipment for residential, commercial, industrial, central-station, or marine instal- lations. (In the latter installation the ‘‘sewer’’ is the sea.) The typical range of blowdown heat recovery is in the 80 to 90 percent area. In view of the rapid rise in fuel prices, this range of heat recovery is significant. Hence, much wider use of blowdown heat recovery can be expected in all types of steam-generating plants. To reduce scale buildup in boilers, low cycles of boiler water concentration are preferred. This means that high blowdown rates will be used. To prevent wasting expensive heat present in the blowdown, heat-recovery equipment such as that dis- cussed above is used. In industrial plants (which are subject to many sources of condensate contamination), cycles of concentration are seldom allowed to exceed 50 (2 percent blowdown). In the above application, the cycles of concentration ⫽ 20, or 5 percent blowdown. To prevent boiler scale buildup, good pretreatment of the makeup is recom- mended. Typical current selections for pretreatment equipment, by using the boiler operating pressure as the main criterion, are thus: This procedure is the work of A. A. Askew, Betz Laboratories, Inc., as reported in Combustion magazine. BOILER BLOWDOWN PERCENTAGE The allowable concentration in a certain drum is 2000 ppm. Pure condensate is fed to the drum at the rate of 85,000 gal/h (89.4 L/s). Make-up, containing 50 grains (gr)/gal (856 mg/L) of sludge-producing impurities, is also delivered to the drum at the rate of 1500 gal/h (1.58 L/s). Calculate the blowdown as a percentage of the boiler steaming capacity. Calculation Procedure: 1. Compute the ppm of impurities per gallon of make-up water There are 58,410 gr/gal (106 mg/L). See the Related Calculations of this procedure for the basis of this factor. Parts per million of impurities, ppmi ⫽ [(gr/gal)i /(gr/ gal)i /(gr/gal)] ⫻ 106 , where (gr/gal)i ⫽ quality of, or impurities in, the make-up water, gr/gal (mg/L); (gr/gal) ⫽ grains/gal (mg/L). Hence, ppmi ⫽ (50/58,410) ⫻ 106 ⫽ 50 ⫻ 17.12 ⫽ 856. 2. Compute the blowdown rate To maintain impurities in the drum at a certain concentration, the parts fed to the drum ⫽ parts discharged by the blowdown. Thus, ppmi ⫽ (gal/h)m ⫽ ppma ⫻ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 248. 4.70 POWER GENERATION (gal/h)a, where the subscripts stand for i ⫽ impurities; m ⫽ make-up; a ⫽ allow- able; b ⫽ blowdown. Then, 856 ⫻ 1500 ⫽ 2000 ⫻ (gal/h)b. Solving: (gal/h)b ⫽ 856 ⫻ 1500/2000 ⫽ 642 (0.675 L/s). 3. Compute the boiler steaming capacity The boiler steaming capacity (gal/h)s ⫽ (gal/h)ƒ ⫹ (gal/h)m ⫺ (gal/h)b, where (gal/h)ƒ ⫽ feedwater flow rate. Then, (gal/h)s ⫽ 85,000 ⫹ 1500 ⫺ 642 ⫽ 85,858 (90.3 L/s). 4. Compute the blowdown percentage Blowdown percentage ⫽ [(gal/h)b /(gal/h)s] ⫻ 100 ⫽ (642/85,858) ⫻ 100 ⫽ 0.747 percent. Related Calculations. The gr/gal factor in step 1 is based on the density of impurities being considered as equal to the maximum density of clean fresh water, 8.3443 lb/gal (1.0 kg/L). Since 1 lb ⫽ 7000 gr, then 8.3443 ⫻ 7000 ⫽ 58,410 gr/gal (106 mg/L). SIZING FLASH TANKS TO CONSERVE ENERGY Determine the dimensions required for a commercial flash tank if the flash tank pressure is 5 lb/in2 (gage) (34.5 kPa) and 14,060 lb/h (1.77 kg/s) of flash steam is available. Would the flash tank be of the centrifugal or top-inlet type? Calculation Procedure: Two major types of flash tanks are in use today: top-inlet and centrifugal-inlet tanks, as shown in Fig. 21. Tank and overall height and outside diameter are also shown in Fig. 21. 1. Determine the rating and type of flash tank required Refer to Table 9. Locate the 5-lb/in2 (gage) (34.5-kPa) flash tank pressure column, and project downward to the minimum value that exceeds 14,060 lb/h (1.77 kg/ s). Note that a no. 5 centrifugal flash tank with a maximum rating of 20,000 lb/h (2.5 kg/s) of flash steam is appropriate, and no standard top-inlet type has sufficient capacity at this pressure for this flow rate. 2. Determine the dimensions of the tank In Table 9 locate tank no. 5, and read the dimensions horizontally to the right. Hence, the dimensions required for the tank are 60-in (152.4-cm) OD, 78-in (198.1- cm) tank height, 88-in (223.5-cm) overall height, inlet pipe size of 6 in (15.2 cm), steam outlet pipe of 8 in (20.3 cm), and a water outlet pipe of 6 in (15.2 cm). Related Calculations. Use this procedure for choosing a flash tank for a variety of applications—industrial power plants, central stations, marine steam plants, and nuclear stations. Flash tanks can conserve energy by recovering steam that might otherwise be wasted. This steam can be used for space heating, feedwater heating, industrial processes, etc. Condensate remaining after the flashing can be used as boiler feedwater because it is usually pure and contains valuable heat. Or the con- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 249. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.71 FIGURE 21 Centrifugal and top-inlet flash- tank dimensions. (Chemical Engineering.) densate may be used in an industrial process requiring pure water at an elevated temperature. Flashing steam can cause a violent eruption of the liquid from which the steam is formed. Hence, any flash tank must be large enough to act as a separator to remove entrained moisture from the steam. The dimensions given in Table 9 are for flash tanks of proven design. Hence, the values obtained from Table 9 are satisfactory for all normal design activities. The procedure given here is the work of T. R. MacMillan, as reported in Chemical Engineering. FLASH TANK OUTPUT A boiler operating with a drum pressure of 1400 lb/in2 absolute (9650 kPa) delivers 200,000 lb (90,720 kg) of steam per hour and has a continuous blowdown of 2 percent of its output in order to keep the boiler water at proper dissolved solids. The water blowdown passes to a flash tank operating at slightly above atmospheric pressure in which part of the water flashes to steam, which in turn passes to an open feedwater heater. How much steam is flashed per hour? Calculation Procedure: 1. Determine the amount of blowdown Amount of blowdown B ⫽ 0.02 D, where D is the steam delivery. Hence, B ⫽ 0.02 ⫻ 200,000 ⫽ 4000 lb/h (30 kg/s). 2. Find the enthalpy of the blowdown-saturated liquid Blowdown water leaves the boiler at point d in Fig. 22a as saturated liquid, point d in Fig. 22b. Blowdown at a pressure of pd ⫽ 1400 lb/in2 (9650 kPa) has, from Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 250. 4.72 TABLE 9 Maximum Ratings for Centrifugal and Top-Inlet Flash Tanks, 1000 lb / h (1000 kg / s)* Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 251. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.73 FIGURE 22 (a) Boiler blowdown flow diagram. (b) Temperature-entropy schematic for blow- down. saturated steam tables mentioned under Related Calculations, an enthalpy hd ⫽ 598.7 Btu/lb ⫽ (1392 kJ/kg). 3. Find the enthalpy of the blowdown fluid at the flash tank The blowdown fluid is assumed to undergo an isenthalpic, or constant-enthalpy, throttling process from point d to the point e on Fig. 22b where, at the flash tank, he ⫽ hd, found above. 4. Find the enthalpy of saturated liquid within the flash tank From the saturated steam tables, at pe ⫽ 15 lb/in2 (103 kPa), slightly above at- mospheric pressure, the enthalpy of the saturated liquid at point ƒ on Fig. 22b, hƒ ⫽ 181.1 Btu/lb (421 kJ/kg). 5. Find the enthalpy of evaporation within the flash tank From the saturated steam tables, the heat required to evaporate 1 lb (0.45 kg) of water under the pressure pe within the flash tank is hg ⫺ hƒ ⫽ hƒg ⫽ 969.7 Btu/lb (2254 kJ/kg). 6. Calculate the amount of steam flashed per hour The tank flashes steam at the rate of F ⫽ B[(he ⫺ hƒ)] ⫽ 4000[598.7 ⫺ 181.1)/ 969.7] ⫽ 1723 lb/h (13 kg/s). Related Calculations. Saturation steam tables appear in Thermodynamic Prop- erties of Water Including Vapor, Liquid, and Solid Phases. 1969, John Wiley Sons, Inc. The equation for F in step 6 stems from the presumption of an adiabatic heat balance where F ⫻ hƒg ⫽ B(he ⫺ hƒ). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 252. 4.74 POWER GENERATION DETERMINING WASTE-HEAT BOILER FUEL SAVINGS An industrial plant has 3000 standard ft3 /min (1.42 m3 /day) of waste gas at 1500⬚F (816⬚C) available. How much steam can be generated by this waste gas if the waste- heat boiler has an efficiency of 85 percent, the specific heat of the gas is 0.0178 Btu/(standard ft3 䡠 ⬚F) (1.19 kJ/cm2 ), the exit gas temperature is 400⬚F (204⬚C), and the enthalpy of vaporization of the steam to be generated is 970.3 Btu/lb (2256.9 kJ/kg)? What fuel savings will be obtained if the plant burns no. 6 fuel oil having a heating value of 140,000 Btu/gal (39,200 kJ/L) and a current cost of $1.00 per gallon ($1 per 3.785 L) and a future cost of $1.35 per gallon ($1.35 per 3.785 L)? The waste-heat boiler is expected to operate 24 h/day, 330 days/year. Efficiency of fuel boilers in this plant is 80 percent. Calculation Procedure: 1. Compute the steam production rate from the waste heat Use the relation S ⫽ CvV(T ⫺ t)60E/hv, where S ⫽ steam production rate, lb/h; Cv ⫽ specific heat of gas, Btu/(standard ft3 䡠 ⬚F); V ⫽ volumetric flow rate of waste gas, standard ft3 /min; T ⫽ waste-gas temperature at boiler exit,⬚F; E ⫽ waste-heat boiler efficiency, expressed as a decimal; hv ⫽ heat of vaporization of the steam being generated by the waste gas, Btu/lb. Substituting gives S ⫽ 0.0178(3000)(1500 ⫺ 400)60(0.85)/970.3 ⫽ 3087.7 lb/h (1403.3 kg/h). 2. Find the present and future fuel savings potential The cost equivalent C dollars per hour of the savings produced by using the waste- heat gas can be found from C ⫽ ShvK/Eb, where the symbols are as given earlier and K ⫽ fuel cost, $ per Btu as fired ($ per 1.055 kJ), Eb ⫽ efficiency of fuel-fired boilers in the plant. Substituting for the current fuel cost of $1 per gallon, we find C ⫽ 3087.4(970.3)($1/140,000)/0.8 ⫽ $26.75. Since the waste-heat boiler will operate 24 h/day, the daily savings will be 24($26.75) ⫽ $642. With 330-days/ year operation, the annual savings is (330 days)($642 per day) ⫽ $211,860. This saving could be used to finance the investment in the waste-heat boiler. Where the exit gas temperature from the waste-heat boiler will be different from 400⬚F (204.4⬚C), adjust the steam output and dollar savings by using the difference in the equation in step 1. Related Calculations. This procedure can be used for finding the savings pos- sible from recovering heat from a variety of gas streams such as diesel-engine and gas-turbine exhausts, process-gas streams, refinery equipment exhausts, etc. To ap- ply the procedure, several factors must be known or assumed: waste-heat boiler steam pressure, feedwater temperature, final exit gas temperature, heating value of fuel being saved, and operating efficiency of the waste-heat and fuel-fired boilers in the plant. Note that the exit gas temperature must be higher than the saturation temperature of the steam generated in the waste-heat boiler for heat transmission between the waste gas and the water in the boiler to occur. As a guide, the exit gas temperature should be 100⬚F (51.1⬚C) above the steam temperature in the waste-heat boiler. For economic reasons, the temperature differ- ence should be at least 150⬚F (76.6⬚C). Otherwise, the amount of heat transfer area required in the waste-heat boiler will make the investment uneconomical. This procedure is the work of George V. Vosseller, P. E. Toltz, King, Durvall, Anderson and Associates, Inc., as reported in Chemical Engineering magazine. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 253. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.75 FIGURING FLUE-GAS REYNOLDS NUMBER BY SHORTCUTS A low-sulfur No. 2 distillate fuel oil has a chemical composition of 87.4 percent carbon and 12.6 percent hydrogen by weight, ignoring sulfur. The fuel’s higher heating value (HHV), or ⌬Hgross, the standard (60⬚F) (15.5⬚C) heat of combustion (based on stoichiometric air usage), is 18,993 Btu/lb (44.148 kJ/kg) of the fuel. With a volumetric proportion of 79 percent atmospheric nitrogen, including rare gases, to 21 percent oxygen, the molar ratio of N2 to O2 in air is 3.76:1. What is the Reynolds number for the flow of the flue gas produced by that fuel if it is completely burned in 50 percent excess air at the rate of 25.3 lb/h (11.5 kg/h) and the flue gas leaves a 1-ft (0.3-m) diameter duct at 2000⬚F (1093⬚C). Calculation Procedure: 1. Compute the volume flow rate of the flue gas Based on stoichiometric air usage, the standard (32⬚F, 1-atm) (0⬚C, 101.3-kPa) vol- ume of flue gas (std ft3 /lb) (std m3 /kg) of fuel burned is Vstd ⫽ (⌬Hgross /100)[1 ⫹ (percent excess air)/100 percent]. Then, Vstd ⫽ (18,993/100)[1 ⫹ (50/100)] ⫽ 285 std ft3 /lb (17.79 std m3 /kg). Adjust for temperature expansion by using the ideal-gas law to get the per lb (kg) of fuel actual amount of flue gas, V⬘ ⫽ Vstd(460 ⫹ T)/(460 ⫹ 32), where T is the flue-gas temperature in⬚F. Thus, V⬘ ⫽ 285(460 ⫹ 2000)/(492) ⫽ 1425 ft3 /lb (89 m3 /kg). The metric result can be verified as follows: Vstd ⫽ 17.79[273 ⫹ (2000 ⫺ 32)/1.8]/273 ⫽ 89 m3 /kg. The flue-gas approximate flow rate, V ⫽ V⬘W/3600, where W is the given hourly burning rate of the fuel. Hence, V ⫽ 1425 ⫻ 25.3/3600 ⫽ 10.0 ft3 /s (0.28 m3 /s). 2. Determine the viscosity of the flue gas Boiler and incinerator flue gases are composed of several gases, hence a precise calculation of the Reynolds number can be cumbersome. By assuming that the flue gas behaves like nitrogen, it is possible to obtain fast and accurate preliminary approximations for both boilers and incinerators. Then, by means of a graph in Fig. 23, read the dynamic viscosity of nitrogen as ⬘ ⫽ 0.054 cp (54 ⫻ 10⫺6 Pa). 3. Compute the Reynolds number of the flue gas By algebraic manipulations, as mentioned under Related Calculations of this pro- cedure, the shortcut formula for an estimate of the Reynolds number is found to be Re ⫽ 73,700/[D⬘(460 ⫹ T)], where the given duct diameter, D ⫽ 1.0 ft (0.3 m) and the other values are as previously determined. Thus, Re ⫽ 72,700 ⫻ 10.0[(1)(0.054)(460 ⫹ 2000)] ⫽ 5470. Related Calculations. The shortcut formula for the value of Re is derived by algebraic manipulation of four expressions listed below. Symbols adequately de- fined previously are not redefined. (1) Reynolds number Re ⫽ DU/, where D ⫽ diameter of a circular cross- section, or equivalent diameter of some other cross-section, ft (m); ⫽ density of the gas, lb/ft3 (kg/m3 ); U ⫽ average linear velocity of the gas, ft/s (m/s); ⫽ viscosity of the gas, lb/(ft 䡠 s) (Pa 䡠 s). (2) At high temperatures, the average densities and viscosities of flue gas closely approximate those of nitrogen alone. Thus, and for nitrogen can be used to Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 254. 4.76 POWER GENERATION FIGURE 23 Dynamic viscosity of nitrogen gas. (Chemical Engineering.) TABLE 10 Flue-gas components estimate Re with no significant error. Hence, from the ideal-gas law an estimate of the density of nitrogen, ⫽ (28)(460 ⫹ 32)/[(359)(460 ⫹ T)], where 28 is the molecular weight of nitrogen; 359 is the volume, ft3 , of 1 lb 䡠 mol of gas at 32⬚F (0⬚C) and at atmospheric pressure, 14.7 lb/in2 (6.89 kPa). In SI units the factor is 22.41 m3 /kg 䡠 mol. (3) U ⫽ 4V/D2 (4) ⫽ ⬘/1488 Turbulent flow in a boiler or incinerator assures adequate mixing and near- complete or complete combustion. Flow is turbulent when its Reynolds number is greater than 2000 or 3000, and is more turbulent when Re is greater. This reduces the amount of excess air required for complete combustion and hence increases boiler and incinerator efficiency. A review of the shortcut equations reveals that, for a given set of values for D, M, and T and that ⬘ depends on T, the degree of accuracy of the shortcut value Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 255. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.77 of Re is reflected by the precision of the value of Vstd found in step 1 of this procedure. This can be done by checking the stoichiometry of the combustion process per lb (kg) of the fuel with the given percentages of C and H2, as follows: lb 䡠 mol C/lb oil ⫽ 0.87/12 ⫽ 0.0728; lb 䡠 mol H2 /lb ⫽ 0.126/2 ⫽ 0.0630. Then, 0.0728 C ⫹ 0.0630 H2 ⫹ [1 ⫹ (percent excess air/100 percent] (0.0728 ⫹ 0.0630/s) O2 ⫹ [1 ⫹ (percent excess air/100 percent](3.76)(0.0728 ⫹ 0.0630/s) N2 → 0.0728 CO2 ⫹ 0.0630 H2O ⫹ (percent excess air/100 per- cent)(0.1043) O2 ⫹ [1 ⫹ (percent excess air/100 percent)](3.76)(0.1043) N2. Table 11 indicates that the lb 䡠 mol/lb oil is 0.7763; hence Vstd ⫽ 0.7763 ⫻ 359 ⫽ 279 std ft3 /lb (17.42 std m3 /kg) of oil. This shows the error in the shortcut estimate for Vstd to be about 2 percent in this case. Also, Table 10 shows that this flue gas has a composition of 9.4 percent CO2; 8.1 percent H2O; 6.7 percent O2; 75.8 percent N2; and has an average molecular weight of 28.96. By the method shown on page 3.279 in Perry’s Chemical Engi- neers’ Handbook, 6th edition, McGraw-Hill, this flue gas has a calculated mixture viscosity of ⬘ ⫽ 0.0536 cP (53.6 ⫻ 10⫺6 Pa). Using this value and the average molecular weight of 28.96 instead of 28 in the gas density formula in step 2, the Re estimate would be 5700. This indicates the shortcut estimate of 5470 to be in error by about 4 percent in this case. There are other factors that could contribute to errors in the shortcut calculations. In practice, wood and municipal sold waste contain considerable amounts of moisture, which reduces their heating values that refer to dry conditions, only. The shortcut calculations are very accurate for fossil fuels, such as coal, fuel oil, and natural gas, and wood. They are useful for wastes or waste-fuel mixtures. Errors by shortcut calculations seldom exceed Ⳳ10 percent when excess air is less than 150 to 200 percent. Though errors for fossil fuels with 100 percent or less excess air are generally 5 percent or less, there are factors that increase the error of the shortcut method: (1) High water content in the fuel or waste; (2) high halogen content; (3) excess air above 100 percent. However, the shortcut method can still be used to give a quick approximation even when these factors are present. This shortcut method is based on two articles written by Irwin Frankel of The Mitrer Corp., Metrek Div., 1820 Dolley Madison Blvd., McLean, VA 22102. The articles, ‘‘Shortcut calculations for fluegas volume’’ and ‘‘Figure fluegas Reynolds number,’’ appeared in the Chemical Engineering magazine issues of June 1, 1981, and August 24, 1981, respectively. DETERMINING THE FEASIBILITY OF FLUE-GAS RECIRCULATION FOR NOx CONTROL IN PACKAGED BOILERS Determine if it is feasible to recirculate flue gas in packaged boilers* to reduce NOx emissions to comply with EPA and local environmental requirements. Find the ranges of applicable parameters for packaged boilers in their typical applications. *Defined as shop-assembled steam generators usually designed for oil and/or natural-gas firing in either watertube or firetube types. Packaged watertube boilers have capacities ranging up to 600,000 lb/h (75.6 kg/s) at pressures from 125 to 2000 lb/in2 (gage) (860 to 13,800 kPa) with temperatures from 353 to 950⬚F (78 to 510⬚C). Higher capacities, pressures, and temperatures are possible. Firetube packaged boiler capac- ities can range up to some 50,000 lb/h (6.3 kg/s) at pressures up to 250 lb/in2 (gage) (1720 kPa) with possible higher capacities, pressures, and temperatures. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 256. 4.78 POWER GENERATION FIGURE 24 Compact system design adds little to space requirements for packaged boilers using FGR to control NOx emissions. (Power.) Calculate the reduction in NOx emissions for a 350-hp (261.1-kW) packaged boiler operating at rated capacity when flue-gas recirculation is increased from zero to 10 percent of the total flow. Calculation Procedure: 1. Determine the suitability of flue-gas recirculation for packaged boilers Flue-gas recirculation (FGR) for NOx-emission control has been successfully ap- plied on utility oil/gas-fired boilers and on industrial solid-fuel-fired units. Some engineers think that FGR is also appropriate for waste-to-energy facilities. On oil/gas-fired steam generators, FGR acts as a flame quencher, reducing com- bustion temperatures by thermal dilution. In doing so, it significantly reduces excess-air requirements and flue-gas heat loss and provides a method of combustion staging. For stoker-fired units, FGR helps improve mixing of fuel and air in the fuel-bed area. Thus, it can help to reduce NOx emissions and improve boiler effi- ciency. Lowering excess-air requirements minimizes the formation of thermal NOx. Note that the technique does not affect formation of NOx from fuel-bound nitrogen. Recent developments in FGR have increased its range of suitable applications to include packaged firetube and watertube boilers of virtually any size. In these applications FGR functions as it does in a utility oil/gas-fired boiler—as a flame- quenching strategy, Fig. 24. The higher the recirculation rate, the greater the re- duction of NOx. Typically, a system that recirculates 10 percent of the flue gas can reduce NOx by about 45 percent. At 20 percent recirculation, NOx is reduced by up to 70 percent. 2. Evaluate the relationship between FGR rate and NOx emissions Plots of the relationship between FGR rate and NOx emissions at various boiler capacities are available from packaged-boiler manufacturers. Figure 25 shows such plots for two typical packaged boilers of 350-hp (261.1-kW) and 200-hp (149.2- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 257. STEAM GENERATION EQUIPMENT AND AUXILIARIES 4.79 FIGURE 25 Choice of FGR rate is important for reducing NOx emis- sions. Smaller units (b) require less recirculation to achieve desired NOx reduction than larger units (a). (Power.) kW). These two plots show that, in general, the NOx emission decreases as the FGR rate, as a percentage of the total flue-gas flow, is increased. Certain limitations on the FGR rate apply, as discussed in this calculation procedure. 3. Compute the reduction in NOx produced by FGR Using Fig. 25 for the 350-hp (261.1-kW) packaged boiler at rated capacity, we see that the NOx is reduced from 89 ppm to 60 ppm when the FGR rate is increased from 0 to 10 percent of the total flue-gas flow. This is a reduction of ([89 ⫺ 60]/ 80)(100) ⫽ 32.6 percent. A reduction of this magnitude in NOx emission is signif- icant. 4. Evaluate recirculation rates and burners to use Recirculation rates can vary, depending on a particular unit’s NOx production and size. On natural gas, FGR is usually limited to 20 percent; and on oil-fired units to 10 to 12 percent at high fire. These limits are set to eliminate two common Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 258. 4.80 POWER GENERATION problems with FGR: (1) Too much cooling which can quench the flame; (2) higher velocities which can push the flame away from the burner. Natural gas is far more responsive to FGR because it contains negligible amounts of fuel-bound nitrogen. Proper introduction and recirculation of the flue gas is necessary to reduce NOx emissions to the desired level. If the gas steam is brought into the suction side of an existing forced-draft fan, the amount of recirculated gas will be limited by the capacity of a single fan. Other factors to consider are condensation, dirt, soot col- lection, corrosion, and a highly variable supply of combustion air, which can reduce the capacity of the unit. Related Calculations. FGR is applicable to almost any type of packaged boiler used in industrial, commercial, residential, portable, marine, hotel, or other service. With ever-increasing emphasis on NOx environmental concerns, FGR is winning more converts. On average, uncontrolled gas-fired boilers emit 80 to 100 ppm NOx, while av- erage uncontrolled oil-fired boilers emit 150 to 300 ppm. FGR offers a potentially inexpensive alternative to add-on controls, such as selective catalytic and nonca- talytic reduction, and more elaborate combustion modifications, including water/ steam injection into the furnace, to control NOx. Further, research into FGR reveals an additional benefit—a reduction in CO formation. This occurs as a result of the added turbulence and mixing around the flame. Data and illustrations in this procedure are the work of Gene Tompkins, Aqua- Chem Inc., Cleaver-Brooks Div., as reported in Power magazine, and edited by Elizabeth A. Bretz. SI values were added by the handbook editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. STEAM GENERATION EQUIPMENT AND AUXILIARIES
- 259. 5.1 SECTION 5 FEEDWATER HEATING METHODS Steam-Plant Feedwater-Heating cycle Analysis 5.1 Direct-Contact Feedwater Heater Analysis 5.2 Closed Feedwater Heater Analysis and Selection 5.3 Power-Plant Heater Extraction-Cycle Analysis 5.8 Feedwater Heating with Diesel-Engine Repowering of a Steam Plant 5.13 STEAM-PLANT FEEDWATER-HEATING CYCLE ANALYSIS The high-pressure cylinder of a turbogenerator unit receives 1,000,000 lb per h (454,000 kg/h) of steam at initial conditions of 1800 psia (12,402 kPa) and 1050⬚F (565.6⬚C). At exit from the cylinder the steam has a pressure of 500 psia (3445 kPa) and a temperature of 740⬚F (393.3⬚C). A portion of this 500-psia (3445-kPa) steam is used in a closed feedwater heater to increase the temperature of 1,000,000 lb per h (454,000 kg/h) of 2000-psia (13,780-kPa) feedwater from 350⬚F (176.6⬚C) to 430⬚F (221.1⬚C); the remainder passes through a reheater in the steam generator and is admitted to the intermediate-pressure cylinder of the turbine at a pressure of 450 psia (3101 kPa) and a temperature of 1000⬚F (537.8⬚C). The intermediate cyl- inder operates nonextraction. Steam leaves this cylinder at 200 psia (1378 kPa) and 500⬚F (260⬚C). Find (a) flow rate to the feedwater heater, assuming no subcooling; (b) work done, in kW, by the high-pressure cylinder; (c) work done, in kW, by the intermediate-pressure cylinder; (d) heat added by the reheater. Calculation Procedure: 1. Find the flow rate to the feedwater heater (a) Construct the flow diagram, Fig. 1. Enter the pressure, temperature, and enthalpy values using the data given and the steam tables. Write an equation for flow across the feedwater heater, or (H2 ⫺ H7) ⫽ water (H6 ⫺ H5). Substituting using the enthalpy data from the flow diagram, flow to heater ⫽ (1 ⫻ 106 )(409 ⫺ 324.4)/ (1379.3 ⫺ 449.4) ⫽ 90.977.5 lb/h (41,303.8 kg/h). 2. Determine the work done by the high-pressure cylinder (b) The work done ⫽ (steam flow rate, lb/h)(H1 ⫺ H2)/3413 ⫽ (1 ⫻ 106 )(1511.3 ⫺ 1379.3)/3414 ⫽ 38,675.7 kW. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 260. 5.2 POWER GENERATION 1,000,000 lb per hr 1,800 psia 1050°F H1 = 1,511.3 1,000,000 lb per hr 2,000 psia 430°F H6 = 409 Reheater 450 psia 1000°F H3 = 1,521 Intermediate- pressure cylinder High-pressure cylinder 500 psia 740°F H2 = 1,379.3 908,900 lb per hr 200 psia 500°F H4 = 1,269 908,900 lb per hr 91,100 lb per hr 1,000,000 lb per hr 2,000 psia 350°F H5 = 324.4 H7 = 449.4 Heater 1,000,000 lb/hr (454,000 kg/hr) 1800 psia (12,402 kPa) 1050°F (565°C) 500 psia (3445 kPa) 740°F (393°C) 1379.3 Btu/lb (3214 kJ/kg) 1511.3 Btu/lb (3521 k? 2000 psia (13,780 kPa) 430°F (221°C) 409 (953 kJ/kg) 350°F (177°C) 324.4 (756 kJ/kg) 450 psia (3101 kPa) 1000°F (538°C) 1521 Btu/lb (3544 kJ/kg) 500°F (260°C) 200 psia (1378 kPa) 1269 Btu/lb (2933 kJ/kg) 324.5 Btu/lb (756 kJ/kg) 908,900 lb/hr (412,641 kg/hr) 91,100 lb/hr (41,359 kg/hr) 324.4 Btu/lb (756 kJ/kg) 449.4 Btu/lb (1047 kJ/kg) FIGURE 1 Feedwater heating flow diagram. 3. Find the work done by the intermediate-pressure cylinder (c) The work done ⫽ (steam flow through the cylinder)(H3 ⫺ H4)/3413 ⫽ (1 ⫻ 106 – 90.977.5 ⫻ 106 )(1521 ⫺ 1269)/3413 ⫽ 67,118 kW. 4. Compute the heat added by the reheater (d) Heat added by the reheater ⫽ (steam flow through the reheater)(H3 ⫺ H2) ⫽ (1 ⫻ 106 ⫺ 90,977.5)(1521 ⫺ 1379.3) ⫽ 128.8 ⫻ 106 Btu/h (135.9 kJ/h). Related Calculations. Use this general procedure to determine the flow through feedwater heaters and reheaters for utility, industrial, marine, and com- mercial steam power plants of all sizes. The method given can also be used for combined-cycle plants using both a steam turbine and a gas turbine along with a heat-recovery steam generator (HRSG) in combination with one or more feedwater heaters and reheaters. DIRECT-CONTACT FEEDWATER HEATER ANALYSIS Determine the outlet temperature of water leaving a direct-contact open-type feed- water heater if 250,000 lb/h (31.5 kg/s) of water enters the heater at 100⬚F Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 261. FEEDWATER HEATING METHODS 5.3 (37.8⬚C). Exhaust steam at 10.3 lb/in2 (gage) (71.0 kPa) saturated flows to the heater at the rate of 25,000 lb/h (31.5 kg/s). What saving is obtained by using this heater if the boiler pressure is 250 lb/in2 (abs) (1723.8 kPa)? Calculation Procedure: 1. Compute the water outlet temperature Assume the heater is 90 percent efficient. Then to ⫽ tiww ⫹ 0.9wshg /(ww ⫹ 0.9ws), where to ⫽ outlet water temperature, ⬚F; ti ⫽ inlet water temperature, ⬚F; ww ⫽ weight of water flowing through heater, lb/h; 0.9 ⫽ heater efficiency, expressed as a decimal; ws ⫽ weight of steam flowing to the heater, lb/h; hg ⫽ enthalpy of the steam flowing to the heater, Btu/lb. For saturated steam at 10.3 lb/in2 (gage) (71.0 kPa), or 10.3 ⫹ 14.7 ⫽ 25 lb/ in2 (abs) (172.4 kPa), hg ⫽ 1160.6 Btu/lb (2599.6 kJ/kg), from the saturation pressure steam tables. Then 100(250,000) ⫹ 0.9(25,000)(1160.6) t ⫽ ⫽ 187.5⬚F (86.4⬚C) o 250,000 ⫹ 0.9(25,000) 2. Compute the savings obtained by feed heating The percentage of saving, expressed as a decimal, obtained by heating feedwater is (ho ⫺ hi)/( hb ⫺ hi) where ho and hi ⫽ enthalpy of the water leaving and entering the heater, respectively, Btu/lb; hb ⫽ enthalpy of the steam at the boiler operating pressure, Btu/lb. For this plant from the steam tables ho ⫺ hi /(hb ⫺ hi) ⫽ 155.44 ⫺ 67.97/(1201.1 ⫺ 67.97) ⫽ 0.077, or 7.7 percent. A popular rule of thumb states that for every 11⬚F (6.1⬚C) rise in feedwater temperature in a heater, there is approximately a 1 percent saving in the fuel that would otherwise be used to heat the feedwater. Checking the above calculation with this rule of thumb shows reasonably good agreement. 3. Determine the heater volume With a capacity of W lb/h of water, the volume of a direct-contact or open-type heater can be approximated from v ⫽ W/10,000, where v ⫽ heater internal volume, ft3 . For this heater v ⫽ 250,000/10,000 ⫽ 25 ft3 (0.71 m3 ). Related Calculations. Most direct-contact or open feedwater heaters store in 2-min supply of feedwater when the boiler load is constant, and the feedwater supply is all makeup. With little or no makeup, the heater volume is chosen so that there is enough capacity to store 5 to 30 min feedwater for the boiler. CLOSED FEEDWATER HEATER ANALYSIS AND SELECTION Analyze and select a closed feedwater heater for the third stage of a regenerative steam-turbine cycle in which the feedwater flow rate is 37,640 lb/h (4.7 kg/s), the desired temperature rise of the water during flow through the heater is 80⬚F (44.4⬚C) (from 238 to 318⬚F or, 114.4 to 158.9⬚C), bleed heating steam is at 100 lb/in2 (abs) (689.5 kPa) and 460⬚F (237.8⬚C), drains leave the heater at the saturation temper- ature corresponding to the heating steam pressure [110 lb/in2 (abs) or 689.5 kPa], and 5 ⁄8-in (1.6-cm) OD admiralty metal tubes with a maximum length of 6 ft (1.8 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 262. 5.4 POWER GENERATION m) are used. Use the Standards of the Bleeder Heater Manufacturers Association, Inc., when analyzing the heater. Calculation Procedure: 1. Determine the LMTD across heater When heat-transfer rates in feedwater heaters are computed, the average film tem- perature of the feedwater is used. In computing this the Standards of the Bleeder Heater Manufacturers Association specify that the saturation temperature of the heating steam be used. At 100 lb/in2 (abs) (689.5 kPa), ts ⫽ 327.81⬚F (164.3⬚C). Then (t ⫺ t ) ⫺ (t ⫺ t ) s i s o LMTD ⫽ t ⫽ m ln [t ⫺ t /(t ⫺ t )] s i s o where the symbols are as defined in the previous calculation procedure. Thus, (327.81 ⫺ 238) ⫺ (327.81 ⫺ 318) t ⫽ m ln [327.81 ⫺ 238/(327.81 ⫺ 318)] ⫽ 36.5⬚F (20.3⬚C) The average film temperature tf for any closed heater is then t ⫽ t ⫺ 0.8t f s m ⫽ 327.81 ⫺ 29.2 ⫽ 298.6⬚F (148.1⬚C) 2. Determine the overall heat-transfer rate Assume a feedwater velocity of 8 ft/s (2.4 m/s) for this heater. This velocity value is typical for smaller heaters handling less than 100,000-lb/h (12.6-kg/s) feedwater flow. Enter Fig. 2 at 8 ft/s (2.4 m/s) on the lower horizontal scale, and project vertically upward to the 250⬚F (121.1⬚C) average film temperature curve. This curve is used even though tf ⫽ 298.6⬚F (148.1⬚C), because the standards recommend that heat-transfer rates higher than those for a 250⬚F (121.1⬚C) film temperature not be used. So, from the 8-ft/s (2.4 m/s) intersection with the 250⬚F (121.1⬚C) curve in Fig. 2, project to the left to read U ⫽ the overall heat-transfer rate ⫽ 910 Btu/ (ft2 䡠 ⬚F 䡠 h) [5.2 k]/m2 䡠 ⬚C 䡠 s)]. Next, check Table 1 for the correction factor for U. Assume that no. 18 BWG 5 ⁄8-in (1.6-cm) OD arsenical copper tubes are used in this exchanger. Then the correction factor from Table 1 is 1.00, and Ucorr ⫽ 910(1.00) ⫽ 910. If no. 9 BWG tubes are chosen, Ucorr ⫽ 910(0.85) ⫽ 773.5 Btu/(ft2 䡠 ⬚F 䡠 h) [4.4 kJ/(m2 䡠 ⬚C 䡠 s)], given the correction factor from Table 1 for arsenical copper tubes. 3. Compute the amount of heat transferred by the heater The enthalpy of the entering feedwater at 238⬚F (114.4⬚C) is, from the saturation- temperature steam table, hfi ⫽ 206.32 Btu/lb (479.9 kJ/kg). The enthalpy of the leaving feedwater at 318⬚F (158.9⬚C) is, from the same table, hfo ⫽ 288.20 Btu/lb (670.4 kJ/kg). Then the heater transferred Ht Btu/h is Ht ⫽ ww(hfo ⫺ hfi), where ww ⫽ feedwater flow rate, lb/h. Or, Ht ⫽ 37,640(288.20 ⫺ 206.32) ⫽ 3,080,000 Btu/h (902.7 kW). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 263. FEEDWATER HEATING METHODS 5.5 FIGURE 2 Heat-transfer rates for closed feedwater heaters. (Standards of Bleeder Heater Manufacturers Association, Inc.) TABLE 1 Multipliers for Base Heat-Transfer Rates [For tube OD 5 ⁄8 to 1 in (1.6 to 2.5 cm) inclusive] Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 264. 5.6 POWER GENERATION 4. Compute the surface area required in the exchanger The surface area required A ft2 ⫽ Ht /Utm. Then A ⫽ 3,080,000/[910)(36.5)] ⫽ 92.7 ft2 (8.6 m2 ). 5. Determine the number of tubes per pass Assume the heater has only one pass, and compute the number of tubes required. Once the number of tubes is known, a decision can be made about the number of passes required. In a closed heater, number of tubes ⫽ ww (passes) (ft3 /s per tube)/[v(ft2 per tube open area)], where ww ⫽ lb/h of feedwater passing through heater; v ⫽ feedwater velocity in tubes, ft/s. Since the feedwater enters the heater at 238⬚F (114.4⬚C) and leaves at 318⬚F (158.9⬚C), its specific volume at 278⬚F (136.7⬚C), midway between ti and to, can be considered the average specific volume of the feedwater in the heater. From the saturation-pressure steam table, vf ⫽ 0.01691 ft3 /lb (0.0011 m3 /kg) at 278⬚F (136.7⬚C). Convert this to cubic feet per second per tube by dividing this specific volume by 3600 (number of seconds in 1 h) and multiplying by the pounds per hour of feedwater per tube. Or, ft3 /s per tube ⫽ (0.01691/3600)(lb/h per tube). Since no. 18 BWG 5 ⁄8-in (1.6-cm) OD tubes are being used, ID ⫽ 0.625 ⫺ 2(thickness) ⫽ 0.625 ⫺ 2(0.049) ⫽ 0.527 in (1.3 cm). Then, open area per tube ft2 ⫽ (d2 /4)/144 ⫽ 0.7854(0.527)2 /144 ⫽ 0.001525 ft2 (0.00014 m2 ) per tube. Alternatively, this area could be obtained from a table of tube properties. With these data, compute the total number of tubes from number of tubes ⫽ [(37,640)(1)(0.01681/3600)]/[(8)(0.001525)] ⫽ 14.29 tubes. 6. Compute the required tube length Assume that 14 tubes are used, since the number required is less than 14.5. Then, tube length l, ft ⫽ A/(number of tubes per pass)(passes)(area per ft of tube). Or, tube length for 1 pass ⫽ 92.7/[(14)(1)(0.1636)] ⫽ 40.6 ft (12.4 m). The area per ft of tube length is obtained from a table of tube properties or computed from 12(OD)/144 ⫽ 12(0.625)/155 ⫽ 0.1636 ft2 (0.015 m2 ). 7. Compute the actual number of passes and the actual tube length Since the tubes in this heater cannot exceed 6 ft (1.8 m) in length, the number of passes required ⫽ (length for one pass, ft)/(maximum allowable tube length, ft) ⫽ 40.6/6 ⫽ 6.77 passes. Since a fractional number of passes cannot be used and an even number of passes permit a more convenient layout of the heater, choose eight passes. From the same equation for tube length as in step 6, l ⫽ tube length ⫽ 92.7/ [(14)(8)(0.1636)] ⫽ 5.06 ft (1.5 m). 8. Determine the feedwater pressure drop through heater In any closed feedwater heater, the pressure loss ⌬p lb/in2 is ⌬p ⫽ F1F2(L ⫹ 5.5D)N/D1.24 , where ⌬p ⫽ pressure drop in the feedwater passing through the heater, lb/in2 ; F1 and F2 ⫽ correction factors from Fig. 3; L ⫽ total lin ft of tubing divided by the number of tube holes in one tube sheet; D ⫽ tube ID; N ⫽ number of passes. In finding F2, the average water temperature is taken as ts ⫺ tm. For this heater, using correction factors from Fig. 3, 5.06(8)(14) 8 ⌬p ⫽ (0.136)(0.761) ⫹ 5.5(0.527) 冋 册 1.24 (8)(14) 0.527 2 ⫽ 14.6 lb/in (100.7 kPa) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 265. FEEDWATER HEATING METHODS 5.7 FIGURE 3 Correction factors for closed feedwater heaters. (Standards of Bleeder Heater Manufacturers Association, Inc.) 9. Find the heater shell outside diameter The total number of tubes in the heater ⫽ (number of passes)(tubes per pass) ⫽ 8(14) ⫽ 112 tubes. Assume that there is 3 ⁄8-in (1.0-cm) clearance between each tube and the tube alongside, above, or below it. Then the pitch or center-to-center distance between the tubes ⫽ pitch ⫹ tube OD ⫽ 3 ⁄8 ⫹ 5 ⁄8 ⫽ 1 in (2.5 cm). The number of tubes per ft2 of tube sheet ⫽ 166/(pitch)2 , or 166/12 ⫽ 166 tubes per ft2 (1786.8 per m2 ). Since the heater has 112 tubes, the area of the tube sheet ⫽ 112/166 ⫽ 0.675 ft2 , or 97 in2 (625.8 cm2 ). The inside diameter of the heater shell ⫽ (tube sheet area, in2 /0.7854)0.5 ⫽ (97/0.7854)0.5 ⫽ 11.1 in (28.2 cm). With a 0.25-in (0.6-cm) thick shell, the heater shell OD ⫽ 11.1 ⫹ 2(0.25) ⫽ 11.6 in (29.5 cm). 10. Compute the quantity of heating steam required Steam enters the heater at 100 lb/in2 (abs) (689.5 kPa) and 460⬚F (237.8⬚C). The enthalpy at this pressure and temperature is, from the superheated steam table, hg ⫽ 1258.8 Btu/lb (2928.0 kJ/kg). The steam condenses in the heater, leaving as condensate at the saturation temperature corresponding to 100 lb/in2 (abs) (689.5 kPa), or 327.81⬚F (164.3⬚C). The enthalpy of the saturated liquid at this temperature is, from the steam tables, hf ⫽ 298.4 Btu/lb (694.1 kJ/kg). The heater steam consumption for any closed-type feedwater heater is W, lb/ h ⫽ ww(⌬t)(hg ⫺ hf), where ⌬t ⫽ temperature rise of feedwater in heater, ⬚F, c ⫽ specific heat of feedwater, Btu/(lb 䡠 ⬚F). Assume c ⫽ 1.00 for the temperature range in this heater, and W ⫽ (37,640)(318 ⫺ 238)(1.00)/(1258.8 ⫺ 298.4) ⫽ 3140 lb/ h (0.40 kg/s). Related Calculations. The procedure used here can be applied to closed feed- water heaters in stationary and marine service. A similar procedure is used for selecting hot-water heaters for buildings, marine, and portable service. Various au- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 266. 5.8 POWER GENERATION thorities recommend the following terminal difference (heater condensate temper- ature minus the outlet feedwater temperature) for closed feedwater heaters: POWER-PLANT HEATER EXTRACTION-CYCLE ANALYSIS A steam power plant operates at a boiler-drum pressure of 460 lb/in2 (abs) (3171.7 kPa), a turbine throttle pressure of 415 lb/in2 (abs) (2861.4 kPa) and 725⬚F (385.0⬚C), and a turbine capacity of 10,000 kW (or 13,410 hp). The Rankine-cycle efficiency ratio (including generator losses) is: full load, 75.3 percent; three-quarters load, 74.75 percent; half load, 71.75 percent. The turbine exhaust pressure is 1 inHg absolute (3.4 kPa); steam flow to the steam-jet air ejector is 1000 lb/h (0.13 kg/s). Analyze this cycle to determine the possible gains from two stages of ex- traction for feedwater heating, with the first stage a closed heater and the second stage a direct-contact or mixing heater. Use engineering-office methods in analyzing the cycle. Calculation Procedure: 1. Sketch the power-plant cycle Figure 4a shows the plant with one closed heater and one direct-contact heater. Values marked on Fig. 4a will be computed as part of this calculation procedure. Enter each value on the diagram as soon as it is computed. 2. Compute the throttle flow without feedwater heating extraction Use the superheated steam tables to find the throttle enthalpy hf ⫽ 1375.5 Btu/lb (3199.4 kJ/kg) at 415 lb/in2 (abs) (2861.4 kPa) and 725⬚F (385.0⬚C). Assume an irreversible adiabatic expansion between throttle conditions and the exhaust pressure of 1 inHg (3.4 kPa). Compute the final enthalpy H2s by the same method used in earlier calculation procedures by finding y2s, the percentage of moisture at the exhaust conditions with 1-inHg absolute (3.4-kPa) exhaust pressure. Do this by setting up the ratio y2s ⫽ (sy ⫺ S1)/sfg, where sg and sfg are entropies at the exhaust pressure; S1 is entropy at throttle conditions. From the steam tables, y2s ⫽ 2.0387 ⫺ 1.6468/1.9473 ⫽ 0.201. Then H2s ⫽ hg ⫺ y2shfg, where hg and hfg are enthalpies at 1 inHg absolute (3.4 kPa). Substitute values from the steam table for 1 inHg absolute (3.4 kPa); or, H2s ⫽ 1096.3 ⫺ 0.201(1049.2) ⫽ 885.3 Btu/lb (2059.2 kJ/kg). The available energy in this irreversible adiabatic expansion is the difference between the throttle and exhaust conditions, or 1375.5 ⫺ 885.3 ⫽ 490.2 Btu/lb Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 267. FEEDWATER HEATING METHODS 5.9 FIGURE 4 (a) Two stages of feedwater heating in a steam plant; (b) Mollier chart of the cycle in (a). (1140.2 kJ/kg). The work at full load on the turbine is: (Rankine-cycle effi- ciency)(adiabatic available energy) ⫽ (0.753)(490.2) ⫽ 369.1 Btu/lb (858.5 kJ/ kg). Enthalpy at the exhaust of the actual turbine ⫽ throttle enthalpy minus full- load actual work, or 1375.5 ⫺ 369.1 ⫽ 1006.4 Btu/lb (2340.9 kJ/kg). Use the Mollier chart to find, at 1.0 inHg absolute (3.4 kPa) and 1006.4 Btu/lb (2340.9 kJ/kg), that the exhaust steam contains 9.5 percent moisture. Now the turbine steam rate SR ⫽ 3413(actual work output, Btu). Or, SR ⫽ 3413/369.1 ⫽ 9.25 lb/kWh (4.2 kg/kWh). With the steam rate known, the nonex- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 268. 5.10 POWER GENERATION traction throttle flow is (SR)(kW output) ⫽ 9.25(10,000) ⫽ 92,500 lb/h (11.7 kg/ s). 3. Determine the heater extraction pressures With steam extraction from the turbine for feedwater heating, the steam flow to the main condenser will be reduced, even with added throttle flow to compensate for extraction. Assume that the final feedwater temperature will be 212⬚F (100.0⬚C) and that the heating range for each heater is equal. Both assumptions represent typical prac- tice for a moderate-pressure cycle of the type being considered. Feedwater leaving the condenser hotwell at 1 inHg absolute (3.4 kPa) is at 79.03⬚F (26.1⬚C). This feedwater is pumped through the air-ejector intercondensers and aftercondensers, where the condensate temperature will usually rise 5 to 15⬚F (2.8 to 8.3⬚C), depending on the turbine load. Assume that there is a 10⬚F (5.6⬚C) rise in condensate temperature from 79 to 89⬚F (26.1 to 31.7⬚C). Then the temper- ature range for the two heaters is 212 ⫺ 89 ⫽ 123⬚F (68.3⬚C). The temperature rise per heater is 123/2 ⫽ 61.5⬚F (34.2⬚C), since there are two heaters and each will have the same temperature rise. Since water enters the first-stage closed heater at 89⬚F (31.7⬚C), the exit temperature from this heater is 89 ⫹ 61.5 ⫽ 150.5⬚F (65.8⬚C). The second-stage heater is a direct-contact unit operating at 14.7 lb/in2 (abs) (101.4 kPa), because this is the saturation pressure at an outlet temperature of 212⬚F (100.0⬚C). Assume a 10 percent pressure drop between the turbine and heater steam inlet. This is a typical pressure loss for an extraction heater. Extraction pressure for the second-stage heater is then 1.1(14.7) ⫽ 16.2 lb/in2 (abs) (111.7 kPa). Assume a 5⬚F (2.8⬚C) terminal difference for the first-stage heater. This is a typical terminal difference, as explained in an earlier calculation procedure. The saturated steam temperature in the heater equals the condensate temperature ⫽ 150.5⬚F (65.8⬚C) exit temperature ⫹ 5⬚F (2.8⬚C) terminal difference ⫽ 155.5⬚F (68.6⬚C). From the saturation-temperature steam table, the pressure at 155.5⬚F (68.6⬚C) is 4.3 lb/in2 (abs) (29.6 kPa). With a 10 percent pressure loss, the extrac- tion pressure ⫽ 1.1(4.3) ⫽ 4.73 lb/in2 (abs) (32.6 kPa). 4. Determine the extraction enthalpies To establish the enthalpy of the extracted steam at each stage, the actual turbine- expansion line must be plotted. Two points—the throttle inlet conditions and the exhaust conditions—are known. Plot these on a Mollier chart, Fig. 4. Connect these two points by a dashed straight line, Fig. 4. Next, measure along the saturation curve 1 in (2.5 cm) from the intersection point A back toward the enthalpy coordinate, and locate point B. Now draw a gradually sloping line from the throttle conditions to point B; from B increase the slope to the exhaust conditions. The enthalpy of the steam at each extraction point is read where the lines of constant pressure cross the expansion line. Thus, for the second-stage direct-contact heater where p ⫽ 16.2 lb/in2 (abs) (111.7 kPa), hg ⫽ 1136 Btu/lb (2642.3 kJ/kg). For the first-stage closed heater where p ⫽ 4.7 lb/in2 (abs) (32.4 kPa), hg ⫽ 1082 Btu/lb (2516.7 kJ/kg). When the actual expansion curve is plotted, a steeper slope is used between the throttle super-heat conditions and the saturation curve of the Mollier chart, because the turbine stages using superheated steam (stages above the saturation curve) are more efficient than stages using wet steam (stages below the saturation curve). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 269. FEEDWATER HEATING METHODS 5.11 5. Compute the extraction steam flow To determine the extraction flow rates, two assumptions must be made—condenser steam flow rate and first-stage closed-heater extraction flow rate. The complete cycle will be analyzed, and the assumption checked. If the assumptions are incorrect, new values will be assumed, and the cycle analyzed again. Assume that the condenser steam flow from the turbine is 84,000 lb/h (10.6 kg/s) when it is operating with extraction. Note that this value is less than the nonextraction flow of 92,500 lb/h (11.7 kg/s). The reason is that extraction of steam will reduce flow to the condenser because the steam is bled from the turbine after passage through the throttle but before the condenser inlet. Then, for the first-stage closed heater, condensate flow is as follows: The value of 5900 lb/h (0.74 kg/s) of condensate from the first-stage heater is the second assumption made. Since it will be checked later, an error in the assumption can be detected. Assume a 2 percent heat radiation loss between the turbine and heater. This is a typical loss. Then Compare the required extraction, 5950 lb/h (0.75 kg/s), with the assumed ex- traction, 5900 lb/h (0.74 kg/s). The difference is only 50 lb/h (0.006 kg/s), which is less than 1 percent. Therefore, the assumed flow rate is satisfactory, because estimates within 1 percent are considered sufficiently accurate for all routine anal- yses. For the second-stage direct-contact heater, condensate flow, lb/h is as follows: The required extraction, calculated in the same way as for the first-stage heater, is (90,900)(61.7/932.2) ⫽ 6050 lb/h (0.8 kg/s). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 270. 5.12 POWER GENERATION FIGURE 5 Diagram of turbine-expansion line. The computed extraction flow for the second-stage heater is not compared with an assumed value because an assumption was not necessary. 6. Compare the actual condenser steam flow Sketch a vertical line diagram, Fig. 5, showing the enthalpies at the throttle, heaters, and exhaust. From this diagram, the work lost by the extracted steam can be com- puted. As Fig. 5 shows, the total enthalpy drop from the throttle to the exhaust is 369 Btu/lb (389.3 kJ/kg). Each pound of extracted steam from the first- and sec- ond-stage bleed points causes a work loss of 75.7 Btu/lb (176.1 kJ/kg) and 129.7 Btu/lb (301.7 kJ/kg), respectively. To carry the same load, 10,000 kW, with ex- tractions, it will be necessary to supply the following additional compensation steam to the turbine throttle: (heater flow, lb/h)(work loss, Btu/h)/(total work, Btu/h). Then Check the assumed condenser flow using nonextraction throttle flow ⫹ addi- tional throttle flow ⫺ heater extraction ⫽ condenser flow. Set up a tabulation of the flows as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 271. FEEDWATER HEATING METHODS 5.13 Compare this actual flow, 83,840 lb/h (10.6 kg/s), with the assumed flow, 84,000 lb/h (10.6 kg/s). The difference, 160 lb/h (0.02 kg/s), is less than 1 percent. Since an accuracy within 1 percent is sufficient for all normal power-plant calcu- lations, it is not necessary to recompute the cycle. Had the difference been greater than 1 percent, a new condenser flow would be assumed and the cycle recomputed. Follow this procedure until a difference of less than 1 percent is obtained. 7. Determine the economy of the extraction cycle For a nonextraction cycle operating in the same pressure range, Heat chargeable to turbine ⫽ (throttle flow ⫹ air-ejector flow)(heat supplied by boiler)/(kW output of turbine) ⫽ (92,500 ⫹ 1000)(1328.3)/10,000 ⫽ 12,410 Btu/kWh (13,093.2 kJ/kWh), which is the actual heat rate HR of the nonextraction cycle. For the extraction cycle using two heaters, As before, heat chargeable to turbine ⫽ (95,840 ⫹ 1000)(1195.3)/10,000 ⫽ 11,580 Btu/kWh (12,217.5 kJ/kWh). Therefore, the improvement ⫽ (nonextraction HR ⫺ extraction HR)/nonextraction HR ⫽ (12,410 ⫺ 11,580)/12,410 ⫽ 0.0662, or 6.62 percent. Related Calculations. (1) To determine the percent improvement in a steam cycle resulting from additional feedwater heaters in the cycle, use the same pro- cedure as given above for three, four, five, six, or more heaters. Plot the percent improvement vs. number of stages of extraction, Fig. 6, to observe the effect of additional heaters. A plot of this type shows the decreasing gains made by addi- tional heaters. Eventually the gains become so small that the added expenditure for an additional heater cannot be justified. (2) Many simple marine steam plants use only two stages of feedwater heating. To analyze such a cycle, use the procedure given, substituting the hp output for the kW output of the turbine. (3) Where a marine plant has more than two stages of feedwater heating, follow the procedure given in (1) above. FEEDWATER HEATING WITH DIESEL-ENGINE REPOWERING OF A STEAM PLANT Show the economies and environmental advantages possible with Diesel-engine repowering of steam boiler/turbine plants using feedwater heating as the entree. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 272. 5.14 POWER GENERATION FIGURE 6 Percentage of improvement in turbine heat rate vs. stages of extraction. Give the typical temperatures and flow rates encountered in such installations using gas and/or oil fuels. Calculation Procedure: 1. Determine the output ranges possible with today’s diesel engines Medium-speed Diesel engines are available in sizes exceeding 16 MW. While this capacity may seem small when compared to gas turbines, it is appropriate for repowering of steam plants up to 600 MW via boiler feedwater heating. Modern Diesel engines can attain simple cycle efficiencies of over 47 percent burning natural gas or heavy fuel oil (HFO). The ability to burn natural gas in Diesels is a key factor when coupled with coal-fired boilers. Since the Clean Air Act Amendments of 1990 (CAA) require these boilers to reduce both NOx and SO2 emissions on a lb/million Btu-fired basis (kg/MJ), a boiler feedwater heating sys- tem that can help make these reductions while simultaneously improving overall plant efficiency is attractive. Diesel engines offer these reductions when used in repowering and feedwater heating. Today Diesel engines convert about 45 percent of mechanical energy to elec- tricity; 30 percent becomes exhaust-gas heat; 12 percent is lost to jacket-water heat; and 6 percent is used to cool the lube oil. The remaining energy lost is generally not recoverable. 2. Show how the diesel engine can be used in the feedwater heating cycle Modern steam-turbine reheat cycles, Fig. 7, use an array of feedwater heaters in a regenerative feedwater heating system. The heaters progressively increase the con- densate temperature until it approaches the steam saturation temperature. Conden- sate then enters the final economizer and evaporator sections of the boiler. Using the waste heat from Diesel engines to partially replace the feedwater heaters is almost completely non-intrusive to the operation of the existing system, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 273. FEEDWATER HEATING METHODS 5.15 FIGURE 7 In repowering, Diesel exhaust is adjusted in temperature to the same levels expected from feedwater heaters in the existing plant. (Power.) but causes several significant alterations in the cycle. Two particular cycle altera- tions are: (1) Jacket water temperature from a Diesel engine is available at about 195⬚F (91⬚C). The lube-oil cooling system produces water at about 170⬚F (77⬚C). These temperatures are appropriate for partial displacement of the boiler’s low- temperature feedwater heaters. (2) A gas/Diesel engine can operate on about 97 percent natural gas/3 percent HFO and has an exhaust temperature of 680⬚F (360⬚C). The exhaust gas can be ducted through an economizer that is equipped with selective catalytic reduction (SCR) and has heat-transfer sections that can adjust the exit temperature to match the preheated-burner-windbox air temperature. The SCR reduces NOx emissions from the engine to about 25 ppm on leaving the economizer. This exhaust econo- mizer, Fig. 7, also elevates the temperature of the feedwater after it leaves the deaerator. 3. Explain the environmental impact of using diesels in the feedwater heating loop Exhaust gas from the economizer sections, Fig. 7, is ducted to the boiler windbox. This gas serves the same function as flue-gas recirculation (FGR) in a low-NOx burner. In the installation in Fig. 7, the two Diesel generators produce 351,600 lb/h (159,626 kg/h) of exhaust gas. Most of this gas is ducted to the boiler windbox to achieve a 17.5 percent O2 level needed for the low NOx burners. The balance enters the boiler as overfire air. 4. Determine the heat-rate improvement possible Diesel engines are highly efficient on a simple-cycle basis. When combined with a steam turbine, as described, the cycle efficiency reaches about 56 percent on an incremental basis. In the example here, the incremental heat rate of the engine combined with the additional output from the turbine is 6060 Btu/kWh (6393 kJ/ kWh). This heat rate represents about 25 percent of the total system power and can Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 274. 5.16 POWER GENERATION be averaged with the heat rate of the associated plant. Total system heat rate may be improved by as much as 10 percent as a result of repowering in this fashion. 5. Evaluate system turndown possible with this type of feedwater heating Typically, a coal-fired boiler can be turned down to about 60 percent load while maintaining superheat and reheat temperatures. Adding Diesel feedwater heat in- creases system output by about 25 percent. More important, the system is almost completely non-intrusive, and can return to normal operation when the Diesel out- put is not required. Thus, the total turndown of the plant is increased from 40 to 52 percent, making plant operation more flexible. 6. Compare diesels vs. gas turbines for feedwater heating Comparing Diesels vs. gas turbines (GT) in this application, it appears that the major differences are in the temperature of the exhaust gas and the quantities of exhaust gas that must be introduced to the boiler. Most GTs have fairly high ex- haust-gas rates on a per-kilowatt basis, varying from 25 to over 30 lb/kW (9 to 13.6 kg/kW). GT exhaust may contain from 14.5 to 15.5 percent O2. Conversely, Diesels have exhaust-gas rates of 15 to 16 lb/kW (6.8 to 7.3 kg/ kW). The O2 concentrations for Diesels vary between 11 percent for spark-ignited gas engines up to 13 percent for gas/Diesels or HFO-fired Diesels. Thus, when providing inlet gases to the boiler and adjusting the windbox concentrations to 17.5 percent O2, the volume of gas has to be even further increased with GTs. 7. Evaluate the cost of this type of feedwater heating Capital cost for modifying the boiler is largely dependent on the site and boiler. Cost for a turnkey-installed Diesel facility is about $850/kW. For a Diesel plant connected with an existing power system, net output of the existing system is increased, as noted, because of increasing flow to the steam turbine’s condenser. This increased output offsets the cost of interconnection to the boiler. Related Calculations. The data and procedure given here represent a new ap- proach to feedwater heating and repowering. Because three function are served—namely feedwater heating, repowering, and environmental compliance, the approach is unique. Calculation of the variables is simple because basic heat- transfer relations—covered elsewhere in this handbook—are used. The date and methods given in this procedure are the work of F. Mack Shelor, Wartsila Diesel Inc., as reported in Power magazine (June 1995). SI values were added by the handbook editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FEEDWATER HEATING METHODS
- 275. 6.1 SECTION 6 INTERNAL-COMBUSTION ENGINES Determining the Economics of Reciprocating I-C Engine Cogeneration 6.1 Diesel Generating Unit Efficiency 6.7 Engine Displacement, Mean Effective Pressure, and Efficiency 6.8 Engine Mean Effective Pressure and Horsepower 6.9 Selection of an Industrial Internal- Combustion Engine 6.10 Engine Output at High Temperatures and High Altitudes 6.11 Indicator Use on Internal-Combustion Engines 6.12 Engine Piston Speed, Torque, Displacement, and Compression Ratio 6.13 Internal-Combustion Engine Cooling- Water Requirements 6.14 Design of a Vent System for an Engine Room 6.18 Design of a Bypass Cooling System for an Engine 6.21 Hot-Water Heat-Recovery System Analysis 6.26 Diesel Fuel Storage Capacity and Cost 6.27 Power Input to Cooling-Water and Lube- Oil Pumps 6.29 Lube-Oil Cooler Selection and Oil Consumption 6.31 Quantity of Solids Entering an Internal- Combustion Engine 6.31 Internal-Combustion Engine Performance Factors 6.32 Volumetric Efficiency of Diesel Engines 6.34 Selecting Air-Cooled Engines for Industrial Applications 6.37 DETERMINING THE ECONOMICS OF RECIPROCATING I-C ENGINE COGENERATION Determine if an internal-combustion (I-C) engine cogeneration facility will be ec- onomically attractive if the required electrical power and steam services can be served by a cycle such as that in Fig. 1 and the specific load requirements are those shown in Fig. 2. Frequent startups and shutdowns are anticipated for this system. Calculation Procedure: 1. Determine the sources of waste heat available in the typical I-C engine There are three primary sources of waste heat available in the usual I-C engine. These are: (1) the exhaust gases from the engine cylinders; (2) the jacket cooling water; (3) the lubricating oil. Of these three sources, the quantity of heat available is, in descending order: exhaust gases; jacket cooling water; lube oil. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 276. 6.2 POWER GENERATION FIGURE 1 Reciprocating-engine cogeneration system waste heat from the exhaust, and jacket a oil cooling, are recovered. (Indeck Energy Services, Inc.) FIGURE 2 Low-speed Diesel-engine cogeneration. (Indeck Energy Services, Inc.) 2. Show how to compute the heat recoverable from each source For the exhaust gases, use the relation, HA ⫽ W(⌬t)(cg), where WA ⫽ rate of gas flow from the engine, lb/h (kg/h); ⌬t ⫽ temperature drop of the gas between the heat exchanger inlet and outlet, ⬚F (⬚C); cg ⫽ specific heat of the gas, Btu/lb ⬚F (J/kg ⬚C). For example, if an I-C engine exhausts 100,000 lb/h (45,400 kg/h) at 700⬚F (371⬚C) to a HRSG (heat-recovery steam generator), leaving the HRSG at 330⬚F (166⬚C), and the specific heat of the gas is 0.24 Btu/lb ⬚F (1.0 kJ/kg ⬚C), Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 277. INTERNAL-COMBUSTION ENGINES 6.3 the heat recoverable, neglecting losses in the HRSG and connecting piping, is HA ⫽ 100,000(700 ⫺ 330)(0.24) ⫽ 8,880,000 Btu/h (2602 MW). With an average heat of vaporization of 1000 Btu/lb (2330 kJ/kg) of steam, this exhaust gas flow could generate 8,880,000/1000 ⫽ 8880 lb/h (4032 kg/h) of steam. If oil with a heating value of 145,000 Btu/gal (40,455 kJ/L) were used to generate this steam, the quantity required would be 8,880,000/145,000 ⫽ 61.2 gal/h (232 L/h). At a cost of 90 cents per gallon, the saving would be $0.90(61.2) ⫽ $55.08/h. Assuming 5000 hours of operation per year, or 57 percent load, the saving in fuel cost would be 5000($55.08) ⫽ $275,400. This is a significant saving in any plant. And even if heat losses in the ductwork and heat-recovery boiler cut the savings in half, the new would still exceed one hundred thousand dollars a year. And as the operating time increases, so too do the savings. 3. Compute the savings potential in jacket-water and lube-oil heat recovery A similar relation can be used to compute jacket-water and lube-oil heat recovery. The flow rate can be expressed in either pounds (kg) per hour or gallons (L) per minute, depending on the designer’s choice. Since water has a specific heat of unity, the heat-recovery potential of the jacket water is HW ⫽ w(⌬tw), where w ⫽ weight of water flow, lb per h (kg/h); ⌬tw ⫽ change in temperature of the jacket water when flowing through the heat exchanger, ⬚F (⬚C). Thus, if the jacket-water flow is 25,000 lb/h (11,350 kg/h) and the tem- perature change during flow of the jacket water through and external heat exchanger is 190 to 70⬚F (88 to 21⬚C), the heat given up by the jacket water, neglecting losses is Hw ⫽ 25,000(190 ⫺ 70) ⫽ 3,000,000 Btu/h (879 MW). During 25 h the heat recovery will be 24(3,000,000) ⫽ 72,000,000 Btu (75,960 MJ). This is a significant amount of heat which can be used in process or space heating, or to drive an air- conditioning unit. If the jacket-water flow rate is expressed in gallons per minute instead of pounds per hour (L/min instead of kg/h), the heat-recovery potential, Hwg ⫽ gpm(⌬t)(8.33) where 8.33 ⫽ lb/gal of water. With a water flow rate of 50 gpm and the same temperature range as above, Hwg ⫽ 50(120)(8.33) ⫽ 49,980 Btu/min (52,279 kJ/ min). 4. Find the amount of heat recoverable from the lube oil During I-C engine operation, lube-oil temperature can reach high levels—in the 300 to 400⬚F (149 to 201⬚C) range. And with oil having a typical specific heat of 0.5 Btu/lb ⬚F (2.1 kJ/kg ⬚C), the heat-recovery potential for the lube oil is ⫽ Hwo wo(⌬t)(co), where wo ⫽ oil flow in lb/h (kg/h); ⌬t ⫽ temperature change of the oil during flow through the heat-recovery heat exchanger ⫽ oil inlet temperature ⫺ oil outlet temperature, ⬚F or ⬚C; co ⫽ specific heat of oil ⫽ 0.5 Btu/lb ⬚F (kJ/kg ⬚C). With an oil flow of 2000 lb/h (908 kg/h), a temperature change of 140⬚F (77.7⬚C), Ho ⫽ 2000(140)(0.50) ⫽ 140,000 Btu/h (41 kW). Thus, as mentioned earlier, the heat recoverable from the lube oil is usually the lowest of the three sources. With the heat flow rates computed here, an I-C engine cogeneration facility can be easily justified, especially where frequent startups and shutdowns are anticipated. Reciprocating Diesel engines are preferred over gas and steam turbines where fre- quent startups and shutdowns are required. Just the fuel savings anticipated for recovery of heat in the exhaust gases of this engine could pay for it in a relatively short time. Related Calculations. Cogeneration, in which I-C engines are finding greater use throughout the world every year, is defined by Michael P. Polsky, President, Indeck Energy Services, Inc., as ‘‘the simultaneous production of useful thermal Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 278. 6.4 POWER GENERATION energy and electric power from a fuel source or some variant thereof. It is more efficient to produce electric power and steam or hot water together than electric power alone, as utilities do, or thermal energy alone, which is common in industrial, commercial, and institutional plants.’’ Figures 1 and 2 in this procedure are from the firm of which Mr. Polsky is president. With the increased emphasis on reducing environmental pollution, conserving fuel use, and operating at lower overall cost, cogeneration—especially with Diesel engines—is finding wider acceptance throughout the world. Design engineers should consider cogeneration whenever there is a concurrent demand for electricity and heat. Such demand is probably most common in industry but is also met in commercial (hotels, apartment houses, stores) and institutional (hospital, prison, nursing-home) installations. Often, the economic decision is not over whether co- generation should be used, but what type of prime mover should be chosen. Three types of prime movers are usually considered for cogeneration—steam turbines, gas turbines, or internal-combustion engines. Steam and/or gas turbines are usually chosen for large-scale utility and industrial plants. For smaller plants the Diesel engine is probably the most popular choice today. Where natural gas is available, reciprocating internal-combustion engines are a favorite choice, especially with frequent startups and shutdowns. Recently, vertical modular steam engines have been introduced for use in co- generation. Modules can be grouped to increase the desired power output. These high-efficiency units promise to compete with I-C engines in the growing cogen- eration market. Guidelines used in estimating heat recovery from I-C engines, after all heat loses, include these: (1) Exhaust-gas heat recovery ⫽ 28 percent of heat in fuel; (2) Jacket- water heat recovery ⫽ 27 percent of heat in fuel; (3) Lube-oil heat recovery ⫽ 9 percent of the heat in the fuel. The Diesel Engine Manufacturers Association (DEMA) gives these values for heat disposition in a Diesel engine at three-quarters to full load: (1) Fuel consumption ⫽ 7366 Btu/bhp 䡠 h (2.89 kW/kW); (2) Useful work ⫽ 2544 Btu/bhp 䡠 h (0.999 kW/kW); (3) Loss in radiation, etc. ⫽ 370 Btu/ bhp 䡠 h (0.145 kW/kW); (4) To cooling water ⫽ 2195 Btu/bhp 䡠 h (0.862 kW/kW); (5) To exhaust ⫽ 2258 Btu/bhp 䡠 h (0.887 kW/kW). The sum of the losses is 1 Btu/bhp 䡠 h greater than the fuel consumption because of rounding of the values. Figure 3 shows a proposed cogeneration, desiccant-cooling, and thermal-storage integrated system for office buildings in the southern California area. While directed at the micro-climates in that area, similar advantages for other micro-climates and building types should be apparent. The data presented here for this system were prepared by The Meckler Group and are based on a thorough engineering and economic evaluation for the Southern California Gas Co. of the desiccant- cooling/thermal-energy-storage/cogeneration system, a proprietary design devel- oped for pre- and post-Title-24 mid-rise office buildings. Title 24 is a section of the State of California Administrative Code that deals with energy-conservation standards for construction applicable to office buildings. A summary of the study was presented in Power magazine by Milton Meckler. In certain climates, office buildings are inviting targets for saving energy via evaporative chilling. When waste heat is plentiful, desiccant cooling and cogener- ation become attractive. In coupling the continuously available heat-rejection capacity of packaged cogeneration units, Fig. 4, with continuously operating re- generator demands, the use of integrated components for desiccant cooling, thermal- energy storage, and cogeneration increases. The combination also ensures a rea- sonable constant, cost-effective supply of essentially free electric power for general building use. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 279. INTERNAL-COMBUSTION ENGINES 6.5 FIGURE 3 Integrated system is a proposed off-peak desiccant/evaporative-cooling configu- ration with cogeneration capability. (Power and The Meckler Group.) Recoverable internal-combustion engine heat should at least match the heat re- quirement of the regenerator, Fig. 3. The selected engine size (see a later procedure in this section), however, should not cause the cogeneration system’s Purpa (Public Utility Regulatory Policies Act) efficiency to drop below 42.5 percent. (Purpa efficiency decreases as engine size increases.) An engine size is selected to give the most economical performance and still have a Purpa efficiency of greater than 42.5 percent. The utility study indicated a favorable payout period and internal rate of return both for retrofits of pre-Title-24 office buildings and for new buildings in compli- ance with current Title-24 requirements (nominal 200 to 500 cooling tons). Al- though the study was limited to office-building occupancies, it is likely that other building types with high ventilation and electrical requirements would also offer attractive investment opportunities. Based on study findings, fuel savings ranged from 3300 to 7900 therms per year. Cost savings ranged from $322,000 to $370,000 for the five-story-building case studies and from $545,000 to $656,000 for 12-story-building case studies where the synchronously powered, packaged cogeneration unit was not used for emer- gency power. Where the cogeneration unit was also used for emergency power, the initial cost decreased from $257,000 to $243,000, representing a 31 percent drop in average cost for the five-story-building cases; and from $513,000 to $432,000, a 22 percent Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 280. 6.6 POWER GENERATION FIGURE 4 Packaged cogeneration I-C engine unit supplies waste heat to desiccant regenerator. (Power and The Meckler Group.) dip in average cost for the 12-story-building cases. The average cost decrease shifts the discounted payback period an average of 5.6 and 5.9 years for the five- and 12- story-building cases, respectively. Study findings were conservatively reported, since no credit was taken for po- tential income resulting from Purpa sales to the serving utility at off-peak hours, when actual building operating requirements fall below rated cogenerator output. This study is another example of the importance of the internal-combustion engine in cogeneration around the world today. Worldwide there is a movement toward making internal-combustion engines, and particularly diesel engines, cleaner-running. In general, this means reducing partic- ulate emissions from diesel-engine exhaust gases. For cities with large numbers of diesel-powered buses, exhaust emissions can be particularly unpleasant. And some medical personnel say that diesel exhaust gases can be harmful to the health of people breathing them. The approach to making diesel engines cleaner takes two tacts: (1) improving the design of the engine so that fewer particulates are emitted and (2) using cleaner fuel to reduce the particulate emissions. Manufacturers are using both approaches to comply with the demands of federal and state agencies regulating emissions. Today’s engineers will find that ‘‘cleaning up’’ diesel engines is a challenging and expensive procedure. However, cleaner-operating diesels are being introduced every year. *Elliott, Standard Handbook of Power Plant Engineering, McGraw-Hill, 1989. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 281. INTERNAL-COMBUSTION ENGINES 6.7 DIESEL GENERATING UNIT EFFICIENCY A 3000-kW diesel generating unit performs thus: fuel rate, 1.5 bbl (238.5 L) of 25⬚ API fuel for a 900-kWh output; mechanical efficiency, 82.0 percent; generator efficiency, 92.0 percent. Compute engine fuel rate, engine-generator fuel rate, in- dicated thermal efficiency, overall thermal efficiency, brake thermal efficiency. Calculation Procedure: 1. Compute the engine fuel rate The fuel rate of an engine driving a generator is the weight of fuel, lb, used to generate 1 kWh at the generator input shaft. Since this engine burns 1.5 bbl (238.5 L) of fuel for 900 kW at the generator terminals, the total fuel consumption is (1.5 bbl)(42 gal/bbl) ⫽ 63 gal (238.5 L), at a generator efficiency of 92.0 percent. To determine the weight of this oil, compute its specific gravity s from s ⫽ 141.5/(131.5 ⫹ ⬚API), where ⬚API ⫽ API gravity of the fuel. Hence, s ⫽ 141.5(131.5 ⫹ 25) ⫽ 0.904. Since 1 gal (3.8 L) of water weighs 8.33 lb (3.8 kg) at 60⬚F (15.6⬚C), 1 gal (3.8 L) of this oil weighs (0.904)(8.33) ⫽ 7.529 lb (3.39 kg). The total weight of fuel used when burning 63 gal is (63 gal)(7.529 lb/gal) ⫽ 474.5 lb (213.5 kg). The generator is 92 percent efficient. Hence, the engine actually delivers enough power to generate 900/0.92 ⫽ 977 kWh at the generator terminals. Thus, the engine fuel rate ⫽ 474.5 lb fuel/977 kWh ⫽ 0.485 lb/kWh (0.218 kg/kWh). 2. Compute the engine-generator fuel rate The engine-generator fuel rate takes these two units into consideration and is the weight of fuel required to generate 1 kWh at the generator terminals. Using the fuel-consumption data from step 1 and the given output of 900 kW, we see that engine-generator fuel rate ⫽ 474.5 lb fuel/900 kWh output ⫽ 0.527 lb/kWh (0.237 kg/kWh). 3. Compute the indicated thermal efficiency Indicated thermal efficiency is the thermal efficiency based on the indicated horse- power of the engine. This is the horsepower developed in the engine cylinder. The engine fuel rate, computed in step 1, is the fuel consumed to produce the brake or shaft horsepower output, after friction losses are deducted. Since the mechanical efficiency of the engine is 82 percent, the fuel required to produce the indicated horsepower is 82 percent of that required for the brake horsepower, or (0.82)(0.485) ⫽ 0.398 lb/kWh (0.179 kg/kWh). The indicated thermal efficiency of an internal-combustion engine driving a gen- erator is ei ⫽ 3413/ƒi(HHV), where ei ⫽ indicated thermal efficiency, expressed as a decimal; ƒi ⫽ indicated fuel consumption, lb/kWh; HHV ⫽ higher heating value of the fuel, Btu/lb. Compute the HHV for a diesel fuel from HHV ⫽ 17,680 ⫹ 60 ⫻ ⬚API. For this fuel, HHV ⫽ 17,680 ⫹ 60(25) ⫽ 19,180 Btu/lb (44,612.7 kJ/kg). With the HHV known, compute the indicated thermal efficiency from ei ⫽ 3,413/[(0.398)(19,180)] ⫽ 0.447 or 44.7 percent. 4. Compute the overall thermal efficiency The overall thermal efficiency eo is computed from eo ⫽ 3413/ƒo(HHV), where ƒo ⫽ overall fuel consumption, Btu/kWh; other symbols as before. Using the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 282. 6.8 POWER GENERATION engine-generator fuel rate from step 2, which represents the overall fuel consump- tion eo ⫽ 3413/[(0.527)(19,180)] ⫽ 0.347, or 34.7 percent. 5. Compute the brake thermal efficiency The engine fuel rate, step 1, corresponds to the brake fuel rate ƒb. Compute the brake thermal efficiency from eb ⫽ 3413/ƒb(HHV), where ƒb ⫽ brake fuel rate, Btu/kWh; other symbols as before. For this engine-generator set, eb ⫽ 3413/ [(0.485)(19,180)] ⫽ 0.367, or 36.7 percent. Related Calculations. Where the fuel consumption is given or computed in terms of lb/(hp 䡠 h), substitute the value of 2545 Btu/(hp 䡠 h) (1.0 kW/kWh) in place of the value 3413 Btu/kWh (3600.7 kJ/kWh) in the numerator of the ei, eo, and eb equations. Compute the indicated, overall, and brake thermal efficiencies as before. Use the same procedure for gas and gasoline engines, except that the higher heating value of the gas or gasoline should be obtained from the supplier or by test. ENGINE DISPLACEMENT, MEAN EFFECTIVE PRESSURE, AND EFFICIENCY A 12 ⫻ 18 in (30.5 ⫻ 44.8 cm) four-cylinder four-stroke single-acting diesel engine is rated at 200 bhp (149.2 kW) at 260 r/min. Fuel consumption at rated load is 0.42 lb/(bhp 䡠 h) (0.25 kg/kWh). The higher heating value of the fuel is 18,920 Btu/lb (44,008 kJ/kg). What are the brake mean effective pressure, engine dis- placement in ft3 /(min 䡠 bhp), and brake thermal efficiency? Calculation Procedure: 1. Compute the brake mean effective pressure Compute the brake mean effective pressure (bmep) for an internal-combustion en- gine from bmep ⫽ 33,000 bhpn /LAn, where bmep ⫽ brake mean effective pressure, lb/in2 ; bhpn ⫽ brake horsepower output delivered per cylinder, hp; L ⫽ piston stroke length, ft; a ⫽ piston area, in2 ; n ⫽ cycles per minute per cylinder ⫽ crank- shaft rpm for a two-stroke cycle engine, and 0.5 the crankshaft rpm for a four- stroke cycle engine. For this engine at its rated hbp, the output per cylinder is 200 bhp/4 cylinders ⫽ 50 bhp (37.3 kW). Then bmep ⫽ 33,000(50)/[(18/12)(12)2 (/4)(260/2)] ⫽ 74.8 lb/in2 (516.1 kPa). (The factor 12 in the denominator converts the stroke length from inches to feet.) 2. Compute the engine displacement The total engine displacement Vd ft3 is given by Vd ⫽ LAnN, where A ⫽ piston area, ft2 ; N ⫽ number of cylinders in the engine; other symbols as before. For this engine, Vd ⫽ (18/12)(12/12)2 (/4)(260/2)(4) ⫽ 614 ft3 /min (17.4 m3 /min). The displacement is in cubic feet per minute because the crankshaft speed is in r/min. The factor of 12 in the denominators converts the stroke and area to ft and ft2 , respectively. The displacement per bhp ⫽ (total displacement, ft3 /min)/bhp output of engine ⫽ 614/200 ⫽ 3.07 ft3 /(min 䡠 bhp) (0.12 m3 /kW). 3. Compute the brake thermal efficiency The brake thermal efficiency eb of an internal-combustion engine is given by eb ⫽ 2545/(sfc)(HHV), where sfc ⫽ specific fuel consumption, lb/(bhp 䡠 h); HHV ⫽ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 283. INTERNAL-COMBUSTION ENGINES 6.9 higher heating value of fuel, Btu/lb. For this engine, eb ⫽ 2545/[(0.42)(18,920)] ⫽ 0.32, or 32.0 percent. Related Calculations. Use the same procedure for gas and gasoline engines. Obtain the higher heating value of the fuel from the supplier, a tabulation of fuel properties, or by test. ENGINE MEAN EFFECTIVE PRESSURE AND HORSEPOWER A 500-hp (373-kW) internal-combustion engine has a brake mean effective pressure of 80 lb/in2 (551.5 kPa) at full load. What are the indicated mean effective pressure and friction mean effective pressure if the mechanical efficiency of the engine is 85 percent? What are the indicated horsepower and friction horsepower of the engine? Calculation Procedure: 1. Determine the indicated mean effective pressure Indicated mean effective pressure imep lb/in2 for an internal-combustion engine is found from imep ⫽ bmep/em, where bmep ⫽ brake mean effective pressure, lb/ in2 ; em ⫽ mechanical efficiency, percent, expressed as a decimal. For this engine, imep ⫽ 80/0.85 ⫽ 94.1 lb/in2 (659.3 kPa). 2. Compute the friction mean effective pressure For an internal-combustion engine, the friction mean effective pressure ƒmep lb/ in2 is found from ƒmep ⫽ imep ⫺ bmep, or ƒmep ⫽ 94.1 ⫺ 80 ⫽ 14.1 lb/in2 (97.3 kPa). 3. Compute the indicated horsepower of the engine For an internal-combustion engine, the mechanical efficiency em ⫽ bhp/ihp, where ihp ⫽ indicated horsepower. Thus, ihp ⫽ bhp/em, or ihp ⫽ 500/0.85 ⫽ 588 ihp (438.6 kW). 4. Compute the friction hp of the engine For an internal-combustion engine, the friction horsepower is ƒhp ⫽ ihp ⫺ bhp. In this engine, ƒhp ⫽ 588 ⫺ 500 ⫽ 88 fhp (65.6 kW). Related Calculations. Use a similar procedure to determine the indicated en- gine efficiency eei ⫽ ei /e, where e ⫽ ideal cycle efficiency; brake engine efficiency, eeb ⫽ ebe; combined engine efficiency or overall engine thermal efficiency eeo ⫽ eo⫽ eoe. Note that each of these three efficiencies is an engine efficiency and cor- responds to an actual thermal efficiency, ei, eb, and eo. Engine efficiency ee ⫽ et /e, where et ⫽ actual engine thermal efficiency. Where desired, the respective actual indicated brake, or overall, output can be substituted for ei, eb, and eo in the numerator of the above equations if the ideal output is substituted in the denominator. The result will be the respective engine efficiency. Output can be expressed in Btu per unit time, or horsepower. Also, ee ⫽ actual mep/ideal mep, and eei ⫽ imep/ideal mep; eeb ⫽ bmep/ideal mep; eeo ⫽ overall mep/ideal mep. Further, eb ⫽ emei, and bmep ⫽ em(imep). Where the actual heat supplied by the fuel, HHV Btu/lb, is known, compute eieb and eo by the method given in the previous calculation procedure. The above relations apply to any re- ciprocating internal-combustion engine using any fuel. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 284. 6.10 POWER GENERATION TABLE 1 Internal-Combustion Engine Rating Table SELECTION OF AN INDUSTRIAL INTERNAL-COMBUSTION ENGINE Select an internal-combustion engine to drive a centrifugal pump handling 2000 gal/min (126.2 L/s) of water at a total head of 350 ft (106.7 m). The pump speed will be 1750 r/min, and it will run continuously. The engine and pump are located at sea level. Calculation Procedure: 1. Compute the power input to the pump The power required to pump water is hp ⫽ 8.33GH/33,000e, where G ⫽ water flow, gal/min; H ⫽ total head on the pump, ft of water; e ⫽ pump efficiency, expressed as a decimal. Typical centrifugal pumps have operating efficiencies rang- ing from 50 to 80 percent, depending on the pump design and condition and liquid handled. Assume that this pump has an efficiency of 70 percent. Then hp ⫽ 8.33(2000)/(350)/[(33,000)(0.70)] ⫽ 252 hp (187.9 kW). Thus, the internal- combustion engine must develop at least 252 hp (187.9 kW) to drive this pump. 2. Select the internal-combustion engine Since the engine will run continuously, extreme care must be used in its selection. Refer to a tabulation of engine ratings, such as Table 1. This table shows that a diesel engine that delivers 275 continuous brake horsepower (205.2 kW) (the near- est tabulated rating equal to or greater than the required input) will be rated at 483 bhp (360.3 kW) at 1750 r/min. The gasoline-engine rating data in Table 1 show that for continuous full load at a given speed, 80 percent of the tabulated power can be used. Thus, at 1750 r/min, the engine must be rated at 252/0.80 ⫽ 315 bhp (234.9 kW). A 450-hp (335.7- kW) unit is the only one shown in Table 1 that would meet the needs. This is too large; refer to another builder’s rating table to find an engine rated at 315 to 325 bhp (234.9 to 242.5 kW) at 1750 r/min. The unsuitable capacity range in the gasoline-engine section of Table 1 is a typical situation met in selecting equipment. More time is often spent in finding a Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 285. INTERNAL-COMBUSTION ENGINES 6.11 TABLE 2 Correction Factors for Altitude and Temperature suitable unit at an acceptable price than is spent computing the required power output. Related Calculations. Use this procedure to select any type of reciprocating internal-combustion engine using oil, gasoline, liquified-petroleum gas, or natural gas for fuel. ENGINE OUTPUT AT HIGH TEMPERATURES AND HIGH ALTITUDES An 800-hp (596.8-kW) diesel engine is operated 10,000 ft (3048 m) above sea level. What is its output at this elevation if the intake air is at 80⬚F (26.7⬚C)? What will the output at 10,000-ft (3048-m) altitude be if the intake air is at 110⬚F (43.4⬚C)? What would the output be if this engine were equipped with an exhaust turbine-driven blower? Calculation Procedure: 1. Compute the engine output at altitude Diesel engines are rated at sea level at atmospheric temperatures of not more than 90⬚F (32.3⬚C). The sea-level rating applies at altitudes up to 1500 ft (457.2 m). At higher altitudes, a correction factor for elevation must be applied. If the atmospheric temperature is higher than 90⬚F (32.2⬚C), a temperature correction must be applied. Table 2 lists both altitude and temperature correction factors. For an 800-hp (596.8-kW) engine at 10,000 ft (3048 m) above sea level and 80⬚F (26.7⬚C) intake air, hp output ⫽ (sea-level hp) (altitude correction factor), or output ⫽ (800)(0.68) ⫽ 544 hp (405.8 kW). 2. Compute the engine output at the elevated temperature When the intake air is at a temperature greater than 90⬚F (32.3⬚C), a temperature correction factor must be applied. Then output ⫽ (sea-level hp)(altitude correction factor)(intake-air-temperature correction factor), or output ⫽ (800)(0.68)(0.95) ⫽ 516 hp (384.9 kW), with 110⬚F (43.3⬚C) intake air. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 286. 6.12 POWER GENERATION TABLE 3 Atmospheric Pressure at Various Altitudes 3. Compute the output of a supercharged engine A different altitude correction is used for a supercharged engine, but the same temperature correction factor is applied. Table 2 lists the altitude correction factors for supercharged diesel engines. Thus, for this supercharged engine at 10,000-ft (3048-m) altitude with 80⬚F (26.7⬚C) intake air, output ⫽ (sea-level hp)(altitude correction factor) ⫽ (800)(0.74) ⫽ 592 hp (441.6 kW). At 10,000-ft (3048-m) altitude with 110⬚F (43.3⬚C) inlet air, output ⫽ (sea-level hp)(altitude correction factor)(temperature correction factor) ⫽ (800)(0.74)(0.95) ⫽ 563 hp (420.1 kW). Related Calculations. Use the same procedure for gasoline, gas, oil, and liq- uefied-petroleum gas engines. Where altitude correction factors are not available for the type of engine being used, other than a diesel, multiply the engine sea-level brake horsepower by the ratio of the altitude-level atmospheric pressure to the atmospheric pressure at sea level. Table 3 lists the atmospheric pressure at various altitudes. An engine located below sea level can theoretically develop more power than at sea level because the intake air is denser. However, the greater potential output is generally ignored in engine-selection calculations. INDICATOR USE ON INTERNAL-COMBUSTION ENGINES An indicator card taken on an internal-combustion engine cylinder has an area of 5.3 in2 (34.2 cm2 ) and a length of 4.95 in (12.7 cm). What is the indicated mean effective pressure in this cylinder? What is the indicated horsepower of this four- cycle engine if it has eight 6-in (15.6-cm) diameter cylinders, an 18-in (45.7-cm) stroke, and operates at 300 r/min? The indicator spring scale is 100 lb/in (1.77 kg/mm). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 287. INTERNAL-COMBUSTION ENGINES 6.13 Calculation Procedure: 1. Compute the indicated mean effective pressure For any indicator card, imep ⫽ (card area, in2 ) (indicator spring scale, lb)/(length of indicator card, in) where imep ⫽ indicated mean effective pressure, lb/in2 . Thus, for this engine, imep ⫽ (5.3)(100)/4.95 ⫽ 107 lb/in2 (737.7 kPa). 2. Compute the indicated horsepower For any reciprocating internal-combustion engine, ihp ⫽ (imep)LAn/33,000, where ihp ⫽ indicated horsepower per cylinder; L ⫽ piston stroke length, ft; A ⫽ piston area, in2 , n ⫽ number of cycles/min. Thus, for this four-cycle engine where n ⫽ 0.5 r/min, ihp ⫽ (107)(18/12)(6)2 (/4)(300/2)/33,000 ⫽ 20.6 ihp (15.4 kW) per cylinder. Since the engine has eight cylinders, total ihp ⫽ (8 cylinders)(20.6 ihp per cylinder) ⫽ 164.8 ihp (122.9 kW). Related Calculations. Use this procedure for any reciprocating internal- combustion engine using diesel oil, gasoline, kerosene, natural gas, liquefied- petroleum gas, or similar fuel. ENGINE PISTON SPEED, TORQUE, DISPLACEMENT, AND COMPRESSION RATIO What is the piston speed of an 18-in (45.7-cm) stroke 300 ⫽ r/min engine? How much torque will this engine deliver when its output is 800 hp (596.8 kW)? What are the displacement per cylinder and the total displacement if the engine has eight 12-in (30.5-cm) diameter cylinders? Determine the engine compression ratio if the volume of the combustion chamber is 9 percent of the piston displacement. Calculation Procedure: 1. Compute the engine piston speed For any reciprocating internal-combustion engine, piston speed ⫽ ƒpm ⫽ 2L(rpm), where L ⫽ piston stroke length, ft; rpm ⫽ crankshaft rotative speed, r/min. Thus, for this engine, piston speed ⫽ 2(18/12)(300) ⫽ 9000 ft/min (2743.2 m/min). 2. Determine the engine torque For any reciprocating internal-combustion engine, T ⫽ 63,000(bhp)/rpm, where T ⫽ torque developed, in 䡠 lb; bhp ⫽ engine brake horsepower output; rpm ⫽ crankshaft rotative speed, r/min. Or T ⫽ 63,000(800)/300 ⫽ 168,000 in 䡠 lb (18.981 N 䡠 m). Where a prony brake is used to measure engine torque, apply this relation: T ⫽ (Fb ⫺ Fo)r, where Fb ⫽ brake scale force, lb, with engine operating; Fo ⫽ brake scale force with engine stopped and brake loose on flywheel; r ⫽ brake arm, in ⫽ distance from flywheel center to brake knife edge. 3. Compute the displacement The displacement per cylinder dc in3 of any reciprocating internal-combustion en- gine is dc ⫽ Li Ai where Li ⫽ piston stroke, in; A ⫽ piston head area, in2 . For this engine, dc ⫽ (18)(12)2 (/4) ⫽ 2035 in3 (33,348 cm3 ) per cylinder. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 288. 6.14 POWER GENERATION The total displacement of this eight-cylinder engine is therefore (8 cylin- ders)(2035 in3 per cylinder) ⫽ 16,280 in3 (266,781 cm3 ). 4. Compute the compression ratio For a reciprocating internal-combustion engine, the compression ratio rc ⫽ Vb /Va, where Vb ⫽ cylinder volume at the start of the compression stroke, in3 or ft3 ; Va ⫽ combustion-space volume at the end of the compression stroke, in3 or ft3 . When this relation is used, both volumes must be expressed in the same units. In this engine, Vb ⫽ 2035 in3 (33,348 cm3 ); Va ⫽ (0.09)(2035) ⫽ 183.15 in3 . Then rc ⫽ 2035/183.15 ⫽ 11.1:1. Related Calculations. Use these procedures for any reciprocating internal- combustion engine, regardless of the fuel burned. INTERNAL-COMBUSTION ENGINE COOLING-WATER REQUIREMENTS A 1000-hbp (746-kW) diesel engine has a specific fuel consumption of 0.360 lb/ (bhp 䡠 h) (0.22 kg/kWh). Determine the cooling-water flow required if the higher heating value of the fuel is 10,350 Btu/lb (24,074 kJ/kg). The net heat rejection rates of various parts of the engine are, in percent: jacket water, 11.5; turbo- charger, 2.0; lube oil. 3.8; aftercooling, 4.0; exhaust, 34.7; radiation, 7.5. How much 30 lb/in2 (abs) (206.8 kPa) steam can be generated by the exhaust gas if this is a four-cycle engine? The engine operates at sea level. Calculation Procedure: 1. Compute the engine heat balance Determine the amount of heat used to generate 1 bhp 䡠 h (0.75 kWh) from: heat rate, Btu/bhp 䡠 h) ⫽ (sfc)(HHV), where sfc ⫽ specific fuel consumption, lb/(bhp 䡠 h); HHV ⫽ higher heating value of fuel, Btu/lb. Or, heat rate ⫽ (0.36)(19.350) ⫽ 6967 Btu/(bhp 䡠 h) (2737.3 W/kWh). Compute the heat balance of the engine by taking the product of the respective heat rejection percentages and the heat rate as follows: Then the power output ⫽ 6967 ⫺ 4422 ⫽ 2545 Btu/(bhp 䡠 h) (999.9 W/kWh), or 2545/6967 ⫽ 0.365, or 36.5 percent. Note that the sum of the heat losses and power generated, expressed in percent, is 100.0. 2. Compute the jacket cooling-water flow rate The jacket water cools the jackets and the turbocharger. Hence, the heat that must be absorbed by the jacket water is 800 ⫹ 139 ⫽ 939 Btu/(bhp 䡠 h) (369 W/kWh), Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 289. INTERNAL-COMBUSTION ENGINES 6.15 using the heat rejection quantities computed in step 1. When the engine is devel- oping its full rated output of 1000 bhp (746 kW), the jacket water must absorb [939 Btu/(bhp 䡠 h)(1000 bhp) ⫽ 939,000 Btu/h (275,221 W). Apply a safety factor to allow for scaling of the heat-transfer surfaces and other unforeseen difficulties. Most designers use a 10 percent safety factor. Applying this value of the safety factor for this engine, we see the total jacket-water heat load ⫽ 939,000 ⫹ (0.10)(939,000) ⫽ 1,032,900 Btu/h (302.5 kW). Find the required jacket-water flow from G ⫽ H/500⌬t, where G ⫽ jacket-water flow, gal/min; H ⫽ heat absorbed by jacket water, Btu/h; ⌬t ⫽ temperature rise of the water during passage through the jackets, ⬚F. The usual temperature rise of the jacket water during passage through a diesel engine is 10 to 20⬚F (5.6 to 11.1⬚C). Using 10⬚F for this engine we find G ⫽ 1,032,900/[(500)(10)] ⫽ 206.58 gal/min (13.03 L/s), say 207 gal/min (13.06 L/s). 3. Determine the water quantity for radiator cooling In the usual radiator cooling system for large engines, a portion of the cooling water is passed through a horizontal or vertical radiator. The remaining water is recirculated, after being tempered by the cooled water. Thus, the radiator must dissipate the jacket, turbocharger, and lube-oil cooler heat, Fig. 5. The lube oil gives off 264 Btu/(bhp 䡠 h) (103.8 W/kWh). With a 10 percent safety factor, the total heat flow is 264 ⫹ (0.10)(264) ⫽ 290.4 Btu/(bhp 䡠 h) (114.1 W/kWh). At the rated output of 1000 bhp (746 kW), the lube-oil heat load ⫽ [290.4 Btu/(bhp 䡠 h)](1000 bhp) ⫽ 290,400 Btu/h (85.1 kW). Hence, the total heat load on the radiator ⫽ jacket ⫹ lube-oil heat load ⫽ 1,032,900 ⫹ 290,400 ⫽ 1,323,300 Btu/h (387.8 kW) Radiators (also called fan coolers) serving large internal-combustion engines are usually rated for a 35⬚F (19.4⬚C) temperature reduction of the water. To remove 1,323,300 Btu/h (387.8 kW) with a 35⬚F (19.4⬚C) temperature decrease will require a flow of G ⫽ H/(500⌬t) ⫽ 1,323,300/[(500)(35)] ⫽ 76.1 gal/min (4.8 L/s). 4. Determine the aftercooler cooling-water quantity The aftercooler must dissipate 278 Btu/(bhp 䡠 h) (109.2 W/kWh). At an output of 1000 bhp (746 kW), the heat load ⫽ [278 Btu/(bhp 䡠 h)](1000 bhp) ⫽ 278,000 Btu/h (81.5 kW). In general, designers do not use a factor of safety for the after- cooler because there is less chance of fouling or other difficulties. With a 5⬚F (2.8⬚C) temperature rise of the cooling water during passage through the after-cooler, the quantity of water required G ⫽ H/(500⌬t) ⫽ 278,000/ [(500)(5)] ⫽ 111 gal/min (7.0 L/s). 5. Compute the quantity of steam generated by the exhaust Find the heat available in the exhaust by using He ⫽ Wc⌬te, where He ⫽ heat available in the exhaust, Btu/h; W ⫽ exhaust-gas flow, lb/h; c ⫽ specific heat of the exhaust gas ⫽ 0.252 Btu/(lb 䡠 ⬚F) (2.5 kJ/kg); ⌬te ⫽ exhaust-gas temperature at the boiler inlet, ⬚F ⫺ exhaust-gas temperature at the boiler outlet, ⬚F. The exhaust-gas flow from a four-cycle turbocharged diesel is about 12.5 lb/ (bhp 䡠 h) (7.5 kg/kWh). At full load this engine will exhaust [12.5 lb/(bhp 䡠 h)](1000 bhp) ⫽ 12,500 lb/h (5625 kg/h). The temperature of the exhaust gas will be about 750⬚F (399⬚C) at the boiler inlet, whereas the temperature at the boiler outlet is generally held at 75⬚F (41.7⬚C) higher than the steam temperature to prevent condensation of the exhaust gas. Steam at 30 lb/in2 (abs) (206.8 kPa) has a temperature of 250.33⬚F (121.3⬚C). Thus, the Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 290. 6.16 POWER GENERATION FIGURE 5 Internal-combustion engine cooling systems: (a) radiator type; (b) evaporating cooling tower; (c) cooling tower. (Power.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 291. INTERNAL-COMBUSTION ENGINES 6.17 FIGURE 6 Slant diagrams for internal-combustion engine heat exchangers. (Power.) exhaust-gas outlet temperature from the boiler will be 250.33 ⫹ 75 ⫽ 325.33⬚F (162.9⬚C), say 325⬚F (162.8⬚C). Then He ⫽ (12,500)(0.252)(750 ⫺ 325) ⫽ 1,375,000 Btu/h (403.0 kW). At 30 lb/in2 (abs) (206.8 kPa), the enthalpy of vaporization of steam is 945.3 Btu/lb (2198.9 kJ/kg), found in the steam tables. Thus, the exhaust heat can gen- erate 1,375,000/945.3 ⫽ 1415 lb/h (636.8 kg/h) if the boiler is 100 percent effi- cient. With a boiler efficiency of 85 percent, the steam generated ⫽ (1415 lb/ h)(0.85) ⫽ 1220 lb/h (549.0 kg/h), or (1200 lb/h)/1000 bhp ⫽ 1.22 lb/(bhp 䡠 h) (0.74 kg/kWh). Related Calculations. Use this procedure for any reciprocating internal- combustion engine burning gasoline, kerosene, natural gas, liquified-petroleum gas, or similar fuel. Figure 1 shows typical arrangements for a number of internal- combustion engine cooling systems. When ethylene glycol or another antifreeze solution is used in the cooling sys- tem, alter the denominator of the flow equation to reflect the change in specific gravity and specific heat of the antifreeze solution, a s compared with water. Thus, with a mixture of 50 percent glycol and 50 percent water, the flow equation in step 2 becomes G ⫽ H/(436⌬t). With other solutions, the numerical factor in the de- nominator will change. This factor ⫽ (weight of liquid lb/gal)(60 min/h), and the factor converts a flow rate of lb/h to gal/min when divided into the lb/h flow rate. Slant diagrams, Fig 6, are often useful for heat-exchanger analysis. Two-cycle engines may have a larger exhaust-gas flow than four-cycle engines because of the scavenging air. However, the exhaust temperature will usually be 50 to 100⬚F (27.7 to 55.6⬚C) lower, reducing the quantity of steam generated. Where a dry exhaust manifold is used on an engine, the heat rejection to the cooling system is reduced by about 7.5 percent. Heat rejected to the aftercooler cooling water is about 3.5 percent of the total heat input to the engine. About 2.5 percent of the total heat input to the engine is rejected by the turbocharger jacket. The jacket cooling water absorbs 11 to 14 percent of the total heat supplied. From 3 to 6 percent of the total heat supplied to the engine is rejected in the oil cooler. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 292. 6.18 POWER GENERATION The total heat supplied to an engine ⫽ (engine output, bhp)[heat rate, Btu/ (bhp 䡠 h)]. A jacket-water flow rate of 0.25 to 0.60 gal/(min 䡠 bhp) (0.02 to 0.05 kg/kW) is usually recommended. The normal jacket-water temperature rise is 10⬚F (5.6⬚C); with a jacket-water outlet temperature of 180⬚F (82.2⬚C) or higher, the temperature rise of the jacket water is usually held to 7⬚F (3.9⬚C) or less. To keep the cooling-water system pressure loss within reasonable limits, some designers recommend a pipe velocity equal to the nominal pipe size used in the system, or 2ft/s for 2-in pipe (0.6 m/s for 50.8-mm); 3 ft/s for 3-in pipe (0.9 m/ s for 76.2-mm); etc. The maximum recommended velocity is 10 ft/s for 10 in (3.0 m/s for 254 mm) and larger pipes. Compute the actual pipe diameter from d ⫽ (G/2.5v)0.5 , where G ⫽ cooling-water flow, gal/min; v ⫽ water velocity, ft/s. Air needed for a four-cycle high-output turbocharged diesel engine is about 3.5 ft3 /(min 䡠 bhp) (0.13 m3 /kW); 4.5 ft3 /(min 䡠 bhp)(0.17 m3 /kW) for two-cycle en- gines. Exhaust-gas flow is about 8.4 ft3 /(min 䡠 bhp) (0.32 m3 /kW) for a four-cycle diesel engine; 13 ft3 /(min 䡠 bhp) (0.49 m3 /kW) for two-cycle engines. Air velocity in the turbocharger blower piping should not exceed 3300 ft/min (1006 m/min); gas velocity in the exhaust system should not exceed 6000 ft/min (1828 m/min). The exhaust-gas temperature should not be reduced below 275⬚F (135⬚C), to prevent condensation. The method presented here is the work of W. M. Kauffman, reported in Power. DESIGN OF A VENT SYSTEM FOR AN ENGINE ROOM A radiator-cooled 60-kW internal-combustion engine generating set operates in an area where the maximum summer ambient temperature of the inlet air is 100⬚F (37.8⬚C). How much air does this engine need for combustion and for the radiator? What is the maximum permissible temperature rise of the room air? How much heat is radiated by the engine-alternator set if the exhaust pipe is 25 ft (7.6 m) long? What capacity exhaust fan is needed for this engine room if the engine room has two windows with an area of 30 ft2 (2.8 m2 ) each, and the average height between the air inlet and the outlet is 5 ft (1.5 m)? Determine the rate of heat dissipation by the windows. The engine is located at sea level. Calculation Procedure: 1. Determine engine air-volume needs Table 4 shows typical air-volume needs for internal-combustion engines installed indoors. Thus, a 60-kW set requires 390 ft3 /min (11.0 m3 /min) for combustion and 6000 ft3 /min (169.9 m3 /min) for the radiator. Note that in the smaller ratings, the combustion air needed is 6.5 ft3 /(min 䡠 kW)(0.18 m3 /kW), and the radiator air re- quirement is 150 ft3 /(min 䡠 kW)(4.2 m3 /kW). 2. Determine maximum permissible air temperature rise Table 4 also shows that with an ambient temperature of 95 to 105⬚F (35 to 40.6⬚C), the maximum permissible room temperature rise is 15⬚C (8.3⬚C). When you deter- mine this value, be certain to use the highest inlet air temperature expected in the engine locality. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 293. INTERNAL-COMBUSTION ENGINES 6.19 TABLE 4 Total Air Volume Needs* TABLE 5 Heat Radiated from Typical Internal-Combustion Units, Btu/min (W)* 3. Determine the heat radiated by the engine Table 5 shows the heat radiated by typical internal-combustion engine generating sets. Thus, a 60-kW radiator-and fan-cooled set radiates 2625 Btu/min (12.8 W) when the engine is fitted with a 25-ft (7.6-m) long exhaust pipe and a silencer. 4. Compute the airflow produced by the windows The two windows can be used to ventilate the engine room. One window will serve as the air inlet; the other, as the air outlet. The area of the air outlet must at least equal the air-inlet area. Airflow will be produced by the stack effect resulting from the temperature difference between the inlet and outlet air. The airflow C ft3 /min resulting from the stack effect is C ⫽ 9.4A(h⌬ta)0.5 , where A ⫽ free air of the air inlet, ft2 ; h ⫽ height from the middle of the air-inlet opening to the middle of the air-outlet opening, ft; ⌬ta ⫽ difference between the average indoor air temperature at point H and the temperature of the incoming air, ⬚F. In this plant, the maximum permissible air temperature rise is 15⬚F (8.3⬚C), from step 2. With a 100⬚F (37.8⬚C) outdoor temperature, the maximum indoor temperature would be 100 ⫹ 15 ⫽ 115⬚F (46.1⬚C). Assume that the difference between the temperature of the incoming and outgoing air is 15⬚F (8.3⬚C). Then C ⫽ 9.4(30)(5 ⫻ 15)0.5 ⫽ 2445 ft3 /min (69.2 m3 /min). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 294. 6.20 POWER GENERATION TABLE 6 Range of Discharge Temperature* 5. Compute the cooling airflow required This 60-kW internal-combustion engine generating set radiates 2625 Btu/min (12.8 W), step 3. Compute the cooling airflow required from C ⫽ HK/⌬ta, where C ⫽ cooling airflow required, ft3 /min; H ⫽ heat radiated by the engine, Btu/min; K ⫽ constant from Table 6; other symbols as before. Thus, for this engine with a fan discharge temperature of 111 to 120⬚F (43.9 to 48.9⬚C), Table 6, K ⫽ 60; ⌬ta ⫽ 15⬚F (8.3⬚C) from step 4. Then C ⫽ (2625)(60)/15 ⫽ 10,500 ft3 /min (297.3 m3 / min). The windows provide 2445 ft3 /min (69.2 m3 /min), step 4, and the engine ra- diator gives 6000 ft3 /min (169.9 m3 /min), step 1, or a total of 2445 ⫹ 6000 ⫽ 8445 ft3 /min (239.1 m3 /min). Thus, 10,500 ⫺ 8445 ⫽ 2055 ft3 /min (58.2 m3 /min) must be removed from the room. The usual method employed to remove the air is an exhaust fan. An exhaust fan with a capacity of 2100 ft3 /min (59.5 m3 /min) would be suitable for this engine room. Related Calculations. Use this procedure for engines burning any type of fuel—diesel, gasoline, kerosene, or gas—in any type of enclosed room at sea level or elevations up to 1000 ft (304.8 m). Where windows or the fan outlet are fitted with louvers, screens, or intake filters, be certain to compute the net free area of the opening. When the radiator fan requires more air than is needed for cooling the room, an exhaust fan is unnecessary. Be certain to select an exhaust fan with a sufficient discharge pressure to over- come the resistance of exhaust ducts and outlet louvers, if used. A propeller fan is usually chosen for exhaust service. In areas having high wind velocity, an axial- flow fan may be needed to overcome the pressure produced by the wind on the fan outlet. Table 6 shows the pressure developed by various wind velocities. When the engine is located above sea level, use the multiplying factor in Table 7 to correct the computed air quantities for the lower air density. An engine radiates 2 to 5 percent of its total heat input. The total heat input ⫽ (engine output, bhp) [heat rate, Btu/(bhp 䡠 h)]. Provide 12 to 20 air changes per hour for the engine room. The most effective ventilators are power-driven exhaust fans or roof ventilators. Where the heat load is high, 100 air changes per hour may be provided. Auxiliary-equipment rooms require 10 air changes per hour. Windows, louvers, or power-driven fans are used. A four-cycle engine requires 3 to 3.5 ft3 / min of air per bhp (0.11 to 0.13 m3 /kW); a two-cycle engine, 4 to 5 ft3 /(min 䡠 bhp) (0.15 to 0.19 m3 /kW). The method presented here is the work of John P. Callaghan, reported in Power. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 295. INTERNAL-COMBUSTION ENGINES 6.21 TABLE 7 Air Density at Various Elevations* FIGURE 7 Engine cooling-system hookup. (Mechanical Engineering.) DESIGN OF A BYPASS COOLING SYSTEM FOR AN ENGINE The internal-combustion engine in Fig. 7 is rated at 402 hp (300 kW) at 514 r/min and dissipates 3500 Btu/(bhp 䡠 h) (1375 W/kW) at full load to the cooling water from the power cylinders and water-cooled exhaust manifold. Determine the re- quired cooling-water flow rate if there is a 10⬚F (5.6⬚C) temperature rise during passage of the water through the engine. Size the piping for the cooling system, using the head-loss data in Fig. 8, and the pump characteristic curve, Fig. 9. Choose a surge tank of suitable capacity. Determine the net positive suction head require- ments for this engine. The total length of straight piping in the cooling system is 45 ft (13.7 m). The engine is located 500 ft (152.4 m) above sea level. Calculation Procedure: 1. Compute the cooling-water quantity required The cooling-water quantity required is G ⫽ H/(500⌬t, where G ⫽ cooling-water flow, gal/min; H ⫽ heat absorbed by the jacket water, Btu/h ⫽ (maximum engine Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 296. 6.22 POWER GENERATION FIGURE 8 Head-loss data for engine cooling-system components. (Mechanical Engineering.) FIGURE 9 Pump and system characteristics for engine cooling system. (Mechanical Engineer- ing.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 297. INTERNAL-COMBUSTION ENGINES 6.23 hp) [heat dissipated, Btu/(bhp 䡠 h)]; ⌬t ⫽ temperature rise of the water during pas- sage through the engine, ⬚F. Thus, for this engine, G ⫽ (402)(3500)/[500(10)] ⫽ 281 gal/min (17.7 L/s). 2. Choose the cooling-system valve and pipe size Obtain the friction head-loss data for the engine, the heat exchanger, and the three- way valve from the manufacturers of the respective items. Most manufacturers have curves or tables available for easy use. Plot the head losses, as shown in Fig. 8, for the engine and heat exchanger. Before the three-way valve head loss can be plotted, a valve size must be chosen. Refer to a three-way valve capacity tabulation to determine a suitable valve size to handle a flow of 281 gal/min (17.7 L/s). Once such tabulation recommends a 3- in (76.2-mm) valve for a flow of 281 gal/min (17.7 L/s). Obtain the head-loss data for the valve, and plot it as shown in Fig. 8. Next, assume a size for the cooling-water piping. Experience shows that a water velocity of 300 to 600 ft/min (91.4 to 182.9 m/min) is satisfactory for internal- combustion engine cooling systems. Using the Hydraulic Institute’s Pipe Friction Manual or Cameron’s Hydraulic Data, enter at 280 gal/min (17.6 L/s), the ap- proximate flow, and choose a pipe size to give a velocity of 400 to 500 ft/min (121.9 to 152.4 m/min), i.e., midway in the recommended range. Alternatively, compute the approximate pipe diameter from d ⫽ 4.95 [gpm/ velocity, ft/min]0.5 . With a velocity of 450 ft/min (137.2 m/min), d ⫽ 4.95(281/ 450)0.5 ⫽ 3.92, say 4 in (101.6 mm). The Pipe Friction Manual shows that the water velocity will be 7.06 ft/s (2.2 m/s), or 423.6 ft/min (129.1 m/min), in a 4- in (101.6 mm) schedule 40 pipe. This is acceptable. Using a 31 ⁄2-in (88.9-mm) pipe would increase the cost because the size is not readily available from pipe suppliers. A 3-in (76.2-mm) pipe would give a velocity of 720 ft/min (219.5 m/min), which is too high. 3. Compute the piping-system head loss Examine Fig. 7, which shows the cooling system piping layout. Three flow con- ditions are possible: (a) all the jacket water passes through the heat exchanger, (b) a portion of the jacket water passes through the heat exchanger, and (c) none of the jacket water passes through the heat exchanger—instead, all the water passes through the bypass circuit. The greatest head loss usually occurs when the largest amount of water passes through the longest circuit (or flow condition a). Compute the head loss for this situation first. Using the method given in the piping section of this handbook, compute the equivalent length of the cooling-system fitting and piping, as shown in Table 8. Once the equivalent length of the pipe and fittings is known, compute the head loss in the piping system, using the method given in the piping section of this handbook with a Hazen-Williams constant of C ⫽ 130 and a rounded-off flow rate of 300 gal/min (18.9 L/s). Summarize the results as shown in Table 8. The total head loss is produced by the water flow through the piping, fittings, engine, three-way valve, and heat exchanger. Find the head loss for the last com- ponents in Fig. 8 for a flow of 300 gal/min (18.9 L/s). List the losses in Table 8, and find the sum of all the losses. Thus, the total circuit head loss is 57.61 ft (17.6 m) of water. Compute the head loss for 0, 0.2, 0.4, 0.6, and 0.8 load on the engine, using the same procedure as in steps 1, 2, and 3 above. Plot on the pump characteristic curve, Fig. 9, the system head loss for each load. Draw a curve A through the points obtained, Fig. 9. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 298. 6.24 POWER GENERATION TABLE 8 Sample Calculation for Full Flow through Cooling Circuit* (Fittings and Piping in Circuit) Compute the system head loss for condition b with half the jacket water [150 gal/min (9.5 L/s)] passing through the heat exchanger and half [150 gal/min (9.5 L/s)] through the bypass circuit. Make the same calculation for 0, 0.2, 0.4, 0.6, and 0.8 load on the engine. Plot the result as curve B, Fig 9. Perform a similar calculation for condition c—full flow through the bypass cir- cuit. Plot the results as curve C, Fig. 9. 4. Compute the actual cooling-water flow rate Find the points of intersection of the pump total-head curve and the three system head-loss curves A, B, and C, Fig. 9. These intersections occur at 314, 325, and 330 gal/min (19.8, 20.5, and 20.8 L/s), respectively. The initial design assumed a 10⬚F (5.6⬚C) temperature rise through the engine with a water flow rate of 281 gal/min (17.7 L/s). Rearranging the equation in step 1 gives ⌬t ⫽ H/(400G). Substituting the flow rate for condition a gives an actual temperature rise of ⌬t ⫽ (402)(3500)/[(500)(314)] ⫽ 8.97⬚F (4.98⬚C). If a 180⬚F Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 299. INTERNAL-COMBUSTION ENGINES 6.25 (82.2⬚C) rated thermostatic element is used in the three-way valve, holding the outlet temperature to to 180⬚F (82.2⬚C), the inlet temperature ti will be ⌬t ⫽ to ⫺ ti ⫽ 8.97; 180 ⫺ ti ⫽ 8.97; ti ⫽ 171.03⬚F (77.2⬚C). 5. Determine the required surge-tank capacity The surge tank in a cooling system provides storage space for the increase in volume of the coolant caused by thermal expansion. Compute this expansion from E ⫽ 62.4g⌬V, where E ⫽ expansion, gal (L); g ⫽ number of gallons required to fill the cooling system; ⌬V ⫽ specific volume, ft3 /lb (m3 /kg) of the coolant at the operating temperature ⫺ specific volume of the coolant, ft3 /lb (m3 /kg) at the filling temperature. The cooling system for this engine must have a total capacity of 281 gal (1064 L), step 1. Round this to 300 gal (1136 L) for design purposes. The system oper- ating temperature is 180⬚F (82.2⬚C), and the filling temperature is usually 60⬚F (15.6⬚C). Using the steam tables to find the specific volume of the water at these temperatures, we get E ⫽62.4(300)(0.01651 ⫺ 0.01604) ⫽ 8.8 gal (33.3 L). Usual design practice is to provide two to three times the required expansion volume. Thus, a 25-gal (94.6-L) tank (nearly three times the required capacity) would be chosen. The extra volume provides for excess cooling water that might be needed to make up water lost through minor leaks in the system. Locate the surge tank so that it is the highest point in the cooling system. Some engineers recommend that the bottom of the surge tank be at least 10 ft (3 m) above the pump centerline and connected as close as possible to the pump intake. A 11 ⁄2- or 2-in (38.1- or 50.8-mm) pipe is usually large enough for connecting the surge tank to the system. The line should be sized so that the head loss of the vented fluid flowing back to the pump suction will be negligible. 6. Determine the pump net positive suction head The pump characteristic curve, Fig 9, shows the net positive suction head (NSPH) required by this pump. As the pump discharge rate increases, so does the NPSH. this is typical of a centrifugal pump. The greatest flow, 330 gal/min (20.8 L/s), occurs in this system when all the coolant is diverted through the bypass circuit, Figs. 4 and 5. At a 330-gal/min (20.8-L/s) flow rate through the system, the required NPSH for this pump is 8 ft (2.4 m), Fig 9. This value is found at the intersection of the 330-gal/min (20.8 L /s) ordinate and the NPSH curve. Compute the existing NPSH, ft (m), from NPSH ⫽ Hs ⫺ Hf ⫹ 2.31(Ps ⫺ Pv)/ s, where Hs ⫽ height of minimum surge-tank liquid level above the pump centerline, ft (m); Hf ⫽ friction loss in the suction line from the surge-tank connection to the pump inlet flange, ft (m) of liquid; Ps ⫽ pressure in surge tank, or atmospheric pressure at the elevation of the installation, lb/in2 (abs) (kPa); Pv ⫽ vapor pressure of the coolant at the pumping temperature, lb /in2 (abs) (kPa); s ⫽ specific gravity of the coolant at the pumping temperature. 7. Determine the operating temperature with a closed surge tank A pressure cap on the surge tank, or a radiator, will permit operation at temperatures above the atmospheric boiling point of the coolant. At a 500-ft (152.4-m) elevation, water boils at 210⬚F (98.9⬚C). Thus, without a closed surge tank fitted with a pres- sure cap, the maximum operating temperature of a water-cooled system would be about 200⬚F (93.3⬚C). If a 7-lb/in2 (gage) (48.3 kPa) pressure cap were used at the 500-ft (152.4-m) elevation, then the pressure in the vapor space of the surge tank could rise to Ps ⫽ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 300. 6.26 POWER GENERATION FIGURE 10 Internal-combustion engine cooling system. 14.4 ⫹ 7.0 ⫽ 21.4 lb/in2 (abs) (147.5 kPa). The steam tables show that water at this pressure boils at 232⬚F (111.1⬚C). Checking the NPSH at this pressure shows that NPSH ⫽ (10 ⫺ 1.02) ⫹ 2.31(21.4 ⫺ 21.4)/0.0954 ⫽ 8.98 ft (2.7 m). This is close to the required 8-ft (2.4-m) head. However, the engine could be safely op- erated at a slightly lower temperature, say 225⬚F (107.2⬚C). 8. Compute the pressure at the pump suction flange The pressure at the pump suction flange P lb/in2 (gage) ⫽ 0.433s(Hs ⫺ Hf) ⫽ (0.433)(0.974)(10.00 ⫺ 1.02) ⫽ 3.79 lb/in2 (gage) (26.1 kPa). A positive pressure at the pump suction is needed to prevent the entry of air along the shaft. To further ensure against air entry, a mechanical seal can be used on the pump shaft in place of packing. Related Calculations. Use this general procedure in designing the cooling sys- tem for any type of reciprocating internal-combustion engine—gasoline, diesel, gas, etc. Where a coolant other than water is used, follow the same procedure but change the value of the constant in the denominator of the equation of step 1. Thus, for a mixture of 50 percent glycol and 50 percent water, the constant ⫽ 436, instead of 500. The method presented here is the work of Duane E. Marquis, reported in Me- chanical Engineering. HOT-WATER HEAT-RECOVERY SYSTEM ANALYSIS An internal-combustion engine fitted with a heat-recovery silencer and a jacket- water cooler is rated at 1000 bhp (746 kW). It exhausts 13.0 lb/(bhp 䡠 h) [5.9 kg/ (bhp 䡠 h)] of exhaust gas at 700⬚F (371.1⬚C). To what temperature can hot water be heated when 500 gal/min (31.5 L/s) of jacket water is circulated through the hookup in Fig. 10 and 100 gal/min (6.3 L/s) of 60⬚F (15.6⬚C) water is heated? The jacket water enters the engine at 170⬚F (76.7⬚C) and leaves at 180⬚F (82.2⬚C). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 301. INTERNAL-COMBUSTION ENGINES 6.27 Calculation Procedure: 1. Compute the exhaust heat recovered Find the exhaust-heat recovered from He ⫽ Wc⌬te, where the symbols are the same as in the previous calculation procedures. Since the final temperature of the exhaust gas is not given, a value must be assumed. Temperatures below 275⬚F (135⬚C) are undesirable because condensation of corrosive vapors in the silencer may occur. Assume that the exhaust-gas outlet temperature from the heat-recovery silencer is 300⬚F (148.9⬚C). The He ⫽ (1000)(13)(0.252)(700 ⫺ 300) ⫽ 1,310,000 Btu/h (383.9 kW). 2. Compute the heated-water outlet temperature from the cooler Using the temperature notation in Fig. 10, we see that the heated-water outlet temperature from the jacket-water cooler is tz ⫽ (wz /w1)(t4 ⫺ t5) ⫹ t1), where w1 ⫽ heated-water flow, lb/h; wz ⫽ jacket-water flow, lb/h; the other symbols are indi- cated in Fig. 10. To convert gal/min of water flow to lb/h, multiply by 500. Thus, w1 ⫽ (100 gal/min)(500) ⫽ 50,000 lb/h (22,500 kg/h), and wz ⫽ (500 gal/ min)(500) ⫽ 250,000 lb/h (112,500 kg/h). Then tz ⫽ (250,000/50,000)(180 ⫺ 170) ⫹ 60 ⫽ 110⬚F (43.4⬚C). 3. Compute the heated-water outlet temperature from the silencer The silencer outlet temperature t3 ⫽ He /w1 ⫹ tz, or t3 ⫽ 1,310,000/50,000 ⫹ 110 ⫽ 136.2⬚F (57.9⬚C). Related Calculations. Use this method for any type of engine—diesel, gaso- line, or gas—burning any type of fuel. Where desired, a simple heat balance can be set up between the heat-releasing and heat-absorbing sides of the system instead of using the equations given here. However, the equations are faster and more direct. DIESEL FUEL STORAGE CAPACITY AND COST A diesel power plant will have six 1000-hp (746-kW) engines and three 600-hp (448-kW) engines. The annual load factor is 85 percent and is nearly uniform throughout the year. What capacity day tanks should be used for these engines? If fuel is delivered every 7 days, what storage capacity is required? Two fuel supplies are available; a 24⬚ API fuel at $0.0825 per gallon ($0.022 per liter) and a 28⬚ API fuel at $0.0910 per gallon ($0.024 per liter). Which is the better buy? Calculation Procedure: 1. Compute the engine fuel consumption Assume, or obtain from the engine manufacturer, the specific fuel consumption of the engine. Typical modern diesel engines have a full-load heat rate of 6900 to 7500 Btu/(bhp 䡠 h) (2711 to 3375 W/kWh), or about 0.35 lb/(bhp 䡠 h) of fuel (0.21 kg/kWh). Using this value of fuel consumption for the nine engines in this plant, we see the hourly fuel consumption at 85 percent load factor will be (6 en- gines)(1000 hp)(0.35)(0.85) ⫹ (3 engines)(600 hp)(0.35)(0.85) ⫽ 2320 lb/h (1044 kg/h). Convert this consumption rate to gal/h by finding the specific gravity of the diesel oil. The specific gravity s ⫽ 141.5/(131.5 ⫹ ⬚API). For the 24⬚ API oil, s ⫽ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 302. 6.28 POWER GENERATION 141.5/(131.5 ⫹ 24) ⫽ 0.910. Since water at 60⬚F (15.6⬚C) weighs 8.33 lb/gal (3.75 kg/L), the weight of this oil is (0.910)(8.33) ⫽ 7.578 lb/gal (3.41 kg/L). For the 28⬚ API oil, s ⫽ 141.5/(131.5 ⫹ 28) ⫽ 0.887, and the weight of this oil is (0.887)(8.33) ⫽ 7.387 lb/gal (3.32 kg/L). Using the lighter oil, since this will give a larger gal/h consumption, we get the fuel rate ⫽ (2320 lb/h)/(7.387 lb/gal) ⫽ 315 gal/h (1192 L/h). The daily fuel consumption is then (24 h/day)(315 gal/h) ⫽ 7550 gal/day (28,577 L/day). In 7 days the engines will use (7 days)(7550 gal/day) ⫽ 52,900, say 53,000 gal (200,605 L). 2. Select the tank capacity The actual fuel consumption is 53,000 gal (200,605 L) in 7 days. If fuel is delivered exactly on time every 7 days, a fuel-tank capacity of 53,000 gal (200,605 L) would be adequate. However, bad weather, transit failures, strikes, or other unpredictable incidents may delay delivery. Therefore, added capacity must be provided to prevent engine stoppage because of an inadequate fuel supply. Where sufficient space is available, and local regulations do not restrict the storage capacity installed, use double the required capacity. The reason is that the additional storage capacity is relatively cheap compared with the advantages gained. Where space or storage capacity is restricted, use 11 ⁄2 times the required capacity. Assuming double capacity is used in this plant, the total storage capacity will be (2)(53,000) ⫽ 106,000 gal (401,210 L). At least two tanks should be used, to permit cleaning of one without interrupting engine operation. Consult the National Board of Fire Underwriters bulletin Storage Tanks for Flammable Liquids for rules governing tank materials, location, spacing, and fire- protection devices. Refer to a tank capacity table to determine the required tank diameter and length or height depending on whether the tank is horizontal or ver- tical. Thus, the Buffalo Tank Corporation Handbook shows that a 16.5-ft (5.0-m) diameter 33.5-ft (10.2-m) long horizontal tank will hold 53,600 gal (202,876 L) when full. Two tanks of this size would provide the desired capacity. Alternatively, a 35-ft (10.7-m) diameter 7.5-ft (2.3-m) high vertical tank will hold 54,000 gal (204,390 L) when full. Two tanks of this size would provide the desired capacity. Where a tank capacity table is not available, compute the capacity of a cylin- drical tank from capacity ⫽ 5.87D2 L, where D ⫽ tank diameter, ft; L ⫽ tank length or height, ft. Consult the NBFU or the tank manufacturer for the required tank wall thickness and vent size. 3. Select the day-tank capacity Day tanks supply filtered fuel to an engine. The day tank is usually located in the engine room and holds enough fuel for a 4- to 8-h operation of an engine at full load. Local laws, insurance requirements, or the NBFU may limit the quantity of oil that can be stored in the engine room or a day tank. One day tank is usually used for each engine. Assume that a 4-h supply will be suitable for each engine. Then the day tank capacity for a 1000-hp (746-kW) engine ⫽ (1000 hp) [0.35 lb/(bhp 䡠 h) fuel] (4 h) ⫽ 1400 lb (630 kg), or 1400/7.387 ⫽ 189.6 gal (717.6 L), given the lighter-weight fuel, step 1. Thus, one 200-gal (757-L) day tank would be suitable for each of the 1000-hp (746-kW) engines. For the 600-hp (448-kW) engines, the day-tank capacity should be (600 hp)[0.35 lb/(bhp 䡠 h) fuel](4 h) ⫽ 840 lb (378 kg), or 840/7.387 ⫽ 113.8 gal (430.7 L). Thus, one 125-gal (473-L) day tank would be suitable for each of the 600-hp (448- kW) engines. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 303. INTERNAL-COMBUSTION ENGINES 6.29 4. Determine which is the better fuel buy Compute the higher heating value HHV of each fuel from HHV ⫽ 17,645 ⫹ 54(⬚API), or for 24⬚ fuel, HHV ⫽ 17,645 ⫹ 54(24) ⫽ 18,941 Btu/lb (44,057 kJ/ kg). For the 28⬚ fuel, HHV ⫽ 17,645 ⫹ 54(28) ⫽ 19,157 Btu/lb (44,559 kJ/kg). Compare the two oils on the basis of cost per 10,000 Btu (10,550 kJ), because this is the usual way of stating the cost of a fuel. The weight of each oil was computed in step 1. Thus the 24⬚ API oil weighs 7,578 lb/gal (0.90 kg/L), while the 28⬚ API oil weighs 7.387 lb/gal (0.878 kg/L). Then the cost per 10,000 Btu (10,550 kJ) ⫽ (cost, $/gal)/[HHV, Btu/lb)/ 10,000](oil weight, lb/gal). For the 24⬚ API oil, cost per 10,000 Btu (10,550 kJ) ⫽ (cost, $/gal)/[(HHV, Btu/lb)/10,000](oil weight, lb/gal). For the 24⬚ API oil, cost per 10,000 Btu (10,550 kJ) ⫽ $0.0825/[(18.941/10,000)(7.578)] ⫽ $0.00574, or 0.574 cent per 10,000 Btu (10,550 kJ). For the 28⬚ API oil, cost per 10,000 Btu ⫽ $0.0910/[(19,157/10,000)(7387)] ⫽ $0.00634, or 0.634 cent per 10,000 Btu (10,550 kJ). Thus, the 24⬚ API is the better buy because it costs less per 10,000 Btu (10,550 kJ). Related Calculations. Use this method for engines burning any liquid fuel. Be certain to check local laws and the latest NBFU recommendations before ordering fuel storage or day tanks. Low-sulfur diesel amendments were added to the federal Clean Air Act in 1991. These amendments required diesel engines to use low-sulfur fuel to reduce atmos- pheric pollution. Reduction of fuel sulfur content will not require any change in engine operating procedures. If anything, the lower sulfur content will reduce en- gine maintenance requirements and costs. The usual distillate fuel specification recommends a sulfur content of not more than 1.5 percent by weight, with 2 percent by weight considered satisfactory. Re- fineries are currently producing diesel fuel that meets federal low-sulfur require- ments. While there is a slight additional cost for such fuel at the time of this writing, when the regulations went into effect, predictions are that the price of low-sulfur fuel will decline as more is manufactured. Automobiles produce 50 percent of the air pollution throughout the developed world. The Ozone Transport Commission, set up by Congress as part of the 1990 Clear Air Act, is enforcing emission standards for new automobiles and trucks. To date, the cost of meeting such standards has been lower than anticipated. By the year 2003, all new automobiles will be pollution-free—if they comply with the requirements of the act. Stationary diesel plants using low-sulfur fuel will emit extremely little pollution. POWER INPUT TO COOLING-WATER AND LUBE-OIL PUMPS What is the required power input to a 200-gal/min (12.6-L/s) jacket-water pump if the total head on the pump is 75 ft (22.9 m) of water and the pump has an efficiency of 70 percent when it handles freshwater and saltwater? What capacity lube-oil pump is needed for a four-cycle 500-hp (373-kW) turbocharged diesel engine having oil-cooled pistons? What is the required power input to this pump if the discharge pressure is 80 lb/in2 (551.5 kPa) and the efficiency of the pump is 68 percent? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 304. 6.30 POWER GENERATION Calculation Procedure: 1. Determine the power input to the jacket-water pump The power input to jacket-water and raw-water pumps serving internal-combustion engines is often computed from the relation hp ⫽ Gh/Ce, where hp ⫽ hp input; G ⫽ water discharged by pump, gal/min; h ⫽ total head on pump, ft of water; C ⫽ constant ⫽ 3960 for freshwater having a density of 62.4 lb/ft3 (999.0 kg/m3 ); 3855 for saltwater having a density of 64 lb/ft3 (1024.6 kg/m3 ). For this pump handling freshwater, hp ⫽ (200)(75)/(3960)(0.70) ⫽ 5.42 hp (4.0 kW). A 7.5-hp (5.6-kW) motor would probably be selected to handle the rated capacity plus any overloads. For this pump handling saltwater, hp ⫽ (200)(75/[(3855)(0.70)] ⫽ 5.56 hp (4.1 kW). A 7.5-hp (5.6-kW) motor would probably be selected to handle the rated capacity plus any overloads. Thus, the same motor could drive this pump whether it handles freshwater or saltwater. 2. Compute the lube-oil pump capacity The lube-oil pump capacity required for a diesel engine is found from G ⫽ H/ 200⌬t, where G ⫽ pump capacity, gal/min; H ⫽ heat rejected to the lube oil, Btu/(bhp 䡠 h); ⌬t ⫽ lube-oil temperature rise during passage through the engine, ⬚F. Usual practice is to limit the temperature rise of the oil to a range of 20 to 25⬚F (11.1 to 13.9⬚C), with a maximum operating temperature of 160⬚F (71.1⬚C). The heat rejection to the lube oil can be obtained from the engine heat balance, the engine manufacturer, or Standard Practices for Stationary Diesel Engines, pub- lished by the Diesel Engine Manufacturers Association. With a maximum heat rejection rate of 500 Btu/(bhp 䡠 h) (196.4 W/kWh) from Standard Practices and an oil-temperature rise of 20⬚F (11.1⬚C), G ⫽ [500 Btu/(bhp 䡠 h)](1000 hp)/ [(200)(20)] ⫽ 125 gal/min (7.9 L/s). By using the lowest temperature rise and the highest heat rejection rate, a safe pump capacity is obtained. Where the pump cost is a critical factor, use a higher temperature rise and a lower heat rejection rate. Thus, with a heat rejection, the above pump would have a capacity of G ⫽ (300)(1000)/[(200)(25)] ⫽ 60 gal/min (3.8 L/s). 3. Compute the lube-oil pump power input The power input to a separate oil pump serving a diesel engine is given by hp ⫽ Gp/1720e, where G ⫽ pump discharge rate, gal/min; p ⫽ pump discharge pressure, lb/in2 , e ⫽ pump efficiency. For this pump, hp ⫽ (125)(80)/[(1720)(0.68)] ⫽ 8.56 hp (6.4 kW). A 10-hp (7.5-kW) motor would be chosen to drive this pump. With a capacity of 60 gal/min (3.8 L/s), the input is hp ⫽ (60)(80)/ [(1720)(0.68)] ⫽ 4.1 hp (3.1 kW). A 5-hp (3.7-kW) motor would be chosen to drive this pump. Related Calculations. Use this method for any reciprocating diesel engine, two- or four-cycle. Lube-oil pump capacity is generally selected 10 to 15 percent oversize to allow for bearing wear in the engine and wear of the pump moving parts. Always check the selected capacity with the engine builder. Where a bypass- type lube-oil system is used, be sure to have a pump of sufficient capacity to handle both the engine and cooler oil flow. Raw-water pumps are generally duplicates of the jacket-water pump, having the same capacity and head ratings. Then the raw-water pump can serve as a standby jacket-water pump, if necessary. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 305. INTERNAL-COMBUSTION ENGINES 6.31 LUBE-OIL COOLER SELECTION AND OIL CONSUMPTION A 500-hp (373-kW) internal-combustion engine rejects 300 to 600 Btu/(bhp 䡠 h) (118 to 236 W/kWh) to the lubricating oil. What capacity and type of lube-oil cooler should be used for this engine if 10 percent of the oil is bypassed? If this engine consumes 2 gal (7.6 L) of lube oil per 24 h at full load, determine its lube- oil consumption rate. Calculation Procedure: 1. Determine the required lube-oil cooler capacity Base the cooler capacity on the maximum heat rejection rate plus an allowance for overloads. The usual overload allowance is 10 percent of the full-load rating for periods of not more than 2 h in any 24 h period. For this engine, the maximum output with a 10 percent overload is 500 ⫹ (0.10)(500) ⫽ 550 hp (410 kW). Thus, the maximum heat rejection to the lube oil would be (500 hp)[600 Btu/(bhp 䡠 h)] ⫽ 330,000 Btu/h (96.7 kW). 2. Choose the type and capacity of lube-oil cooler Choose a shell-and-tube type heat exchanger to serve this engine. Long experience with many types of internal-combustion engines shows that the shell-and-tube heat exchanger is well suited for lube-oil cooling. Select a lube-oil cooler suitable for a heat-transfer load of 330,000 Btu/h (96.7 kW) at the prevailing cooling-water temperature difference, which is usually as- sumed to be 10⬚F (5.6⬚C). See previous calculation procedures for the steps in selecting a liquid cooler. 3. Determine the lube-oil consumption rate The lube-oil consumption rate is normally expressed in terms of bhp 䡠 h/gal. Thus, if this engine operates for 24 h and consumes 2 gal (7.6 L) of oil, its lube-oil consumption rate ⫽ (24 h)(500 bhp)/2 gal ⫽ 6000 bhp 䡠 h/gal (1183 kWh/L). Related Calculations. Use this procedure for any type of internal-combustion engine using any fuel. QUANTITY OF SOLIDS ENTERING AN INTERNAL-COMBUSTION ENGINE What weight of solids annually enters the cylinders of a 1000-hp (746-kW) internal- combustion engine if the engine operates 24 h/day, 300 days/year in an area having an average dust concentration of 1.6 gr per 1000 ft3 of air (28.3 m3 )? The engine air rate (displacement) is 3.5 ft3 /(min 䡠 bhp) (0.13 m3 /kW). What would the dust load be reduced to if an air filter fitted to the engine removed 80 percent of the dust from the air? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 306. 6.32 POWER GENERATION Calculation Procedure: 1. Compute the quantity of air entering the engine Since the engine is rated at 1000 hp (746 kW) and uses 3.5 ft3 /(min 䡠 bhp) [0.133 m3 /(min 䡠 kW)], the quantity of air used by the engine each minute is (1000 hp)[3.5 ft3 /(min 䡠 hp)] ⫽ 3500 ft3 /min (99.1 m3 /min). 2. Compute the quantity of dust entering the engine Each 1000 ft3 (28.3 m3 ) of air entering the engine contains 1.6 gr (103.7 mg) of dust. Thus, during every minute of engine operation, the quantity of dust entering the engine is (3500/1000)(1.6) ⫽ 5.6 gr (362.8 mg). The hourly dust intake ⫽ (60 min/h)(5.6 gr/min) ⫽ 336 gr/h (21,772 mg/h). During the year the engine operates 24 h/day for 300 days. Hence, the annual intake of dust is (24 h/day)(300 days/year)(336 gr/h) ⫽ 2,419,200 gr (156.8 kg). Since there is 7000 gr/lb, the weight of dust entering the engine per year ⫽ 2,419,200 gr/(7000 gr/lb) ⫽ 345.6 lb/year (155.5 kg/year). 3. Compute the filtered dust load With the air filter removing 80 percent of the dust, the quantity of dust reaching the engine is (1.00⫺ 0.80)(345.6 lb/year) ⫽ 69.12 lb/year (31.1 kg/year). This shows the effectiveness of an air filter in reducing the dust and dirt load on an engine. Related Calculations. Use this general procedure to compute the dirt load on an engine from any external source. INTERNAL-COMBUSTION ENGINE PERFORMANCE FACTORS Discuss and illustrate the important factors in internal-combustion engine selection and performance. In this discussion, consider both large and small engines for a full range of usual applications. Calculation Procedure: 1. Plot typical engine load characteristics Figure 11 shows four typical load patterns for internal-combustion engines. A con- tinuous load, Fig. 11a, is generally considered to be heavy-duty and is often met in engines driving pumps or electric generators. Intermittent heavy-duty loads, Fig. 11b, are often met in engines driving concrete mixers, batch machines, and similar loads. Variable heavy-duty loads, Fig. 11c, are encountered in large vehicles, process machinery, and similar applications. Variable light-duty loads, Fig. 11d, are met in small vehicles like golf carts, lawn mowers, chain saws, etc. 2. Compute the engine output torque Use the relation T ⫽ 5250 bhp/(r/min) to compute the output torque of an internal- combustion engine. In this relation, bhp ⫽ engine bhp being developed at a crank- shaft speed having rotating speed of rpm. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 307. INTERNAL-COMBUSTION ENGINES 6.33 FIGURE 11 Typical internal-combustion engine load cycles: (a) continuous, heavy-duty; (b) intermittent, heavy-duty; (c) variable, heavy-duty; (d) variable, light-duty. (Product Engineering.) 3. Compute the hp output required Knowing the type of load on the engine (generator, pump, mixer, saw blade, etc.), compute the power output required to drive the load at a constant speed. Where a speed variation is expected, as in variable-speed drives, compute the average power needed to accelerate the load between two desired speeds in a given time. 4. Choose the engine output speed Internal-combustion engines are classified in three speed categories: high (1500 r/ min or more), medium (750 to 1500 r/min), and low (less than 750 r/min). Base the speed chosen on the application of the engine. A high-speed engine can be lighter and smaller for the same hp rating, and may cost less than a medium- speed or slow-speed engine serving the same load. But medium-speed and slow- speed engines, although larger, offer a higher torque output for the equivalent hp rating. Other advantages of these two speed ranges include longer service life and, in some instances, lower maintenance costs. Usually an application will have its own requirements, such as allowable engine weight, available space, output torque, load speed, and type of service. These re- quirements will often indicate that a particular speed classification must be used. Where an application has no special speed requirements, the speed selection can be made on the basis of cost (initial, installation, maintenance, and operating costs), type of parts service available, and other local conditions. 5. Analyze the engine output torque required In some installations, an engine with good lugging power is necessary, especially in tractors, harvesters, and hoists, where the load frequently increases above normal. For good lugging power, the engine should have the inherent characteristic of in- creasing torque with drooping speed. The engine can then resist the tendency for increased load to reduce the output speed, giving the engine good lugging qualities. One way to increase the torque delivered to the load is to use a variable-ratio hydraulic transmission. The transmission will amplify the torque so that the engine will not be forced into the lugging range. Other types of loads, such as generators, centrifugal pumps, air conditioners, and marine drives, may not require this lugging ability. So be certain to consult the engine power curves and torque characteristic curve to determine the speed at which the maximum torque is available. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 308. 6.34 POWER GENERATION 6. Evaluate the environmental conditions Internal-combustion engines are required to operate under a variety of environmen- tal conditions. The usual environmental conditions critical in engine selection are altitude, ambient temperature, dust or dirt, and special or abnormal service. Each of these, except the last, is considered in previous calculation procedures. Special or abnormal service includes such applications as fire fighting, emer- gency flood pumps and generators, and hospital standby service. In these applica- tions, an engine must start and pick up a full load without warmup. 7. Compare engine fuels Table 9 compares four types of fuels and the internal-combustion engines using them. Note that where the cost of the fuel is high, the cost of the engine is low; where the cost of the fuel is low, the cost of the engine is high. This condition prevails for both large and small engines in any service. 8. Compare the performance of small engines Table 10 compares the principal characteristics of small gasoline and diesel engines rated at 7 hp (5 kW) or less. Note that engine life expectancy can vary from 500 to 25,000 h. With modern, mass-produced small engines it is often just as cheap to use short-life replaceable two-stroke gasoline engines instead of a single long- life diesel engine. Thus, the choice of a small engine is often based on other con- siderations, such as ease and convenience of replacement, instead of just hours of life. Chances are, however, that most long-life applications of small engines will still require a long-life engine. But the alternative must be considered in each case. Related Calculations. Use the general data presented here for selecting inter- nal-combustion engines having ratings up to 200 hp (150 kW). For larger engines, other factors such as weight, specific fuel consumption, lube-oil consumption, etc., become important considerations. The method given here is the work of Paul F. Jacobi, as reported in Product Engineering. VOLUMETRIC EFFICIENCY OF DIESEL ENGINES A four-cycle six-cylinder Diesel engine of 4.25-in (11.4-cm) bore and 60-in (15.2- cm) stroke running at 1200 rpm has 9 percent CO2 present in the exhaust gas. The fuel consumption is 28 lb (12.7 kg) per hour. Assuming that 13.7 percent CO2 indicates an air-fuel ratio of 15 lb of air to 1 lb (6.6 kg to 0.45 kg), calculate the volumetric efficiency of the engine. Intake air temperature is 60⬚F (15.6⬚C) and the barometric pressure is 29.8 in (79.7 cm). Calculation Procedure: 1. Find the percentage of N2 in the exhaust gas Atmospheric air contains 76.9 percent nitrogen by weight. If an analysis of the fuel oil shows zero nitrogen before combustion, all the nitrogen in the exhaust gas must come from the air. Therefore, with 13.7 percent CO2 by volume in the dry exhaust the nitrogen content is: N2 ⫽ (76.9/100)(15) ⫽ 11.53 lb (5.2 kg) N2 per lb (0.454 kg) of fuel oil. Converting to moles, 11.53 lb (5.2 kg) N2 /28 lb (12.7 kg) fuel per hour ⫽ 0.412 mole N2 per lb (0.454 kg) of fuel oil. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 309. 6.35 TABLE 9 Comparison of Fuels for Internal-Combustion Engines* Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 310. 6.36 TABLE 10 Performance Table for Small Internal-Combustion Engines [Less than 7 hp (5 kW)]* Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 311. INTERNAL-COMBUSTION ENGINES 6.37 2. Compute the weight of N2 in the exhaust Use the relation, percentage of CO2 in the exhaust gases ⫽ (CO2)/(N2 ⫹ CO2) in moles. Substituting, (13.7)/(100) ⫽ (CO2)/(N2 ⫹ 0.412). Solving for CO2 we find CO2 ⫽ 0.0654 mole. Now, since mole percent is equal to volume percent, for 9 percent CO2 in the exhaust gases, 0.09 ⫽ (CO2)/(CO2 ⫹ N2) ⫽ 0.0654/(0.0654 ⫹ N2). Solving for N2, we find N2 ⫽ 0.661 mole. The weight of N2 therefore ⫽ 0.661 ⫻ 28 ⫽ 18.5 lb (8.399 kg). 3. Calculate the amount of air required for combustion The air required for combustion is found from (N2) ⫽ 18.5/0.769, where 0.769 ⫽ percent N2 in air, expressed as a decimal. Solving, N2 ⫽ 24.057 lb (10.92 kg) per lb (0.454 kg) of fuel oil. 4. Find the weight of the actual air charge drawn into the cylinder Specific volume of the air at 60⬚F (15.6⬚C) and 29.8 in (75.7 cm) Hg is 13.03 ft3 (0.368 m3 ) per lb. Thus, the actual charge drawn into the cylinder ⫽ (lb of air per lb of fuel)(specific volume of the air, ft3 /lb)(fuel consumption, lb/h)/3600 s/h. Or 24.1(13.02)(28)/3600 ⫽ 2.44 ft3 (0.69 m3 ) per second. 5. Compute the volumetric efficiency of this engine Volumetric efficiency is defined as the ratio of the actual air charge drawn into the cylinder divided by the piston displacement. The piston displacement for one cyl- inder of this engine is (bore area)(stroke length)(1 cylinder)/1728 in3 /ft3 . Solving, piston displacement ⫽ 0.785(4.25)2 (6)(1)/1728 ⫽ 0.0492 ft3 (0.00139 m3 ). The number of suction strokes per minute ⫽ rpm/2. The volume displaced per second by the engine ⫽ (piston displacement per cylinder)(number of cylinders)(rpm/2)/60 s/min. Substituting, engine displacement ⫽ 0.0492(6)(1200/ 2)/60 ⫽ 2.952 ft3 /s (0.0084 m3 /s). Then, the volumetric efficiency of this engine ⫽ actual charge drawn into the cylinder/engine displacement ⫽ 2.45/2.952 ⫽ 0.8299, or 82.99 percent. Related Calculations. Use this general procedure to determine the volumetric efficiency of reciprocating internal-combustion engines—both gasoline and Diesel. The procedure is also used for determining the fuel consumption of such engines, using test data from actual engine runs. SELECTING AIR-COOLED ENGINES FOR INDUSTRIAL APPLICATIONS Choose a suitable air-cooled gasoline engine to replace a 10-hp (7.46 kW) electric motor driving a municipal service sanitary pump at an elevation of 8000 ft (2438 m) where the ambient temperature is 90⬚F (32.2⬚C). Find the expected load duty for this engine; construct a typical load curve for it. Calculation Procedure: 1. Determine the horsepower (kW) rating required of the engine Electric motors are rated on an entirely different basis than are internal-combustion engines. Most electric motors will deliver 25 percent more power than their rating Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 312. 6.38 POWER GENERATION during a period of one or two hours. For short periods many electric motors may carry 50 percent overload. Gasoline engines, by comparison, are rated at the maximum power that a new engine will develop on a dynamometer test conducted at an ambient temperature of 60⬚F (15.6⬚C) and a sea-level barometric pressure of 29.92 in (759.97 mm) of mercury. For every 10⬚F (5.56⬚C) rise in the intake ambient air temperature there will be a 1 percent reduction in the power output. And for every 1-in (2.5-cm) drop in barometric pressure there will be a 3.5 percent power output loss. For every 1000 ft (304.8 m) of altitude above sea level a 3.5 percent loss in power output also occurs. Thus, for average atmospheric conditions, the actual power of a gasoline engine is about 5 to 7 percent less than the standard rating. And if altitude is a factor, the loss can be appreciable, reaching 35 percent at 10,000 ft (3048 m) altitude. Also, in keeping with good industrial practice, a gasoline engine is not generally operated continuously at maximum output. This practice provides a factor of safety in the form of reserve power. Most engine manufacturers recommend that this factor of safety be 20 to 25 percent below rated power. This means that the engine will be normally operated at 75 to 80 percent of its standard rated output. The duty cycle, however, can vary with different applications, as Table 11 shows. For the 10-hp (7.46 kW) electric motor we are replacing with a gasoline engine, the motor can deliver—as discussed—25 percent more than its rating, or in this instance, 12.5 hp (9.3 kW) for short periods. On the basis that the gasoline engine is to operate at not over 75 percent of its rating, the replacement engine should have a rating of 12.5/0.75 ⫽ 16.7 hp (12.4 kW). In summary, the gasoline engine should have a rating at least 67 percent greater than the electric motor it replaces. This applies to both air- and liquid-cooled en- gines for sea-level operation under standard atmospheric conditions. If the engine is to operate at altitude, a further allowance must be made, resulting—in some instances—in an engine having twice the power rating of the electric motor. 2. Find the power required at the installed altitude and inlet-air temperature As noted above, altitude and inlet-air temperature both influence the required rating of a gasoline engine for a given application. Since this engine will be installed at an altitude of 8000 ft (2438 m), the power loss will be (8000/1000)(3.5) ⫽ 28 percent. Further, the increased inlet-air temperature of 90⬚F (32.2⬚C) vs. the standard of 60⬚F (15.6⬚C), or a 30⬚ difference will reduce the power output by (30/ 10)(1.0) ⫽ 3.0 percent. Thus, the total power output reduction will be 28 ⫹ 3 ⫽ 31 percent. Therefore, the required rating of this gasoline engine will be at least (1.31)(16.7) ⫽ 21.87 hp (16.3 kW). Once the power requirements of a design are known, the next consideration is engine rotative speed, which is closely related to the horsepower and service life. Larger engines with their increased bearing surfaces and lower speeds, naturally require less frequent servicing. Such engines give longer, more trouble-free life than the smaller, high-speed engines of the same horsepower (kW) rating. The initial cost of a larger engine is greater but more frequent servicing can easily bring the cost of a smaller engine up to that of the larger one. Conversely, the smaller, higher-speed engine has advantages where lighter weight and smaller installation dimensions are important, along with a relatively low first cost. Torque is closely associated with engine rotative speed. For most installations an engine with good lugging power is desirable, and in some installations, essential. This is especially true in tractors, harvesters, and hoists, where the load frequently increases considerably above normal. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 313. 6.39 TABLE 11 Duty Ratings for Combustion Engine Application 100 80 60 40 20 0 Load, percent Time 100 80 60 40 20 0 Load, percent Time 100 80 60 40 20 0 Load, percent Time 100 80 60 40 20 0 Load, percent Time (A) — Continuous Heavy Duty (B) — Typical Intermittent Heavy Duty (C) — Typical Variable Load—Heavy (D) — Typical Variable Load—Light Key: 1—Continuous Duty 2—Intermittant heavy duty 3—Variable load duty, heavy 4—Variable load duty, light INDUSTRIAL SERVICE 1—Standby units 3—Air compressors 3—Floor sanders 4—Shop trucks and welders MUNICIPAL SERVICE 3—Street sweepers and flushers 3—Sanitary pumps 3—Pipe thawing rigs 4—Diesel starting units MINING 3—Horizontal diamond drills 3—Rocker shovels RAILWAY MAINTENANCE 3—Tampers 3—Tie adzing machines 3—Railway maintenance cars 3—Rail grinders 3—Weed cutters 4—Rail leveling machines HIGHWAY MAINTENANCE 1—Road rollers 1—Bituminous sprayers 2—Concrete cutters OIL FIELD EQUIPMENT 1—Well drills and pumps 1—High pressure pumps 2—Pipe wrapping machines 3—Pipe straightening machines AGRICULTURAL EQUIPMENT 1—Irrigation pumps 3—Combine harvesters 3—Hay balers, tractors 3—Insecticide sprayers 3—Rotary tillers 3—Potato harvesters 3—Mowers 3—Spreaders, dusters MARINE 1—Lighthouse units 1—Water oxygenation units 3—Inboard marine engines 3—Underwater weed cutters CONTRUCTION MACHINERY 1—Centrifugal pumps 2—Concrete mixers 3—Concrete vibrators 3—Concrete surfacing machines CONTRUCTION MACHINERY (Cont.) 3—Diaphragm pumps 4—Hoists and power saws SPECIALIZED SERVICE 1—Airport service units such as air compressors, hydraulic pumps and generators 1—Weed burners 2—Refrigerated trucks 2—Paint sprayers 2—Portable fire fighting equipment 3—Miniature railways 3—Water purification units for armed forces 3—Cable reelers 3—Lawn mowers and rollers 3—Post peelers 3—Portable showers for armed forces Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 314. 6.40 POWER GENERATION If the characteristics of the engine output curve are such that the torque will increase with reducing engine speed, the tendency for the increasing load to reduce engine speed is resisted and the engine will ‘‘hang on.’’ In short, it will have good lugging qualities, as shown in Fig. 12a. If the normal operating speed of the engine is 2000 to 2200 r/min, the maximum lugging qualities will result. Sanitary-pump drives do not—in general—require heavy lugging. If, however, with the same curve, Fig. 12a, the normal operating speed of the engine is held at 1400 r/min or below, stalling of the engine may occur easily when the load is increased. Such an increase will cause engine speed to reduce, resulting in a decrease in torque and causing further reduction in speed until the engine finally stalls abruptly, unless the load can be quickly released. Figure 12b shows performance curves for a typical high-speed engine with max- imum power output at top speed. The torque curve for this engine is flat and the engine is not desirable for industrial or agricultural type installations. 3. Determine the duty rating; draw a load curve for the engine Refer to Table 11 for municipal service. There you will see that sanitary pumps have a variable load, heavy duty rating. Figure 12 shows a plot of the typical load variation in such an engine when driving a sanitary pump in municipal service. 4. Select the type of drive for the engine A variety of power takeoffs are used for air-cooled gasoline engines, Fig. 13. For a centrifugal pump driven by a gasoline engine, a flange coupling is ideal. The same is true for engines driving electric generators. Both the pump and generator run at engine speed. When a plain-flange coupling is used, the correct alignment of the gasoline engine and driven machine is extremely important. Flexible cou- plings and belt drives eliminate alignment problems. In many instances a clutch is required between the engine and equipment so that the power may be engaged or disengaged at will. A manually engaged clutch is the most common type in use on agricultural and industrial equipment. Where automatic engagement and disengagement are desired, a centrifugal clutch may be used. These clutches can be furnished to engage at any speed be- tween 500 and 1200 r/min and the load pick-up is smooth and gradual. Typical applications for such clutches are refrigerating machines with thermostatic control for starting and stopping the engine. Clutches also make starting of the engine easier. It is often impossible to start an internal-combustion engine rigidly connected to the load. There are many applications where a speed reduction between the engine and machine is necessary. If the reduction is not too great, it may be accomplished by belt drive. But often a gear reduction is preferable. Gear reductions can be furnished in ratios up to 4 for larger engines, and up to 6 for smaller sizes. Many of these reductions can be furnished in either enginewise or counter-enginewise rotation, and either with or without clutches. Related Calculations. Table 11 shows 54 different applications and duty rat- ings for small air-cooled gasoline engines. With this information the engineer has a powerful way to make a sensible choice of engine, drive, speed, torque, and duty cycle. Important factors to keep in mind when choosing small internal-combustion engines for any of the 54 applications shown are: (1) Engines should have sufficient capacity to ensure a factor of safety of 20 to 25 percent for the power output. (2) Between high- and low-speed engines, the latter have longer life, but first cost is higher. (3) In take-off couplings, the flexible types are preferred. (4) A clutch is Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 315. INTERNAL-COMBUSTION ENGINES 6.41 35 30 25 20 15 1100 1050 1000 950 900 850 Horsepower Torque, inch-lb. 1,000 1,400 Speed, rpm 1,800 2,200 (a) 35 30 25 20 15 950 900 800 Horsepower Torque, inch-lb. 1,000 1,400 Speed, rpm 1,800 2,200 (b) SI Values hp kW in.-lb Nm 15 11.2 850 96.1 20 14.9 900 101.7 25 18.7 950 107.4 30 22.4 1000 113.0 35 26.1 1050 118.7 FIGURE 12 Torque curves for typical air-cooled internal-combustion engines. (a) Engine with good lugging quality will ‘‘hang on’’ as load increases. (b) Performance curve for a high-speed engine with maximum power output at top speed. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 316. 6.42 POWER GENERATION Flange coupling Flexible coupling Reduction gear without clutch Reduction gear with clutch Manually engaged clutch Centrifugal clutch with V belt drive V belt sheave FIGURE 13 Power take-offs for air-cooled engines. Fluid couplings are also used to cushion shock loads in certain specialized applications. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 317. INTERNAL-COMBUSTION ENGINES 6.43 desirable, especially in heavier equipment, to disconnect the load and to make engine starting easier. (5) In operations where the intake air is dusty or contains chaff, intake screens should be used. (6) An oil-bath type air cleaner should always be used ahead of the carburetor. (7) Design engine mountings carefully and locate them to avoid vibration. (8) Provide free flow of cooling air to the flywheel fan inlet and also to the hot-air outlet from the engine. Carefully avoid recirculation of the hot air by the flywheel. (9) If the engine operation is continuous and heavy, Stellite exhaust valves and valve-seat inserts should be used to ensure long life. Valve rotators are also of considerable value in prolonging valve life, and with Stellite valves, constitute an excellent combination for heavy service. Exclusive of aircraft, air-cooled engines are usually applied in size ranges from 1 to 30 hp (0.75 to 22.4 kW). Larger engines are being built and, depending on the inherent cooling characteristics of the system, performing satisfactorily. How- ever, the bulk of applications are on equipment requiring about 30 hp (22.4 kW), or less. The smaller engines up to about 8 or 9 hp (5.9 to 6.7 kW) are usually single-cylinder types; from 8 to 15 hp (5.9 to 11.2 kW) two-cylinder engines are prevalent, while above 15 hp (11.2 kW), four-cylinder models are commonly used. Within these ranges, air-cooled engines have several inherent advantages: they are light-weight, with weight varying from about 14 to 20 lb/hp (8.5 to 12.2 kg/ kW) for a typical single-cylinder engine operating at 2600 r/min to about 12 to 15 lb/h (7.3 to 9.1 kg/kW) for a typical four-cylinder unit running at 1800 r/min. Auxiliary power requirements for these engines are low since there is no radiator fan or water pump; there is no danger of the engine boiling or freezing, and no maintenance of fan bearings, or water pumps; and first cost is low. In selecting an engine of this type, the initial step is to determine the horsepower requirements of the driven load. On equipment of entirely new design, it is often difficult to ascertain the amount of power necessary. In such instances, a rough estimate of the horsepower range (kW range) is made and one or more sample engines bracketing the range obtained for use on experimental models of the equipment. In other applications, it is possible to calculate the torque required, from which the horsepower (kW) can be deter- mined. Or, as is not uncommon, the new piece of equipment may be another size in a line of machines. In this case, the power determination can be made on a proportional basis. This procedure is the work of A. F. Milbrath, Vice President and Chief Engineer, Wisconsin Motor Corporation, as reported in Product Engineering magazine. SI values were added by the Handbook editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 318. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. INTERNAL-COMBUSTION ENGINES
- 319. P • A • R • T 2 PLANT AND FACILITIES ENGINEERING Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 320. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLANT AND FACILITIES ENGINEERING
- 321. 7.3 SECTION 7 PUMPS AND PUMPING SYSTEMS PUMP OPERATING MODES AND CRITICALITY 7.3 Series Pump Installation Analysis 7.3 Parallel Pumping Economics 7.5 Using Centrifugal Pump Specific Speed to Select Driver Speed 7.10 Ranking Equipment Criticality to Comply with Safety and Environmental Regulations 7.12 PUMP AFFINITY LAWS, OPERATING SPEED, AND HEAD 7.16 Similarity or Affinity Laws for Centrifugal Pumps 7.16 Similarity or Affinity Laws in Centrifugal Pump Selection 7.17 Specific Speed Considerations in Centrifugal Pump Selection 7.18 Selecting the Best Operating Speed for a Centrifugal Pump 7.19 Total Head on a Pump Handling Vapor-Free Liquid 7.21 Pump Selection for any Pumping System 7.26 Analysis of Pump and System Characteristic Curves 7.33 Net Positive Suction Head for Hot- Liquid Pumps 7.41 Condensate Pump Selection for a Steam Power Plant 7.43 Minimum Safe Flow for a Centrifugal Pump 7.46 Selecting a Centrifugal Pump to Handle a Viscous Liquid 7.47 Pump Shaft Deflection and Critical Speed 7.49 Effect of Liquid Viscosity on Regenerative-Pump Performance 7.51 Effect of Liquid Viscosity on Reciprocating-Pump Performance 7.52 Effect of Viscosity and Dissolved Gas on Rotary Pumps 7.53 Selection of Materials for Pump Parts 7.56 Sizing a Hydropneumatic Storage Tank 7.56 Using Centrifugal Pumps as Hydraulic Turbines 7.57 Sizing Centrifugal-Pump Impellers for Safety Service 7.62 Pump Choice to Reduce Energy Consumption and Loss 7.65 SPECIAL PUMP APPLICATIONS 7.68 Evaluating Use of Water-Jet Condensate Pumps to Replace Power-Plant Vertical Condensate Pumps 7.68 Use of Solar-Powered Pumps in Irrigation and Other Services 7.83 Pump Operating Modes and Criticality SERIES PUMP INSTALLATION ANALYSIS A new plant addition using special convectors in the heating system requires a system pumping capability of 45 gal/min (2.84 L/s) at a 26-ft (7.9-m) head. The Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 322. 7.4 PLANT AND FACILITIES ENGINEERING pump characteristic curves for the tentatively selected floor-mounted units are shown in Fig. 1; one operating pump and one standby pump, each 0.75 hp (0.56 kW) are being considered. Can energy be conserved, and how much, with some other pumping arrangement? Calculation Procedure: 1. Plot the characteristic curves for the pumps being considered Figure 2 shows the characteristic curves for the proposed pumps. Point 1 in Fig. 1 is the proposed operating head and flow rate. An alternative pump choice is shown at Point 2 in Fig. 1. If two of the smaller pumps requiring only 0.25 hp (0.19 kW) each are placed in series, they can generate the required 26-ft (7.9-m) head. 2. Analyze the proposed pumps To analyze properly the proposal, a new set of curves, Fig. 2, is required. For the proposed series pumping application, it is necessary to establish a seriesed pump curve. This is a plot of the head and flow rate (capacity) which exists when both pumps are running in series. To construct this curve, double the single-pump head values at any given flow rate. Next, to determine accurately the flow a single pump can deliver, plot the system-head curve using the same method fully described in the previous calcula- tion procedure. This curve is also plotted on Fig. 2. Plot the point of operation for each pump on the seriesed curve, Fig. 2. The point of operation of each pump is on the single-pump curve when both pumps are operating. Each pump supplies half the total required head. When a single pump is running, the point of operation will be at the intersection of the system-head curve and the single-pump characteristic curve, Fig. 2. At this point both the flow and the hp (kW) input of the single pump decrease. Series pumping, Fig. 2, requires the input motor hp (kW) for both pumps; this is the point of maximum power input. 3. Compute the possible savings If the system requires a constant flow of 45 gal/min (2.84 L/s) at 26-ft (7.9-m) head the two-pump series installation saves (0.75 hp ⫺ 2 ⫻ 0.25 hp) ⫽ 0.25 hp (0.19 kW) for every hour the pumps run. For every 1000 hours of operation, the system saves 190 kWh. Since 2000 hours are generally equal to one shift of op- eration per year, the saving is 380 kWh per shift per year. If the load is frequently less than peak, one-pump operation delivers 32.5 gal/ min (2.1 L/s). This value, which is some 72 percent of full load, corresponds to doubling the saving. Related Calculations. Series operation of pumps can be used in a variety of designs for industrial, commercial, residential, chemical, power, marine, and similar plants. A series connection of pumps is especially suitable when full-load demand is small; i.e., just a few hours a week, month, or year. With such a demand, one pump can serve the plant’s needs most of the time, thereby reducing the power bill. When full-load operation is required, the second pump is started. If there is a need for maintenance of the first pump, the second unit is available for service. This procedure is the work of Jerome F. Mueller, P.E., of Mueller Engineering Corp. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 323. PUMPS AND PUMPING SYSTEMS 7.5 0 35 30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 80 0 2.5 5.0 7.5 10.0 1 2 3 4 5 3/4 HP PUMP (0.56 kW) 1/2 HP PUMP (0.37 kW) 1/4 HP PUMP (0.19 kW) 1/6 HP PUMP (0.12 kW) 1 2 GPM L/s HEAD - FEET Head, m FIGURE 1 Pump characteristic curves for use in series installation. PARALLEL PUMPING ECONOMICS A system proposed for heating a 20,000-ft2 (1858-m2 ) addition to an industrial plant using hot-water heating requires a flow of 80 gal/min (7.4 L/s) of 200⬚F (92.5⬚C) water at a 20⬚F (36⬚C) temperature drop and a 13-ft (3.96-m) system head. The Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 324. 7.6 PLANT AND FACILITIES ENGINEERING 0 35 30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 80 0 2.5 5.0 7.5 10.0 1 2 3 4 5 GPM L/s HEAD - FEET Head, m OPERATING POINT OF EACH PUMP WHEN BOTH ARE RUNNING SINGLE PUMP OPERATING POINT SINGLE PUMP CURVE SYSTEM CURVE SERIESED PUMP CURVE DESIGN OPERATING CONDITION FIGURE 2 Seriesed-pump characteristic and system-head curves. required system flow can be handled by two pumps, one an operating unit and one a spare unit. Each pump will have an 0.5-hp (0.37-kW) drive motor. Could there be any appreciable energy saving using some other arrangement? The system re- quires 50 hours of constant pump operation and 40 hours of partial pump operation per week. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 325. PUMPS AND PUMPING SYSTEMS 7.7 0 10 5 25 20 15 10 5 0 0 20 40 60 80 100 120 140 160 9.0 7.5 6.0 4.5 3.0 1.5 0 1/2 HP PUMP (0.37 kW) SYSTEM LOAD 1/4 HP PUMP (0.10 kW) GALLONS PER MINUTE FEET OF HEAD Head, m L/s FIGURE 3 Typical pump characteristic curves. Calculation Procedure: 1. Plot characteristic curves for the proposed system Figure 3 shows the proposed hot-water heating-pump selection for this industrial building. Looking at the values of the pump head and capacity in Fig. 3, it can be seen that if the peak load of 80 gal/min (7.4 L/s) were carried by two pumps, then each would have to pump only 40 gal/min (3.7 L/s) in a parallel arrangement. 2. Plot a characteristic curve for the pumps in parallel Construct the paralleled-pump curve by doubling the flow of a single pump at any given head, using data from the pump manufacturer. At 13-ft head (3.96-m) one pump produces 40 gal/min (3.7 L/s); two pumps 80 gal/min (7.4 L/s). The re- sulting curve is shown in Fig. 4. The load for this system could be divided among three, four, or more pumps, if desired. To achieve the best results, the number of pumps chosen should be based on achieving the proper head and capacity requirements in the system. 3. Construct a system-head curve Based on the known flow rate, 80 gal/min (7.4 L/s) at 13-ft (3.96-m) head, a system-head curve can be constructed using the fact that pumping head varies as the square of the change in flow, or Q2 /Q1 ⫽ H2 /H1, where Q1 ⫽ known design flow, gal/min (L/s); Q2 ⫽ selected flow, gal/min (L/s); H1 ⫽ known design head, ft (m); H2 ⫽ resultant head related to selected flow rate, gal/min (L/s) Figure 5 shows the plotted system-head curve. Once the system-head curve is plotted, draw the single-pump curve from Fig. 3 on Fig. 5, and the parallelled- pump curve from Fig. 4. Connect the different pertinent points of concern with dashed lines, Fig. 5. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 326. 7.8 PLANT AND FACILITIES ENGINEERING 25 20 15 10 5 0 5 0 10 L/s 9.0 7.5 6.0 4.5 3.0 1.5 0 Head, m 0 20 40 60 80 100 120 140 160 GALLONS PER MINUTE ONE PUMP TWO PUMPS Paralleled FIGURE 4 Single- and dual-parallel pump characteristic curves. 25 20 15 10 5 0 9.0 7.5 6.0 4.5 3.0 1.5 0 5 0 10 L/s FEET OF HEAD Head, m 0 20 40 60 80 100 120 140 160 TWO PUMPS SINGLE PUMP SYSTEM CURVE GALLONS PER MINUTE FIGURE 5 System-head curve for parallel pumping. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 327. PUMPS AND PUMPING SYSTEMS 7.9 The point of crossing of the two-pump curve and the system-head curve is at the required value of 80 gal/min (7.4 L/s) and 13-ft (3.96-m) head because it was so planned. But the point of crossing of the system-head curve and the single-pump curve is of particular interest. The single pump, instead of delivering 40 gal/min (7.4 L/s) at 13-ft (3.96-m) head will deliver, as shown by the intersection of the curves in Fig. 5, 72 gal/min (6.67 L/s) at 10-ft (3.05-m) head. Thus, the single pump can effectively be a standby for 90 percent of the required capacity at a power input of 0.5 hp (0.37 kW). Much of the time in heating and air conditioning, and frequently in industrial processes, the system load is 90 percent, or less. 4. Determine the single-pump horsepower input In the installation here, the pumps are the inline type with non-overload motors. For larger flow rates, the pumps chosen would be floor-mounted units providing a variety of horsepower (kW) and flow curves. The horsepower (kW) for—say a 200- gal/min (18.6 L/s) flow rate would be about half of a 400-gal/min (37.2 L/s) flow rate. If a pump were suddenly given a 300-gal/min (27.9 L/s) flow-rate demand at its crossing point on a larger system-head curve, the hp required might be excessive. Hence, the pump drive motor must be chosen carefully so that the power required does not exceed the motor’s rating. The power input required by any pump can be obtained from the pump characteristic curve for the unit being considered. Such curves are available free of charge from the pump manufacturer. The pump operating point is at the intersection of the pump characteristic curve and the system-head curve in conformance with the first law of thermodynamics, which states that the energy put into the system must exactly match the energy used by the system. The intersection of the pump characteristic curve and the system-head curve is the only point that fulfills this basic law. There is no practical limit for pumps in parallel. Careful analysis of the system- head curve versus the pump characteristic curves provided by the pump manufac- turer will frequently reveal cases where the system load point may be beyond the desired pump curve. The first cost of two or three smaller pumps is frequently no greater than for one large pump. Hence, smaller pumps in parallel may be more desirable than a single large pump, from both the economic and reliability stand- points. One frequently overlooked design consideration in piping for pumps is shown in Fig. 6. This is the location of the check valve to prevent reverse-flow pumping. Figure 6 shows the proper location for this simple valve. 5. Compute the energy saving possible Since one pump can carry the fluid flow load about 90 percent of the time, and this same percentage holds for the design conditions, the saving in energy is 0.9 ⫻ (0.5 kW ⫺ .25 kW) ⫻ 90 h per week ⫽ 20.25 kWh/week. (In this com- putation we used the assumption that 1 hp ⫽ 1 kW.) The annual savings would be 52 weeks ⫻ 20.25 kW/week ⫽ 1053 kWh/yr. If electricity costs 5 cents per kWh, the annual saving is $0.05 ⫻ 1053 ⫽ $52.65/yr. While a saving of some $51 per year may seem small, such a saving can become much more if: (1) larger pumps using higher horsepower (kW) motors are used; (2) several hundred pumps are used in the system; (3) the operating time is longer—168 hours per week in some systems. If any, or all, these conditions prevail, the savings can be substantial. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 328. 7.10 PLANT AND FACILITIES ENGINEERING FIGURE 6 Check valve locations to prevent reverse flow. Related Calculations. This procedure can be used for pumps in a variety of applications: industrial, commercial, residential, medical, recreational, and similar systems. When analyzing any system the designer should be careful to consider all the available options so the best one is found. This procedure is the work of Jerome F. Mueller, P.E., of Mueller Engineering Corp. USING CENTRIFUGAL PUMP SPECIFIC SPEED TO SELECT DRIVER SPEED A double-suction condenser circulator handling 20,000 gal/min (75,800 L/min) at a total head of 60 ft (18.3 m) is to have a 15-ft (4.6-m) lift. What should be the rpm of this pump to meet the capacity and head requirements? Calculation Procedure: 1. Determine the specific speed of the pump Use the Hydraulic Institute specific-speed chart, Fig. 7, page 7.11. Entering at 60 ft (18.3 m) head, project to the 15-ft suction lift curve. At the intersection, read the specific speed of this double-suction pump as 4300. 2. Use the specific-speed equation to determine the pump operating rpm Solve the specific-speed equation for the pump rpm. Or rpm ⫽ Ns ⫻ 0.75 0.5 H /Q , where Ns ⫽ specific speed of the pump, rpm, from Fig. 7; H ⫽ total head on pump, ft (m); Q ⫽ pump flow rate, gal/min (L/min). Solving, rpm ⫽ 4300 ⫻ ⫽ 655.5 r/min. The next common electric motor rpm 0.75 0.5 60 /20,000 is 660; hence, we would choose a motor or turbine driver whose rpm does not exceed 660. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 329. PUMPS AND PUMPING SYSTEMS 7.11 FIGURE 7 Upper limits of specific speeds of single-stage, single- and double-suction centrifugal pumps handling clear water at 85⬚F (29.4⬚C) at sea level. (Hydraulic Institute.) The next lower induction-motor speed is 585 r/min. But we could buy a lower- cost pump and motor if it could be run at the next higher full-load induction motor speed of 700 r/min. The specific speed of such a pump would be: Ns ⫽ [700 ⫽ 4592. Referring to Fig. 7, the maximum suction lift with a 0.5 0.75 (20,000) ]/60 specific speed of 4592 is 13 ft (3.96 m) when the total head is 60 ft (18.3). If the pump setting or location could be lowered 2 ft (0.6 m), the less expensive pump and motor could be used, thereby saving on the investment cost. Related Calculations. Use this general procedure to choose the driver and pump rpm for centrifugal pumps used in boiler feed, industrial, marine, HVAC, and similar applications. Note that the latest Hydraulic Institute curves should be used. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 330. 7.12 PLANT AND FACILITIES ENGINEERING RANKING EQUIPMENT CRITICALITY TO COMPLY WITH SAFETY AND ENVIRONMENTAL REGULATIONS Rank the criticality of a boiler feed pump operating at 250⬚F (121⬚C) and 100 lb/ in2 (68.9 kPa) if its Mean Time Between Failures (MTBF) is 10 months, and vi- bration is an important element in its safe operation. Use the National Fire Protec- tion Association (NFPA) ratings of process chemicals for health, fire, and reactivity hazards. Show how the criticality of the unit is developed. Calculation Procedure: 1. Determine the Hazard Criticality Rating (HCR) of the equipment Process industries of various types—chemical, petroleum, food, etc.—are giving much attention to complying with new process safety regulations. These efforts center on reducing hazards to people and the environment by ensuring the me- chanical and electrical integrity of equipment. To start a program, the first step is to evaluate the most critical equipment in a plant or factory. To do so, the equipment is first ranked on some criteria, such as the relative importance of each piece of equipment to the process or plant output. The Hazard Criticality Rating (HCR) can be determined from a listing such as that in Table 1. This tabulation contains the analysis guidelines for assessing the process chemical hazard (PCH) and the Other Hazards (O). The ratings for such a table of hazards should be based on the findings of an experienced team thoroughly familiar with the process being evaluated. A good choice for such a task is the plant’s Process Hazard Analysis (PHA) Group. Since a team’s familiarity with a process is highest at the end of a PHA study, the best time for rating the criticality of equipment is toward the end of such safety evaluations. From Table 1, the NFPA rating, N, of process chemicals for Health, Fire, and Reactivity, is N ⫽ 2, because this is the highest of such ratings for Health. The Fire and Reactivity ratings are 0, 0, respectively, for a boiler feed pump because there are no Fire or Reactivity exposures. The Risk Reduction Factor (RF), from Table 1, is RF ⫽ 0, since there is the potential for serious burns from the hot water handled by the boiler feed pump. Then, the Process Chemical Hazard, PCH ⫽ N ⫺ RF ⫽ 2 ⫺ 0 ⫽ 2. The rating of Other Hazards, O, Table 1, is O ⫽ 1, because of the high tem- perature of the water. Thus, the Hazard Criticality Rating, HCR ⫽ 2, found from the higher numerical value of PCH and O. 2. Determine the Process Criticality Rating, PCR, of the equipment From Table 2, prepared by the PHA Group using the results of its study of the equipment in the plant, PCR ⫽ 3. The reason for this is that the boiler feed pump is critical for plant operation because its failure will result in reduced capacity. 3. Find the Process and Hazard Criticality Rating, PHCR The alphanumeric PHC value is represented first by the alphabetic character for the category. For example, Category A is the most critical, while Category D is the least critical to plant operation. The first numeric portion represents the Hazard Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 331. PUMPS AND PUMPING SYSTEMS 7.13 TABLE 1 The Hazard Criticality Rating (HCR) is Determined in Three Steps* Hazard Criticality Rating 1. Assess the Process Chemical Hazard (PCH) by: ● Determining the NFPA ratings (N) of process chemicals for: Health, Fire, Reactivity hazards ● Selecting the highest value of N ● Evaluating the potential for an emissions release (0 to 4): High (RF ⫽ 0): Possible serious health, safety or environmental effects Low (RF ⫽ 1): Minimal effects None (RF ⫽ 4): No effects ● Then, PCH ⫽ N ⫺ RF. (Round off negative values to zero.) 2. Rate Other Hazards (O) with an arbitrary number (0 to 4) if they are: ● Deadly (4), if: Temperatures ⬎ 1000⬚F Pressures are extreme Potential for release of regulated chemicals is high Release causes possible serious health safety or environmental effects Plant requires steam turbine trip mechanisms, fired-equipment shutdown systems, or toxic- or combustible-gas detectors† Failure of pollution control system results in environmental damage† ● Extremely dangerous (3), if: Equipment rotates at ⬎5000 r/min Temperatures ⬎500⬚F Plant requires process venting devices Potential for release of regulated chemicals is low Failure of pollution control system may result in environmental damage† ● Hazardous (2), if: Temperatures ⬎300⬚F; Extended failure of pollution control system may cause damage† ● Slightly hazardous (1), if: Equipment rotates at ⬎3600 r/min Temperatures ⬎ 140⬚F or pressures ⬎ 20 lb/in2 (gage) ● Not hazardous (0), if: No hazards exist 3. Select the higher value of PCH and O as the Hazard Criticality Rating *Chemical Engineering. †Equipment with spares drop one category rating. A spare is an inline unit that can be immediately serviced or be substituted by an alternative process option during the repair period. Criticality Rating, HCR, while the second numeric part the Process Criticality Rat- ing, PCR. These categories and ratings are a result of the work of the PHA Group. From Table 3, the Process and Hazard Criticality Rating, PHCR ⫽ B23. This is based on the PCR ⫽ 3 and HCR ⫽ 2, found earlier. 4. Generate a criticality list by rating equipment using its alphanumeric PHCR values Each piece of equipment is categorized, in terms of its importance to the process, as: Highest Priority, Category A; High Priority, Category B; Medium Priority, Cat- egory C; Low Priority, Category D. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 332. 7.14 PLANT AND FACILITIES ENGINEERING TABLE 2 Process Criticality Rating* Process Criticality Rating Essential (4) The equipment is essential if failure will result in shutdown of the unit, unacceptable product quality, or severely reduced process yield Critical (3) The equipment is critical if failure will result in greatly reduced capacity, poor product quality, or moderately reduced process yield Helpful (2) The equipment is helpful if failure will result in slightly reduced capacity, product quality or reduced process yield Not critical (1) The equipment is not critical if failure will have little or no process conse- quences *Chemical Engineering. TABLE 3 The Process and Hazard Criticality Rating* PHC Rankings Process Criticality Rating Hazard Criticality Rating 4 3 2 1 0 4 A44 A34 A24 A14 A04 3 A43 B33 B23 B13 B03 2 A42 A32 C22 C12 C02 1 A41 B31 C21 CD11 D01 Note: The alphanumeric PHC value is represented first by the alphabetic character for the category (for example, category A is the most critical while D is the least critical). The first numeric portion represents the Hazard Criticality Rating, and the second numeric part the Process Criticality Rating. *Chemical Engineering. Since the boiler feed pump is critical to the operation of the process, it is a Category B, i.e., High Priority item in the process. 5. Determine the Criticality and Repetitive Equipment, CRE, value for this equipment This pump has an MTBF of 10 months. Therefore, from Table 4, CRE ⫽ b1. Note that the CRE value will vary with the PCHR and MTBF values for the equipment. 6. Determine equipment inspection frequency to ensure human and environmental safety From Table 5, this boiler feed pump requires vibration monitoring every 90 days. With such monitoring it is unlikely that an excessive number of failures might occur to this equipment. 7. Summarize criticality findings in spreadsheet form When preparing for a PHCR evaluation, a spreadsheet, Table 6, listing critical equipment, should be prepared. Then, as the various rankings are determined, they can be entered in the spreadsheet where they are available for easy reference. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 333. PUMPS AND PUMPING SYSTEMS 7.15 TABLE 4 The Criticality and Repetitive Equipment Values* CRE Values PHCR Mean time between failures, months 0–6 6–12 12–24 ⬎24 A a1 a2 a3 a4 B a2 b1 b2 b3 C a3 b2 c1 c2 D a4 b3 c2 d1 *Chemical Engineering. TABLE 5 Predictive Maintenance Frequencies for Rotating Equipment Based on Their CRE Values* Maintenance cycles CRE Frequency, days 7 30 90 360 a1, a2 VM LT a3, a4 VM LT b1, b3 VM c1, d1 VM VM: Vibration monitoring. LT: Lubrication sampling and testing. *Chemical Engineering. TABLE 6 Typical Spreadsheet for Ranking Equipment Criticality* Spreadsheet for calculating equipment PHCRS Equipment number Equipment description NFPA rating H F R RF PCH Other HCR PCR PHCR TKO Tank 4 4 0 0 4 0 4 4 A44 TKO Tank 4 4 0 1 3 3 3 4 A34 PU1BFW Pump 2 0 0 0 2 1 2 3 B23 *Chemical Engineering. Enter the PCH, Other, HCR, PCR, and PHCR values in the spreadsheet, as shown. These data are now available for reference by anyone needing the infor- mation. Related Calculations. The procedure presented here can be applied to all types of equipment used in a facility—fixed, rotating, and instrumentation. Once all the equipment is ranked by criticality, priority lists can be generated. These lists can Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 334. 7.16 PLANT AND FACILITIES ENGINEERING then be used to ensure the mechanical integrity of critical equipment by prioritizing predictive and preventive maintenance programs, inventories of critical spare parts, and maintenance work orders in case of plant upsets. In any plant, the hazards posed by different operating units are first ranked and prioritized based on a PHA. These rankings are then used to determine the order in which the hazards need to be addressed. When the PHAs approach completion, team members evaluate the equipment in each operating unit using the PHCR sys- tem. The procedure presented here can be used in any plant concerned with human and environmental safety. Today, this represents every plant, whether conventional or automated. Industries in which this procedure finds active use include chemical, petroleum, textile, food, power, automobile, aircraft, military, and general manu- facturing. This procedure is the work of V. Anthony Ciliberti, Maintenance Engineer, The Lubrizol Corp., as reported in Chemical Engineering magazine. Pump Affinity Laws, Operating Speed, and Head SIMILARITY OR AFFINITY LAWS FOR CENTRIFUGAL PUMPS A centrifugal pump designed for a 1800-r/min operation and a head of 200 ft (60.9 m) has a capacity of 3000 gal/min (189.3 L/s) with a power input of 175 hp (130.6 kW). What effect will a speed reduction to 1200 r/min have on the head, capacity, and power input of the pump? What will be the change in these variables if the impeller diameter is reduced from 12 to 10 in (304.8 to 254 mm) while the speed is held constant at 1800 r/min? Calculation Procedure: 1. Compute the effect of a change in pump speed For any centrifugal pump in which the effects of fluid viscosity are negligible, or are neglected, the similarity or affinity laws can be used to determine the effect of a speed, power, or head change. For a constant impeller diameter, the laws are Q1 /Q2 ⫽ N1 /N2; H1 /H2 ⫽ (N1 /N2)2 ; P1 /P2 ⫽ (N1 /N2)3 . For a constant speed, Q1 / Q2 ⫽ D1 /D2; H1 /H2 ⫽ (D1 /D2)2 ; P1 /P2 ⫽ (D1 /D2)3 . In both sets of laws, Q ⫽ capacity, gal/min; N ⫽ impeller rpm; D ⫽ impeller diameter, in; H ⫽ total head, ft of liquid; P ⫽ bhp input. The subscripts 1 and 2 refer to the initial and changed conditions, respectively. For this pump, with a constant impeller diameter, Q1 /Q2 ⫽ N1 /N2; 3000/ Q2 ⫽ 1800/1200; Q2 ⫽ 2000 gal/min (126.2 L/s). And, H1 /H2 ⫽ (N1 / N2)2 ⫽ 200/H2 ⫽ (1800/1200)2 ; H2 ⫽ 88.9 ft (27.1 m). Also, P1 /P2 ⫽ (N1 / N2)3 ⫽ 175/P2 ⫽ (1800/1200)3 ; P2 ⫽ 51.8 bhp (38.6 kW). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 335. PUMPS AND PUMPING SYSTEMS 7.17 2. Compute the effect of a change in impeller diameter With the speed constant, use the second set of laws. Or, for this pump, Q1 / Q2 ⫽ D1 /D2; 3000/Q2 ⫽ 12 ⁄10; Q2 ⫽ 2500 gal/min (157.7 L/s). And H1 / H2 ⫽ (D1 /D2)2 ; 200/H2 ⫽ (12 ⁄10)2 ; H2 ⫽ 138.8 ft (42.3 m). Also, P1 /P2 ⫽ (D1 / D2)3 ; 175/P2 ⫽ (12 ⁄10)3 ; P2 ⫽ 101.2 bhp (75.5 kW). Related Calculations. Use the similarity laws to extend or change the data obtained from centrifugal pump characteristic curves. These laws are also useful in field calculations when the pump head, capacity, speed, or impeller diameter is changed. The similarity laws are most accurate when the efficiency of the pump remains nearly constant. Results obtained when the laws are applied to a pump having a constant impeller diameter are somewhat more accurate than for a pump at constant speed with a changed impeller diameter. The latter laws are more accurate when applied to pumps having a low specific speed. If the similarity laws are applied to a pump whose impeller diameter is increased, be certain to consider the effect of the higher velocity in the pump suction line. Use the similarity laws for any liquid whose viscosity remains constant during passage through the pump. However, the accuracy of the similarity laws decreases as the liquid viscosity increases. SIMILARITY OR AFFINITY LAWS IN CENTRIFUGAL PUMP SELECTION A test-model pump delivers, at its best efficiency point, 500 gal/min (31.6 L/s) at a 350-ft (106.7-m) head with a required net positive suction head (NPSH) of 10 ft (3 m) a power input of 55 hp (41 kW) at 3500 r/min, when a 10.5-in (266.7-mm) diameter impeller is used. Determine the performance of the model at 1750 r/min. What is the performance of a full-scale prototype pump with a 20-in (50.4-cm) impeller operating at 1170 r/min? What are the specific speeds and the suction specific speeds of the test-model and prototype pumps? Calculation Procedure: 1. Compute the pump performance at the new speed The similarity or affinity laws can be stated in general terms, with subscripts p and m for prototype and model, respectively, as Qp ⫽ Hp ⫽ 3 2 2 K N Q ; K K H ; d n m d n m NPSHp ⫽ Pp ⫽ where Kd ⫽ size factor ⫽ prototype 2 2 5 5 K K NPSH ; K K P , 2 n m d n m dimension/model dimension. The usual dimension used for the size factor is the impeller diameter. Both dimensions should be in the same units of measure. Also, Kn ⫽ (prototype speed, r/min)/(model speed, r/min). Other symbols are the same as in the previous calculation procedure. When the model speed is reduced from 3500 to 1750 r/min, the pump dimen- sions remain the same and Kd ⫽ 1.0; Kn ⫽ 1750/3500 ⫽ 0.5. Then Q ⫽ (1.0)(0.5)(500) ⫽ 250 r/min; H ⫽ (1.0)2 (0.5)2 (350) ⫽ 87.5 ft (26.7 m); NPSH ⫽ (1.0)2 (0.5)2 (10) ⫽ 2.5 ft (0.76 m); P ⫽ (1.0)5 (0.5)3 (55) ⫽ 6.9 hp (5.2 kW). In this computation, the subscripts were omitted from the equations because the same pump, the test model, was being considered. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 336. 7.18 PLANT AND FACILITIES ENGINEERING 2. Compute performance of the prototype pump First, Kd and Kn must be found: Kd ⫽ 20/10.5 ⫽ 1.905; Kn ⫽ 1170/3500 ⫽ 0.335. Then Qp ⫽ (1.905)3 (0.335)(500) ⫽ 1158 gal/min (73.1 L/s); Hp ⫽ (1.905)2 (0.335)2 (350) ⫽ 142.5 ft (43.4 m); NPSHp ⫽ (1.905)2 (0.335)2 (10) ⫽ 4.06 ft (1.24 m); Pp ⫽ (1.905)5 (0.335)3 (55) ⫽ 51.8 hp (38.6 kW). 3. Compute the specific speed and suction specific speed The specific speed or, as Horwitz1 says, ‘‘more correctly, discharge specific speed,’’ is Ns ⫽ N while the suction specific speed S ⫽ 0.5 0.75 0.5 0.75 (Q) /(H) , N(Q) /(NPSH) , where all values are taken at the best efficiency point of the pump. For the model, Ns ⫽ ⫽ 965; S ⫽ 0.5 0.75 0.5 3500(500) /(350) 3500(500) / ⫽ 13,900. For the prototype, Ns ⫽ ⫽ 965; 0.75 0.5 0.75 (10) 1170(1158) /(142.5) S ⫽ ⫽ 13,900. The specific speed and suction specific 0.5 0.75 1170(1156) /(4.06) speed of the model and prototype are equal because these units are geometrically similar or homologous pumps and both speeds are mathematically derived from the similarity laws. Related Calculations. Use the procedure given here for any type of centrifugal pump where the similarity laws apply. When the term model is used, it can apply to a production test pump or to a standard unit ready for installation. The procedure presented here is the work of R. P. Horwitz, as reported in Power magazine.1 SPECIFIC SPEED CONSIDERATIONS IN CENTRIFUGAL PUMP SELECTION What is the upper limit of specific speed and capacity of a 1750-r/min single-stage double-suction centrifugal pump having a shaft that passes through the impeller eye if it handles clear water at 85⬚F (29.4⬚C) at sea level at a total head of 280 ft (85.3 m) with a 10-ft (3-m) suction lift? What is the efficiency of the pump and its approximate impeller shape? Calculation Procedure: 1. Determine the upper limit of specific speed Use the Hydraulic Institute upper specific-speed curve, Fig. 7, for centrifugal pumps or a similar curve, Fig. 8, for mixed- and axial-flow pumps. Enter Fig. 7 at the bottom at 280-ft (85.3-m) total head, and project vertically upward until the 10-ft (3-m) suction-lift curve is intersected. From here, project horizontally to the right to read the specific speed NS ⫽ 2000. Figure 8 is used in a similar manner. 2. Compute the maximum pump capacity For any centrifugal, mixed- or axial-flow pump, NS ⫽ where 0.5 0.75 (gpm) (rpm)/H , t Ht ⫽ total head on the pump, ft of liquid. Solving for the maximum capacity, we get gpm ⫽ /rpm)2 ⫽ /1750)2 ⫽ 6040 gal/min (381.1 0.75 0.75 (N H (2000 ⫻ 280 S t L/s). 1 R. P. Horwitz, ‘‘Affinity Laws and Specific Speed Can Simplify Centrifugal Pump Selection,’’ Power, November 1964. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 337. PUMPS AND PUMPING SYSTEMS 7.19 FIGURE 8 Upper limits of specific speeds of single-suction mixed-flow and axial-flow pumps. (Hydraulic Institute.) 3. Determine the pump efficiency and impeller shape Figure 9 shows the general relation between impeller shape, specific speed, pump capacity, efficiency, and characteristic curves. At NS ⫽ 2000, efficiency ⫽ 87 per- cent. The impeller, as shown in Fig. 9, is moderately short and has a relatively large discharge area. A cross section of the impeller appears directly under the NS ⫽ 2000 ordinate. Related Calculations. Use the method given here for any type of pump whose variables are included in the Hydraulic Institute curves, Figs. 7 and 8, and in similar curves available from the same source. Operating specific speed, computed as above, is sometimes plotted on the performance curve of a centrifugal pump so that the characteristics of the unit can be better understood. Type specific speed is the operating specific speed giving maximum efficiency for a given pump and is a number used to identify a pump. Specific speed is important in cavitation and suction-lift studies. The Hydraulic Institute curves, Figs. 7 and 8, give upper limits of speed, head, capacity and suction lift for cavitation-free operation. When making actual pump analyses, be certain to use the curves (Figs. 7 and 8) in the latest edition of the Standards of the Hydraulic Institute. SELECTING THE BEST OPERATING SPEED FOR A CENTRIFUGAL PUMP A single-suction centrifugal pump is driven by a 60-Hz ac motor. The pump delivers 10,000 gal/min (630.9 L/s) of water at a 100-ft (30.5-m) head. The available net Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 338. 7.20 PLANT AND FACILITIES ENGINEERING FIGURE 9 Approximate relative impeller shapes and efficiency variations for various specific speeds of centrifugal pumps. (Worthington Corporation.) positive suction head ⫽ 32 ft (9.7 m) of water. What is the best operating speed for this pump if the pump operates at its best efficiency point? Calculation Procedure: 1. Determine the specific speed and suction specific speed Ac motors can operate at a variety of speeds, depending on the number of poles. Assume that the motor driving this pump might operate at 870, 1160, 1750, or 3500 r/min. Compute the specific speed NS ⫽ N ⫽ 0.5 0.75 (Q) /(H) ⫽ 3.14N and the suction specific speed S ⫽ 0.5 0.75 0.5 N(10,000) /(100) N(Q) / ⫽ ⫽ 7.43N for each of the assumed speeds. Tabu- 0.75 0.5 0.75 (NPSH) N(10,000) /(32) late the results as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 339. PUMPS AND PUMPING SYSTEMS 7.21 TABLE 7 Pump Types Listed by Specific Speed* TABLE 8 Suction Specific-Speed Ratings* 2. Choose the best speed for the pump Analyze the specific speed and suction specific speed at each of the various oper- ating speeds, using the data in Tables 7 and 8. These tables show that at 870 and 1160 r/min, the suction specific-speed rating is poor. At 1750 r/min, the suction specific-speed rating is excellent, and a turbine or mixed-flow type pump will be suitable. Operation at 3500 r/min is unfeasible because a suction specific speed of 26,000 is beyond the range of conventional pumps. Related Calculations. Use this procedure for any type of centrifugal pump handling water for plant services, cooling, process, fire protection, and similar re- quirements. This procedure is the work of R. P. Horwitz, Hydrodynamics Division, Peerless Pump, FMC Corporation, as reported in Power magazine. TOTAL HEAD ON A PUMP HANDLING VAPOR-FREE LIQUID Sketch three typical pump piping arrangements with static suction lift and sub- merged, free, and varying discharge head. Prepare similar sketches for the same pump with static suction head. Label the various heads. Compute the total head on each pump if the elevations are as shown in Fig. 10 and the pump discharges a maximum of 2000 gal/min (126.2 L/s) of water through 8-in (203.2-mm) schedule 40 pipe. What hp is required to drive the pump? A swing check valve is used on the pump suction line and a gate valve on the discharge line. Calculation Procedure: 1. Sketch the possible piping arrangements Figure 10 shows the six possible piping arrangements for the stated conditions of the installation. Label the total static head, i.e., the vertical distance from the surface Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 340. 7.22 PLANT AND FACILITIES ENGINEERING FIGURE 10 Typical pump suction and discharge piping arrangements. of the source of the liquid supply to the free surface of the liquid in the discharge receiver, or to the point of free discharge from the discharge pipe. When both the suction and discharge surfaces are open to the atmosphere, the total static head equals the vertical difference in elevation. Use the free-surface elevations that cause the maximum suction lift and discharge head, i.e., the lowest possible level in the supply tank and the highest possible level in the discharge tank or pipe. When the supply source is below the pump centerline, the vertical distance is called the static suction lift; with the supply above the pump centerline, the vertical distance is called static suction head. With variable static suction head, use the lowest liquid level in the supply tank when computing total static head. Label the diagrams as shown in Fig. 10. 2. Compute the total static head on the pump The total static head Hts ft ⫽ static suction lift, hsl ft ⫹ static discharge head hsd ft, where the pump has a suction lift, s in Fig. 10a, b, and c. In these installations, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 341. PUMPS AND PUMPING SYSTEMS 7.23 Hts ⫽ 10 ⫹ 100 ⫽ 110 ft (33.5 m). Note that the static discharge head is computed between the pump centerline and the water level with an underwater discharge, Fig. 10a; to the pipe outlet with a free discharge, Fig. 10b; and to the maximum water level in the discharge tank, Fig. 10c. When a pump is discharging into a closed compression tank, the total discharge head equals the static discharge head plus the head equivalent, ft of liquid, of the internal pressure in the tank, or 2.31 ⫻ tank pressure, lb/in2 . Where the pump has a static suction head, as in Fig. 10d, e, and ƒ, the total static head Hts ft ⫽ hsd ⫺ static suction head hsh ft. In these installations, Ht ⫽ 100 ⫺ 15 ⫽ 85 ft (25.9 m). The total static head, as computed above, refers to the head on the pump without liquid flow. To determine the total head on the pump, the friction losses in the piping system during liquid flow must be also determined. 3. Compute the piping friction losses Mark the length of each piece of straight pipe on the piping drawing. Thus, in Fig. 10a, the total length of straight pipe Lt ft ⫽ 8 ⫹ 10 ⫹ 5 ⫹ 102 ⫹ 5 ⫽ 130 ft (39.6 m), if we start at the suction tank and add each length until the discharge tank is reached. To the total length of straight pipe must be added the equivalent length of the pipe fittings. In Fig. 10a there are four long-radius elbows, one swing check valve, and one globe valve. In addition, there is a minor head loss at the pipe inlet and at the pipe outlet. The equivalent length of one 8-in (203.2-mm) long-radius elbow is 14 ft (4.3 m) of pipe, from Table 9. Since the pipe contains four elbows, the total equivalent length ⫽ 4(14) ⫽ 56 ft (17.1 m) of straight pipe. The open gate valve has an equivalent resistance of 4.5 ft (1.4 m); and the open swing check valve has an equivalent resistance of 53 ft (16.2 m). The entrance loss he ft, assuming a basket-type strainer is used at the suction- pipe inlet, is he ft ⫽ Kv2 /2g, where K ⫽ a constant from Fig. 11; v ⫽ liquid velocity, ft/s; g ⫽ 32.2 ft/s2 (980.67 cm/s2 ). The exit loss occurs when the liquid passes through a sudden enlargement, as from a pipe to a tank. Where the area of the tank is large, causing a final velocity that is zero, hex ⫽ v2 /2g. The velocity v ft/s in a pipe ⫽ gpm /2.448d2 . For this pipe, v ⫽ 2000/ [(2.448)(7.98)2 ] ⫽ 12.82 ft/s (3.91 m/s). Then he ⫽ 0.74(12.82)2 /[2(32.2)] ⫽ 1.89 ft (0.58 m), and hex ⫽ (12.82)2 /[(2)(32.2)] ⫽ 2.56 ft (0.78 m). Hence, the total length of the piping system in Fig. 10a is 130 ⫹ 56 ⫹ 4.5 ⫹ 53 ⫹ 1.89 ⫹ 2.56 ⫽ 247.95 ft (75.6 m), say 248 ft (75.6 m). Use a suitable head-loss equation, or Table 10, to compute the head loss for the pipe and fittings. Enter Table 10 at an 8-in (203.2-mm) pipe size, and project horizontally across to 2000 gal/min (126.2 L/s) and read the head loss as 5.86 ft of water per 100 ft (1.8 m/30.5 m) of pipe. The total length of pipe and fittings computed above is 248 ft (75.6 m). Then total friction-head loss with a 2000 gal/min (126.2-L/s) flow is ft ⫽ Hƒ (5.86)(248/100) ⫽ 14.53 ft (4.5 m). 4. Compute the total head on the pump The total head on the pump Ht ⫽ Hts ⫹ For the pump in Fig. 10a, H . ƒ Ht ⫽ 110 ⫹ 14.53 ⫽ 124.53 ft (37.95 m), say 125 ft (38.1 m). The total head on the pump in Fig. 10b and c would be the same. Some engineers term the total head on a pump the total dynamic head to distinguish between static head (no-flow vertical head) and operating head (rated flow through the pump). The total head on the pumps in Fig. 10d, c, and ƒ is computed in the same way as described above, except that the total static head is less because the pump has Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 342. 7.24 TABLE 9 Resistance of Fittings and Valves (length of straight pipe giving equivalent resistance) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 343. PUMPS AND PUMPING SYSTEMS 7.25 FIGURE 11 Resistance coefficients of pipe fittings. To convert to SI in the equation for h, v2 would be measured in m/s and feet would be changed to meters. The following values would also be changed from inches to millimeters: 0.3 to 7.6, 0.5 to 12.7, 1 to 25.4, 2 to 50.8, 4 to 101.6, 6 to 152.4 10 to 254, and 20 to 508. (Hydraulic Institute.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 344. 7.26 PLANT AND FACILITIES ENGINEERING TABLE 10 Pipe Friction Loss for Water (wrought-iron or steel schedule 40 pipe in good condition) a static suction head. That is, the elevation of the liquid on the suction side reduces the total distance through which the pump must discharge liquid; thus the total static head is less. The static suction head is subtracted from the static discharge head to determine the total static head on the pump. 5. Compute the horsepower required to drive the pump The brake hp input to a pump bhpi ⫽ (gpm)(Ht )(s)/3960e, where s ⫽ specific gravity of the liquid handled; e ⫽ hydraulic efficiency of the pump, expressed as a decimal. The usual hydraulic efficiency of a centrifugal pump is 60 to 80 percent; reciprocating pumps, 55 to 90 percent; rotary pumps, 50 to 90 percent. For each class of pump, the hydraulic efficiency decreases as the liquid viscosity increases. Assume that the hydraulic efficiency of the pump in this system is 70 percent and the specific gravity of the liquid handled is 1.0. Then bhpi ⫽ (2000)(127)(1.0)/(3960)(0.70) ⫽ 91.6 hp (68.4 kW). The theoretical or hydraulic horsepower hph ⫽ (gpm)(Ht )(s)/3960, or hph ⫽ (2000) ⫽ (127)(1.0)/3900 ⫽ 64.1 hp (47.8 kW). Related Calculations. Use this procedure for any liquid—water, oil, chemical, sludge, etc.—whose specific gravity is known. When liquids other than water are being pumped, the specific gravity and viscosity of the liquid, as discussed in later calculation procedures, must be taken into consideration. The procedure given here can be used for any class of pump—centrifugal, rotary, or reciprocating. Note that Fig. 11 can be used to determine the equivalent length of a variety of pipe fittings. To use Fig. 11, simply substitute the appropriate K value in the relation h ⫽ Kv2 /2g, where h ⫽ equivalent length of straight pipe; other symbols as before. PUMP SELECTION FOR ANY PUMPING SYSTEM Give a step-by-step procedure for choosing the class, type, capacity, drive, and materials for a pump that will be used in an industrial pumping system. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 345. PUMPS AND PUMPING SYSTEMS 7.27 FIGURE 12 (a) Single-line diagrams for an industrial pipeline; (b) single-line diagram of a boiler-feed system. (Worthington Corporation.) Calculation Procedure: 1. Sketch the proposed piping layout Use a single-line diagram, Fig. 12, of the piping system. Base the sketch on the actual job conditions. Show all the piping, fittings, valves, equipment, and other units in the system. Mark the actual and equivalent pipe length (see the previous calculation procedure) on the sketch. Be certain to include all vertical lifts, sharp bends, sudden enlargements, storage tanks, and similar equipment in the proposed system. 2. Determine the required capacity of the pump The required capacity is the flow rate that must be handled in gal/min, million gal/day, ft3 /s, gal/h, bbl/day, lb/h, acre ft/day, mil/h, or some similar measure. Obtain the required flow rate from the process conditions, for example, boiler feed Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 346. 7.28 PLANT AND FACILITIES ENGINEERING rate, cooling-water flow rate, chemical feed rate, etc. The required flow rate for any process unit is usually given by the manufacturer or can be computed by using the calculation procedures given throughout this handbook. Once the required flow rate is determined, apply a suitable factor of safety. The value of this factor of safety can vary from a low of 5 percent of the required flow to a high of 50 percent or more, depending on the application. Typical safety factors are in the 10 percent range. With flow rates up to 1000 gal/min (63.1 L/s), and in the selection of process pumps, it is common practice to round a computed required flow rate to the next highest round-number capacity. Thus, with a required flow rate of 450 gal/min (28.4 L/s) and a 10 percent safety factor, the flow of 450 ⫹ 0.10(450) ⫽ 495 gal/min (31.2 L/s) would be rounded to 500 gal/min (31.6 L/s) before the pump was selected. A pump of 500-gal/min (31.6-L/s), or larger, capacity would be selected. 3. Compute the total head on the pump Use the steps given in the previous calculation procedure to compute the total head on the pump. Express the result in ft (m) of water—this is the most common way of expressing the head on a pump. Be certain to use the exact specific gravity of the liquid handled when expressing the head in ft (m) of water. A specific gravity less than 1.00 reduces the total head when expressed in ft (m) of water; whereas a specific gravity greater than 1.00 increases the total head when expressed in ft (m) of water. Note that variations in the suction and discharge conditions can affect the total head on the pump. 4. Analyze the liquid conditions Obtain complete data on the liquid pumped. These data should include the name and chemical formula of the liquid, maximum and minimum pumping temperature, corresponding vapor pressure at these temperatures, specific gravity, viscosity at the pumping temperature, pH, flash point, ignition temperature, unusual character- istics (such as tendency to foam, curd, crystallize, become gelatinous or tacky), solids content, type of solids and their size, and variation in the chemical analysis of the liquid. Enter the liquid conditions on a pump selection form like that in Fig. 13. Such forms are available from many pump manufacturers or can be prepared to meet special job conditions. 5. Select the class and type of pump Three classes of pumps are used today—centrifugal, rotary, and reciprocating, Fig. 14. Note that these terms apply only to the mechanics of moving the liquid—not to the service for which the pump was designed. Each class of pump is further subdivided into a number of types, Fig. 14. Use Table 11 as a general guide to the class and type of pump to be used. For example, when a large capacity at moderate pressure is required, Table 11 shows that a centrifugal pump would probably be best. Table 11 also shows the typical characteristics of various classes and types of pumps used in industrial process work. Consider the liquid properties when choosing the class and type of pump, be- cause exceptionally severe conditions may rule out one or another class of pump at the start. Thus, screw- and gear-type rotary pumps are suitable for handling viscous, nonabrasive liquid, Table 11. When an abrasive liquid must be handled, either another class of pump or another type of rotary pump must be used. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 347. PUMPS AND PUMPING SYSTEMS 7.29 FIGURE 13 Typical selection chart for centrifugal pumps. (Worthington Corporation.) Also consider all the operating factors related to the particular pump. These factors include the type of service (continuous or intermittent), operating-speed preferences, future load expected and its effect on pump head and capacity, main- tenance facilities available, possibility of parallel or series hookup, and other con- ditions peculiar to a given job. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 348. 7.30 PLANT AND FACILITIES ENGINEERING FIGURE 14 Modern pump classes and types. Once the class and type of pump is selected, consult a rating table (Table 12) or rating chart, Fig. 15, to determine whether a suitable pump is available from the manufacturer whose unit will be used. When the hydraulic requirements fall be- tween two standard pump models, it is usual practice to choose the next larger size of pump, unless there is some reason why an exact head and capacity are required for the unit. When one manufacturer does not have the desired unit, refer to the engineering data of other manufacturers. Also keep in mind that some pumps are custom-built for a given job when precise head and capacity requirements must be met. Other pump data included in manufacturer’s engineering information include characteristic curves for various diameter impellers in the same casing, Fig. 16, and variable-speed head-capacity curves for an impeller of given diameter, Fig. 17. Note that the required power input is given in Figs. 15 and 16 and may also be given in Fig. 17. Use of Table 12 is explained in the table. Performance data for rotary pumps are given in several forms. Figure 18 shows a typical plot of the head and capacity ranges of different types of rotary pumps. Reciprocating-pump capacity data are often tabulated, as in Table 13. 6. Evaluate the pump chosen for the installation Check the specific speed of a centrifugal pump, using the method given in an earlier calculation procedure. Once the specific speed is known, the impeller type and approximate operating efficiency can be found from Fig. 9. Check the piping system, using the method of an earlier calculation procedure, to see whether the available net positive suction head equals, or is greater than, the required net positive suction head of the pump. Determine whether a vertical or horizontal pump is more desirable. From the standpoint of floor space occupied, required NPSH, priming, and flexibility in Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 349. PUMPS AND PUMPING SYSTEMS 7.31 TABLE 11 Characteristics of Modern Pumps changing the pump use, vertical pumps may be preferable to horizontal designs in some installations. But where headroom, corrosion, abrasion, and ease of mainte- nance are important factors, horizontal pumps may be preferable. As a general guide, single-suction centrifugal pumps handle up to 50 gal/min (3.2 L/s) at total heads up to 50 ft (15.2 m); either single- or double-suction pumps are used for the flow rates to 1000 gal/min (63.1 L/s) and total heads to 300 ft (91.4 m); beyond these capacities and heads, double-suction or multistage pumps are generally used. Mechanical seals are becoming more popular for all types of centrifugal pumps in a variety of services. Although they are more costly than packing, the mechanical seal reduces pump maintenance costs. Related Calculations. Use the procedure given here to select any class of pump—centrifugal, rotary, or reciprocating—for any type of service—power plant, atomic energy, petroleum processing, chemical manufacture, paper mills, textile mills, rubber factories, food processing, water supply, sewage and sump service, air conditioning and heating, irrigation and flood control, mining and construction, marine services, industrial hydraulics, iron and steel manufacture. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 350. 7.32 PLANT AND FACILITIES ENGINEERING TABLE 12 Typical Centrifugal-Pump Rating Table FIGURE 15 Composite rating chart for a typical centrifugal pump. (Goulds Pumps, Inc.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 351. PUMPS AND PUMPING SYSTEMS 7.33 FIGURE 16 Pump characteristics when impeller diameter is varied within the same casing. FIGURE 17 Variable-speed head-capacity curves for a centrifugal pump. ANALYSIS OF PUMP AND SYSTEM CHARACTERISTIC CURVES Analyze a set of pump and system characteristic curves for the following conditions: friction losses without static head; friction losses with static head; pump without lift; system with little friction, much static head; system with gravity head; system with different pipe sizes; system with two discharge heads; system with diverted flow; and effect of pump wear on characteristic curve. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 352. 7.34 PLANT AND FACILITIES ENGINEERING FIGURE 18 Capacity ranges of some rotary pumps. (Worthington Corpora- tion.) TABLE 13 Capacities of Typical Horizontal Duplex Plunger Pumps Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 353. PUMPS AND PUMPING SYSTEMS 7.35 FIGURE 19 Typical system-friction curve. Calculation Procedure: 1. Plot the system-friction curve Without static head, the system-friction curve passes through the origin (0,0), Fig. 19, because when no head is developed by the pump, flow through the piping is zero. For most piping systems, the friction-head loss varies as the square of the liquid flow rate in the system. Hence, a system-friction curve, also called a friction- head curve, is parabolic—the friction head increases as the flow rate or capacity of the system increases. Draw the curve as shown in Fig. 19. 2. Plot the piping system and system-head curve Figure 20a shows a typical piping system with a pump operating against a static discharge head. Indicate the total static head, Fig. 20b, by a dashed line—in this installation Hts ⫽ 110 ft. Since static head is a physical dimension, it does not vary with flow rate and is a constant for all flow rates. Draw the dashed line parallel to the abscissa, Fig. 20b. From the point of no flow—zero capacity—plot the friction-head loss at various flow rates—100, 200, 300 gal/min (6.3, 12.6, 18.9 L/s), etc. Determine the friction- head loss by computing it as shown in an earlier calculation procedure. Draw a curve through the points obtained. This is called the system-head curve. Plot the pump head-capacity (H-Q) curve of the pump on Fig. 20b. The H-Q curve can be obtained from the pump manufacturer or from a tabulation of H and Q values for the pump being considered. The point of intersection A between the H-Q and system-head curves is the operating point of the pump. Changing the resistance of a given piping system by partially closing a valve or making some other change in the friction alters the position of the system-head curve and pump operating point. Compute the frictional resistance as before, and plot the artificial system-head curve as shown. Where this curve intersects the H-Q curve is the new operating point of the pump. System-head curves are valuable for analyzing the suitability of a given pump for a particular application. 3. Plot the no-lift system-head curve and compute the losses With no static head or lift, the system-head curve passes through the origin (0,0), Fig. 21. For a flow of 900 gal/min (56.8 L/s) in this system, compute the friction loss as follows, using the Hydraulic Institute Pipe Friction Manual tables or the method of earlier calculation procedures: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 354. 7.36 PLANT AND FACILITIES ENGINEERING FIGURE 20 (a) Significant friction loss and lift; (b) system-head curve superimposed on pump head-capacity curve. (Peerless Pumps.) FIGURE 21 No lift; all friction head. (Peerless Pumps.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 355. PUMPS AND PUMPING SYSTEMS 7.37 FIGURE 22 Mostly lift; little friction head. (Peerless Pumps.) Compute the friction loss at other flow rates in a similar manner, and plot the system-head curve, Fig. 21. Note that if all losses in this system except the friction in the discharge pipe were ignored, the total head would not change appreciably. However, for the purposes of accuracy, all losses should always be computed. 4. Plot the low-friction, high-head system-head curve The system-head curve for the vertical pump installation in Fig. 22 starts at the total static head, 15 ft (4.6 m), and zero flow. Compute the friction head for 15,000 gal/min as follows: Hence, almost 90 percent of the total head of 15 ⫹ 2 ⫽ 17 ft (5.2 m) at 15,000- gal/min (946.4-L/s) flow is static head. But neglect of the pipe friction and exit losses could cause appreciable error during selection of a pump for the job. 5. Plot the gravity-head system-head curve In a system with gravity head (also called negative lift), fluid flow will continue until the system friction loss equals the available gravity head. In Fig. 23 the avail- able gravity head is 50 ft (15.2 m). Flows up to 7200 gal/min (454.3 L/s) are obtained by gravity head alone. To obtain larger flow rates, a pump is needed to Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 356. 7.38 PLANT AND FACILITIES ENGINEERING FIGURE 23 Negative lift (gravity head). (Peerless Pumps.) FIGURE 24 System with two different pipe sizes. (Peerless Pumps.) overcome the friction in the piping between the tanks. Compute the friction loss for several flow rates as follows: Using these three flow rates, plot the system-head curve, Fig. 23. 6. Plot the system-head curves for different pipe sizes When different diameter pipes are used, the friction loss vs. flow rate is plotted independently for the two pipe sizes. At a given flow rate, the total friction loss for the system is the sum of the loss for the two pipes. Thus, the combined system- head curve represents the sum of the static head and the friction losses for all portions of the pipe. Figure 24 shows a system with two different pipe sizes. Compute the friction losses as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 357. PUMPS AND PUMPING SYSTEMS 7.39 FIGURE 25 System with two different discharge heads. (Peerless Pumps.) Compute the total head at other flow rates, and then plot the system-head curve as shown in Fig. 24. 7. Plot the system-head curve for two discharge heads Figure 25 shows a typical pumping system having two different discharge heads. Plot separate system-head curves when the discharge heads are different. Add the flow rates for the two pipes at the same head to find points on the combined system- head curve, Fig. 25. Thus, The flow rate for the combined system at a head of 88 ft (26.8 m) is 1150 ⫹ 550 ⫽ 1700 gal/min (107.3 L/s). To produce a flow of 1700 gal/min (107.3 L/s) through this system, a pump capable of developing an 88-ft (26.8-m) head is required. 8. Plot the system-head curve for diverted flow To analyze a system with diverted flow, assume that a constant quantity of liquid is tapped off at the intermediate point. Plot the friction loss vs. flow rate in the normal manner for pipe 1, Fig. 26. Move the curve for pipe 3 to the right at zero head by an amount equal to Q2, since this represents the quantity passing through Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 358. 7.40 PLANT AND FACILITIES ENGINEERING FIGURE 26 Part of the fluid flow is diverted from the main pipe. (Peerless Pumps.) FIGURE 27 Effect of pump wear on pump capacity. (Peerless Pumps.) pipes 1 and 2 but not through pipe 3. Plot the combined system-head curve by adding, at a given flow rate, the head losses for pipes 1 and 3. With Q ⫽ 300 gal/min (18.9 L/s), pipe 1 ⫽ 500 ft (152.4 m) of 10-in (254-mm) pipe, and pipe 3 ⫽ 50 ft (15.2 m) of 6-in (152.4-mm) pipe. 9. Plot the effect of pump wear When a pump wears, there is a loss in capacity and efficiency. The amount of loss depends, however, on the shape of the system-head curve. For a centrifugal pump, Fig. 27, the capacity loss is greater for a given amount of wear if the system-head curve is flat, as compared with a steep system-head curve. Determine the capacity loss for a worn pump by plotting its H-Q curve. Find this curve by testing the pump at different capacities and plotting the corresponding head. On the same chart, plot the H-Q curve for a new pump of the same size, Fig. 27. Plot the system-head curve, and determine the capacity loss as shown in Fig. 27. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 359. PUMPS AND PUMPING SYSTEMS 7.41 Related Calculations. Use the techniques given here for any type of pump—centrifugal, reciprocating, or rotary—handling any type of liquid—oil, wa- ter, chemicals, etc. The methods given here are the work of Melvin Mann, as reported in Chemical Engineering, and Peerless Pump Division of FMC Corp. NET POSITIVE SUCTION HEAD FOR HOT-LIQUID PUMPS What is the maximum capacity of a double-suction condensate pump operating at 1750 r/min if it handles 100⬚F (37.8⬚C) water from a hot well in a condenser having an absolute pressure of 2.0 in (50.8 mm) Hg if the pump centerline is 10 ft (30.5 m) below the hot-well liquid level and the friction-head loss in the suction piping and fitting is 5 ft (1.52 m) of water? Calculation Procedure: 1. Compute the net positive suction head on the pump The net positive suction head hn on a pump when the liquid supply is above the pump inlet ⫽ pressure on liquid surface ⫹ static suction head ⫺ friction-head loss in suction piping and pump inlet ⫺ vapor pressure of the liquid, all expressed in ft absolute of liquid handled. When the liquid supply is below the pump centerline—i.e., there is a static suction lift—the vertical distance of the lift is subtracted from the pressure on the liquid surface instead of added as in the above relation. The density of 100⬚F (37.8⬚C) water is 62.0 lb/ft3 (992.6 kg/m3 ), computed as shown in earlier calculation procedures in this handbook. The pressure on the liquid surface, in absolute ft of liquid ⫽ (2.0 inHg)(1.133)(62.4/62.0) ⫽ 2.24 ft (0.68 m). In this calculation, 1.133 ⫽ ft of 39.2⬚F (4⬚C) water ⫽ 1 inHg; 62.4 ⫽ lb/ft3 (999.0 kg/m3 ) of 39.2⬚F (4⬚C) water. The temperature of 39.2⬚F (4⬚C) is used because at this temperature water has its maximum density. Thus, to convert inHg to ft absolute of water, find the product of (inHg)(1.133)(water density at 39.2⬚F)/(water density at operating temperature). Express both density values in the same unit, usually lb/ft3 . The static suction head is a physical dimension that is measured in ft (m) of liquid at the operating temperature. In this installation, hsh ⫽ 10 ft (3 m) absolute. The friction-head loss is 5 ft (1.52 m) of water. When it is computed by using the methods of earlier calculation procedures, this head loss is in ft (m) of water at maximum density. To convert to ft absolute, multiply by the ratio of water den- sities at 39.2⬚F (4⬚C) and the operating temperature, or (5)(62.4/62.0) ⫽ 5.03 ft (1.53 m). The vapor pressure of water at 100⬚F (37.8⬚C) is 0.949 lb/in2 (abs) (6.5 kPa) from the steam tables. Convert any vapor pressure to ft absolute by finding the result of [vapor pressure, lb/in2 (abs)] (144 in2 /ft2 )/liquid density at operating tem- perature, or (0.949)(144)/62.0 ⫽ 2.204 ft (0.67 m) absolute. With all the heads known, the net positive suction head is hn ⫽ 2.24 ⫹ 10 ⫺ 5.03 ⫺ 2.204 ⫽ 5.01 ft (1.53 m) absolute. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 360. 7.42 PLANT AND FACILITIES ENGINEERING FIGURE 28 Capacity and speed limitations of condensate pumps with the shaft through the impeller eye. (Hydraulic Institute.) 2. Determine the capacity of the condensate pump Use the Hydraulic Institute curve, Fig. 28, to determine the maximum capacity of the pump. Enter at the left of Fig. 28 at a net positive suction head of 5.01 ft (1.53 m), and project horizontally to the right until the 3500-r/min curve is intersected. At the top, read the capacity as 278 gal/min (17.5 L/s). Related Calculations: Use this procedure for any condensate or boiler-feed pump handling water at an elevated temperature. Consult the Standards of the Hydraulic Institute for capacity curves of pumps having different types of construc- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 361. PUMPS AND PUMPING SYSTEMS 7.43 tion. In general, pump manufacturers who are members of the Hydraulic Institute rate their pumps in accordance with the Standards, and a pump chosen from a catalog capacity table or curve will deliver the stated capacity. A similar procedure is used for computing the capacity of pumps handling volatile petroleum liquids. When you use this procedure, be certain to refer to the latest edition of the Stan- dards. CONDENSATE PUMP SELECTION FOR A STEAM POWER PLANT Select the capacity for a condensate pump serving a steam power plant having a 140,000 lb/h (63,000 kg/h) exhaust flow to a condenser that operates at an absolute pressure of 1.0 in (25.4 mm) Hg. The condensate pump discharges through 4-in (101.6-mm) schedule 40 pipe to an air-ejector condenser that has a frictional resis- tance of 8 ft (2.4 m) of water. From here, the condensate flows to and through a low-pressure heater that has a frictional resistance of 12 ft (3.7 m) of water and is vented to the atmosphere. The total equivalent length of the discharge piping, in- cluding all fittings and bends, is 400 ft (121.9 m), and the suction piping total equivalent length is 50 ft (15.2 m). The inlet of the low pressure heater is 75 ft (22.9 m) above the pump centerline, and the condenser hot-well water level is 10 ft (3 m) above the pump centerline. How much power is required to drive the pump if its efficiency is 70 percent? Calculation Procedure: 1. Compute the static head on the pump Sketch the piping system as shown in Fig. 29. Mark the static elevations and equiv- alent lengths as indicated. The total head on the pump Ht ⫽ Hts ⫹ where the symbols are the same as H , ƒ in earlier calculation procedures. The total static head Hts ⫽ hsd ⫺ hsh . In this installation, hsd ⫽ 75 ft (22.9 m). To make the calculation simpler, convert all the heads to absolute values. Since the heater is vented to the atmosphere, the pressure acting on the surface of the water in it ⫽ 14.7 lb/in2 (abs) (101.3 kPa), or 34 ft (10.4 m) of water. The pressure acting on the condensate in the hot well is 1 in (25.4 mm) Hg ⫽ 1.133 ft (0.35 m) of water. [An absolute pressure of 1 in (25.4 mm) Hg ⫽ 1.133 ft (0.35 m) of water.] Thus, the absolute discharge static head ⫽ 75 ⫹ 34 ⫽ 109 ft (33.2 m), whereas the absolute suction head ⫽ 10 ⫹ 1.13 ⫽ 11.13 ft (3.39 m). Then Hts ⫽ hhd ⫺ hsh ⫽ 109.00 ⫺ 11.13 ⫽ 97.87 ft (29.8 m), say 98 ft (29.9 m) of water. 2. Compute the friction head in the piping system The total friction head ⫽ pipe friction ⫹ heater friction. The pipe friction loss Hƒ is found first, as shown below. The heater friction loss, obtained from the manu- facturer or engineering data, is then added to the pipe-friction loss. Both must be expressed in ft (m) of water. To determine the pipe friction, use Fig. 30 of this section and Table 17 and Fig. 6 of the Piping section of this handbook in the following manner. Find the product of the liquid velocity, ft/s, and the pipe internal diameter, in, or vd. With an exhaust flow of 140,000 lb/h (63,636 kg/h) to the condenser, the condensate flow is the same, or 140,000 lb/h (63,636 kg/h) at a temperature of 79.03⬚F (21.6⬚C), corre- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 362. 7.44 PLANT AND FACILITIES ENGINEERING FIGURE 29 Condensate pump serving a steam power plant. sponding to an absolute pressure in the condenser of 1 in (25.4 mm) Hg, obtained from the steam tables. The specific volume of the saturated liquid at this temperature and pressure is 0.01608 ft3 /lb (0.001 m3 /kg). Since 1 gal (0.26 L) of liquid occupies 0.13368 ft3 (0.004 m3 ), specific volume, gal/lb, is (0.01608/0.13368) ⫽ 0.1202 (1.01 L/kg). Therefore, a flow of 140,000 lb/h (63,636 kg/h) ⫽ a flow of (140,000)(0.1202) ⫽ 16,840 gal/h (63,739.4 L/h), or 16,840/60 ⫽ 281 gal/min (17.7 L/s). Then the liquid velocity v ⫽ gpm /2.448d2 ⫽ 281/2.448(4.026)2 ⫽ 7.1 ft/s (2.1 m/s), and the product vd ⫽ (7.1)(4.026) ⫽ 28.55. Enter Fig. 30 at a temperature of 79⬚F (26.1⬚C), and project vertically upward to the water curve. From the intersection, project horizontally to the right to vd ⫽ 28.55 and then vertically upward to read R ⫽ 250,000. Using Table 17 and Fig. 6 of the Piping section and R ⫽ 250,000, find the friction factor ƒ ⫽ 0.0185. Then the head loss due to pipe friction ⫽ (L/D)(v2 /2g) ⫽ 0.0185 (450/4.026/ Hƒ 12)/[(7.1)2 /2(32.2)] ⫽ 19.18 ft (5.9 m). In this computation, L ⫽ total equivalent length of the pipe, pipe fittings, and system valves, or 450 ft (137.2 m). 3. Compute the other head losses in the system There are two other head losses in this piping system: the entrance loss at the square-edged hot-well pipe leading to the pump and the sudden enlargement in the low-pressure heater. The velocity head v2 /2g ⫽ (7.1)2 /2(32.2) ⫽ 0.784 ft (0.24 m). Using k values from Fig. 11 in this section, he ⫽ kv2 /2g ⫽ (0.5)(0.784) ⫽ 0.392 ft (0.12 m); hex ⫽ v2 /2g ⫽ 0.784 ft (0.24 m). 4. Find the total head on the pump The total head on the pump Ht ⫽ Hts ⫹ ⫽ 97.87 ⫹ 19.18 ⫹ 8 ⫹ 12 ⫹ Hƒ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 363. 7.45 FIGURE 30 Kinematic viscosity and Reynolds number chart. (Hydraulic Institute.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 364. 7.46 PLANT AND FACILITIES ENGINEERING 0.392 ⫹ 0.784 ⫽ 138.226 ft (42.1 m), say 140 ft (42.7 m) of water. In this calcu- lation, the 8-(2.4-m) and 12-ft (3.7-m) head losses are those occurring in the heaters. With a 25 percent safety factor, total head ⫽ (1.25)(140) ⫽ 175 ft (53.3 m). 5. Compute the horsepower required to drive the pump The brake horsepower input bhpi ⫽ (gpm)(Ht )(s)/3960e, where the symbols are the same as in earlier calculation procedures. At 1 in (25.4 mm) Hg, 1 lb (0.45 kg) of the condensate has a volume of 0.01608 ft3 (0.000455 m3 ). Since density ⫽ 1/ specific volume, the density of the condensate ⫽ 1/0.01608 ⫽ 62.25 ft3 /lb (3.89 m3 /kg). Water having a specific gravity of unity weighs 62.4 lb/ft3 (999 kg/m3 ). Hence, the specific gravity of the condensate is 62.25/62.4 ⫽ 0.997. Then, assum- ing that the pump has an operating efficiency of 70 percent, we get bhpi ⫽ (281)(175) ⫻ (0.997)/[3960(0.70)] ⫽ 17.7 bhp (13.2 kW). 6. Select the condensate pump Condensate or hot-well pumps are usually centrifugal units having two or more stages, with the stage inlets opposed to give better axial balance and to subject the sealing glands to positive internal pressure, thereby preventing air leakage into the pump. In the head range developed by this pump, 175 ft (53.3 m), two stages are satisfactory. Refer to a pump manufacturer’s engineering data for specific stage head ranges. Either a turbine or motor drive can be used. Related Calculations. Use this procedure to choose condensate pumps for steam plants of any type—utility, industrial, marine, portable, heating, or process—and for combined steam-diesel plants. MINIMUM SAFE FLOW FOR A CENTRIFUGAL PUMP A centrifugal pump handles 220⬚F (104.4⬚C) water and has a shutoff head (with closed discharge valve) of 3200 ft (975.4 m). At shutoff, the pump efficiency is 17 percent and the input brake horsepower is 210 (156.7 kW). What is the minimum safe flow through this pump to prevent overheating at shutoff? Determine the min- imum safe flow if the NPSH is 18.8 ft (5.7 m) of water and the liquid specific gravity is 0.995. If the pump contains 500 lb (225 kg) of water, determine the rate of the temperature rise at shutoff. Calculation Procedure: 1. Compute the temperature rise in the pump With the discharge valve closed, the power input to the pump is converted to heat in the casing and causes the liquid temperature to rise. The temperature rise t ⫽ (1 ⫺ e) ⫻ Hs /778e, where t ⫽ temperature rise during shutoff, ⬚F; e ⫽ pump efficiency, expressed as a decimal; Hs ⫽ shutoff head, ft. For this pump, t ⫽ (1 ⫺ 0.17)(3200)/[778(0.17)] ⫽ 20.4⬚F (36.7⬚C). 2. Compute the minimum safe liquid flow For general-service pumps, the minimum safe flow M gal/min ⫽ 6.0(bhp input at shutoff)/t. Or, M ⫽ 6.0(210)/20.4 ⫽ 62.7 gal/min (3.96 L/s). This equation in- cludes a 20 percent safety factor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 365. PUMPS AND PUMPING SYSTEMS 7.47 Centrifugal boiler-feed pumps usually have a maximum allowable temperature rise of 15⬚F (27⬚C). The minimum allowable flow through the pump to prevent the water temperature from rising more than 15⬚F (27⬚C) is 30 gal/min (1.89 L/s) for each 110-bhp (74.6-kW) input at shutoff. 3. Compute the temperature rise for the operating NPSH An NPSH of 18.8 ft (5.73 m) is equivalent to a pressure of 18.8(0.433)(0.995) ⫽ 7.78 lb/in2 (abs) (53.6 kPa) at 220⬚F (104.4⬚C), where the factor 0.433 converts ft of water to lb/in2 . At 220⬚F (104.4⬚C), the vapor pressure of the water is 17.19 lb/in2 (abs) (118.5 kPa), from the steam tables. Thus, the total vapor pressure the water can develop before flashing occurs ⫽ NPSH pressure ⫹ vapor pressure at operating temperature ⫽ 7.78 ⫹ 17.19 ⫽ 24.97 lb/in2 (abs) (172.1 kPa). Enter the steam tables at this pressure, and read the cor- responding temperature as 240⬚F (115.6⬚C). The allowable temperature rise of the water is then 240 ⫺ 220 ⫽ 20⬚F (36.0⬚C). Using the safe-flow relation of step 2, we find the minimum safe flow is 62.9 gal/min (3.97 L/s). 4. Compute the rate of temperature rise In any centrifugal pump, the rate of temperature rise tr ⬚F/min ⫽ 42.4(bhp input at shutoff)/wc, where w ⫽ weight of liquid in the pump, lb; c ⫽ specific heat of the liquid in the pump, Btu/(lb ⬚F). For this pump containing 500 lb (225 kg) of water with a specific heat, c ⫽ 1.0, tr ⫽ 42.4(210)/[500(1.0)] ⫽ 17.8⬚F/min (32⬚C/min). This is a very rapid temperature rise and could lead to overheating in a few minutes. Related Calculations. Use this procedure for any centrifugal pump handling any liquid in any service—power, process, marine, industrial, or commercial. Pump manufacturers can supply a temperature-rise curve for a given model pump if it is requested. This curve is superimposed on the pump characteristic curve and shows the temperature rise accompanying a specific flow through the pump. SELECTING A CENTRIFUGAL PUMP TO HANDLE A VISCOUS LIQUID Select a centrifugal pump to deliver 750 gal/min (47.3 L/s) of 1000-SSU oil at a total head of 100 ft (30.5 m). The oil has a specific gravity of 0.90 at the pumping temperature. Show how to plot the characteristic curves when the pump is handling the viscous liquid. Calculation Procedure: 1. Determine the required correction factors A centrifugal pump handling a viscous liquid usually must develop a greater ca- pacity and head, and it requires a larger power input than the same pump handling water. With the water performance of the pump known—from either the pump characteristic curves or a tabulation of pump performance parameters—Fig. 31, prepared by the Hydraulic Institute, can be used to find suitable correction factors. Use this chart only within its scale limits; do not extrapolate. Do not use the chart for mixed-flow or axial-flow pumps or for pumps of special design. Use the chart only for pumps handling uniform liquids; slurries, gels, paper stock, etc., may cause Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 366. 7.48 PLANT AND FACILITIES ENGINEERING FIGURE 31 Correction factors for viscous liquids handled by centrifugal pumps. (Hydraulic Institute.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 367. PUMPS AND PUMPING SYSTEMS 7.49 incorrect results. In using the chart, the available net positive suction head is as- sumed adequate for the pump. To use Fig. 31, enter at the bottom at the required capacity, 750 gal/min (47.3 L/s), and project vertically to intersect the 100-ft (30.5-m) head curve, the required head. From here project horizontally to the 1000-SSU viscosity curve, and then vertically upward to the correction-factor curves. Read CE ⫽ 0.635; CQ ⫽ 0.95; CH ⫽ 0.92 for 1.0QNW . The subscripts E, Q, and H refer to correction factors for efficiency, capacity, and head, respectively; and NW refers to the water capacity at a particular efficiency. At maximum efficiency, the water capacity is given as 1.0QNW ; other efficiencies, expressed by numbers equal to or less than unity, give different capacities. 2. Compute the water characteristics required The water capacity required for the pump Qw ⫽ /CQ where ⫽ viscous ca- Q Q v v pacity, gal/min. For this pump, Qw ⫽ 750/0.95 ⫽ 790 gal/min (49.8 L/s). Like- wise, water head Hw ⫽ /CH, where ⫽ viscous head. Or, Hw ⫽ 100/ H H v v 0.92 ⫽ 108.8 (33.2 m), say 109 ft (33.2 m) of water. Choose a pump to deliver 790 gal/min (49.8 L/s) of water at 109-ft (33.2-m) head of water, and the required viscous head and capacity will be obtained. Pick the pump so that it is operating at or near its maximum efficiency on water. If the water efficiency Ew ⫽ 81 percent at 790 gal/min (49.8 L/s) for this pump, the efficiency when handling the viscous liquid ⫽ Ew CE . Or, ⫽ E E v v 0.81(0.635) ⫽ 0.515, or 51.5 percent. The power input to the pump when handling viscous liquids is given by P ⫽ v where s ⫽ specific gravity of the viscous liquid. For this pump, Q H s /3960E , v v v ⫽ (750) ⫻ (100)(0.90)/[3960(0.515)] ⫽ 33.1 hp (24.7 kW). Pv 3. Plot the characteristic curves for viscous-liquid pumping Follow these eight steps to plot the complete characteristic curves of a centrifugal pump handling a viscous liquid when the water characteristics are known: (a) Se- cure a complete set of characteristic curves (H, Q, P, E) for the pump to be used. (b) Locate the point of maximum efficiency for the pump when handling water. (c) Read the pump capacity, Q gal/min, at this point. (d) Compute the values of 0.6Q, 0.8Q, and 1.2Q at the maximum efficiency. (e) Using Fig. 31, determine the correction factors at the capacities in steps c and d. Where a multistage pump is being considered, use the head per stage (⫽ total pump head, ft/number of stages), when entering Fig. 31. (ƒ) Correct the head, capacity, and efficiency for each of the flow rates in c and d, using the correction factors from Fig. 31. (g) Plot the corrected head and efficiency against the corrected capacity, as in Fig. 32. (h) Compute the power input at each flow rate and plot. Draw smooth curves through the points obtained, Fig. 32. Related Calculations. Use the method given here for any uniform viscous liquid—oil, gasoline, kerosene, mercury, etc—handled by a centrifugal pump. Be careful to use Fig. 31 only within its scale limits; do not extrapolate. The method presented here is that developed by the Hydraulic Institute. For new developments in the method, be certain to consult the latest edition of the Hydraulic Institute Standards. PUMP SHAFT DEFLECTION AND CRITICAL SPEED What are the shaft deflection and approximate first critical speed of a centrifugal pump if the total combined weight of the pump impellers is 23 lb (10.4 kg) and the pump manufacturer supplies the engineering data in Fig. 33? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 368. 7.50 PLANT AND FACILITIES ENGINEERING FIGURE 32 Characteristics curves for water (solid line) and oil (dashed line). (Hydraulic Institute.) FIGURE 33 Pump shaft deflection and critical speed. (Goulds Pumps, Inc.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 369. PUMPS AND PUMPING SYSTEMS 7.51 Calculation Procedure: 1. Determine the deflection of the pump shaft Use Fig. 33 to determine the shaft deflection. Note that this chart is valid for only one pump or series of pumps and must be obtained from the pump builder. Such a chart is difficult to prepare from test data without extensive test facilities. Enter Fig. 33 at the left at the total combined weight of the impellers, 23 lb (10.4 kg), and project horizontally to the right until the weight-deflection curve is intersected. From the intersection, project vertically downward to read the shaft deflection as 0.009 in (0.23 mm) at full speed. 2. Determine the critical speed of the pump From the intersection of the weight-deflection curve in Fig. 33 project vertically upward to the critical-speed curve. Project horizontally right from this intersection and read the first critical speed as 6200 r/min. Related Calculations. Use this procedure for any class of pump—centrifugal, rotary, or reciprocating—for which the shaft-deflection and critical-speed curves are available. These pumps can be used for any purpose—process, power, marine, in- dustrial, or commercial. EFFECT OF LIQUID VISCOSITY ON REGENERATIVE-PUMP PERFORMANCE A regenerative (turbine) pump has the water head-capacity and power-input char- acteristics shown in Fig. 34. Determine the head-capacity and power-input char- acteristics for four different viscosity oils to be handled by the pump—400, 600, 900, and 1000 SSU. What effect does increased viscosity have on the performance of the pump? Calculation Procedure: 1. Plot the water characteristics of the pump Obtain a tabulation or plot of the water characteristics of the pump from the man- ufacturer or from their engineering data. With a tabulation of the characteristics, enter the various capacity and power points given, and draw a smooth curve through them, Fig. 34. 2. Plot the viscous-liquid characteristics of the pump The viscous-liquid characteristics of regenerative-type pumps are obtained by test of the actual unit. Hence, the only source of this information is the pump manu- facturer. Obtain these characteristics from the pump manufacturer or their test data, and plot them on Fig. 34, as shown, for each oil or other liquid handled. 3. Evaluate the effect of viscosity on pump performance Study Fig. 34 to determine the effect of increased liquid viscosity on the perform- ance of the pump. Thus at a given head, say 100 ft (30.5 m), the capacity of the pump decreases as the liquid viscosity increases. At 100-ft (30.5-m) head, this pump has a water capacity of 43.5 gal/min (2.74 L/s), Fig. 34. The pump capacity for the various oils at 100-ft (30.5-m) head is 36 gal/min (2.27 L/s) for 400 SSU; 32 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 370. 7.52 PLANT AND FACILITIES ENGINEERING FIGURE 34 Regenerative pump performance when handling water and oil. (Aurora Pump Di- vision, The New York Air Brake Company.) gal/min (2.02 L/s) for 600 SSU; 28 gal/min (1.77 L/s) for 900 SSU; and 26 gal/ min (1.64 L/s) for 1000 SSU, respectively. There is a similar reduction in capacity of the pump at the other heads plotted in Fig. 34. Thus, as a general rule, the capacity of a regenerative pump decreases with an increase in liquid viscosity at constant head. Or conversely, at constant capacity, the head developed decreases as the liquid viscosity increases. Plots of the power input to this pump show that the input power increases as the liquid viscosity increases. Related Calculations. Use this procedure for a regenerative-type pump han- dling any liquid—water, oil, kerosene, gasoline, etc. A decrease in the viscosity of a liquid, as compared with the viscosity of water, will produce the opposite effect from that of increased viscosity. EFFECT OF LIQUID VISCOSITY ON RECIPROCATING-PUMP PERFORMANCE A direct-acting steam-driven reciprocating pump delivers 100 gal/min (6.31 L/s) of 70⬚F (21.1⬚C) water when operating at 50 strokes per minute. How much 2000- SSU crude oil will this pump deliver? How much 125⬚F (51.7⬚C) water will this pump deliver? Calculation Procedure: 1. Determine the recommended change in pump performance Reciprocating pumps of any type—direct-acting or power—having any number of liquid-handling cylinders—one to five or more—are usually rated for maximum Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 371. PUMPS AND PUMPING SYSTEMS 7.53 TABLE 14 Speed-Correction Factors delivery when handling 250-SSU liquids or 70⬚F (21.1⬚C) water. At higher liquid viscosities or water temperatures, the speed—strokes or rpm—is reduced. Table 14 shows typical recommended speed-correction factors for reciprocating pumps for various liquid viscosities and water temperatures. This table shows that with a liquid viscosity of 2000 SSU the pump speed should be reduced 20 percent. When 125⬚F (51.7⬚C) water is handled, the pump speed should be reduced 25 percent, as shown in Table 14. 2. Compute the delivery of the pump The delivery capacity of any reciprocating pump is directly proportional to the number of strokes per minute it makes or to its rpm. When 2000-SSU oil is used, the pump strokes per minute must be reduced 20 percent, or (50)(0.20) ⫽ 10 strokes/min. Hence, the pump speed will be 50 ⫺ 10 ⫽ 40 strokes/min. Since the delivery is directly proportional to speed, the delivery of 2000-SSU oil ⫽ (40/50)(100) ⫽ 80 gal/min (5.1 L/s). When handling 125⬚F (51.7⬚C) water, the pump strokes/min must be reduced 25 percent, or (50)(0.5) ⫽ 12.5 strokes/min. Hence, the pump speed will be 50.0 ⫺ 12.5 ⫽ 37.5 strokes/min. Since the delivery is directly proportional to speed, the delivery of 125⬚F (51.7⬚C) water ⫽ (37.5/50)(10) ⫽ 75 gal/min (4.7 L/s). Related Calculations. Use this procedure for any type of reciprocating pump handling liquids falling within the range of Table 14. Such liquids include oil, kerosene, gasoline, brine, water, etc. EFFECT OF VISCOSITY AND DISSOLVED GAS ON ROTARY PUMPS A rotary pump handles 8000-SSU liquid containing 5 percent entrained gas and 10 percent dissolved gas at a 20-in (508-mm) Hg pump inlet vacuum. The pump is rated at 1000 gal/min (63.1 L/s) when handling gas-free liquids at viscosities less than 600 SSU. What is the output of this pump without slip? With 10 percent slip? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 372. 7.54 PLANT AND FACILITIES ENGINEERING TABLE 15 Rotary Pump Speed Reduction for Various Liquid Viscosities Calculation Procedure: 1. Compute the required speed reduction of the pump When the liquid viscosity exceeds 600 SSU, many pump manufacturers recommend that the speed of a rotary pump be reduced to permit operation without excessive noise or vibration. The speed reduction usually recommended is shown in Table 15. With this pump handling 8000-SSU liquid, a speed reduction of 40 percent is necessary, as shown in Table 15. Since the capacity of a rotary pump varies directly with its speed, the output of this pump when handling 8000-SSU liquid ⫽ (1000 gal/min) ⫻ (1.0 ⫺ 0.40) ⫽ 600 gal/min (37.9 L/s). 2. Compute the effect of gas on the pump output Entrained or dissolved gas reduces the output or a rotary pump, as shown in Table 16. The gas in the liquid expands when the inlet pressure of the pump is below atmospheric and the gas occupies part of the pump chamber, reducing the liquid capacity. With a 20-in (508-mm) Hg inlet vacuum, 5 percent entrained gas, and 10 percent dissolved gas, Table 16 shows that the liquid displacement is 74 percent of the rated displacement. Thus, the output of the pump when handling this viscous, gas- containing liquid will be (600 gal/min) (0.74) ⫽ 444 gal/min (28.0 L/s) without slip. 3. Compute the effect of slip on the pump output Slip reduces rotary-pump output in direct proportion to the slip. Thus, with 10 percent slip, the output of this pump ⫽ (444 gal/min)(1.0 ⫺ 0.10) ⫽ 369.6 gal/ min (23.3 L/s). Related Calculations. Use this procedure for any type of rotary pump—gear, lobe, screw, swinging-vane, sliding-vane, or shuttle-block, handling any clear, vis- cous liquid. Where the liquid is gas-free, apply only the viscosity correction. Where the liquid viscosity is less than 600 SSU but the liquid contains gas or air, apply the entrained or dissolved gas correction, or both corrections. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 373. 7.55 TABLE 16 Effect of Entrained or Dissolved Gas on the Liquid Displacement of Rotary Pumps (liquid displacement: percent of displacement) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 374. 7.56 PLANT AND FACILITIES ENGINEERING SELECTION OF MATERIALS FOR PUMP PARTS Select suitable materials for the principal parts of a pump handling cold ethylene chloride. Use the Hydraulic Institute recommendation for materials of construction. Calculation Procedure: 1. Determine which materials are suitable for this pump Refer to the data section of the Hydraulic Institute Standards. This section contains a tabulation of hundreds of liquids and the pump construction materials that have been successfully used to handle each liquid. The table shows that for cold ethylene chloride having a specific gravity of 1.28, an all-bronze pump is satisfactory. In lieu of an all-bronze pump, the principal parts of the pump—casing, impeller, cylinder, and shaft—can be made of one of the following materials: austenitic steels (low-carbon 18-8; 18-8/Mo; highly alloyed stainless); nickel-base alloys containing chromium, molybdenum, and other ele- ments, and usually less than 20 percent iron; or nickel-copper alloy (Monel metal). The order of listing in the Standards does not necessarily indicate relative superi- ority, since certain factors predominating in one instance may be sufficiently over- shadowed in others to reverse the arrangement. 2. Choose the most economical pump Use the methods of earlier calculation procedures to select the most economical pump for the installation. Where the corrosion resistance of two or more pumps is equal, the standard pump, in this instance an all-bronze unit, will be the most economical. Related Calculations. Use this procedure to select the materials of construction for any class of pump—centrifugal, rotary, or reciprocating—in any type of service—power, process, marine, or commercial. Be certain to use the latest edition of the Hydraulic Institute Standards, because the recommended materials may change from one edition to the next. SIZING A HYDROPNEUMATIC STORAGE TANK A 200-gal/min (12.6-L/s) water pump serves a pumping system. Determine the capacity required for a hydropneumatic tank to serve this system if the allowable high pressure in the tank and system is 60 lb/in2 (gage) (413.6 kPa) and the allow- able low pressure is 30 lb/in2 (gage) (206.8 kPa). How many starts per hour will the pump make if the system draws 3000 gal/min (189.3 L/s) from the tank? Calculation Procedure: 1. Compute the required tank capacity If the usual hydropneumatic system, a storage-tank capacity in gal of 10 times the pump capacity in gal/min is used, if this capacity produces a moderate run- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 375. PUMPS AND PUMPING SYSTEMS 7.57 ning time for the pump. Thus, this system would have a tank capacity of (10)(200) ⫽ 2000 gal (7570.8 L). 2. Compute the quantity of liquid withdrawn per cycle For any hydropneumatic tank the withdrawal, expressed as the number of gallons (liters) withdrawn per cycle, is given by W ⫽ (vL ⫺ vH )/C, where vL ⫽ air volume in tank at the lower pressure, ft3 (m3 ); vH ⫽ volume of air in tank at higher pressure, ft3 (m3 ); C ⫽ conversion factor to convert ft3 (m3 ) to gallons (liters), as given below. Compute VL and VH using the gas law for vH and either the gas law or the reserve percentage for vL . Thus, for vH, the gas law gives vH ⫽ pL vL /pH, where pL ⫽ lower air pressure in tank, lb/in2 (abs) (kPa); pH ⫽ higher air pressure in tank lb/in2 (abs) (kPa); other symbols as before. In most hydropneumatic tanks a liquid reserve of 10 to 20 percent of the total tank volume is kept in the tank to prevent the tank from running dry and damaging the pump. Assuming a 10 percent reserve for this tank, vL ⫽ 0.1 V, where V ⫽ tank volume in ft3 (m3 ). Since a 2000-gal (7570-L) tank is being used, the volume of the tank is 2000/7.481 ft3 /gal ⫽ 267.3 ft3 (7.6 m3 ). With the 10 percent reserve at the 44.7 lb/in2 (abs) (308.2-kPa) lower pressure, vL ⫽ 0.9 (267.3) ⫽ 240.6 ft3 (6.3 m3 ), where 0.9 ⫽ V ⫺ 0.1 V. At the higher pressure in the tank, 74.7 lb/in2 (abs) (514.9 kPa), the volume of the air will be, from the gas law, vH ⫽ pL vL /pH ⫽ 44.7 (240.6)/74.7 ⫽ 143.9 ft3 (4.1 m3 ). Hence, during withdrawal, the volume of liquid removed from the tank will be Wg ⫽ (240.6 ⫺ 143.9)/0.1337 ⫽ 723.3 gal (2738 L). In this relation of the constant converts from cubic feet to gallons and is 0.1337. To convert from cubic meters to liters, use the constant 1000 in the denominator. 3. Compute the pump running time The pump has a capacity of 200 gal/min (12.6 L/s). Therefore, it will take 723/ 200 ⫽ 3.6 min to replace the withdrawn liquid. To supply 3000 gal/h (11,355 L/h) to the system, the pump must start 3000/723 ⫽ 4.1, or 5 times per hour. This is acceptable because a system in which the pump starts six or fewer times per hour is generally thought satisfactory. Where the pump capacity is insufficient to supply the system demand for short periods, use a smaller reserve. Compute the running time using the equations in steps 2 and 3. Where a larger reserve is used—say 20 percent—use the value 0.8 in the equations in step 2. For a 30 percent reserve, the value would be 0.70, and so on. Related Calculations. Use this procedure for any liquid system having a hy- dropneumatic tank—well drinking water, marine, industrial, or process. USING CENTRIFUGAL PUMPS AS HYDRAULIC TURBINES Select a centrifugal pump to serve as a hydraulic turbine power source for a 1500- gal/min (5677.5-L/min) flow rate with 1290 ft (393.1 m) of head. The power application requires a 3600-r/min speed, the specific gravity of the liquid is 0.52, and the total available exhaust head is 20 ft (6.1 m). Analyze the cavitation potential and operating characteristics at an 80 percent flow rate. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 376. 7.58 PLANT AND FACILITIES ENGINEERING Calculation Procedure: 1. Choose the number of stages for the pump Search of typical centrifugal-pump data shows that a head of 1290 ft (393.1 m) is too large for a single-stage pump of conventional design. Hence, a two-stage pump will be the preliminary choice for this application. The two-stage pump chosen will have a design head of 645 ft (196.6 m) per stage. 2. Compute the specific speed of the pump chosen Use the relation Ns ⫽ pump rpm where Ns ⫽ specific speed of the 0.5 0.75 (Q) /H , pump; rpm ⫽ r/min of pump shaft; Q ⫽ pump capacity or flow rate, gal/min; H ⫽ pump head per stage, ft. Substituting, we get Ns ⫽ 0.5 3600(1500) / ⫽ 1090. Note that the specific speed value is the same regardless of the 0.75 (645) system of units used—USCS or SI. 3. Convert turbine design conditions to pump design conditions To convert from turbine design conditions to pump design conditions, use the pump manufacturer’s conversion factors that relate turbine best efficiency point (bep) per- formance with pump bep performance. Typically, as specific speed Ns varies from 500 to 2800, these bep factors generally vary as follows: the conversion factor for capacity (gal/min or L/min) CQ, from 2.2 to 1.1; the conversion factor for head (ft or m) CH, from 2.2 to 1.1; the conversion factor for efficiency CE, from 0.92 to 0.99. Applying these conversion factors to the turbine design conditions yields the pump design conditions sought. At the specific speed for this pump, the values of these conversion factors are determined from the manufacturer to be CQ ⫽ 1.24; CH ⫽ 1.42; CE ⫽ 0.967. Given these conversion factors, the turbine design conditions can be converted to the pump design conditions thus: Qp ⫽ Qt /CQ, where Qp ⫽ pump capacity or flow rate, gal/min or L/min; Qt ⫽ turbine capacity or flow rate in the same units; other symbols are as given earlier. Substituting gives Qp ⫽ 1500/1.24 ⫽ 1210 gal/min (4580 L/min). Likewise, the pump discharge head, in feet of liquid handled, is Hp ⫽ Ht /CH . So Hp ⫽ 645/1.42 ⫽ 454 ft (138.4 m). 4. Select a suitable pump for the operating conditions Once the pump capacity, head, and rpm are known, a pump having its best bep at these conditions can be selected. Searching a set of pump characteristic curves and capacity tables shows that a two-stage 4-in (10-cm) unit with an efficiency of 77 percent would be suitable. 5. Estimate the turbine horsepower developed To predict the developed hp, convert the pump efficiency to turbine efficiency. Use the conversion factor developed above. Or, the turbine efficiency Et ⫽ Ep CE ⫽ (0.77)(0.967) ⫽ 0.745, or 74.5 percent. With the turbine efficiency known, the output brake horsepower can be found from bhp ⫽ Qt Ht Ets /3960, where s ⫽ fluid specific gravity; other symbols as before. Substituting, we get bhp ⫽ 1500(1290)(0.745)(0.52)/3960 ⫽ 198 hp (141 kW). 6. Determine the cavitation potential of this pump Just as pumping requires a minimum net positive suction head, turbine duty requires a net positive exhaust head. The relation between the total required exhaust head Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 377. PUMPS AND PUMPING SYSTEMS 7.59 FIGURE 35 Cavitation constant for hydraulic turbines. (Chemical Engineer- ing.) (TREH) and turbine head per stage is the cavitation constant r ⫽ TREH/H. Figure 35 shows r vs. Ns for hydraulic turbines. Although a pump used as a turbine will not have exactly the same relationship, this curve provides a god estimate of r for turbine duty. To prevent cavitation, the total available exhaust head (TAEH) must be greater than the TREH. In this installation, Ns ⫽ 1090 and TAEH ⫽ 20 ft (6.1 m). From Fig. 35, r ⫽ 0.028 and TREH ⫽ 0.028(645) ⫽ 18.1 ft (5.5 m). Because TAEH ⬎ TREH, there is enough exhaust head to prevent cavitation. 7. Determine the turbine performance at 80 percent flow rate In many cases, pump manufacturers treat conversion factors as proprietary infor- mation. When this occurs, the performance of the turbine under different operating conditions can be predicted from the general curves in Figs. 36 and 37. At the 80 percent flow rate for the turbine, or 1200 gal/min (4542 L/min), the operating point is 80 percent of bep capacity. For a specific speed of 1090, as before, the percentages of bep head and efficiency are shown in Figs. 36 and 37: 79.5 percent of bep head and percent of bep efficiency. To find the actual perform- ance, multiply by the bep values. Or, Ht ⫽ 0.795(1290) ⫽ 1025 ft (393.1 m); Et ⫽ 0.91(74.5) ⫽ 67.8 percent. The bhp at the new operating condition is then bhp ⫽ 1200 (1025)(0.678)(0.52)/3960 ⫽ 110 hp (82.1 kW). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 378. 7.60 PLANT AND FACILITIES ENGINEERING FIGURE 36 Constant-speed curves for turbine duty. (Chemi- cal Engineering.) FIGURE 37 Constant-speed curves for turbine duty. (Chemi- cal Engineering.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 379. PUMPS AND PUMPING SYSTEMS 7.61 FIGURE 39 Constant-head curves for turbine only. (Chemical Engineering.) FIGURE 38 Constant-head curves for turbine duty. (Chemical Engineering.) In a similar way, the constant-head curves in Figs. 38 and 39 predict turbine performance at different speeds. For example, speed is 80 percent of bep speed at 2880 r/min. For a specific speed of 1090, the percentages of bep capacity, effi- ciency, and power are 107 percent of the capacity, 94 percent of the efficiency, and 108 percent of the bhp. To get the actual performance, convert as before: Qt ⫽ 107(1500) ⫽ 1610 gal/min (6094 L/min); Et ⫽ 0.94(74.5) ⫽ 70.0 percent; bhp ⫽ 1.08(189) ⫽ 206 hp (153.7 kW). Note that the bhp in this last instance is higher than the bhp at the best efficiency point. Thus more horsepower can be obtained from a given unit by reducing the speed and increasing the flow rate. When the speed is fixed, more bhp cannot be obtained from the unit, but it may be possible to select a smaller pump for the same application. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 380. 7.62 PLANT AND FACILITIES ENGINEERING Related Calculations. Use this general procedure for choosing a centrifugal pump to drive—as a hydraulic turbine—another pump, a fan, a generator, or a compressor, where high-pressure liquid is available as a source of power. Because pumps are designed as fluid movers, they may be less efficient as hydraulic turbines than equipment designed for that purpose. Steam turbines and electric motors are more economical when steam or electricity is available. But using a pump as a turbine can pay off in remote locations where steam or electric power would require additional wiring or piping, in hazardous locations that require nonsparking equipment, where energy may be recovered from a stream that otherwise would be throttled, and when a radial-flow centrifugal pump is im- mediately available but a hydraulic turbine is not. In the most common situation, there is a liquid stream with fixed head and flow rate and an application requiring a fixed rpm; these are the turbine design condi- tions. The objective is to pick a pump with a turbine bep at these conditions. With performance curves such as Fig. 34, turbine design conditions can be converted to pump design conditions. Then you select from a manufacturer’s catalog a model that has its pump bep at those values. The most common error in pump selection is using the turbine design conditions in choosing a pump from a catalog. Because catalog performance curves describe pump duty, not turbine duty, the result is an oversized unit that fails to work prop- erly. This procedure is the work of Fred Buse, Chief Engineer, Standard Pump Al- drich Division of Ingersoll-Rand Co., as reported in Chemical Engineering maga- zine. SIZING CENTRIFUGAL-PUMP IMPELLERS FOR SAFETY SERVICE Determine the impeller size of a centrifugal pump that will provide a safe contin- uous-recirculation flow to prevent the pump from overheating at shutoff. The pump delivers 320 gal/min (20.2 L/s) at an operating head of 450 ft (137.2 m). The inlet water temperature is 220⬚F (104.4⬚C), and the system has an NPSH of 5 ft (1.5 m). Pump performance curves and the system-head characteristic curve for the dis- charge flow (without recirculation) are shown in Fig. 35, and the piping layout is shown in Fig. 42. The brake horsepower (bhp) of an 11-in (27.9-cm) and an 11.5- in (29.2-cm) impeller at shutoff is 53 and 60, respectively. Determine the permis- sible water temperature rise for this pump. Calculation Procedure: 1. Compute the actual temperature rise of the water in the pump Use the relation P0 ⫽ where P0 ⫽ pressure corresponding to the actual P ⫹ P , v NPSH liquid temperature in the pump during operation, lb/in2 (abs) (kPa); ⫽ vapor Pv pressure in the pump at the inlet water temperature, lb/in2 (abs) (kPa); ⫽ pressure created by the net positive suction head on the pumps, lb/in2 PNPSH (abs) (kPa). The head in feet (meters) must be converted to lb/in2 (abs) (kPa) by the relation lb/in2 (abs) ⫽ (NPSH, ft) (liquid density at the pumping temperature, lb/ft3 )/(144 in2 /ft2 ). Substituting yields P0 ⫽ 17.2 lb/in2 (abs) ⫹ 5(59.6)/ 144 ⫽ 19.3 lb/in2 (abs) (133.1 kPa). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 381. PUMPS AND PUMPING SYSTEMS 7.63 FIGURE 40 Performance of a pump at constant speed in pump duty and turbine duty. (Chemical Engineering.) Using the steam tables, find the saturation temperature Ts corresponding to this absolute pressure as Ts ⫽ 226.1⬚F (107.8⬚C). Then the permissible temperature rise is Tp ⫽ Ts ⫺ Top, where Top ⫽ water temperature in the pump inlet. Or, Tp ⫽ 226.1 ⫺ 220 ⫽ 6.1⬚F (3.4⬚C). 2. Compute the recirculation flow rate at the shutoff head From the pump characteristic curve with recirculation, Fig. 43, the continuous- recirculation flow QB for an 11.5-in (29.2-cm) impeller at an operating head of 450 ft (137.2 m) is 48.6 gal/min (177.1 L/min). Find the continuous-recirculation flow at shutoff head Hs ft (m) of 540 ft (164.6 m) from Qs ⫽ QB (Hs /Hop )0.5 , where Hop ⫽ operating head, ft (m). Or Qs ⫽ 48.6(540/450) ⫽ 53.2 gal/min (201.4 L/min). 3. Find the minimum safe flow for this pump The minimum safe flow, lb/h, is given by wmin ⫽ 2545bhp /[Cp Tp ⫹ (1.285 ⫻ 10⫺3 )Hs ], where Cp ⫽ specific head of the water; other symbols as before. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 382. 7.64 PLANT AND FACILITIES ENGINEERING FIGURE 41 System-head curves without recirculation flow. (Chemical Engineering.) FIGURE 42 Pumping system with a con- tinuous-recirculation line. (Chemical Engi- neering.) Substituting, we find wmin ⫽ 2545(60)/[1.0(6.1) ⫹ (1.285 ⫻ 10⫺3 )(540)] ⫽ 22,476 lb/h (2.83 kg/s). Converting to gal/min yields Qmin ⫽ wmin /[(ft3 /h)(gal/min)(lb/ ft3 )] for the water flowing through the pump. Or, Qmin ⫽ 22,476/[(8.021)(59.6)] ⫽ 47.1 gal/min (178.3 L/min). 4. Compare the shutoff recirculation flow with the safe recirculation flow Since the shutoff recirculation flow Qs ⫽ 53.2 gal/min (201.4 L/min) is greater than Qmin ⫽ 47.1 gal/min (178.3 L/min), the 11.5-in (29.2-cm) impeller is adequate to provide safe continuous recirculation. An 11.25-in (28.6-cm) impeller would not be adequate because Qmin ⫽ 45 gal/min (170.3 L/min) and Qs ⫽ 25.6 gal/min (96.9 L/min). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 383. PUMPS AND PUMPING SYSTEMS 7.65 FIGURE 43 System-head curves with recirculation flow. (Chemical Engineering.) Related Calculations. Safety-service pumps are those used for standby service in a variety of industrial plants serving the chemical, petroleum, plastics, aircraft, auto, marine, manufacturing, and similar businesses. Such pumps may be used for fire protection, boiler feed, condenser cooling, and related tasks. In such systems the pump is usually oversized and has a recirculation loop piped in to prevent overheating by maintaining a minimum safe flow. Figure 41 shows a schematic of such a system. Recirculation is controlled by a properly sized orifice rather than by valves because an orifice is less expensive and highly reliable. The general procedure for sizing centrifugal pumps for safety service, using the symbols given earlier, is this: (1) Select a pump that will deliver the desired flow QA, using the head-capacity characteristic curves of the pump and system. (2) Choose the next larger diameter pump impeller to maintain a discharge flow of QA to tank A, Fig. 41, and a recirculation flow QB to tank B, Fig. 41. (3) Compute the recirculation flow Qs at the pump shutoff point from Qs ⫽ QB (Hs /Hop )0.5 . (4) Cal- culate the minimum safe flow Qmin for the pump with the larger impeller diameter. (5) Compare the recirculation flow Qs at the pump shutoff point with the minimum safe flow Qmin . If Qs ⱖ Qmin, the selection process has been completed. If Qs ⬍ Qmin, choose the next larger size impeller and repeat steps 3, 4, and 5 above until the impeller size that will provide the minimum safe recirculation flow is determined. This procedure is the work of Mileta Mikasinovic and Patrick C. Tung, design engineers, Ontario Hydro, as reported in Chemical Engineering magazine. PUMP CHOICE TO REDUCE ENERGY CONSUMPTION AND LOSS Choose an energy-efficiency pump to handle 1000 gal/min (3800 L/min) of water at 60⬚F (15.6⬚C) at a total head of 150 ft (45.5 m). A readily commercially available pump is preferred for this application. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 384. 7.66 PLANT AND FACILITIES ENGINEERING FIGURE 44 Selection guide is based mainly on specific speed, which indicates impeller geom- etry. (Chemical Engineering.) Calculation Procedure: 1. Compute the pump horsepower required For any pump, bhpi ⫽ (gpm)(Ht )(s)/3960e, where bhpi ⫽ input brake (motor) horsepower to the pump; Ht ⫽ total head on the pump, ft; s ⫽ specific gravity of the liquid handled; e ⫽ hydraulic efficiency of the pump. For this application where s ⫽ 1.0 and a hydraulic efficiency of 70 percent can be safely assumed, bhpi ⫽ (1000)(150)(1)/(3960)(0.70) ⫽ 54.1 bhp (40.3 kW). 2. Choose the most energy-efficient pump Use Fig. 44, entering at the bottom at 1000 gal/min (3800 L/min) and projecting vertically upward to a total head of 150 ft (45.5 m). The resulting intersection is within area 1, showing from Table 17 that a single-stage 3500-r/min electric-motor- driven pump would be the most energy-efficiency. Related Calculations. The procedure given here can be used for pumps in a variety of applications—chemical, petroleum, commercial, industrial, marine, aer- onautical, air-conditioning, cooling-water, etc., where the capacity varies from 10 to 1,000,000 gal/min (38 to 3,800,000 L/min) and the head varies from 10 to 10,000 ft (3 to 3300 m). Figure 44 is based primarily on the characteristic of pump specific speed Ns ⫽ NQ2 / where N ⫽ pump rotating speed, r/min; 3 / 4 H , Q ⫽ capacity, gal/min (L/min); H ⫽ total head, ft (m). When Ns is less than 1000, the operating efficiency of single-stage centrifugal pumps falls off dramatically; then either multistage or higher-speed pumps offer the best efficiency. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 385. PUMPS AND PUMPING SYSTEMS 7.67 TABLE 17 Type of Pump for Highest Energy Efficiency* Area 1 of Fig. 44 is the densest, crowded both with pumps operating at 1750 and 3500 r/min, because years ago 3500-r/min pumps were not thought to be as durable as 1750-r/min ones. Since the adoption of the AVS standard in 1960 (su- perseded by ANSI B73.1), pumps with stiffer shafts have been proved reliable. Also responsible for many 1750-r/min pumps in area 1 has been the impression that the higher (3500-r/min) speed causes pumps to wear out faster. However, because impeller tip speed is the same at both 3500 and 1750 r/min [as, for ex- ample, a 6-in (15-cm) impeller at 3500 r/min and a 12-in (30-cm) one at 1750 r/ min], so is the fluid velocity, and so should be the erosion of metal surface. Another reason for not limiting operating speed is that improved impeller inlet design allows operation at 3500 r/min to capacities of 5000 gal/min (19,000 L/min) and higher. Choice of operating speed also may be indirectly limited by specifications per- taining to suction performance, such as that fixing the top suction specific speed S directly or indirectly by choice of the sigma constant or by reliance on Hydraulic Institute charts. Values of S below 8000 to 10,000 have long been accepted for avoiding cavi- tation. However, since the development of the inducer, S values in the range of 20,000 to 25,000 have become commonplace, and values as high as 50,000 have become practical. The sigma constant, which relates NPSH to total head, is little used today, and Hydraulic Institute charts (which are being revised) are conservative. In light of today’s designs and materials, past restrictions resulting from suction performance limitations should be reevaluated or eliminated entirely. Even if the most efficient pump has been selected, there are a number of cir- cumstances in which it may not operate at peak efficiency. Today’s cost of energy has made these considerations more important. A centrifugal pump, being a hydrodynamic machine, is designed for a single peak operating-point capacity and total head. Operation at other than this best ef- ficiency point (bep) reduces efficiency. Specifications now should account for such factors as these: 1. A need for a larger number of smaller pumps. When a process operates over a wide range of capacities, as many do, pumps will often work at less than full capacity, hence at lower efficiency. This can be avoided by installing two or three pumps in parallel, in place of a single large one, so that one of the smaller pumps can handle the flow when operations are at a low rate. 2. Allowance for present capacity. Pump systems are frequently designed for full flow at some time in the future. Before this time arrives, the pumps will operate far from their best efficiency points. Even if this interim period lasts only 2 or Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 386. 7.68 PLANT AND FACILITIES ENGINEERING 3 years, it may be more economical to install a smaller pump initially and to replace it later with a full-capacity one. 3. Inefficient impeller size. Some specifications call for pump impeller diameter to be no larger than 90 or 95 percent of the size that a pump could take, so as to provide reserve head. If this reserve is used only 5 percent of the time, all such pumps will be operating at less than full efficiency most of the time. 4. Advantages of allowing operation to the right of the best efficiency point. Some specifications, the result of such thinking as that which provides reserve head, prohibit the selection of pumps that would operate to the right of the best effi- ciency point. This eliminates half of the pumps that might be selected and results in oversized pumps operating at lower efficiency. This procedure is the work of John H. Doolin, Director of Product Development, Worthington Pumps, Inc., as reported in Chemical Engineering magazine. Special Pump Applications EVALUATING USE OF WATER-JET CONDENSATE PUMPS TO REPLACE POWER-PLANT VERTICAL CONDENSATE PUMPS Evaluate the economic and application feasibility of replacing the vertical conden- sate pumps in a typical 1100-MW pressurized-water-reactor steam power plant hav- ing a feedwater train of two feedwater pumps, two heater drain pumps, and three vertical condensate pumps, with a water-jet pump in combination with a horizontal centrifugal pump. The flow rates, pressure heads, and related characteristics of the plant being considered are shown in Table 18. Calculation Procedure: 1. Develop the performance parameters for the water-jet pump During the past two decades, turbine generator sizes increased from about 100 MW in the 1950s to 300 MW in the 1960s, and then up to about 750 MW in the early 1970s. At this writing (1997), generator sizes for both nuclear and fossil-fuel plants are even larger than the 750 MW cited here. This drastic increase in size, plus the introduction of low pressure nuclear power cycles, brought about an increase in condensate flow to more than 10 ⫻ 106 lb/h (4.5 ⫻ 106 kg/h). Actually, in a typical 1300-MWe nuclear thermal cycle today, the condensate flow from condenser ho- twell may be as high as 12 ⫻ 106 lb/h (5.5 ⫻ 106 kg/h). Current practice in condensate pumping system design is to either increase the pump capacity or in- crease the number of pumps operating in parallel to meet the flow requirements. However, these measures may increase: • Initial fabrication and installation costs • Probability of pump failures • Routine maintenance and repair costs (and equally, if not more important, the attendant costs of plant down time). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 387. 7.69 TABLE 18A Feedwater System Pressure of a 1100-MW PWR Plant Percent of max. design load 100 95.9 75 50 25 15 Percent of guarantee load 104.3 100 78.2 52.1 26.1 15.6 Turbine output—kW 1,210,081 1,160,596 907,560 605,040 302,519 181,512 Feedwater flow—lb/h 15,886,500 15,155,582 11,669,947 8,006,481 4,535,192 2,980,230 Feedwater flow—gal/min 36,996 35,181 26,653 17,937 9,889 6,409 Feed pump suction temp.—⬚F 403 398.8 377 348.1 305.7 279 Steam generator press.—lb/in2 (abs) 990 990 1,036 1,073 1,091 1,093 Feed pump discharge press.—lb/in2 (abs) 1,149 1,144 1,156 1,160 1,146 1,136 Feed pump suction press.—lb/in2 (abs) 459 475 522 559 582 589 Feed pump TDH—lb/in2 690 669 634 601 564 547 Feed pump TDH—ft 1,856 1,794 1,673 1,555 1,420 1,360 Condensate flow—lb/h 10,500,000 10,150,000 7,820,000 5,475,000 3,125,000 2,200,000 Condensate flow—gal/min 21,212 20,505 15,798 11,061 6,313 4,444 Condensate pump discharge press—lb/in2 547.2 549.4 565.2 578.1 589.3 593.6 Condensate pump discharge press—ft 1,275 1,280 1,317 1,347 1,373 1,383 Condensate system loss—lb/in2 88.2 74.4 43.2 19.1 7.3 4.6 Notes: 1. Based on three condensate pumps and two heater drain pumps operating through the full load range. 2. The conversion factors from English Units to SI Units are tabulated below: English Unit 1 lb/h 1 gal/min 1⬚F 1 lb/in2 1 ft SI unit 1.26 ⫻ 10⫺4 kg/s 6.309 ⫻ 10⫺5 m3 /s .5556 K 6.895 ⫻ 103 Pa .3048 M Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 388. 7.70 TABLE 18B SI Values for Feedwater System Pressure of an 1100-MW PWR Plant Percent max. design load 100 95.9 75 50 25 15 Percent guarantee load 104.3 100 78.2 52.1 26.1 15.6 Turbine output, kW 1,210,081 1,160,596 907,560 605,040 302,519 181,512 Feedwater flow, kg/h 7,212,471 6,880,634 5,298,k56 3,634,932 2,058,977 1,353,024 Feedwater flow, L/s 2524 2220 1682 1132 624 404 Feedwater pump suction temp, ⬚C 206 203.8 191.7 175.6 152.1 137.2 Steam generator press, kPa 6821 6821 7138 7393 7517 7531 Feedwater pump disch press, kPa 7917 7882 7965 7992 7896 7827 Feedwater pump suction press, kPa 3163 3273 3997 3852 4010 4058 Feed pump TDH, kPa 4754 4609 4368 4141 3886 3769 Feed pump TDH, m 566 547 510 474 433 415 Condensate flow, kg/h 4,767,000 4,608,100 3,550,280 2,485,650 1,418,750 998,800 Condensate flow, L/s 1338 1294 997 698 398 280 Condensate pump disch press kPa 3770 3785 3894 3983 4060 4090 Condensate pump disch press, m 389 390 401 411 418 422 Condensate system loss, kPa 608 513 298 132 50 32 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 389. PUMPS AND PUMPING SYSTEMS 7.71 H1 = –– + –– + Z1 P1 V1 2 2g ␥ Hd = –– + –– + Zd Pd Vd2 2g ␥ H2 = –– + –– + Z2 P2 V2 2g ␥ 2 CONDENSER HOT WELL Z2 Q2 Q1 h1 LOSS h2 LOSS b a ⌬P1 V1A1 V5,A5 Vj, Aj FIGURE 45 Water jet-centrifugal pump. The purpose of this calculation procedure is to examine the technical feasibility of using a water jet pump in combination with a horizontal centrifugal pump to replace a vertical centrifugal pump. In comparison with a typical vertical pump installation, the horizontal pump installation appears to offer a relatively higher system availability factor which should stem directly from the greater accessibility offered by the typical horizontal pump installation over the typical vertical pump installation. Marked improvement in both preventive and corrective maintenance times, even where equivalent failures or failure rates are assumed for both types of pump installations, invoke serious economic factors which cannot be overlooked in consideration of the current and inordinately high costs of plant down time. More- over, in combination with the water jet pump, the horizontal configuration appears to offer a solution to the related NPSH problems. The combination also appears pertinent in the design of other systems, such as the heater drain systems in the feedwater cycle, where similar conditions may obtain. A water-jet pump, Fig. 45, consists of a centrifugal pump discharging through a nozzle located at the bottom of the condenser hotwell. The operating principle of the water-jet pump is based on the transfer of momentum from one stream of fluid to another. Water jet pumps were incorporated into the flow recirculation system of boiling water reactor design in 1965. The pumps were selected in lieu of conventional centrifugal pumps because of their basic simplicity and the economic incentives resulting from the possible reduction in the number of coolant loops and vessel nozzles by placing the water jet pump inside the pressure vessel. The reduction in the number of coolant loops permits a smaller drywell so that both primary and secondary containment structure can be designed more compactly. Concurrently, the efficiency of the water jet pump has been markedly improved through extensive development and testing programs pursued by the manufacturer. An efficiency of 41.5 percent has been obtained at a suction flow to driving flow ratio of 2.55 in the manufacturer’s second generation jet pumps (1). Figure 45 shows the proposed arrangement of a water jet pump and horizontal centrifugal pump combination to replace the conventional vertical condensate pump. The high momentum jet stream ejected from the recirculation nozzle is mixed with a low momentum stream from the condenser hotwell in the throat. This mixed flow slows down in the diffuser section where part of its momentum (kinetic energy) is converted into pressure. The flow is then led to the centrifugal pump suction through a short piping section. The pressure of the fluid is increased through the pump and a major part of this flow is then directed through the lower pressure heaters and Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 390. 7.72 PLANT AND FACILITIES ENGINEERING FIGURE 46 Thermal cycle arrangement with water jet-centrifugal pump. finally to the suction of the feedwater pump, or to the deaerator, to provide the required feedwater flow to the stream generator. The remaining flow is led through the recirculation line back to the jet pump throat to induce the suction flow from the condenser hotwell; and thereby provide for continuous recirculation. A possible turbine cycle arrangement with a water jet and horizontal centrifugal condensate pump is shown in Fig. 46. The characteristics of a water-jet pump are defined by the following equations and nomenclature, using data from Fig. 45: Q ⫽ A V , Q ⫽ A V , Q ⫹ Q ⫽ A V ⫽ Q (1) 1 j j 2 s s 1 2 t t t Q2 ⫽ M (2) Q1 Aj ⫽ R (3) At 2g(H ⫺ Pa /␥) 1 Q ⫽ A (4) 1 j 冪 1 ⫹Kj Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 391. PUMPS AND PUMPING SYSTEMS 7.73 FIGURE 47 Water jet pump characteristic curve. 2g(H ⫺ Pa /␥) 2 Q ⫽ A (5) 2 s 冪 1 ⫹ Ks 2 2 Pa V V t t ⫹ ⫽ H ⫹ K (6) d d ␥ 2g 2g Following Gosline and O’Brien (3), the head ratio, N, depends upon six param- eters; M, R, Ks , Kj , Kt , and Kd , or: H ⫺ H d 2 N ⫽ ⫽ ƒ(M, R, K , K , K , K ) (7) s j t d H ⫺ H 1 d which vary with the design of the water jet pump itself and with the length of the connecting pipes. Once the design of the water jet pump is fixed, these parameters are known functions of flow. Based upon their extensive testing, the manufacturer suggests the use of the M-N curve shown in Fig. 47 for the water jet pump design evaluation. This M-N correlation is essentially a straight line and can be represented by: N0 N ⫽ N ⫺ M (8) 0 M0 where N0 is the value of N at M ⫽ 0 and M0 is the value of M when N ⫽ 0. The M-N curve of the water jet pump shown in Fig. 45 may be represented by: N ⫽ .246 ⫺ .04125M (9) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 392. 7.74 PLANT AND FACILITIES ENGINEERING Nomenclature A ⫽ cross-section area, ft2 (0.0929 m2 ) bhp ⫽ pump brake horsepower D ⫽ diameter, ft (0.3048 m) e ⫽ centrifugal pump efficiency ƒ ⫽ friction factor F ⫽ brake horsepower ratio g ⫽ acceleration of gravity, 32.2 ft/s2 (9.815 m/s2 ) H ⫽ hydraulic head, ft (0.3048 m) hp ⫽ horsepower ⌬Hp ⫽ pump total dynamic head, ft (0.3048 m) K ⫽ friction parameter ⫽ fL/D kW ⫽ kilowatt L ⫽ length, ft (0.3048 m) M ⫽ induced flow and driving flow ratio ⫽ Q2 /Q1 M0 ⫽ constant n ⫽ pump speed, rpm N ⫽ head ratio ⫽ Hd ⫺ H2 /H1 ⫺ Hd N0 ⫽ constant NHR ⫽ net heat rate, Btu/kWh (1054 J/kWh) NPSH ⫽ net positive suction head P ⫽ pressure, lb/ft2 (47.88 Newton/m2 ) qin ⫽ thermal energy input, kW Q ⫽ flow rate, ft3 /s (0.02832 m3 /s) R ⫽ ratio of area of nozzle to area of throat ⫽ Aj /At V ⫽ velocity, ft/s (0.3048 m/s) Z ⫽ elevation, ft (0.3048 m) ⫽ jet pump efficiency ␥ ⫽ specific weight of liquid, lb ft/ft3 (16.02 kg/m3 ) Subscripts a ⫽ entrance of throat b ⫽ end of throat c ⫽ centrifugal pump d ⫽ jet pump discharge j ⫽ tip of the nozzle s ⫽ annular area surrounding tip of nozzle t ⫽ throat of mixing chamber v ⫽ vertical condensate pump j-c ⫽ water jet-centrifugal pump combination The water jet pump efficiency, , is defined as the ratio of the total energy increase of the suction flow to the total energy decrease of the driving flow, or: ⫽ M N 100 (10) This definition of efficiency is different from the centrifugal pump efficiency, ec, which is defined as: pump output Q␥H e ⫽ ⫽ (11) c bhp 550 ⫻ bhp Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 393. PUMPS AND PUMPING SYSTEMS 7.75 2. Define the performance of the centrifugal pump associated with the water- jet pump The performance of the horizontal centrifugal pump is defined in the manufacturer supplied pump characteristic curve and pump affinity laws: 2 Q n H n ⫽ and ⫽ (12) 冉 冊 Q n H n 0 0 0 0 From Fig. 45, the pump discharge head, H1, is: H ⫽ H ⫺ h ⫹ ⌬Hp (13) 1 d 1 where h1 is the head loss from the water jet pump exit to the centrifugal pump suction. Hydraulic Horsepower of Water Jet-Centrifugal Pump. From Fig. 45, the total flow, Qt, through the horizontal centrifugal pump is: Q ⫽ Q ⫹ Q (14) t 1 2 The hydraulic horsepower of the pump is: Q (H ⫺ H ) t 1 d (Hydraulic hp) ⫽ (15) j-c 550 From Eqs. (7) and (8): N0 N ⫺ M H ⫹ H 冉 冊 0 1 2 M0 H ⫽ (16) d N0 1 ⫹ N ⫺ M 0 M0 Substituting Eq. (16) into Eq. (15), we have: Q ␥ H ⫺ H t 1 2 (Hydraulic hp) ⫽ (17) j-c 550 N 冢 冣 0 1 ⫹ N ⫺ M 0 M0 For the conventional vertical condensate pump, the hydraulic horsepower is: Q ␥(H ⫺ H ) 2 1 2 (Hydraulic hp) ⫽ (18) v 550 From Eqs. (17) and (18): 1 1 ⫹ (Hydraulic hp) M j-c ⫽ (19) (Hydraulic hp) N v 0 1 ⫹ N ⫺ M 0 M0 The hydraulic hp ratio for different values of M is shown in Fig. 48. The man- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 394. 7.76 PLANT AND FACILITIES ENGINEERING 3 2 1 0 0 1 2 3 4 5 6 (HYDRAULIC HP)j – v (HYDRAULIC HP)j – c (HYDRAULIC HP)j – v (HYDRAULIC HP)j – c 1 + 1 + N0 – M = 1 — M N0 M0 N0 = .246 M0 = 5.964 M FIGURE 48 Hydraulic hp ratio vs. M. ufacturer’s suggested M-N curve, Fig. 47, is used in the calculation. From equation (11), the brake hp ratio is: (bhp) (Hydraulic hp) e j-c j-c c ⫽ (20) (bhp) (Hydraulic hp) e v v j-c Since the centrifugal pump efficiencies within a normal operating range do not change significantly, ⬇ ec, and: ej-c 1 1 ⫹ (bhp) M j-c ⬇ ⫽ F (21) (bhp) N v 0 1 ⫹ N ⫺ M 0 M0 3. Compute the effect of the water-jet pump on the net heat rate The net heat rate is defined as: qin NHR ⫽ New Heat Rate ⫽ (22) kW ⫺ kW ⫺ kW E AUX CON where: qin ⫽ total thermal energy input kWE ⫽ generator output kWAUX ⫽ total plant auxiliary power excluding the power to condensate pumps kWCON ⫽ total power required to drive a motor driven condensate pump. If we further define that kW ⫽ kWE ⫺ kWAUX, qin (Net Heat Rate) ⫽ (23) v kW ⫺ kWCON If a water jet-centrifugal pump combination is used to replace the vertical con- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 395. PUMPS AND PUMPING SYSTEMS 7.77 (NET HEAT RATE)j – c (NET HEAT RATE) v 1.01 1.008 1.006 1.004 1.002 1.0 0 1 2 3 M 4 5 6 N0 = .246 M0 = 5.964 ec = .78 kW = 1160596 kWCON FIGURE 49 Net-heat rate ratio vs. M. densate pump while keeping qin and kW unchanged, the net heat rate can be shown as: qin (Net Heat Rate) ⫽ (24) j-c kW ⫺ kW ⫻ F CON From the foregoing equations: (NHR) kW (F ⫺ 1) j-c CON ⫽ 1 ⫹ (25) (NHR) kW ⫺ kW ⫻ F v CON The effect of M on net heat ratio is calculated according to Eq. (25) and this is shown in Fig. 49. The following data of a 1100-MW PWR plant have been used in the calculation kW ⫽ 1,160,596 3 Condensate flow⫽ 20,505 gal/min (1.294 m /s) Condensate pump TDH ⫽ 1280 ft (390.14 m) Pump efficiency ⫽ 0.78 gpm ⫻ H kW ⫽ ⫻ 0.746 CON 3960 ⫻ ec ⫽ 6339 kW As shown in Fig. 49, the increase in heat rate caused by the water jet pump is approximately 0.1 percent at M ⫽ 3 and is less than 0.2 percent at M ⫽ 2. However, the heat rate ratio increases very rapidly with a further decrease in M. 4. Develop the performance calculations for this installation The performance calculations of a water jet-centrifugal pump combination in a power plant must be developed from an overall analysis of the feedwater-condensate system. To initiate the design analysis, the M-N relationship developed in Eq. (9) and shown in Fig. 47 is examined and, for obvious reasons, the peak efficiency Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 396. 7.78 PLANT AND FACILITIES ENGINEERING Total dynamic head, m TOTAL DYNAMIC HEAD, FT (⌬ Hp) 1000 800 600 400 200 0 1500 1400 1300 1200 1100 1000 900 800 700 600 500 0 0 0 100 200 300 400 500 600 1 1FT = .3048 M 1GPM = 6.309 x 105 M3/SEC 2 3 4 5 6 7 8 9 10 100% 95% 90% 85% 80% SCHEME D SCHEME A SCHEME B 1170 RPM 1112 RPM 1053 RPM 995 RPM 936 RPM FLOW Qp GPM x 103 L/sec FIGURE 50 Condensate pump head-capacity curve. point (M ⫽ 3) is selected for the design of the water jet pump. At this design point, the efficiency of the water jet pump is about 37 percent. The head and flow characteristics of the horizontal centrifugal pump are shown in Fig. 50, and the following simplifying assumptions are made in the development of the performance calculations (refer also to Fig. 45). 1. K2 ⫽ 0. 2. Vs is small such that H2 ⬇ Pa/␥ ⫽ 5 lb/in2 (34.5 kPa) or 11.65 ft (3.6 m) of water. 3. h1 loss is 10 lb/in2 (68.9 kPa) (or 23.3 ft) (7.1 m) and is a constant under all loading conditions. 4. h2 loss is 25 lb/in2 (172.2 kPa) (or 58.25 ft) (17.8 m) and is a constant under all loading conditions. From Eqs. (7), (9), and (13), we have: H ⫽ H ⫹ (1.246 ⫺ .04125M) ⫻ (⌬Hp ⫺ h ) (26) 1 2 1 To match the vertical condensate pump at maximum design condition, the total dynamic head of the horizontal pump (⌬Hp) shall be such that H1 ⫽ 1275 ft (388.6 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 397. PUMPS AND PUMPING SYSTEMS 7.79 m). With h1 ⫽ 23.3 ft (7.1 m), H2 ⫽ 11.65 ft (3.6 m), and M ⫽ 3, the required ⌬Hp from Eq. (26) is: ⌬Hp ⫽ (H ⫺ H ) ⫼ (1.246 ⫺ .04125M) ⫹ h ⫽ 1149 ft (350.2 m) 1 2 1 and Hd from Eq. (13) is: H ⫽ H ⫹ h ⫺ ⌬Hp ⫽ 149.3 ft (45.5 m) d 1 1 From Eqs. (2) and (14): 1 Q ⫽ Q 1 ⫹ (27) 冉 冊 t 2 M with M ⫽ 3, the required condensate flow of three horizontal centrifugal pump Qt is: 1 6 Q ⫽ 10.5 ⫻ 10 ⫻ 1 ⫹ 冉 冊 t 3 6 3 ⫽ 14 ⫻ 10 lb s/h or 28.283 gal/min (1.784 m /s) From Fig. 50, the condensate pump characteristic curve, which is generated by pump affinity law, the horizontal centrifugal pump will be running at 1147 r/min. The jet nozzle area, Aj, can be calculated from: Q ⫽ CA 兹2g(H ⫺ h ⫺ H ) (28) 1 j 1 2 2 which is another form of Eq. (4). Assume that nozzle flow coefficient C ⫽ 0.9, we have: 2 2 A ⫽ .020948 ft (.00195 m ) j or: d ⫽ 1.96 in (.0498 m) j Substitute the value of Aj and C into Eq. (28), we have: Q ⫽ .1513 兹H ⫺ 69.9 (29) 1 1 Accordingly, Eqs. (2), (26), (27), and (29) define the water jet-centrifugal pump performance. 5. Determine the best drive for the water-jet centrifugal condensate pump Pump DRIVING Schemes. Four possible schemes of driving the water jet- centrifugal condensate pump are examined. For each scheme, the water jet- centrifugal pump is designed to duplicate the head and capacity performance of the corresponding vertical condensate pump at the maximum design condition and then the schemes are examined for continuous operation at other loading conditions; namely, 100, 75, 50, 25, and 15 percent. Schematic arrangements of each scheme and sample calculations at 75 percent load are shown in Table 20. Scheme A: Variable Speed Motor Drive and Variable M Ratio. In this scheme, variable speed electric motor is used to drive the water jet-centrifugal pump so that the condensate flow and the pressure head at the feed pump suction are identical to that of the base case which uses conventional vertical condensate pumps. To Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 398. 7.80 PLANT AND FACILITIES ENGINEERING TABLE 19 M Ratio vs. Load Variation Load Condition VWO 100% 75% 50% 25% 15% M Ratio 3 2.894 2.2 1.52 .86 .6021 satisfy equation (4), the water jet pump flow ratio M changes from 3 at maximum design condition to 0.6021 at 15 percent load as shown in Table 19. At maximum design condition, the horizontal centrifugal pump is running at 1147 r/min and 9428 gal/min (594.9 L/s). For the partial load operation, the pump will follow the curve labeled Scheme A in Fig. 50. From equations (21) and (25), the increase in net heat rate will be higher at the 25 and 15 percent partial loading operation. If long-term partial load operation is expected, this scheme should be avoided. Scheme B: Variable Speed Motor Drive and Constant M Ratio. In this scheme, the pump drive is identical to Scheme A except that a flow regulating control valve is installed in the water jet pump recirculation line to maintain a constant M ratio at all loading conditions. In this case, the speed of the horizontal centrifugal pump will vary according to the curve labeled Scheme B in Fig. 50. The feedwater pump operation is identical to that of Scheme A and the base case. Scheme C: Constant Speed Motor Drive and Constant M Ratio. In this scheme, the horizontal centrifugal pump is running at a constant speed of 1147 r/min. A control valve is used to keep the flow ratio M ⫽ 3. In this case, the pressure head at the water jet-centrifugal pump discharge is higher than that of the vertical con- densate pump. Consequently, the feedwater pump will be running at a lower speed and lower total dynamic head to keep the steam generator pressure identical to that of the base case. The required feed pump total dynamic head and corresponding speed are shown as the curve labeled Scheme C in Fig. 51. Scheme D: Turbine Drive Jet-Centrifugal Pump and Constant M Ratio. In this scheme, the feedwater pump and water jet-centrifugal pump are running at a con- stant speed ratio and both are driven by the auxiliary turbine. A control valve is used to keep the flow ratio M ⫽ 3. Under these conditions, the water jet-centrifugal pump and the feedwater pump will follow the curves labeled Scheme D in Figs. 50 and 51, respectively, to produce the identical steam generator conditions in Table 18. It should be noted that the auxiliary turbine driven feedwater pump has been shown to have a better cycle efficiency than a motor driven pump in the same application for large power plants (4); intuitively, the auxiliary turbine driven jet- centrifugal pump arrangement may also provide certain gains in cycle efficiency over other water jet-centrifugal pump drive schemes. 6. Summarize the findings for this pump application It has been shown that a water jet-centrifugal pump can be used to replace the conventional vertical condensate pump in a steam power plant feedwater system. All four schemes discussed in the preceding section are feasible means of driving the water jet-centrifugal pump combination. While the resulting auxiliary power requirements for the jet-centrifugal pump system will be slightly higher, the increase will be insignificant if the flow rate M is kept greater than 2. The proposed change from conventional vertical pump to a water jet-centrifugal pump may have advantages: 1. Increased feedwater system reliability and reduced plant downtime 2. Easier maintenance operations, reduced cost of maintenance Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 399. TABLE 20 Pump Driving Schemes From Table 18 @ 75% load Flow ⫽ 7.82 ⫻ 106 labs/h Q2 ⫽ 5266 gal/min H1 ⫽ 565.2 lb/in2 ⫽ 1317 ft From Eq. 29 Q1 ⫽ .1513兹1317 ⫺ 69.9 ⫽ 5.343 ft3 /s ⫽ 2398 gal/min M ⫽ ⫽ ⫽ 2.196 Q 5266 2 Q 2398 1 From Eq. 26 ⌬Hp ⫽ 1317⫺11.65 1.246⫺.04125⫻2.196 ⫹ 23.3 ⫽ 1153 ft Qt ⫽ Q1 ⫹ Q2 ⫽ 7664 gal/ min From Fig. 50 Pump speed ⫽ 1123 r/min From Eq. 13 Hd ⫽ 1317 ⫹ 23.3 ⫺ 1153 ⫽ 187.3 ft From Table 18 @ 75% load Q2 ⫽ 5266 gal/min H1 ⫽ 565.2 lb/in2 ⫽ 1317 ft Qt ⫽ Q2 1 1 ⫹ 冉 冊 M ⫽ 7021 gal/min From Eq. 26 ⌬H ⫽ 1186.5 ft From Eq. 6 Pump speed ⫽ 1131 RPM From Eq. 13 Hd ⫽ 1317 ⫹ 23.3 ⫺ 1186.5 ⫽ 153.8 ft From Table 18 @ 75% load Q2 ⫽ 5266 gal/min Qt ⫽ 7021 gal/min From Fig. 50 @ 1147 RPM ⌬Hp ⫽ 1229 ft From Eq. 26 H1 ⫽ 11.65 ⫹ 1.12225 (1229 ⫺23.3) ⫽ 1364.8 ft ⫽ 585.7 lb/in2 From Table 18 condensate System head loss ⫽ 43.2 lb/in2 Feed pump suction pressure ⫽ 542.3 lb/in2 Feed Pump ⌬Hp ⫽ 1156 ⫺ 542.3 ⫽ 613.7 lb/in2 ⫽ 1619 ft From Eq. 13 Hd ⫽ 1364.8 ⫹ 23.3 ⫺ 1229 ⫽ 159.1 ft Feed Pump speed ⫽ 4830 RPM Condensate pump speed ⫽ 1147 RPM Gear reduction ratio ⫽ 4.2 1 Qt ⫽ 7021 gal/min Iterative Procedure is Used To find pump running SPeeds Try Feed Pump Speed ⫽ 4500 RPM Condensate Pump RPM ⫽ ⫽ 1071 rptn 4500 4.2 From Fig. 50 ⌬Hp ⫽ 1061 ft From Eq. 26 H1 ⫽ 1176.2 ft ⫽ 504.8 lb/ in2 Feed Pump Suction Pressure ⫽ 504.8 ⫺ 43.3 ⫽ 461.6 lb/in2 Feed Pump ⌬Hp ⫽ 1156 ⫺ 461.6 ⫽ 694.4 lb/in2 ⫽ 1832.0 ft From Fig. 511 Feed pump speed ⫽ 4990 rpm Very close to assumed 4500 r/min Hd ⫽ 1176.2 ⫹ 23.3 ⫺ 1061 ⫽ 138.5 ft SI Values A gal/min L/s A lb/in2 kPa A ft m 5266 332.3 565.2 3894 1317 401.4 2398 151.3 187.3 57.1 7664 483.6 B B B 5266 332.3 565.2 3894 1317 401.4 7021 443.0 1186.5 361.6 153.8 46.9 C C C 5266 332.3 585.7 4035.5 1229 374.6 7021 443.0 43.2 297.6 1364.8 415.9 542.3 3736.4 1619 493.5 613.7 4228.4 159.1 48.5 D D D 7021 443.0 504.8 3478.1 1061 323.4 461.6 3180.4 1832 558.4 694.4 4784.4 138.5 42.2 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 400. 7.82 PLANT AND FACILITIES ENGINEERING 900 600 300 0 3000 2500 2000 1500 1000 500 0 Net dynamic head, m NET DYNAMIC HEAD – FEET 0 0 4 8 12 16 20 24 28 32 36 40 WITH VERTICAL CONDENSATE PUMP 1000 2000 CAPACITY – 1000 GPM L/sec 1FT = .3048 M 1GPM = 6.309 x 105 M3/SEC 15% 25% 50% 75% 100% VW0 5100 RPM 5000 RPM 4500 RPM HEAD–TWO PUMPS 4000 RPM 3500 RPM SCHEME D SCHEME C FIGURE 51 Feed pump head-capacity curve (two pumps). 3. More flexibility in plant layout which, in turn, may favorably effect on conden- sate system piping costs. With the present high cost of plant outage, the improvement in system reliability alone may provide sufficient economic incentive for considering the water jet- centrifugal pump combination. Related Calculations. While the study here was directed at a PWR steam power plant, the approach used is valid for any steam power plant—utility, indus- trial, commercial, or marine—using the types of pumps considered. The water-jet pump, developed in the mid-1800s, has many inherent advantages which can be used in today’s highly competitive power-generation industry. In every such instal- lation, the condensate pump in the feedwater system of the steam electric generating power plant takes suction from the condenser hotwell and delivers the condensate through the tube side of the lower pressure feedwater heaters to the deaerator, or to the suction of the feed pump. The continuous operation of the entire plant de- pends upon the proper functioning of the condensate pumps. It should also be noted that the condensate pumping system consumes a significant portion of the auxiliary power, and represents a measurable portion of the plant first cost. In power plant applications, multiple parallel pumping arrangements are em- ployed to provide a flexible operational system. Condensate pumps are of constant speed motor-driven, vertical centrifugal type, and are located in a pit near the con- denser. The difference in fluid elevations between the condenser hotwell and the first stage of the centrifugal pump is the only NPSH available to the pump because the condensate in the hotwell is always saturated. This procedure is the work of E. N. Chu, Engineering Specialist, and F. S. Ku, Assistant Chief Mechanical Engineer, Bechtel Power Corporation, as reported in Combustion magazine and presented at the IEEE-ASME Joint Power Generation Conference. SI values were added by the handbook editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 401. PUMPS AND PUMPING SYSTEMS 7.83 REFERENCES 1. Kudrika, A. A. and Gluntz, D. M., ‘‘Development of Jet Pumps for Boiling Water Reactor Recirculation Systems,’’ Journal of Engineering for Power, Transactions of the ASME, Jan. (1974). 2. Anon., ‘‘Design and Performance of General Electric Boiling Water Reactor Jet Pumps,’’ General Electric Company Report APED-5460, Sept. (1968). 3. Gosline, J. E. and O’Brien, M. P., ‘‘The Water Jet Pump,’’ University of California Publi- cation, Vol. 3, No. 3, 167 (1934). 4. Goodell, J. H. and Leung, P., ‘‘Boiler Feed Pump Drives,’’ ASME Paper No. 64-PWR-7, IEEE-ASME National Power Conference (1964). USE OF SOLAR-POWERED PUMPS IN IRRIGATION AND OTHER SERVICES Devise a solar-powered alternative energy source for driving pumps for use in ir- rigation to handle 10,000 gal/min (37.9 m3 /min) at peak output with an input of 50 hp (37.3 kW). Show the elements of such a system and how they might be interconnected to provide useful output. Calculation Procedure: 1. Develop a suitable cycle for this application Figure 52 shows a typical design of a closed-cycle solar-energy powered system suitable for driving turbine-powered pumps. In this system a suitable refrigerant is chosen to provide the maximum heat absorption possible from the sun’s rays. Water is pumped under pressure to the solar collector, where it is heated by the sun. The water then flows to a boiler where the heat in the water turns the liquid refrigerant into a gas. This gas is used to drive a Rankine-cycle turbine connected to an irri- gation pump, Fig. 52. The rate of gas release in such a closed system is a function of (a) the unit enthalpy of vaporization of the refrigerant chosen, (b) the temperature of the water leaving the solar collector, and (c) the efficiency of the boiler used to transfer heat from the water to the refrigerant. While there will be some heat loss in the piping and equipment in the system, this loss is generally considered negligible in a well- designed layout. 2. Select, and size, the solar collector to use The usual solar collector chosen for systems such as this is a parabolic tracking- type unit. The preliminary required area for the collector is found by using the rule of thumb which states: For parabolic tracking-type solar collectors the required sun- exposure area is 0.55 ft2 per gal/min pumped (0.093 m2 per 0.00379 m3 /min) at peak output of the pump and collector. Another way of stating this rule of thumb is: Required tracking parabolic solar collector area ⫽ 110 ft2 per hp delivered (13.7 m2 /kW delivered). Thus, for a solar collector designed to deliver 10,000 gal/min (37.9 m3 /min) at peak output, the preliminary area chosen for this parabolic tracking solar collector Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 402. 7.84 PLANT AND FACILITIES ENGINEERING FIGURE 52 Closed-cycle system gassifies refrigerant in boiler to drive Ran- kine-cycle turbine for pumping water. (Product Engineering, Battelle Me- morial Institute, and Northwestern Mutual Life Insurance Co.) will be, Ap ⫽ (10,000 gal/min)(0.55 ft2 /gal/min) ⫽ 550 ft2 (511 m2 ). Or, using the second rule of thumb, Ap ⫽ (110)(50) ⫽ 5500 ft2 (511 m2 ). Final choice of the collector area will be based on data supplied by the collector manufacturer, refrigerant choice, refrigerant properties, and the actual operating efficiency of the boiler chosen. In this solar-powered pumping system, water is drawn from a sump basin and pumped to an irrigation canal where it is channeled to the fields. The 50-hp (37.3- kW) motor was chosen because it is large enough to provide a meaningful dem- onstration of commercial size and it can be scaled up to 200 to 250 hp (149.2 to 186.5 kW) quickly and easily. Sensors associated with the solar collector aim it at the sun in the morning, and, as the sun moves across the sky, track it throughout the day. These same sensing Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 403. PUMPS AND PUMPING SYSTEMS 7.85 devices also rotate the collectors to a storage position at night and during storms. This is done to lessen the chance of damage to the reflective surfaces of the col- lectors. A backup control system is available for emergencies. 3. Predict the probable operating mode of this system In June, during the longest day of the year, the system will deliver up to 5.6 million gallons (21,196 m3 ) over a 9.5-h period. Future provisions for energy storage can be made, if needed. Related Calculations. Solar-powered pumps can have numerous applications beyond irrigation. Such applications could include domestic water pumping and storage, ornamental fountain water pumping and recirculation, laundry wash water, etc. The whole key to successful solar power for pumps is selecting a suitable application. With the information presented in this procedure the designer can check the applicability and economic justification of proposed future designs. In today’s environmentally-conscious design world, the refrigerant must be care- fully chosen so it is acceptable from both an ozone-depletion and from a thermo- dynamic standpoint. Banned refrigerants should not, of course, be used, even if attractive from a thermodynamic standpoint. This procedure is the work of the editorial staff of Product Engineering maga- zine reporting on the work of Battelle Memorial Institute and the Northwestern Mutual Life Insurance Co. The installation described is located at MMLI’s Gila River Ranch, southwest of Phoenix, AZ. SI values were added by the handbook editor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 404. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PUMPS AND PUMPING SYSTEMS
- 405. 8.1 SECTION 8 PIPING AND FLUID FLOW PRESSURE SURGE IN FLUID PIPING SYSTEMS 8.2 Pressure Surge in a Piping System From Rapid Valve Closure 8.2 Piping Pressure Surge with Different Material and Fluid 8.5 Pressure Surge in Piping System with Compound Pipeline 8.6 PIPE PROPERTIES, FLOW RATE, AND PRESSURE DROP 8.8 Quick Calculation of Flow Rate and Pressure Drop in Piping Systems 8.8 Fluid Head-Loss Approximations for All Types of Piping 8.10 Pipe-Wall Thickness and Schedule Number 8.11 Pipe-Wall Thickness Determination by Piping Code Formula 8.12 Determining the Pressure Loss in Steam Piping 8.15 Piping Warm-Up Condensate Load 8.18 Steam Trap Selection for Industrial Applications 8.20 Selecting Heat Insulation for High- Temperature Piping 8.27 Orifice Meter Selection for a Steam Pipe 8.29 Selection of a Pressure-Regulating Valve for Steam Service 8.30 Hydraulic Radius and Liquid Velocity in Water Pipes 8.33 Friction-Head Loss in Water Piping of Various Materials 8.33 Chart and Tabular Determination of Friction Head 8.36 Relative Carrying Capacity of Pipes 8.39 Pressure-Reducing Valve Selection for Water Piping 8.41 Sizing a Water Meter 8.42 Equivalent Length of a Complex Series Pipeline 8.43 Equivalent Length of a Parallel Piping System 8.44 Maximum Allowable Height for a Liquid Siphon 8.45 Water-Hammer Effects in Liquid Pipelines 8.47 Specific Gravity and Viscosity of Liquids 8.47 Pressure Loss in Piping Having Laminar Flow 8.48 Determining the Pressure Loss in Oil Pipes 8.49 Flow Rate and Pressure Loss in Compressed-Air and Gas Piping 8.56 Flow Rate and Pressure Loss in Gas Pipelines 8.57 Selecting Hangers for Pipes at Elevated Temperatures 8.58 Hanger Spacing and Pipe Slope for an Allowable Stress 8.66 Effect of Cold Spring on Pipe Anchor Forces and Stresses 8.67 Reacting Forces and Bending Stress in Single-Plane Pipe Bend 8.68 Reacting Forces and Bending Stress in a Two-Plane Pipe Bend 8.75 Reacting Forces and Bending Stress in a Three-Plane Pipe Bend 8.77 Anchor Force, Stress, and Deflection of Expansion Bends 8.79 Slip-Type Expansion Joint Selection and Application 8.80 Corrugated Expansion Joint Selection and Application 8.84 Design of Steam Transmission Piping 8.88 Steam Desuperheater Analysis 8.98 Steam Accumulator Selection and Sizing 8.100 Selecting Plastic Piping for Industrial Use 8.102 Analyzing Plastic Piping and Lining for Tanks, Pumps and Other Components for Specific Applications 8.104 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: HANDBOOK OF MECHANICAL ENGINEERING CALCULATIONS
- 406. 8.2 PLANT AND FACILITIES ENGINEERING Friction Loss in Pipes Handling Solids in Suspension 8.111 Desuperheater Water Spray Quantity 8.112 Sizing Condensate Return Lines for Optimum Flow Conditions 8.114 Estimating Cost of Steam Leaks from Piping and Pressure Vessels 8.116 Quick Sizing of Restrictive Orifices in Piping 8.117 Steam Tracing a Vessel Bottom to Keep Its Contents Fluid 8.118 Designing Steam-Transmission Lines Without Steam Traps 8.119 Line Sizing for Flashing Steam Condensate 8.124 Determining the Friction Factor for Flow of Bingham Plastics 8.127 Time Needed to Empty a Storage Vessel with Dished Ends 8.130 Time Needed to Empty a Vessel Without Dished Ends 8.133 Time Needed to Drain a Storage Tank Through Attached Piping 8.134 Pressure Surge in Fluid Piping Systems PRESSURE SURGE IN A PIPING SYSTEM FROM RAPID VALVE CLOSURE Oil, with a specific weight of 52 lb/ft3 (832 kg/m3 ) and a bulk modulus of 250,000 lb/in2 (1723 MPa), flows at the rate of 40 gal/min (2.5 L/s) through stainless steel pipe. The pipe is 40 ft (12.2 m) long, 1.5 in (38.1 mm) O.D., 1.402 in (35.6 mm) I.D., 0.049 in (1.24 mm) wall thickness, and has a modulus of elasticity, E, of 29 ⫻ 106 lb/in2 (199.8 kPa ⫻ 106 ). Normal static pressure immediately upstream of the valve in the pipe is 500 lb/in2 (abs) (3445 kPa). When the flow of the oil is reduced to zero in 0.015 s by closing a valve at the end of the pipe, what is: (a) the velocity of the pressure wave; (b) the period of the pressure wave; (c) the amplitude of the pressure wave; and (d) the maximum static pressure at the valve? Calculation Procedure: 1. Find the velocity of the pressure wave when the valve is closed (a) Use the equation 68.094 a ⫽ 兹␥[(1/K) ⫹ (D/Et)] where the symbols are as given in the notation below. Substituting, 68.094 a ⫽ 4 6 兹52 [(1/25 ⫻ 10 ) ⫹ (1.402/29 ⫻ 10 ⫻ 0.049] ⫽ 4228 ft/s (1288.7 m/s) An alternative solution uses Fig. 1. With a D/t ratio ⫽ 1.402/0.049 ⫽ 28.6 for Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 407. PIPING AND FLUID FLOW 8.3 5,000 4,000 3,000 2,000 1524 1219 914 610 Bulk modulus K = 250,000 psi Specific weight = 52 lb per cu ft ␥ Stainless steel pipe, E = 29 x 106 psi Copper pipe, E = 17 x 106 psi Alum inum pipe, E = 10.7 x 106 psi 0 140 120 100 80 60 40 20 a, Velocity of pressure wave, ft per sec Velocity, m/sec D / t; I.D. of pipe / wall thickness 250,000 psi (1723 GPa) 300,000 psi (2.07 GPa) 52 lb/ft3 (832 kg/m3) 62.42 lb/ft3 (998.7 kg/m3) 29 ⫻ 106 psi (199.8 GPa) 17 ⫻ 106 psi (117.1 (GPa) 10.7 ⫻ 106 psi (73.7 GPa) FIGURE 1 Velocity of pressure wave in oil column in pipe of different diameter-to- wall thickness ratios. (Product Engineering.) stainless steel pipe, the velocity, a, of the pressure wave is 4228 ft/s (1288.7 m/ s). 2. Compute the time for the pressure wave to make one round trip in the pipe b) The time for the pressure wave to make one round trip between the pipe ex- tremities, or one interval, is: 2L/a ⫽ 2(40)/4228 ⫽ 0.0189 s, and the period of the pressure wave is: 2(2L/a) ⫽ 2(0.0189) ⫽ 0.0378 s. 3. Calculate the pressure surge for rapid valve closure c) Since the time of 01015 s for valve closure is less than the internal time 2L/a equal to 0.0189 s, the pressure surge can be computed from: ⌬p ⫽ ␥aV/144g for rapid valve closure. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 408. 8.4 PLANT AND FACILITIES ENGINEERING The velocity of flow, V ⫽ [(40)(231)(4)]/[(60)()(1.4022 )(12)] using the stan- dard pipe flow relation, or V ⫽ 8.3 ft/s (2.53 m/s). Then, the amplitude of the pressure wave, using the equation above is: 52 ⫻ 4228 ⫻ 8.3 2 ⌬p ⫽ ⫽ 393.5 lb/in (2711.2 kPa). 144 ⫻ 32.2 4. Determine the resulting maximum static press in the pipe d) The resulting maximum static pressure in the line, pmax ⫽ p ⫹ ⌬p ⫽ 500 ⫹ 393.5 ⫽ 893.5 lb/in2 (abs) (6156.2 kPa). Related Calculations. In an industrial hydraulic system, such as that used in machine tools, hydraulic lifts, steering mechanisms, etc., when the velocity of a flowing fluid is changed by opening or closing a valve, pressure surges result. The amplitude of the pressure surge is a function of the rate of change in the velocity of the mass of fluid. This procedure shows how to compute the amplitude of the pressure surge with rapid valve closure. The procedure is the work of Nils M. Sverdrup, Hydraulic Engineer, Aerojet- General Corporation, as reported in Product Engineering magazine. SI values were added by the handbook editor. Notation a ⫽ velocity of pressure wave, ft/s (m/s) aE ⫽ effective velocity of pressure wave, ft/s (m/s) A ⫽ cross-sectional area of pipe, in2 (mm2 ) Ao ⫽ area of throttling orifice before closure, in2 (mm2 ) c ⫽ velocity of sound, ft/s (m/s) CD ⫽ coefficient of discharge D ⫽ inside diameter of pipe, in (mm) E ⫽ modulus of elasticity of pipe material, lb/in2 (kPa) F ⫽ force, lb (kg) g ⫽ gravitational acceleration, 32.2 ft/s2 K ⫽ bulk modulus of fluid medium, lb/in2 (kPa) L ⫽ length of pipe, ft (m) m ⫽ mass, slugs N ⫽ T/(2L/a) ⫽ number of pressure wave intervals during time of valve clo- sure p ⫽ normal static fluid pressure immediately upstream of valve when the fluid velocity is V, lb/in2 (absolute) (kPa) ⌬p ⫽ amplitude of pressure wave, lb/in2 (kPa) pmax ⫽ maximum static pressure immediately upstream of valve, lb/in2 (absolute) (kPa) pd ⫽ static pressure immediately downstream of the valve, lb/in2 (absolute) (kPa) Q ⫽ volume rate of flow, ft3 /s (m3 /s) t ⫽ wall thickness of pipe, in (mm) T ⫽ time in which valve is closed, s v ⫽ fluid volume, in3 (mm3 ) vA ⫽ air volume, in3 (mm3 ) V ⫽ normal velocity of fluid flow in pipe with valve wide open, ft/s (m/s) VE ⫽ equivalent fluid velocity, ft/s (m/s) Vn ⫽ velocity of fluid flow during interval n, ft/s (m/s) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 409. PIPING AND FLUID FLOW 8.5 W ⫽ work, ft 䡠 lb (W) ␥ ⫽ specific weight, lb/ft3 (kg/m3 ) n ⫽ coefficient dependent upon the rate of change in orifice area and dis- charge coefficient ⫽ period of oscillation of air cushion in a sealed chamber, s PIPING PRESSURE SURGE WITH DIFFERENT MATERIAL AND FLUID (a) What would be the pressure rise in the previous procedure if the pipe were aluminum instead of stainless steel? (b) What would be the pressure rise in the system in the previous procedure if the flow medium were water having a bulk modulus, K, of 300,000 lb/in2 (2067 MPa) and a specific weight of 62.42 lb/ft3 (998.7 kg/m3 )? Calculation Procedure: 1. Find the velocity of the pressure wave in the pipe (a) From Fig. 2, for aluminum pipe having a D/t ratio of 28.6, the velocity of the pressure wave is 3655 ft/s (1114.0 m/s). Alternatively, the velocity could be com- puted as in step 1 in the previous procedure. 2. Compute the time for one interval of the pressure wave As before, in the previous procedure, 2 L/a ⫽ 2 (40/3655) ⫽ 0.02188 s. 3. Calculate the pressure rise in the pipe Since the time of 0.015 s for the valve closure is less than the interval time of 2 L/a equal to 0.02188, the pressure rise can be computed from: ⌬p ⫽ ␥aV/144g or, 52 ⫻ 3655 ⫻ 8.3 2 ⌬p ⫽ ⫽ 340.2 lb/in (2343.98 kPa) 144 ⫻ 32.2 4. Find the maximum static pressure in the line Using the pressure-rise relation, pmax ⫽ 500 ⫹ 340.2 ⫽ 840.2 lb/in2 (abs) (5788.97 kPa). 5. Determine the pressure rise for the different fluid (b) For water, use Fig. 2 for stainless steel pipe having a D/t ratio of 28.6 to find a ⫽ 4147 ft/s (1264 m/s). Alternatively, the velocity could be calculated as in step 1 of the previous procedure. 6. Compute the time for one internal of the pressure wave Using 2 L/a ⫽ 2 (40)/4147 ⫽ 0.012929 s. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 410. 8.6 PLANT AND FACILITIES ENGINEERING 5,000 4,000 3,000 2,000 1524 1219 914 610 a, Velocity of pressure wave, ft per sec Velocity, m/sec 0 140 120 100 80 60 40 20 D / t; I.D. of pipe / wall thickness See Fig. 1 for SI values Bulk modulus K = 250,000 psi Specific weight = 52 lb per cu ft ␥ Stainless steel pipe, E = 29 x 106 psi Copper pipe, E = 17 x 106 psi Alum inum pipe, E = 10.7 x 106 psi FIGURE 2 Velocity of pressure wave in water column in pipe of different diameter-to- wall thickness ratios. (Product Engineering.) 7. Find the pressure rise and maximum static pressure in the line Since the time of 0.015 s for valve closure is less than the interval time 2 L/a equal to 0.01929 s, the pressure rise can be computed from ⌬p ⫽ ␥aV/144g for rapid valve closure. Therefore, the pressure rise when the flow medium is water is: 62.42 ⫻ 4147 ⫻ 8.3 2 ⌬p ⫽ ⫽ 463.4 lb/in (3192.8 kPa) 144 ⫻ 32.2 The maximum static pressure, pmax ⫽ 500 ⫹ 463.4 ⫽ 963.4 lb/in2 (abs) (6637.8 kPa). Related Calculations. This procedure is the work of Nils M. Sverdrup, as detailed in the previous procedure. PRESSURE SURGE IN PIPING SYSTEM WITH COMPOUND PIPELINE A compound pipeline consisting of several stainless-steel pipes of different diam- eters, Fig. 3, conveys 40 gal/min (2.5 L/s) of water. The length of each section of Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 411. PIPING AND FLUID FLOW 8.7 FIGURE 3 Compound pipeline consists of pipe sections having dif- ferent diameters. (Product Engineer- ing.) pipe is: L1 ⫽ 25 ft (7.6 m); L2 ⫽ 15 ft (4.6 m); L3 ⫽ 10 ft (3.0 m); pipe wall thickness in each section is 0.049 in (1.24 mm); inside diameter of each section of pipe is D1 ⫽ 1.402 in (35.6 mm); D2 ⫽ 1.152 in (29.3 mm); D3 ⫽ 0.902 in (22.9 mm). What is the equivalent fluid velocity and the effective velocity of the pressure wave on sudden valve closure? Calculation Procedure: 1. Determine fluid velocity and pressure-wave velocity in the first pipe D1 /t1 ratio of the first pipe ⫽ 1.402/0.049 ⫽ 28.6. Then, the fluid velocity in the pipe can be found from V1 ⫽ 0.4085(Gn /(Dn )2 , where the symbols are as shown below. Substituting, V1 ⫽ 0.4085(40)/(1.402)2 ⫽ 8.31 ft/s (2.53 m/s). Using these two computed values, enter Fig. 2 to find the velocity of the pressure wave in pipe 1 as 4147 ft/s (1264 m/s). 2. Find the fluid velocity and pressure-wave velocity in the second pipe The D2 /t2 ratio for the second pipe ⫽ 1.152/0.049 ⫽ 23.51. Using the same ve- locity equation as in step 1, above V2 ⫽ 0.4085(40)/(1.152)2 ⫽ 12.31 ft/s (3.75 m/s). Again, from Fig. 2, a2 ⫽ 4234 ft/s (1290.5 m/s). Thus, there is an 87-ft/s (26.5- m/s) velocity increase of the pressure wave between pipes 1 and 2. 3. Compute the fluid velocity and pressure-wave velocity in the third pipe Using a similar procedure to that in steps 1 and 2 above, V3 ⫽ 20.1 ft/s (6.13 m/ s); s3 ⫽ 4326 ft/s (1318.6 m/s). 4. Find the equivalent fluid velocity and effective pressure-wave velocity for the compound pipe Use the equation L V ⫹ L V ⫹ 䡠 䡠 䡠 ⫹ L V 1 1 2 2 n n V ⫽ E L ⫹ L ⫹ 䡠 䡠 䡠 ⫹ L 1 2 n to find the equivalent fluid velocity in the compound pipe. Substituting, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 412. 8.8 PLANT AND FACILITIES ENGINEERING 25 ⫻ 8.3 ⫹ 15 ⫻ 12.3 ⫹ 10 ⫻ 20.1 V ⫽ E 25 ⫹ 15 ⫹ 10 ⫽ 11.9 ft/s (3.63 m/s) To find the effective velocity of the pressure wave, use the equation L ⫹ L ⫹ 䡠 䡠 䡠 L 1 2 n a ⫽ g (L /a ) ⫹ (L /a ) ⫹ 䡠 䡠 䡠 ⫹ (L /a ) 1 1 2 2 n n Substituting, 25 ⫹ 15 ⫹ 10 a ⫽ g (25/4147) ⫹ (15/4234) ⫹ (10/4326) ⫽ 4209 ft/s (1282.9 m/s) Thus, equivalent fluid velocity and effective velocity of the pressure wave in the compound pipe are both less than either velocity in the individual sections of the pipe. Related Calculations. Compound pipes find frequent application in industrial hydraulic systems. The procedure given here is useful in determining the velocities produced by sudden closure of a valve in the line. L1, L2, . . . , Ln ⫽ length of each section of pipe of constant diameter, ft (m) a1, a2, . . . , an ⫽ velocity of pressure wave in the respective pipe sections, ft/s (m/s) ag ⫽ effective velocity of the pressure wave, ft/s V1, V2, . . Vn ⫽ velocity of fluid in the respective pipe sections, ft/s (m/s) VE ⫽ equivalent fluid velocity, ft/s (m/s) Gn ⫽ rate of flow in respective section, U.S. gal/min (L/s) Dn ⫽ inside diameter of respective pipe, in (mm) The fluid velocity in an individual pipe is 2 V ⫽ 0.4085G /D n n n This procedure is the work of Nils M. Sverdrup, as detailed earlier. Pipe Properties, Flow Rate, and Pressure Drop QUICK CALCULATION OF FLOW RATE AND PRESSURE DROP IN PIPING SYSTEMS A 3-in (76-mm) Schedule 40S pipe has a 300-gal/min (18.9-L/s) water flow rate with a pressure loss of 8 lb/in2 (55.1 kPa)/100 ft (30.5 m). What would be the flow rate in a 4-in (102-mm) Schedule 40S pipe with the same pressure loss? What would be the pressure loss in a 4-in (102-mm) Schedule 40S pipe with the same Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 413. PIPING AND FLUID FLOW 8.9 flow rate, 300 gal/min (18.9 L/s)? Determine the flow rate and pressure loss for a 6-in (152-mm) Schedule 40S pipe with the same pressure and flow conditions. Calculation Procedure: 1. Determine the flow rate in the new pipe sizes Flow rate in a pipe with a fixed pressure drop is proportional to the ratio of (new pipe inside diameter/known pipe inside diameter)2.4 . This ratio is defined as the flow factor, F. To use this ratio, the exact inside pipe diameters, known and new, must be used. Take the exact inside diameter from a table of pipe properties. Thus, with a 3-in (76-mm) and a 4-in (102-mm) Schedule 40S pipe conveying water at a pressure drop of 8 lb/in2 (55.1 kPa)/100 ft (30.5 m), the flow factor F ⫽ (4.026/3.068)2.4 ⫽ 1.91975. Then, the flow rate, FR, in the large 4-in (102- mm) pipe with the 8 lb/in2 (55.1 kPa) pressure drop/100 ft (30.5 m), will be, FR ⫽ 1.91975 ⫻ 300 ⫽ 575.9 gal/min (36.3 L/s). For the 6-in (152-mm) pipe, the flow rate with the same pressure loss will be (6.065/3.068)2.4 ⫻ 300 ⫽ 1539.8 gal/min (97.2 L/s). 2. Compute the pressure drops in the new pipe sizes The pressure drop in a known pipe size can be extrapolated to a new pipe size by using a pressure factor, P, when the flow rate is held constant. For this condition, P ⫽ (known inside diameter of the pipe/new inside diameter of the pipe)4.8 . For the first situation given above, P ⫽ (3.068/4.026)4.8 ⫽ 0.27134. Then, the pressure drop, PDN, in the new 4-in (102-mm) Schedule 40S pipe with a 300-gal/ min (18.9-L/s) flow will be PDN ⫹ P(PDK ), where PDK ⫽ pressure drop in the known pipe size. Substituting, PDN ⫽ 0.27134(8) ⫽ 2.17 lb/in2 /100 ft (14.9 kPa/ 30.5 m). For the 6-in (152-mm) pipe, using the same approach, PDN ⫽ (3.068/6.065)4.8 (8) ⫽ 0.303 lb/in2 /100 ft (2.1 kPa/30.5 m). Related Calculations. The flow and pressure factors are valuable timesavers in piping system design because they permit quick determination of new flow rates or pressure drops with minimum time input. When working with a series of pipe- size possibilities of the same Schedule Number, the designer can compute values for F and P in advance and apply them quickly. Here is an example of such a calculation for Schedule 40S piping of several sizes: Nominal pipe size, new/known Flow factor, F Nominal pipe size, known/new Pressure factor, P 2/1 5.092 1/2 0.0386 3/2 2.58 2/3 0.150 4/3 1.919 3/4 0.271 6/4 2.674 4/6 0.1399 8/6 1.933 6/8 0.267 10/8 1.726 8/10 0.335 12/10 1.542 10/12 0.421 When computing such a listing, the actual inside diameter of the pipe, taken from a table of pipe properties, must be used when calculating F or P. The F and P values are useful when designing a variety of piping systems for chemical, petroleum, power, cogeneration, marine, buildings (office, commercial, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 414. 8.10 PLANT AND FACILITIES ENGINEERING residential, industrial), and other plants. Both the F and P values can be used for pipes conveying oil, water, chemicals, and other liquids. The F and P values are not applicable to steam or gases. Note that the ratio of pipe diameters is valid for any units of measurement— inches, cm, mm—provided the same units are used consistently throughout the calculation. The results obtained using the F and P values usually agree closely with those obtained using exact flow or pressure-drop equations. Such accuracy is generally acceptable in everyday engineering calculations. While the pressure drop in piping conveying a liquid is inversely proportional to the fifth power of the pipe diameter ratio, turbulent flow alters this to the value of 4.8, according to W. L. Nelson, Technical Editor, The Oil and Gas Journal. FLUID HEAD-LOSS APPROXIMATIONS FOR ALL TYPES OF PIPING Using the four rules for approximating head loss in pipes conveying fluid under turbulent flow conditions with a Reynolds number greater than 2100, find: (a) A 4-in (101.6-mm) pipe discharges 100 gal/min (6.3 L/s); how much fluid would a 2-in (50.8-mm) pipe discharge under the same conditions? (b) A 4-in (101.6-mm) pipe has 240 gal/min (15.1 L/s) flowing through it. What would be the friction loss in a 3-in (76.2-mm) pipe conveying the same flow? (c) A flow of 10 gal/min (6.3 L/s) produces 50 ft (15.2 m) of friction in a pipe. How much friction will a flow of 200 gal/min (12.6 L/s) produce? (d) A 12-in (304.8-mm) diameter pipe has a friction loss of 200 ft (60.9 m)/1000 ft (304.8 M). What is the capacity of this pipe? Calculation Procedure: 1. Use the rule: At constant head, pipe capacity is proportional to d2.5 (a) Applying the constant-head rule for both pipes: 42.5 ⫽ 32.0; 22.5 ⫽ 5.66. Then, the pipe capacity ⫽ (flow rate, gal/min or L/s)(new pipe size2.5 )/(previous pipe size2.5 ) ⫽ (100)(5.66)/32 ⫽ 17.69 gal/min (1.11 L/s). Thus, using this rule you can approximate pipe capacity for a variety of con- ditions where the head is constant. This approximation is valid for metal, plastic, wood, concrete, and other piping materials. 2. Use the rule: At constant capacity, head is proportional to 1/d5 (b) We have a 4-in (101.6-mm) pipe conveying 240 gal/min (15.1 L/s). If we reduce the pipe size to 3 in (76.2 mm) the friction will be greater because the flow area is smaller. The head loss ⫽ (flow rate, gal/min or L/s)(larger pipe diameter to the fifth power)/(smaller pipe diameter to the fifth power). Or, head ⫽ (240)(45 )/(35 ) ⫽ 1011 ft/1000 ft of pipe (308.3 m/304.8 m of pipe). Again, using this rule you can quickly and easily find the friction in a different size pipe when the capacity or flow rate remains constant. With the easy availability of handheld calculators in the field and computers in the design office, the fifth power of the diameter is easily found. 3. Use the rule: At constant diameter, head is proportional to gal/min (L/s)2 (c) We know that a flow of 100 gal/min (6.3 L/s) produces 50-ft (15.2-m) friction, h, in a pipe. The friction, with a new flow will be, h ⫽ (friction, ft or m, at known Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 415. PIPING AND FLUID FLOW 8.11 flow rate)(new flow rate, gal/min or L/s2 )/(previous flow rate, gal/min or L/s2 ). Or, h ⫽ (50)(2002 )/(1002 ) ⫽ 200 ft (60.9 m). Knowing that friction will increase as we pump more fluid through a fixed- diameter pipe, this rule can give us a fast determination of the new friction. You can even do the square mentally and quickly determine the new friction in a matter of moments. 4. Use the rule: At constant diameter, capacity is proportional to friction, h0.5 (d) Here the diameter is 12 in (304.8 mm) and friction is 200 ft (60.9 m)/1000 ft (304.8 m). From a pipe friction chart, the nearest friction head is 84 ft (25.6 m) for a flow rate of 5000 gal/min (315.5 L/s). The new capacity, c ⫽ (known ca- pacity, gal/min or L/s)(known friction, ft or m0.5 )/(actual friction, ft or m0.5 ). Or, c ⫽ 5000(2000.5 )/(840.5 ) ⫽ 7714 gal/min (486.6 L/s). As before, a simple calculation, the ratio of the square roots of the friction heads times the capacity will quickly give the new flow rates. Related Calculations. Similar laws for fans and pumps give quick estimates of changed conditions. These laws are covered elsewhere in this handbook in the sections on fans and pumps. Referring to them now will give a quick comparison of the similarity of these sets of laws. PIPE-WALL THICKNESS AND SCHEDULE NUMBER Determine the minimum wall thickness tm in (mm) and schedule number SN for a branch steam pipe operating at 900⬚F (482.2⬚C) if the internal steam pressure is 1000 lb/in2 (abs) (6894 kPa). Use ANSA B31.1 Code for Pressure Piping and the ASME Boiler and Pressure Vessel Code valves and equations where they apply. Steam flow rate is 72,000 lb/h (32,400 kg/h). Calculation Procedure: 1. Determine the required pipe diameter When the length of pipe is not given or is as yet unknown, make a first approxi- mation of the pipe diameter, using a suitable velocity for the fluid. Once the length of the pipe is known, the pressure loss can be determined. If the pressure loss exceeds a desirable value, the pipe diameter can be increased until the loss is within an acceptable range. Compute the pipe cross-sectional area a in2 (cm2 ) from a ⫽ 2.4Wv/V, where W ⫽ steam flow rate, lb/h (kg/h); v ⫽ specific volume of the steam, ft3 /lb (m3 / kg); V ⫽ steam velocity, ft/min (m/min). The only unknown in this equation, other than the pipe area, is the steam velocity V. Use Table 1 to find a suitable steam velocity for this branch line. Table 1 shows that the recommended steam velocities for branch steam pipes range from 6000 to 15,000 ft/min (1828 to 4572 m/min). Assume that a velocity of 12,000 ft/min (3657.6 m/min) is used in this branch steam line. Then, by using the steam table to find the specific volume of steam at 900⬚F (482.2⬚C) and 1000 lb/in2 (abs) (6894 kPa), a ⫽ 2.4(72,000)(0.7604)/12,000 ⫽ 10.98 in2 (70.8 cm2 ). The inside diameter of the pipe is then d ⫽ 2(a/)0.5 ⫽ 2(10.98/)0.5 ⫽ 3.74 in (95.0 mm). Since pipe is not ordinarily made in this fractional internal diameter, round it to the next larger size, or 4-in (101.6-mm) inside diameter. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 416. 8.12 PLANT AND FACILITIES ENGINEERING TABLE 1 Recommended Fluid Velocities in Piping 2. Determine the pipe schedule number The ANSA Code for Pressure Piping, commonly called the Piping Code, defines schedule number as SN ⫽ 1000 Pi /S, where Pi ⫽ internal pipe pressure, lb/in2 (gage); S ⫽ allowable stress in the pipe, lb/in2 , from Piping Code. Table 2 shows typical allowable stress values for pipe in power piping systems. For this pipe, assuming that seamless ferritic alloy steel (1% Cr, 0.55% Mo) pipe is used with the steam at 900⬚F (482⬚C), SN ⫽ (1000)(1014.7)/13,100 ⫽ 77.5. Since pipe is not ordinarily made in this schedule number, use the next highest readily available schedule number, or SN ⫽ 80. [Where large quantities of pipe are required, it is sometimes economically wise to order pipe of the exact SN required. This is not usually done for orders of less than 1000 ft (304.8 m) of pipe.] 3. Determine the pipe-wall thickness Enter a tabulation of pipe properties, such as in Crocker and King—Piping Hand- book, and find the wall thickness for 4-in (101.6-mm) SN 80 pipe as 0.337 in (8.56 mm). Related Calculations. Use the method given here for any type of pipe—steam, water, oil, gas, or air—in any service—power, refinery, process, commercial, etc. Refer to the proper section of B31.1 Code for Pressure Piping when computing the schedule number, because the allowable stress S varies for different types of service. The Piping Code contains an equation for determining the minimum required pipe-wall thickness based on the pipe internal pressure, outside diameter, allowable stress, a temperature coefficient, and an allowance for threading, mechanical strength, and corrosion. This equation is seldom used in routine piping-system de- sign. Instead, the schedule number as given here is preferred by most designers. PIPE-WALL THICKNESS DETERMINATION BY PIPING CODE FORMULA Use the ANSA B31.1 Code for Pressure Piping wall-thickness equation to deter- mine the required wall thickness for an 8.625-in (219.1-mm) OD ferritic steel plain- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 417. 8.13 TABLE 2 Allowable Stresses (S Values) for Alloy-Steel Pipe in Power Piping Systems* (Abstracted from ASME Power Boiler Code and Code for Pressure Piping, ASA B31.1 ) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 418. 8.14 PLANT AND FACILITIES ENGINEERING end pipe if the pipe is used in 900⬚F (482⬚C) 900-lb/in2 (gage) (6205-kPa) steam service. Calculation Procedure: 1. Determine the constants for the thickness equation Pipe-wall thickness to meet ANSA Code requirements for power service is com- puted from tm ⫽ {DP/[2(S ⫹ YP)]} ⫹ C, where tm ⫽ minimum wall thickness, in; D ⫽ outside diameter of pipe, in; P ⫽ internal pressure in pipe, lb/in2 (gage); S ⫽ allowable stress in pipe material, lb/in2 ; Y ⫽ temperature coefficient; C ⫽ end- condition factor, in. Values of S, Y, and C are given in tables in the Code for Pressure Piping in the section on Power Piping. Using values from the latest edition of the Code, we get S ⫽ 12,500 lb/in2 (86.2 MPa) for ferritic-steel pipe operating at 900⬚F (482⬚C); Y ⫽ 0.40 at the same temperature; C ⫽ 0.065 in (1.65 mm) for plain-end steel pipe. 2. Compute the minimum wall thickness Substitute the given and Code values in the equation in step 1, or tm ⫽ [(8.625)(900)]/[2(12,500 ⫹ 0.4 ⫻ 900)] ⫹ 0.065 ⫽ 0.367 in (9.32 mm). Since pipe mills do not fabricate to precise wall thicknesses, a tolerance above or below the computed wall thickness is required. An allowance must be made in specifying the wall thickness found with this equation by increasing the thickness by 121 ⁄2 percent. Thus, for this pipe, wall thickness ⫽ 0.367 ⫹ 0.125(0.367) ⫽ 0.413 in (10.5 mm). Refer to the Code to find the schedule number of the pipe. Schedule 60 8-in (203-mm) pipe has a wall thickness of 0.406 in (10.31 mm), and schedule 80 has a wall thickness of 0.500 in (12.7 mm). Since the required thickness of 0.413 in (10.5 mm) is greater than schedule 60 but less than schedule 80, the higher schedule number, 80, should be used. 3. Check the selected schedule number From the previous calculation procedure, SN ⫽ 1000 Pi /S. From this pipe, SN ⫽ 1000(900)/12,500 ⫽ 72. Since piping is normally fabricated for schedule numbers 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160, the next larger schedule number higher than 72, that is 80, will be used. This agrees with the schedule number found in step 2. Related Calculations. Use this method in conjunction with the appropriate Code equation to determine the wall thickness of pipe conveying air, gas, steam, oil, water, alcohol, or any other similar fluids in any type of service. Be certain to use the correct equation, which in some cases is simpler than that used here. Thus, for lead pipe, tn ⫽ Pd/2S, where P ⫽ safe working pressure of the pipe, lb/in2 (gage); d ⫽ inside diameter of pipe, in; other symbols as before. When a pipe will operate at a temperature between two tabulated Code values, find the allowable stress by interpolating between the tabulated temperature and stress values. Thus, for a pipe operating at 680⬚F (360⬚C), find the allowable stress at 650⬚F (343⬚C) [⫽ 9500 lb/in2 (65.5 MPa)] and 700⬚F (371⬚C) [⫽ 9000 lb/in2 (62.0 MPa)]. Interpolate thus: allowable stress at 680⬚F (360⬚C) ⫽ [(700⬚F ⫺ 680⬚F)/(700⬚F ⫺ 650⬚F)](9500 ⫺ 9000) ⫹ 9000 ⫽ 200 ⫹ 9000 ⫽ 9200 lb/in2 (63.4 MPa). The same result can be obtained by interpolating downward from 9500 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 419. PIPING AND FLUID FLOW 8.15 lb/in2 (65.5 MPa), or allowable stress at 680⬚F (360⬚C) ⫽ 9500 ⫺ [(680 ⫺ 650)/ (700 ⫺ 650)](9500 ⫺ 9000) ⫽ 9200 lb/in2 (63.4 MPa). DETERMINING THE PRESSURE LOSS IN STEAM PIPING Use a suitable pressure-loss chart to determine the pressure loss in 510 ft (155.5 m) of 4-in (101.6-mm) flanged steel pipe containing two 90⬚ elbows and four 45⬚ bends. The schedule 40 piping conveys 13,000 lb/h (5850 kg/h) of 20-lb/in2 (gage) (275.8-kPa) 350⬚F (177⬚C) superheated steam. List other methods of determining the pressure loss in steam piping. Calculation Procedure: 1. Determine the equivalent length of the piping The equivalent length of a pipe Le ft ⫽ length of straight pipe, ft ⫹ equivalent length of fittings, ft. Using data from the Hydraulic Institute, Crocker and King—Piping Handbook, earlier sections of this handbook, or Fig. 4, find the equivalent length of a 90⬚ 4-in (101.6-mm) elbow as 10 ft (3 m) of straight pipe. Likewise, the equivalent length of a 45⬚ bend is 5 ft (1.5 m) of straight pipe. Substituting in the above relation and using the straight lengths and the number of fittings of each type, we get Le ⫽ 510 ⫹ (2)(10) ⫹ 4(5) ⫽ 550 ft (167.6 m) of straight pipe. 2. Compute the pressure loss, using a suitable chart Figure 2 presents a typical pressure-loss chart for steam piping. Enter the chart at the top left at the superheated steam temperature of 350⬚F (177⬚C), and project vertically downward until the 40-lb/in2 (gage) (275.8-kPa) superheated steam pres- sure curve is intersected. From here, project horizontally to the right until the outer border of the chart is intersected. Next, project through the steam flow rate, 13,000 lb/h (5900 kg/h) on scale B, Fig. 5, to the pivot scale C. From this point, project through 4-in (101.6-mm) schedule 40 pipe on scale D, Fig. 5. Extend this line to intersect the pressure-drop scale, and read the pressure loss as 7.25 lb/in2 (50 kPa)/100 ft (30.4 m) of pipe. Since the equivalent length of this pipe is 550 ft (167.6 m), the total pressure loss in the pipe is (550/100)(7.25) ⫽ 39.875 lb/in2 (274.9 kPa), say 40 lb/in2 (275.8 kPa). 3. List the other methods of computing pressure loss Numerous pressure-loss equations have been developed to compute the pressure drop in steam piping. Among the better known are those of Unwin, Fritzche, Spitz- glass, Babcock, Guttermuth, and others. These equations are discussed in some detail in Crocker and King—Piping Handbook and in the engineering data pub- lished by valve and piping manufacturers. Most piping designers use a chart to determine the pressure loss in steam piping because a chart saves time and reduces the effort involved. Further, the accuracy obtained is sufficient for all usual design practice. Figure 3 is a popular flowchart for determining steam flow rate, pipe size, steam pressure, or steam velocity in a given pipe. Using this chart, the designer can Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 420. 8.16 PLANT AND FACILITIES ENGINEERING FIGURE 4 Equivalent length of pipe fittings and valves. (Crane Company.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 421. 8.17 FIGURE 5 Pressure loss in steam pipes based on the Fritzche formula. (Power.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 422. 8.18 PLANT AND FACILITIES ENGINEERING determine any one of the four variables listed above when the other three are known. In solving a problem on the chart in Fig. 6, use the steam-quantity lines to intersect pipe sizes and the steam-pressure lines to intersect steam velocities. Here are two typical applications of this chart. Example. What size schedule 40 pipe is needed to deliver 8000 lb/h (3600 kg/h) of 120-lb/in2 (gage) (827.3-kPa) steam at a velocity of 5000 ft/min (1524 m/min)? Solution. Enter Fig. 6 at the upper left at a velocity of 5000 ft/min (1524 m/ min), and project along this velocity line until the 120-lb/in2 (gage) (827.3-kPa) pressure line is intersected. From this intersection, project horizontally until the 8000-lb/h (3600-kg/h) vertical line is intersected. Read the nearest pipe size as 4 in (101.6 mm) on the nearest pipe-diameter curve. Example. What is the steam velocity in a 6-in (152.4-mm) pipe delivering 20,000 lb/h (9000 kg/h) of steam at 85 lb/in2 (gage) (586 kPa)? Solution. Enter the bottom of the chart, Fig. 6, at the flow rate of 20,000 lb/h (9000 kg/h), and project vertically upward until the 6-in (152.4-mm) pipe curve is intersected. From this point, project horizontally to the 85-lb/in2 (gage) (586-kPa) curve. At the intersection, read the velocity as 7350 ft/min (2240.3 m/ min). Table 3 shows typical steam velocities for various industrial and commercial applications. Use the given values as guides when sizing steam piping. PIPING WARM-UP CONDENSATE LOAD How much condensate is formed in 5 min during warm-up of 500 ft (152.4 m) of 6-in (152.4-mm) schedule 40 steel pipe conveying 215-lb/in2 (abs) (1482.2-kPa) saturated steam if the pipe is insulated with 2 in (50.8 mm) of 85 percent magnesia and the minimum external temperature is 35⬚F (1.7⬚C)? Calculation Procedure: 1. Compute the amount of condensate formed during pipe warm-up For any pipe, the condensate formed during warm-up Ch lb/h ⫽ 60(Wp )(⌬t)(s)/ , where Wp ⫽ total weight of pipe, lb; ⌬t ⫽ difference between final and initial h N ƒg temperature of the pipe, ⬚F; s ⫽ specific heat of pipe material, Btu/(lb 䡠 ⬚F); ⫽ enthalpy of vaporization of the steam, Btu/lb; N ⫽ warm-up time, min. hƒg A table of pipe properties shows that this pipe weighs 18.974 lb/ft (28.1 kg/ m). The steam table shows that the temperature of 215-lb/in2 (abs) (1482.2-kPa) saturated steam is 387.89⬚F (197.7⬚C), say 388⬚F (197.8⬚C); the enthalpy ⫽ 837.4 Btu/lb (1947.8 kJ/kg). The specific heat of steel pipe s ⫽ 0.144 Btu hƒg /(lb 䡠 ⬚F) [0.6 kJ/(kg 䡠 ⬚C)]. Then Ch ⫽ 60(500 ⫻ 18.974)(388 ⫺ 35)(0.114)/ [(837.4)(5)] ⫽ 5470 lb/h (2461.5 kg/h). 2. Compute the radiation-loss condensate load Condensate is also formed by radiation of heat from the pipe during warm-up and while the pipe is operating. The warm-up condensate load decreases as the radiation load increases, the peak occurring midway (21 ⁄2 min in this case) through the warm- up period. For this reason, one-half the normal radiation load is added to the warm- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 423. 8.19 FIGURE 6 Spitzglass chart for saturated steam flowing in schedule 40 pipe. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 424. 8.20 PLANT AND FACILITIES ENGINEERING TABLE 3 Steam Velocities Used in Pipe Design up load. Where the radiation load is small, it is often disregarded. However, the load must be computed before its magnitude can be determined. For any pipe, Cr ⫽ (L)(A)(⌬t)(H)/ , where L ⫽ length of pipe, ft; A ⫽ external hƒg area of pipe, ft2 /ft of length; H ⫽ heat loss through bare pipe or pipe insulation, Btu/(ft2 䡠 h 䡠 ⬚F), from the piping or insulation tables. This 6-in (152.4-mm) schedule 40 pipe has an external area A ⫽ 1.73 ft2 /ft (0.53 m2 /m) of length. The heat loss through 2 in (50.8 mm) of 85 percent magnesia, from insulation tables, is H ⫽ 0.286 Btu/(ft2 䡠 h 䡠 ⬚F) [1.62 W/(m2 䡠 ⬚C)]. Then Cr ⫽ (500) ⫻ (1.73)(388 ⫺ 35)(0.286)/837.4 ⫽ 104.2 lb/h (46.9 kg/h). Adding half the radiation load to the warm-up load gives 5470 ⫹ 52.1 ⫽ 5522.1 lb/h (2484.9 kg/h). 3. Apply a suitable safety factory to the condensate load Trap manufacturers recommend a safety factor of 2 for traps installed between a boiler and the end of a steam main; traps at the end of a long steam main or ahead of pressure-regulating or shutoff valves usually have a safety factor of 3. With a safety factor of 3 for this pipe, the steam trap should have a capacity of at least 3(5522.1) ⫽ 16,566.3 lb/h (7454.8 kg/h), say 17,000 lb/h (7650.0 kg/h). Related Calculations. Use this method to find the warm-up condensate load for any type of steam pipe—main or auxiliary—in power, process, heating, or vacuum service. The same method is applicable to other vapors that form condensate—Dowtherm, refinery vapors, process vapors, and others. STEAM TRAP SELECTION FOR INDUSTRIAL APPLICATIONS Select steam traps for the following four types of equipment: (1) the steam directly heats solid materials as in autoclaves, retorts, and sterilizers; (2) the steam indirectly heats a liquid through a metallic surface, as in heat exchangers and kettles, where the quantity of liquid heated is known and unknown; (3) the steam indirectly heats a solid through a metallic surface, as in dryers using cylinders or chambers and platen presses; and (4) the steam indirectly heats air through metallic surfaces, as in unit heaters, pipe coils, and radiators. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 425. PIPING AND FLUID FLOW 8.21 TABLE 5 Use These Specific Heats to Calculate Condensate Load TABLE 4 Factors P ⫽ (T ⫺ t)/L to Find Condensate Load Calculation Procedure: 1. Determine the condensate load The first step in selecting a steam trap for any type of equipment is determination of the condensate load. Use the following general procedure. a. Solid materials in autoclaves, retorts, and sterilizers. How much condensate is formed when 2000 lb (900.0 kg) of solid material with a specific heat of 1.0 is processed in 15 min at 240⬚F (115.6⬚C) by 25-lb/in2 (gage) (172.4-kPa) steam from an initial temperature of 60⬚F in an insulated steel retort? For this type of equipment, use C ⫽ WsP, where C ⫽ condensate formed, lb/ h; W ⫽ weight of material heated, lb; s ⫽ specific heat, Btu/(lb 䡠 ⬚F); P ⫽ factor from Table 4. Thus, for this application, C ⫽ (2000)(1.0)(0.193) ⫽ 386 lb (173.7 kg) of condensate. Note that P is based on a temperature rise of 240 ⫺ 60 ⫽ 180⬚F (100⬚C) and a steam pressure of 25 lb/in2 (gage) (172.4 kPa). For the retort, using the specific heat of steel from Table 5, C ⫽ (4000)(0.12)(0.193) ⫽ 92.6 lb of condensate, say 93 lb (41.9 kg). The total weight of condensate formed in 15 min is 386 ⫹ 93 ⫽ 479 lb (215.6 kg). In 1 h, 479(60/15) ⫽ 1916 lb (862.2 kg) of condensate is formed. A safety factor must be applied to compensate for radiation and other losses. Typical safety factors used in selecting steam traps are as follows: Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 426. 8.22 PLANT AND FACILITIES ENGINEERING With a safety factor of 4 for this process retort, the trap capacity ⫽ (4)(1916) ⫽ 7664 lb/h (3449 kg/h), say 7700 lb/h (3465 kg/h). b(1). Submerged heating surface and a known quantity of liquid. How much condensate forms in the jacket of a kettle when 500 gal (1892.5 L) of water is heated in 30 min from 72 to 212⬚F (22.2 to 100⬚C) with 50-lb/in2 (gage) (344.7- kPa) steam? For this type of equipment, C ⫽ GwsP, where G ⫽ gal of liquid heated; w ⫽ weight of liquid, lb/gal. Substitute the appropriate values as follows: C ⫽ (500)(8.33)(1.0) ⫻ (0.154) ⫽ 641 lb (288.5 kg), or (641)(60/3) ⫽ 1282 lb/h (621.9 kg/h). With a safety factor of 3, the trap capacity ⫽ (3)(1282) ⫽ 3846 lb/h (1731 kg/h), say 3900 lb/h (1755 kg/h). b(2). Submerged heating surface and an unknown quantity of liquid. How much condensate is formed in a coil submerged in oil when the oil is heated as quickly as possible from 50 to 250⬚F (10 to 121⬚C) by 25-lb/in2 (gage) (172.4-kPa) steam if the coil has an area of 50 ft2 (4.66 m2 ) and the oil is free to circulate around the coal? For this condition, C ⫽ UAP, where U ⫽ overall coefficient of heat transfer, Btu/(h 䡠 ft2 䡠 ⬚F), from Table 6; A ⫽ area of heating surface, ft2 . With free convection and a condensing-vapor-to-liquid type of heat exchanger, U ⫽ 10 to 30. With an average value of U ⫽ 20, C ⫽ (20)(50)(0.214) ⫽ 214 lb/h (96.3 kg/h) of conden- sate. Choosing a safety factor 3 gives trap capacity ⫽ (3)(214) ⫽ 642 lb/h (289 kg/h), say 650 lb/h (292.5 kg/h). b(3). Submerged surfaces having more area than needed to heat a specified quantity of liquid in a given time with condensate withdrawn as rapidly as formed. Use Table 7 instead of step b(1) or b(2). Find the condensate rate by multiplying the submerged area by the appropriate factor from Table 7. Use this method for heating water, chemical solutions, oils, and other liquids. Thus, with steam at 100 lb/in2 (gage) (689.4 kPa) and a temperature of 338⬚F (170⬚C) and heating oil from 50 to 226⬚F (10 to 108⬚C) with a submerged surface having an area of 500 ft2 (46.5 m2 ), the mean temperature difference (Mtd) ⫽ steam temperature minus the average liquid temperature ⫽ 338 ⫺ (50 ⫹ 226/2) ⫽ 200⬚F (93.3⬚C). The factor from Table 7 for 100 lb/in2 (gage) (689.4 kPa) steam and a 200⬚F (93.3⬚C) Mtd is 56.75. Thus, the condensate rate ⫽ (56.75)(500) ⫽ 28,375 lb/h (12,769 kg/h). With a safety factor of 2, the trap capacity ⫽ (2)(28.375) ⫽ 56,750 lb/h (25,538 kg/h). c. Solids indirectly heated through a metallic surface. How much condensate is formed in a chamber dryer when 1000 lb (454 kg) of cereal is dried to 750 lb (338 kg) by 10-lb/in2 (gage) (68.9-kPa) steam? The initial temperature of the cereal is 60⬚F (15.6⬚C), and the final temperature equals that of the steam. For this condition, C ⫽ 970(W ⫺ D)/ ⫹ WP, where D ⫽ dry weight of the hƒg material, lb; ⫽ enthalpy of vaporization of the steam at the trap pressure, hƒg Btu/lb. From the steam tables and Table 4, C ⫽ 970(1000 ⫺ 750)/952 ⫹ (1000)(0.189) ⫽ 443.5 lb/h (199.6 kg/h) of condensate. With a safety factor of 4, the trap capacity ⫽ (4)(443.5) ⫽ 1774 lb/h (798.3 kg/h). d. Indirect heating of air through a metallic surface. How much condensate is formed in a unit heater using 10-lb/in2 (gage) (68.9-kPa) steam if the entering-air temperature is 30⬚F (⫺1.1⬚C) and the leaving-air temperature is 130⬚F (54.4⬚C)? Airflow is 10,000 ft3 /min (281.1 m3 /min). Use Table 8, entering at a temperature difference of 100⬚F (37.8⬚C) and pro- jecting to a steam pressure of 10 lb/in2 (gage) (68.9 kPa). Read the condensate formed as 122 lb/h (54.9 kg/h) per 1000 ft3 /min (28.3 m3 /min). Since 10,000 ft3 /min (283.1 m3 /min) of air is being heated, the condensate rate ⫽ (10,000/ Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 427. 8.23 TABLE 6 Ordinary Ranges of Overall Coefficients of Heat Transfer Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 428. 8.24 PLANT AND FACILITIES ENGINEERING TABLE 7 Condensate Formed in Submerged Steel* Heating Elements, lb/(ft2 䡠 h) [kg/(m2 䡠 min)] TABLE 8 Steam Condensed by Air, lb/h at 1000 ft3 /min (kg/h at 28.3 m3 /min)* 1000)(122) ⫽ 1220 lb/h (549 kg/h). With a safety factor of 3, the trap capacity ⫽ (3)(1220) ⫽ 3660 lb/h (1647 kg/h), say 3700 lb/h (1665 kg/h). Table 9 shows the condensate formed by radiation from bare iron and steel pipes in still air and with forced-air circulation. Thus, with a steam pressure of 100 lb/ in2 (gage) (689.4 kPa) and an initial air temperature of 75⬚F (23.9⬚C), 1.05 lb/h (0.47 kg/h) of condensate will be formed per ft2 (0.09 m2 ) of heating surface in still air. With forced-air circulation, the condensate rate is (5)(1.05) ⫽ 5.25 lb/(h 䡠 ft2 ) [25.4 kg/(h 䡠 m2 )] of heating surface. Unit heaters have a standard rating based on 2-lb/in2 (gage) (13.8-kPa) steam with entering air at 60⬚F (15.6⬚C). If the steam pressure or air temperature is dif- ferent from these standard conditions, multiply the heater Btu/h capacity rating by the appropriate correction factor form, Table 10. Thus, a heater rated at 10,000 Btu/h (2931 W) with 2-lb/in2 (gage) (13.8-kPa) steam and 60⬚F (15.6⬚C) air would have an output of (1.290)(10,000) ⫽ 12,900 Btu/h (3781 W) with 40⬚F (4.4⬚C) inlet air and 10-lb/in2 (gage) (68.9-kPa) steam. Trap manufacturers usually list heater Btu ratings and recommend trap model numbers and sizes in their trap en- gineering data. This allows easier selection of the correct trap. 2. Select the trap size based on the load and steam pressure Obtain a chart or tabulation of trap capacities published by the manufacturer whose trap will be used. Figure 7 is a capacity chart for one type of bucket trap manu- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 429. PIPING AND FLUID FLOW 8.25 TABLE 9 Condensate Formed by Radiation from Bare Iron and Steel, lb/(ft2 䡠 h) [kg/(m2 䡠 h)] TABLE 10 Unit-Heater Correction Factors factured by Armstrong Machine Works. Table 11 shows typical capacities of im- pulse traps manufactured by the Yarway Company. To select a trap from Fig. 7, when the condensate rate is uniform and the pressure across the trap is constant, enter at the left at the condensation rate, say 8000 lb/h (3600 kg/h) (as obtained from step 1). Project horizontally to the right to the vertical ordinate representing the pressure across the trap [⫽ ⌬p ⫽ steam-line pres- sure, lb/in2 (gage) ⫺ return-line pressure with with trap valve closed, lb/in2 (gage)]. Assume ⌬p ⫽ 20 lb/in2 (gage) (138 kPa) for this trap. The intersection of the horizontal 8000-lb/h (3600-kg/h) projection and the vertical 20-lb/in2 (gage) (137.9-kPa) projection is on the sawtooth capacity curve for a trap having a 9 ⁄16-in (14.3-mm) diameter orifice. If these projections intersected beneath this curve, a 9 ⁄16-in (14.3-mm) orifice would still be used if the point were between the verticals for this size orifice. The dashed lines extending downward from the sawtooth curves show the ca- pacity of a trap at reduced ⌬p. Thus, the capacity of a trap with a 3 ⁄8-in (9.53-mm) orifice at ⌬p ⫽ 30 lb/in2 (gage) (207 kPa) is 6200 lb/h (2790 kg/h), read at the intersection of the 30-lb/in2 (gage) (207-kPa) ordinate and the dashed curve ex- tended from the 3 ⁄8-in (9.53-mm) solid curve. To select an impulse trap from Table 11, enter the table at the trap inlet pressure, say 125 lb/in2 (gage) (862 kPa), and project to the desired capacity, say 8000 lb/ h (3600 kg/h), determined from step 1. Table 11 shows that a 2-in (50.8-mm) trap Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 430. 8.26 PLANT AND FACILITIES ENGINEERING FIGURE 7 Capacities of one type of bucket steam trap. (Armstrong Machine Works.) TABLE 11 Capacities of Impulse Traps, lb/h (kg/h) [Maximum continuous discharge of condensate, based on condensate at 30⬚F (16.7⬚C) below steam temperature.] having an 8530-lb/h (3839-kg/h) capacity must be used because the next smallest size has a capacity of 5165 lb/h (2324 kg/h). This capacity is less than that re- quired. Some trap manufacturers publish capacity tables relating various trap models to specific types of equipment. Such tables simplify trap selection, but the condensate rate must still be computed as given here. Related Calculations. Use the procedure given here to determine the trap ca- pacity required for any industrial, commercial, or domestic application including acid vats, air dryers, asphalt tanks, autoclaves, baths (dyeing), belt presses, bleach tanks, blenders, bottle washers, brewing kettles, cabinet dryers, calenders, can wash- ers, candy kettles, chamber dryers, chambers (reaction), cheese kettles, coils (cook- ing, kettle, pipe, tank, tank-car), confectioners’ kettles, continuous dryers, conveyor Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 431. PIPING AND FLUID FLOW 8.27 dyers, cookers (nonpressure and pressure), cooking coils, cooking kettles, cooking tanks, cooking vats, cylinder dryers, cylinders (jacketed), double-drum dryers, drum dryers, drums (dyeing), dry cans, dry kilns, dryers (cabinet, chamber, continuous, conveyor, cylinder, drum, festoon, jacketed, linoleum, milk, paper, pulp, rotary, shelf, stretch, sugar, tray, tunnel), drying rolls, drying rooms, drying tables, dye vats, dyeing baths and drums, dryers (package), embossing-press platens, evapo- rators, feed waterheaters, festoon dryers, fin-type heaters, fourdriniers, fuel-oil pre- heaters, greenhouse coils, heaters (steam), heat exchangers, heating coils and ket- tles, hot-break tanks, hot plates, kettle coils, kettles (brewing, candy, cheese, confectioners’, cooking, heating, process), kiers, kilns (dry), liquid heaters, mains (steam), milk-bottle washers, milk-can washers, milk dryers, mixers, molding press platens, package dryers, paper dryers, percolators, phonograph-record press platens, pipe coils (still- and circulating-air), platens, plating tanks, plywood press platens, preheaters (fuel-oil), preheating tanks, press platens, pressure cookers, process ket- tles, pulp dryers, purifiers, reaction chambers, retorts, rotary dryers, steam mains (risers, separators), stocking boarders, storage-tank coils, storage water heaters, stretch dryers, sugar dryers, tank-car coils, tire-mold presses, tray dryers, tunnel dryers, unit heaters, vats, veneer press platens, vulcanizers, and water stills. Hospital equipment—such as autoclaves and sterilizers—can be analyzed in the same way, as can kitchen equipment—bain marie, compartment cooker, egg boiler, kettles, steam table, and urns; and laundry equipment—blanket dryers, curtain dryers, flat- work ironers, presses (dry-cleaning, laundry) sock forms, starch cookers, tumblers, etc. When using a trap capacity diagram or table, be sure to determine the basis on which it was prepared. Apply any necessary correction factors. Thus, cold-water capacity ratings must be corrected for traps operating at higher condensate tem- peratures. Correction factors are published in trap engineering data. The capacity of a trap is greater at condensate temperatures less than 212⬚F (100⬚C) because at or above this temperature condensate forms flash steam when it flows into a pipe or vessel at atmospheric [14.7 lb/in2 (abs) (101.3 kPa)] pressure. At altitudes above sea level, condensate flashes into steam at a lower temperature, depending on the altitude. The method presented here is the work of L. C. Campbell, Yarway Corporation, as reported in Chemical Engineering. SELECTING HEAT INSULATION FOR HIGH- TEMPERATURE PIPING Select the heat insulation for a 300-ft (91.4-m) long 10-in (254-mm) turbine lead operating at 570⬚F (299⬚C) for 8000 h/year in a 70⬚F (21.1⬚C) turbine room. How much heat is saved per year by this insulation? The boiler supplying the turbine has an efficiency of 80 percent when burning fuel having a heating value of 14,000 Btu/lb (32.6 MJ/kg). Fuel costs $6 per ton ($5.44 per metric ton). How much money is saved by the insulation each year? What is the efficiency of the insulation? Calculation Procedure: 1. Choose the type of insulation to use Refer to an insulation manufacturer’s engineering data or Crocker and King— Piping Handbook for recommendations about a suitable insulation for a pipe op- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 432. 8.28 PLANT AND FACILITIES ENGINEERING TABLE 12 Recommended Insulation Thickness erating in the 500 to 600⬚F (260 to 316⬚C) range. These references will show that calcium silicate is a popular insulation for this temperature range. Table 12 shows that a thickness of 3 in (76.2 mm) is usually recommended for 10-in (254-mm) pipe operating at 500 to 599⬚F (260 to 315⬚C). 2. Determine heat loss through the insulation Refer to an insulation manufacturer’s engineering data to find the heat loss through 3-in (76.2-mm) thick calcium silicate as 0.200 Btu/(h 䡠 ft2 䡠 ⬚F) [1.14 W/(m2 䡠 ⬚C)]. Since 10-in (254-mm) pipe has an area of 2.817 ft2 /ft (0.86 m2 /m) of length and since the temperature difference across the pipe is 570 ⫺ 70 ⫽ 500⬚F (260⬚C), the heat loss per hour ⫽ (0.200)(2.817)(50)⫽ 281.7 Btu/(h 䡠 ft) (887.9 W/m2 ). The heat loss from bare 10-in (254-mm) pipe with a 500⬚F (260⬚C) temperature differ- ence is, from an insulation manufacturer’s engineering data, 4.640 Btu/(h 䡠 ft2 䡠 ⬚F) [26.4 W/(m2 䡠 ⬚C)], or (4.64)(2.817)(500) ⫽ 6510 Btu/(h 䡠 ft) (6.3 kW/m). 3. Determine annual heat saving The heat saved ⫽ bare-pipe loss, Btu/h ⫺ insulated-pipe loss, Btu/ h ⫽ 6510 ⫺ 281.7 ⫽ 6228.3 Btu/(h 䡠 ft) (5989 W/m) of pipe. Since the pipe is 300 ft (91.4 m) long and operates 8000 h per year, the annual heat saving ⫽ (300)(8000)(6228.3) ⫽ 14,940,000,000 Btu/year (547.4 kW). 4. Compute the money saved by the heat insulation The heat saved in fuel as fired ⫽ (annual heat saving, Btu/year)/(boiler efficiency) ⫽ 14,940,000,000/0.80 ⫽ 18,680,000,000 Btu/year (5473 MW). Weight of fuel saved ⫽ (annual heat saving, Btu/year)/(heating value of fuel, Btu/lb)(2000 lb/ton) ⫽ 18,680,000,000/[(14,000)(2000)] ⫽ 667 tons (605 t). At $6 per ton ($5.44 per metric ton), the monetary saving is ($6)(667) ⫽ $4002 per year. 5. Determine the insulation efficiency Insulation efficiency ⫽ (bare-pipe loss ⫺ insulated-pipe loss)/bare pipe loss, all expressed in Btu/h, or bare-pipe loss ⫽ (6510.0 ⫺ 281.7)/6510.0 ⫽ 0.957, or 95.7 percent. Related Calculations. Use this method for any type of insulation—magnesia, fiber-glass, asbestos, felt, diatomaceous, mineral wool, etc.—used for piping at el- evated temperatures conveying steam, water, oil, gas, or other fluids or vapors. To Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 433. PIPING AND FLUID FLOW 8.29 coordinate and simplify calculations, become familiar with the insulation tables in a reliable engineering handbook or comprehensive insulation catalog. Such famil- iarity will simplify routine calculations. ORIFICE METER SELECTION FOR A STEAM PIPE Steam is metered with an orifice meter in a 10-in (254-mm) boiler lead having an internal diameter of dp ⫽ 9.760 in (247.9 mm). Determine the maximum rate of steam flow that can be measured with a steel orifice plate having a diameter of d0 ⫽ 5.855 in (148.7 mm) at 70⬚F (21.1⬚C). The upstream pressure tap is 1D ahead of the orifice, and the downstream tap is 0.5D past the orifice. Steam pressure at the orifice inlet pp ⫽ 250 lb/in2 (gage) (1724 kPa), temperature is 640⬚F (338⬚C). A differential gage fitted across the orifice has a maximum range of 120 in (304.8 cm) of water. What is the steam flow rate when the observed differential pressure is 40 in (101.6 cm) of water? Use the ASME Research Committee on Fluid Meters method in analyzing the meter. Atmospheric pressure is 14.696 lb/in2 (abs) (101.3 kPa). Calculation Procedure: 1. Determine the diameter ratio and steam density For any orifice, meter, diameter ratio ⫽  ⫽ meter orifice diameter, in/pipe internal diameter, in ⫽ 5.855/9.760 ⫽ 0.5999. Determine the density of the steam by entering the superheated steam table at 250 ⫹ 14.696 ⫽ 264.696 lb/in2 (abs) (1824.8 kPa) and 640⬚F (338⬚C) and reading the specific volume as 2.387 ft3 /lb (0.15 m3 /kg). For steam, the density ⫽ 1/ specific volume ⫽ ds ⫽ 1/2.387 ⫽ 0.4193 lb/ft3 (6.7 kg/m3 ). 2. Determine the steam viscosity and meter flow coefficient From the ASME publication, Fluid Meters—Their Theory and Application, the steam viscosity gu1 for a steam system operating at 640⬚F (338⬚C) is gu1 ⫽ 0.0000141 in 䡠 lb/(⬚F 䡠 s 䡠 ft2 ) [0.000031 N 䡠 m/(⬚C 䡠 s 䡠 m2 )]. Find the flow coefficient K from the same ASME source by entering the 10-in (254-mm) nominal pipe diameter table at  ⫽ 0.5999 and projecting to the appro- priate Reynolds number column. Assume that the Reynolds number ⫽ 107 , ap- proximately, for the flow conditions in this pipe. Then K ⫽ 0.6486. Since the Reynolds number for steam pressures above 100 lb/in2 (689.4 kPa) ranges from 106 to 107 , this assumption is safe because the value of K does not vary appreciably in this Reynolds number range. Also, the Reynolds number cannot be computed yet because the flow rate is unknown. Therefore, assumption of the Reynolds num- ber is necessary. The assumption will be checked later. 3. Determine the expansion factor and the meter area factor Since steam is a compressible fluid, the expansion factor Y1 must be determined. For superheated steam, the ratio of the specific heat at constant pressure cp to the specific heat at constant volume is k ⫽ ⫽ 1.3. Also, the ratio of the c c /c v p v differential maximum pressure reading hw, in of water, to the maximum pressure in the pipe, lb/in2 (abs) ⫽ 120/246.7 ⫽ 0.454. From the expansion-factor curve in the ASME Fluid Meters, Y1 ⫽ 0.994 for  ⫽ 0.5999 and the pressure ratio ⫽ 0.454. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 434. 8.30 PLANT AND FACILITIES ENGINEERING And, from the same reference, the meter area factor Fa ⫽ 1.0084 for a steel meter operating at 640⬚F (338⬚C). 4. Compute the rate of steam flow For square-edged orifices, the flow rate, lb/s ⫽ w ⫽ 0.0997Fa Kd2 Y1(hw ds )0.5 ⫽ (0.0997)(1.0084)(0.6486)(5.855)2 (0.994)(120 ⫻ 0.4188)0.5 ⫽ 15.75 lb/s (7.1 kg/s). 5. Compute the Reynolds number for the actual flow rate For any steam pipe, the Reynolds number R ⫽ 48w/(dp gu1) ⫽ 48(15.75)/ [(3.1416)(0.760)(0.0000141)] ⫽ 1,750,000. 6. Adjust the flow coefficient for the actual Reynolds number In step 2, R ⫽ 107 was assumed and K ⫽ 0.6486. For R ⫽ 1,750,000, K ⫽ 0.6489, from ASME Fluid Meters, by interpolation. Then the actual flow rate wh ⫽ (computed flow rate)(ratio of flow coefficients based on assumed and actual Reynolds numbers) ⫽ (15.75)(0.6489/0.6486)(3.600) ⫽ 56,700 lb/h (25,515 kg/ h), closely, where the value 3600 is a conversion factor for changing lb/s to lb/h. 7. Compute the flow rate for a specific differential gage deflection For a 40-in (101.6-cm) H2O deflection, Fa is unchanged and equals 1.0084. The expansion factor changes because hw /pp ⫽ 40/264.7 ⫽ 0.151. From the ASME Fluid Meters, Y1 ⫽ 0.998. By assuming again that R ⫽ 107 , K ⫽ 0.6486, as before, w ⫽ (0.0997) (1.0084)(0.6486)(5.855)2 (0.998)(40 ⫻ 0.4188)0.5 ⫽ 9.132 lb/s (4.1 kg/s). Computing the Reynolds number as before, gives R ⫽ (40)(0.132)/ [(3.1416)(0.76)(0.0000141)] ⫽ 1,014,000. The value of K corresponding to this value, as before, is from ASME—Fluid Meters: K ⫽ 0.6497. Therefore, the flow rate for a 40 in (101.6 cm) H2O reading, in lb/h ⫽ wh ⫽ (0.132)(0.6497/ 0.6486)(3600) ⫽ 32,940 lb/h (14,823 kg/h). Related Calculations. Use these steps and the ASME Fluid Meters or com- prehensive meter engineering tables giving similar data to select or check an orifice meter used in any type of steam pipe—main, auxiliary, process, industrial, marine, heating, or commercial, conveying wet, saturated, or superheated steam. SELECTION OF A PRESSURE-REGULATING VALVE FOR STEAM SERVICE Select a single-seat spring-loaded diaphragm-actuated pressure-reducing valve to deliver 350 lb/h (158 kg/h) of steam at 50 lb/in2 (gage) (344.7 kPa) when the initial pressure is 225 lb/in2 (gage) (1551 kPa). Also select an integral pilot- controlled piston-operated single-seat pressure-regulating valve to deliver 30,000 lb/h (13,500 kg/h) of steam at 40 lb/in2 (gage) (275.8 kPa) with an initial pressure of 225 lb/in2 (gage) (1551 kPa) saturated. What size pipe must be used on the downstream side of the valve to produce a velocity of 10,000 ft/min (3048 m/ min)? How large should the pressure-regulating valve be if the steam entering the valve is at 225 lb/in2 (gage) (1551 kPa) and 600⬚F (316⬚C)? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 435. PIPING AND FLUID FLOW 8.31 TABLE 14 Pressure-Regulating-Valve Capacity TABLE 13 Pressure-Reducing-Valve Capacity, lb/h (kg/h) Calculation Procedure: 1. Compute the maximum flow for the diaphragm-actuated valve For best results in service, pressure-reducing valves are selected so that they operate 60 to 70 percent open at normal load. To obtain a valve sized for this opening, divide the desired delivery, lb/h, by 0.7 to obtain the maximum flow expected. For this valve then, the maximum flow ⫽ 350/0.7 ⫽ 500 lb/h (225 kg/h). 2. Select the diaphragm-actuated valve size Using a manufacturer’s engineering data for an acceptable valve, enter the appro- priate valve capacity table at the valve inlet steam pressure, 225 lb/in2 (gage) (1551 kPa), and project to a capacity of 500 lb/h (225 kg/h), as in Table 13. Read the valve size as 3 ⁄4 in (19.1 mm) at the top of the capacity column. 3. Select the size of the pilot-controlled pressure-regulating valve Enter the capacity table in the engineering data of an acceptable pilot-controlled pressure-regulating valve, similar to Table 14, at the required capacity, 30,000 lb/ h (13,500 kg/h). Project across until the correct inlet steam pressure column, 225 lb/in2 (gage) (1551 kPa), is intercepted, and read the required valve size as 4 in (101.6 mm). Note that it is not necessary to compute the maximum capacity before entering the table, as in step 1, for the pressure-reducing valve. Also note that a capacity table such as Table 14 can be used only for valves conveying saturated steam, unless the table notes state that the values listed are valid for other steam conditions. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 436. 8.32 PLANT AND FACILITIES ENGINEERING TABLE 15 Equivalent Saturated Steam Values for Superheated Steam at Various Pressures and Temperatures 4. Determine the size of the downstream pipe Enter Table 14 at the required capacity, 30,000 lb/h (13,500 kg/h); project across to the valve outlet pressure, 40 lb/in2 (gage) (275.8 kPa); and read the required pipe size as 8 in (203.2 mm) for a velocity of 10,000 ft/min (3048 m/min). Thus, the pipe immediately downstream from the valve must be enlarged from the valve size, 4 in (101.6 mm), to the required pipe size, 8 in (203.2 mm), to obtain the desired steam velocity. 5. Determine the size of the valve handling superheated steam To determine the correct size of a pilot-controlled pressure-regulating valve han- dling superheated steam, a correction must be applied. Either a factor or a tabulation of corrected pressures, Table 15, may be used. to use Table 15, enter at the valve inlet pressure, 225 lb/in2 (gage) (1551.2 kPa), and project across to the total tem- perature, 600⬚F (316⬚C), to read the corrected pressure, 165 lb/in2 (gage) (1137.5 kPa). Enter Table 14 at the next highest saturated steam pressure, 175 lb/in2 (gage) (1206.6 kPa) project down to the required capacity, 30,000 lb/h (13,500 kg/h); and read the required valve size as 5 in (127 mm). Related Calculations. To simplify pressure-reducing and pressure-regulating valve selection, become familiar with two or three acceptable valve manufacturers’ engineering data. Use the procedures given in the engineering data or those given here to select valves for industrial, marine, utility, heating, process, laundry, kitchen, or hospital service with a saturated or superheated steam supply. Do not oversize reducing or regulating valves. Oversizing causes chatter and excessive wear. When an anticipated load on the downstream side will not develop for several months after installation of a valve, fit to the valve a reduced-area disk sized to handle the present load. When the load increases, install a full-size disk. Size the valve for the ultimate load, not the reduced load. Where there is a wide variation in demand for steam at the reduced pressure, consider installing two regulators piped in parallel. Size the smaller regulator to handle light loads and the larger regulator to handle the difference between 60 percent of the light load and the maximum heavy load. Set the larger regulator to open when the minimum allowable reduced pressure is reached. Then both regu- lators will be open to handle the heavy load. Be certain to use the actual regulator inlet pressure and not the boiler pressure when sizing the valve if this is different Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 437. PIPING AND FLUID FLOW 8.33 from the inlet pressure. Data in this calculation procedure are based on valves built by the Clark-Reliance Corporation, Cleveland, Ohio. Some valve manufacturers use the valve flow coefficient for valve sizing. Cv This coefficient is defined as the flow rate, lb/h, through a valve of given size when the pressure loss across the valve is 1 lb/in2 (6.89 kPa). Tabulations like Tables 13 and 14 incorporate this flow coefficient and are somewhat easier to use. These tables make the necessary allowances for downstream pressure less than the critical pres- sure (⫽ 0.55 ⫻ absolute upstream pressure, lb/in2 , for superheated steam and hy- drocarbon vapors; and 0.58 ⫻ absolute upstream pressure, lb/in2 , for saturated steam). The accuracy of these tabulations equals that of valve sizes determined by using the flow coefficient. HYDRAULIC RADIUS AND LIQUID VELOCITY IN WATER PIPES What is the velocity of 1000 gal/min (63.1 L/s) of water flowing through a 10-in (254-mm) inside-diameter cast-iron water main? What is the hydraulic radius of this pipe when it is full of water? When the water depth is 8 in (203.2 mm)? Calculation Procedure: 1. Compute the water velocity in the pipe For any pipe conveying water, the liquid velocity is v ft/s ⫽ gal/min/(2.448d2 ), where d ⫽ internal pipe diameter, in. For this pipe, v ⫽ 1000/[2.448(10)] ⫽ 4.08 ft/s (1.24 m/s), or (60)(4.08) ⫽ 244.8 ft/min (74.6 m/min). 2. Compute the hydraulic radius for a full pipe For any pipe, the hydraulic radius is the ratio of the cross-sectional area of the pipe to the wetted perimeter, or d /4. For this pipe, when full of water, the hydraulic radius ⫽ 10/4 ⫽ 2.5. 3. Compute the hydraulic radius for a partially full pipe Use the hydraulic radius tables in King and Brater—Handbook of Hydraulics, or compute the wetted perimeter by using the geometric properties of the pipe, as in step 2. From the King and Brater table, the hydraulic radius ⫽ Fd, where F ⫽ table factor for the ratio of the depth of water, in/diameter of channel, in ⫽ 8/10 ⫽ 0.8. For this ratio, F ⫽ 0.304. Then, hydraulic radius ⫽ (0.304)(10) ⫽ 3.04 in (77.2 mm). Related Calculations. Use this method to determine the water velocity and hydraulic radius in any pipe conveying cold water—water supply, plumbing, pro- cess, drain, or sewer. FRICTION-HEAD LOSS IN WATER PIPING OF VARIOUS MATERIALS Determine the friction-head loss in 2500 ft (762 m) of clean 10-in (254-mm) new tar-dipped cast-iron pipe when 2000 gal/min (126.2 L/s) of cold water is flowing. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 438. 8.34 PLANT AND FACILITIES ENGINEERING TABLE 16 Values of C in Hazen-Williams Formula What is the friction-head loss 20 years later? Use the Hazen-Williams and Manning formulas, and compare the results. Calculation Procedure: 1. Compute the friction-head loss by the Hazen-Williams formula The Hazen-Williams formula is ⫽ [v /(1.318 where ⫽ friction- 0.63 1.85 h CR )] , h ƒ h ƒ head loss per ft of pipe, ft of water; v ⫽ water velocity, ft/s; C ⫽ a constant depending on the condition and kind of pipe; Rh ⫽ hydraulic radius of pipe, ft. For a water pipe, v ⫽ gal/min/(2.44d2 ); for this pipe, v ⫽ 2000/ [2.448(10)2 ] ⫽ 8.18 ft/s (2.49 m/s). From Table 16 or Crocker and King—Piping Handbook, C for new pipe ⫽ 120; for 20-year-old pipe, C ⫽ 90; Rh ⫽ d /4 for a full-flow pipe ⫽ 10/4 ⫽ 2.5 in, or 2.5/12 ⫽ 0.208 ft (63.4 mm). Then ⫽ [8.18/(1.318 ⫻ 120 ⫻ 0.208 ⫽ 0.0263 ft (8.0 mm) of water per ft (m) 0.63 1.85 h )] ƒ of pipe. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽ 2500(0.0263) ⫽ 65.9 ft (20.1 m) of water for the new pipe. For 20-year-old pipe and the same formula, except with C ⫽ 90, ⫽ 0.0451 hƒ ft (13.8 mm) of water per ft (m) of pipe. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽ 2500(0.0451) ⫽ 112.9 ft (34.4 m) of water. Thus, the friction- head loss nearly doubles [from 65.9 to 112.9 ft (20.1 to 34.4 m)] in 20 years. This shows that it is wise to design for future friction losses; otherwise, pumping equip- ment may become overloaded. 2. Compute the friction-head loss from the Manning formula The Manning formula is where n ⫽ a constant depending on 2 2 4 / 3 h ⫽ n v /2.208R , ƒ h the condition and kind of pipe, other symbols as before. Using n ⫽ 0.011 for new coated cast-iron pipe from Table 17 or Crocker and King—Piping Handbook, we find ⫽ (0.011)2 (8.18)2 /[2.208 ⫽ 0.0295 4 / 3 h (0.208) ] ƒ ft (8.9 mm) of water per ft (m) of pipe. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽ 2500(0.0295) ⫽ 73.8 ft (22.5 m) of water, as compared with 65.9 ft (20.1 m) of water computed with the Hazen-Williams formula. For coated cast-iron pipe in fair condition, n ⫽ 0.013, and ⫽ 0.0411 ft (12.5 hƒ mm) of water. For 2500 ft (762 m) of pipe, the total friction-head loss ⫽ 2500(0.0411) ⫽ 102.8 ft (31.3 m) of water, as compared with 112.9 ft (34.4 m) of Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 439. PIPING AND FLUID FLOW 8.35 TABLE 17 Roughness Coefficients (Manning’s n) for Closed Conduits water computed with the Hazen-Williams formula. Thus, the Manning formula gives results higher than the Hazen-Williams in one case and lower in another. However, the differences in each case are not excessive; (73.8 ⫺ 65.9)/65.9 ⫽ 0.12, or 12 percent higher, and (112.9 ⫺ 102.8)/102.8 ⫽ 0.0983, or 9.83 percent lower. Both these differences are within the normal range of accuracy expected in pipe friction-head calculations. Related Calculations. The Hazen-Williams and Manning formulas are popular with many piping designers for computing pressure losses in cold-water piping. To simplify calculations, most designers use the precomputed tabulated solutions avail- able in Crocker and King—Piping Handbook, King and Brater—Handbook of Hy- draulics, and similar publications. In the rush of daily work these precomputed solutions are also preferred over the more complex Darcy-Weisbach equation used in conjunction with the friction factor ƒ, the Reynolds number R, and the roughness- diameter ratio. Use the method given here for sewer lines, water-supply pipes for commercial, industrial, or process plants, and all similar applications where cold water at tem- peratures of 33 to 90⬚F (0.6 to 32.2⬚C) flows through a pipe made of cast iron, riveted steel, welded steel, galvanized iron, brass, glass, wood-stove, concrete, vit- Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 440. 8.36 PLANT AND FACILITIES ENGINEERING FIGURE 8 Typical industrial piping system. rified, common clay, corrugated metal, unlined rock, or enameled steel. Thus, either of these formulas, used in conjunction with a suitable constant, gives the friction- head loss for a variety of piping materials. Suitable constants are given in Tables 16 and 17 and in the above references. For the Hazen-Williams formula, the con- stant C varies from about 70 to 140, while n in the Manning formula varies from about 0.017 for C ⫽ 70 to 0.010 for C ⫽ 140. Values obtained with these formulas have been used for years with satisfactory results. At present, the Manning formula appears the more popular. CHART AND TABULAR DETERMINATION OF FRICTION HEAD Figure 8 shows a process piping system supplying 1000 gal/min (63.1 L/s) of 70⬚F (21.1⬚C) water. Determine the total friction head, using published charts and pipe- friction tables. All the valves and fittings are flanged, and the piping is 10-in (254- mm) steel, schedule 40. Calculation Procedure: 1. Determine the total length of the piping Mark the length of each piping run on the drawing after scaling it or measuring it in the field. Determine the total length by adding the individual lengths, starting at Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 441. PIPING AND FLUID FLOW 8.37 the supply source of the liquid. In Fig. 8, beginning at the storage sump, the total length of piping ⫽ 10 ⫹ 20 ⫹ 40 ⫹ 50 ⫹ 75 ⫹ 105 ⫽ 300 ft (91.4 m). Note that the physical length of the fittings is included in the length of each run. 2. Compute the equivalent length of each fitting The frictional resistance of pipe fittings (elbows, tees, etc.) and valves is greater than the actual length of each fitting. Therefore, the equivalent length of straight piping having a resistance equal to that of the fittings must be determined. This is done by finding the equivalent length of each fitting and taking the sum for all the fittings. Use the equivalent length table in the pump section of this handbook or in Crocker and King—Piping Handbook, Baumeister and Marks—Standard Hand- book for Mechanical Engineers, or Standards of the Hydraulic Institute. Equivalent length values will vary slightly from one reference to another. Starting at the supply source, as in step 1, for 10-in (254-mm) flanged fittings throughout, we see the equivalent fitting lengths are: bell-mouth inlet, 2.9 ft (0.88 m); 90⬚ ell at pump, 14 ft (4.3 m); gate valve, 3.2 ft (0.98 m); swing check valve, 120 ft (36.6 m); 90⬚ ell, 14 ft (4.3 m); tee, 30 ft (9.1 m); 90⬚ ell, 14 ft (4.3 m); 90⬚ ell, 14 ft (4.3 m); globe valve, 310 ft (94.5 m); swing check valve, 120 ft (36.6 m); sudden enlargement ⫽ (liquid velocity, ft/s)2 /2g ⫽ (4.07)2 /2(32.2) ⫽ 0.257 ft (0.08 m), where the terminal velocity is zero, as in the tank. Find the liquid velocity as shown in a previous calculation procedure in this section. The sum of the fitting equivalent lengths is 2.9 ⫹ 14 ⫹ 3.2 ⫹ 120 ⫹ 14 ⫹ 30 ⫹ 14 ⫹ 14 ⫹ 310 ⫹ 120 ⫹ 0.257 ⫽ 642.4 ft (159.8 m). Adding this to the straight length gives a total length of 642.4 ⫹ 300 ⫽ 942.4 ft (287.3 m). 3. Compute the friction-head loss by using a chart Figure 9 is a popular friction-loss chart for fairly rough pipe, which is any ordinary pipe after a few years’ use. Enter at the left at a flow of 1000 gal/min (63.1 L/s), and project to the right until the 10-in (254-mm) diameter curve is intersected. Read the friction-head loss at the top or bottom of the chart as 0.4 lb/in2 (2.8 kPa), closely, per 100 ft (30.5 m) of pipe. Therefore, total friction-head loss ⫽ (0.4)(942.4/100) ⫽ 3.77 lb/in2 (26 kPa). Converting gives (3.77)(2.31) ⫽ 8.71 ft (2.7 m) of water. 4. Compute the friction-head loss from tabulated data Using the Standards of the Hydraulic Institute pipe-friction table, we find that the friction head of water per 100 ft (30.5 m) of pipe ⫽ 0.500 ft (0.15 m). Hence, hƒ the total friction head ⫽ (0.500)(942.4/100) ⫽ 4.71 ft (1.4 m) of water. The Institute recommends that 15 percent be added to the tabulated friction head, or (1.15)(4.71) ⫽ 5.42 ft (1.66 m) of water. Using the friction-head tables in Crocker and King—Piping Handbook, the fric- tion head ⫽ 6.27 ft (1.9 m) per 1000 ft (304.8 m) of pipe with C ⫽ 130 for new, very smooth pipe. For this piping system, the friction-head loss ⫽ (942.4/ 1000)(6.27) ⫽ 5.91 ft (1.8 m) of water. 5. Use the Reynolds number method to determine the friction head In this method, the friction factor is determined by using the Reynolds number R and the relative roughness of the pipe /D, where ⫽ pipe roughness, ft, and D ⫽ pipe diameter, ft. For any pipe, R ⫽ Dv /v, where v ⫽ liquid velocity, ft/s, and v ⫽ kinematic viscosity, ft2 /s. Using King and Brater—Handbook of Hydraulics, v ⫽ 4.07 ft/s Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 442. 8.38 PLANT AND FACILITIES ENGINEERING FIGURE 9 Friction loss in water piping. (1.24 m/s), and v ⫽ 0.00001059 ft2 /s (0.00000098 m2 /s) for water at 70⬚F (21.1⬚C). Then R ⫽ (10/12)(4.07)/0.00001059 ⫽ 320.500. From Table 18 or the above reference, ⫽ 0.00015, and /D ⫽ 0.00015/(10/ 12) ⫽ 0.00018. From the Reynolds-number, relative-roughness, friction-factor curve Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 443. PIPING AND FLUID FLOW 8.39 TABLE 18 Abslute Roughness Classification of Pipe Surfaces for Selection of Friction Factor ƒ in Fig. 10. in Fig. 10 or in Baumeister—Standard Handbook for Mechanical Engineers, the friction factor ƒ ⫽ 0.016. Apply the Darcy-Weisbach equation ⫽ ƒ(l/D)(v2 /2g), where l ⫽ total pipe hƒ length, including the fittings’ equivalent length, ft. Then ⫽ (0.016)(942.4/10/ hƒ 12)(4.07)2 /(2 ⫻ 32.2) ⫽ 4.651 ft (1.43 m) of water. 6. Compare the results obtained Three different friction-head values were obtained: 8.71, 5.91, and 4.651 ft (2.7, 1.8, and 1.4 m) of water. The results show the variations that can be expected with the different methods. Actually, the Reynolds number method is probably the most accurate. As can be seen, the other two methods give safe results—i.e., the com- puted friction head is higher. The Pipe Friction Manual, published by the Hydraulic Institute, presents excellent simplified charts for use with the Reynolds number method. Related Calculations. Use any of these methods to compute the friction-head loss for any type of pipe. The Reynolds number method is useful for a variety of liquids other than water—mercury, gasoline, brine, kerosene, crude oil, fuel oil, and lube oil. It can also be used for saturated and superheated steam, air, methane, and hydrogen. RELATIVE CARRYING CAPACITY OF PIPES What is the equivalent steam-carrying capacity of a 24-in (609.6-mm) inside- diameter pipe in terms of a 10-in (254-mm) inside-diameter pipe? What is the equivalent water-carrying capacity of a 23-in (584.2-mm) inside-diameter pipe in terms of a 13.25-in (336.6-mm) inside-diameter pipe? Calculation Procedure: 1. Compute the relative carrying capacity of the steam pipes For steam, air, or gas pipes, the number N of small pipes of inside diameter d2 in equal to one pipe of larger inside diameter d1 in is N ⫽ 3 3 (d 兹d ⫹ 3.6)/(d ⫹ 1 2 2 For this piping system, N ⫽ (243 ⫹ ⫹ 3.6)/ 兹d ⫹ 3.6). 兹10 1 (103 ⫹ ⫹ 3.6) ⫽ 9.69, say 9.7. Thus, a 24-in (609.6-mm) inside-diameter 兹24 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 444. 8.40 PLANT AND FACILITIES ENGINEERING FIGURE 10 Friction factors for laminar and turbulent flow. steam pipe has a carrying capacity equivalent to 9.7 pipes having a 10-in (254- mm) inside diameter. 2. Compute the relative carrying capacity of the water pipes For water, N ⫽ ⫽ ⫽ 3.97. Thus, one 23-in (584-cm) inside- 2.5 2.5 (d /d ) (23/13.25) 2 1 diameter pipe can carry as much water as 3.97 pipes of 13.25-in (336.6-mm) inside diameter. Related Calculations. Crocker and King—Piping Handbook and certain pip- ing catalogs (Crane, Walworth, National Valve and Manufacturing Company) con- tain tabulations of relative carrying capacities of pipes of various sizes. Most piping designers use these tables. However, the equations given here are useful for ranges not covered by the tables and when the tables are unavailable. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 445. PIPING AND FLUID FLOW 8.41 TABLE 19 Maximum Capacities of Water Pressure-Reducing Valves, gal/h (L/s) PRESSURE-REDUCING VALVE SELECTION FOR WATER PIPING What size pressure-reducing valve should be used to deliver 1200 gal/h (1.26 L/ s) of water at 40 lb/in2 (275.8 kPa) if the inlet pressure is 140 lb/in2 (965.2 kPa)? Calculation Procedure: 1. Determine the valve capacity required Pressure-reducing valves in water systems operate best when the nominal load is 60 to 70 percent of the maximum load. Using 60 percent, we see that the maximum load for this valve ⫽ 1200/0.6 ⫽ 2000 gal/h (2.1 L/s). 2. Determine the valve size required Enter a valve capacity table in suitable valve engineering data at the valve inlet pressure, and project to the exact, or next higher, valve capacity. Thus, enter Table 19 at 140 lb/in2 (965.2 kPa) and project to the next higher capacity, 2200 gal/h (2.3 L/s), since a capacity of 2000 gal/h (2.1 L/s) is not tabulated. Read at the top of the column the required valve size as 1 in (25.4 mm). Some valve manufacturers present the capacity of their valves in graphical in- stead of tabular form. One popular chart, Fig. 11, is entered at the difference be- tween the inlet and outlet pressures on the abscissa, or 140 ⫺ 40 ⫽ 100 lb/in2 (689.4 kPa). Project vertically to the flow rate of 2000/60 ⫽ 33.3 gal/min (2.1 L /s). Read the valve size on the intersecting valve capacity curve, or on the next curve if there is no intersection with the curve. Figure 11 shows that a 1-in (25.4- mm) valve should be used. This agrees with the tabulated capacity. Related Calculations. Use this method for pressure-reducing valves in any type of water piping—process, domestic, commercial—where the water tempera- ture is 100⬚F (37.8⬚C) or less. Table 19 is from data prepared by the Clark-Reliance Corporation, Fig. 11 is from Foster Engineering Company data. Some valve manufacturers use the valve flow coefficient for valve sizing. Cv This coefficient is defined as the flow rate, gal/min, through a valve of given size when the pressure loss across the valve is 1 lb/in2 (6.9 kPa). Tabulations like Table 19 and flowcharts like Fig. 11 incorporate this flow coefficient and are somewhat easier to use. Their accuracy equals that of the flow coefficient method. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 446. 8.42 PLANT AND FACILITIES ENGINEERING FIGURE 11 Pressure-reducing valve flow capacity. (Foster Engineering Company.) SIZING A WATER METER A 6 ⫻ 4 in (152.4 ⫻ 101.6 mm) Venturi tube is used to measure water flow rate in a piping system. The dimensions of the meter are: inside pipe diameter dp ⫽ 6.094 in (154.8 mm); throat diameter d ⫽ 4.023 in (102.2 mm). The differ- ential pressure is measured with a mercury manometer having water on top of the mercury. The average manometer reading for 1 h is 10.1 in (256.5 mm) of mercury. The temperature of the water in the pipe is 41⬚F (5.0⬚C), and that of the room is 77⬚F (25⬚C). Determine the water flow rate in lb/h, gal/h, and gal/min. Use the ASME Research Committee on Fluid Meters method in analyzing the meter. Calculation Procedure: 1. Convert the pressure reading to standard conditions The ASME meter equation constant is based on a manometer liquid temperature of 68⬚F (20.0⬚C). Therefore, the water and mercury density at room temperature, 77⬚F (25⬚C), and the water density at 68⬚F (20.0⬚C), must be used to convert the manometer reading to standard conditions by the equation hw ⫽ hm (md ⫺ wd )/ws, where hw ⫽ equivalent manometer reading, in (mm) H2O at 68⬚F (20.0⬚C); hm ⫽ manometer reading at room temperature, in mercury; md ⫽ mercury density at room temperature, lb/ft3 ; wd ⫽ water density at room temperature, lb/ft3 ; ws ⫽ water density at standard conditions, 68⬚F (20.0⬚C), lb/ft3 . From density val- ues from the ASME publication Fluid Meters: Their Theory and Application, hw ⫽ 10.1(844.88 ⫺ 62.244)/62.316 ⫽ 126.8 in (322.1 cm) of water at 68⬚F (20.0⬚C). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 447. PIPING AND FLUID FLOW 8.43 FIGURE 12 Complex series pipeline. 2. Determine the throat-to-pipe diameter ratio The throat-to-pipe diameter ratio  ⫽ 4.023/6.094 ⫽ 0.6602. Then 1/(1 ⫺ 4 )0.5 and 1/(1 ⫺ 0.66024 )0.5 ⫽ 1.1111. 3. Assume a Reynolds number value, and compute the flow rate The flow equation for a Venturi tube is w lb/h ⫽ 359.0(Cd2 / , 4 0.5 兹1 ⫺  )(w h ) dp w where C ⫽ meter discharge coefficient, expressed as a function of the Reynolds number; wdp ⫽ density of the water at the pipe temperature, lb/ft3 . With a Reynolds number greater than 250,000, C is a constant. As a first trial, assume R ⬎ 250,000 and C ⫽ 0.984 from Fluid Meters. Then w ⫽ 359.0(0.984)(4.023)2 (1.1111) (62.426 ⫻ 126.8)0.5 ⫽ 565,020 lb/h (254,259 kg/h), or 565,020/8.33 lb/gal ⫽ 67,800 gal/h (71.3 L/s), or 67,800/60 min/h ⫽ 1129 gal/min (71.23 L/s). 4. Check the discharge coefficient by computing the Reynolds numbers For a water pipe, R ⫽ 48ws /(dp gu), where ws ⫽ flow rate, lb/s ⫽ w /3600; u ⫽ coefficient of absolute viscosity. Using Fluid Meters data for water at 41⬚F (5⬚C), we find R ⫽ 48(156.95)/[( ⫻ 6.094)(0.001004)] ⫽ 391,900. Since C is constant for R ⬎ 250,000, use of C ⫽ 0.984 is correct, and no adjustment in the computations is necessary. Had the value of C been incorrect, another value would be chosen and the Reynolds number recomputed. Continue this procedure until a satisfactory value for C is obtained. 5. Use an alternative solution to check the results Fluid Meters gives another equation for Venturi meter flow rate, that is w lb/ s ⫽ 0.525(Cd2 / where p1 ⫺ p2 is the manometer dif- 4 0.5 兹1 ⫺  )[w (p ⫺ p )] , dp 1 2 ferential pressure in lb/in2 . Using the conversion factor in Fluid Meters for con- verting in of mercury under water at 77⬚F (25⬚C) to lb/in2 (kPa), we get p1 ⫺ p2 ⫽ (10.1)(0.4528) ⫽ 4.573 lb/in2 (31.5 kPa). Then w ⫽ (0.525)(0.984) (4.023)2 (1.1111)(62.426 ⫻ 4.573)0.5 ⫽ 156.9 lb/s (70.6 kg/s), or (156.9)(3600 s/ h) ⫽ 564,900 lb/h (254,205 kg/h), or 564,900/8.33 lb/gal ⫽ 67,800 gal/h (71.3 L/s), or 67,800/60 min/h ⫽ 1129 gal/min (71.2 L/s). This result agrees with that computed in step 3 within 1 part in 5600. This is much less than the probable uncertainties in the values of the discharge coefficient and the differential pressure. Related Calculations. Use this method for any Venturi tube serving cold-water piping in process, industrial, water-supply, domestic, or commercial service. EQUIVALENT LENGTH OF A COMPLEX SERIES PIPELINE Figure 12 shows a complex series pipeline made up of four lengths of different size pipe. Determine the equivalent length of this pipe if each size of pipe has the same friction factor. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 448. 8.44 PLANT AND FACILITIES ENGINEERING Calculation Procedure: 1. Select the pipe size for expressing the equivalent length The usual procedure in analyzing complex pipelines is to express the equivalent length in terms of the smallest, or next to smallest, diameter pipe. Choose the 8-in (203.2-mm) size as being suitable for expressing the equivalent length. 2. Find the equivalent length of each pipe For any complex series pipeline having equal friction factors in all the pipes, Le ⫽ equivalent length, ft, of a section of constant diameter ⫽ (actual length of section, ft) (inside diameter, in, of pipe used to express the equivalent length/inside diameter, in, of section under consideration)5 . For the 16-in (406.4-mm) pipe, Le ⫽ (1000)(7.981/15.000)5 ⫽ 42.6 ft (12.9 m). The 12-in (304.8-mm) pipe is next; for it Le ⫽ (3000)(7.981/12.00)5 ⫽ 390 ft (118.9 m). For the 8-in (203.2-mm) pipe, the equivalent length ⫽ actual length ⫽ 2000 ft (609.6 m). For the 4-in (101.6-mm) pipe, Le ⫽ (10)(7.981/4.026)5 ⫽ 306 ft (93.3 m). Then the total equivalent length of 8-in (203.2-mm) pipe ⫽ sum of the equiv- alent lengths ⫽ 42.6 ⫹ 390 ⫹ 2000 ⫹ 306 ⫽ 2738.6 ft (834.7 m); or, by rounding off, 2740 ft (835.2 m) of 8-in (203.2-mm) pipe will have a frictional resistance equal to the complex series pipeline shown in Fig. 12. To compute the actual frictional resistance, use the methods given in previous calculation procedures. Related Calculations. Use this general procedure for any complex series pipe- line conveying water, oil, gas, steam, etc. See Crocker and King—Piping Handbook for derivation of the flow equations. Use the tables in Crocker and King to simplify finding the fifth power of the inside diameter of a pipe. The method of the next calculation procedure can also be used if a given flow rate is assumed. Choosing a flow rate of 1000 gal/min (63.1 L/s) and using the tables in the Hydraulic Institute Pipe Friction Manual give an equivalent length of 2770 ft (844.3 m) for the 8-in (203.2-mm) pipe. This compares favorably with the 2740 ft (835.2 m) computed above. The difference of 30 ft (9.1 m) is negligible and can be ac- counted for by calculator variations. The equivalent length is found by summing the friction-head loss for 1000-gal/ min (63.1-L/s) flow for each length of the four pipes—16, 12, 8, and 4 in (406, 305, 203, and 102 mm)—and dividing this by the friction-head loss for 1000 gal/ min (63.1 L/s) flowing through an 8-in (203.2-mm) pipe. Be careful to observe the units in which the friction-head loss is stated, because errors are easy to make if the units are ignored. EQUIVALENT LENGTH OF A PARALLEL PIPING SYSTEM Figure 13 shows a parallel piping system used to supply water for industrial needs. Determine the equivalent length of a single pipe for this system. All pipes in the system are approximately horizontal. Calculation Procedure: 1. Assume a total head loss for the system To determine the equivalent length of a parallel piping system, assume a total head loss for the system. Since this head loss is assumed for computation purposes only, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 449. PIPING AND FLUID FLOW 8.45 FIGURE 13 Parallel piping system. its value need not be exact or even approximate. Assume a total head loss of 50 ft of water for each pipe in this system. 2. Compute the flow rate in each pipe in the system Assume that the roughness coefficient C in the Hazen-Williams formula is equal for each of the pipes in the system. This is a valid assumption. Using the assumed value of C, compute the flow rate in each pipe. To allow for possible tuberculation of the pipe, assume that C ⫽ 100. The Hazen-Williams formula is given in a previous calculation procedure and can be used to solve for the flow rate in each pipe. A more rapid way to make the computation is to use the friction-loss tabulations for the Hazen-Williams formula in Crocker and King—Piping Handbook, the Hydraulic Institute—Pipe Friction Manual, or a similar set of tables. Using such a set of tables, enter at the friction-head loss equal to 50 ft (15.2 m) per 5000 ft (1524 m) of pipe for the 6-in (152.4-mm) line. Find the corresponding flow rate Q gal/min. Using the Hydraulic Institute tables, Qa ⫽ 270 gal/min (17.0 L/s); Qb ⫽ 580 gal/min (36.6 L/s); Qc ⫽ 1000 gal/min (63.1 L/s). Hence, the total flow ⫽ 270 ⫹ 580 ⫹ 1000 ⫽ 1850 gal/min (116.7 L/s). 3. Find the equivalent size and length of the pipe Using the Hydraulic Institute tables again, look for a pipe having a 50-ft (15.2 m) head loss with a flow of 1850 gal/min (116.7 L/s). Any pipe having a discharge equal to the sum of the discharge rates for all the pipes, at the assumed friction head, is an equivalent pipe. Interpolating friction-head values in the 14-in (355.6-mm) outside-diameter [13.126-in (333.4-mm) inside-diameter] table shows that 5970 ft (1820 m) of this pipe is equivalent to the system in Fig. 13. This equivalent size can be used in any calculations related to this system—selection of a pump, determination of head loss with longer or shorter mains, etc. If desired, another equivalent-size pipe could be found by entering a different pipe-size table. Thus, 5310 ft (1621.5 m) of 14-in (355.6-mm) pipe [12.814-in (326.5-mm) inside diameter] is also equivalent to this system. Related Calculations. Use this procedure for any liquid—water, oil, gasoline, brine—flowing through a parallel piping system. The pipes are assumed to be full at all times. MAXIMUM ALLOWABLE HEIGHT FOR A LIQUID SIPHON What is the maximum height h ft (m), Fig. 14, that can be used for a siphon in a water system if the length of the pipe from the water source to its highest point is Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 450. 8.46 PLANT AND FACILITIES ENGINEERING FIGURE 14 Liquid siphon piping system. 500 ft (152.4 m), the water velocity is 13.0 ft/s (3.96 m/s), the pipe diameter is 10 in (254 mm), and the water temperature is 70⬚F (21.1⬚C) if 3200 gal/min (201.9 L/s) is flowing? Calculation Procedure: 1. Compute the velocity of the water in the pipe From an earlier calculation procedure, v ⫽ gpm /(2.448d2 ). With an internal diameter of 10.020 in (254.5 mm), v ⫽ 3200/[(2.448)(10.02)2 ] ⫽ 13.0 ft/s (3.96 m/s). 2. Determine the vapor pressure of the water Using a steam table, we see that the vapor pressure of water at 70⬚F (21.1⬚C) is ⫽ 0.3631 lb/in2 (abs) (2.5 kPa), or (0.3631) (144 in2 /ft2 ) ⫽ 52.3 lb/ft2 (2.5 pv kPa). The specific volume of water at 70⬚F (21.1⬚C) is, from a steam table, 0.01606 ft3 /lb (0.001 m3 /kg). Converting this to density at 70⬚F (21.1⬚C), density ⫽ 1/0.01606 ⫽ 62.2 lb/ft3 (995.8 kg/m3 ). The vapor pressure in ft of 70⬚F (21.1⬚C) water is then ⫽ (52.3 lb/ft2 )/(62.2 lb/ft3 ) ⫽ 0.84 ft (0.26 m) of water. ƒv 3. Compute or determine the friction-head loss and velocity head From the reservoir to the highest point of the siphon, B, Fig. 14, the friction head in the pipe must be overcome. Use the Hazen-Williams or a similar formula to determine the friction head, as given in earlier calculation procedures or a pipe- friction table. From the Hydraulic Institute Pipe Friction Manual, ⫽ 4.59 ft per hƒ 100 ft (1.4 m per 3.5 m), or (500/100)(4.59) ⫽ 22.95 ft (7.0 m). From the same table, velocity head ⫽ 2.63 ft/s (0.8 m/s). 4. Determine the maximum height for the siphon For a siphon handling water, the maximum allowable height h at sea level with an atmospheric pressure of 14.7 lb/in2 (abs) (101.3 kPa) ⫽ [14.7 ⫻ (144 in2 /ft2 )/ (density of water at operating temperature, lb/ft3 ) ⫺ (vapor pressure of water at operating temperature, ft ⫹ 1.5 ⫻ velocity head, ft ⫹ friction head, ft)]. For this pipe, h ⫽ 14.7 ⫻ 144/62.2 ⫺ (0.84 ⫹ 1.5 ⫻ 2.63 ⫹ 22.95) ⫽ 11.32 ft (3.45 m). In actual practice, the value of h is taken as 0.75 to 0.8 the computed value. Using 0.75 gives h ⫽ (0.75)(11.32) ⫽ 8.5 ft (2.6 m). Related Calculations. Use this procedure for any type of siphon conveying a liquid—water, oil, gasoline, brine, etc. Where the liquid has a specific gravity dif- ferent from that of water, i.e., less than or greater than 1.0, proceed as above, expressing all heads in ft of liquid handled. Divide the resulting siphon height by the specific gravity of the liquid. At elevations above atmospheric, use the actual atmospheric pressure instead of 14.7 lb/in2 (abs) (101.3 kPa). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 451. PIPING AND FLUID FLOW 8.47 WATER-HAMMER EFFECTS IN LIQUID PIPELINES What is the maximum pressure developed in a 200-lb/in2 (1378.8-kPa) water pipe- line if a valve is closed nearly instantly or pumps discharging into the line are all stopped at the same instant? The pipe is 8-in (203.2-mm) schedule 40 steel, and the water flow rate is 2800 gal/min (176.7 L/s). What maximum pressure is de- veloped if the valve closes in 5 s and the line is 5000 ft (1524 m) long? Calculation Procedure: 1. Determine the velocity of the pressure wave For any pipe, the velocity of the pressure wave during water hammer is found from vw ⫽ 4720/(1 ⫹ Kd/Et)0.5 , where vw ⫽ velocity of the pressure wave in the pipe- line, ft/s; K ⫽ bulk modulus of the liquid in the pipeline ⫽ 300,000 for water; d ⫽ internal diameter of pipe, in; E ⫽ modulus of elasticity of pipe material, lb/ in2 ⫽ 30 ⫻ 106 lb/in2 (206.8 Gpa) for steel; t ⫽ pipe-wall thickness, in. For 8-in (203.2-mm) schedule 40 steel pipe and data from a table of pipe properties, vw ⫽ 4720/[1 ⫹ 300,000 ⫻ 7.981/(30 ⫻ 106 ⫻ 0.322)]0.5 ⫽ 4225.6 ft/s (1287.9 m/s). 2. Compute the pressure increase caused by water hammer The pressure increase p1 lb/in2 due to water hammer ⫽ vw v /[32.2(2.31)], where v ⫽ liquid velocity in the pipeline, ft/s; 32.2 ⫽ acceleration due to gravity, ft/s2 ; 2.31 ft of water ⫽ 1-lb/in2 (6.9-kPa) pressure. For this pipe, v ⫽ 0.4085 gpm/d2 ⫽ 0.4085(2800)/(7.981)2 ⫽ 18.0 ft/s (5.5 m/s). Then pi ⫽ (4225.6)(18)/[32.2(2.31)] ⫽ 1022.56 lb/in2 (7049.5 kPa). The maximum pressure developed in the pipe is then p1 ⫹ pipe operating pressure ⫽ 1022.56 ⫹ 200 ⫽ 1222.56 lb/in2 (8428.3 kPa). 3. Compute the hammer pressure rise caused by valve closure The hammer pressure rise caused by valve closure lb/in2 ⫽ 2pi L /vw T, where pv L ⫽ pipeline length, ft; T ⫽ valve closing time, s. For this pipeline, ⫽ 2(1022.56)(5000)/[(4225.6)(5)] ⫽ 484 lb/in2 (3336.7 kPa). Thus, the maxi- pv mum pressure in the pipe will be 484 ⫹ 200 ⫽ 648 lb/in2 (4467.3 kPa). Related Calculations. Use this procedure for any type of liquid—water, oil, etc.—in a pipeline subject to sudden closure of a valve or stoppage of a pump or pumps. The effects of water hammer can be reduced by relief valves, slow-closing check valves on pump discharge pipes, air chambers, air spill valves, and air in- jection into the pipeline. SPECIFIC GRAVITY AND VISCOSITY OF LIQUIDS An oil has a specific gravity of 0.8000 and a viscosity of 200 SSU (Saybolt Seconds Universal) at 60⬚F (15.6⬚C). Determine the API gravity and Bé gravity of this oil and its weight in lb/gal (kg/L). What is the kinematic viscosity in cSt? What is the absolute viscosity in cP? Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 452. 8.48 PLANT AND FACILITIES ENGINEERING Calculation Procedure: 1. Determine the API gravity of the liquid For any oil at 60⬚F (15.6⬚C), its specific gravity S, in relation to water at 60⬚F (15.6⬚C), is S ⫽ 141.5/(131.5 ⫹ ⬚API); or ⬚API ⫽ (141.5 ⫺ 131.5S)/S. For this oil, ⬚API ⫽ [141.5 ⫺ 131.5(0.80)]/0.80 ⫽ 45.4 ⬚API. 2. Determine the Bé gravity of the liquid For any liquid lighter than water, S ⫽ 140/(130 ⫹ Bé); or Bé ⫽ (140 ⫺ 130S)/S. For this oil, Bé ⫽ [140 ⫺ 130(0.80)]/0.80 ⫽ 45 Bé. 3. Compute the weight per gal of liquid With a specific gravity of S, the weight of 1 ft3 of oil ⫽ (S)[weight of 1 ft3 (1 m3 ) of fresh water at 60⬚F (15.6⬚C)] ⫽ (0.80)(62.4) ⫽ 49.92 lb/ft3 (799.2 kg/m3 ). Since 1 gal (3.8 L) of liquid occupies 0.13368 ft3 the weight of this oil is (49.92)(0.13368) ⫽ 6.66 lb/gal (0.79 kg/L). 4. Compute the kinematic viscosity of the liquid For any liquid having an SSU viscosity greater than 100 s, the kinematic viscosity k ⫽ 0.220 (SSU) ⫽ 135/SSU cSt. For this oil, k ⫽ 0.220(200) ⫺ 135/200 ⫽ 43.325 cSt. 5. Convert the kinematic viscosity to absolute viscosity For any liquid, the absolute viscosity, cP ⫽ (kinematic viscosity, cSt)(density). Thus, for this oil, the absolute viscosity ⫽ (43.325)(49.92) ⫽ 2163 cP. Related Calculations. For liquids heavier than water, S ⫽ 145/(145 ⫺ Bé). When the SSU viscosity is between 32 and 99 SSU, k ⫽ 0.226 (SSU) ⫺ 195/SSU cSt. Modern terminology for absolute viscosity is dynamic viscosity. Use these relations for any liquid—brine, gasoline, crude oil, kerosene, Bunker C, diesel oil, etc. Consult the Pipe Friction Manual and Crocker and King—Piping Handbook for tabulations of typical viscosities and specific gravities of various liquids. PRESSURE LOSS IN PIPING HAVING LAMINAR FLOW Fuel oil at 300⬚F (148.9⬚C) and having a specific gravity of 0.850 is pumped through a 30,000-ft (9144-m) long 24-in (609.6-mm) pipe at the rate of 500 gal/ min (31.6 L/s). What is the pressure loss if the viscosity of the oil is 75 cP (0.075 Pa 䡠 s)? Calculation Procedure: 1. Determine the type of flow that exists Flow is laminar (also termed viscous) if the Reynolds number R for the liquid in the pipe is less than 1200. Turbulent flow exists if the Reynolds number is greater than 2500. Between these values is a zone in which either condition may exist, depending on the roughness of the pipe wall, entrance conditions, and other factors. Avoid sizing a pipe for flow in this critical zone because excessive pressure drops result without a corresponding increase in the pipe discharge. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 453. PIPING AND FLUID FLOW 8.49 TABLE 20 Reynolds Number Compute the Reynolds number from R ⫽ 3.162G/kd, where G ⫽ flow rate gal/ min (L/s); k ⫽ kinematic viscosity of liquid, cSt ⫽ viscosity z, cP/specific gravity of the liquid S; d ⫽ inside diameter of pipe, in (cm). From a table of pipe properties, d ⫽ 22.626 in (574.7 mm). Also, k ⫽ z/S ⫽ 75/0.85 ⫽ 88.2 cSt. Then R ⫽ 3162(500)/[88.2(22.626)] ⫽ 792. Since R ⬍ 1200, laminar flow exists in this pipe. 2. Compute the pressure loss by using the Poiseuille formula The Poiseuille formula gives the pressure drop pd lb/in2 (kPa) ⫽ 2.73(10⫺4 )luG/ d4 , where l ⫽ total length of pipe, including equivalent length of fittings, ft; u ⫽ absolute viscosity of liquid, cP (Pa 䡠 s); G ⫽ flow rate gal/min (L/s); d ⫽ inside diameter of pipe, in (cm). For this pipe, pd ⫽ 2.73(10⫺4 )(10,000)(75)(500)/ 262,078 ⫽ 1.17 lb/in2 (8.1 kPa). Related Calculations. Use this procedure for any pipe in which there is laminar flow of the liquid. Other liquids for which this method can be used include water, molasses, gasoline, brine, kerosene, and mercury. Table 20 gives a quick summary of various ways in which the Reynolds number can be expressed. The symbols in Table 20, in the order of their appearance, are D ⫽ inside diameter of pipe, ft (m); v ⫽ liquid velocity, ft/s (m/s); p ⫽ liquid density, lb/ft3 (kg/m3 ); ⫽ absolute viscosity of liquid, lb mass/(ft 䡠 s) [kg/(m 䡠 s)]; d ⫽ inside diameter of pipe, in (cm). From a table of pipe properties, d ⫽ 22.626 in (574.7 mm). Also, k ⫽ z/S liquid flow rate, lb/h (kg/h); B ⫽ liquid flow rate, bbl/h (L/s); k ⫽ kinematic viscosity of the liquid, cSt; q liquid flow rate, ft3 (m3 /s); Q ⫽ liquid flow rate, ft3 /min (m3 /min). Use Table 20 to find the Reynolds number for any liquid flowing through a pipe. DETERMINING THE PRESSURE LOSS IN OIL PIPES What is the pressure drop in a 5000-ft (1524-m) long 6-in (152.4-mm) oil pipe conveying 500 bbl/h (22.1 L/s) of kerosene having a specific gravity of 0.813 at Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 454. 8.50 PLANT AND FACILITIES ENGINEERING 65⬚F (18.3⬚C), which is the temperature of the liquid in the pipe? The pipe is schedule 40 steel. Calculation Procedure: 1. Determine the kinematic viscosity of the oil Use Fig. 15 and Table 21 or the Hydraulic Institute—Pipe Friction Manual kine- matic viscosity and Reynolds number chart to determine the kinematic viscosity of the liquid. Enter Table 12 at kerosene, and find the coordinates as X ⫽ 10.2, Y ⫽ 16.9. Using these coordinates, enter Fig. 15 and find the absolute viscosity of kerosene at 65⬚F (18.3⬚C) as 2.4 cP. By the method of a previous calculation pro- cedure, the kinematic viscosity ⫽ absolute viscosity, cP/specific gravity of the liquid ⫽ 2.4/0.813 ⫽ 2.95 cSt. This value agrees closely with that given in the Pipe Friction Manual. 2. Determine the Reynolds number of the liquid The Reynolds number can be found from the Pipe Friction Manual chart mentioned in step 1 or computed from R ⫽ 2214B /(dk) ⫽ 2214(500)/[(6.065)(2.95)] ⫽ 61,900. To use the Pipe Friction Manual chart, compute the velocity of the liquid in the pipe by converting the flow rate to ft3 /s. Since there is 42 gal/bbl (0.16 L) and 1 gal (0.00379 L) ⫽ 0.13368 ft3 (0.00378 m3 ), 1 bbl ⫽ (42)(0.13368) ⫽ 5.6 ft3 (0.16 m3 ). With a flow rate of 500 bbl/h (79.5 m3 /h) the equivalent flow ⫽ (500)(5.6) ⫽ 2800 ft3 /h (79.3 m3 /h), or 2800/3600 s/h ⫽ 0.778 ft3 /s (0.02 m3 /s). Since 6-in (152.4-mm), schedule 40 pipe has a cross-sectional area of 0.2006 ft2 (0.02 m2 ) internally, the liquid velocity ⫽ 0.778/0.2006 ⫽ 3.88 ft/s (1.2 m/s). Then, the product (velocity, ft/s)(internal diameter, in) ⫽ (3.88)(6.065) ⫽ 23.75 ft/s. In the Pipe Friction Manual, project horizontally from the kerosene specific- gravity curve to the vd product of 23.75, and read the Reynolds number as 61,900, as before. In general, the Reynolds number can be found more quickly by com- puting it using the appropriate relation given in an earlier calculation procedure, unless the flow velocity is already known. 3. Determine the friction factor of this pipe Enter Fig. 16 at the Reynolds number value of 61,900, and project to the curve 4 as indicated by Table 22. Read the friction factor as 0.0212 at the left. Alternatively, the Pipe Friction Manual friction-factor chart could be used, if desired. 4. Compute the pressure loss in the pipe Use the Fanning formula pd ⫽ 1.06(10⫺4 )ƒlB2 /d5 . In this formula, ⫽ density of the liquid, lb/ft3 . For kerosene, ⫽ (density of water, lb/ft3 )(specific gravity of the kerosene) ⫽ (62.4)(0.813) ⫽ 50.6 lb/ft3 (810.1 kg/m3 ). Then pd ⫽ 1.06(10⫺4 ) ⫻ (0.0212)(50.6)(5000)(500)2 /8206 ⫽ 17.3 lb/in2 (119.3 kPa). Related Calculations. The Fanning formula is popular with oil-pipe designers and can be stated in various ways: (1) with velocity v ft/s, pd ⫽ 1.29(10⫺3 )ƒV2 l/ d; (2) with velocity V ft/min, pd ⫽ 3.6(10⫺7 )ƒV2 l/d; (3) with flow rate in G gal /min, pd ⫽ 2.15(10⫺4 )ƒlG2 /d2 ; (4) with the flow rate in W lb/h, pd ⫽ 3.36(10⫺6 )ƒlW2 /d5 . Use this procedure for any petroleum product—crude oil, kerosene, benzene, gasoline, naphtha, fuel oil, Bunker C, diesel oil, toluene, etc. The tables and charts presented here and in the Pipe Friction Manual save computation time. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 455. PIPING AND FLUID FLOW 8.51 FIGURE 15 Viscosities of liquids at 1 atm. For coordinates, see Table 21. Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 456. TABLE 21 Viscosities of Liquids Coordinates for use with Fig. 15 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 457. 8.53 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 458. 8.54 FIGURE 16 Friction-factor curves. (Mechanical Engineering.) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 459. 8.55 TABLE 22 Data for Fig. 16 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 460. 8.56 PLANT AND FACILITIES ENGINEERING TABLE 23 Gas Constants FLOW RATE AND PRESSURE LOSS IN COMPRESSED-AIR AND GAS PIPING Dry air at 80⬚F (26.7⬚C) and 150 lb/in2 (abs) (1034 kPa) flows at the rate of 500 ft3 /min (14.2 m3 /min) through a 4-in (101.6-mm) schedule 40 pipe from the dis- charge of an air compressor. What are the flow rate in lb/h and the air velocity in ft/s? Using the Fanning formula, determine the pressure loss if the total equivalent length of the pipe is 500 ft (152.4 m). Calculation Procedure: 1. Determine the density of the air or gas in the pipe For air or a gas, pV ⫽ MRT, where p ⫽ absolute pressure of the gas, lb/ft2 (abs); V ⫽ volume of M lb of gas, ft3 ; M ⫽ weight of gas, lb; R ⫽ gas constant, ft 䡠 lb/ (lb 䡠 ⬚F); T ⫽ absolute temperature of the gas, ⬚R. For this installation, using 1 ft3 of air, M ⫽ pV /(RT), M ⫽ (150)(144)/[(53.33)(80 ⫹ 459.7)] ⫽ 0.750 lb/ft3 (12.0 kg/m3 ). The value of R in this equation was obtained from Table 23. 2. Compute the flow rate of the air or gas For air or a gas, the flow rate Wh lb/h ⫽ (60) (density, lb/ft3 )(flow rate, ft3 /min); or Wh ⫽ (60)(0.750)(500) ⫽ 22,500 lb/h (10,206 kg/h). 3. Compute the velocity of the air or gas in the pipe For any air or gas pipe, velocity of the moving fluid v ft/s ⫽ 183.4 Wh /3600 d2 , where d ⫽ internal diameter of pipe, in; ⫽ density of fluid, lb/ft3 . For this system, v ⫽ (183.4)(22,500)/[(3600)(4.026)2 (0.750)] ⫽ 94.3 ft/s (28.7 m/s). Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 461. PIPING AND FLUID FLOW 8.57 4. Compute the Reynolds number of the air or gas The viscosity of air at 80⬚F (26.7⬚C) is 0.0186 cP, obtained from Crocker and King—Piping Handbook, Perry et al.—Chemical Engineers’ Handbook, or a sim- ilar reference. Then, by using the Reynolds number relation given in Table 20, R ⫽ 6.32W /(dz) ⫽ (6.32)(22,500)/[(4.026)(0.0186)] ⫽ 1,899,000. 5. Compute the pressure loss in the pipe Using Fig. 16 or the Hydraulic Institute Pipe Friction Manual, we get ƒ ⫽ 0.0142 to 0.0162 for a 4-in (101.6-mm) schedule 40 pipe when the Reynolds number ⫽ 3,560,000. From the Fanning formula from an earlier calculation pro- cedure and the higher value of ƒ, pd ⫽ 3.36(10⫺6 )ƒlW2 /d5 , or pd ⫽ 3.36(10⫺6 )(0.0162)(500)(22,500)2 /[(4.026)5 (0.750)] ⫽ 17.37 lb/in2 (119.8 kPa). Related Calculations. Use this procedure to compute the pressure loss, veloc- ity, and flow rate in compressed-air and gas lines of any length. Gases for which this procedure can be used include ammonia, carbon dioxide, carbon monoxide, ethane, ethylene, hydrogen, hydrogen sulfide, isobutane, methane, nitrogen, n-butane, oxygen, propane, propylene, and sulfur dioxide. Alternate relations for computing the velocity of air or gas in a pipe are v ⫽ 144Ws /a; v ⫽ 183.4Ws /d2 ; v ⫽ 0.0509 Ws vg /d2 , where Ws ⫽ flow rate, lb/s; a ⫽ cross-sectional area of pipe, in2 , vg ⫽ specific volume of the air or gas at the operating pressure and temperature, ft3 /lb. FLOW RATE AND PRESSURE LOSS IN GAS PIPELINES Using the Weymouth formula, determine the flow rate in a 10-mi (16.1-km) long 4-in (101.6-mm) schedule 40 gas pipeline when the inlet pressure is 200 lb/in2 (gage) (1378.8 kPa), the outlet pressure is 20 lb/in2 (gage) (137.9 kPa), the gas has a specific gravity of 0.80, a temperature of 60⬚F (15.6⬚C), and the atmospheric pressure is 14.7 lb/in2 (abs) (101.34 kPa). Calculation Procedure: 1. Compute the flow rate from the Weymouth formula The Weymouth formula for flow rate is Q ⫽ 28.05[ where 2 2 5.33 0.5 ( p ⫺ p )d /sL] , i 0 pi ⫽ inlet pressure, lb/in2 (abs); p0 ⫽ outlet pressure, lb/in2 (abs); d ⫽ inside diameter of pipe, in; s ⫽ specific gravity of gas; L ⫽ length of pipeline, mi. For this pipe, Q ⫽ 28.05 ⫻ [(214.72 ⫺ 34.72 )4.026 ⫽ 86,500 lb/h 5.33 0.5 /0.8 ⫻ 10] (39,925 kg/h). 2. Determine if the acoustic velocity limits flow If the outlet pressure of a pipe is less than the critical pressure pc lb/in2 (abs), the flow rate in the pipe cannot exceed that obtained with a velocity equal to the criti- cal or acoustic velocity, i.e., the velocity of sound in the gas. For any gas, pc ⫽ Q(Ti )0.5 /d2 C, where Ti ⫽ inlet temperature, ⬚R; C ⫽ a constant for the gas being considered. Using C ⫽ 2070 from Table 23, or Crocker and King—Piping Handbook, pc ⫽ (86,500)(60 ⫹ 460)0.5 /[(4.026)2 (2070)] ⫽ 58.8 lb/in2 (abs) (405.4 kPa). Since Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 462. 8.58 PLANT AND FACILITIES ENGINEERING FIGURE 17 Typical complex pipe operating at high temperature. the outlet pressure p0 ⫽ 34.7 lb/in2 (abs) (239.2 kPa), the critical or acoustic ve- locity limits the flow in this pipe because pc ⬎ p0. When pc ⬍ p0, critical velocity does not limit the flow. Related Calculations. Where a number of gas pipeline calculations must be made, use the tabulations in Crocker and King—Piping Handbook and Bell—Petroleum Transportation Handbook. These tabulations will save much time. Other useful formulas for gas flow include the Panhandle, Unwin, Fritsche, and rational. Results obtained with these formulas agree within satisfactory limits for normal engineering practice. Where the outlet pressure is unknown, assume a value for it and compute the flow rate that will be obtained. If the computed flow is less than desired, check to see that the outlet pressure is less than the critical. If it is, increase the diameter of the pipe. Use this procedure for natural gas from any gas field, manufactured gas, or any other similar gas. To find the volume of gas that can be stored per mile of pipe, solve Vm ⫽ 1.955pm d2 K, where pm ⫽ mean pressure in pipe, lb/in2 (abs) ⬇ (pi ⫹ p0)/2; K ⫽ (1/Z)0.5 , where Z ⫽ super compressibility factor of the gas, as given in Bau- meister and Marks—Standard Handbook for Mechanical Engineers and Perry— Chemical Engineer’s Handbook. For exact computation of pm, use pm ⫽ (2 ⁄3) ( pi ⫹ p0 ⫺ pi p0 /pi ⫹ p0). SELECTING HANGERS FOR PIPES AT ELEVATED TEMPERATURES Select the number, capacity, and types of pipe hanger needed to support the 6-in (152.5-mm) schedule 80 pipe in Fig. 17 when the installation temperature is 60⬚F (15.6⬚C) and the operating temperature is 700⬚F (371.1⬚C). The pipe is insulated with 85 percent magnesia weighing 11.4 lb/ft (16.63 N/m). The pipe and unit served by the pipe have a coefficient of thermal expansion of 0.0575 in/ft (0.48 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 463. PIPING AND FLUID FLOW 8.59 FIGURE 18 Pipe shapes commonly used in power and process plants assume the ap- proximate forms shown by the dotted lines when the pipe temperature rises. (Power.) cm/m) between the 60⬚F (15.6⬚C) installation temperature and the 700⬚F (371.1⬚C) operating temperature. Calculation Procedure: 1. Draw a freehand sketch of the pipe expansion Use Fig. 18 as a guide and sketch the expanded pipe, using a dashed line. The sketch need not be exactly to scale; if the proportions are accurate, satisfactory results will be obtained. The shapes shown in Fig. 18 cover the 11 most common situations met in practice. 2. Tentatively locate the required hangers Begin by locating hangers H-1 and H-5 close to the supply and using units, Fig. 17. Keeping a hanger close to each unit (boiler, turbine, pump, engine, etc.) prevents overloading the connection on the unit. Space intermediate hangers H-2, H-3, and H-4 so that the recommended dis- tances in Table 24 or hanger engineering data (e.g., Grinnell Corporation Pipe Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 464. 8.60 PLANT AND FACILITIES ENGINEERING TABLE 24 Maximum Recommended Spacing between Pipe Hangers Hanger Design and Engineering) are not exceeded. Indicate the hangers on the piping drawing as shown in Fig. 17. 3. Adjust the hanger locations to suit structural conditions Study the building structural steel in the vicinity of the hanger locations, and adjust these locations so that each hanger can be attached to a support having adequate strength. 4. Compute the load each hanger must support From a table of pipe properties, such as in Crocker and King—Piping Handbook, find the weight of 6-in (152.4-mm) schedule 80 pipe as 28.6 lb/ft (41.7 N/m). The insulation weighs 11.4 lb/ft (16.6 N/m), giving a total weight of insulated pipe of 28.6 ⫹ 11.4 ⫽ 40.0 lb/ft (58.4 N/m). Compute the load on the hangers supporting horizontal pipes by taking half the length of the pipe on each side of the hanger. Thus, for hanger H-1, there is (2 ft)(1 ⁄2) ⫹ (16 ft) ⫻ (1 ⁄2) ⫽ 9 ft (2.7 m) of horizontal pipe, Fig. 17, which it supports. Since this pipe weighs 40 lb/ft (58.4 N/m), the total load on hanger H-1 ⫽ (9 ft)(40 lb/ft) ⫽ 360 lb (1601.4 N). A similar analysis for hanger H-2 shows that it supports (8 ⫹ 1)(40) ⫽ 360 lb (1601.4 N). Hanger H-3 supports the entire weight of the vertical pipe, 30 ft (9.14 m), plus 1 ft (0.3 m) at the top bend and 1 ft (0.3 m) at the bottom bend, or a total of 1 ⫹ 30 ⫹ 1 ⫽ 32 ft (9.75 m). The total load on hanger H-3 is therefore (32)(40) ⫽ 1280 lb (5693.7 N). Hanger H-4 supports (1 ⫹ 8)(40) ⫽ 360 lb (1601.4 N), and hanger H-5 supports (8 ⫹ 6)(40) ⫽ 560 lb (2491 N). As a check, compute the total weight of the pipe and compare it with the sum of the endpoint and hanger loads. Thus, there is 100 ft (30.5 m) of pipe weighing (80)(40) ⫽ 3200 lb (14.2 kN). The total load the hangers will support is 360 ⫹ 360 ⫹ 1280 ⫹ 360 ⫹ 560 ⫽ 2920 lb (12.9 kN). The first endpoint will support (1)(40) ⫽ 40 lb (177.9 N), and the anchor will support (6)(40) ⫽ 240 lb (1067 N). The total hanger and endpoint support ⫽ 2920 ⫹ 40 ⫹ 240 ⫽ 3200 lb (14.2 kN); therefore, the pipe weight ⫽ the hanger load. 5. Sketch the shape of the hot pipe Use Fig. 18 as a guide, and draw a dotted outline of the approximate shape the pipe will take when hot. Start with the first corner point nearest the unit on the left, Fig. 19. This point will move away from the unit, as in Fig. 19. Do the same for the first corner point near the other unit served by the pipe and for intermediate corner points. Use arrows to indicate the probable direction of pipe movement at each corner. When sketching the shape of the hot pipe, remember that a straight pipe expanding against a piece of pipe at right angles to itself will bend the latter. The distance that various lengths of pipe will bend while producing a tensile stress Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 465. PIPING AND FLUID FLOW 8.61 FIGURE 19 Expansion of the various parts of the pipe shown in Fig. 17. (Power.) TABLE 25 Deflection, in (mm), that Produces 14,000-lb/in2 (96,530-kPa) Tensile Stress in Pipe Legs Acting as a Cantilever Beam, Load at Free End of 14,000 lb/in2 (96.5 MPa) is given in Table 25. This stress is a typical allowable value for pipes in industrial systems. 6. Determine the thermal movement of units served by the pipe If either or both fixed units (boiler, turbine, etc.) operate at a temperature above or below atmospheric, determine the amount of movement at the flange of the unit to which the piping connects, using the thermal data in Table 26. Do this by applying the thermal expansion coefficient for the metal of which the unit is made. Determine the vertical and horizontal distance of the flange face from the point of no move- ment of the unit. The point of no movement is the point or surface where the unit is fastened to cold structural steel or concrete. The flange, point a, Fig. 19, is 8 ft (2.4 m) above the bolted end of the unit and directly in line with the bolt, Fig. 17. Since the bolt and flange are on a common vertical line, there will not be any horizontal movement of the flange because the bolt is the no-movement point of the unit. Since the flange is 8 ft (2.4 m) away from the point of no movement, the amount that the flange will move ⫽ (distance away from the point of no movement, ft)(coefficient of thermal expansion, in/ft) ⫽ (8)(0.0575) ⫽ 0.46 in (11.7 mm) away (up) from the point of no movement. If the unit were operating at a temperature Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 466. 8.62 PLANT AND FACILITIES ENGINEERING TABLE 26 Thermal Expansion of Pipe, in/ ft (mm/m) (Carbon and Carbon-Moly Steel and WI) less than atmospheric, it would contract and the flange would move toward (down) the point of no movement. Mark the flange movement on the piping sketch, Fig. 19. Anchor, d, Fig. 19, does not move because it is attached to either cold structural steel or concrete. 7. Compute the amount of expansion in each pipe leg Expansion of the pipe, in ⫽ (pipe length, ft)(coefficient of linear expansion, in/ft). For length ab, Fig. 17, the expansion ⫽ (20)(0.0575) ⫽ 1.15 in (29.2 mm); for bc, (30)(0.0575) ⫽ 1.73 in (43.9 mm); for cd, (30)(0.0575) ⫽ 1.73 in (43.9 mm). Mark the amount and direction of expansion on Fig. 19. 8. Determine the allowable deflection for each pipe leg Enter Table 25 at the nominal pipe size and find the allowable deflection for a 14,000-lb/in2 (96.5-MPa) tensile stress for each pipe leg. Thus, for ab, the allowable deflection ⫽ 2.80 in (71.1 mm) for a 20-ft (6.1-m) long leg; for bc, 6.30 in (160 mm) for a 30-ft (9.1-m) long leg; for cd, 6.30 in (160 mm) for a 30-ft (9.1-m) long leg. Mark these allowable deflections on Fig. 19, using dashed arrows. 9. Compute the actual vertical and horizontal deflections Sketch the vertical deflection diagram, Fig. 20a, by drawing a triangle showing the total expansion in each direction in proportion to the length of the parts at right angles to the expansion. Thus, the 0.46-in (11.7-mm) upward expansion at the flange, a, is at right angles to leg ab and is drawn as the altitude of the right triangle. Lay off 20 t (6.1 m), ab, on the base of the triangle. Since bc is parallel to the direction of the flange movement, it is shown as a point, bc, on the base of this triangle. From point bc, lay off cd on the base of the triangle, Fig. 20a, since it is at right angles to the expansion of point a. Then, by similar triangles, 50:46 ⫽ 30:x; x ⫽ 0.28 in (7.1 mm). Therefore, leg bc moves upward 0.28 in (7.1 mm) because of the flange movement at a. Now draw the deflection diagram, Fig. 20b, showing the upward movement of leg ab and the downward movement of leg cd along the length of each leg, or 20 and 30 ft (6.1 and 9.1 m), respectively. Solve the similar triangles, or 20:x1 ⫽ 30:(1.73 ⫺ x1); x1 ⫽ 0.69 in (17.5 mm). Therefore, point b moves up 0.69 in (17.5 mm) as a result of the expansion of leg bc. Then 1.73 ⫺ x1 ⫽ 1.73 ⫺ 0.69 ⫽ 1.04 in (26.4 mm). Thus, point c moves down 1.04 in (26.4 mm) as a result of the expansion of bc. The total distance b moves up ⫽ 0.28 ⫹ 0.69 ⫽ 0.97 in (24.6 Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 467. PIPING AND FLUID FLOW 8.63 FIGURE 20 (a), (b) Vertical deflection diagrams for the pipe in Fig. 17; (c), (d) horizontal deflection diagrams for the pipe in Fig. 17. (Power.) mm), whereas the total distance c moves down ⫽ 1.04 ⫺ 0.28 ⫽ 0.76 in (19.3 mm). Mark these actual deflections on Fig. 19. Find the actual horizontal deflections in a similar fashion by constructing the triangle, Fig. 20c, formed by the vertical pipe bc and the horizontal pipe ab. Since point a does not move horizontally but point b does, lay off leg ab at right angles to the direction of movement, as shown. From point b lay off leg bc. Then, since leg bc expands 1.73 in (43.9 mm), lay this distance off perpendicular to ac, Fig. 20c. By similar triangles, 20 ⫹ 30:1.73 ⫽ 20:y; y ⫽ 0.69 in (17.5 mm). Hence, point b deflects 0.69 in (17.5 mm) in the direction shown in Fig. 19. Follow the same procedure for leg cd, constructing the triangle in Fig. 20d. Beginning with point b, lay off legs bc and cd. The altitude of this right triangle is then the distance point c moves when leg ab expands, or 1.15 in (29.2 mm). By similar triangles, 30 ⫹ 30:1.15 ⫽ 30:y1; y1 ⫽ deflection of point c ⫽ 0.58 in (14.7 mm). 10. Select the type of pipe hanger to use Figure 21 shows several popular types of pipe hangers, together with the movements that they are designed to absorb. For hangers H-1 and H-2, use type E, Fig. 21, Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PIPING AND FLUID FLOW
- 468. 8.64 FIGURE 21 Pipe hangers chosen depend on the movement expected. Hangers A and B are suitable for pipe movement in one horizontal direction. Hangers C and D permit pipe movement in two horizontal directions. Vertical and horizontal movement requires use of hangers such as E for horizontal pipes and F for vertical pipes. (G) Cantilever support; (H) sliding movement in two hor