thermal systems and applications

2
Syllabus
Module 1
Steam Engineering: Properties of steam - wet, dry and superheated steam -
dryness fraction - enthalpy and internal energy - entropy of steam - temperature
entropy diagram - process - Mollier chart - Rankine cycle for wet, dry and
superheated steam. Steam Generators - classification - modern steam generators -
boiler mountings and accessories.
Module 2
Steam nozzles - Mass flow rate - throat pressure for maximum discharge - throat
area - effect of friction - super saturated flow.
Steam turbines: velocity triangles, work done, governing, and efficiencies.
Module 3
Gas turbine Plants - Open and closed cycles - thermodynamics cycles -
regeneration, re heating - inter cooling - efficiency and performance of gas
turbines. Rotary Compressors - Analysis of rotary compressors - centrifugal and
axial compressors. Combustion - combustion chambers of gas turbines -
cylindrical, annular and industrial type combustion chamber - combustion
intensity - combustion chambers efficiency - pressure loss combustion process
and stability loop.
Module 4
Introduction to solar energy - solar collectors - Liquid flat plate collectors -
principle - thermal losses and efficiency - characteristics - overall loss coefficient
- thermal analysis - useful heat gained by fluid - mean plate temperature -
performance - focussing type solar collectors - solar concentrators and receivers
- sun tracking system - characteristics - optical losses - thermal performance -
solar pond - solar water heating - solar thermal power generation (Description
Only)

3
Module 5
Thermal power plants: layout and operation of steam and diesel power plants - coal
burners - stockers - cooling ponds & towers - chimneys - draught - dust collectors -
precipitators - feed water heaters - evaporators - steam condensers - coal handling - ash
handling.

4
MODULE 1
Steam Engineering
Formation of steam -
Consider a cylinder fitted with a piston which can move freely upwards and
downwards in it.
(a) Let 1 kg of water at 0o
C under the piston
Let the piston is loaded with load w to ensure heating at constant pressure.
Now if heat is imparted to water, a rise in temperature will be noticed and this
rise will continue till boiling point is reached.
B.P of water, at normal atmospheric pressure of 1.01325 bar is 100o
C. But it
increases with the increase in pressure.
(b) The volume of water will increase slightly with the increase in temperature, but
increase in volume of water (or work) is generally neglected for all types of calculations.
The boiling temperature is known as the temperature of formation of steam or
saturation temperature.
(c) Now, if supply of heat to water is continued, it will be notices that rise of
temperature after the boiling point is reached nil but piston starts moving upwards which
indicates that there is increase in volume which is only possible if steam formation
occurs.
The heat being supplied does not show any rise of temperature but changes
water into vapour state (steam) and is known as Latent heat or hidden heat.

5
So long as the steam is in contact with water, it is called wet steam.
(d) If heating of steam is further progressed such that all the water particles
associated with steam are evaporated, the steam so obtained is called dry and saturated
steam.
If vg m3
is the volume of 1 kg of dry and saturated steam then work done on the
piston will be
P (Vg - Vf), where ‗P‘ is the constant pressure (due to weight ‗W‘ on the
piston).
(e) If the supply of heat to the dry and saturated steam is continued at constant
pressure, there will be increase in temperature and volume of steam.
The steam so obtained is called super heated steam and it behaves like a perfect
gas.
Temperature Vs Total Heat Graph during steam formation
A represents the initial condition of water at 0oC and pressure p (in bar)
During the formation of the super heated steam, from water at freezing point,
the heat is absorbed in the following 3 stages.
The heating of water upto boiling temperature or saturation temperature (ts) is

6
shown by AB.
AP known as sensible heat, liquid heat or total heat of water.
The change of state from liquid to steam is sown by BC PQ, latent heat of
vaporisation.
The super heating process is CD.
QR known as the heat of superheat.
LINE, AR represents the total heat of the super heated steam.
If the pressure is increased, the boiling temperature also increases.
The line passing through the points A, B, E, K  Saturated liquid line.
The line passing through the points L, F, C  Dry saturated steam line.
[Some times, these terms are briefly written as liquid line and dry steam line.
but the word saturated is always understood].
Note:
When the pressure and saturation temperature increases, the latent heat of
vaporisation decreases, it becomes ZERO at a point (N), where liquid and dry steam
lines meet.
The point N is known as critical point and at this point, the liquid and vapour
phases merge, and become identical in every respect.
The temperature corresponding to critical point N is known as critical
temperature and the pressure is known as critical pressure.
For steam, the critical temperature is 374.15o
C and critical pressure is 220.9 bar
Pc = 220.9 bar
Tc = 374.15oC
At critical point and above, there is no definite transition from liquid to vapour
and two phases cannot be distinguished visually. The latent heat of vaporisation is zero
at critical point and has no meaning at pressure higher than critical.

7
At T = 273.16 k and P = 0.006113 bar ice, water and steam co-exist in the
thermodynamic equilibrium in a closed vessel and bcf (Belleni - 200) is called triple
point line. At lower pressures than this, ice sublimates to steam.
IMPORTANT TERMS RELATING STEAM FORMATION
1. Sensible Heat of water (hf)
It is defined as the quantity of heat absorbed by 1 kg of water when it is heated
from 0oC (freezing point) to boiling point.
If i kg of water is heated from 0o
C to 100o
C the sensible heat added to it will be
4.18 × 100 = 418 kJ
But if water is at say 20o
C initially then sensible heat added will be 4.18 × (100-
20) = 334.7 kJ
This type of heat is denoted by letter hf and its value can be directly read from
the steam tables.
The value of specific heat of water may be taken as 4.18 kJ/kg K at low
pressures but at high pressures it is different from this value.
2. Latent Heat or Hidden Heat (hfg)
It is the amount of heat required to convert water at a given temperature and
pressure into steam at the same temperature and pressure.
The value of L.H is not constant and varies according to pressure variation.
3. Dryness Fraction (x)
It is related with wet steam
Mass of dry saturated vapour to the total mass of the mixture.
x =
g g
g f
m m
m m m


mg = Mass of actual Dry steam
mf = Mass of water in suspension

8
m = Mass of mixture = mg + mf
eg:- If in 1 kg of wet steam 0.9 kg is the dry steam and 0.1 kg water particles
then x = 0.9.
No steam can be completely dry and saturated, so long as it is in contact with
the water from which it is being formed.
The steam is called saturated when the molecules escaping from the liquid
become equal to the molecules returning to it.
Saturated steam may be dry or wet. When the saturated vapour contains
particles of liquid evenly distributed over the entire mass of vapour, it is called wet
saturated steam.
Wet steam is characterised by its dryness fraction.
Dryness fraction, x =
mass of day saturated vapour
mass of mixture
=
mg
m
x =
mass of dry vapour in the mixture
mass of the mixture
Q. Calculate the dryness fraction of steam which has 1.25 kg of water in suspension
with 40 kg of steam
=
g
g f
m
m m
=
40
0.97
40 1.25
4. Total heat or enthalpy of wet steam (h)
It is defined as the quantity of heat required to convert 1 kg of water at 0o
C into
steam at constant pressure.
5. Total heat of dry saturated steam
If steam is dry saturated, x = 1 and hg = hf + hfg

9
6. Superheated steam
Total heat of super heating is always carried out at constant pressure.
It represents the quantity of heat required to convert 1 kg of water at 0o
C into
super heated steam at constant pressure.
 sup f fg ps sup sh h h c T T   
The value of specific heat of steam at constant pressure Cps depends upon the
degree of superheat and the pressure of steam generation. Its average value is taken from
2 to 2.1 kJ/kg K.
Water boils at 12o
C if pressure on the surface of water is kept at 0.014 bar.
7o
C if pressure 0.01 bar.
Advantages obtained by using ‘super heated’ steam
1. By super heating steam, its heat content and have its capacity to do work is
increased without having increase its pressure.
2. High temperature use of super heated steam results in an increase in thermal
efficiency.
3. Super heating is done in a super heater which obtains its heat from waste
furnace gases which would have otherwise passed uselessly up the chimney.
Volume of wet and dry steam
If steam has a dryness fraction of x.
1 kg of this steam will contain x kg of dry steam and (1 - x) kg of water.
Let ,
fv  volume of 1 kg of water
gv  volume of 1 kg of perfect dry steam
fv = specific volume of saturated liquid
fgv = specific volume of evaporation

10
gv = specific volume of dry steam, then
[specific volume of a fluid is the volume occupied by a unit mass of the fluid]
Volume of 1 kg of wet steam = volume of dry steam + volume of water
[Since vf is very small as compared to gv , therefore the expression (1 - x) vf
may be neglected.
 Volume of 1 kg of wet steam = 3
gx v m
 g fx v 1 x v  
g f fxv v xv  
 f g fv x v v  
f fgv xv 
f fg fg fgv xv v v   
   f fg fgv v 1 x v   
 g fgv 1 x v  
Super heated Steam
The superheated steam behaves like a perfect gas and therefore, its volume can
be worked out by applying Charles law to steam at the beginning and at the end of super
heating process.
vg = Specific volume of dry steam at pressure P
Ts = Saturation temperature in K
Tsup = Temperature of super heated steam in K
Vsup = Volume of 1 kg of super heated steam at pressure P.
Then
g sup
s sup
PV PV
T T


11
g sup
sup
s
V T
V
T

Internal Energy of steam
The actual Heat energy above the freezing point of water stored in steam is
known as internal energy of steam.
The work of evaporation is not stored in the steam as it is utilised in during
external work.
So the internal energy of steam could be found by subtracting work of
evaporation from the total heat.
u = h - pv
For wet steam
 f fg gu h xh pxv  
=  f fg gh h 100pxv  kJ/kg
Pressure on the piston in bar
= P × 105 N/m2
1 bar = 105 N/m2
For dry saturated steam
 f fg gu h h pv  
g gh 100pv  kJ/kg
For super heated steam
 f fg ps sup s supu h h C T T PV    
 g ps sup s suph C T T 100PV    
 
Entropy of steam
1. The entropy of water at 0oC is taken as zero. The water is heated and

12
evaporated at constant pressure. The steam is also super heated at constant pressure in
super heaters.
2. So the entropy of steam can be calculated from the formula for the change of
entropy at constant pressure.
Entropy of water
p
s
C dTdQ
d
T T

 
The total increase in entropy of water from freezing point to boiling point, may
be obtained by integrating the above expression within the limits 273 K and Ts K.
ss T p dT
so 273
s
C
d
T

 
s s
f p e p
T T
S C log 2.3C log
273 273
 
   
    
   
The value of Sf may be directly seen from the steam tables
Entropy Increase during Evaporation
When the water is completely evaporated into steam, it absorbs full latent heat
(hfg) at constant temperature T, corresponding to the given pressure.
Entropy =
Heat absorbed
Absolute temperature
 Increase of entropy during evaporation
fg
fg
h
S
T

If the steam is wet with dryness fraction x, the evaporation will be partial.
i.e., if evaporation is partial,
Heat absorbed = x hfg
 Increase of entropy,
fg
fg
xh
S
T


13
Entropy of wet and dry steam
Entropy of wet and dry steam =
Entropy of water + Entropy during evaporation
=
fg
f f fg
xh
S S xS
T
   (wet steam)
=
fg
f f fg g
h
S S S S
T
    (dry steam)
Entropy of super heated steam
Heat absorbed; dQ = Cps dT
psdT
s
C
d
T
 [value taken × 1.67 kJ/kg K to 2.5 kJ/kg K]
sup sup
g s
S T
s pS T
dT
d C
T
  or
sup sup
sup g ps e p
T T
S S C log 2.3C log
T T
   
     
   
where  sup gS S is the increase in entropy.
Entropy of 1 kg of superheated steam is

sup
sup g ps
T
S S 2.3C log
T
 
   
 
TEMPERATURE - ENTROPY (T.S) DIAGRAM

14
STEAM TABLES
The generation of steam at different pressures has been studied experimentally
and various properties of steam have been obtained at different conditions. The
properties have been listed in tables called steam tables. The steam tables are available
for
1. Saturated water and steam - on pressure basis.
2. Saturated water and steam - on temperature basis.
3. Super heated steam - on pressure and temperature basis for enthalpy,
entropy and specific volume.
4. Supercritical steam - on pressure and temperature basis above 221.2 bar
and 374.15o
C for enthalpy, entropy and specific volume.
Some important points regarding Steam Tables
(a) The steam table gives values for 1 kg of water and 1 kg of steam.
(b) The steam table gives values of properties from the triple point of water to
the critical point of steam.
(c) For getting values of thermodynamic properties, either saturation pressure or
saturation temperature need to be known. Pressure based steam table (i.e., extreme left
pressure column is placed) is used when pressure value is known, similarly temperature
based steam table is used when temperature value is known.

15
(d) At low pressure the volume of saturated liquid is very small as compared to
the volume of dry steam and usually the specific volume of the liquid is neglected. but at
very high pressure the volume of liquid is comparable and should not be neglected.
(e) The specific enthalpy and specific entropy at 0o
C are both taken as zero and
measurements are made from 0o
C onwards.
(f) In computing properties for wet steam it should be noted that only hfg and sfg
are affected by dryness fraction but hf and sf are not affected by dryness fraction. This
means that for steam with dryness fraction x,
g f fgh h xh 
g f fgS S xS 
Property Table
Property Wet steam Dry steam Super heated steam
Volume   f g1 x v x v  gv sup
g
s
T
v .
T
Enthalpy f gfh xh f fg gh h h   g ps sup sah C T T 
Entropy f fgS xS f fg gS S S  sup
g ps n
s
T
S C l
T

Enthalpy - Entropy chart (Mollier chart)
Most of the thermodynamic systems deal with flow of steam in steady
condition where change in enthalpy is encountered.

16
The most convenient method of computing change in enthalpy is the enthalpy-
entropy chart.
Saturated liquid region is not required for solving engineering problems and
therefore only a part of chart near saturated vapour region and super heat region is
shown.
This chart is very useful for solving problems on nozzles and steam power
plants.
1. Dryness fraction lines
2. Constant volume lines
3. Constant pressure line
4. Isothermal lines
5. Isentropic lines
6. Throttling lines
RANKINE CYCLE
M.Rankine (1820-1872), a Professor at Glasgow University
It is also a reversible cycle but it differs from the Carnot cycle in the following
respects:
(i) The condensation process is allowed to proceed to completion; the exhaust
steam from the engine/turbine is completely condensed. At the end of condensation
process the working fluid is only liquid and not a mixture of liquid and vapour.

17
(ii) The pressure of liquid water can be easily raised to the boiler pressure
(pressure at which steam is being generated in the boiler) by employing a small sized
pump.
In addition, the steam may be super heated in the boiler so as to obtain exhaust
steam of higher quality. That will prevent pitting and erosion of turbine blades.
Steam power plant working on ideal Rankine cycle
The various elements are:
A boiler which generates steam at constant pressure
An engine or turbine in which steam expands isentropically and work is done.
A condenser in which heat is removed from the exhaust steam and it is
completely converted into water at constant pressure
A hot well in which the under state is collected
A pump which raises the pressure of liquid water to the boiler pressure and
pumps it into the boiler for conversion into steam.

18
Consider a steady flow conditions at all states and 1 kg of steam is circulating
through the cycle.
The heat supplied by the boiler per kg of steam generated
Heat absorbed = Q1 = (h2 – h1) = (h2 – h4) - (h1 – h4)
where,
Wp = (h1 – h4) is called pump work per kg of steam.
Heat rejected into the condenser = Q2 = (h3 – h4)
Net work done per kg of steam = Q1 - Q2
= (h2 – h4) - Wp - (h3 – h4)
= (h2 – h3) - Wp
= WT - WP
Where,
WT = Turbine work = (h2 – h3) = isentropic enthalpy drop during expansion
Rankine efficiency = R
1
Network done W
Heat supplied Q
  
=
 
 
1 2 P
1 3 P
h h W
h h W
 
 
The pump work (WP) is very small as compared to turbine work (h2 – h3) and
heat added (h2 – h1), therefore it can be fairly neglected.
WP = ( P1 - P2) V4
P1 = Boiler pressure, P2 = Condenser pressure
V4 = Specific volume of saturated liquid at condenser pressure.
The field pump handles liquid water which is in compressed, which means with
the increase in pressure its density or specific volume undergoes a little change. Using
general property relation for reversible adiabatic compression, we get,

19
Tds = dh - vdp
ds = 0
dh = v dp
 h = v  P ... (since change in specific volume is negligible)
hf2 - hf3 = V1 (P1 - P2)
When P is in bar and v is in m3
/kg, we have
hf2 - hf3 = V4 (P1 - P2) × 105
J/kg
The Rankine efficiency without pump work is
1 2
R
1 f 3
h h
h f

 

............ (1)
State 3 (i.e., at the end of isentropic expansion) must be known then only h3 can
be determined. State 3 is located from the steam table by equating entropy S2 and S3 or
by drawing a vertical line on the Mollier chart from State 1 to condenser pressure.
Modified Rankine Cycle (Steam Engine Cycle)
In the steam engine the expansion is not continued up to the point 2 as the stroke
will be too long and as the work obtained is very small at the tail end of the stroke which
is not even sufficient to overcome the frictional resistances near the end of the stroke.
Therefore in actual practice the expansion is terminated at point 5 instead of 2 and the
steam is released at constant volume. This causes a sudden pressure drop from P2 to P2
to Pb (back pressure) at constant volume due to the steam communicating with outside
atmosphere. This is represented by 56 fig. This reduces the stroke length of the engine
without any appreciable change in the work done.

20
Specific Steam Consumption (S.S.C)
It is defined as the steam consumption (kg/s) to produce unit power (kW)
S.S.C =
 1 2
Mass flow rate per hour kg/s 3600
kg/kWhr
Net power output kW h h
 

(h1 - h2) kJ work is obtained from 1 kg of steam.
1 kW hr = 3600 kJ
S.S.C =
 1 2
3600
kg/kWhr
h h
In case of steam power plant, the specific steam consumption is an indicator of
the relative size of the plant.
Work ratio (Wr) : It is the ratio of network done to the turbine work.
 
 
1 2 P
r
1 2
h h W
W
h h
 


Relative Efficiency or Efficiency Ratio
Relative Efficiency =
Thermal Efficiency
Ranking Efficiency
Q. A simple Rankine cycle steam power plant operates between the temperature of
260o
C and 95o
C. The steam is supplied to the turbine at a dry saturated condition.
In the turbine it expands in an isentropic manner. Determine the efficiency of the
Rankine cycle followed by the turbine and the efficiency of the carnot cycle

21
operating between these two temperature limits. Draw the T - S and H - S
diagrams.
Solution:
T1 = 260o
C = 260 + 273 = 533 K ; T2 = 95o
C = 95 + 273 = 368 K.
From steam table, At 260o
C, P2 = 46.94 bar 1 95o
C, P2 = 0.845 bar.
The initial and final conditions of steam are shown in the H-S diagram.
h1 = 2800 kJ/kg;
h2 = 2170 kJ/kg;
From steam tables at temperature 95o
C,
hf3 = 398 kJ/kg
Efficiency of Rankine cycle, 1 2
R
1 f 3
h h
h f

 

=
2800 2170
2800 398


= 0.262 = 26.2%
Efficiency of Carnot cycle, 1 2
c
1
T T
T

 
=
533 368
533

= 0.3096 = 30.96%
Ranking cycle for wet dry and super heated steam
The value of h1 and h2 may be determined by using steam tables
h1 = hg = 2796.4 kJ/kg ; Sg = 6.001 kJ/kg
hf3 = hf = 398 kJ/kg = 2270.2 kJ/kg
Sf3 = Sf = 1.25 kJ/kg ; Sfg = 6.167 kJ/kg K

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Dryness fraction at 2
S1 = S2
6.001 = 1.25 + x × (6.167)  x = 0.77
h2 = hf + x hfg
= 398 + 0.77 × 2270 - 2 = 2146 kJ/kg
Specific Steam Consumption
It is the mass of steam that must be supplied to a steam engine or turbine in
order to develop a unit amount of work or power out put.
The amount of work or power out put is usually expresses in kilowatt hour
(kWh).
W = J/s
S.S.C =
1 2
Mass flow rate per hour kg/s kg 3600
Net power out put kW kWS h h
  

=
 1 2
3600
kg/kWhr
h h
Q. A steam power plant uses steam at a pressure of 50 bar and temperature 500o
C
and exhausted into a condenser where a pressure of 0.05 bar is maintained. The
mass flow rate of the steam is 150 kg/sec. determine (a) the Rankine engine
efficiency (b) Power developed (c) specific steam consumption (d) Heat rejected
into the condenser per hour (e) Carnot efficiency.
P1 = 50 bar, P2 = 0.05 bar
From steam tables:
50 bar 263.99o
C (Saturation Temperature)
Page - 44 (Properties of super heated steam)
h1 = 3433.8 and S1 = 6.9770 kJ
S1 = S2

23
6.977 = f 3 fgS xS
6.977 = 0.4764 + x × 7.9187, x 0.82 
 h2 = hf2 = x hfg = 137.8 + 0.82 × 2423.8 = 2125.316 kJ/kg
Vf3 = 1.005 × 10-3
m3
/kg
hf3 = 137.82 kJ/kg
(a) Rankine Engine Efficiency = 1 2
1 3
h h 3433.8 2125.316
h h 3433.8 137.82
 

 
= 0.3969 = 39.69%
(b) Power developed = ms × work done per kg = 150 × (h1 - h2)
= 150 × 1308.384
= 196257.6 kW = 196.257 mW
(c) S.S.C =
1 2
3600 3600
h h 1308.384


= 2.751 kg/kW hr
(d) Heat Rejected into the condenser = Q2 = ms (h2 - h3)
= 150 (2125.316 - 137.8)
= 298127.4 kJ/s
(e) Carnot efficiency, c =
 
 
2
1
273 32.9T
1 1
T 273 263.9

  

= 0.43 = 13%
P1  Boiler Pressure, P2 = Condenser Pr
V3  Specific volume of saturated liquid at the condenser pressure
WP = (P1 - P2) V3
(f) Ranking cycle efficiency,
 
 
1 2 P
R
1 f 3 P
h h W
h h W
 
 
 

24
=
   
   
3433.7 2125.316 50 0.05 /10
3433.7 137.8 50 0.05 /10
  
  
=
1308.384 4.995
100 39.6%
3295.9 4.995

 

Q. Dry saturated steam at 10 bar is supplied to a prime mover and the exhaust
takes place at 0.2 bar. Determine the Rankine Efficiency, efficiency ratio and
specific steam consumption of the prime mover, if the indicated thermal efficiency
is 20%. Also find the percentage change in the Rankine efficiency, if steam is
initially 90% dry.
From Mollier chart, h1 = 2775 kJ/kg, h2 = 2150 kJ/kg
From steam tables, we find that enthalpy of water at 0.2 bar,
hf3 = 251.5 kJ/kg
Rankine Efficiency, 1 2
R
1 f 3
h h
h h

 

=
2775 2150
2775 251.5


= 0.247 or 24.7%
Efficiency ratio =
Indicated thermal efficiency
Ranking efficiency
0.2
0.247
 = 0.81 or 81%
Specific Steam Consumption =
1 2
3600
h h
=
3600
2775 2150
= 5.76 kg/kWh
Percentage change in the Rankine efficiency if the steam is initially 90% dry
h1 = 2580 kJ/kg, h2 = 2030 kJ/kg
Rankine efficiency,

25
2 3
R
2 f 3
h h
h h

 

=
25080 2030
2580 251.5


= 0.236 or 23.6%
Percentage change in Rankine Efficiency
=
24.7 23.6
100 4.45%
24.7

 
Q. In a Rankine cycle, the steam at turbine inlet is saturated at a pressure of 30 bar
and the exhaust pressure is 0.25 bar. Determine, (i) Pump-Work (ii) Turbine
power. (iii) Rankine efficiency (iv) condenser heat flow (v) dryness at the end
of expansion. Assume flow rate of 10 Kg/s.
P1= 30 bar
P2 = .05 bar
(i) Pump work per 1 Kg.
 p 4 3 fW m P P V 
5
1 (30 .04) .00102 10 3KJ     
Power required for the pump
10 3KJ
30KW
sec

 
(ii) Turbine Power
From steam table for 30 bar, dry sale steam
h1 = kg, 2803 KJ/Kg
at (1) entropy
S1 = Sg1 = 7.831 KJ/kgK
at .2 steam is wet steam
2 f 2 2 2 2S S x Sfg 0.893 x x...   
Since 1-2 is an isentropic process
S1 = S2

26
 7.831= 0.893 + x2 × .........
x2 = 0.763
Enthalpy at 2, (wet steam of x2 dry)
2 f 2 2 2h h x hfg 
= 272 + 0.763 × 2346
Turbine power = 10× (2803-2062) KJ/s. = 7410 KW
(iii) Rankine Efficiency
1 2 p
1 3 p
h h W
h (h w )
 

 
=
 
(2803 2062) 3
2803 272 3
 
 
=0.292 or 29.2%
(iv) heat flow rate in the condenser
= m(h2–h3) = 10× (2062–272) = 17900 KW
(v) Dryness at the end of expansion = 0.763 = 76.3%
Thermodynamic Processes of steam
Constant volume process
V1 = x1Vg1 , V2 = x2Vg2
(i) W 1 – 2 = 0 dv = 0
(ii) U1 = h1 – 100P1V1 = h1–100P1 X1 Vg1
U2 = h2–P2V2100 = h2–P2X2Vg2100.....(Wet)
= h2–P2Vg2 100.........(dry saturated)
= h2–P2 Vsup 100 ...........(super heated)
(iii) heat absorbed, q12 = du + w 1–2 = U2–U1
Applying first law energy equation
2
1
Q u pdv  

27
 2 1 2 1U U P(V – V )  
if initially steam is wel. V1 = X1Vg1
Finally super heated V2 = Vsup
Constant Temperature Process
 in wet steam region (hynerbolic in super heated steam region)
 will be a constant pressure process also during
 Condensation & evaporation
Q = h2 – h1
W = P1 (V2–V1)
 Limited to wet steam region only
Hyperbolic Process
 Process PV = C
 Hyperbolic process is also an isothermal process in the superheated
steam regions.
2 2
2
11 1
vc
W pdv dv clog
v v
 
   
 
 
2
1
1
V
W P V1log
V

Q u w 
  2
2 1 1 1
1
V
U U P V loge
V
 
    
 
    2
2 2 2 1 1 1 1 1
1
V
h P V h P V P V log
V
 
      
 
  2
2 1 1 1
1
V
Q h – h P V log
V
 
   
 

28
Isentropic Process
Q u w 
Q = O adiabalic
W = U1–U2
Steady flow reversible
W = h1–h2
1 1 1 2 2 2
1 2
u P V Q W U P V
h h
   

Polytropic Process
steam follows the low PVn
= C
Work done 1 1 2 2P V P V
W
n 1



Applying first law energy equation to the non flow process.
Q u W 
=   1 1 2 2
2 1
P V P V
U U
n 1
 
  
 
    1 1 2 2
2 2 2 1 1 1
P V P V
h P V h P V
n 1

    

   2 1 1 1 2 2
1
h h P V P V 1
n 1
 
     
 
   2 1 1 1 2 2
n
Q h h P V P V
n 1
   

Throttling Process
 Const. enthalpy in the absence of heat and work transfer enthalpy
remains constant.
h1 = h2
 during throttling pressure always falls

29
Process Wo Qn
Isochoric O U2–U1
Isobaric  2 1P V V 2 1h h
Isothermal 2 1P(V V ) h2–h1
Hyper bolic 2
1 1
1
V
P V loge
V
 
 
 
  2
2 1 1 1
1
V
h h P V loge
V
 
   
 
Isentropic U2–U1 0
Polytropic 1 1 2 2P V P V
n 1


   2 1 1 1 2 2
n
h h P V P V
n 1
  

throttling process h1 = h2
STEAM GENERATORS
In simple a boiler may be defined as a closed vessel in which steam is produced
from water by combustion of fuel.
According to American Society of Mechanical Engineers (A.S.M.E.) a „steam
generating unit‟ is defined as: “A combination of apparatus for producing, furnishing or
recovering heat together with the apparatus for transferring the heat so made available
to the fluid being heated and vapourised”.
The steam generated is employed for the following purposes:
For generating power in steam engines or steam turbines.
(ii) In the textile industries for sizing and bleaching etc. and many other industries like
sugar mills ; chemical industries.
(iii) For heating the buildings in cold weather and for producing hot water for hot water
supply.
The primary requirements of steam generators or boilers are

30
The water must be contained safely.
(ii) The steam must be safely delivered in desired condition (as regards its pressure,
temperature, quality and required rate).
CLASSIFICATION OF BOILERS
The boilers may be classified as follows:
1. Horizontal, Vertical or Inclined
If the axis of the boiler is horizontal, the boiler is called as horizontal, if the axis is
vertical, it is called vertical boiler and if the axis is inclined it is known as inclined
boiler. The parts of a horizontal boiler can be inspected and repaired easily but it
occupies more space. The vertical boiler occupies less floor area.
2. Fire Tube and Water Tube
In the fire tube boilers, the hot gases are inside the tubes and the water surrounds the
tubes. Examples : Cochran, Lancashire and Locomotive boilers.
Boiler Terms
Shell. The shell of a boiler consists of one or more steel plates bent into a cylindrical
form and riveted or welded together. The shell ends are closed with the end plates.
Setting. The primary function of setting is to confine heat to the boiler and form a
passage for gases. It is made of brickwork and may form the wall of the furnace and the
combustion chamber. It also provides support in some types of boilers (e.g., Lancashire
boilers).
Grate. It is the platform in the furnace upon which fuel is burnt and it is made of cast
iron
bars. The bars are so arranged that air may pass on to the fuel for combustion. The area
of the grate on which the fire rests in a coal or wood fired boiler is called grate surface.
Furnace. It is a chamber formed by the space above the grate and below the boiler shell,
in which combustion takes place. It is also called a fire-box.

31
Water space and steam space. The volume of the shell that is occupied by the water is
termed water space while the entire shell volume less the water and tubes (if any) space
is called steam space.
Mountings. The items such as stop valve, safety valves, water level gauges, fusible
plug, blow-off cock, pressure gauges, water level indicator etc. are termed as mountings
and a boiler cannot work safely without them.
Accessories. The items such as superheaters, economisers, feed pumps etc. are termed
as accessories and they form integral part of the boiler. They increase the efficiency of
the boiler.
Water level. The level at which water stands in the boiler is called water level. The
space above the water level is called steam space.
FIRE TUBE BOILERS
The various fire tube boilers are described as follows:
Simple Vertical Boiler
It consists of a cylindrical shell, the greater portion of which is full of water
(which surrounds the fire box also) and remaining is the steam space. At the bottom of
the fire box is grate on which fuel is burnt and the ash from it falls in the ash pit.
The fire box is provided with two cross tubes. This increases the heating surface and the
circulation of water. The cross tubes are fitted inclined. This ensures efficient circulation
of water. At the ends of each cross tube are provided hand holes to give access for
cleaning these tubes. The combustion gases after heating the water and thus converting it
into steam escape to the atmosphere through the chimney. Man hole, is provided to clean
the interior of the boiler and exterior of the combustion chamber and chimney. The
various mountings shown in Figure are (i) Pressure gauge, (ii) Water level gauge or
indicator, (iii) Safety valve, (iv) Steam stop valve, (v) Feed check valve, and (vi) Man
hole. Flow of combustion gases and circulation of water in water jackets are indicated by
arrows

32
The rate of production in such a boiler normally does not exceed 2500 kg/hr and
pressure is normally limited to 7.5 to 10 bar.
A simple vertical boiler is self-contained and can be transported easily.
Cochran Boiler
It is one of the best types of vertical multi-tubular boiler, and has a number of horizontal
Dimensions, working pressure, capacity, heating surface and efficiency are given below:
Shell diameter 2.75 m
Height 5.79m
Working pressure 6.5 bar (max. pressure = 15 bar)
steam capacity 3500 kg/hr (max. capacity = 4000 kg/hr)
Heating surface 120m2
Efficiency 70 to 75% (depending on the fuel used)

33
Cochran boiler consists of a cylindrical shell with a dome shaped top where the
space is provided for steam. The furnace is one piece construction and is seamless. Its
crown has a hemispherical shape and thus provides maximum volume of space. The fuel
is burnt on the grate and ash is collected and disposed of from ash pit. The gases of
combustion produced by burning of fuel enter the combustion chamber through the flue
tube and strike against fire brick lining which directs them to pass through number of
horizontal tubes, being surrounded by water. After which the gases escape to the
atmosphere through smoke box and chimney. A number of hand-holes are provided
around the outer shell for cleaning purposes.
The various boiler mountings shown in Figure are : (i) Water level gauge, (ii)
Safety valve, (iii) Steam stop valve, (iv) Blow off cock, (v) Man hole and, (vi) Pressure
gauge.

34
The path of combustion of gases and circulation of water are shown by arrows in Fig.
11.2.
Cornish Boiler
This form of boiler was first adopted by Trevithick, the Cornish engineer, at the time of
introduction of high-pressure steam to the early Cornish engine, and is still used. The
specifications of Cornish boiler are given below
No. of flue tubes One
Diameter of the shell 1.25 w 1.75 m
Length of the shell 4 to 7 m
Pressure of the steam 10.5 bar
Steam capacity 6500 kg/h.
It consists of a cylindrical shell with flat ends through which passes a smaller
flue tube containing the furnace. The products of combustion pass from the fire grate
forward over the brickwork bridge to the end of the furnace tube; they then return by the
two side flues to the front end of the boiler, and again pass to the back end of a flue
along the bottom of the boiler to the chimney

35
The various boiler mountings which are used on this boiler are : (i) Steam stop
valve, (ii) Pressure gauge, (iii) Water gauge, (iv) Fusible plug, (v) Blow off cock, (vi)
High steam low water safety valve, (vii) Feed check valve and (viii) Man hole.
The advantage possessed by this type of boiler is that the sediment contained in
the water falls to the bottom, where the plates are not brought into contact with the
hottest portion of the furnace gases. The reason for carrying the product of combustion
first through the side flues, and lastly through the bottom flue, is because the gases,
having parted with much of their heat by the time they reach the bottom flue, are less
liable to unduly heat the plates in the bottom of the boiler, where the sediment may have
collected.
Lancashire Boiler
This boiler is reliable, has simplicity of design, ease of operation and less
operating and maintenance costs. It is commonly used in sugar-mills and textile
industries where alongwith the power steam and steam for the process work is also
needed. In addition this boiler is used where larger reserve of water and steam are
needed.
The specifications of Lancashire boiler are given below
Diameter of the shell 2 to 3 m
Length of the shell 7 to 9 m
Maximum working pressure
Steam capacity 9000 kg/h
Efficiency 50 to 70%
The Lancashire boiler consists of a cylindrical shell inside which two large tubes
are placed. The shell is constructed with several rings of cylindrical from and it is placed
horizontally over a brickwork which forms several channels for the flow of hot gases.
These two tubes are also constructed with several rings of cylindrical form. They pass

36
from one and of the shell to the other and are covered with water. The furnace is placed
at the front end of each tube and they are known as furnace tubes. The coal is introduced
through the fire hole into the grate. There is low brickwork fire bridge at the back of the
gate to prevent the entry of the burning coal and ashes into the interior of the furnace
tubes.
The combustion products from the grate pass up to the back end of the furnace
tubes, and then in downward direction. Thereafter they move through the bottom
channel or bottom flue up to the front end of the boiler where they are divided and pass
up to the side flues. Now they move along the two side flues and come to the chimney
flue from where they lead to the chimney. To control the flow of hot gases to the
chimney, dampers (in the form of sliding doors) are provided. As a result the flow of
airto the grate can be controlled. The various mountings used on the boiler are shown in
Figure.
In Cornish and Lancashire boilers, conical shaped cross tubes known as galloway
tubes (not shown) may be fitted inside the furnace tubes to increase their heating
surfaces and circulation of water. But these tubes have now become absolete for their
considerable cost of fitting. Moreover, they cool the furnace gases and retard
combustion.

37
Locomotive Boiler
It is mainly employed in locomotives though it may also be used as a stationary
boiler. It is compact and its capacity for steam production is quite high for its size as it
can raise large quantity of steam rapidly.
Dimensions and the specifications of the locomotive boilers (made at Chitranjan
works in India) are given below

38
Barrel diameter 2.095 m
Length of the barrel 5.206 m
Size of the tubes (superheater) 14cm
The locomotive boiler consists of a cylindrical barrel with a rectangular fire box
at one end and a smoke box at the other end. The coal is introduced through the fire hole
into the grate which is placed at the bottom of the fire box. The hot gases which are
generated due to burning of the coal are deflected by an arch of fire bricks, so that walls
of the fire box may be heated properly. The fire box is entirely surrounded by water
except for the fire hole and the ash pit which is situated below the fire box which is
fitted with dampers at its front and back ends. The dampers control the flow of air to the
grate. The hot gases pass from the fire box to the smoke box through a series of fire
tubes and then they are discharged into the atmosphere through the chimney. The fire
tubes are placed inside the barrel. Some of these tube are of larger diameter and the
others of smaller diameter. The superheater tubes are placed inside the fire tubes of
larger diameter. The heat of the hot gases is transmitted into the water through the
heating surface of the fire tubes. The steam generated is collected over the water surface.

39
A dome shaped chamber known as steam dome is fitted on the upper part of the
barrel, from where the steam flows through a steam. pipe into the chamber. The flow of
steam is regulated by means of a regulator. From the chamber it passes through the super
heater tubes and returns to the superheated steam chamber (not shown) from which it is
led to the cylinders through the pipes, one to each cylinder.
In this boiler natural draught cannot be obtained because it requires a very high
chimney which cannot be provided on a locomotive boiler since it has to run on rails.
Thus some artificial arrangement has to be used to produce a correct draught. As such
the draught here is produced by exhaust steam from the cylinder which is discharged
through the blast pipe to the chimney. When the locomotive is standing and no exhaust
steam is available from the engine fresh steam from the boiler is used for the purpose.
The various boiler mountings include
Safety valves, pressure gauge, water level indicator, fusible plug, man hole, blow-off
cock and feed check valve.
Merits
1. High steam capacity.
2. Low cost of construction.
3. Portability.
4. Low installation cost.
5. Compact.
Demerits
1. There are chances to corrosion and scale formation in the water legs due to the
accumulation of sediments and the mud particles.
2. It is difficult to clean some water spaces.
3. Large flat surfaces need bracing.
4. It cannot carry high overlo1ds without being damaged by overheating.
5. There are practical constructional limits for pressure and capacity which do not meet
requirements.

40
Scotch boiler
The scotch type marine boiler is probably the most popular boiler for steaming
capacities upto about 1000 kg/hr and pressure of about 17 bar. It is of compact size and
occupies small floor space.
Figure shows a single ended scotch type marine boiler. It consists of a cylindrical
shell in which are incorporated one to four cylindrical, corrugated steel furnaces. The
furnaces are internally fired and surrounded by water. A combustion chamber is located
at the back end of the furnace and is also surrounded by water. Usually each furnace has
its own combustion chamber. A nest of fire tubes run from the front tube plate to the
back tube plate. The hot gases produced due to burning of fuel move to the combustion
chambers (by means of the draught). Then they travel to the smoke box through the fire
tubes and finally leave the boiler via uptake and the chimney.
In a double ended scotch boiler furnaces are provided at each end. They look like
single ended boilers placed back to back. A doub‘e ended boiler for same evaporation
capacity, is cheaper and occupies less space as compared to single ended boiler.

41
WATER TUBE BOILERS
The types of water tube boilers are given below
Babcock and Wilcox Water-tube Boiler
The water tube boilers are used exclusively, when pressure above 10 bar and
capacity in excess of 7000 kg of steam per hour is required. Babcock and Wilcox water-
tube boiler is an example of horizontal straight tube boiler and may be designed for
stationary or marine purposes.
The particulars (dimensions, capacity etc.) relating to this boiler are given below
Diameter of the drum 1.22 to 1.83 m
Length 6.096 to 9.144 m
Size of the water tubes 7.62 to 10.16 cm
Size of superheater tubes 3.84 to 5.71 cm
Working pressure 40 bar (max.)
Steaming capacity 40000 kg/h (max.)
Efficiency 60 to 80%
Figure shows a Babcock and Wilcox boiler with longitudinal drum. It consists of
a drum connected to a series of front end and rear end header by short riser tubes. To
these headers are connected a series of inclined water tubes of solid drawn mild steel.
The angle of inclination of the water tubes to the horizontal is about 15° or more.
A hand hole is provided in the header in front of each tube for cleaning and inspection of

42
tubes. A feed valve is provided to fill the drum and inclined tubes with water the level of
which is indicated by the water level indicator. Through the fire door the fuel is supplied
to grate where it is burnt. The hot gases are forced to move upwards between the tubes
by baffle plates provided. The water from the drum flows through the inclined tubes via
downtake header and goes back into the shell in the form -of water and steam via uptake
header. The steam gets collected in the steam space of the drum. The steam then enters
through the antipriming pipe and flows in the superheater tubes where it is further heated
and is finally taken out through the main stop valve and supplied to the engine when
needed.
At the lowest point of the boiler is provided a mud collector to remove the mud
particles through a blow-down-cock.
The entire boiler except the furnace are hung by means of metallic slings or
straps or wrought iron girders supported on pillars. This arrangement enables the drum
and the tubes to expand or contract freely. The brickwork around the boiler encloses the
furnace and the hot gases.

43
The various mountings used on the boiler are shown in Figure.
A Babcock Wilcox water tube boiler with cross draw differs from longitudinal
drum boiler in a way that how drum is placed with reference to the axis of the water
tubes of the boiler. The longitudinal drum restricts the number of tubes that can be
connected to one drum circumferentially and limits the capacity of the boiler. In the
cross drum there is no limitation of the number of connecting tubes. The pressure of
steam in case of cross drum boiler may be as high as 100 bar and steaming capacity upto
27000 kg/h.
Stirling Boiler
Stirling water tube boiler is an example of bent tube boiler. The main elements of
a bent type water tube boiler are essentially drum or drums and headers connected by
bent tubes. For large central power stations these boilers are very popular. They have
steaming capacities as high as 50000 kg/h and pressure as high as 60 bar.
Figure shows a small-sized stirling water tube boiler. It consists of two upper
drums known as steam drums and a lower drum known as mud or water drum. The
steam drums are connected to mud drum by banks of bent tubes. The steam and water
space of the steam drums are interconnected with each other, so that balance of water
and steam may be obtained. For carrying out cleaning operation a man hole at one end of
each drum is provided. The feed water from the economiser (not shown) is delivered to
the steam drum-i which is fitted with a baffle. The baffle deflects the water to move
downwards into the drum. The water flows from the drum 1 to the mud drum through
the rearmost water tubes at the backside. So the mud particles and other impurities will
move to the mud drum, where these particles may be deposited. As this drum is not
subjected to high temperature, so the impurities may not cause harm to the drum. The
blow-off cock blows off the impurities. The baffle provided at the mud drum deflects the
pure water to move upwards to the drum 1 through the remaining half of the water tubes
at the back. The water also flows from it to the drum 2 through the water tubes which are
just over the furnace. So they attain a higher temperature than the remaining portion of
the boiler and a major portion of evaporation takes place in these tubes. The steam is

44
taken from the drum 1 through a steam pipe and then it passes through the superheater
tubes where the steam is superheated, Finally the steam moves to the stop valve from
where it can be supplied for further use. The combustion products ensuing from the grate
move in the upward and downward directions due to the brickwall baffles and are finally
discharged through the chimney into the atmosphere. Fire brick arch gets incandescent
hot and helps in combustion and preventing the chilling of the furnace when fire door is
opened and cold air rushes in. The steam drums and mud drum are supported on steel
beams independent of the brickwork. It is lighter and more flexible than the straight tube
boilers. But it is comparatively more difficult to clean and inspect the bent tubes.

45
BOILER MOUNTINGS AND ACCESSORIES
Boiler Mountings. These are different fittings and devices which are necessary
for the operation and safety of a boiler. Usually these devices are mounted over boiler
shell. In accordance with the Indian boiler regulation the following mountings should be
fitted to the boilers
Two safety valves
• Two water level indicators
• A pressure gauge
• A steam stop valve
• A feed check valve
• A blow-off cock ;1
.An attachment for inspector‘s test gauge
A man hole
• Mud holes or sight holes.
Boilers of Lancashire and Cornish type should be fitted with a high pressure and
low water safety valve
All land boilers should have a fusible plug in each furnace.
Boiler Accessories. These are auxiliary plants required for steam boilers for
their proper operation and for the increase of their efficiency. Commonly used boiler
accessories are
• Feed pumps
• Injector

46
• Economiser
• Air preheater
• Superheater
• Steam separator
• Steam trap.
BOILER MOUNTINGS
The various boiler mountings are discussed as follows
Water Level Indicator
The function of a water level indicator is to indicate, the level of water in the
boiler con8tdntly. It is also called water gauge. Normally two water level indicators are
fitted at the front end of every boiler. Where the boiler drum is situated at considerable
height from the floor, the water gauge is often inclined to make the water level visible
from any position. When the water being boated in the boiler transforms into steam the
level of water in the boiler shell goes on decreasing. For the proper working of the
boiler, the water must be kept at safe-level. If the water level falls below the safe level
and the boiler goes on producing steam without the addition of feed water, great damage
like crack and leak can occur to the parts of the boiler which get uncovered from water.
This can result in the stoppage of steam generation and boiler operation.
Figure shows a Hopkinson‘s water gauge. It is a common form of glass tube
water-level gauge. A is the front end plate of the boiler. F is a very hard glass tube
indicating water level and is connected to the boiler plate through stuffing boxes in
hollow gun metal castings (B, C) having flanges X, Y for bolting the plate.

47
For controlling the passage of steam and water cocks D and E are
provided. When these cocks are opened the water stands in the glass tube at the same
level as in the boiler. K is the drain cock to blow out water at intervals so as not to allow
any sediments to accumulate. Upper and lower stuffing boxes are connected by a hollow
metal column G. Balls J and H rest in the position shown in the normal working of the
gauge. When the glass tube breaks due to rush of water in the bottom passage the balls
move to dotted positions and shut off the water and steam. Then the cocks D and E can
be safely closed and broken glass tube replaced. M, N, P and .R are screwed caps for
internal cleaning of the passage after dismantling. L is the guard glass ; it is tough and
does not give splinters on breaking. Thus when the gauge glass breaks, and this guard
glass which normally will hold flying pieces, also gives way, the pieces will not fly one
and hurt the attendant.
Pressure Gauge
The function of a pressure gauge is to measure the pressure exerted inside the
vessel. The gauge is usually mounted on the front top of the shell or the drum. It is
usually constructed to indicate upto double the maximum working pressure. Its dial is

48
graduated to read pressures in kg‘cm2 (or bar) gauge (i.e., above atmospheric). There are
two types of pressure gauges: (i) Bourdon tube pressure gauge and (ii) Diaphragm type
pressure gauge. A pointer, which rotates over a circular graduated scale, indicates the
pressure.
A pressure gauge is known as compound pressure gauge if it is designed in such
a fashion so as to measure pressures above and below the atmosphere on the same dial.
Figure shows a Bourdon pressure gauge (single tube) a common type of pressure
gauge used. The essential feature of this gauge is the elliptical spring tube which is made
of a special quality of bronze and is solid drawn. One end A is closed by a plug and the
other is connected with a block C, the block is connected with a syphon tube (which is
full of condensed water). The steam pressure forces the water from the syphon tube into
elliptical tube and this causes the tube to become circular is cross-section. As the tube is
fixed at C, the other end A moves outwards. This outward movement is magnified by the
rod R and transmitted to toothed sector T. This toothed sector is hinged at the point H
and meshes with the pinion P fixed to the spindle of the pointer N. Thus the pointer
moves and registers the pressure on a graduated dial.
The movement of the free end of the elliptical tube is proportional to the
difference between external and internal pressure on the tube. Since the outside pressure

49
on the tube is atmospheric, the movement of the free end is a measure of the boiler
pressurô above atmospheric i.e., gauge pressure.
Figure shows a U-tube syphon which connects the gauge to the boiler. The U-
tube syphon is connected to the steam space of the boiler and contains condensed steam
which enters the gauge tube. The condensed water transmits pressure to the gauge, and
at the sametime prevents steam from entering the pressure gauge. In case steam passes
into the gauge tube it will expand the tube and reading obtained will be false.
Furthermore metal may be affected. Plug R is used for connecting the inspector‘s
standard gauge and testing accuracy of boiler pressure gauge while in service. Plug Z is
employed for cleaning the syphon. Three way cock S is used for either connecting the
boiler pressure gauge to steam space or inspector‘s pressure gauge to the steam space.
The double-tube Bourdon gauge is more rigid than the single tube and more suitable for
locomotive
and portable boilers.
Safety Valves
The function of a safety valve is to release the excess steam when the pressure of
steam inside the boiler exceeds the rated pressure. As soon as the pressure of steam
inside the boiler exceeds the rated pressure the safety valve automatically opens and

50
excess steam rushes out into the atmosphere till the pressure drops down to the normal
value. A safety valve is generally mounted on the top of the shell.
As per boiler regulations every boiler must be fitted at least with two safety
valves.
The various types of safety valves are enumerated and discussed as follows:
1. Dead weight safety valve.
2. Lever safety valve.
3. Spring loaded safety valve.
4. High steam and low water safety valve.
Dead Weight Safety Valve
Figure shows a dead weight safety valve. A is the vertical cast iron pipe through
which steam pressure acts. B is the bottom flange directly connected to seating block on
the boiler shell communicating to the steam space. V is the gun metal valve and VS is
the gun metal valve seat. D is another cast iron pipe for discharge of excess steam from
the boiler. W are the weights in the form of cylindrical disc of cast iron. WC is the
weight carrier carrying the weights W. The cover plate C covers these weights. The
steam pressure acts in the upward direction and is balanced by the force of the dead
weights W. The total dead-weights consist of the sum of the weights W, weight of the
valve V, weight of the weight carrier and weight of the cover plate C.
When the steam pressure is greater than the working pressure it lifts the valve
with its weights. So the steam escapes from the boiler and the steam pressure thereby
decreases.

51
Merits of dead weight safety valve
1. Simplicity of design.
2. Gives quite a satisfactory performance during operation.
3. It cannot be easily tempered from the pressure adjustment view-point.
Demerits:
1. Unsuitable for use on any boiler where extensive vibration and movement are
experienced (e.g. locomotive and marine work).
2. It is not suitable for high pressure boilers because a large amount of weight is required
to balance the steam pressure.
Uses. It is mainly used for low pressures, low capacity, stationary boilers of the Cornish
and Lancashire types.
Lever Safety Valve
It consists of a lever and weight W. The valve (r1ade of gun metal) rests on the
valve seat (gun metal) which is screwed into the valve body ; the valve seat can be
replaced if required. The valve body is fitted on the boiler shell. One end of the lever is
hinged while at the other is suspended a weight W. The strut presses against the valve

52
on seat against the steam pressure below the valve. The slotted lever guide allows
vertical movement to the lever.
When the steam pressure becomes greater than the normal working pressure, the
valve is lifted with the lever and the weight. Consequently, the steam escapes through
the passages between the valve and seat and the steam pressure decreases. The
disadvantages of this valve is that it admits of being tempered with, and the effect of a
small addition to the weight is magnified considerably in its action on the valve. Figure
shows the loading arrangement on the lever
Economiser
An economiser is a device in which the waste heat of the flue gases is utilised for
heating the feed water.
Economiser are of the two types (i) Independent type, and (ii) Integral type.
Former is installed in chamber apart from the boiler setting. The chamber is situated at
the passage of the flow of the flue gases from the boiler or boiler to the chimney. Latter
is a part of the boiler heating surface and is installed within the boiler setting.
Figure shows an independent type vertical tube economiser (called Green‘s
economiser). It is employed for boilers of medium pressure range upto about 25 bar. It
consists of a large number of vertical cast iron pipes P which are connected with two
horizontal pipes, one at the top and the other at the bottom. A is the bottom pipe through
which the feed water is pumped into the economiser. The water comes into the top pipe

53
B from the bottom pipe (via vertical pipes) and finally flows to the boiler, The flue gases
move around the pipes in the direction opposite to the flow of water. Consequently, heat
transfer through the surfaces of the pipes takes place and water is thereby heated. A
blow-off cock is provided at the back end of vertical pipes to remove sediments
deposited in the bottom boxes. The soot of the flue gases which gets deposited on the
pipes reduces the efficiency of the economiser. To prevent the soot deposit, the scrapers
S move up and down to keep the external surface of the pipe clean (for better heat
transfer). By-pass arrangement enables to isolate or include the economiser in the path
of flue gases.

54
The use of an economiser entails the following advantages
1. The temperature range between various parts of the boiler is reduced which results in
reduction of stresses due to unequal expansion.
2. If the boiler is fed with cold water it may result in chilling the boiler metal. Hot feed
water checks it.
3. Evaporative capacity of the boiler is increased.
4. Overall efficiency of the plant is increased.
Air Preheater
The function of the air pre-heater is to increase the temperature of air before it
enters the furnace. It is generally placed after the economiser ; so the flue gases pass
through the economiser and then to the air preheater. An air-preheater consists of plates
or tubes with hot gases on one side and air on the other. It preheats the air to be supplied
to the furnace. Preheated air accelerates the combustion and facilitates the burning of
coal.
Degree of preheating depends on
Type of fuel,
(iii) Rating at which the boiler and furnace are operated.
There are three types of air preheaters
1. Tubular type

55
2. Plate type
3. Storage type.
Figure shows a tubular type air preheater. After leaving the boiler or
economiser the gaseous products of combustion travel through the inside of the tubes of
air preheater in a direction opposite to that of air travel and transfer some of their heat to
the air to be supplied to the furnace. Thus the air gets initially heated before being
supplied to the furnace. The gases reverse their direction near the bottom of the air
heater, and a soot hopper is fitted to the bottom of air heater casing to collect soot.
In the plate type air preheater the air absorbs heat from the hot gases being
swept through the heater at high velocity on the opposite side of a plate. Figure shows a
self explanatory sketch of a storage type air preheater (heat exchanger).

56
Finally the gases escape to the atmosphere through the stack (chimney). The
temperature of the gases leaving the stack should be kept as low as possible so that there
is minimum loss of heat to the stack. Storage type air preheaters are employed widely in
larger plants.

57
MODULE II
Steam Nozzles & Steam Turbines
Introduction
A steam turbine converts the energy of high-pressure, high temperature steam
produced by a steam generator into shaft work. The energy conversion is brought about
in the following ways: The highpressure, high-temperature steam first expands in the
nozzles emanates as a high velocity fluid stream.
1. The high velocity steam coming out of the nozzles impinges on the blades
mounted on a wheel. The fluid stream suffers a loss of momentum while flowing
past the blades that is absorbed by the rotating wheel entailing production of
torque.
2. The moving blades move as a result of the impulse of steam (caused by the
change of momentum) and also as a result of expansion and acceleration of the
steam relative to them. In other words they also act as the nozzles.
A steam turbine is basically an assembly of nozzles fixed to a stationary casing and
rotating blades mounted on the wheels attached on a shaft in a row-wise manner. In
1878, a Swedish engineer, Carl G. P. de Laval developed a simple impulse turbine, using
a convergent-divergent (supersonic) nozzle which ran the turbine to a maximum speed
of 100,000 rpm. In 1897 he constructed a velocity-compounded impulse turbine (a two-
row axial turbine with a row of guide vane stators between them.
Auguste Rateau in France started experiments with a de Laval turbine in 1894, and
developed the pressure compounded impulse turbine in the year 1900. In the USA ,
Charles G. Curtis patented the velocity compounded de Lavel turbine in 1896 and
transferred his rights to General Electric in 1901. In England , Charles A. Parsons
developed a multi-stage axial flow reaction turbine in 1884.
Steam turbines are employed as the prime movers together with the electric generators in
thermal and nuclear power plants to produce electricity. They are also used to propel

58
large ships, ocean liners, submarines and to drive power absorbing machines like large
compressors, blowers, fans and pumps.
Turbines can be condensing or non-condensing types depending on whether the back
pressure is below or equal to the atmosphere pressure.
Flow through Nozzles
A nozzle is a duct that increases the velocity of the flowing fluid at the expense
of pressure drop. A duct which decreases the velocity of a fluid and causes a
corresponding increase in pressure is a diffuser . The same duct may be either a nozzle
or a diffuser depending upon the end conditions across it. If the cross-section of a duct
decreases gradually from inlet to exit, the duct is said to be convergent. Conversely if the
cross section increases gradually from the inlet to exit, the duct is said to be divergent. If
the cross-section initially decreases and then increases, the duct is called a convergent-
divergent nozzle. The minimum cross-section of such ducts is known as throat. A fluid
is said to be compressible if its density changes with the change in pressure brought
about by the flow. If the density does not changes or changes very little, the fluid is said
to be incompressible. Usually the gases and vapors are compressible, whereas liquids are
incompressible .
Steam Nozzles
 A steam nozzle is a passage of varying resection, which converts heat energy of
steam into Kinetic Energy as the steam expands from higher pressure to lower
pressure.
Purpose
 to produce high velocity jet of steam to run in steam turbines.
 The amount of energy so converted depends upon the pressure ratio and the type
of expansion
 Isentropic expansion provides the maximum expansion
 Generally nozzles are so shaped that isentropic expansion is obtained.

59
Types of Nozzles
(1)Convergent Nozzle
 area diminishes from inlet section to at let section
 useful up to a pressure ratio of 0.58 using saturated steam.
(2)Divergent Nozzle
(3)Convergent Divergent
 Nozzle with divergent part in addition to the convergent part to obtain more
pressure drop acceleration is .....
 divergent portion is long
 T is divergent angle
 Least cross section is called throat.
Two Functions of turbine nozzle
(i) a portion heat energy to kinetic energy
(ii) In Impulse turbine directs high velocity steam to turbine blades.
Reaction turbines – nozzle movable
Flow of Steam Trhough Nozzle
Consider a unit mass flow of steam through a nozzle.
Applying steady flow energy equation to the sections 1 and 2.
2 2
1 2
1 2
V V1 1
h R h W
1000 2 2 1000
     
h = enthalpy
V = velocity
W = work transfer
Q = heat transfer
Since expansion. is isentropic and there is no external work done during the flow of

60
steam W = Q = O
2 2
2 1
1 2
V V1
h h
1000 2 2
 
   
 
 2 2
2 1 1 2V V h h 2000   
2
2 1 1 2V V 2000(h h )  
Since V1 <<V2
2 1 2V 2000(h h )  = d44.72 h
This is the general energy equation irrespective of the shape of the nozzle.
Mass of steam discharged through nozzle
The flow of steam through the nozzle may be represented by an eqn of the form
Pvn
= constant
n = 1.135 for saturated steam
= 1.3 for superheated steam
Steam performs works upon itself by accelerating itself to a high velocity.
As the steam pressure drops its enthalpy is reduced. This reduction of the
enthalpy must be equal to the increase in KE.
heat drop = work done percentage of steam during cycle.
2 2
2 1
1 1 2 2
V V n
(P V P V )
2 2 n 1
  

V1<<V2
2
2
1 1 2 2
V n
(P V P V )
2 n 1
 

2 2
1 1
1 1
P Vn
P V (1
n 1 P V
 


61
we know that n n
1 1 2 2P V P V
2 1
1 2
V P
1/ n
V P
 
 
 

2
2 2 1
1 1
1 2
V P Pn
P V 1 1/ n
2 n 1 P P
  
       
2
1 1
1
Pn n 1
P V 1
n 1 P n
    
         
2
2 1 1
1
Pn n 1
V 2 P V 1
n 1 P n
   
   
   
Volume of steam flowing per second
= A × V2
Specific volume of steam V2 m3
/Kg
mass of steam discharged per second
2
Volumeof steamdischargedper
Specificvolumeof 1Kgof steant at P

2
2
AV
V

n 1
n
2
1 1
2 1
PA n
2 P V 1
V n 1 P
 
  
        
1/ n
2 1
1 2
V P
V P
 
 
 
½
1
2 1
2
P
V V
P
 
  
 
1/ n
1
1 2
P1 1
V2 V P
 
  
 

62
1/ n
1 2
1 1
1 2 1
P PA n n 1
m 2 P V 1
V P n 1 P n

     
       
    
n 1
1/ n
n
2 2
1 1
1 1 1
P PA 2n
P V 1
V P n 1 P

  
    
          
 
n 1
2/ n
n
1 2 2
1 1 1
P P P2n
A 1
n 1 V P P
 
    
            
 
n 1
n
1 2 2
1 1 1
P P P2n
A 2/ n
n 1 V P P
 
    
           
Condition for Maximum Discharge through a nozzle (critical pressure ratio)
n 1
2/ n
n
1 2 2
1 1 1
P P P2n
m A
n 1 V P P
 
    
           
A nozzle is designed for maximum discharge by designing a certain throat pressure.
There is only one value of the ratio 2
1
P
P
, which produces maximum discharge.
The portion of the equation which contains 2
1
P
P
is differentiated and equated to zero, for
maximum discharge.
n 1
n
2 2
1 12
1
P Pd 2
0
P n PP
d
P
 
    
              
 
2 n 1
1 1
n n
2 2
1 1
P P2 n 1
0
n P n P

 
   
    
   

63
1
n
2 2
1 1
P P2 2 n n 1
n P n n P
    
   
   
2 n
1/ n
n
2 2
1 1
P P N 1 n
P P n 2


    
     
   
1 n
n
2
1
P N 1
P 2

  
 
 
 
n n
1 n 1 n2
1
P n 1 n 1
P 2 2

      
    
   
n
n 1n 1
2

 
 
 
n
n 12
n 1
 
 
 
P2
P1
is called critical pressure ratio and the pressure P2 at the throat is known as
critical pressure.
STAGNATION, SONIC PROPERTIES AND ISENTROPIC EXPANSION IN NOZZLE
The stagnation values are useful reference conditions in a compressible flow.
Suppose the properties of a flow (such as T, p, ρ etc.) are known at a point. The
stagnation properties at a point are defined as those which are to be obtained if the local
flow were imagined to cease to zero velocity isentropically. The stagnation values are
denoted by a subscript zero. Thus, the stagnation enthalpy is defined as
For a calorically perfect gas, this yields,

64
which defines the stagnation temperature. It is meaningful to express the ratio of
in the form
or,
If we know the local temperature (T) and Mach number (Ma), we can fine out the
stagnation temperature . Consequently, isentropic relations can be used to obtain
stagnation pressure and stagnation density as.
In general, the stagnation properties can vary throughout the flow field.
However, if the flow is adiabatic, then is constant throughout the flow. It
follows that the and are constant throughout an adiabatic flow, even in the
presence of friction. Here a is the speed of sound and the suffix signifies the stagnation
condition. It is understood that all stagnation properties are constant along an isentropic
flow. If such a flow starts from a large reservoir where the fluid is practically at rest,
then the properties in the reservoir are equal to the stagnation properties everywhere in
the flow (Fig. 1.1).

65
Fig 1.1 An isentropic process starting from a reservoir
There is another set of conditions of comparable usefulness where the flow is sonic,
Ma=1.0. These sonic, or critical properties are denoted by asterisks: and. .
These properties are attained if the local fluid is imagined to expand or compress
isentropically until it reachers Ma=1.
We have already discussed that the total enthalpy, hence , is conserved so long the
process is adiabatic, irrespective of frictional effects. In contrast, the stagnation pressure
and density decrease if there is friction.
From Eq.(1), we note that
or,
is the relationship between the fluid velocity and local temperature (T), in an adiabatic
flow. The flow can attain a maximum velocity of

66
As it has already been stated, the unity Mach number, Ma=1, condition is of special
significance in compressible flow, and we can now write from Eq.(2), (3) and (4).
For diatomic gases, like air , the numerical values are
The fluid velocity and acoustic speed are equal at sonic condition and is
or,
We shall employ both stagnation conditions and critical conditions as reference
conditions in a variety of one dimensional compressible flows.
Effect of Area Variation on Flow Properties in Isentropic Flow
In considering the effect of area variation on flow properties in isentropic flow, we shall
concern ourselves primarily with the velocity and pressure. We shall determine the
effect of change in area, A, on the velocity V, and the pressure p.
From Bernoulli's equation, we can write

67
or,
Dividing by , we obtain
---- 1.1
A convenient differential form of the continuity equation as
Substituting from Eq. (1.1)
-----1.2
Invoking the relation ( ) for isentropic process in Eq. (1.2), we get
-----1.3
From Eq. (1.3), we see that for Ma<1 an area change causes a pressure change of
the same sign, i.e. positive dA means positive dp for Ma<1. For Ma>1, an area change
causes a pressure change of opposite sign.
Again, substituting from Eq.(1.1) into Eq. (1.3), we obtain
-------1.4

68
From Eq. (1.4), we see that Ma<1 an area change causes a velocity change of
opposite sign, i.e. positive dA means negative dV for Ma<1. For Ma>1, an area change
causes a velocity change of same sign.
These results are summarized in Fig.1.1, and the relations (1.3) and (1.4) lead to
the following important conclusions about compressible flows:
1. At subsonic speeds (Ma<1) a decrease in area increases the speed of flow. A
subsonic nozzle should have a convergent profile and a subsonic diffuser should
possess a divergent profile. The flow behaviour in the regime of Ma<1 is
therefore qualitatively the same as in incompressible flows.
2. In supersonic flows (Ma>1), the effect of area changes are different. According
to Eq. (1.4), a supersonic nozzle must be built with an increasing area in the flow
direction. A supersonic diffuser must be a converging channel. Divergent nozzles
are used to produce supersonic flow in missiles and launch vehicles.
Fig 1.2 Shapes of nozzles and diffusersin subsonic and supersonic regimes
Suppose a nozzle is used to obtain a supersonic stream staring from low speeds
at the inlet (Fig.1.2). Then the Mach number should increase from Ma=0 near the inlet to
Ma>1 at the exit. It is clear that the nozzle must converge in the subsonic portion and
diverge in the supersonic portion. Such a nozzle is called a convergent-divergent nozzle.

69
A convergent-divergent nozzle is also called a de Laval nozzle, after Carl G.P. de Laval
who first used such a configuration in his steam turbines in late nineteenth century (this
has already been mentioned in the introductory note). From Fig.1.2 it is clear that the
Mach number must be unity at the throat, where the area is neither increasing nor
decreasing. This is consistent with Eq. (1.4) which shows that dV can be non-zero at the
throat only if Ma=1. It also follows that the sonic velocity can be achieved only at the
throat of a nozzle or a diffuser.
Fig 1.3 A convergent-divergent nozzle
The condition, however, does not restrict that Ma must necessarily be unity at the throat,
According to Eq. (1.4), a situation is possible where at the throat if dV=0 there.
For an example, the flow in a convergent-divergent duct may be subsonic everywhere
with Ma increasing in the convergent portion and decreasing in the divergent portion
with at the throat (see Fig.1.3). The first part of the duct is acting as a nozzle,
whereas the second part is acting as a diffuser. Alternatively, we may have a convergent-
divergent duct in which the flow is supersonic everywhere with Ma decreasing in the
convergent part and increasing in the divergent part and again at the throat (see
Fig. 1.4).

70
Fig 1.3 Convergent-divergent duct with at throat
Fig 1.4 Convergent-divergent duct with at throat
Isentropic Flow of a vapor or gas through a nozzle
First law of thermodynamics:

71
(if )
where is enthalpy drop across the nozzle
Again we know, Tds = dh - νdp
For the isentropic flow, dh = νdp
or,
or,
Assuming that the pressure and volume of steam during expansion obey the law pνn
=
constant, where n is the isentropic index

72
Now, mass flow rate
Therefore, the mass flow rate at the exit of the nozzle
=
The exit pressure, p2 determines the for a given inlet condition. The mass flow rate is
maximum when,
For maximum ,

73
n = 1.4, for diatomic gases
for super saturated steam
for dry saturated steam
for wet steam with dryness fraction x
For , (50%drop in inlet pressure)
If we compare this with the results of sonic properties, as described in the earlier
section, we shall observe that the critical pressure occurs at the throat for Ma = 1. The
critical pressure ratio is defined as the ratio of pressure at the throat to the inlet pressure,
for checked flow when Ma = 1
Expansion of Steam in a Nozzle
Figure 1.5 Super Saturated Expansion of Steam in a Nozzle
 The process 1-2 is the isentropic expansion. The change of phase will begin to
occur at point 2
 vapour continues to expand in a dry state
 Steam remains in this unnatural superheated state untit its density is about eight
times that of the saturated vapour density at the same pressure
 When this limit is reached, the steam will suddenly condense

74
 Point 3 is achieved by extension of the curvature of constant pressure line
from the superheated region which strikes the vertical expansion line at 3 and
through which Wilson line also passes. The point 3 corresponds to a metastable
equilibrium state of the vapour.
 The process 2-3 shows expansion under super-saturation condition which is not
in thermal equilibrium
 It is also called under cooling
 At any pressure between and i.e., within the superheated zone, the
temperature of the vapous is lower than the saturation temperature corresponding
to that pressure
 Since at 3, the limit of supersaturation is reached, the steam will now condense
instantaneously to its normal state at the constant pressure, and constant enthalpy
which is shown by the horizontal line where is on normal wet area
pressure line of the same pressure .
 is again isentropic, expansion in thermal equilibrium.
 To be noted that 4 and are on the same pressure line.
Thus the effect of supersaturation is to reduce the enthalpy drop slightly during
the expansion and consequently a corresponding reduction in final velocity. The
final dryness fraction and entropy are also increased and the measured discharge
is greater than that theoretically calculated.
Degree of super heat =
= limiting saturation pressure
= saturation pressure at temperature shown on T-s diagram
degree of undercooling - -

75
is the saturation temperature at
= Supersaturated steam temperature at point 3 which is the limit of supersaturation.
Supersaturated vapour behaves like supersaturated steam and the index to expansion
Problems
Qn.1. Steam is expanded in a set of nozzles from 10 bar and 2000C to 5 bar. What type
of nozzle is it? Neglecting the initial velocity find minimum area of the nozzle required
to allow a flow of 3 kg/s under the given conditions. Assume that expansion of steam to
be isentropic.
Solution. Steam pressure at the entry to the steam nozzles,
p1 = 10 bar, 200o
C
Steam exit pressure, p1 = 5 bar
We know that,
 
13n
0.3n 12
1
p 2 2
p n 1 1.3 1
   
    
    
4.333
2
0.5457
2.3
 
  
 
2 1p p 0.5457 10 0.5457 5.5 bar    
Since throat pressure (p2) is greater than the exit pressure, the nozzle used is
convergent divergent nozzle. The minimum area will be at throat, where the pressure is

76
5.5 bar.
From Mollier chart, 1 2h h 120 kJ / kg
Specific volume, 3
u 0.345 m / kg
Velocity at the throat, 2C 44.72 120 489.88 m/s 
Throat area, 2
2
2
mv 3 0.345
A 0.0021 m
C 489.88

  
Qn.2. Steam having pressure of 10.5 bar and 0.95 dryness is expanded through
convergent-divergent nozzle and the pressure of steam leaving the nozzle is 0.85 bar.
Find the velocity at the throat for maximum discharge conditions. Index of expansion
may be assumed as 135. Calculate mass rate of flow of steam through the nozzle.
Solution. The pressure at throat for maximum discharge,
n 1.135
n 1 1.135 1
2 1
2 2
p p 10.5
n 1 1.135 1
    
    
    
8.41
2
10.5 6.06 bar
2.135
 
  
 
The velocity C2 at throat for maximum discharge is given by (eqn. 11)
 5
2 1 1
n 1.135
C 2 p v 2 10.5 10 0.95 0.185
n 1 1.135 1
     
 
443 m/s
[C2 can also be obtained with the help of steam tables or Mollier chart also]
n n
1 1 2 2p v p v
 1.135 1.135
210.5 0.95 0.185 6.06 v  
3
2v 0.285 m / kg
Mass flow rate, 2 2
2
A C 1 443
m
u 0.285

 
2
1554.4 kg/ m of throat area

77
Qn. 3 A convergent-divergent nozzle is to be designed in which steam initially at 14
bar and 800C of superheat is to be expanded down to a back pressure of 1.05 bar.
Determine the necessary throat and exit diameters of the nozzle for a steam discharge of
500 kg/hour, assuming that the expansion is in thermal equilibrium throughout and
friction reheat amounting to 12% of the total isentropic enthalpy drop to be effective in
the divergent part of the nozzle.
Solution. o
1 sup sp 14 bar, t t 80 C  
o
sup s 3t t 80 195 80 275 C; p 1.05bar     
We know that,
n 1.3
n 1 1.3 12
1
p 2 2
0.546
p n 1 1.3 1
    
     
    
ie, 2 1p p 0.546 14 0.546 7.64 bar    
From Mollier chart, h1 = 2980 kJ/kg, h2 = 2850 kJ/kg
h3 = 2490 kJ/kg, 3x 0.921 
u2 = 0.287 m3
/kg (From Mollier chart)
d 1 2h h h 2980 2850 130 kJ / kg    
d 1 3h h h 2980 2490 490 kJ / kg     

78
For throat:
2 dC 44.72 h 44.72 130 509.8 m/s  
Now, 2 2 2
2
A C A 509.8
m
u 0.287

 
6 2
2
m 0.287 500 0.287
A 7.82 10 m
509.8 3600 509.8
 
    

ie, 2 5
2D 7.82 10
4

 
or
1/ 25
2
7.82 10 4
D 0.009978 m or 9.9 mm
  
    
ie, Throat diameter = 9.9 mm.
At exit:
 3 dC 44.72 kh 44.72 1 0.12 490 928.6m/s     
3
3
3 3 gu x u 0.921 1.69 1.556 m / kg    
23
3
3
m u 500 1.556
A 0.0002327 m
3600 928.6c
 
   

ie, 2
3D 0.0002327
4


or,
1/ 2
3
0.0002327 4
D 0.0172m or 17.2 mm
 
  
 
Qn. 4 Dry saturated steam enters the Steam nozzle at a pressure of 15 bar and is
discharged at a pressure of 2.0 bar. If the dryness fraction of discharge steam is 0.96,
what will be the final velocity of steam? Neglect initial velocity of steam.
If 10% of heat drop is lost in friction, find the percentage reduction in the final
velocity.

79
Solution: Initial pressure of steam, p1 = 16 bar, x1 = 1.
Final pressure of steam, p2 = 2.0 bar, x2 = 0.96
From steam tables:
At p1 = 15 bar, x1 = 1 : h1 = hg = 2789.9 kJ/kg.
At p2 = 2 bar: 2fh 504.7 kJ / kg , 2fgh 2201.6 kJ / kg
2 22 f 2 fh h x h 504.7 0.96 2201.6 2618.2kJ / kg     
The velocity of steam at discharge from nozzle in S.I. units is given by:
 2 d 1 2C 44.72 h 44.72 h h  
 44.72 2789.9 2618.2 585.9 m/s  
ie, Final velocity of steam = 585.9 m/s.
In case 10% of heat drop is lost in friction, nozzle co-efficient.
= 1.0 – 0.1 = 0.9
Hence the velocity of steam = d44.72 kh
 44.72 0.9 2789.9 2618.2 555.9 m/s  
Percentage reduction in velocity =
585.9 555.9
100 5.12%
585.9

 
Qn. 5. Steam initially dry and saturated is expanded in a nozzle from 15 bar at 3000C to
1.0 bar. If the frictional loss in the nozzle is 12% of the total heat drop calculate the mass
of steam discharged when exit diameter of the nozzle is 15 mm.
Solution: Pressure, p1 = 15 bar, 300o
C
Pressure, p2 = 1.0 bar
Frictional loss in nozzle = 12%

80
-efficient, k = 1 – 0.12 = 0.88.
Exit diameter of nozzle, d2 =15 mm
Neglecting the velocity of steam at inlet to the nozzle, the velocity of steam at
exit from the nozzle is given by
 2 d 1 2C 44.72 kh 44.72 0.88 h h    
 44.72 0.88 3037 2515 958.5 m/s   
Dryness fraction of steam at discharge pressure, 2x 0.93 
Specific volume of dry saturated steam at 1.0 bar, 2
3
gv 1.694 m / kg .
Hence mass of steam discharged through nozzle per hour
 
2
2
2 2
2 g
/ 4 15/1000A C
3600 3600 387 kg / h
x u 0.93 1.694
  
    


81
STEAM TURBINES
Turbines
 We shall consider steam as the working fluid
 Single stage or Multistage
 Axial or Radial turbines
 Atmospheric discharge or discharge below atmosphere in condenser
 Impulse/and Reaction turbine
Impulse Turbines
Impulse turbines (single-rotor or multirotor) are simple stages of the
turbines. Here the impulse blades are attached to the shaft. Impulse blades can be
recognized by their shape. They are usually symmetrical and have entrance and
exit angles respectively, around 20 ° . Because they are usually used in the
entrance high-pressure stages of a steam turbine, when the specific volume of
steam is low and requires much smaller flow than at lower pressures, the impulse
blades are short and have constant cross sections.
The Single-Stage Impulse Turbine
The single-stage impulse turbine is also called the de Laval turbine after
its inventor. The turbine consists of a single rotor to which impulse blades are
attached. The steam is fed through one or several convergent-divergent nozzles
which do not extend completely around the circumference of the rotor, so that
only part of the blades is impinged upon by the steam at any one time. The
nozzles also allow governing of the turbine by shutting off one or more them.
The velocity diagram for a single-stage impulse has been shown in Fig. 2.1.
Figure 2.2 shows the velocity diagram indicating the flow through the turbine
blades.

82
Figure 2.1 Schematic diagram of an Impulse Trubine
and = Inlet and outlet absolute velocity
and = Inlet and outlet relative velocity (Velocity relative to the rotor blades.)
U = mean blade speed
= nozzle angle, = absolute fluid angle at outlet
It is to be mentioned that all angles are with respect to the tangential velocity ( in the
direction of U )
Figure 2.2 Velocity diagram of an Impulse Turbine

83
and = Inlet and outlet blade angles
and = Tangential or whirl component of absolute velocity at inlet and outlet
and = Axial component of velocity at inlet and outlet
Tangential force on a blade,
(mass flow rate X change in velocity in tangential direction)
or,
Power developed =
Blade efficiency or Diagram efficiency or Utilization factor is given by
or,
stage efficiency
or,
or,

84
Optimum blade speed of a single stage turbine
where, = friction coefficient
= Blade speed ratio
is maximum when also
or,
or,
is of the order of 180
to 220

85
Now, (For single stage impulse turbine)
The maximum value of blade efficiency
For equiangular blades,
If the friction over blade surface is neglected
Compounding in Impulse Turbine
If high velocity of steam is allowed to flow through one row of moving blades, it
produces a rotor speed of about 30000 rpm which is too high for practical use.
It is therefore essential to incorporate some improvements for practical use and
also to achieve high performance. This is possible by making use of more than one set of
nozzles, and rotors, in a series, keyed to the shaft so that either the steam pressure or the
jet velocity is absorbed by the turbine in stages. This is called compounding. Two types
of compounding can be accomplished: (a) velocity compounding and (b) pressure
compounding
Either of the above methods or both in combination are used to reduce the high
rotational speed of the single stage turbine.

86
The Velocity - Compounding of the Impulse Turbine
The velocity-compounded impulse turbine was first proposed by C.G. Curtis to solve the
problems of a single-stage impulse turbine for use with high pressure and temperature
steam. The Curtis stage turbine, as it came to be called, is composed of one stage of
nozzles as the single-stage turbine, followed by two rows of moving blades instead of
one. These two rows are separated by one row of fixed blades attached to the turbine
stator, which has the function of redirecting the steam leaving the first row of moving
blades to the second row of moving blades. A Curtis stage impulse turbine is shown in
Fig. 23.1 with schematic pressure and absolute steam-velocity changes through the
stage. In the Curtis stage, the total enthalpy drop and hence pressure drop occur in the
nozzles so that the pressure remains constant in all three rows of blades.
Figure 2.3 Velocity Compounding arrangement
Velocity is absorbed in two stages. In fixed (static) blade passage both pressure
and velocity remain constant. Fixed blades are also called guide vanes. Velocity
compounded stage is also called Curtis stage. The velocity diagram of the velocity-
compound Impulse turbine is shown in Figure 2.3.

87
Figure 2.4 Velocity diagrams for the Velocity-Compounded Impulse
turbine
The fixed blades are used to guide the outlet steam/gas from the previous stage in
such a manner so as to smooth entry at the next stage is ensured.
K, the blade velocity coefficient may be different in each row of blades
Work done =
End thrust =
The optimum velocity ratio will depend on number of stages and is given by
• Work is not uniformly distributed (1st >2nd )

88
• The fist stage in a large (power plant) turbine is velocity or pressure compounded
impulse stage.
Pressure Compounding or Rateau Staging
The Pressure - Compounded Impulse Turbine
To alleviate the problem of high blade velocity in the single-stage impulse
turbine, the total enthalpy drop through the nozzles of that turbine are simply divided up,
essentially in an equal manner, among many single-stage impulse turbines in series
(Figure 2.5). Such a turbine is called a Rateau turbine , after its inventor. Thus the inlet
steam velocities to each stage are essentially equal and due to a reduced Δh.

89
Figure 2.5 Pressure-Compounded Impulse Turbine
Pressure drop - takes place in more than one row of nozzles and the increase in kinetic
energy after each nozzle is held within limits. Usually convergent nozzles are used
We can write
where is carry over coefficient
Reaction Turbine
A reaction turbine, therefore, is one that is constructed of rows of fixed and
rows of moving blades. The fixed blades act as nozzles. The moving blades move as a
result of the impulse of steam received (caused by change in momentum) and also as a
result of expansion and acceleration of the steam relative to them. In other words, they
also act as nozzles. The enthalpy drop per stage of one row fixed and one row moving
blades is divided among them, often equally. Thus a blade with a 50 percent degree of
reaction, or a 50 percent reaction stage, is one in which half the enthalpy drop of the
stage occurs in the fixed blades and half in the moving blades. The pressure drops will
not be equal, however. They are greater for the fixed blades and greater for the high-
pressure than the low-pressure stages.
The moving blades of a reaction turbine are easily distinguishable from those of
an impulse turbine in that they are not symmetrical and, because they act partly as
nozzles, have a shape similar to that of the fixed blades, although curved in the opposite
direction. The schematic pressure line (Fig. 2.5) shows that pressure continuously drops
through all rows of blades, fixed and moving. The absolute steam velocity changes

90
within each stage as shown and repeats from stage to stage. Figure 2.6 shows a typical
velocity diagram for the reaction stage.
Figure 2.5 Three stages of reaction turbine indicating pressure and velocity
distribution
Pressure and enthalpy drop both in the fixed blade or stator and in the moving blade or
Rotor
Degree of Reaction =
or,
A very widely used design has half degree of reaction or 50% reaction and this is known
as Parson's Turbine. This consists of symmetrical stator and rotor blades.

91
Figure 2.7 The velocity diagram of reaction blading
The velocity triangles are symmetrical and we have
Energy input per stage (unit mass flow per second)
From the inlet velocity triangle we have,

92
Work done (for unit mass flow per second)
Therefore, the Blade efficiency
Reaction Turbine, Continued
Put then
For the maximum efficiency and we get
from which finally it yields
Figure 2.8 Velocity diagram for maximum efficiency

93
Absolute velocity of the outlet at this stage is axial (see figure 2.8). In this case,
the energy transfer
can be found out by putting the value of in the
expression for blade efficiency
is greater in reaction turbine. Energy input per stage is less, so there are more
number of stages.
Stage Efficiency and Reheat factor
The Thermodynamic effect on the turbine efficiency can be best understood by
considering a number of stages between two stages 1 and 2 as shown in Figure 25.2
Figure 2.9 Different stage of a steam turbine

94
The total expansion is divided into four stages of the same efficiency and pressure
ratio.
The overall efficiency of expansion is . The actual work during the expansion from 1
to 2 is
Reheat factor (R.F.)=
Problems
Qn. 1 In a De Laval turbine steam issues from the nozzle with a velocity of 1200 m/s.
The nozzle angle is 200, the mean blade velocity is 400 m/s, and the inlet and outlet
angles of blades are equal. The mass of steam flowing through the turbine per hour is
1000 kg.
Calculate:
(i) Blade angles.
(ii) Relative velocity of steam entering the blades.
(iii) Tangential force on the blades.
(iv) Power developed.
(v) Blade efficiency.
Take blade velocity co-efficient as 0.8.

95
Solution. Absolute velocity of steam entering the blade, C1 = 1200 m/s
o
Mean blade velocity, Cbl = 400 m/s
Blade velocity co-efficient, K = 0.8
Mass of steam flowing through the turbine, ms = 1000 kg/h.
Ref. Procedure of drawing the inlet and outlet triangles (LMS and LMN
respectively is as follows:)
Select a suitable scale and draw line LM to represent Cbl (= 400 m/s)
At point L make angle of 20o
1 =
(1200 m/s). Join MS produces M to meet the perpendicular drawn from S at P. Thus
inlet triangle is completed.
By measurement: 1
o
r30 , C 830 m/s  
o
30   
Now, 2 1r rC KC 0.8 830 664 m/s   
At point M make an angle of 30o
cut the length MN to represent
 0rC 664m/s . Join LN. Produce L to meet the perpendicular drawn from N at Q.
Thus outlet triangle is completed.
o
30  
(ii) Relative velocity of steam entering the blade, 1rC
1rC MS 830 m/s 
(iii) Tangential force on the blades:
Tangential force    1 0s w w
1000
m C C 1310 363.8 N
60 60
   


96
(iv) Power developed, P:
 1 2s w w bl
1000 1310 400
P m C C C kW 145.5 kW
60 60 1000

    

(v) Blade efficiency, bl
 1 2bl w w
bl 2 2
1
2C C C 2 400 1310
72.8%
C 1200
  
   
Qn. 2 A stage of a turbine with Parson‘s blading delivers dry saturated steam at 2.7 bar
from the fixed blades at 90 m/s. The mean blade height is 40 mm, and the moving blade
exit angle is 200. The axial velocity of steam is ¾ of the blade velocity at the mean
radius. Steam is supplied to the stage at the rate of 9000 kg/h. The effect of the blade tip
thickness on the annulus area can be neglected. Calculate:
(i) The wheel speed in r.p.m.;
(ii) The diagram power;
(iii) The diagram efficiency;
(iv) The enthalpy drop of the steam in this stage.
Solution. The velocity diagram is shown in Fig. 19.47 (…) and the blade wheel annulus
is represented in Fig. 19.47 (b).
Pressure = 2.7 bar, x = 1, C1 = 90 m/s, h = 40 mm = 0.04 m.
1 0
o
f f bl20 , C C 3/ 4C      = 9000 kg/h
Rate of steam supply
(i) Wheel speed, N:
o o
f bt 1C 3/ 4 C C sin 20 90sin 20 30.78 m/s   
blC 30.78 4/3 41.04 m/s  

97
The mass flow of steam is given by : f
2
C A
m
u

(where A is the annulus area, and u is the specific volume of the steam)
In this case, gu u at 2.7 bar = 0.6686 m3
/kg
s
9000 30.78
m
3600 0.6686
   or 29000 0.6686
A 0.054 m
3600 30.78

 

(where D is the mean diameter, and h is the mean blade height)
0.054 D 0.04    or
0.054
D 0.43 m
0.04
 

Also, bl
DN
C
60

 or
0.43 N
41.04
60
 

41.04 60
N 1823 r.p.m.
0.43

 


98
(ii) The diagram power:
Diagram power s blm C C
Now, 1 blC 2C cos C    o
2 90 cos20 41.04 128.1 m/s    
 Diagram power =
9000 128.1 41.04
13.14 kW
3600 1000
 


(iii) The diagram efficiency:
Rate of doing work per kg/s = blC C 128.1 41.04Nm/s  
Also, energy input to the moving blades per statge
0 1 1 1
2 2 2 2 22 2 2
r r 1 r r21 1 1
1
C C C C CC C C
C
2 2 2 2 2 2
 
        0r 1C C
Referring to ... we have
1
2 2 2
r 1 bl 1 blC C C 2C C cos   
2 2 o
90 41.04 2 90 41.04 cos20     
8100 1684.28 6941.69  
1rC 53.3 m/s 
Energy input =
2
2 53.3
90 6679.5 Nm per kg /s
2
 
 Diagram efficiency =
128.1 41.04
0.787 or 78.7%
6679.5


(iv) Enthalpy drop in the stage:
Enthalpy drop in the moving blades
0 1
2 2 2 2
r rC C 90 53.3
2.63 kJ / kg
2 2 1000
 
  

 0 1r rC C
 Total enthalpy drop per stage = 2 × 2.63 = 5.26 kJ/kg

99
Module III
GAS TURBINES
The gas turbines are mainly divided into two groups:
1. Constant pressure combustion gas turbine
(a) Open cycle constant pressure gas turbine
(b) Closed cycle constant pressure gas turbine
2. Constant volume combustion gas turbine.
In almost all the fields open cycle gas turbine plants are used. Closed cycle plants were
introduced at one stage because of their ability to burn cheap fuel
Merits of gas turbines
(I) Merits over IC engines:
1. The mechanical efficiency of a gas turbine (95%) is quite high as compared with
IC engines (85%0 since the IC engine has a large number of sliding parts.
2. A gas turbine does not require a fly wheel as the torque on the shaft is continuous
and uniform. Whereas a flywheel is a must in case of an IC engine.
3. The weight of gas turbine per H.P developed is less than that of an I.C engine.
4. The gas turbine can be driven at very high speeds (40000 r.p.m) whereas this is
not possible with I.C engines.
5.the components of gas turbine can be made lighter since the pressure used in it are
very low, say 5 bar compared with I.C engine say 60 bar.

100
6. In the gas turbine the ignition and lubrication systems are much simpler as
compared with I.C engines.
7. Cheaper fuels such as par affine type, residue oils or powdered coal can be used
whereas special grade fuels are employed in petrol engine to check knocking or
pinking.
8. The exhaust from gas turbine is less polluting comparatively since excess air is
used for combustion.
9. Because of low specific weight the gas turbines are particularly suitable for use in
aircrafts.
Demerits of gas turbines
1. The thermal efficiency of a simple turbine cycle is low (15 to 20%) as
compared with I.C engines (25 to 30%).
2. With wide operating speeds the fuel control is comparatively difficult.
3. Due to higher operating speeds it is imperative to have a speed reduction
device.
4. It is difficult to start a gas turbine as compared to an I.C engine.
5. The gas turbine valves need a special cooling system.
6. One of the main demerits of a gas turbine is its very poor thermal efficiency at
part loads, as the quantity of air remains same irrespective of load, and output
is reduced by reducing the quantity of fuel supplied.
7. Owing to the use of nickel chromium alloy, the manufacture of the blades is
difficult and costly.

101
8. For the same output of the gas turbine produces five times exhaust gases than
I.C engine
CONSTANT PRESSURE COMBUSTION GAS TURBINES
Open Cycle Gas Turbines
Refer Figure. The fundamental gas turbine unit is one operating on the open
cycle in which a rotary compressor and a turbine are mounted on a common shaft. Air is
drawn into the compressor and after compression passes to a combustion chamber.
Energy is supplied in the combustion chamber by spraying fuel into the air stream, and
the resulting hot gases expand through the turbine to the atmosphere. In order to achieve
net work output from the unit, the turbine must develop more gross work output than is
required to drive the compressor and to overcome mechanical losses in the drive. The
products of combustion coming out from the turbine are exhausted to the atmosphere as
they cannot be used any more. The working fluids (air and fuel) must be replaced
continuously as they are exhausted into the atmosphere.
If pressure loss in the combustion chamber is neglected, this cycle may be
drawn on a T-s diagram as shown in Figure
• 1-2 represents: irreversible adiabatic compression.

102
• 2-3 represents: constant pressure heat supply in the combustion chamber.
• 3-4 represents: irreversible adiabatic expansion.
• 1-2 represents: ideal isentropic compression.
• 3-4 represents: ideal isentropic expansion.
Assuming change in kinetic energy between the various points in the cycle to
be negligibly mall compared with enthalpy changes and then applying the flow equation
to each part of cycle, or unit mass, we have
Work input (compressor) = cp ( T2 - T1 )
Heat supplied (combustion chamber) = cp ( T3 - T2 )
Work output (turbine) = cp ( T3 - T4 )
Net work output = Work output - Work input
= cp (T3 - T4) - cp(T2 - T1)
and thermal
Net work output
Heat supplied
 

103
=
   
 
p 3 4 p 2 1
p 3 2
c T T c T T
c T T
   

Compressor isentropic efficiency, comp
=
Work input required in isentropic compression
Actual work required
=
 
 
p 2 1 2 1
2 1p 2 1
c T T T T
T Tc T T
 

 
... (1)
Turbine isentropic efficiency, turbine
=
Actual work output
Isentropic work output
=
 
 
p 3 4
3 4
p 3 4 3 4
c T T T T
c T T T T
 

 
... (2)
Note. With the variation in temperature, the value of the specific heat of a real
gas varies, and also in the open cycle, the specific heat of the gases in the combustion
chamber and in turbine is different from that in the compressor because fuel has been
added and a chemical change has taken place. Curves showing the variation of cp with
temperature and air/fuel ratio can be used and a suitable mean value of cp and hence 
can be found out. It is usual in gas turbine practice to assume fixed mean value of cp and
 for the expansion process, and fixed mean values of cp and  for the compression
process. In an open cycle gas turbine unit the mass flow of gases in turbine is greater
than that in compressor due to mass of fuel burned, but it is possible to neglect mass of
fuel, since the air/ fuel ratios used are large. Also, in many cases, air is bled from the
compressor for cooling purposes, or in the case of air-craft at high altitudes, bled air is
used for de-icing and cabin air-conditioning. This amount of air bled is approximately
the same as the mass of fuel injected therein.

104
Methods for Improvement of Thermal Efficiency of Open Cycle Gas Turbine Plant
The following methods are employed to increase the specific output and
thermal efficiency of the plant :
1. Intercooling 2. Reheating 3. Regeneration.
1. Intercooling. A compressor in a gas turbine cycle utilises the major
percentage of power developed by the gas turbine. The work required by the compressor
can b'e reduced by compressing the air in two stages and incorporating an intercooler
between the two as shown in Figure. The corresponding T-s diagram for the unit is
shown in figure. The actual processes take place as follows:
1-2' ... L.P. (Low pressure) compression
2'-3 ... Intercooling
3-4' ... H.P. (High pressure) compression
4'-5 ... C.C. (Combustion chamber)-heating
5-6' ... T (Turbine)-expansion
The ideal cycle for this arrangement is 1-2-3-4-5-6 ; the compression process

105
without intercooling is shown as 1-L' in the actual case, and 1-L in the ideal isentropic
case.
Now,
Work input (with intercooling)
=    p 2 1 p 4 3c T T c T T    ... (3)
Work input (without intercooling)
=      p L 1 p 2 1 p L 2c T T c T T c T T        ... (4)
By comparing equation (4) with equation (3) it can be observed that the work
input with intercooling is less than the work input with no intercooling, when cp (T4 -
T3) is less than cp(TL - T2). This is so if it is assumed that isentropic efficiencies of the
two compressors, operating separately, are each equal to the isentropic efficiency of the
single compressor which would be required if no intercooling were used. Then (T4- T3)
< (TL - T2) since the pressure lines diverge on the T-s diagram from left to the right.
Again, work ratio =
Net work output
Gross work output
=
Work of expansion - Work of compression
Work of expansion

106
From this we may conclude that when the compressor work input is reduced
then the work ratio is increased.
However the heat supplied in the combustion chamber when intercooling is
used in the cycle, is given by,
Heat supplied with intercooling = cp(T5 - T4)
Also the heat supplied when intercooling is not used, with the same maximum
cycle temperature T5, is given by
Heat supplied without intercooling = cp (T5 - TL)
Thus, the heat supplied when intercooling is used is greater than with no
intercooling. Although the net work output is, increased by intercooling it is found in
general that the increase in heat to be supplied causes the thermal efficiency to decrease.
When intercooling is used a supply of cooling water must be readily available. The
additional bulk of the unit may offset the advantage to be gained by increasing the work
ratio.
2. Reheating. The output of a gas turbine can be amply improved by expanding
the gases in two stages with a reheater between the two as shown in figure. The H.P.
turbine drives the compressor and the L.P. turbine provides the useful power output. The
corresponding T-s diagram is shown in figure. The line 4-L represents the expansion in
the L.P. turbine if reheating is not employed.
Neglecting mechanical losses the work output of the H.P. turbine must be
exactly equal to the work input required for the compressor i.e., cpa (T2 - T1) = cpg (T3 -
T4)

107
The work output (net output) of L.P. turbine is given by,
Net work output (with reheating) = cpg (T5 - T6)
and Net work output (without reheating) = cpg (T4 - TL)
Since the pressure lines diverge to the right on T-s diagram it can be seen that
the temperature difference (T5 - T6) is always greater than (T4 - TL), so that reheating
increases the network output.
Although net work is increased by reheating the heat to be supplied is also
increased, and the net effect can be to reduce the thermal efficiency
Heat supplied = cpg (T3 - T2) + cpg (T5 - T4).

108
Note. cpa and cpg stand for specific heats of air and gas respectively at constant
pressure.
3. Regeneration. The exhaust gases from a gas turbine carry a large quantity of
heat with them since their temperature is far above the ambient temperature. They can be
used to heat the air coming from the compressor thereby reducing the mass of fuel
supplied in the combustion chamber. figure shows a gas turbine plant with a regenerator.
The corresponding T-s diagram is shown in figure. 2-3 represents the heat flow into the
compressed air during its passage through the heat exchanger and 3-4 represents the heat
taken in from the combustion of fuel. Point 6 represents the temperature of exhaust gases
at discharge from the heat exchanger. The maximum temperature to which the air could
be heated in the heat exchanger is ideally that of exhaust gases, but less than this is
obtained in practice because a temperature gradient must exist for an unassisted transfer
of energy. The effectiveness of the heat exchanger is given by:
Effectiveness,
Increase in enthalpy per kg of air
Available increase in enthalpy per kg of air
 
= 3 2
5 2
(T -T )
(T T )

 
... (25.5)

109
(assuming cpa and cpg to be equal)
A heat exchanger is usually used in large gas turbine units for marine
propulsion or industrial power.
Effect of Operating Variables on Thermal Efficiency
The thermal efficiency of actual open cycle depends on the following
thermodynamic variables:
(i) Pressure ratio
(ii) Turbine inlet temperature (T3)
(iii) Compressor inlet temperature (T1)
(iv) Efficiency of the turbine ( turbine )
(v) Efficiency of the compressor ( comp )
Effect of turbine inlet temperature and pressure ratio :
If the permissible turbine inlet-temperature (with the other variables being
constant) of an open cycle gas turbine power plant is increased its thermal efficiency is
amply improved. A practical limitation to increasing the turbine inlet temperature,
however, is the ability of the material available for the turbine blading to withstand the
high rotative and thermal stresses.

110
Refer figure. For a given turbine inlet temperature, as the pressure ratio
increases, the heat supplied as well as the heat rejected are reduced. But the ratio of
change of heat supplied is not the same as the ratio of change heat rejected. As a
consequence, there exists an optimum pressure ratio producing maximum thermal
efficiency for a given turbine inlet temperature.
As the pressure ratio increases, the thermal efficiency also increases until it
becomes maximum and then it drops off with a further increase in pressure ratio.
Further, as the turbine inlet temperature increases, the peaks of the curves flatten out
giving a greater range of ratios of pressure optimum efficiency.
.

111
Following particulars are worthnoting :
Gas temperatures Efficiency (gas turbine)
550 to 600°C 20 to 22%
900 to 1000°C 32 to 35%
Above 1300°C more than 50%
Effect of turbine and compressor efficiencies:
Refer figure. The thermal efficiency of the actual gas turbine cycle is very
sensitive to variations in the efficiencies of the compressor and turbine. There is a
particular pressure ratio at which maximum efficiencies occur. For lower efficiencies,
the peak of the thermal efficiency occurs at lower pressure ratios and vice versa.

112
Effect of compressor inlet temperature:
Refer figure. With the decrease in the compressor inlet temperature there is
increase in thermal efficiency of the plant. Also the peaks of thermal efficiency occur at
high pressure ratios and the curves become flatter giving thermal efficiency over a wider
pressure ratio range.

113
Closed Cycle Gas Turbine (Constant pressure or joule cycle).
Figure shows a gas turbine operating on a constant pressure cycle in which the
closed system consists of air behaving as an ideal gas. The various operations are as
follows: Refer figures.
Operation 1-2: The air is compressed isentropically from the lower pressure P1
to the upper pressure P2, the temperature rising from T1 to T2.
No heat flow occurs.
Operation 2-3: Heat flow into the system increasing the volume from V2 to V3
and temperature from T2 to T3 whilst the pressure remains
constant at P2.
Heat received = mcp (T3 - T2).
Operation 3-4: The air is expanded isentropically from P2 to P1, the
temperature falling from T3 to T4. No heat flow occurs.
Operation 4-1 : Heat is rejected from the system as the volume decreases from
V4 to V1 and the temperature from T4 to T1 whilst the pressure
remains constant at P1. Heat rejected = mcp (T4 - T1)
air-standard
Work done
Heat received
 
=
Heat received/cycle - Heat rejected/cycle
Heat received/cycle
=
   
 
p 3 2 p 4 1 4 1
p 3 2 3 2
mc T T mc T T T T
1
mc T T T T
   
 
 
Now, from isentropic expansion
1
2 2
1 1
T p
T p

 
  
 

114
 p
2 1 r
1
T T
 


, where rp = Pressure ratio
Similarly
1
3 2
4 1
T p
T p

 
  
 
or
 p
3 4 r
1
T T
 



115

     p p p
4 1
air standard 1 1 1
4 r 1 r r
T T 1
1 1
T T
   
  

    

... (6)
The expression shows that the efficiency of the ideal joule cycle increases with
the pressure ratio. The absolute limit of pressure is determined by the limiting
temperature of the material of the turbine at the point at which this temperature is
reached by the compression process alone, no further heating of the gas in the
combustion chamber would be permissible and the work of expansion would ideally just
balance the work of compression so that no excess work would be available for external
use.
Now we shall prove that the pressure ratio for maximum work is a function of
the limiting temperature ratio.

116
Work output during the cycle
= Heat received/cycle - Heat rejected/cycle
= mcp (T3 - T2) - mcp (T4 - T1) = mcp (T3 - T4) - mcp (T2 - T1)
= mcpT3) 4 2
1
3 1
T T
1 T 1
T T
   
     
  
In case of a given turbine the minimum temperature T1 and the maximum
temperature T3 are prescribed, T1 being the temperature of the atmosphere and T3 the
maximum temperature which the metals of turbine would withstand. Consider the
specific heat at constant pressure cp to be constant. Then,
Since,
 p
1
3 2
r
4 1
T T
T T


 
Using the constant ‗z‘ =
1

,
we have, work output/cycle W = K  z
3 1 pz
p
1
T 1 T r 1
r
  
    
    
Differentiating with respect to rp
 
 z 1
3 1 p
p p
dW z
K T T zr
dr r z 1

 
   
  
= 0 for a maximum

    3
1 pz 1
p
zT
T z r z 1
r

 
 2z 3
p
1
T
r
T

    
 2 1
1/ 2z
p 3 1 p 3 1r T /T i.e.,r T /T


 
Thus the pressure ratio for maximum work is a function of the limiting
temperature ratio. Fig. 25.16 shows an arrangement of closed cycle stationary gas
turbine plant in which air is continuously circulated. This ensures that the air is not

117
polluted by the addition of combustion waste product, since the heating of air is carried
out in the form of heat exchanger shown in the diagram as air heater. The air exhausted
from the power turbine is cooled before readmission to L.P. compressor. The various
operations as indicated on T-s diagram (Fig. 25.17) are as follows:
Operation 1-2': Air is compressed from P1 to Px in the L.P. compressor.
Operation 2'-3: Air is cooled in the intercooler at constant pressure Px.
Operation 3-4' : Air is compressed in the H.P. compressor from Px to P2
Operation 4'-5 : High pressure air is heated at constant pressure by exhaust
gases from power turbine in the heat exchanger to T5
Operation 5-6 : High pressure air further heated at constant pressure to the
maximum temperature T6 by an air heater (through external
combustion).
Operation 6-7' : The air is expanded in the H.P. turbine from P2 to Px
producing work to drive the compressor.
Operation 7'-8 : Exhaust air from the H.P. turbine is heated at constant
pressure in the air heater (through external combustion) to the
maximum temperature T8(= T 6)
Operation 8-9' : The air is expanded in the L.P. turbine from Px to P1
producing energy for a flow of work externally.
Operation 9'-10 : Air from L.P. turbine is passed to the heat exchanger where
energy is transferred to the air delivered from the H.P.
compressor. The temperature of air leaving the heat
exchanger and entering the cooler is T10.
Fig. 25.17. T-s diagram for the plant.
Operation 10-11 : Air cooled to Tl by the cooler before entering the L.P.
compressor.

118
The energy balance for the whole plant is as follows:
Q1 + Q2 - Q3 - Q4 = W
In a closed cycle plant, in practice, the control of power output is achieved by
varying the mass flow by the use of a reservoir in the circuit. The reservoir maintains the
design pressure and temperature and therefore achieves an approximately constant level
of efficiency for varying loads. In this cycle since it is closed, gases other than air with
favourable properties can be used; furthermore it is possible to burn solid fuels in the
combustion heaters. The major factor responsible for inefficiency in this cycle is the
large irreversible temperature drop which occurs in the air heaters between the furnace

119
and circulating gas.
Note 1. In a closed cycle gas turbines, although air has been extensively used,
the use of ‗helium‘ which though of a lower density, has been inviting the attention of
manufacturers for its use, for large output gas turbine units. The specific heat of helium
at constant pressure is about ‗five times‘ that of air, therefore for each kg mass flow the
heat drop and hence energy dealt with in helium machines is nearly five times of those
in case of air. The surface area of the heat exchanger for helium can be kept as low as
1/3 of that required for gas turbine plant using air as working medium. For the same
temperature ratio and for the plants of the same output the cross-sectional area required
for helium is much less than that for air. It may therefore be concluded that the size of
helium unit is considerably small comparatively.
2. Some gas turbine plants work on a combination of two cycles the open cycle
and the closed cycle. Such a combination is called the semi-closed cycle. Here a part of
the working fluid is confined within the plant and another part flows from and to
atmosphere.
Merits and Demerits of Closed Cycle Gas Turbine Over Open Cycle Gas Turbine
Merits of closed cycle:
1. Higher thermal 2. Reduced size
3. No contamination 4. Improved heat transmission
5. Improved part load efficiency 6. Lesser fluid friction
7. No loss of working medium 8. Greater output
9. Inexpensive fuel.
Demerits of closed cycle:
1. Complexity
2. Large amount of cooling water is required. This limits its use to stationary
installation or marine use where water is available in abundance.
3. Dependent system.
4. The weight of the system per H.P. developed is high comparatively,

120
therefore not economical for moving vehicles.
5. Requires the use of a very large air heater.
Problems
Qn. In a closed cycle gas turbine there is two-stage compressor and a two stage turbine.
All the components are mounted on the same shaft. The pressure and temperature at the
inlet of the first-stage compressor are 1.5 bar and 20°C. The maximum cycle
temperature and pressure are limited to 750o
C and 6 bar. A perfect intercooler is used
between the two-stage compressors and a reheater is used between the two turbines.
Gases are heated in the reheater to 750o
C before entering into the L.P. turbine. Assuming
the compressor and turbine efficiencies as 0.82, calculate :
(i) The efficiency of the cycle without regenerator.
(ii) The efficiency of the cycle with a regenerator whose effectiveness is 0.70.
(iii) The mass of the fluid circulated if the power developed by the plant is 350
kW,
The working fluid used in the cycle is air.
p = 1,005 kj/kg K.
Solution.
Given : T1 = 20 = 278 = 293 K, T5 = T7 = 750 + 273 = 1023 K,
P1 = 1.5 bar, P2 = 6 bar, compressor turbine 0.82   
Effectiveness of regenerator,  = 0.70, Power developed, P = 350 kW.
For air : cp

121
As per given conditions: 1 3 2 4T T , T T  
1
2 2
1 1
T p
T p

 
  
 
and x 1 2p p p 1.5 6 3bar   
Now
1 1.4 1
1.42
2 1
1
p 3
T T 293 357 K
p 1.5
 
   
       
  
 
2 1
compressor L.P.
2 1
T T
T T

 
 
2
357 293
0.82
T 293


 
2 2 4
357 293
T 293 371 K ie, T T 371K
0.82
      
Now,
1 1.4 1
1.4
5 5 2
6 6 x
T p p
T p p
 
   
    
  
 5 2 6 xp p ; p p 
0.286
6
1023 6
1.219
T 3
 
  
 

122
6
1023
T 839K
1.219
  
5 6
turbine(H.P.)
5 6
T T
T T

 

61023 T
0.82
1023 839



 6T 1023 0.82 1023 839 872K    
   8 6 turbine H.P. turbine L.P.T T 872Kas     
and 7 5T T 1023 K 
Effectiveness of regenerator, 4
8 4
T T
T T

 
 
Where T‘ is the temperature of air coming out of regenerator.
T 371
0.70
872 371

 

i,e  T 0.70 872 371 371 722K    
Net work available,        net T L.P. T L.P. C H.P. C L.P.W W W W W      
   
   T LP C L.P.2 W W  
 
as the work developed by each turbine is came and
work absorbed by each compressor is same.
   net p 5 6 2 1W 2c T T T T     
  
   2 1.005 1023 371 1023 872       = 807 kJ/kg of air.
Heat supplied per kg of air without regenerator
   p 5 4 p 7 6c T T c T T   
   1.005 1023 371 1023 872 807 kJ / kg of air      

123
Heat supplied per kg of air with regenerator
   p 5 p 7 6c T T c T T   
   1.005 1023 722 1023 872      = 454.3 kJ/kg
(i) thermal(without regenerator)
146.73
0.182 or 18.2%
807
  
(ii)  thermal with regenerator
146.73
0.323 or 32.3%
454.3
 
(iii) Mass of fluid circulated, m:
Power developed P 146.73 m kW 
350 146.73 m  
ie,
350
m 2.38 kg /s
146.73
 
ie, Mass of fluid circulated = 2.38 kg/s.
COMPRESSORS
The density of the fluids changes with a change in pressure as well as in
temperature as they pass through the machines. These machines are called 'compressible
flow machines' and more popularly 'turbo machines'. Apart from the change in density
with pressure, other features of compressible flow, depending upon the regimes, are also
observed in course of flow of fluids through turbo machines. Therefore, the basic
equation of energy transfer (Euler's equation, as discussed before) along with the
equation of state relating the pressure, density and temperature of the working fluid and
other necessary equations of compressible flow, are needed to describe the performance
of a turbo machine. However, a detailed discussion on all types of turbo machines is
beyond the scope of this book. We shall present a very brief description of, compressors,
in this module. In practice two kinds of compressors: centrifugal and axial are generally
in use.

124
Centrifugal Compressors
A centrifugal compressor is a radial flow rot dynamic fluid machine that uses
mostly air as the working fluid and utilizes the mechanical energy imparted to the
machine from outside to increase the total internal energy of the fluid mainly in the form
of increased static pressure head.
During the second world war most of the gas turbine units used centrifugal
compressors. Attention was focused on the simple turbojet units where low power-plant
weight was of great importance. Since the war, however, the axial compressors have
been developed to the point where it has an appreciably higher isentropic efficiency.
Though centrifugal compressors are not that popular today, there is renewed interest in
the centrifugal stage, used in conjunction with one or more axial stages, for small
turbofan and turboprop aircraft engines.
A centrifugal compressor essentially consists of three components.
1. A stationary casing
2. A rotating impeller as shown in Fig.1 (a) which imparts a high velocity to the
air. The impeller may be single or double sided as show in Fig.1 (b) and (c), but
the fundamental theory is same for both.
3. A diffuser consisting of a number of fixed diverging passages in which the air
is decelerated with a consequent rise in static pressure.
Figure 1(a)

125
(b) (c) (d)
Figure 1 Schematic views of a centrifugal compressor
Figure 2 Single entry and single outlet centrifugal compressor
Figure 2 is the schematic of a centrifugal compressor, where a single entry radial
impeller is housed inside a volute casing.
Principle of operation: Air is sucked into the impeller eye and whirled outwards at
high speed by the impeller disk. At any point in the flow of air through the impeller the
centripetal acceleration is obtained by a pressure head so that the static pressure of the
air increases from the eye to the tip of the impeller. The remainder of the static pressure
rise is obtained in the diffuser, where the very high velocity of air leaving the impeller
tip is reduced to almost the velocity with which the air enters the impeller eye.

126
Usually, about half of the total pressure rise occurs in the impeller and the other
half in the diffuser. Owing to the action of the vanes in carrying the air around with the
impeller, there is a slightly higher static pressure on the forward side of the vane than on
the trailing face. The air will thus tend to flow around the edge of the vanes in the
clearing space between the impeller and the casing. This results in a loss of efficiency
and the clearance must be kept as small as possible. Sometimes, a shroud attached to the
blades as shown in (Figure.2 ) may eliminate such a loss, but it is avoided because of
increased disc friction loss and of manufacturing difficulties.
The straight and radial blades are usually employed to avoid any undesirable
bending stress to be set up in the blades. The choice of radial blades also determines that
the total pressure rise is divided equally between impeller and diffuser.
Before further discussions following points are worth mentioning for a centrifugal
compressor.
(i) The pressure rise per stage is high and the volume flow rate tends to be low. The
pressure rise per stage is generally limited to 4:1 for smooth operations.
(ii) Blade geometry is relatively simple and small foreign material does not affect much
on operational characteristics.
(iii) Centrifugal impellers have lower efficiency compared to axial impellers and when
used in aircraft engine it increases frontal area and thus drags. Multistage is also difficult
to achieve in case of centrifugal machines.
Surging and choking
Before describing a typical set of characteristics, it is desirable to consider what
might be expected to occur when a valve placed in the delivery line of the compressor
running at a constant speed, is slowly opened. When the valve is shut and the mass flow
rate is zero, the pressure ratio will have some value. Figure3. Indicates a theoretical
characteristics curve ABC for a constant speed.

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The centrifugal pressure head produced by the action of the impeller on the air
trapped between the vanes is represented by the point 'A' in Figure 3. As the valve is
opened, flow commences and diffuser begins to influence the pressure rise, for which
the pressure ratio increases. At some point 'B', efficiency approaches its maximum and
the pressure ratio also reaches its maximum. Further increase of mass flow will result in
a fall of pressure ratio. For mass flows greatly in excess of that corresponding to the
design mass flow, the air angles will be widely different from the vane angles and
breakaway of the air will occur. In this hypothetical case, the pressure ratio drops to
unity at 'C' , when the valve is fully open and all the power is absorbed in overcoming
internal frictional resistances.
In practice, the operating point 'A' could be obtained if desired but a part of the
curve between 'A' and 'B' could not be obtained due to surging. It may be explained in
the following way. If we suppose that the compressor is operating at a point 'D' on the
part of characteristics curve (Figure 3.) having a positive slope, then a decrease in mass
flow will be accompanied by a fall in delivery pressure. If the pressure of the air
downstream of the compressor does not fall quickly enough, the air will tend to reverse
its direction and will flow back in the direction of the resulting pressure gradient. When
this occurs, the pressure ratio drops rapidly causing a further drop in mass flow until the
point 'A' is reached, where the mass flow is zero. When the pressure downstream of the
compressor has reduced sufficiently due to reduced mass flow rate, the positive flow
becomes established again and the compressor picks up to repeat the cycle of events
which occurs at high frequency.
This surging of air may not happen immediately when the operating point moves
to the left of 'B' because the pressure downstream of the compressor may at first fall at a
greater rate than the delivery pressure. As the mass flow is reduced further, the flow
reversal may occur and the conditions are unstable between 'A' and 'B'. As long as the
operating point is on the part of the characteristics having a negative slope, however,
decrease in mass flow is accompanied by a rise in delivery pressure and the operation is
stable.

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Figure 3. The theoretical characteristic curve
There is an additional limitation to the operating range, between 'B' and 'C'. As
the mass flow increases and the pressure decreases, the density is reduced and the radial
component of velocity must increase. At constant rotational speed this means an increase
in resultant velocity and hence an angle of incidence at the diffuser vane leading edge.
At some point say 'E', the position is reached where no further increase in mass flow can
be obtained no matter how wide open the control valve is. This point represents the
maximum delivery obtainable at the particular rotational speed for which the curve is
drawn. This indicates that at some point within the compressor sonic conditions have
been reached, causing the limiting maximum mass flow rate to be set as in the case of
compressible flow through a converging diverging nozzle. Choking is said to have taken
place. Other curves may be obtained for different speeds, so that the actual variation of
pressure ratio over the complete range of mass flow and rotational speed will be shown
by curves such as those in Figure. 4.. The left hand extremities of the constant speed
curves may be joined up to form surge line, the right hand extremities indicate choking
(Figure 4.).

129
Figure 4. Variations of pressure ratio over the complete range of
mass flow for different rotational speeds
AXIAL FLOW COMPRESSORS
The basic components of an axial flow compressor are a rotor and stator, the
former carrying the moving blades and the latter the stationary rows of blades. The
stationary blades convert the kinetic energy of the fluid into pressure energy, and also
redirect the flow into an angle suitable for entry to the next row of moving blades. Each
stage will consist of one rotor row followed by a stator row, but it is usual to provide a
row of so called inlet guide vanes. This is an additional stator row upstream of the first
stage in the compressor and serves to direct the axially approaching flow correctly into
the first row of rotating blades. For a compressor, a row of rotor blades followed by a
row of stator blades is called a stage. Two forms of rotor have been taken up, namely
drum type and disk type. A disk type rotor illustrated in Figure 5. The disk type is used
where consideration of low weight is most important. There is a contraction of the flow
annulus from the low to the high pressure end of the compressor. This is necessary to
maintain the axial velocity at a reasonably constant level throughout the length of the

130
compressor despite the increase in density of air. Figure6. Illustrate flow through
compressor stages. In an axial compressor, the flow rate tends to be high and pressure
rise per stage is low. It also maintains fairly high efficiency.
Figure 5. Disk type axial flow compressor
The basic principle of acceleration of the working fluid, followed by diffusion to
convert acquired kinetic energy into a pressure rise, is applied in the axial compressor.
The flow is considered as occurring in a tangential plane at the mean blade height where
the blade peripheral velocity is U . This two dimensional approach means that in general
the flow velocity will have two components, one axial and one peripheral denoted by
subscript w , implying a whirl velocity. It is first assumed that the air approaches the
rotor blades with an absolute velocity, , at and angle to the axial direction. In
combination with the peripheral velocity U of the blades, its relative velocity will be
at and angle as shown in the upper velocity triangle. After passing through the
diverging passages formed between the rotor blades which do work on the air and
increase its absolute velocity, the air will emerge with the relative velocity of at
angle which is less than . This turning of air towards the axial direction is, as
previously mentioned, necessary to provide an increase in the effective flow area and is

131
brought about by the camber of the blades. Since is less than due to diffusion,
some pressure rise has been accomplished in the rotor. The velocity in combination
with U gives the absolute velocity at the exit from the rotor at an angle to the
axial direction. The air then passes through the passages formed by the stator blades
where it is further diffused to velocity at an angle which in most designs equals to
so that it is prepared for entry to next stage. Here again, the turning of the air towards
the axial direction is brought about by the camber of the blades.
Figure 6. Flow through stages
COMUSTION CHAMBER
Design Factors
Over a period of five decades, the basic factors influencing the design of
combustion systems for gas turbines have not changed, although recently some new
requirements have evolved. The key issues may be summarized as follows.
1. The temperature of the gases after combustion must be comparatively controlled to
suit the highly stressed turbine materials. Development of improved materials and

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methods of blade cooling, however, has enabled permissible combustor outlet
temperatures to rise from about 1100K to as much as 1850 K for aircraft applications.
2. At the end of the combustion space the temperature distribution must be of known
form if the turbine blades are not to suffer from local overheating. In practice, the
temperature can increase with radius over the turbine annulus, because of the strong
influence of temperature on allowable stress and the decrease of blade centrifugal stress
from root to tip.
3. Combustion must be maintained in a stream of air moving with a high velocity in the
region of 30-60 m/s, and stable operation is required over a wide range of air/fuel ratio
from full load to idling conditions. The air/fuel ratio might vary from about 60:1 to
120:1 for simple cycle gas turbines and from 100:1 to 200:1 if a heat-exchanger is used.
Considering that the stoichiometric ratio is approximately 15:1, it is clear that a high
dilution is required to maintain the temperature level dictated by turbine stresses
4. The formation of carbon deposits ('coking') must be avoided, particularly the hard
brittle variety. Small particles carried into the turbine in the high-velocity gas stream can
erode the blades and block cooling air passages; furthermore, aerodynamically excited
vibration in the combustion chamber might cause sizeable pieces of carbon to break free
resulting in even worse damage to the turbine.
5. In aircraft gas turbines, combustion must be stable over a wide range of chamber
pressure because of the substantial change in this parameter with a altitude and forward
speed. Another important requirement is the capability of relighting at high altitude in
the event of an engine flame-out.
6. Avoidance of smoke in the exhaust is of major importance for all types of gas turbine;
early jet engines had very smoky exhausts, and this became a serious problem around
airports when jet transport aircraft started to operate in large numbers. Smoke trails in
flight were a problem for military aircraft, permitting them to be seen from a great
distance. Stationary gas turbines are now found in urban locations, sometimes close to
residential areas.

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7. Although gas turbine combustion systems operate at extremely high efficiencies, they
produce pollutants such as oxides of nitrogen , carbon monoxide (CO) and
unburned hydrocarbons (UHC) and these must be controlled to very low levels. Over the
years, the performance of the gas turbine has been improved mainly by increasing the
compressor pressure ratio and turbine inlet temperature (TIT). Unfortunately this results
in increased production of . Ever more stringent emissions legislation has led to
significant changes in combustor design to cope with the problem.
Probably the only feature of the gas turbine that eases the combustion designer's
problem is the peculiar interdependence of compressor delivery air density and mass
flow which leads to the velocity of the air at entry to the combustion system being
reasonably constant over the operating range.
For aircraft applications there are the additional limitations of small space and
low weight, which are, however, slightly offset by somewhat shorter endurance
requirements. Aircraft engine combustion chambers are normally constructed of light-
gauge, heat-resisting alloy sheet (approx. 0.8 mm thick), but are only expected to have a
life of some 10000 hours. Combustion chambers for industrial gas turbine plant may be
constructed on much sturdier lines but, on the other hand, a life of about 100000 hours is
required. Refractory linings are sometimes used in heavy chambers, although the
remarks made above regarding the effects of hard carbon deposits breaking free apply
with even greater force to refractory material.
The combustion process
The overall air fuel ratio is in the region of 100:1, but the stoichiometric ratio is
approx. 15;1, so the air should be introduced in stages. In the first stage, called primary
zone 15 to 20% of the air is introduced around the jet fuel, to provide the necessary high
temperature for rapid combustion in the second stage called secondary zone, 305 of the
total air is introduced through holes in the flame tube to complete the combustion.In the
third stage called tertiary or dilution zone, the remaining air is mixed with the products
of combustion to cool them down to the temperature required at inlet to the turbine.

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Sufficient turbulence must be promoted so that the hot cold streams can thoroughly
mixed to give the desired outlet temperature distribution with no hot streaks which
would damage the turbine blades.
Figure 7. Combustion chamber with swirl vanes
Figure7. Indicates the schematic of a combustion chamber. The primary air is
introduced through twisted radial vanes known as 'swirl vanes‘ that result in a vortex
motion with a low-pressure region along the axis of the chamber. The fuel is injected in
the same direction of air. The vortex motion is some time enhanced by injecting the
secondary air through short tangential chutes in the flame tube. The burning gases tend
to flow towards the region of low pressure and some portion of them swept round
towards the jet of fuel as indicated by the arrow. The objective is to obtain a stable
flame.
There are mainly Three types of combustion chambers
1. Cylindrical Combustion Chamber.
In case of aircraft practice these chambers are placed around the shaft connecting the
compressor and the turbine as shown in figure. Each chamber is supplied with separate
stream of air from compressor and it is having its own fuel jet from a common supply
line. This type of layout is particularly suitable for gas turbines using centrifugal
compressor because the air stream is already divided by the diffuser vanes

135
2. Annular Combustion Chamber
This type of combustion chamber is more suitable for use with axial flow compressors
which surrounds the rotor shaft. Although large number of fuel jets can be employed but
is very difficult to obtain an even air fuel distribution.
2. Industrial Combustion Chamber
The space occupied by the combustion system of an industrial gas turbine is usually only
of secondary importance and combustion is carried out in one or two large cylindrical
chambers feeding the turbine via a scroll or volute. The chambers are often situated at
one end of the heat exchangers when this computer is included in the cycle.

136
Combustion chamber performance
The main factors in assessing the combustion chamber performance are
(a) Pressure loss
(b) Combustion efficiency
(c) Stability limits
(d) Combustion intensity
Pressure loss
The combustion chamber pressure loss is due to two distinct causes,
(a) Hot loss or fundamental loss
(b) Cold loss (losses due to skin friction and turbulence
The stagnation pressure loss associated with hot loss arises because of an increase
in temperature implies a decrease in density and consequently an increase in velocity
and momentum of the stream. A pressure force (∆p x A) must be present to impart the
increase in momentum.
One of the standard idealized cases considered in gas dynamics is that of a
heated gas stream flowing without friction in a duct of constant cross-sectional area. The
stagnation pressure ratio in this situation can, for any given temperature pressure rise,
can be predicted with the aid of the Rayleigh-line functions. When the velocity is low
and the fluid flow can be treated as incompressible, a simple equation for the pressure
drop will be,
– ρ – 1)
The pressure loss due to skin friction and turbulence is called cold loss. The
stagnation drop due to cold loss is higher than that due to hot loss. Turbulence is created
by the devices ,ie , swirl vanes, to stabilize the flame. In addition there is the turbulence
induced by the jets of secondary and dilution air. It is for good mixing of the secondary

137
air with the burning gases to avoid chilling, and for good mixing of the dilution air to
avoid hot streaks.
The more effective the mixing the higher the pressure loss.
The overall pressure can be expressed by using the term pressure loss factor.
P.L.F = pressure drop in combustion chamber ÷ inlet dynamic head.
ρ
Where ρ = inlet density
m = mass flow of air
Am = maximum cross-sectional area of the
chamber
–1]
Combustion efficiency

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The efficiency of a combustion process may be found from a chemical
analysis of the combustion products. Knowing the air fuel ratio used and the
proportion of incompletely burnt constituents, it is possible to calculate the ratio of
the actual energy released to the theoretical quantity available. This approach via
chemical analysis is not easy, because not only is it difficult to obtain truly
representative samples from the high velocity stream, but also owing to the high air
fuel ratios employed in gas turbines, the unburnt constituents to be measured are a
very small proportion of the whole sample. Ordinary gas analysis apparatus, such as
the Orsat, is not adequate and much more elaborate techniques have had to be
developed.
If an overall combustion efficiency is all that is required, however, and
not an investigation of the state of the combustion process at different stages, it is
easier to conduct development work on a test rig on the basis of the combustion
efficiency
η = theoretical efficiency for actual ∆T ’ actual efficiency for actual ∆T
For this purpose the only measurement required are those necessary for
determining the fuel air ratio and the mean stagnation temperatures at inlet and outlet
of the chamber. The theoretical efficiency can be obtained from curves.
Stability limits
For any particular combustion chamber there is both a rich and a weak
limit to the air fuel ratio beyond which the flame is unstable. Usually the limit is
taken as the air fuel ratio at which the flame blows out, although instability often
occurs before this limit is reached. Such instability takes the form of rough running,
which not only indicates poor combustion, but sets aerodynamic vibration which
reduces the life of the chamber and causes vlade vibration problems. The range of air
fuel ratio between the rich and weak limits is reduced with increase of air velocity,
and if the air mass flow is increased beyond a certain value it is impossible to initiate
combustion at all. A typical stability loop is shown in figure, where the limiting air

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fuel ratio is plotted against air mass flow. If a combustion chamber is to be suitable,
its operating range defined by stability loop must obviously cover the required range
of air fuel ratio and mass flow of the gas turbine for which it is intended.
Furthermore, allowance must be made for conditions which prevail when the engine
is accelerated or decelerated. For example, on acceleration there will be a rapid
increase in fuel flow as the ―throttle‖ is opened while the air flow will not reach its
new equilibrium value until the engine has reached its new speed. Momentarily the
combustion chamber will be operating with a very low air fuel ratio. Most control
systems have a built-in device which places an upper limit on the rate of change of
fuel flow; not only to avoid blow-out, but also to avoid transient high temperatures in
the turbine.
The stability loop is a function of the pressure in the chamber: a decrease
in pressure reduces the rate at which the chemical reactions process, and
consequently it narrows the stability limits. For aircraft engines it is important to
check that the limits are sufficiently wide with a chamber pressure equal to the
compressor delivery pressure which exists at the highest operating altitude. Engines
of high pressure ratio present less of a problem to the combustion chamber designer
than those of loss pressure ratio. If the stability limits are too narrow, changes must
be made to improve the circulation pattern in the primary zone.
Combustion intensity

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The size of the combustion chamber is determined primarily by the rate
of heat release required. The nominal heat release rate can be found from mfQnet , m
is the air mass flow, f the fuel air ratio and Qnet the net calorific value of the fuel.
Enough has been said for the reader to appreciate that the larger the volume which
can be provided the easier it will be to achieve a low pressure drop, high efficiency,
good outlet temperature distribution and satisfactory stability characteristics.
The design problem is also eased by an increase in pressure and
temperature of the air entering the chamber, for two reasons. Firstly, an increase will
reduce the time necessary for the ―preparation‖ of the fuel and air mixture
(evaporation of droplets, etc.) making more time available for the combustion process
itself. Note that since the compressor delivery temperature is a function of the
compressor delivery pressure, is an adequate measure of both. Secondly, we know
that the combustion chamber pressure is important because of its effect on the rate at
which the chemical reactions proceed.
Combustion intensity = heat release rate ÷ (combustion volume x pressure) kW / m³
atm
The lower the value of the combustion intensity the easier it is to design a
combustion system which will meet all the desired requirements. It is quite
inappropriate to compare the performance of different systems on the basis of
efficiency, pressure loss, etc., if they are operating with widely differing orders of
combustion intensity is in the region of 2-5 x 10⁴ kW / m³ atm. While in industrial
gas turbines the figure can be much lower because of the larger volume of
combustion space available; a further reduction would result if a heat exchanger were
used, requiring a significantly smaller heat release in the combustor.

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MODULE IV
INTRODUCTION TO SOLAR ENERGY
Strictly speaking, all forms of energy on the earth are derived from the sun.
However, the more conventional forms of energy, the fossil fuels received their solar
energy input eons ago and possess the energy in a greatly concentrated form. These
highly concentrated solar energy sources are being used as such at a rapid rate that they
will be depleted in not-too distant future.
There are four primary sources of energy viz., petroleum, natural gas and natural-
gas-liquids, coal and wood. Excepting wood, all these common sources have finite
supplies. The life-time is estimated to range from 15 years for a natural gas to nearly 300
years for coal. Therefore, as these non-renewable sources are consumed, the mankind
must turn its attention to longer-term, permanent type of energy sources. The two most
significant such sources are nuclear and solar energy. Nuclear energy requires advanced
technology and costly means for its safe and reliable utilization and may have
undesirable side effects. Solar energy, on the other hand, shows promise of becoming a
dependable energy source without new requirement of a highly technical and specialized
nature for its wide spread utilization. In addition, there appear to be no significant
polluting effects from its use.
Modern scientific research in the utilization of solar energy commenced in 1855
when C. Guntur, an Austrian, invented a solar boiler using mirrors. In 1876 an American
inventor, John Ericsson who invented several types of hot air engines prior to this date,
visualised that at some time in the near future, a chain of solar power stations across
North Africa, the Middle East India, Australia and Central America would be set up. In
the latter half of the last century and during the first of this century, progress in the field
of energy research was fairly slow. This was mainly due to availability ‗of cheap fossil
fuels. The waning solar energy research was revived in 1940 when God fray Cabot left a
large sum of money for research projects at the Massachusetts Institute of Technology.
During the recent energy panic research in the utilization of solar energy has gathered
considerable momentum, especially in industrialized countries such as U.S.A., U.S.S.R.,

142
France, Australia and Canada. The near future will certainly show some major
breakthrough in solar energy‘ technology.
All countries in the world receive some solar energy. This amount varies from a
few hundred hours per year as in the northern countries and the lower part of South
America, to four thousands hours per year as in the case in most of the Arabian
peninsula and the Sahara Desert. In estimating the amount of solar energy falling on the
earth, let us consider first of all the natural deserts of the world. This area is about 20 x
106 2 with average solar insolation of 583.30 W/m2/day (500 gin cal/cm2/day). Another
30 x 106 km2 receive about 291.65 W/m2/day (250 gin cal/cm2/day). Let us ignore all
the areas of sea and the rest of the land. Therefore, the amount of solar energy received
by this 50 x 106 2 is 162.2 x 1012 kWh/day, assuming eight hours of sunshine, or
approximately 60 x 1015 kWh/year. Using 5%, this energy will result in 300 x 1013
kWh and comparing this with the estimated world energy demand in the year 2000 (50 x
1012 kWh/year), it can be seen that it is 60 times what the world will require then. Solar
energy, which is the ultimate source of most forms ofenergy used now, is clean, safe and
exists in viable quantities in many countries. The drawbacks in using solar radiation as
energy, as have been pointed out, are that it cannot be stored and it is a dilute form of
energy. This is however, an outdated argument since the energy can be stored by
producing hydrogen, or by storing in other mechanical or electrical storage devices, the
energy can be concentrated in solar furnaces, for example which can achieved
temperatures in the region of 5000°C. In addition to the thousands of ways in which the
sun‘s energy has been used by both nature and man throughout the time to grow food, to
see by, to get a suntan, to dry clothes, it has also been deliberately harnessed to perform
a number of other ‗chores‘. Solar energy is used to heat and cool buildings, to heat water
and swimming pools, to power refrigerators; and to operate engines, pumps and sewage
treatment plants. It powers cars, ovens, water stills, furnaces, distillation equipment, crop
dryers, and sludge dryers powered by solar energy. Wind is used to generate electricity
and mechanical power and solar-converted electricity is used both on earth and in space.
Stoves and cars run on solar-made methane gas, power plants operate on organic trash
and sewage plants produce methane gas.

143
Solar electrolizers convert water to clean hydrogen gas (a fuel).
None of these uses, however, can be comprehended without knowledge of the basic
principles of solar energy. Most of the energy we receive from the sun comes in the form
of light, a short wave‘ radiation, not all of which is visible to the human eye. When the
radiation strikes a solid or liquid, it is absorbed and transformed in heat energy ; the
material becomes warm and stores the heat conducts it to surroundings materials (air,
water, other solids liquids), or re-radiates it to other materials of lower temperature. TI
re-radiation is a large-wave radiation. Glass easily transmits short-wave radiation, which
means that poses little interference to incoming solar energy, but it is a very po
transmitter of long wave radiation. Once the sun‘s energy has passi through the glass
windows and has been absorbed by some materi inside, the heat will not be re-radiated
back outside. Glass, therefoi acts as a heat trap, a phenomenon which has been
recognized some time in the construction of green houses, which can get qui warm on
sunny days, even in the middle of winter; this has come be known, in fact, as the ―green
house effect‖. Solar collectors for hot heating, usually called flat-plate collectors, almost
always have o or more glass covers, although various plastic and other transparent
materials are often used instead of glass.
Beneath the cover plate, collectors commonly have another ph which absorbs the
sun‘s rays hitting it. This absorber plate is usually made of copper, aluminium, steel or
another suitable material and usually coated with a substance like black paint or one of
the mc sophisticated selective coatings available that will help it absorb t most heat,
rather than reflect or re-radiate it. Once the heat absorbed, it can be picked up and used.
The glass cover plates to reduce the loss of heat through the front while insulation
reduces heat loss through the back.
From the absorber plate, heat is transferred by conduction to a transfer fluid,
usually a liquid or air, which flows by the absorber plate, often with the help of a pump
or blower. The liquids (water or a non-freezing fluid such as ethylene glycol) flows over
the black surface or through tubes incorporated into the absorber plate. If air is used, it is
blown across the surfaces of the absorber plate, which should have many small irregular
surfaces with which the air can come in contact.
In some cases, it is possible to move the fluids (whether liquid or air) without

144
mechanical aid, by natural convection or thermo siphoning. As the fluid is heated, it
tends to rise, and cooler fluid flows in, to take its place. If the collector is tilted or
vertical, this effect will move fluid across the collector plate and off without any external
help. Some of the simplest systems work this way and in the right application, they are
very effective pumping, however, usually gives greater collection efficiencies and
allows more versatile use of the collected heat.
The applications of solar energy (other than on space craft enjoying most success
today are:
1.Heating of buildings. .
2. Cooling of buildings.
3. Solar water heating and solar air heating.
4. Salt production by evaporation of sea water or inland brines
5.Solar distillation on a small community scale.
6.Solar drying or agricultural products. .
7.Solarcookers.
8.Solarenginesforwater-pumping
9.Foodrefrigeration.
10.Photo-voltaicconversion.
11.Solarfurnaces.
12.Solarthermalpowergeneration.
13.Industrialprocessheat.
14. Indirect source of solar energy conversion, i.e. in the form of wind, through blo-
conversion tides.
The heat from solar collectors is directly used for warming the living spaces of a
building in conventional ways e.g., through radiators and hot air registers. When the

145
building does not require heat, the warmed air or liquid from the collector can be
moved to a heat storage container. In the case of air, the storage is often a pile of
rocks, or some’other heat holding material ; in the case of liquid, it is usually a large,
well-insulated tank of water, which has considerable heat capacity. Heat is also stored
in containers of chemicals called eutectic or phase changing salts. The salts, which
store large quantities of heat in a relatively small volume, melt when they are heated
and release heat later as they cool and crystallize. When the building needs heat, the
air or water from its heating system passes through the storage is warmed, and is then
fed through the conventional heaters to warm the space. For sunless days or cloudy
days, an auxiliary system as a back-up, is always required. The same is true for solar
cooling systems.
The heat from solar energy can be used to cool buildings, using the absorption
cooling principle imperative in gas-fired refrigerators. Presently available equipment,
however usually requires extremely high operating temperatures far above those for
efficient solar collection. A great deal of current research is being devoted to
developing systems requiring lower operating temperatures and collectors which are
more efficient at higher temperatures, but it will probably be several years before solar
collectors will be commercially viable.
Solar energy units for heating domestic water are commercially available and
are used by millions of people in various parts of the world. The Australian government
requires electrically supplemented solar water heaters for all new housing in the
northern part of the country. In Israel, solar water heaters are widely used, and simple,
plastic, non-supplemented water heaters have been widely used in Japan. There is a
thriving, though small, solar water heater industry in Florida and California. Because of
the low price of competitive fuels and difficulty of designing solar water heaters which
can operate successfully during freezing weather, solar domestic water heating has not
been widely used in northern climates. However, with rising fuel prices and the
increased development of solar collectors, solar heating of domestic water in cold
climates is being adopted

146
A solar water heater commonly comprises a blackened flat plate metal collector
with an associated metal tubing, facing the general direction of the sun. The collector is
provided with a transparent glass cover and a layer of thermal insulation beneath the
plate. The collector tubing is connected by a pipe to an insulated tank that stores hot
water during.in-sunny periods. The collector absorbs solar radiation and by
transferresulting heat to the water circulating through the tubing by gravitor by a
pump, hot water is supplied to the storage tank. The materials commonly used in flat
plate collectors are copper, roll-bond aluminium, galvanised iron or mild steel. Various
configurations of the plate and tubing using these materials have been experimented
with, and the performance of a flat plate collector depends on the selective coating on
the absorber plate. A solar collector area of one square meter can provide about 75
litres of hot water at about 60°C on an average sunny day. The present costs of various
types of flat plate collectors range from Rs. 1000 to 1500 per square metre.
Solar water heating systems for domestic, industrial and commercial
applications are at present available. Except in the hilly regions and in the northern
latitudes, the potential for domestic hater heaters is somewhat limited. In commercial
establishments howver, there is great potential especially in hotels, hospitals, guest
houses, tourist bungalows, Canteen etc. For industrial applications, solar water heating
system can meet the low and medium temperature process heat requirement hot
water upto 90°C, hot air upto 110°C and low pressure steam upto 140°C. These are
especially useful in engineering, textile, chemicals, pharmaceutical, food processing,
sugar, dairy and other industries. Hot water systems have relevance for many
agricultural and village industries also, such as for handloom fabrics, sen-culture,
leather tanning and handmade paper. Pharmaceutical industry demands steam from
coal and electricity. Not to depend entirely on such high grade energy some companies
in Maharashtra state have gone in for solar flat plate collectors to supply water at 60°C.
Availability of solar system for 250 days in a year have shown about a 5% saving in
furnace oil in Hoechst pharmaceutical, with an annual saving of Rs. 20,000 in 1979. The

147
company has planned to take advantage of the system and have gone for additional
units and hope to ensure a payback period, of 6 to 7 years
.
Hospitals on one hand, use the low temperature hot water as such or heat it
further by electrical means. Cleaning, washing and sterilisation needs are thus partly
met with. Jahangir Textile Mills, Ahmedabad have preferred to install solar hot air
system for supplying 50 kg/mm. of hot air at 80°C. Solar heated hot air is used in the
cheese drying and as preheated air for printing float dryer. Hot water, steam and hot
air could well be used in textile industry in the following areas at varying temperatures
Dyeinganddrying(80°—100°C)
SizingandKiers(100—85°C)
Washing,Mercerisingand
drying(80—100°C)
Washing,Drying(100°C)
Drumdrying(100°C)
Aging(50°C)
Sanforising(125°C)
Stenters(150°C)
Calendering(90°C)
Solar energy could be used for preheating water up to 50—60°C, with further
heating of process steam to 90°C and above being done in boiler, resulting in 15 to 25%
saving in the fuel cost, Madural coats, Madura and Jahangir Mills have paved the way
for use of solar water heaters and air heaters. Jahangir Mill have installed collectors
with 180 m2 to provide 5500 liters’ of hot water at80°Cdaily.Hot air ispredominantly
used for drying cloth and yarn. Normally steam heating is resorted to and there is
considerable loss of steam. Here solar air heaters, are advantageously employed. Chest
yearn) dryer and printing float dryers are the machines best suited £br the use of solar
hot air. The total installed cost of the system in the mill is Rs. 250,000 (say). This system
annually save 35 tonnes of coal or 15 kilo litres of oil and the pay back-period for the

148
heater ranges from 5 to 7 years. Solar hot air systems, if they prove to be practically
adaptable in textile mills, have great potential for being retrofitted in a large number of
textile mills. The mills also require hot water for humidification, which could adopt a
solar system. Of the total energy consumption in composite textile mills, more than
50% is accounted for thermal energy requirements. Steam, hot water and hot air
requirements per kg of fabric, may be around 20, 40 and 30 kg respectively, in the
temperature range of 80—110°C. Solar evaporation is historical and traditional method
of obtaining salt from sea water or brine. Modern developments have been concerned
mainly with improved pond construction. The basic method of solar distillation is to
admit solar radiation through a transparent cover in a shallow, covered brine basin;
water evaporates from the brine and the vapour condenses on the covers which are so
arranged that the condensate flows therefrom into côllection troughs and thence into a
product-water storage tank. In arid, semi-arid, or coastal areas, there is abundant sun
light that can be used for converting brackish or saline water into potable distilled
water. Solar stills can produce 3 to 5 litres of distilled water per square meter on an
average sunny day. The solar distillation technology to convert brackish water into
potable water is simple and small solar stills can be fabricated locally in rural areas.
A traditional and wide-spread use of solar energy is for drying particularly of
agricultural products. This is a process of substantial economic significance in many
areas. The process is of special interest in the case of soft fruits ; these are particularly
vulnerable to attack by insects, as the sugar concentration increases during drying.
Fruit dryer in which fruit is placed, in carefully designed racks to provide controlled
exposure to solar radiation often improves product quality and saves consider*le time.
A simple cabinetyer consists of a box, insulated at the base, painted black on
the inside and covered with an inclined transparent sheet of glass. Ventilation holes are
provided at the base and at the top of the sides of the box to facilitate a flow of air over
the drying material, which is placed on perforated trays in the interior of the cabinet
base. Large drying systems like grain, paddy, maize, cash crops like ginger, cashew,
pepper etc., spray-drying of milk ; timber and veneer drying ; tobacco curing; fish and

149
fruit drying, etc. have also becdeveloped. A rise in 10—15°C of ambient air with a
reduction in its relative humidity to 60%, is suitable for drying most of the cereal grain8
to the level of the safe moisture content for storage 500 kg of paddy could be dried
from 30 to 14% moisture content in a period of 6 hours on bright sunny day by using air
flow rate or 4 m3/min., with temperaturerise8-10°C. Solar cookers and ovens are
developed for cooking all types of food in 40 to 60 minutes. Two types of solar cookers
have been developed in our country, these are,
(a) A box type closed cooker with glass, a cover and extra booster mirror which
provides concentrated radiation in the oven and traps heat within the small space in
which the food is placed.
(b) A cooker based on concentrating solar energy by a paraboloid mirror
reflector which directly heats the cooking vessels. An inexpensive solar cooker made
out of bamboo cane with aluminium sheet as the reflector surface has also been
developed. On very clear days, temperatures of about 300°C in summer and about
200—250°C in winter can be achieved with the help of this cooker. The food even
remains warm for a few hours after sunset. Roasting, baking and boiling of ingredients
can be achieved within 30 to 90 minutes under clear sky conditions. Factors such
as problems of heat storage, regulation of heat for cooking, the socio-cultura) habits of
the people and inadequate promotional efforts have been the main reasons for the
lack of interest shown in solar cookers in spite of their being functionally satisfactory
and low priced. Solar refrigeration is intended for food preservation (or storage of
biological and medical materials) and deserves top-priority in our country. Solar air-
conditioning can be utilized for space cooling. Solar assisted heat pumps would provide
both cooling and heating. Cold storages are very important for preservation and
conservation of food articles. It estimated that in India, there is a loss of about 30
percent of the produce, due to lack of proper cold storage facilities. There are two
methods of solar refrigeration.

150
(a’) Vapour Absorption Refrigeration Systems that utilize low grade thermal
energy obtained from flat-plate collectors with a little modification.
(hi Concentrating (focusing) collectors to supply heat at a higher temperature to
a heat engine which then drives the compressor of a conventional refrigerator. Solar
refrigeration, therefore, provides an effective solution particularly in sub-tropical and
tropical areas of our country, where matching between the cooling load and the solar
isolation is generally very good. Solar refrigeration with an absorption system is a
better way of direct utilization of energy For a country like India, the preservation of
agricultural products before they are dispatched to the urban areas from villages, could
be considered essential. The vapour absorption system replacing the compressor by a
generator absorber assembly can work with wide range of absorbents and refrigerants.
In absorption system motion power required is very small, but still C.O.P. of the system
is low.
The system efficiency could be improved, if it absorbs waste heat (flue-gas from
boiler) or uses alow grade energy. Such absorption chillers are common in process
industry. Solar energy based absorption system could very well be compared with
vapour compression system using high grade energy. In such an analysis if the overall
efficiency right from power generation ii considered the vapour compression system
may be no way better than solar absorption system. Still for the application of this low
grade nonpolluting energy, the huge initial cost comes as a hurdle for large scale
adoption .
Solar absorption chilling system for food preservation has gained momentum, in
northern part for preservation of potato and onions. With proper modifications of roof-
cum collector type panels, the initial cost is brought down. V
Using concentrating type collectors to heat fluids which can be used to operate
heat engines which in turn drive generators to produce electricity. Low temperature
flat-plate collectors are not adequate for heating building and water and with increased
development will be adequate for powering solar absorption type cooling equipment.

151
However, they are insufficient for the high efficiency production of electricity or for
making artificial fuels by thermal processes. For this, high temperature collectors such
as concentrating collectors are required. Solar energy is focused from a relatively large
area into small, from which it is carried to storage. Such concentrators are usually
parabolic or cylindrical in shape. Temperatures up to 500°C V and more are attainable
.
Seasonal efficiencies of most focusing collectors are often lower than that of flat
plate collectors because of these higher operating temperatures ; in addition, because
they rely only on direct rather than diffuse radiation, they need clear skies to operate.
On the other hand, flat plate collectors are able to use solar radiation that is not V
nearly as bright as that necessary for concentrating collectors. Concentrating collectors
are usually comparatively expensive.
Scientists have proposed schemes for using the elm’s energy to generate
electricity on a large scale by creating farms of many square km. of concentrating solar
panels in areas where the skies are rarely cloudy .
The solar energy can be used for power generation. Research and
SOLAR COLLECTORS
A solar collector is a device designed to absorb incident solar radiation and to
transfer the energy to a fluid passing in contact with it. Utilization of solar energy
requires solar collectors. There are two general types— the flat-plate collector and the
concentrating (focusing) collector.
Solar collectors may be classified according to their collector characteristics, -
the way in which they are mounted and the type of transfer fluid they employ
(1) Collecting characteristics: & Anon-concentrating or ‘flat-plate’ collector is
one in which the absorbing surface for solar radiation is essentially flat with
no means for concentrating the incoming solar radiation. A concentrating
or-focusing collector is one which usually contains reflectors or employs

152
other optical means to concentrate the energy falling on the aperture on to
a heat exchanger of surface area smaller than the aperture.
(2) Mounting:. A collector can be mounted to remain stationary, be adjustable
as to tilt angle (measured from the horizontal) to follow the change in solar
radiation or be designed to track the sun. Tracking is done by employing
either an equatorial mounting or an equatorial mounting the purpose of
increasing the absorption of the daily solar irradiation.
(3) Types of fluid: A collector will usually use either a liquid or a gas as the
transfer fluid The most common liquids are water or a water-ethylene glycol
solution. The most common gas is air.
Physical Principles of the Conversion of Solar Radiation into Heat
The fundamental process now in general use for heat conversion is the green
house effect, The name comes from its first use in green houses in which It is possible to
grow exotic plants in cold climates through better utilization of the available sunlight.
Figure shows how temperature on earth is affected by the ‗green house. Visible
sunlight is absorbed on the ground at a temperature of 20°C, for example emits infrared
light at a wavelength of about 10ηm but CO2 in the atmosphere absorbs light of that
wavelength and back radiates part of it to earth, (CO2 does not absorb the coming
sunlight winch has a shorter wavelength). Hence the ‗green house effect‘ brings about
an accumulation of, energy on the ground.

153
LIQUID FLAT PLATE COLLECTORS
Figure is a schematic representation of a typical flat-plate solar collector (plate
and tube type). It basically consists of a flat surface with high absorptivity for solar
radiation, called the absorbing surface. Typically a metal plate, usually of copper, steel
or aluminium material with tubing of copper in thermal contact with the plates, are the
most commonly used materials. The absorber plate is usually made from a metal sheet 1
to 2 mm in thickness, while the tubes, which are also of metal, range in diameter from 1
to 1.5 cm. They are soldered, brazed or clamped to the bottom (in some cases, to the top)
of the absorber plate with the pitch ranging from 5 to 15 cm. In some designs, the tubes
are also in line and integral with the absorber plate. For the absorber plate, corrugated
galvanized sheet is a material widely available throughout the world and Figure (a) and
(b) shows two ways in which it has been used.
The use of conventional standard panel radiators shown in Fig. 5.3.2 (c) is one of
the simplest practical applications. The methods of bonding and clamping tubes to flat or
corrugated sheet are shown in figure (d) and (e) while (f) is a tube in strip or roll bond
design, in which the tubes are formed in the sheet, ensuring a good thermal bond
between the sheet and the tube

154
Heat is, transferred from the absorber plate to a point of use by circulation of
fluid (usually water) across the solar heated surface. Thermal insulation of 5 to 10 cm
thickness is usually placed behind the absorber plate to prevent the heat losses from the
rear surface. Insulation material is generally mineral wool or glass wool or a heat
resistant fiber glass.
The front covers are generally glass (may be one or more) that is transparent to
in-coming solar radiation and opaque to the infrared re-radiation from the absorber. The
glass covers act as a convection shield to reduce the losses from the absorber plate
beneath. Glass is generally used for the transparent covers but certain plastic films may
be satisfactory. Glass is the most favourable material. Thickness of 3 or 4 mm are
commonly use

155
The usual practice is to have 1 or 2 covers with a specific ranging from 1.5 to 3
cm. Advantages of second glass which is added above the first one are
(i) Losses due to air convection are further reduced. This is important in windy
areas.
(ii) Radiation losses in the infra-red spectrum are reduced by a further 25%,
because half of the 50% which is emitted outwards from the first glass plate is back-
radiated.
It is not worthwhile to use more than two glass plates. This is due to the fact that
each plate reflects about 15% of the incoming sunlight.
Some plastic glazing‘s have been recommended. Slagwood, fibre glass,
polyurethane foam, hay in polythene bags (to keep the moisture out) are suitable
materials for insulating the sides and bottom of collector. The collector box support all
the components and provides weather protection.
For water streams the absorber plate can be any metal, plastic or rubber sheet
that incorporates water channels, while for air systems the space above or below the
collector plate serves as the conduit. The surface finish of the absorber plate may be a
flat black paint with an appropriate primer. The primer coat should preferably be thin
since a thick under coat of paint would increase the resistance to heat transfer. The
primer should be of the self-etching type. If the primer is not a self-etching type, the
repeated thermal expansion and contraction of the plate may cause the paint to peel after
a year or so. Several types of backed on or chemical finishes are also available. Black
painted absorbers are preferred because they are considerably cheaper.
The liquid heated is generally water. However sometimes mixtures of water and
ethylene glycol are used if ambient temperatures below 0°C are likely to be encountered.
Typical collector dimensions are 2 m x 1 m x 15 cm.
Selection of Materials for Flat-plate Collectors
To design and construct solar collectors for heating and cooling projects, detailed
knowledge of the properties of the materials and characteristics of the various

156
components is necessary to predict the performance and durability of the collector.
Needed-property data can generally be classified into three categories
(i) Thermo physical
(ii) Physical and
(iii) Environment properties.
Thermo physical properties include thermal conductivity, heat capacity and
radiant heat transfer characteristics. Physical properties include density, tensile strength,
melting point and modulus of elasticity. Environmental properties include resistance to
ultraviolet degradation, moisture penetration and degradability due to pollutants in the
atmosphere. All of these data are required to develop collectors that are reliable, durable
and efficient. Durability is the criterion most often overlooked by the beginner
constructing collectors .
Absorber plate. The collector absorber plate should have high thermal
conductivity, adequate tensile and compressive strength, and good corrosion resistance.
Copper is generally preferred because of its extremely high conductivity and resistance
to corrosion. Collectors are also constructed of aluminium, steel and various
thermoplastics. Aluminium and steel require a corrosion-inhibited heat transfer fluid.
Most potable water contain chloride and metal (e.g. copper and iron) ions ; these would
cause pitting in aluminium channels, Also, if aluminium is used, one cannot mix copper
plumbing and aluminium collectors without taking adequate precautions to ensure that
the copper ions from corrosion of the piping and chlorides from the soldering fluxes do
not destroy the aluminium. Galvanic effects can be an important factor in multimetal
systems, so electrical isolation should always be provided between dissimilar metals. If
the flow rate is too high, corrosion also can be produced by the simple erosion process
resulting from high flow rates and turbulence in the fluid passages. Partial blockages of
the flow passages can also cause localized high velocities, resulting in this type of
degradation.
Until recently, absorber plates for flat-plate solar collectors were usually
constructed with tubes soldered or welded onto a metal plate, which was then blackened.

157
Some of the earlier solar water heaters actually had tubes fastened to the plate without
soldering, resulting in poor heat transfer and poor thermal performance. The standard
procedure for fabricating an absorber plate has been to take a sheet of copper oi
aluminium and solder tubes to it; solar radiation falling on the plate would heat the metal
plate and some of the heat was transferred to water flowing through the tubing. One of
the more important advances in solar technology has been the development of internal
tube collector plates, such as the Roll-Bond panel and the tube-in-strip collector plate.
The internal tube collectors have superior heat transfer characteristics and also can be
mass-produced, so that the laborious process of soldering and welding tubes onto a flat-
plate is eliminated. Also the tubes cannot come loose from the plate. Because of these
and other desirable features of the internal tube absorber plates, such absorbers are being
incorporated into many modern collector designs .
Cover Plate. The cover plate (or plates) through which the solar energy must be
transmitted is also extremely important to the function of the collector. The purpose of
the cover plates are
(i) to transmit as much solar energy as possible to the absorber
(ii) to minimize heat loss from the absorber plate to the environ-
(iii) to shield the absorber plate from direct exposure to weathering; and
(iv) to receive as much of the solar energy as possible for the longest period of
time each day.
The most critical factors for the cover plate-materials are strength, durability,
non-degradability and solar energy transmission
Tempered glass is the most common cover material for collectors because of its
proven durability and because it is not affected by ultraviolet radiation from the sun.
Experience has shown that, unless the glass is tempered, the day-to-day thermal cycling
of the cover plate tends to cause breakage. Tempered glass, properly mounted on to a
flat-plate collector, is highly resistance to breakage both from thermal cycling and from
natural events. Glass is also effective in reducing radiated heat loss because it is opaque
to the longer wavelength infra-red (IR) radiation re-emitted by the hot absorber plate.

158
Plastic materials may also be used for cover plates, such as the acrylic poly
carbonate plastics, plastic films to Tedlar and Mylar, and commercial plastics such as
Lexan. Plastic materials tend to have limited life-times because of the effect of
ultraviolet (UV) light in reducing the transmissivity of the plastic. Also, they are usually
partially transmitting to long-wavelength radiation and are therefore less effective in
reducing radiated heat losses from the absorber plate. Some plastics also are unable to
withstand the maximum equilibrium temperatures that are encountered in flat-plate
collectors, especially when the collector is dry. The main advantages of plastic materials
are the resistance to breakage, reduction in weight and, in some cases, reduction in cost.
Most glass and plastic materials of interest have refractive indices of about 1.5.
Unless special coatings or surface treatments are applied, this results in approximately
8% of the normal incidence solar radiation reflecting from the glass away from the
absorber plate from each cover plate, and a greater fraction is reflected at higher
incidence angles. This means that the maximum transmittance is 92% for a single,
perfectly clear, non absorptive sheet of glass. In multi- glazed panels, the reduction in
transmission is about 8% more for each additional sheet.
In addition, there is a transmission reduction due to absorption of solar radiation
within the material itself. The amount of solar radiation reflected can be reduced
considerably by etching or by applying an antireflective coating to the surface. Etching
produces a surface coating with a refractive index lower than 1.5, which results in less
reflection.
The transmissivity of glass depends on its iron content. A normal sheet of
window glass looks green when viewed through the edge because of the iron oxide
within the glass. Water-white crystal glass has the lowest iron content and therefore the
highest transmission of solar energy. Water-white crystal is available annealed or
tempered. Tempered glass has about five times the impact and thermal shock resistance
to ordinary annealed glass.
In selecting the glass for cover plates, the mechanical strength must be adequate
to resist breakage from the maximum expected wind and snow loads, and normally

159
expected impact. The mechanical strength is proportional to the square of the thickness
of the glass. Cover plates for solar collectors normally should be alteast 0.33 cm thick.
Thermal shock to the glass cover plate must also be taken into account. It is
caused by several different processes. First is the day- by-day heating and cooling from
the increase in solar intensity on the collectors during the morning hours and subsequent
decrease in the afternoon. In addition, in partly cloudy weather, glass temperatures can
rise and fall by 50°C or more in a matter of minutes as clouds pass overhead. The central
area of the collectors is subjected to greater heating than the edges of the glass, since
normally the edges are enclosed in flashing and are not exposed at all to direct sunlight.
This results in a thermal stress in the glass at the edges which may be estimated at
60,000 kg/m2-°C temperature difference between the heated centre and the cooler edge
of the glass plate. Thicker glass plates are more subject to thermal shock than thinner
plates. Additional stress can occur when a single collector is partially shaded. In this
case, part of the glass plate is subjected to high temperatures while the shaded area is
not. These processes can easily result in breakage of non annealed glass, and accounts
for the use of temperated glass in solar collectors.
The rigidity of the cover plate is also important. Rigidity is proportional to the
cube of the thickness of the plate. The resistance to fracture under mechanical stress is
especially important when the collector is double-glazed. Some flexure may be desirable
to accommodate the expansion of air within the gap when this type of collector is heated
Thermal Losses and Efficiency of Flat-plate Collector
`The performance of a solar collector is described by an energy balance that
indicates the distribution of incident solar energy into useful energy gain and various
losses. The thermal losses can be separated into three components.
(i) Conductive losses: Conduction through the back and the sides of a collector is
usually negligible if the back and sides of the collectors are well insulated. An overall
heat transfer coefficient value of less than 0.69 W1m2 °K is suggested to minimize back
losses.

160
(ii) Convective losses: Convective losses occur from the absorber plate to the
environment through intermediate convection exchanges between the air enclosed in
each insulating zone and the boundaries of each zone—the collector covers. In the
absence of wind, external convection loss from the outermost cover is by the mechanism
of natural convection; but even in low winds, forced convection occurs and increases the
loss substantially. (Natural convection occurs without an imposed external flow whereas
forced convection occurs in the presence of an external flow). Sizing the air gap between
the collector covers at 1.25 to 2.5 cm reduces internal convective losses to the minimum
possible level.
Convection loss between glass plates can also be inhibited if a honeycomb-type,
cellular structure is placed between the absorber and the outer window plate. However,
in addition to the increase in cost, a cellular structure also reflects a part of the incoming
radiation, thus preventing solar radiation from reaching the absorber plate. A cellular
structure also increases the thermal conductivity of the space between the absorber and
the outer air. A honeycomb that transmits solar radiation, is opaque in the infra-red
spectrum and has a low thermal conductivity could be ideal for a solar collector.
Evacuation of the space between the absorber and the outer cover has been proposed to
reduce internal convection and conduction, but the cost of added supports and
maintenance of a vacuum are excessive
(iii) Radiative losses: Radiative losses from the absorber can be reduced by the
use of spectrally selective absorber coatings. Such coatings have a high absorptance of
about 0.9 in the solar spectrum and a low emittance, usually of the order of 0.1, in the
infra-red spectrum in which the absorber radiates to the environment. Selective absorber
coatings, therefore, decrease heat losses and increase collector efficiency. Selective
black coatings are commercially available from a few sources, but their cost, stability
and direction radiation properties are checked before using them on a large scale. Under
steady conditions, the useful heat delivered by a solar collector is equal to the energy
absorbed in the metal surface minus the heat losses from the surface directly and
indirectly to the surroundings. This principle can be stated in the relationship:
Qu= Ac [HR (τ-α) — UL (tp — ta)]

161
Where Q is the useful energy delivered by collector, Watts W (kcal/hr)
Ac is the collector area, m2
HR is the solar energy received on the upper surface of the sloping collector
structure; W/m2 (kcal/hr m2)
H is rate of incident beam or diffuse radiation on a unit area of surface of any
orientation.
R = Factor to convert beam or diffuse radiation to that on the plane of collector.
[Beam and Diffuse radiations are considered separately. (τ-α) for beam radiation
is determined from the actual angle of incidence (τ-α) for diffuse radiation may be taken
as that for beam radiation at an incidence angle of 600. The symbol HR is used to
represent the sum of Hb,Rb and HdRd].
τ is fraction of the incoming solar radiati9n that reaches the absorbing surface,
transmissivity (dimensionless)
τ is the fraction of the solar energy reaching the surface that is absorbed,
absorptivity (dimensionless)
(τ . α) is effective transmittance-absorptance product of cover system for beam
and diffuse radiation.
UL is the overall heat loss coefficient. Rate of heat transferred to the
surroundings per square meter of exposed collector surface per degree Celsius
difference between average collector surface temperature and the surrounding air
temperature, W/m2 °C (kcal/hr m2 °C)
tp is the average temperature of the upper surface of the absorber plate, °C.
ta is atmosphere temperature °C
A diagrammatic representation of terms in this relationship is shown in Fig.
5.4.1. In order that the performance of a solar collector be as high as economically
practical, design and operating factors that increase the value of HR, (τ -α) in equation

162
(5.4.1) and that reduce the value of UL (tp — ta), are selected. The greater the energy
absorption in the metal surface and lower the heat loss from the surface, the higher is the
useful recovery. If an unglazed absorber plate is used, the heat-loss coefficient to the
atmosphere UL, of 30 to 60 W/m2 °C(25 to 50 kcal.fhr m2 C) is so large that an
absorber temperature of 15 to 30°C above atmospheric temperature is the maximum
achievable under full solar radiation of 1000 W/m2 (860 kcal/hr m2). Under these
conditions, no useful heat is delivered. from the collector because the heat loss is as
large as the solar heat observed. When a fluid is circulated through the collector, useful
heat output requires an even lower delivery temperature. Unless a low temperature
application is involved, such as swimming pool heating, heat losses must, therefore, be
reduced.
5.4.1
To reduce the rate of radiation and convection loss, as already stated, one or
more transparent surfaces, such as glass, are placed above the absorber surface. One
layer of glass can transmit as much as 92 per cent of the solar radiation striking it, while
greatly reducing the heat loss coefficient UL. This reduction is due to the suppression of
convection loss by interposing a relatively stagnant air layer between absorber plate and
glass, and by absorption of long wave thermal radiation emitted by the hot metal
absorber surface. The combined heat-loss coefficient can be reduced to 5 to 10 W/m2 °C

163
(4.30 to 8.60 kcal/hr m2 °C) by the use of one glass cover. Similar benefits can be
achieved by use of certain transparent plastic materials.
The heat loss coefficient can be reduced further by using a second transparent
cover with an air space between the two-surfaces. Two convection barriers are then
present, as well as two surfaces impeding radiation loss coefficient in the range of 4
W/m2 °C (3.85 kcal/hr m2 °C) are then typically obtained.
Radiation losses can be decreased by other techniques, such as by reducing the
radiation-emitting characteristics of the absorber. Thermal radiation emitted by the
absorber plate may also be reduced by reflecting it downward from the lower glass cover
by employing an infrared reflecting coating on the glass. A very thin, optically
transparent layer of tin oxide or Indium oxide deposited on the glass reflects thermal
radiation back to the absorber plate. This coating absorbs some of the solar radiation,
however, so the reduced thermal loss is largely offset by reducing solar energy input to
the absorber plate.
The fore-going discussion has been concerned with methods for reducing UL, the
heat loss coefficient. By so doing, the total heat loss is minimized and the collector
efficiency is increased. It is evident from equation (5.4.1) that losses also decrease as the
difference between average plate temperature and air temperature decreases. The
ambient (outside) air temperature is an uncontrollable factor, but the fact that it varies
with time and with geographic location means that collector efficiency also depends on
these factors. It is clear also, that a collector is more efficient at lower plate temperatures
than at high temperatures. But plate temperature depends on the temperature of the fluid
being circulated in contact with the plate, the rate of fluid circulation and the type of
fluid. Fluid temperature depends on conditions elsewhere in the heat utilization system,
whereas the other factors in Fig. (5.4.1) depend on collector design, operating
conditions, solar energy input, and atmospheric temperature.
Energy Balance Equation. The energy balance on the whole collector can be
written as

164
A {[HR (τ α)b + [HR (τ α)]d }= Qu+ Qt+Qs ...(5.4.2)
Qu = rate of useful heat transfer to a working fluid in the solar exchanger;
Qt = rate of energy losses from the collector to the surroundings by re-radiation,
convection, and by conduction through supports for the absorber plate and so on. The
losses due to reflection from the covers are included in the (ta) term;
Qs = rate of energy storage in the collector.
The energy balance on a flat-plate collector operating at steady state may be written:
HTAC (τ α)s = Qu + Qt
where HT = total solar radiation on the collector
AC = aperture area of the collector
QU= rate of useful heat delivered by the collector
Qt = rate of heat lost from the collector
(τ α)e- effective transmittance absorptance product which is found from:
(τ α)e = τ.α/ ( 1-(1-α)-ρd )
where Pd = diffuse reflectance of the covers, which may be estimated by
calculating the reflectance p for an incident angle of 60°
Transmittance τ of the cover plates given by
Where K = extinction coefficient of the transparent cover material
L = total thickness of N covers θ°
θ2 = angle of refraction, given by Snell‘s law
sin θ2 = (sin θ1)/n

165
where θ1 = angle of incidence
n = index of refraction of the transparent cover plate.
The reflectance from a transparent surface is different for the parallel ρp and
perpendicular ρs component of the radiation striking the surface
For unpolarized, light, the two components of the incident light, are equal, so,
For normal incidence radiation, the two components are reflected equally, and
Where Q = rate of useful heat transfer to a working fluid in the solar exchanger;
The useful heat delivered by the collector is therefore
QU =Ac [HT(τ α)e — UL (Tp — Ta) …………1
where UL = overall loss coefficient equal to the ambient temperature.
UL = Qt/( Tp — Ta))
Tp = average temperature of the upper surface of the absorber plate
Ta = ambient temperature
Collector efficiency η, is the measure of collector performance and is defined as the ratio
of the useful gain over any time period to the incident solar energy over the same time
period

166
Heat Removal factor FR may be introduced here:
FR= Actual useful energy collected / useful energy collected if the entire collector
absorber surface were at the temp of the fluid entering the collector.
Introducing this factor into Equation (1) results in a new performance equation
Where Ti is the collector fluid inlet temperature
If the instantaneous fluid collector efficiency is defined as
ηinst = actual solar energy collected /solar energy incident or intercepted by the
collector
= Qu/ HTAC
This is indicated that if the efficiency is plotted against ( Ti- Ta)/HT a straight line will
result with a slop of FR UL with a y-intercept of FR(τ α)e. this is the way actual
performance data for solar collectors are presented.
with η plotted against (Ti — Ta) FR(τ α)e is the efficiency of the collector would have if
the fluid inlet temperature were equal to the ambient temperature.
The collector heat removed factor may be calculated from:

167
Where m = mass flow rate of fluid
Cp = heat capacity of the fluid
F = collector efficiency factor
In general, the efficiency of flat-plate collectors can be improved by two methods:
1. The transmission of energy through the collector to the working fluid may be
increased. This may be done by improving.
(a) Transmittance (in approximately the 0.4 to 1.9 un spectral range) of the
transparent (glass or plastic) cover plates.
(b) absorptance of the absorber plate to the incident solar radiation (absorptance
approximately 1.0 are obtained by appropriate black or selective coatings) ; and/or
(c) heat transfer coefficients from the absorbing surface to the fluid. These
depend on the thermal conductance resistance through the absorber plate (thermal
conductivity), plate thickness and on the nature of convection in the flow channels, such
as weather the flow is turbulent or laminar, and surface roughness.
2. Decreasing the thermal losses from the collector to the ambient by reducing
conductive, convective and radiative losses.
(a) Conductive losses that occur through the back and sides of the collector can
be reduced by using a sufficiently thick layer of thermal insulation. The main problem is
in the front, where heat is conducted from the absorber plate through the air layer (s)
between the plate and the transparent covers and on out to the ambient air. The increase
in the thickness of the air gaps reduces the loss up to a certain Limit, where further
increases allow significant natural convection. Natural convection transfers heat at
higher rates than does conduction; this leads to higher, rather than lower, heat losses.
Alternatively, several transparent panes could be used to create a number of narrow air

168
gaps. This, however, reduces the transmission of solar energy to the absorber. The
single-pane collector is the most efficient when the absorber temperature is not much
higher than that of the outer cover plate (transmittance dominating-over heat losses) but
becomes rapidly less efficient as this temperature difference increases. Therefore, high
temperature collectors require two transparent covers.
(b) Convective losses separate into internal convective losses from the absorber
plate to the outer cover pane, and external losses from the outer cover pane to the
ambient air. In the absence of wind, the external convective losses are caused by natural
convection. Even low winds, however, dominate convection when they occur. Although
means could and should be introduced to reduce the external convective losses, it would
be most useful to reduce the internal losses, and thus to reduce the temperature of the
outer cover plate. While the available information on natural convection in vertical air
gaps is not conclusive, the convection is very small and comparable in its effect to
conduction for small Grashof or Rayleigh numbers, and it becomes significantly greater
for larger values of these numbers. The nature of the convection also depends on the
specific boundary conditions and the geometric aspects of the enclosure. Besides the
maintenance of narrow air gaps to decrease convection, a cellular structure can be placed
between the absorber and the cover plate. The major problems associated with the
incorporation of cellular structures are : they (i) reflect a part of the solar radiation, thus
preventing it from reaching the absorber plate: (ii) increase the thermal conductivity of
the space between the absorber and the cover plate ; aaid (iii) add to the cost of the
collector. Evacuation of the space between, the absorber and the cover plate practically
eliminates convective losses. This normally can be done only by tubular collectors.
(3) Radiative losses from the absorber to the ambient can be reduced by a
spectrally selective coating on the absorber plate. These coatings have a high
absorptivity in the solar spectrum, but have a substantially lower emissivity usually of
the order of the one-tenth, in the infra-red (IR) spectrum, in which most absorber plates
radiate. The selective absorbers thus decrease heat losses and increase collector
efficiency.

169
Thermal radiation emitted by the absorber plate may also be reduced by
reflecting it downward from the lower glass cover by employing an IR reflecting coating
on the glass. An optically transparent, very thin layer of tin oxide or Indium oxide
deposited on the glass will reduce radiation loss by reflecting it back to the absorber
plate as stated earlier. This coating absorbs a small fraction of the solar radiation,
however, so the reduced thermal loss is largely offset by reduced solar energy input to
the absorber plate.
General Characteristics of Flat-plate Solar Collectors
The most commonly used models of the performance of flat-plate collectors are
those developed by Hottel and Woerts (1942). 1-lottel and Whillir (1958), and Bliss
(1959). The major assumption that has been made in each of these works is that the
thermal capacitance effects of the collector components are negligible.
Examination of a simple configuration as shown in Fig. 5.5.1 gives an
understanding of the temperature gradients. Fig. 5.5.2 shows the region between two
tubes. Some of the solar energy absorbed by the plate must be conducted along the plate
to the region of the tubes. Thus in the vicinity of the tubes the temperature will be lower
than the temperature in the midway between the tubes. The temperature above the tubes
will be nearly uniform because of the presence of the tube and weld metal.
Fig. 5.5.1

170
Fig. 5.5.2
The fluid which is heated, causing a temperature gradient to exist in the direction
of flow. A situation as shown in Fig. 5.5.2 (b) at any location y, is expected in any
region of the collector because the general temperature level is governed by the local
temperature level of the fluid. The general temperature distribution in the direction x, is
as shown in Fig. 5.5.2 (c) and at any location x, the temperature distribution in the
direction will look like as shown in Fig. 5.5.2(d).
The assumptions made in the analysis are:
1. The performance is steady state
2. Sheet and tube type construction
3. The headers cover a small area of the collector can be neglected.
4. The headers provide uniform flow to tubes.
5. The temperature gradients in the glass cover plates are negligible.
6. There is one dimensional heat flow through covers.
7. There is one dimensional heat flow through the back insulation.
8. The sky can be considered as a black body for long wave length radiation at an
equivalent sky temperature.

171
9. Temperature gradients around tubes can be neglected.
10. The temperature gradient in the direction of flow and between the tubes can be
treated independently.
11. Thermal and radiation properties of the collector materials are independent of
temperature.
12. Edge heat losses are negligible compared with heat losses through the collector plate.
13. Losses through the front and back are the same ambient temperature.
14. Dust and dirt on the collector are negligible.
15. Shading of the collector absorbing plate is negligible.
16. Headers contain a disproportionally large volume of water.
Evaluation of Overall Loss Coefficient
The energy loss from the collector plate consists of radiation and convection to
the cover and the edges and the conduction through the back insulation. At some typical
location on the plate where the temperature is t, (mean temperature of the plate), solar
energy of amount S [ HR (τ α)beam + HR (τ α)diffuse] is absorbed by the plate. This
absorbed energy is distributed to losses through the top, bottom and edges and to useful
energy gain.
Thermal network for a 3 cover system and its equivalent thermal network are
shown in Figs. 5.6.1 and 5.6.2 respectively.
The energy loss through the bottom of the collector is shown by two series
resistance R1 and R2. R1 represents resistance to heat flow through the insulation and R2
represents the convection and radiation resistance to the environment. Since R1 >> R2,
we may neglect the value of R2 in calculation. Thus the back loss coefficient Ub, is
approximately
Ub=1/R1=1/(xi/ki)=ki/xi
Where ki = insulation thermal conductivity
xi = insulation thermal thickness.

172
Thermal network for flat plate collector equivalent thermal network
for flat plate collector
If edge insulation thickness is kept equal to bottom insulation thickness, the edge
losses may be estimated by assuming one dimensional sideways heat flow around the
perimeter of the collector system .
The loss coefficient for the top surface is the result of convection and radiation,
between parallel plates. The energy transfer between the plate and first glass cover is
exactly same as between any other two adjacent glass plates and this is approximately
equal to the energy lost to the surroundings from the top glass. The energy loss through
the top per unit area (qloss,top) is then given as
Where hp-c1. = heat transfer coefficient between two inclined parallel plates

173
T = Mean absolute plate temperature °K
Mean Plate Temperature
We have the relation for temperature distribution in the flow direction as
The mean fluid temperature can be found by integrating above expression from 0
to L
Performing the integration process and substituting the values of FR and Qu the
mean fluid temperature was shown by Klein to be .
An approximate relation between mean fluid temperature and the mean plate
temperature is given by .
Where Rpf is the heat transfer resistance between the plate and the fluid. For
liquid flowing in tubes Rpfi is given by .
where n = number of tubes .
L = tube length .
The expressions are used to decide the mean plate temperature with Klein‘s
empirical relation for U. This is also an interactive process.
Collector Performance
The principle design factor affecting collector performance are those related to
heat-loss control and those involving the absorption of solar radiation. For useful heat
gain we have the following equation:

174
Qu = Ac [HR (τ α) - UL (tp - ta)] .
If the numerical values of all the terms in Equation (5.9.1) are known, the rate of
useful recovery Qu can be calculated. In addition to the design characteristics of the
collector, the three operating condition s—solar radiation, average absorber plate
temperature and ambient temperature must be known. With the exception of plate
temperature, these terms can be measured in an operating collector or obtained from
tables or charts for design purposes. Absorber plate temperature however, is seldom
known, nor can it be easily determined. It is affected by the other operating conditions,
inducing the temperature of the fluid being supplied to the collector .
In an operating system composed of collector, storage and space being heated,
the temperature of the fluid in storage can be measured. When a system is being
designed, storage temperature can be calculated or assumed until confirmed. This fluid
is supplied directly to the collector or indirectly via a heat exchanger, thereby, affecting
the absorber plate temperature in Equation (5.9.1). In a typical liquid collector, average
plate temperatures are usually 5 to 10°C above inlet liquid temperature and in air
collectors the temperature difference is 20 to 30°C. For convenience Equation (5.9.1)
can be modified by substituting inlet fluid temperature for the average plate temperature
if a suitable correction factor is applied. The resulting equation is
Qu = FRAc [HR (τ α)- UL (ti - ta)]
Where ti is the temperature of the fluid entering the collector.
FR is a correction factor or ―heat removal factor‖ having a value less tanh 1.0,
such that the useful heat recovery calculated by Equation (5.9.2) is equal to that
calculated by Equation (5.9.1).
The heat removal factor, FR, can be interpreted as the ratio of the heat actually
delivered to that which would be delivered if the collector plate were at a uniform
temperature equal to that of the entering fluid. This temperature equality would
theoretically be possible if the fluid were circulated at such a high rate that there would
be a negligible rise in its temperature as it passed through the collector and if the heat
transfer coefficient were so high that the temperature difference between the absorber

175
surface and the fluid would be negligible. Under such circumstances, the value of FR
would be equal to 1.0.
In equation (5.9.2), the temperature of the inlet fluid is dependent on the
characteristics of the complete solar heating system and the heat demand of the building.
FR, however, is affected only by the solar collector characteristics and the fluid type and
flow rate through the between absorber plate and fluid, the more nearly the fluid
temperature approaches the plate temperature at any one position in the collector.
Similarly, the greater the fluid circulation rate, the smaller is the temperature change
from inlet to outlet and the closer is the inlet fluid temperature to the average plate
temperature. Fig. 5.9.1 shows a typical temperature pattern in a solar collector supplied
with water at 55°C under full sun. Water leaves the collector at about 65°C, absorber
plate temperature is about 5°C above the liquid temperature throughout the collector and
the average plate temperature is about 65°C. If typical values of the collector parameters
are substituted in equations (5.9.1) and (5.9.2), a 55°C inlet fluid temperature in
Equation (5.9.2) and 65°C average plate temperature in Equation (5.9.1) are constant
with a heat removed factor,F8 of about Q.9. If the Coefficient of heat transfer between
the collec liquid is lower, or if a lower liquid circula ion rate is used, the value of FR
would be reduced.

176
A temperature pattern in a typical collector supplied with air from the space
being heated or from the cold end of a pebble bed storage unit at 21°C is also shown in
Fig. 5.9.2. Full sun and a practical air
circulation rate of about 0.6 m3/min per square metre of collector are assumed the
example. The mass flow rate is about the same as that of the liquid at practical pressure
losses. An air temperature rise of 33 to 40°C occur under these conditions, air having
only one-fourth the heat capacity of water. Rather than a moderate 5°C difference
between the plate and liquid temperatures, as in the liquid case, the air collector is
characterized by a 16 to 28°C temperature driving force. The lower heat-transfer
coefficient from the plate to the fluid is responsible for this difference. Under these
conditions chosen, the average plate temperature woiJd be about 65°C approximately,
the same is estimated for the liquid system. Use of Equation (5.9.2) with an inlet
temperature of 21°C results in a heat removal factor FR, typically about 0.7 for an air
collector. Although solar air heaters having heat-transfer surfaces approximately equal to
the solar absorbing area have heat removal factors substantially below those of liquid
collectors, typically differences in fluid inlet temperatures in space-heating systems
result in comparable performance.

177
Equation (5.9.2) may be rewritten as efficiency of solar collection, that is, useful
heat delivery divided by total solar radiation, by dividing both sides of the equation by H
(= HR) and by A. The result is equation (5.9.3)
For a specific collector operating at a constant fluid circulation rate, the values of
Ac, Fft,τ, α and UL are nearly constant regardless of solar and temperature levels. (In
fact values of t and x vary with angle of incidence of solar radiation on the plane of the
collector). Assuming that they are constant, Eq. (5.9.3) represents a straight (ti - ta)/Ht
line on a graph of efficiency versus °. The properties of this line are an intercept (the
intersection of the line with the vertical efficiency axis), equal to the numerical value of
FR < τ α > and a slope of the line, that is, vertical scale change divided by the horizontal
scale change, equal to (— FR UL)
In experimental data can collector heat delivery at various temperatures and solar
conditions are plotted on a graph, with efficiency as the vertically axis, the best straight
line through the data points correlates collector performance with solar and temperature
conditions. Intersection of the line with the vertical axis corresponds to the fluid inlet
temperature being the same as the ambient temperature, where collector efficiency is at
its maximum. At the intersection of the line with the horizontal axis, collection
efficiency is zero. This condition corresponds to such a low radiation level or to such a
high temperature of the fluid supply to the collector that heat losses are equal to solar
absorption and no useful heat is delivei‘ed from the collector.
The linearity of the Equation (5.9.3) rests on the assumption that the values of
FR( τ α) and F1 UL are constant and independent of ti, ta and Ht. Although the influence
is small, FR and UL both depend slightly on collector temperature, represented by t1.
Since radiation loss is a function of the fourth power of the inlet and ambient
temperatures, whereas convection loss is dependent on the first power temperature
difference, the heat loss coefficient increases with rise in collector temperature and with
temperature difference.

178
A graph of efficiency versus (ti - ta)/Ht therefore, must curve slightly
downward as the temperature difference/solar radiation ratio is increased. Fig. 5.9.3
shows the magnitude of this effect, which is usually small enough and in a portion of the
collector operating range seldom encountered, that the linear assumption is adequate for
practical design purposes.
Thermal efficiency curve for a double-glazed flat plate liquid-heating collector
with a selective coating.
In addition to recognition of the linear approximation, useful application of the
equation requires determination and substitution of appropriate values of FR, t, x and
UL. The values of FR and UL are affected by wind velocity across the collector and fluid
velocity through the collector, so both of these flow rates must be known and specified.
Wind velocity has a comparatively small effect on the efficiency of glazed collectors
(and essentially no effect in evacuated tube type collectors) and a ‗standard‘ 4.5 rn/s
wind speed is normally assumed for rating collector efficiencies. The value of FR is not
strongly dependent on flow rate in liquid collectors, but is particularly sensitive to flow
rate in solar air collectors.

179
Selective Absorber Coatings
An effective way to reduce thermal losses from the absorber plate of a solar
beating panel is by using selective absorber coatings. An Ideal selective coating is one
that is a perfect absorber of solar radiation while being a perfect reflector of thermal
radiation. Such a coating will make a surface, a poor emitter of thermal radiation
.
Hence a selective coating increases the temperature of an absorbing surface. If
back losses of an absorbing surface are absent, the steady- state conditions give:
Solar flux absorbed = Thermal flux emitted
The absorptance and emittance of radiation at a given wavelength are equal.
However, at different wavelengths they can vary from near zero to near unity. Since
96% of the sun‘s radiation is concentrated in wavelength ranges of less than 2.5 Jun and
99% of the radiation from a collector surface (operated at less than 400 °K) is in
wavelengths of more than 2.5 jim it is possible to have a surface that will absorb all of
the solar radiation while emitting very little.
A selective surface is a surface that has a high absorptance for short wave
radiation (less than 2.5 jim) and a low emittance of long- wave radiation (more than 2.5
jtm). Although a large number of experimental selective surface treatments and coatings
have been tested, only a few have survived the laboratory
A large number of non-selective coatings are available and are in widespread use
as flat-plate collector coatings. These are primarily organic coatings such as flat black
paints. Most of these coatings have absorptances exceeding 0.95 and emittances of
0.90—0.95. Although the emittances are high, use of these coatings may be justified
economically in some applications where high collector temperatures are not required,
such as hot water systems or swimming pooi heaters. Table 5.10.1 lists properties of
some selective coatings.

180
Table 5.10.1. Selective Coating Properties
Coating Type Absorptance ct Emittanee
Black chrome Electroplated 0.96 0.10
Black Nickel Electroplated 0.90 0.10
Black copper Copper oxide 0.87—0.92 0.07—0.35
Black Anodize Aluminium oxide 0.94 0.07
Solar foil Black chrome over copper 0.96 0.10
Enersorb* Urethane paint 0.97 0.90
Nextel* Paint 0.98 0.89
*Non selective.
Most surfaces that are good absorber for solar radiation are also good radiators
for heat. If, for example, a non selective surface has an absorptance of 0.95 for radiation,
it will radiate heat at a rate of about 95% of that of a black body radiator. Selective
surfaces are capable of absorbing solar radiation effectively while at the same time
radiating little heat. Most selective surfaces are composed of a very thin black metallic
oxide on a bright metal base. The black oxide coating is thick enough to act as a good
solar absorber, with an absorptivity as high as 0.96, but it is essentially transparent to
long-wave thermal radiation emitted by an object at a temperature of several hundred
degrees. Since bright metals have low emissivity for thermal radiation, that is, are poor
heat radiators, and since the thin oxide coating is transparent to such radiation, the
combination is a poor radiator of heat. As a result, the radiation loss from a selective
surface is considerably lower than from a conventional, non-selective surface. The
overall heat loss coefficient UL is reduced when this type of surface is used.
A selective surface should also possess the following characteristics in addition
to the above mentioned spectral characteristics .

181
(i) Its properties should not change with use. A significant degradation in the
values of a and e, has been observed in the case of some selective coatings, largely due
to the effect of exposure to atmospheric humidity .
(ii) It should be able to withstand the temperature levels associated with the
absorber plate surface of a collector over extended periods of time. It should be able to
withstand any short-term temperature rise which may occur when no useful heat is being
removed.
(iii) It should be able to withstand atmospheric corrosion and oxidation.
(iv) It should be of reasonable cost
Effect of Dust and Shading
From long term experiments on collectors, it is found that collector performance
is decreased about 1% due to dirty glass. In India, Garg (1974) found in a experiment
that dust reduced the transmittance by an average of 8% for glass tilted at 45°.
For design purposes without extensive tests, it is suggested that radiation absorbed by
the plate be reduced by a factor of(1 — d), where d is 0.02 to account for dust.
The dust deposit on the cover system reduces the transmissivity of the cover
system and thereby efficiency of the collector. The rate of deposition of dust depends on
the graphical and metrological parameters. The dust factor is related to the dust layer
transmissivity, hence the dust factor reported by investigators depend for places of
investigations. A comparison and correlation of the finding of the above investigators is
possible only if the transmissivity of the dust layer is known as a function of(i) angle of
incidence, (ii) dust deposit rate and (iii)ratio of diffuse to total radiation. Therefore, a
systematic study of the reduction in transmissivity due to dust with reference to above
variables is needed. Whenever the angle of incidence is not normal, some of the
structure will intercept solar radiation. Some of this radiation will be reflected to the
absorbing plate if the side walls are of a high reflectance material. Hottel and Woertz
recommend that the radiation absorbed by the plate be reduced by 3% to account for
shading effects. Hence radiation absorbed by the plate is reduced by (1 — s), where s =
0.03 to account for shading.

182
Therefore, the amount , S=HR< τ α > (1—d)(1—s)=0.951HR < τ α >
SOLAR COLLECTORS : FOCUSING TYPE
Focusing collector is a device to collect solar energy with high intensity of solar
radiation on the energy absorbing surface. Such collectors use optical system in the form
of reflectors or refractors. A focusing collector is a special form of flat plate collector
modified by introducing a reflecting (or refracting) surface (concentrator) between the
solar radiations and the absorber. Focusing collectors can have radiation increase from
low value of 1.5 to 2, high values of the order of 10,000.
The main advantages of concentrator systems over flat-plate type collectors are
1. Reflecting surfaces requires less material and are structurally simpler than flat-
plate collectors. For a concentrator system the cost per unit area of solar
collecting surface is therefore potentially less than that for flat-plate collectors.
2. The absorber area of a concentrator system is smaller than that of a flat-plate
system of same solar energy collection and the insolation intensity is therefore
greater.
3. Because of the area from which heat is lost to the surroundings per unit of the
solar energy collecting area is less than that for a flat-plate collector and because
the insolation on the absorber is more concentrated, the working fluid can attain
higher temperatures in a concentrating system that in a flat-plate collector of the
same solar energy collecting surface.
4. Owing to the small area of absorber per unit of solar energy collecting area,
selective surface treatment and/or vacuum insulation to reduce heat losses and
improve collector efficiency are economicallY feasible.
5. Focusing or concentrating systems can be used for electric power generation
when not used for heating or cooling. The total useful operating time per year
can therefore be large for a concentrator system than for a flat-plate collector and

183
the initial installation cost of the system can be regained by saving in energy in a
shorter period of time.
6. Because the temperature attainable with concentrating system is higher, the
amount of heat which can be stored per unit volume is larger and consequently
the heat storage costs are less for concentrator systems than for flat-plate
collectors.
7. In solar heating and cooling applications, the higher temperature of the working
fluid attainable with a concentrating system makes it possible to attain higher
efficiencies, in the cooling cycle and lower cost for air conditioning with
concentrator systems than with flat-plate collectors.
8. Little or no‘ antifreeze is required to protect the absorber in a concentrator
system whereas the entire solar energy collection surface requires antifreeze
protection in a flat-plate collector.
Bus such collectors present additional problems, i.e. disadvantages are
1. Out of the beam and diffuse solar radiation, components, only beam component
is collected in case of focusing collectors because diffuse component cannot be
reflected and is thus lost.
2. In some stationary reflecting systems it is necessary to have a small absorber to
track the sun image; in others the reflector may have to be adjustable more than
one position if year round operation is desired : in other words costly orienting
systems have to be used to track the sun.
3. Additional requirements of maintenance particularly to retain the quality of
reflecting surface against dirt, weather, oxidation etc
4. Non-uniform flux on the absorber whereas flux in flat-plate collectors is uniform.
5. Additional optical losses such as reflectance loss and the intercept loss, so they
introduce additional factors in energy balances.

184
These problems and consequent high cost have restricted the utility of focusing
collectors and no long time practical applications of focusing collectors other than for
furnaces are being made. New materials and better engineering or systems may make
them of practical importance.
To avoid confusion of terminology the word collector will be applied to the total
system including the receiver and the concentrator. The receiver is that element of the
system where the radiation is absorbed and converted to some other energy form and
includes the absorber, covers and insulation etc. A schematic of a focusing collector iS
shown in Figure. The concentrator or optical system is the part of the collector that
direct (reflects or refracts) radiations on to the receiver.
Solar Concentrators and Receiver Geometries
There is a wide variety of means for increasing the flux of radiation on receivers;
they can be classified as lenses or reflectors, by the types of mounting and orienting
systems, by the concentration of the radiation they are able to accomplish, by materials
of construction, or by orientation. Concentrator is a component used to increase the
intensity of energy flux on a receiver; Concentration ratio (CR), it is the quantity = , the
ratio of the area of the concentrator aperature to the energy absorbing area of the
receiver. It determines the effectiveness of a concentrator. Concentrating collectors can
be classified as follows

185
1. They may be of reflecting type utilizing mirrors or of the refracting type utilizing
Fresnel lenses. The reflecting surfaces may be of parabolic, spherical or flat
configuration. They must be continuous or segmental.
2. As per the optics, the solar concentrators generally can be DIassifled as either
point focus or line focus systems. Point focus ystems have circular symmetry and
are generally used when high brightness concentration factors are required, as in
solar furnaces and central receiver power systems. Line focus systems have
cylindrical symmetry and are generally used when medium concentration is
sufficient to reach the desired operating temperatures.
3. Abroad classification of solar concentrator is based on the field of view of the
concentrator. If the field of view is much larger than the angular size of the sun
which is about ° (32‘), then it is not necessary to continuously orient the
concentrator towards the sun as it moves in the sky. Such concentrators are
referred to as non-tracking concentrators, in contrast to tracking concentrators
which need to track the sun continuously. The concentration ratio achievable by
non-tracking concentratoré, is generally less than that for tracking concentrators.
Based on the above classification, following are some possible concentrating systems.
1. Plane Reflector and Plane Receiver Type. Figure (a) shows that both reflector
and receiver are plane. Such a system is very simple in construction and has the
advantage of absorbing some diffuse component of radiation which falls directly
on the receiver. However the concentration ratios of this type are relatively low,
with a maximum value of four or less than four,
2. Conical Reflector and Cylindrical Receiver type. Figure (b) shows this system
in which reflector is conical and receiver is cylindrical. Concentration ratio is
little higher than that of first case, it may be of the order of 10.
3. Fresnel Reflector. This consists of a parabolic shape reflector made up of small
segments [Figure (c),(d)]. The main advantage is in easy fabrication but this does
incur some additional losses of radiation near the rim of each segment. Its
refracting counterpart is shown in Figure (d). The advantage of linear Fresnel
lenses is that the convenient mass production technique of extrusion of

186
thermoplastic materials can be applied to their fabrication. A concentration ratio
of about 10 is obtainable using them.
4. Parabolic system. In a system consisting of a paraboloid or a parabolic mirror
and having receiver at its focal point. The concentration ratios are very high and
therefore can be used where high temperatures are required. In cylindrical
system, the concentration ratio is lower than paraboloid counterparts. In both the
cases the receiver is placed at the focus, i.e. along the focal line in cylindrical
parabolic system and at the focus point in paraboloidal system.
A modification to parabolic system is shown in Figure. In the case o this case a
double reflection system is used to shift the focus to a convenient point.
Concentration ratios of about 30 to 100 or higher would be needed to achieve
temperatures in the range 300 to 5000°C or higher. Collectors designed for such high

187
concentration ratios necessarily have small angles of field of view and hence need to
track the sun continuously. A broad classification of such collector is,
(i) Central receiver collector, such as the paraboloidal mirror and the tower
power plant using heliostat mirrors.
(ii) The linear focus collector in the form of a parabolic trough the ones
employing faceted mirror strips.
(iii) Spherical and conical mirror (Axicon) with aberrateic foci. The physical
upper limit to the concentration ratios achievable with paraboloids and
parabolic troughs is determjned by their f/d ratios and are about 10,000 and
100 respectively for the two cases. The concentration ratios achieved in
practice are about 1/3 to ½ of the above values because of surface
irregularities of the reflector tracking errors etc.
The concept of central receiver collector is very simple. the cost and heat losses
in transporting a working fluid to location, use sunlight itself as the transfer medium.

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Solar Air Heaters
Solar air heater has an important place among solar heat collectors. It can be used
as sub-systems in many systems meant for the utilization of solar energy) Possible
applications of solar air heaters are drying or curing or agricultural products, space
heating for comfort, regeneration of dehumidifying agents, seasoning of timber, curing
of industrial products such as plastics.(j general solar air heaters are quite suitable for
low and mo4erate temperature applications as their design are relatively simple When
air at high temperature is required, the design of a heater-becomes complicated and
hence very costly. With the recent trend of providing both heating and cooling with solar
energy, liquid heaters for high temperature operation have gained more popularity. As
far as the ultimate application for heating air to maintain a comfortable environment is
concerned, the solar air heater is the most logical choice. Heating a fluid by the sun, then
transferring heat to the air results in loss of temperature potential available. If the
limitations of solar energy applications in terms of attainable temperatures and related
efficiencies as well as the low cost requirements are considered, it would be wiser to
eliminate any heat transfer operations.

189
Direct use of the air circulated through the solar air heater as the working
substance also reduces the number of components required in the system. Solar heated
air could be used more effectively for drying under controlled conditions. Solar heaters
supplying hot air to a conventional drier or special design combining the air heater and
the drying cabinet in one package have cost and efficiency advantages for drying
applications. Solar air heaters also eliminate corrosion and leakage problems which may
be difficult and costly to overcome. The cost of the air heater could be substantially
lower than the liquid systems. Higher pressures experienced in liquid heaters necessitate
the use of heavy-gauge sheet metal or tubes. The air heaters could be designed using less
materials even some scrap of no commercial value.
Air systems are more appealing than liquid system because they require less
plumbing and are, therefore, less costly. The complications of liquid systems stem from
potential freezing problems within the collector, the need to allow for the expansion of
liquid as it heats up in the system, including the possibility for the flashing of the liquid
into gas (such as water into steam) ; the possibility of leakage anywhere within the
system ; and the corrosion of the metal plumbing. The relative simplicity of air systems
is attractive to people who wish to build their own system, but as with all system of
collecting, storing and using the sun‘s energy, their precise design is difficult and all
except the simplest of systems must be designed by some one- knowledge in simple
mechanics, and heat transfer. Air collectors, are however, relatively easy to maintain and
repair. Fans, damper motors, and control may fail, but the large components, including
the collector, the heat storage and the ducting should last indefinitely. The construction
of air collectors and related components and systems is relatively simple when compared
to the tasks of plumbing and of trying to find the and utilize absorber plate compatible
with liquid systems. More easily handled are absorber plates compatible with air systems
: since they are not connected into a plumbing system, they must be leak-proof and since
they do not have to have great care taken in allowing for expansion and contraction, they
do not have to be built with as much precision. in fact, for air type collectors, the
absorber surface need not be metal, as stated earlier. Since in many collectors designs,
the air conies in contact with every surface heated by the sun, heat does not have to be
conducted from one area of the absorber surface to another, as is the case with absorber

190
plates for liquid collectors. Almost any blackened surface which is heated by the sun
will transfer heat to air when the air is blown on it. This heat transfer mechanism opens
up numerous possibilities for absorber surface
There could be a major problem for improperly designed collectors, of the
limitation of the heat transfer from the absorber plate to the working fluid for air heaters.
There are several ways to improve the heat. transfer coefficients, and hence efficiencies
are quite coin- parable with liquid heaters
Disadvantages of solar air heaters are
(1) Need of handling larger volumes of air than liquids due to low density of air as
working substance.
(2) Thermal capacity of the air is low. In cases where the thermal storage is required,
water is superior one. Water may be used both as a heat transfer and a heat storage
substance. The freezing problem is overcome by using a antifreeze solution. Due to
the high cost of the antifreezing solutions, the cost of the thermal storage fluid may
become quite high. The suggested use of antifreeze to water heat exchangers (water
being the storage substance) reduces the storage system cost but the slight loss in
temperature available and the expense of an extra heat exchanger may be justified.
The final choice of the working fluid for solar energy is a complicated matter that
necessitates a careful comparative analysis of thermal performance, operation,
maintenance and economic parameters.
(3) They have relatively high fluid circulation costs (especially if the rock heat-storage
unit is not carefully designed).
(4) They have relatively large volumes of storage (roughly three times as much volume
as for water heat-storage).
(5) They have a higher noise level.
(6) The system has difficulty of adding conventional absorption air-conditioners to air
systems; and

191
(7) The space is required for ducting.
Water-heating solar systems use a common heat transfer and storage medium in
areas of the world where freezing temperatures are not encountered. The water storage
volume is about one-third of the volume of rocks necessary to store equal quantities of
heat for air systems. Liquid systems are rather easily adapted to supply energy to
absorption air-conditioners and are also less noisy than air systems. The energy
requirements for pumping the heat-transfer fluid are much less for water than for air
systems using blowers to circulate air.
Figure represents a typical solar air-heating collector. A conventional air heater
is typically a flat passage between two parallel plates. One of the plates is blackened to
absorb incident solar radiation. One or more transparent covers are located above the
absorbing surface. The air is made to pass through the passage and in doing so, is heated.
Insulation around the sides and base of the unit is necessary to keep heat losses to a
minimum.
There are eight variables that a designer concerns himself with in the construction of an
air heater:
1. Heater configuration is the aspect ratio of the duct and the length of the duct through
which air passes.
2. Airflow: Air must be pumped through the heater; increasing the air velocity results in
higher collection efficiencies, but also in increased operating costs
.
3. The type and number of layers of cover material must be considered and spectral
transmittance properties must be examined. In general, the higher the temperature
required, the more covers are used; the principle underlying the use of multi covers is
that each air layer between two successive covers provides a barrier against heat losses
from the absorbing surface to the atmosphere. The temperature of the outermost cover
plate becomes progressively lower with increases in the number of covers ; hence, the
heat losses from the outermost pane to the atmosphere are reduced. However, with a

192
large number of covers, the reflective losses increase (in addition to the cost), so that
more than two covers are seldom used. Covers of high transmissivity and low
reflectivity are desired to keep the amount of reflected and absorbed radiation row
4. Absorber plate material. Although selective surfaces can significantly improve the
performance of solar air heaters by increasing the collector efficiency, black-painted
solar heaters are commonly used, due to the cost of selective surfaces. The absorber is
coated black to absorb the maximum amount of incident radiation. The absorber need
not be metal, since the air to be heated is in contact with the entire absorbing surface.
This means that the thermal conductivity of the absorber plate is relatively unimportant.
5. Natural convection barriers : A stagnant. air gap interposes a high impedance to
convective heat flow between the absorber plate and the ambient air. The losses, both of
radiation and convection, can be reduced to low values by the use of multiple covers or
honeycombs, but the consequent reduction in transmission of solar radiation makes more
than one air gap of doubtful value.
6. Plate-to-air heat transfer coefficient: The absorber can be roughened and coated to
increase the effective coefficient of heat transfer between the air and the plate. The
roughness ensures a high level of turbulence in the boundary layer of the flowing air
stream. For this reason, crumpled or corrugated sheets and wire screens are attractive as
absorbing materials.
7. Insulation is required at the absorber base to minimize heat losses through the
underside of the heater.
8. Solar radiation data corresponding to the site are needed to evaluate heater
performance. For a typical solar air heating collector, shown in Fig. 6.1.1, we can write
the performance equation as follows useful heat gain per unit area.
qU = HT(τ α)e — UL (Tp — Ta)
(HT =HR and qU = Qu/Qc )

193
The collector efficiency factor F‟ is defined as the ratio of the actual rate of
useful heat collected to the ratio of heat collection that would be possible if the entire
collector surface was at the average fluid temperature, in other words, if there were no
temperature drop between fluid and absorber plate. Obviously, F‘ must be less than one,
because there must be a temperature differential between fluid and plate for heat to flow
from the plate to the fluid. Thus, if T1 is the average fluid temperature, then F‟ may be
written
So
If F and the overall loss coefficient are known, the performance of the collector
can be calculated. Equation (6.1.3) applies to liquid- and air heating collectors. Different
types of collectors can be compared with each other by comparing values of F, Uç and (t
. F‘ is related to the heat removal factor FR by:
where G = mass flow rate per unit area of collector aperture = m/A
Cp = heat capacity of heat transfer fluid (air for air heating co1lector).
Comparing equations we get
qU =FR [HT(τ α)e — UL (Tfi — Ta)
This is the performance equation written in terms of the fluid inlet temperature,
where as equation (6.1.3) is the performance equation written in terms of the average
fluid temperature. Tfi (fluid inlet temperature) is much easier to measure than air flow
over the absorber plate (between the absorber and the cover plate). Average fluid
temperature T1 can be approximately taken as
Tf = (Tfi+ To)/2

194
To= collector outlet temperature
Materials for Flat Plate Air Collectors or Solar Air Heaters :
(i)Sheets of glass .
(ii) Metal scraps (or pieces of metal can) attached to the rigid board, metal lathe,
fibre glass meshes (e.g. air filters), crushed glass or rock, cloth and even paper. Many of
these can be obtained cheaply or as recycled or re-used materials ; however, the entire
surface must be black, must be heated directly by the sun, and must come in contact with
air flowing through the collector.
Types of Air Heaters
Basically air heaters are classified in the following two categories
(i) The first type has a non-porous absorber in which the air stream does not
flow through the absorber plate. Air may flow above and/or behind the
absorber plate, as shown in Fig. 6.2.1
(ii) The second type has a porous absorber that includes slit and expanded
metal, transpired honey-comb and over-lapped glass plate absorber, as
shown in Fig. 6.2.2
(iii) Non-porous absorber plate type collectors. A non-porous absorber may
be cooled by the air stream flowing over both sides of the plate as shown
in Fig. 6.2.1 (a). In most common design the air flows behind the
absorbing surface. Air flow above the upper surface increases the
convection losses from the cover plate and therefore is not recommended
if the air inlet temperature and/or temperature rise at the collector are
large, it is shown in Fig. 6.2.1 (b).

195
Transmission of the solar radiation through the transparent cover system and its
absorption is identical to that of a liquid type flat-plate collector. To improve collection
efficiency selective coating may be applied provided there is no much cost. Due to low
heat transfer rates, efficiencies are lower than liquid solar heaters under the same
radiation intensity and temperature conditions. Performance of air heaters is improved
by
(a) roughening the rear of the plate to promote turbulence and improve the
convective heat transfer coefficient, or

196
(b) adding fins to increase the heat transfer surface. Usually turbulence is also
increased which enhances the convective heat transfer.
Absorption of solar radiation is improved due to surface radiative characteristics
and the geometry of the corrugations, which help in trapping the reflected adiation.
(ii) Collectors with porous absorbers. The main drawback of the non-porous absorber
plate is the necessity of absorbing all incoming radiation over the projected area from a
thin layer over the surface, which is in the order of a few microns. Unless selective
coatings are used, radiative losses from the absorber plate are excessive, therefore, the
collection efficiency cannot be improved. The pressure drop along the duct formed
between the absorber plate and the rear insulation may also be prohibitive especially in
the case of added fins to increase the heat transfer surface and turbulence rate. The
difficulty with turbulence is the pressure drop across the collector. Too many surfaces
and too much restriction to air flow will require a larger fan and a larger amount of
energy to push the air through. The energy required for this cancels out saving from
using solar energy, particularly if fan is electrical and if the amount of energy which is
burned at the power plant to produce the electrical energy is included.
These defects are eliminated in a porous absorber type collectors, in two ways.
(a) The solar radiation penetrates to greater depths and is absorbed gradually
depending on the matrix density. The cool air stream introduced from the upper surface
of the matrix is first heated by the Upper layers which are cooler than the bottom layers.
The air streams warm up, while traversing the matrix layers. The lower matrix layers are
hotter than the upper ones, therefore, the air stream can effectzvely transfer heat from
the matrix. Improper selection of the matrix Porosity and the thickness may result in
reduced efficiencies since the additional matrix layers beyond an optimum may no
longer absorb the solar radiation and heat the air stream further.
Applications of Solar Air Heaters
An heated by using one of the solar heat collectors described in the previous sections
could be mainly used for the following processes:

197
(i) Heating buildings.
(ii) Air conditioning buildings utilizing desiccant beds or an absorption refrigeration
process.
(iii) Drying agricultural produce and lumber.
(iv) Heating green houses.
(v) Using air heaters as the heat source for a heat engine such as a Brayton or Stirling
cycle.
(i) For space heating such as rooms, the direct absorption of solar radiation with
the thermo plane windows is done. Heat of fusion type thermal storage material, which
acts as a thermal buffer in the room is used. Glauber‘s salt (Na2.S04.10H20) and rock
pile are also used as thermal storage media. Solar heat collectors are also employed for
space heating with eutectic salt thermal storage. Collectors generate electricity via CdS
cells in addition of heating air. Finned absorber type design of air heater is preferred.
(ii) Dunkle (1965) has suggested solar air conditioning by using desiccant beds.
Another approach to obtain cooling is by using heat of solar air heaters, in the generator
of an absorption air conditioner. Generator temperature is kept nearly at 95°C.
(iii) The advantages of solar drying over open sun exposure of agricultural
produce have been accepted. Solar driers are expected to be used by farmers with limited
technical skills and small capital therefore, these proposed devices should be simple,
inexpensive and use least land area. Basic solar drying processes and various design
which are suggested are shown in Fig. 6.4.1. Totally enclosed type driers, provide the
protection from dust, dirt and entrance of the insects and enable control of the rate of
drying. The cabinet-type driers may serve family needs, while large scale commercial
applications necessitate tunnel or shelf type driers. An inflated plastic drier is the
simplest to build, but charging the wet produce, turning over during drying and
emptying could be a problem. Shelf-type driers with plastic liners without forced sir
circulation are extensively used in Australia.
(iv) Direct absorption of the solar radiation is employed In the green house which
acts as a solar air heater with no air circulation. The term Green House effect usually

198
refers to solar air or liquid heaters, employing transparent glazings. This phenomenon
was believed to control the green house temperature, namely, transmission of the short
wave solar radiation through the green house glazing‘s by entrapment of the out-going
radiation.
(v) Application of air heaters as the heat source for a heat engine, however,
requires temperatures in the order of hundred of degrees which is beyond the practical
range of operation of the collectors described here. Concentrating type collectors are
suggested.
Typical Air Collector
Fig. 5.3.3 shows a schematic flat-plate collector where an air stream is heated by
the back side of the collector plate. Fins attached to the plate increase the contact
surface. The back side of the collector is heavily insulated with mineral wool or some
other material (detailed analysis of solar air collectors is given in Chapter 6). The most
favorable orientation, of a collector, for heating only, is facing due south at an
inclination angle to the horizontal equal to the latitude plus 15° (s = 0 + 15°).
5.3.3
Air can be passed in contact with black solar absorbing surface such as finned
plates or ducts as mentioned above, corrugated or roughened plates of various materials,

199
several layer of metal screening and overlapped glass plates. Flow may be straight
through, surpentine, above, below or on both sides of the absorber plate, or through a
porous absorber material.
The flat-plate is a simple and effective means of collecting solar energy for
applications that require heat at temperatures below about 100°C. These collectors have
been used successfully for providing domestic hot water, space heating, air conditioning,
power generation, water pumping, cooking and other purposes
The advantages of flat-plate collectors, as compared with focusing collectors are
(i) No complicated tracking mechanisms are involved.
(ii) Construction is relatively simple.
(iii) They can utilize both the diffuse and direct components of the available solar
radiation.
(iv) They are easily manufactured.
The advantages are their temperature limitation (about 100°C) and the fact that
the collector heat exchanger area must equal the collector aperture area.
Japanese pipe solar water heaters
Japanese typically take bath in the evening because of humidity and therefore use
water heaters which are nothing but glass, stainless steel, G.I. or plastic pipes blackened.
Some typical and commercial designs of Solar water heaters are:
(i Natural circulation solar water heater (pressurized)
(ii) Natural circulation solar water heater (non-pressurized)
(iii) Forced circulation solar water heater.
(i) Natural circulation solar water heater (pressurized). A natural circulation
system is shown in Fig. 11.2.5. It consists of a tilted collector, with transparent cover
plates, a separate, highly insulated water storage tank, and well insulated pipes

200
connecting the two. The bottom of storage tank is at least a foot (0.4 m) higher than the
top of the collector, and no auxiliary energy is required to circulate water through it.
Circulation occurs through natural convection, or thermosiphoning. When water in the
collector is heated by the sun, it expands (becomes less dense) and rises up the collector,
through a pipe and into the top of the storage tank. This forces cooler water at the
bottom of the tank out another pipe leading to the bottom of the collector. This water, in
turn, is heated and rises up into the tank. As long as the sun shines the water will quitely
circulate, getting warmer. After sunset, a thermosiphon system can reverse its flow
direction and loss heat to the environment during the night. To avoid reverse flow, the
top heater of the absorber should be at least 1 ft (0.4 m) below the cold leg fitting on the
storage tank, as shown. To provide heat during long, cloudly periods, an electrical
immersion heater can be used as a backup for the solar system. A non-freezing fluid
should be used in the collector circuit. The thermosiphon system is ofle of the least
expensive solar hot-water systems and should be used whenever possible.
(ii) Natural circulation solar water heater (Non-pressurized). Fig. 11.2.6 (a).
The pressurized system is able to supply hot water at locations above the storage tank.
This creates considerable stress on the water channels in the collector which must be
designed accordingly. The non-pressurized systems supply hot water by gravity flow
only to users lower than the tank. If pressurized hot water is required (for showers or
appliances) the difference in height will have to be large enough to meet the
requirements. If the height of difference can not: be accommodated, the only solution is
to install a separate pump and pressure tank. The stresses within non-pressurized system
are lower which allows cheaper and easier construction.

201
(iii) Forced circulation solar water heater. Fig. 11.2.7 s schematically an
example of forced circulation system. Here th no requirements for location of the tank
above the collector additional components would include a pump, motor, and a
controller (a differential thermostat between tank and collect( check valve is needed to
prevent reverse circulation and rest nighttime thermal losses from the collector. In this
example, aux heater is shown as provided to the water leaving the tank and to the load.

202
Description of Solar Water Heaters and Their Installation Details
In thermosiphon solar water heater natural circulation takes through changes of
density of the water caused by heat absorbed solar radiation. The solar heated water rises
into the insu storage tank, and the other colder water from the tank flov natural
convection into the lower part of the solar collector. (For this type of water heater, the
height of the storage above the top of the solar collector is an important design paran For
a small system, the tank should be about 0.6 m above the from the collector (range is
about 0.4 m to 0.6 m). This type of water heater has become very popular in the mid
east, Africa, Australia and other areas of the world where temperature do not
substantially below freezing in the winter. Thermosiphon solar heaters are generally not
used in climates that experience s temperatures well below freezing, since repeated
freezing and ing of water in the collector tubes can result in tube rupture. efforts have
been made to adopt thermosiphon systems to fre climates by providing electrical
resistance heating in the coil during cold whether or using plastic absorbers that can
accommodate freezing in the tubes. The insulation is left off the hot water piping and
storage tank for clarity. The m valves on the upper and lower headers permit the
collectors drained without having to drain the tank. The drain valve is n sary to drain the
collectors. The cold water shut off valve on th of the storage tank and the drain valve on
the bottom are req to drain the tank. The mixing valve is required for safety to pr
delivery of scalding water, and in general to assure that the del temperature does not
exceed some preset valve. The temperature pressure relief valve is a standard
requirement of plumbing c and serves to protect against excessive system steam pressure
can be removed from the system by manually opening this valve copper header as
sloped upward as shown to prevent air traps
SOLAR PONDS
The solar pond combines solar energy collection and sensible heat storage.
Temperature inversions have been observed in natural lakes having high concentration
gradients of dissolved salts (i.e. concentrated solution at the bottom and dilute solution at
the top). This phenomenon suggested the possibility of constructing large-scale
horizontal solar collectors as ponds. No convective solar ponds have been proposed as a

203
simple relatively inexpensive method of collecting and storing solar energy on a large
scale.
The two most fundamental characteristics of solar energy, namely its diluteness
and intermittent nature, are also the reasons why‘ it is not being harnessed on a large
scale at present. First of all, collectors fabricated using materials such as glass, metals,
wood etc. have size limitations and therefore a large number of them with suitable
interconnections will be needed to collect large amount of solar energy. Secondly, to
supply energy ‗on demand‘ will require some sort of energy storage and reconversion
system to smooth out the variations in the isolation due to cloud cover, seasonal and
diurnal effects.
Solar ponds promise an economical way around these two problems by
employing a mass of water for both collection and storage of solar energy. The energy is
stored in low grade (60 to 100°C). Thermal form which, in self, might be suitable for a
variety of application. Such as space heating and industrial process heat. Alternatively,
organic Rankine cycle engines can be used to obtain mechanical: and/or electrical
energy.
Ponds have been studied experimentally and analytically at the National Physical
Laboratory of Israel and by others. Tabor - outlines the general concept and the major
problems. Tabor and I (1965) carried out theoretical investigations of the underlying
physics of the solar pond and laboratory and field tests, to study the many factors
affecting pond performance. At MIT, Stolzenbach (1968) developed numerical methods
to predict temperature distributions within the solar pond. A group of Russian scientists
at the Uzbek SSR Academy of Science (1973) has been actively engaged in very
detailed and sophisticated research of the relevant physics. In India Dr. G.C. Jam (1973)
has designed and is operating a solar pond use in the production of salt at the Central
881t and Marine Chemicals Research Institute at Bhavnagar.
The operation of a 400 rn2 salt gradient solar pond during 1980 and,1981 was
reported by Nielsonnd Kamal (1981). It was found that the boat loss to the earth
exceeded the heat gained by the pond. Weeks aid Bryont (1981) reported that a salt

204
gradient solar pond of the University of New „Mexico reached a temperature of 108°C
and boiled during July of 1980. The stability of the salt and temperature gradients were
substantially disrupted when the pond began to boil. Since this pond was of a relatively
smaller Size (13 m din), it is reasonable to expect that larger ponds would be even more
Susceptible to boiling, unless heat is extracted.
The possibility of using solar ponds to provide hot water for residential
subdivisions was investigated by Leboeuf (1981). The study indicated that it would-be
technically and economically. feasible to use solar ponds to supply the thermal
requirements of a communty. Solar ponds were sized to provide all the heating and
domestic hot water loads in a typical year for communities in. Washington, DC, and Fort
Worth, Texas. It was found, for example, that a I00-m-diameter solar pond with 1-m
storage layer depth would provide sufficient energy to drive an absorption or Rankine
cycle cooling unit to meet the air conditioning needs of 50 homes. Styris et al (1976)
examined applications of non-convecting solar ponds for heating buildings and
providing process heat in Rich land, Washington. They determined that a major cost
factor in the operation of solar pon9 is the quantity of salt necessary to sustain the
salinity gradiant. Koai (1979) reported a numerical analysis of the static alt gradient
solar pond in which he found that the efficiency equation had the same farm as that of a
conventional fiat-plate solar collector.
PrincIple of Operation of Solar Ponds
The solar pond is a simple device for collecting and storing solar heat. Natural
ponds convert solar radiation into heat, but the heat is quickly lost through convection in
the pond and evaporation from its surface. A solar pond, on the other hand, is designed
to reduce convective and evaporative heat losses so that useful amounts of heat can be
collected and stored. Solar ponds may be classified as convecting or non.conevecting.
Convecting Solar Ponds.
A convecting solar pond reduces beat lose by being covered by a transparent
membrane or glazing (Pig.

205
One type of convecting solar pond uses a plastic tube filled vith water, as
illustrated in Fig. 10.2.2. Each pond module includes long, narrow plastic bag measuring
5 x 60 m containing water 5—10 tm deep. The bag has a transparent top to allow
transmission of tunlight and to prevent evaporation losses. The bottom of the bag is llack
to absorb sunlight. A layer of insulation beneath the plastic bag ninimizes heat losses to
the ground. One or two layers may be arched wer the bag of water to suppress
convective and radiative losses.
Cross section of a shallow solar pond.
In this type of solar pond, the hot water is removed late in the afternoon and stored in
insulated reservoirs. Glazing materials for the solar pond may include polyvinyl chloride
(PVC) film and clear acrylic panels. The panels covering the plastic bags screen out
ultraviolet (UV) radiation and greatly increase the life of the plastic bags.

206
Non-convecting Solar Ponds.
Non-convecting solar ponds prevent heat losses by inhibiting the convection to
forces caused by thermal buoyancy. In convecting solar ponds, solar radiation is
transmitted through the water to the bottom, where it is absorbed; in turn, the water
adjacent to the bottom is heated. Natural buoyancy forces cause the heated water to rise,
and the heat is ultimately released to the atmosphere. In a non -convective solar pond,
the warm water is prevented from rising to the surface. Non-convective ponds may be
stabilized by viscosity, a gel or to a salt. The salt gradient pond is the most common type
of non-convecting solar pond, it will be described in the following sections
.
Salt Gradient Ponds. A solar pond is a mass of shallow water about 1—1.5 metre
deep with a large collection area, which acts as a heat trap. It contains dissolved salts to
generate a stable density gradient. Salts has been dissolved in high concentrations near
the bottom, with decreasing concentration toward the surface. The salts most commonly
used for salt gradient ponds are sodium chloride and magnesium chloride, although there
are many other possibilities.
Part of the incident solar radiation entering the pond surface is absorbed
throughout the depth and the remainder which penetrates the pond is absorbed at the
black bottom. If the pond were initially filled with fresh water, the lower layers would
heat up, expand and rise to the surface. Because of the convective mixing and heat loss
at the surface, only a small temperature rise in the. Pond could be realized. On the other
hand, convection can be eliminated by initially creating a sufficient strong salt
concentration gradient. In this case, thermal expansion in the hotter lower layers is
insufficient to des- H stabilize the pond. With convection suppressed, the heat is lost
from the lower layers only by conduction. Because of its relatively low thermal
conductivity, the water acts as an insulator and permits high temperatures (over 90°C) to
develop in the bottom layers. Energy can be extracted from the pond by receiving the
water in the hot layers of the pond through a heat exchanger.
The salt gradient pond consists of three layers. In the top layer, vertical
convection takes place due to effects of wind

207
Salt gradient solar pond
Types of Solar Ponds
Solar ponds are of several types like shallow solar pond, partitioned solar pond,
viscosity stabilized solar pond, membrane stratified solar pond and saturated solar pond.
A brief discussions of each are given below.
1. Shallow Solar Ponds (SSP). A shallow solar pond is body of ) water with
shallow depth acting as large collector and a storage of solar radiation. It is large area,
low cost collector where water is directly exposed to solar radiation and enclosed in a
thermal insulating base material and one or two sheets of glazing. To keep cost low,
polymeric materials are used wherever possible instead of metal and glass. There is a
large number of design options available. In the most popular design, a black pond liner
of a tough material such as butyl rubber, hypalon or chlorinated polyethylene is
stretched over the insulation base and attached to the top of the concrete curbings. Two
layers of clear plastic film are then placed over the black liner and attached to the curbs.
The space between the liner and the lower film is filled with water and the top film is

208
inflated by use of a small blower. In another design recommended for large applications
a two-layer plastic bag is fabricated with a black bottom and clear top. It rests on a
insulation pad and is filled with water. Arched over the top of the bag are semigrid,
corrugated clear plastic sheets secured along the curb edges and also over the top by
steel tie-down straps where the sheet overlaps. Several such SSP can be connected
together and hot water from them can be pumped and stored in a large insulated storage
reservoir to reduce thermal losses during night and bad weather conditions. Water
temperature in the range of 50—75° can be obtained which can be used as industrial
process heat or for electricity generation by employing a secondary fluid (such as Freon)
which will drive a turbine coupled to an electric generator.
2. Partitioned Solar Ponds. In a partitioned solar pond the lower convective zone
and non-convective zone is separated by a transparent portion and the process of
operation remains the same as the conventional salt gradient solar pond. The idea of
partitioned solar pond was given by Rabl and Nielson, so that lower convective zofle can
be used for seasonal storage of heat for house heating. The portion also helps in
maintaining the stability in the pond and heat can be extracted from lower convective
zone without disturbing the non-convective zone. Generally the use of a flexible
membrane is recommended but in this case the overall loading of the partition must be
small to prevent rupture. Which means that either the lower layer density must be such
that the convection zone supports the non-convective layer, or the convective zone must
be given a pressure heat to balance gravitational force on the partition? In the later case
fresh water can be used in the convective zone and thereby eliminating the corrosion
problems which is associated with energy extraction from the brine. In the partitioned
solar pond, the membrane allows for use of considerable less salt because the salt
content is proportional to the square of the depth. But in this case the membrane should
be fixed to the pond walls by a leak-tight seam. it was seen that the temperature in the
convective zone decreases with increasing thickness of the convective layer, but the
mean temperature is independent of the thickness. It is also concluded from the
experiments, that the efficiency of partitioned solar pond is higher than the conventional
non- convecting solar poBd. Optimal efficiency of 37 per cent and 26.9 per cent are
obtained at collection temperature of 50°C and 100°C respectively.

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3. Viscosity Stabilized Solar Ponds. In the viscosity stabilized solar pond a kind
of gel is used in water, making it non-convective. The idea of viscosity stabilized solar
pond was first given by Shafer and the phenomenon can be described as static rather
than stable. It is known that the Rayleigh number which relates buoyant forces and
viscous drag is responsible for the circulation and its critical value for the onset of
natural convection for a layer of fluid bounded top and bottom, and heated from the
bottom is 1707. The Rayleigh number is given as
where β = salt expansion coefficient
T= temperature difference between two layers of liquid
d = distance between two layers
v = kinematic viscosity of fluid
a = coefficient of salt diffusion
g = gravitational constant.
From the above equation it is seen that by increasing the viscosity, the Rayleigh
number can be reduced below the critical number and thereby suppressing the natural
convection. It has been shown that even simple water with soluble gums can produce
syrups that have viscosities in the range of 36.3 to 36.3 x i03 kg/ms suitable in the pond
for nonconvective operation. Materials suitable for viscosity stabilized ponds should
have high transmittance for solar radiatior, high thickening efficiency should be capable
of performing at temperatures upto 70°C. Natural polymers such as gum arabic, locust
beam gum, agin, starch and gelatin are all potentially useful materials. Both the syn.
thetic polymers like polyacrylic acid (salts), polyacrylamide, a carboxy vinyl polymer,
polymers of ethylene oxide etc. and semi-synthetic polymers alike carboxymethyl
cellulose, hydroxy ethyle cellulose, methyl-cellulose, hydroxy propyl-methyl cellulose
etc. can also be used for stabilizing the pond. Several cross linked polymer gels and
detergent oil/water gels can also be prepared which can also be suitably used in ponds.
Shaffer has recommended the use of commercial carboxy vinyl polymer as a thickener
which is found stable even at 70°C and with a proper inhibitor has shown outstanding
photochemical characteristics. The idea of viscosity stabilized solar pond appears to be

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promising but requires studies in depth and presently is not economically competitive
with salt gradient solar ponds.
4. Membrane Stratified Saltless Solar Ponds. A possible alternative of the
conventional salt gradient solar pond is the membrane stratified solar pond. The idea for
it appears to be taken from flat plate collectors where transparent honeycomb is used for
natural convection suppression. In a conventional salt gradient solar pond there are three
zones, upper convection zone (UCZ), non-convection zone (NCZ) and lower convection
zone (LCZ), while in membrane stratified solar pond there are only two zones, the upper
non-convective zone (at the top serving as insulating layer) and lower convective zone at
the bottom (serving as a heat storage layer). The basic difference between the two types
is in the mechanism for maintaining non-convection in the NCZ. In the membrane
stratified solar pond, the convection is suppressed by using transparent membranes in the
NCZ having spacing with each other small enough to suppress convection. A few
advantages of membrane stratified solar pond are listed below:
1. Since no salt is used in this pond, this pond can be made maintenance free and
low cost.
2. There is no environmental or geological hazard with the membrane stratified
solar pond.
3. There is no upper convection zone in this type and thus making the same more
efficient when compared to salt gradient solar pond
4. A larger depth of lower convection zone (LCZ) can be maintained in a
membrane stratified pond resulting in seasonal storage, less diurnal temperature
variation and higher collection efficiency
Three types of membranes are suggested: (i) Horizontal sheets; (U) vertical
tubes; and (iii) vertical sheets .
Use of a thick horizontal membrane at the top of the pond to keep out dust and
debris and to prevent optical fouling of the membranes. Teflon ii suggested to be the
suitable membrane material because of its long life, high transparency, inert to virtu ally

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all chemicals, commercial availability in all sizes and thicknesses etc. Apart from water
as liquid in membrane solar pond other liquids like concentrated sugar solution, ethanol
and combination of water and ethanol are suggested as liquids.
5. Saturated Solar Ponds. Saturated solar ponds are non-convecting bodies of
water, relying upon a density gradient brought about by differential solubility of salt
with temperature. In a saturated solar pond the naturally developed temperature gradient
between top and bottom develops and maintains a density gradient by ion migration. In
such a pond, salt for which the solubility increases quickly with temperature is used. The
pond water is kept saturated with such a salt at all levels and since the pond is hotter in
the bottom than the top, more salt is dissolved in the bottom. In such a pond, vertical
diffusion of salt is prevented and the density gradient is stable thus making the pond
maintenance free. Generally, Na2SO4, MgC12 and Borax are recommended in saturated
solar pond. Na2SO4 shows an increase in density with temperature upto transition at
30.3°C. At this point the density of the solution starts decreasing with increasing
temperature which shows that Na2SO4 is not an acceptable working salt. Both MgCl2
and Borax are the salts which can form a stable saturated solar pond.
Applications of Solar Ponds
Because of large storage of heat and negligible diurnal fluctuations in pond
temperature, solar pond has a variety of applications like heating and cooling of
buildings, swimming pool and green house heating, industrial process heat, desalination,
power production, agricultural crop drying, the production of renewable liquid fuels
such as ethanol for gasohol. Some of the applications are discussed below:
1. Heating and Cooling of Buildings. Because of the large heat storage
capability in the lower convective zone of the solar pond, it has ideal use for
heating even at high latitude stations and for several cloudy days. Many
scientists have attempted and sized the solar pond for a particular required
heating load for house heating. Calculations have shown that a solar pond
with a 100 m diameter and a 1 m deep lower convective zone is sufficient to
drive either an absorption system or chiller capable of meeting 100 percent of

212
the typical cooling load of 50 house community in Fortworth (USA). Even
single storey buildings can be heated economically with solar pond in which
case the area of solar pond can be approximately equal to the floor area of the
house.
2. Power Production. A solar pond can be used to generate electricity by
driving a thermo electric device or an organic Rankine Cycle engine—a
turbine powered by evaporating an organic fluid with a low boiling point.
The concept of solar pond for power production holds great promise in those
areas where there is sufficient insolation and terrain and soil conditions allow
for construction and operation of large area solar ponds necessary to generate
meaningful quantities of electrical energy. Even low temperatures heat that is
obtained from solar pond can be converted into electric power. The
conversion efficiency is limited due to its low operating temperatures (70—
100°C). Because of low temperature, the solar pond power plant (SPPP)
requires organic working fluids which have low boiling points such as
halocarbons (like Freons) or hydrocarbons (such as propane). Atypical SPPP
is shown in Fig. 10.4.1.
3. Desalination. The low cost thermal energy can be used to desalt or otherwise
purify water for drinking or irrigation .
Multiflash desalination units along with a solar pond is an attractive
proposition for getting distilled water because the multi-flash desalination
plant below 100°C which can well be achieved by a solar pond. This system
will be suitable at places where potable water is in short supply and brackish
water is available. It has been estimated that about 4700 m3/day distilled
water can be obtained from a pond of 0.31 2 area with a multi-effect
distillation unit. The cost of distilled water appears to be high for
industrialized countries but can be used in developing countries where there
is a shortage of potable water. Moreover this type of desalination plant
produces five times more distilled water than the conventional basin type
solar still.

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4. Solar-pond systems may be appropriate at other terminal lakes around the
world. In addition to generating electricity and running desalination
operations, such ponds could provide low-cost energy for processing
valuable minerals from the lakes.Site built‖ solar ponds are constructed near
the point of end use rather than at a more advantageous location such as salt
flats or mines. Building these ponds requires digging, leveling and preparing
the ground. A linear made of a synthetic material that will stand up to contact
with hot brine is usually added. Salt or brine also must be transported to the
site by truck, rail or pipe line. Thus, site-built ponds are more costly than
natural ponds and usually smaller. Although site builds ponds are most often
used to provide heat, they may also be used to generate electricity and desalt-
water where energy is very expensive.
5. Most artificially solar ponds have been site built, primary because they have
been constructed either for research or for small scale thermal applications,
such as heating a swimming pool or green house
Building a small, site-built pond does not require a massive capital
investment, while constructing the larger, more advantageously sited solar
ponds requires launching a substantial venture
6. Heating animal housing and drying crops on farms. Low grade heat can be
used in many ways on farms, which have enough land for solar ponds.
Several small demonstration ponds in Ohio, Iowa and Illinois have been used
to heat green houses and hogbarns.
7. Industrial process heat. Industrial process heat is th thérmal energy used
directly in the preparation and or treatment of materials and goods
manufactured by industry. Several scientists have determined the economics
of solar pond for the supply of process. heat in industries. According to them
the solar pond can play a significant role in supplying the process heat to
industries thereby saving oil, natural gas, electricity, and coal. From the
calculations it was concluded that for crop drying and for a paper industry,

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for which economics have been determined, the heat from solar pond is
highly competitive with oil and natural gas.
8. Heat for biomass conversion. Site-built solar ponds could provide heat to
convert biomass to alcohol or methane. While no solar ponds have been used
for this purpose, it is an ideal coupling of two renewable-energy technologies
Rankine Cycle Solar Thermal Power Generation System.
To convert solar energy into electricity through thermal conversion, researches in
the world have done a considerable work in varying capacity power systems. Organic
fluid Rankine cycle has been extensively used in these studies. The development work in
the generation of solar thermal power in higher range, i.e. 10 kW and above upto
megawatts, is in line in various parts of the world. The systems have been developed by
using a solar pond, flat-plate collector, a focussing collector (distribution type) or a
heliostat. The heliostat systems are used normally in a very high range of solar thermal
power production (around megawatts).
A low temperature solar engine, using heated water from fiatnbifp anlar collector
and butane as the working fluid is shown in Figure. which is developed in France for lift
irrigation. The system has array of flat-plate collectors to heat water upto nearly 70°C
and in the heat exchanger, the heat of water is used for boiling butane. The high pressure

215
butane vapour runs a butane turbine which operates a hydraulic pump which pumps the
water from the well used for irrigation. The exhaust butane vapour from butane turbin. is
condensed in a condenser with the help of water which is pumped by the pump. This
condensate is fed to the heat exchanger or butane boiler.
The system is applied for small power plants of about 10 kW capacity. It has the
advantage of simplicity. A Rankine cycle power plant using solar pond has already been
discussed in chapter Solar Pond”.

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MODULE V
STEAM POWER PLANT
Introduction
Steam is an important medium of producing mechanical energy. Steam has the
advantage that it can be raised from water which is available in abundance it does not
react much with the materials of the equipment of power plant and is stable at the
temperature required in the plant. Steam is used to drive steam engines, steam turbines
etc. Steam power station is most suitable where coal is available in abundance. Thermal
electrical power generation is one of the major methods. Out of total power developed in
India about 60% is thermal. For a thermal power plant the range of pressure may vary
from 10 kgi‘cm2 to super critical pressures and the range of temperature may be from
250° C to 650°C. The average all India Plant load factor (P.L.F.) of thermal power
plants in 1987-88 has been worked out to be 56.4% which is the highest P.L.F. recorded
by thermal sector so far.
Essentials of Steam Power Plant Equipment
A steam power plant must have following equipments:
(i) A furnace to burn the fuel.
(ii) Steam generator or boiler containing water. H generated in the furnace is
utilized to convert water steam.
(iii) Main power unit such as an engine or turbine to use heat energy of steam
and perform work.
(iv) Piping system to convey steam and water.
In addition to the above equipment the plant requires van auxiliaries and
accessories depending upon the availability of water, fuel and the service for
which the plant is intended.

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The flow sheet of a thermal power plant consists of the follow four main
circuits:
(i) Feed water and steam flow circuit
(ii) Coal and ash circuit
(iii) Air and gas circuit
(iv) Cooling water circuit.
A steam power plant using steam as working substance works basically
on Rankine cycle.
Steam is generated in a boiler, expanded in the prime mo and condensed
in the condenser and fed into the boiler again.
The different types of systems and components used in steam power plant
are as follows:
(i) High pressure boiler
(ii) Prime mover
(iii) Condensers and cooling towers
(iv) Coal handling system
(u) Ash and dust handling system
(vi) Draught system
(vii) Feed water purification plant
(viii) Pumping system
(ix) Air preheater, econorniser; super heater, feed heaters.

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Figure shows a schematic arrangement of equipment of a steam power station.
Coal received in coal storage yard of power station transferred in the furnace by coal
handling unit. Heat produced d to burning of coal is utilized in converting water
contained in boiler drum into steam at suitable pressure and temperature the steam
generated is passed through the turbine. Super heated steam then flows through the
turbine. After doing work in the turbine pressure of steam is reduced. Steam leaving the
turbine pas through the condenser which maintain the low pressure of steam the exhaust
of turbine. Steam pressure in the condenser depends upon flow rate and temperature of
cooling water and on effectiveness of air removal equipment. Water circulating through
the condenser may be taken from the various sources such as river, lake or sea. If
sufficient quantity of water is not available the hot water coming out of the condenser
may be cooled in cooling towers and circulated again through the condenser.
Bled steam taken from the turbine at suitable extraction points is sent to low
pressure and high pressure water heaters.

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Air taken from the atmosphere is first passed through the air pre-heater, where it
is heated by gases. The hot air then passes through the furnace. The flue gases after
passing over boiler and super heater tubes, flow through the dust collector and then
through economizer, air pre-heater and finally they are exhausted to the atmosphere
through the chimney. Steam condensing system consists of the following:
(i) Condenser (ii) Cooling water
(iii) Cooling tower (iv) H*well
(v) Condenser cooling water pump (vi) Condensate air
extraction pump
(vii) Air extraction pump (viii) Boiler feed pump
(ix) Makeup water pump.
Power Station Design
Power station design required wide experience. A satisfactory design consists of the
following steps:
(i) Selection of site.
(ii) Estimation of capacity of power station.
(iii) Selection of turbines and their auxiliaries.
(iv) Selection of boilers, and their auxiliaries.
(v) Design of fuel handling system.
(vi) Selection of condensers.
(vii) Design of cooling system.
(viii) Design of piping system to carry steam and water.
(ix) Selection of electrical generator.
(x) Design and control of instruments.
(xi) Design of layout of power station.
Quality of coal used in steam power station plays an important rote in the design
of power plant. The various factors to be considered while designing the boilers and
oal handling units are as follows:

220
(i) slagging and erosion properties of ash.
(ii) Moisture in the coal. Excessive moisture creates additional problems
particularly in case of pulverized fuel power
(iii) Burning characteristic of coal.
(iv) Corrosive nature of ash.
Characteristics of Steam Power Plant
The desired characteristics for a steam power plant are as follows:
(1) Higher efficiency. (ii) Lower cost.
(iii) Ability to burn coal especially of high ash content, and inferior coals.
(iv) Reduced environmental impact in terms of air pollution.
(v) Reduced water requirement.
(vi) Higher reliability and availability.
Coal Handling
CoaI delivery equipment is one of the major components of plant co. The various
steps involved in coal handling are as follows:
(j) Coal delivery (ii) Unloading
(iii) Preparation (iv) Transfer
(v) Outdoor storage (in) Covered storage
(vii) In plant handling (viii) Weighing and measuring
(ix) Feeding the coal into furnace.

221
(i) Coal Delivery. The coal from supply points is delivered by ships or boats to
power stations situated near to sea or river whereas coal is supplied by rail or trucks to
the power stations which are situated away from sea or river. The transportation of coal
by trucks is used if the railway facilities are not available.
(ii) Unloading. The type of equipment to be used for unloading the coal received
at the power station depends on how coal is received at the power station. If coal is
delivered by trucks, there is no need of unloading device as the trucks may dump the
coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used.
In case the-coal is brought by railway wagons, ships or boats, the unloading may be
done by car shakes, rotary car dumpers, cranes, grab buckets and coal accelerators.
Rotary car dumpers although costly are quite efficient for unloading closed wagons.

222
(iii) Preparation. When the coal delivered is in the form of big lumps and it is
not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers,
sizers driers and magnetic separators.
(iv) Transfer. After preparation coal is transferred to the dead storage by means
of the following systems:
1. Belt conveyors. 2. Screw conveyors.
3.Bucket elevators. 4. Grab bucket elevators.
5. Skip hoists. 6. .Flight conveyor.
1. Belt conveyor. Figure shows a belt conveyor. It consists of an endless belt
moving over a pair of end drums (rollers). At some distance a supporting roller is
provided at the centre. The belt is made up of rubber or canvas. Belt conveyor is suitable
for the transfer of coal over long distances it is used in medium and large power plants.
The initial cost of the system is not high and power consumption is also low. The
inclination at which coal can be successfully elevated by belt conveyor is about 20.
Average speed of belt conveyors varies between 200—300 r.p.m. This conveyor is
preferred than other types.

223
Advantages of belt conveyor
1. Its operation is smooth and clean.
2. It requires less power as compared to other types of systems.
3.Large quantities of coal can be discharged quickly and continuously.
4. Material can be transported on moderates inclines
2. Screw conveyor. It consists of an endless helicoids screw fitted to a shaft. The
screw while rotating in a trough transfers the coal from feeding end to the discharge end.
This system is suitable, where coal is to be transferred over shorter distance and space
limitations exist. The initial cost of the system is low. It suffers from the drawbacks that
the power consumption is high and there is considerable wear of screw. Rotation of
screw varies between 75—125 r.p.m.
3. Bucket Elivater. It consists of buckets fixed to a chain. The chain moves over
two wheels. The coal is carried by the buckets from bottom and discharged at the top.
4. Grab bucket elevator. It lifts and transfers coal on a single rail o track from
one point to the other. The coal lifted by grab buckets is transferred to overhead bunker
or storage. This system requires less power for operation and requires minimum
maintenance.
The grab bucket conveyor can be used with crane or tower as shown in figure.
Although the initial cost of this system is high but operating cost is less.

224
5. Skip hoist. Consists of a vertical or inclined hoist way a bucket or a car d by a
frame and a cable for hoisting the bucket. The bucket is held in upright position. It is
simple and compact method of elevating coal or ash. Figure shows a skip hoist.
6. Flight conveyor. It consists of one or two strands of chain to which steel
scraper or flights are attached which scrap the coal through a trough having identical
shape. This coal is discharged in the bottom of trough. It is low in first cost but has large
energy consumption. There is considerable wear .
Skip hoist and bucket elevators lift the coal vertically while Belts and flight
conveyors move the coal horizontally or on inclines.

225
Figure shows a flight conveyor. Flight conveyors possess the following
advantages:
(i They can be used to transfer coal as well as ash.
(ii) The speed of conveyor can be regulated easily.
(iii) They have a rugged construction.
(iv) They need little operational care.
Disadvantages. Various disadvantages of flight conveyors are as follows
(i) There is more wear due to dragging action.
(ii) Power consumption is more.
(iii) Maintenance cost is high.
(iv) Due to abrasive nature of material handled the speed of conveyors is low (10
to 30 rn/mm).
(v) Storage of coal. It is desirable that sufficient quantity of coal should be stored.
Storage of coal gives protection against the interruption of coal supplies when there is
delay in transportation of coal or due to strikes in coal mines. Also when the prices are
low, the coal can be purchased and stored for future use. The amount of coal to be stored
depends on the availability of space for storage, transportation facilities, the amount of
coal that will whether away and nearness figure to coal mines of the power station.
Usually coal required for one month operation of power plant is stored in case of power
stations situated at longer distance from the collieries whereas coal need for about 15
days is stored in case of power station situated near to collieries. Storage of coal for

226
longer periods is not advantageous because it blocks the capital and results in
deterioration of the quality of coal.
The coal received at the power station is stored in dead storage in the form of
piles laid directly on the ground.
The coal stored has the tendency to whether (to combine with oxygen of air) and
during this process coal loss some of its heating value and ignition quality. Due to low
oxidation the coal may ignite spontaneously .this is avoided by storing coal in the form
of piles which consist of thick and compact layers of coal so that air cannot pass through
the coal piles. This will minimize the reaction between coal and oxygen. The other
alternative is to allow the air to pass through layers of coal so that air may remove the
heat of reaction and avoid burning. In case the coal is to be stored for longer periods the
outer surface of piles may be sealed with asphalt or fine coal.
The coal is stored by the following methods
(i) Stocking the coal in heats. The coal is piled on the ground up to 10—12 m
height. The pile top should be given a slope in the direction in which the rain may be
drained off. The sealing of stored pile is desirable in order to avoid the oxidation of coal
after packing an air tight layer of coal .
Asphalt, fine coal dust and bituminous coating are the materials commonly used
for this purpose.

227
(ii) Under water storage. The possibility of slow oxidation and spontaneous
combustion can be completely eliminated by storing- the coal under water. Coal should
be stored at a site located on solid ground, well drained, free of standing water
preferably on high ground not subjected to flooding.
(vi) In Plant Handling. From the dead storage the coal is brought to covered storage
(Live storage) (bins or bunkers). A cylindrical bunker shown in Figure. In plant handling
may include the equipment such as belt conveyors, screw conveyors, bucket elevators
etc. to transfer the coal. Weigh Lorries hoppers and automatic scales are used to record
the quantity of coal delivered to the furnace
(vii) Coal weighing methods. .Weigh lorries, hoppers and automatic scales are
used to weigh the quantity coal. The commonly used methods to weigh the coal are as
follows:
(i) Mechanical (ii) Pneumatic (iii) Electronic.
The mechanical method works on a suitable layer system mounted on kp.ife
edges and bearings connected to a resistance in the form of a spring pendulum. The
pneumatic weighters use a pneumatic transmitter weight head and the corresponding air
pressure determined by the load applied. The electronic weighing produce voltage
signals proportional to the load applied
The important factor considered in selecting fuel handling systems are as
follows:
(i) Plant flue rate, (ii) Plant location in respect to fuel shipping, (iii) Storage area
available.
Dewatering of Coal
Excessive surface moisture of coal reduces and heating value of coal and creates
handling problems. The coal should therefore be dewatered to produce clean coal.
Cleaning of coal has the following advantages:

228
i) Improved heating value.
iii) Easier crushing and pulverising.
(iii) Improved boiler performance.
(iv) Less ash to handle.
(v) Easier handling.
(vi) Reduced transportation cost.
Method of Fuel Firing
The solid fuels are fired into the furnace by the following methods
1. Hand firing. 2. Mechanical firing.
Hand Firing
This is a simple method of firing coal into the furnace. It requires no capital investment.
It is used for smaller plants This method of fuel firing discontinuous process and there is
a limit to the size furnace which can be efficiently fired by this method. Adjustments are
to be made every time for the supply of air when fresh coal is fed into furnace
.
Hand Fired Grates. A hand fired grate is used to support the fuel bed and admit
air for combustion. While burning coal the total area of air openings varies from 30 to
50% of the total grate area. The grate area required for an installation depends upon
various factors such as its heating surface, the rating at which it is to be operated and the
type of fuel burnt by it. The width of air openings varies from 3 to 12 mm
.
The construction of the grates should be such that it is kept uniformly cool by incoming
air. It should allow ash to pass freely. Hand fired grates are made up of cast iron. The
various types of hand fired grates are shown in Fig. 3.8. In large furnaces vertical
shaking grates of circular type are used.

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Mechanical Firing (Stokers)
Mechanical stokers are commonly used to feed solid fuels into the furnace in
medium and large size power plants.
The various advantages of stoker firing are as follows:
(i) large quantities of fuel can be fed into the furnace. Thus greater combustion
capacity is achieved.
(ii) Poor grades of fuel can be burnt easily.
(iii) Stoker save labour of handling ash and are self-cleaning.
(iv) By-dsing stokers better furnace conditions can be maintained by feeding coal
at a uniform rate.
(v) Stokers save coal and increase the efficiency of coal firing.
The main disadvantages of stokers are their more costs of operation and repairing
resulting from high furnace temperatures.
Principles of Stokers. The working of various types of stokers is based on the following
two principles:

230
1. Overfeed Principle. According to this principle (Figure) the primary air enters
the grate from the bottom. The air while moving through the gate openings gets heated
up and the grate is cooled. The hot air that moves through a layer of incandescent coke
where oxygen reacts with coke to form CO2 and water vapours accompanying the air
react with incandescent coke to form The gases leaving the surface of fuel bed contain
volatile matter of raw fuel and gases like C02, C0, H2, N2 and H20. Then additional air
known secondary-air is supplied to burn the combustible gases. The combustion gases
entering the boiler consist of N2, C02, 02 and H20 and also CO if the combustion is not
completed.
2. Underfeed Principle: figure show underfeed principle. In underfeed principle
air entering rough the holes in the grate comes in contact with the raw coal (green coal).
Then it passes through the incandescent coke where reactions similar to overfeed system
take place. The gases produced then passes through a layer of ash. The secondary air is
supplied to burn the combustible gases. Under feed principle is suitable for burning the
semi-bituminous and bituminous coals.

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Types of Stokers
The various types of stokers are as follows Stokers
Charging of fuel into the furnace is mechanized+ by means of stokers of various
types. They are installed above the fire doors underneath the bunkers which supply the
fuel. The bunkers receive the fuel from a conveyor
(i) Chain Grate Stoker. Chain grate stoker and travelling grate stoker differ only in
grate Figure consists of an endless chain which forms a support for the fuel bed.
The chain travels over two sprocket wheels, one at the front and one at the rear of
furnace. The travelling chain receives coal at its front end through a hopper and carries it
into the furnace. The ash is tipped from the rear end of chain. The speed of grate (chain)
can be adjusted to suit the firing condition. The air required for combustion enters below

232
the grate. Stokers are used for burning non-coking free burning high volatile high ash
coals. Although initial cost of this stoker is high but operation and maintenance cost is
low.
The travelling grate stoker also uses an endless chain but differs in that it carries
small grate bars which actually support the fuel fed. It is used to burn lignite, very small
sizes of anthracites coke breeze etc.
The stokers are suitable for low ratings because the fuel must be burnt before it
reaches the rear of the furnace. With forced draught, rate of combustion is nearly 30 to
50 lb of coal per square foot of grate area per hour, for bituminous 20 to 35 pounds per
square foot per hour for anthracite.
(ii) Spreader Stoker. A spreader stoker is shown in Fig. 3.13. In this stoker the
coal from the hopper is fed on to a feeder which measures the coal in accordance to the
requirements. Feeder is a rotating drum fitted with blades. Feeders can be reciprocating
rams, endless belts, spiral worms etc. From the feeder the coal drops on to spreader
distributor which spread The spreader system should distribute the coal evenly over the
entire grate area. The spreader speed depends on the size of coal.
Advantages
The various advantages of spreader stoker are as follows:
1. Its operation cost is low.
2. A wide variety of coal can be burnt easily by this stoker.

233
3. A thin fuel bed on the grate is helpful in meeting the fluctuating loads.
4. Ash under the fire is cooled by the incoming air and this minimizes clinkering.
5. The fuel burns rapidly and there is little coking with coking fuels.
Disadvantages
1. The spreader does not work satisfactorily with varying size of coal.
2. In this stoker the coal burns in suspension and due to this fly ash is discharged with
flue gases which requires an efficient dust collecting equipment.
(iii) Multi-retort Stoker. A multi-retort stoker is shown in figure. The coal
falling from the hopper is pushed forward during the inward stroke of stoker ram. The
distributing rams (pushers) then slowly move the entire coal bed down the length of
stoker. The length of stroke of pushers can be varied as desired. The slope of stroke
helps in moving the fuel bed and this fuel bed movement keeps it slightly agitated to
break up clinker formation. The primary air enters the fuel bed from main wind box
situated below the stoker. Partly burnt coal moves on to the extension grate. A thinner
fuel bed on the extension grate requires lower air pressure under it. The air entering from
the main wind box into the extension grate wind box is regulated by an air damper
.
As sufficient amount of coal always remains on the grate, this stoker can be used
under large boilers (upto 500,000 lb per hr capacity) to obtain high rates of combustion.
Due to thick fuel bed the air supplied from the main wind box should be at higher
pressure.

234
Pulverised Coal
Coal is pulverised (powdered) to increase its surface exposure thus permitting
rapid combustion. Efficient use of coal depends greatly on the combusti‘on process
employed.
For large scale generation of energy the efficient method of burning coal is
confined still to pulverised coal combustion. The pulverised coal is obtained by grinding
the raw coal in pulverising mills. The various pulverising mills used are as follows:
(i) Ball mill (ii) Hammer mill
(iii) Ball and race mill (iv) Bowl mill.
The essential functions of pulverising mills are as follows
(i) Drying of the coal (ii) Grinding
(iii) Separation of particles of the desired size.
Proper drying of raw coal which may contain moisture is necessary for effective
grinding.
The coal pulverising mills reduce coal to powder form by three actions as follows
(i) Impact (ii) Attrition (abrasion)
(iii) Crushing.
Most of the mills use all the above mentioned all the three actions in varying
degrees. In impact type mills hammers break the coal into smaller pieces whereas in
attrition type the coal pieces which rub against each other or metal surfaces to
disintegrate. In crushing type mills coal caught between met& rolling surfaces gets
broken into pieces. The crushing mills use steel balls in a container. These balls act as
crushing elements.

235
Pulverised Coal Firing
Pulverised coal firing is done by two system:
(i) Unit System or Direct System.
(ii) Bin or Central System.
Unit System. In this system figure the raw coal from the coal bunker drops on to
the feeder.
Hot air is passed through coal in the feeder to dry the coal. The coal is then
transferred to the pulverising mill where it is pulverised. Primary air is supplied to the
mill, by the fan. The mixture of pulverised coal and primary air then flows to burner
where secondary air is added. The unit system is so called from the fact that each burner
or a burner group and pulveriser constitutes a unit.

236
Advantages
1. The system is simple and cheaper than the central system.
2. There is direct control of combustion from the pulverising mill.
3. Coal transportation system is simple.
Bin or Central System. It is shown in figure. Crushed coal from the raw coal
bunker is fed by gravity to a dryer where hot air is passed through the coal to dry it. The
dryer may use waste flue gases, preheated air or bleeder steam as drying agent. The dry
coal is then transferred to the pulverising mill. The pulverised coal obtained is
transferred to the pulverised coal bunker (bin). The transporting air is separated from the
coal in the cyclone separator. The primary air is mixed with the coal at the feeder and
the mixture is supplied to the burner.
Advantages
1. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed.
2. There is always some coal in reserve. Thus any occasional breakdown in the
coal supply will not effect the coal feed to the burner.
3. For a given boiler capacity pulverising mill of small capacity will be required
as compared to unit system.
Disadvantages
1. The initial cost of the system is high.

237
2. Coal transportation system is quite complicated.
3. The system requires more space.
To a large extent the performance of pulverised fuel system depends upon the
mill performance. The pulverised mill should satisfy the following requirements:
1. It should deliver the rated tonnage of coal.
2. Pulverised coal produced by it should be of satisfactory fineness over a wide
range of capacities.
3. It should be quiet in operation.
4. Its power consumption should be low.
5. Maintenance cost of the mill should be low.
Figure shows the equipments for unit and central system of pulverised coal
handling plant.

238
Pulverised Coal Burners
Burners are used to burn the pulverised coal. The main difference between The
various difference between the various burners lies in the rapidity of air coal mixing i.e.,
turbulence. For bituminous coals the turbulent type of burner is used whereas for low
volatile coals the burners with long flame should be used. A pulverized coal burner
should satisfy the following requirements
(i) I should mix the coal and primary air thoroughly and should bring this
mixture before it enters the furnace in contact with additional air known as secondary air
to create sufficient turbulence.
(ii) It should deliver and air to the furnace in right proportions and should
maintain stable ignition of coal air mixture and control flame shape and travel in the
furnace. The flame shape f o troll air vanes and other control adjustments incorporated
into the burner. Secondary air if supplied in too much quantity may cool the mixture and
prevent its heating to ignition temperature
.
(iii) 1 air mixture should move away from the burner at a rate equal to flame
front travel in order to avoid flashback into the burner .
The various types of burners are as follows:

239
1. Long Flame Burner (U-Flame Burner). In this burner air and coal mixture
travels a considerable distance thus providing sufficient time for complete
combustion.
2. Short Flame Burner (Turbulent Burner). It is shown in figure. The burner
is fitted in the furnace will and the flame enters the furnace horizontally:
3. Tangential burner. A tangential burner is shown in figure in this system one
burner is fitted attach corner of the furnace The inclination of the burner is so
made that the flame produced are tangential to an imaginary circle at the
centre
4. 1oneBUrner.t-is-slTwrrr-Fig12O(di. This burner usag-eiuheLcoLjntend of
pulverised coal. Its advantages are as follows:

240
(i) It saves the cost of pulverisation because of a crusher needs less
(ii) Problem fly ash is reduced. Ash produced is in the molten form and due to
inclination of furnace it flows to an appropriate disposal system.
Ash Disposal
A large quantity of ash is, produced in steam power plants using coal. Ash
produced ii about 10 to 20% of the total coal burnt in the furnace. Handling of ash is a
problem because ash coming out of the furnace is too hot, it is dusty and irritating to

241
handle and is accompanied by some pois6ñöu gases. It is desirable to quench the ash
before handling due to following reasons : (1) Quenching reduces the temperature of
ash. (2) It reduces the corrosive action of ash. (3) Ash forms clinkers by fusing in large
lumps and by quenching clinkers will disintegrate. (4) Quenching reduces the dust
accompanying the ash.
Handling of ash includes its removal from the furnace, loading on the conveyors
and delivered to the fill from where it can be disposed off.
Ash Handling equipment
Mechanical means are required for the disposal of ash. The handling equipment
should perform the following functions: (1) Capital investrnent, operating and
maintenance charges of the equipment should be low. (2) It should be able to handle
large quantities of ash. (3) Clinkers, soot, dust etc. create troubles, the equipment should
be able to handle them smoothly. (4) The equipment used should remove the ash from
the furnace, load it to the conveying system to deliver the ash to a dumping site or
storage and finally it should have means to dispose of the stored ash. (5) The equipment
should be corrosion and wear resistant.
Figure shows a general layout of ash handling and dust collection system. The
commonly used ash handling systems are as follows

242
(i) Hydraulic system (ii) Pneumatic system
(iii) Mechanical system.
The commonly used ash discharge equipment is as follows:
(i) Rail road cars (ii) Motor truck (iii) Barge
The various methods used for the disposal of ash are as follows:
(i) Hydraulic System. In this system, ash from the furnace grate falls into a
system of water possessing high velocity and is carried to he_sii.mps. It is generally
used in large power plants. Hydraulic power system is of two types namely low
pressure hydraulic system used for continuous removal of ash and high pressure system
which is used for intermittent ash disposal. Fig. 3.24 shows hydraulic system.
In this method water at sufficient pressure is used to take away the ash to sump. Where
water and ash are separated. The ash is then transferred to the dump site in wagons, rail
cars or trucks. The loading of ash may be through a belt conveyor, grab buckets. If there
is an ash basement with ash hopper the ash can fall, directly in ash car or conveying
system.
(ii) Water Jetting. Water jetting of ash is shown in Fig. 3.25. In this method a
low pressure jet of water coming out of the quenching nozzle is used to cool the ash. The
ash falls into a trough and is then removed.

243
(iii) Ash Sluice Ways and Ash Sump System. This system shown
diagrammatically in Fig. 3.26 used high pressure (H.P.) pump to supply high pressure
(H.P.) water jets which carry ash from the furnace bottom through ash sluices (channels)
constructed in basement floor to ash sump fitted with screen. The screen divides the ash
sump into compartments for coarse and fine ash. The fine ash passes through the screen
and moves into the dust sump (D.S.). Dust slurry pump (D.S. pump) carries the dust
through dust pump (D.P.), suction pipe and dust delivery (D.D.) pipe to the disposal site.
Overhead crane having grab bucket is used to remove coarse ash. A.F.N. represents ash
feeding nozzle and S.B.N. represents sub way booster nozzle and D.A. means draining
apron

244
.
(iv) Pneumatic system. In this system ash from the boiler furnace outlet falls
into a crusher where larger ash particles are crushed to small sizes. The ash is then
carried by a high velocity air or steam to the point of delivery. Air leaving the ash
separator is passed through filter to remove dust etc. so that the exhauster handles clean
air which will protect the blades of the exhauster.
(v)Mechanical ash handling system. In this system ash cooled by water seal
falls on the belt conveyor and is carried out continuously to the bunker. The ash is then
removed to the dumping site from the ash bunker with the help of trucks.
Smoke and Dust Removal
In coal fed furnaces the products of combustion contain particles of solid matter
floating, in suspension. This may be smoke or dust. The production of smoke indicates
that combustion conditions are faulty and amount of smoke produced can be reduced by
improving the furnace design.
In spreader stokers and pulverised coal fired furnaces the coal is burnt in
suspension and due to this dust in the form of fly ash is produced. The size of dust
particles is designated in microns (1 j.t = 0.001 mm). Dust particles are mainly ash
particles called fly ash intermixed with some quantity of carbon ash material called

245
cinders. Gas borne particles larger than 1 i in diameter are called dust and when such
particles become greater in size than 100.t they are called cinders .
Smoke is produced due to the incomplete combustion of fuels, smoke particles
are less than 10μ in size. .
The disposal smoke to the atmosphere is not desirable due to the following reasons:
1. A smoky atmosphere is less healthful than smoke free air.
2. Smoke is produced due to incomplete combustion of coal. This will create a
big economic loss due to loss of heating value of coal.
3. In a smoky atmosphere lower standards of cleanliness are prevalent.
Buildings, clothings, furniture etc. becomes dirty due to smoke. Smoke
corrodes the metals and darkens the paints.
To avoid smoke nuisance the coal should be completely burnt in the furnace.
The presence of dense smoke indicates poor furnace conditions and a loss in efficiency
and capacity of a boiler p1ant. A small amount of smoke leaving chimney shows good
furnace conditions whereas smokeless chimney does not necessarily mean a better
efficiency in the boiler room.
To avoid the atmospheric pollution the fly a8h must be removed from the
gaseous products of combustion before they leaves the chimney.
The removal of dust and cinders from the flue gas is usually effected by
commercial dust collectors which are installed between the boiler outlet and chimney
usually in the chimney side (of air preheater.
Types of Dust Collectors
The various types of dust collectors are as follows

246
1. Mechanical dust collectors .
2. Electrical dust collectors
Mechanical dust collectors. Mechanical dust collectors are sub-divided into wet
and dry types. In wet type collectors also known as scrubbers water sprays are used to
wash dust from the air. The basic principles of mechanical dust collectors are shown in
figure. As shown in Figure by increasing the cross-sectional area of duct through which
dust gases are passing the velocity of gases is reduced and causes heavier, dust particles
to fall down. Changing the direction of flow of flue gases causes the heavier particles of
settle out. Sometimes baffles are provided as shown in figure to separate the heavier
particles.
Mechanical dust collectors may be wet type or dry type. Wet type dust collectors
called scrubbers make use of water sprays to wash the dust from flue gases.
Dry type dust collectors include gravitational, cyclone, louvered and baffle dust
collectors.
A cyclone dust collector is shown in figure. This collector uses a downward
flowing vortex for dust laden gases along the inner walls. The clean gas leaves from an
inner upward flowing vortex. The dust particles fall to the bottom due to centrifuging
action
Electrostatic Precipitators. It has two sets of electrodes, insulated from each
other, that maintain an electrostatic field between them at high voltage. The flue gases
are made to pass between these two sets of electrodes. The electric field ionizes the dust
particles that pass through it attracting them to the electrode of opposite char the other
electrode is maintained at a negative potential of 30,000 to 60,000 volts. The dust
particles are removed from the collecting electrode by rapping the electrode periodicaIl5
he electrostatic precipitator is costly but has low maintain cost and is frequently
employed with pulverised coal fired power stations for its effectiveness on very fine ash
particles and is superior to that of any other type.

247
COOLING PONDS AND COOLING TOWERS
In the modern fossil-fueled steam power plants, about 10% to 15% of the heat
input is rejected to the atmosphere through boiler chimneys. While 48% to 52% of the
heat input is rejected to a cooling water system through the steam condensers. In nuclear
power plants about 67% to 68% of the heat generated within the reactor is rejected to the
water through steam condensers. A comparative study has been shown in table
It is clear; therefore, that enormous amount of water is required for cooling
purposes in steam condensers. It has been found that approximately 50 gallons of water
per kWh is required for condenser cooling and about 5% additional quantity is needed
for other plant services, e.g. quenching of ash, boiler make up water and bearing cooling
etc. Therefore in sitting of a new steam power plant. The method of condenser cooling
becomes one of the most important factors to be considered. For this purpose, the power
plant can be located near a source of natural water e.g. rivers, lakes and coastal water for
once through condenser cooling. Local beat dissipation to open water is a natural and
attractive proposal provided the water quality standards are maintained. The supply of
ocean water is practically unlimited, but ocean sites are accessible only in few cases.
Very large lakes, bays and estuaries may be used but the hydraulics and ecological
factors may restrict their use. In many areas, high summer temperatures may permit only
small temperature increases. Few rivers have sufficient flow to supply the quantities
desired. Lakes and reservoirs are attractive because of natural stratification where cool

248
water be taken from the deep areas. Warm water is returned at the surface and
evaporation, radiation and conduction and convection dissipate the heat from the
increased surface temperature. The efficiencies of these systems are sensitive to depth
which enhances vertical stratification. In all cases, sufficient make up water must be
supplied to replace evaporation and to flush accumulations of dissolved solids.
Figure shows the Once-through cooling water system. The natural water source
being a river. Pumping station. is situated on the ever bank, and the water is drawn by
the pumps through the screens. A protective dam is built upstream the water intake. To
trap mechanical admixtures, the water first flows through a coarse strainer and then
through a screen. For uninterrupted supply of water, two pressure pipe lines are used.
During winter, a fraction of the heated water from the condensers is directed via by pass
channel to the water intake of the pumping station to prevent its freezing. After
circulating through the condensers, the water flows into drain wells and is discharged
from them through drain channel into the river, 40 to 50 m downstream the pumping
station intake to avoid the intake of hot water by - the pumps. This system is useful if the
river flow during the driest period is about 2 to 3 times the rate of water consumption
.
But the modern trend is to locate the steam power stations as near as possible to
the centre of gravity of the electrical load instead of near a source of natural water to

249
avoid huge transmission costs. Moreover,, the natural water contains free acid, sewage
contamination and other foreign materials and it cannot be used as it is, otherwise there
will be rapid deterioration of the metal parts of the condenser and other apparatus
through which water flows. To overcome this trouble water treatment plants will have to
be installed. Also as the capacity of the modern steam power plants becomes larger and
larger, more and more power plants are set up, the availability of natural water sources
suitable as a heat sink will decrease. Again there may be constraints imposed by the
limitations on thermal discharge to natural waters. Therefore for large power plants
situated away from the source of natural water, enormous quantities of pure water may
not be available for once through condenser cooling and the same supply of water may
have to be used again and again. Therefore there must be some arrangement to recool
the circulating water and for this purpose cooling ponds and cooling towers are needed.
This type of cooling water system is known as closed circuit or circulating cooling water
system
COOLING PONDS
The simplest method of cooling the water is to discharge it through a pipe line to
a pond of sufficient area and exposing the hot water to the atmosphere. The cooling is
effected by the air blowing across the surface of the pond. The heat from the hot water
will be transferred to the air by two processes, convection and evaporation. The
evaporation, of some of the hot water with the absorption of the latent heat of
vaporization will cool the remaining water. Loss of water by evaporation and windage is
about 2 to 3%. To increase the rate of cooling the area of the pond will have to be
increased. To overcome the difficulty, it is necessary to use some device by means of
which the additional contact of the water with atmosphere is obtained., The device
consists cia spraying system, Fig. 8.2. The water is sprayed into the air over the pond by
means of nozzle. The water pressure in the nozzles is from 0.21 to 1.05 bar. The nozzles
break the water into a spray suitable for1 the weather conditions under which the system
is working. A whirling motion is also given to the nozzle to produce better atomizing of
the water. The spray I nozzles should be placed about 1.2 to 2.4 meters above the surface
of water. There should be no interference in the spray path of the nozzles. They should
be located where the wind is not obstructed by the buildings etc

250
Dissipation of heat is influenced by the following factors:
1. Initial temperature of water entering the pond.
2. Atmospheric temperature.
• 3. Relative humidity.
• 4. Air velocity.
5. Solar radiation.
6. Earth temperature.
7. Atmospheric pressure.
8. Area of pond.
9. Depth of the pond.
The disadvantages of cooling ponds are that considerable quantity of water may
be carried away in suspension in air when its velocity is high and loss due to evaporation
and also space consideration is there.
This system is used only in low capacity electric power stations e.g. diesel engine
power stations
COOLING TOWERS
For large power plants, cooling towers are used in place of cooling ponds. A
cooling lower is a wooden or metallic rectangular structure, inside of which is packed
with baffling devices. The hot water is delivered to the top of tower and falls down
through the tower and is broken into small particles while passing over the baffling

251
devices. Air enters the tower at the bottom and flows upward and cools the water. The
air vaporizes a small percentage of water. thereby cooling the remaining water. The air
gets heated and leaves the towers at the top. The cooled water falls down into a tank
below the tower from where it can be aging circulated to the condenser. The
arrangement of the cooling tower is shown in Fig. 8.3. Such cooling towers are known
as ―Wet Cooing Towers. Their performance is limited by wet bulb temperature. They are
more efficient in dry climates than in humid areas.
The heat exchange between air and water through direct contact is from two types of
heat transfer.
1. The evaporative cooling of water.
2. The convective heating of air. Cooling by evaporation is. the greatest. Cooling
takes place more rapidly in breezy weather. Cooling is also increased by the
dryness of atmosphere, low atmospheric pressure, high temperature of air and
water and quick renewal of the air in contact with the water. Make up water must
be added to the tower basin to replenish the water lost through evaporation.
Amount of water evaporated is approximately 0.346 kg to 0.368 kg per 1000 kJ
of heat load from the condenser. For dissipating the condenser heat, the amount
of cooling water can be calculated as below
Depending upon the design and plant loading, the temperature of cooling water
passing through the condenser increases by about 3 to 8°C. It means that about 12 to 35

252
kJ of heat is picked up per kg of water. Now each kg of exhaust steam gives up about
2326 kJ/kg when condensing. For a 500 MW plant with an exhaust steam rate of 3.63 kg
per kWh. the steam flow per hour at full load will be 3.63 x 500 x 1000=18 x io kg.
Therefore to despite so much heat, the amount of cooling water needed is 18 x iO kg per
hour. To cool such enormous quantity of water, huge volumes of air are required. For
example, for dissipating the condenser heat load of 750 MW plant the mechanical
drought cooling tower air mass flow rate ranges from. 38.5 x 106 kg per hour to 45 x
106 kg per hour.
Types of Cooling Towers
The cooling towers can be classified in two ways, according to the material of
their construction and according to the nature of draught of air produced though the
cooling tower. According to the material, the cooling towers are of the following types:
1. Timber
2. Ferro-concrete
3. Multi deck concrete towers
4. Metallic
Timber is used for small towers but has many disadvantages. Ferro - concrete
towers are used on all large capacity stations, but they are high in the initial cost. Multi
deck steel towers are also used for large steam stations.
According to the nature of air draught, the cooling towers are of the following
types:
1. Atmospheric.
2. Natural draught cooling towers
3. Mechanical draught cooling towers
(a) Induced draught cooling towers.
(b) Forced draught cooling towers.
(c) Combined induced and forced draught cooling towers.

253
Atmospheric Cooling Towers
he working of this tower is shown in figure. The hot water is delivered to the
topmost tray or louver and it falls down from one tray to another until it reaches the tank
below the tower. The water is cooled by air flowing across the tower. To increase the
rate of cooling, the water is delivered at the top through spray nozzles. The number of
decks of trays depends upon the load of the plant. This type of cooling tower is used
only for small capacity power plants and air conditioning plants in cinema halls,
hospitals etc.
Natural Draught Cooling Towers
This tower is shown in Fig. 8.5. Some portion of the tower interior is packed
with wooden hurdles and distributing trays for spreading the water and for breaking it to
small particles, The hot water from the condenser is pumped to a height of 9 m to 12 m
and enters the tower and then distributed over the wood work and trays. The hot water
then falls down and the steam vapours which are lighter than air will rise upwards. This
will create natural draught and air will enter from the bottom of the tower which is open
to atmosphere The rising air nih meet the falling spray of hot water and cools it The hot
air along with some vapours will leave the tower at top-and the cooled water will fall
down in the form of rain into the pond below the tower. From this pond, the cooled
water is again circulated through the condenser. The disadvantage of this type is that to
produce large natural draught the towers should be very high The escape of the water
particles by the leaving air is prevented by using water eliminators at the top of the

254
water. These consist of tno layers of nood arranged in such a manner that the water
particles get deposited on the underside of the wooden blades. When these particles have
built up to form drops. the drops fall back into the tower.
Mechanical Draught Cooling Towers
In these towers the draught of air for cooling the water is produced mechanically
by means of propeller fans. These towers are usually built in cells or units, the capacity
depending upon the number of cells used. These lowers require a,sinaller land area and
can be built at most locations. The fans give good control over the air flow and thus the
water temperature. Also they cost less to install than natural draught towers. However
they have drawbacks also:
(1) Local fogging and icing may occur in winter season.
(2) Fan power requirements and maintenance costs make them over expensive to operate
Figure shows a forced draught cooling tower. It is similar to natural draught
tower ?s far as the interior construction is concerned, but the sides of the tower are
closed and form an air and water tight structure, except for fan openings at the base for
the inlet Of fresh air, and the pullet at the top for the escape of air and vapours. There are
hoods at the base projecting from the main portion of the tower where the fans are

255
placed for forcing the air. into the tower. Fig. 8.7 shows an induced draught tower. In
these towers, the fans are placed at the top of the tower and they draw the air in through
louvers extending all around the tower at its base.

256
Comparison of Cooling Towers
The natural draught towers are being gradually replaced by mechanical draught types
because the later requires less pumping head, less space and less wind age loss. Induced
draught tower is considered to be better than forced draught tower because in the later
type, the power requirements are high and the maintenance of fans is costlier. The
induced draught tower occupies less space as the fan drives are placed at the top of the
tower. Moreover, since air is drawn from all the sides of the tower, the cooling effect is
distributed across the entire cross-section of the tower. Also, since the fans handle warm
air, they are non-freezing. Again, as the air leaves the tower at a high speed, this type is
non-circulating.
FEED WATER HEATERS
The function of feed water heaters is to raise the temperature of the feed water by
means of bled steam before the feed water is supplied to the boiler from the hot well.
There are two main types of feed water heaters : Open or contact type and closed or
surface type. In open heaters, feed water mixes with the heating steam and in closed
heaters, the bled steam and feed water don‘t mix with each other.
Open or Contact Heaters: These heaters are usually constructed to remove
noncondensable gases from water and steam alongwith raising the temperature of feed
water and such an heater is termed as ―Deaerator‖. Oxygen and Carbon dioxide
dissolved in the feed water greatly corrode the inner surfaces of the boiler unit and the
pipelines. Therefore, the feed water should be delivered to the boilers after it has
undergone deaeration or degasification The amount of gas dissolved in water depends
upon its temperature. This decreases sharply with increasing temperatures and drops to
almost zero at the boiling point. Hence thermal deaeration of the feed water is usually
practised. A thermal deaeration is nothing but a direct contact or open heater. Open
heaters are of two types: Tray type and Jet type. Tray type open heater which also
removes 02, CO2 and ammonia is shown in Fig. 11.9. Feed water first of all flows
through a vent condenser and then into a spray pipe in the heater shell and is sprayed
upward. The steam enters the chamber and mixes with the water. The mixture of water
and condçnsed steam then flows down over the staggered trays of the heating section.

257
After this, the water flows over a section of air separating trays where the non-
consdensable gases will be separated from water. The steam alongwith these gases flow
upward into the vent condenser and the water flows down at the bottom of the chamber.
In the vent condenser, the steam vapours are condensed by the incoming feed water and
the non-condensable gases are vented out. This condensed steam falls back into the main
chamber.

258
In the jet type, there is no vent condenser but a preheater section in the top of the
shell. In this section, the water is atomised with the help of jet nozzles and most of 02 is
removed. Remaining non-condensable gases are separated in the air separating trays.
The direct contact feed water heaters (open heaters) have got the following advantages:
1. Complete conversion of steam to water is accomplished.
2. Non-Condensable corrosive gases are removed from the feed water.
3. The removal of impurities in the water is possible
4. The water is brought to the temperature of the steam
5. The heater acts as a small reservoir.

259
Because of the stress limitations of the heater shell, the steam pressure is limited
to a few N/ni2 above atmospheric pressure although pressures to about 5 bar absolute
have been used. Consequently, the feed water is rarely heated above 105°C. To prevent
failure from boiling in the suction connection of the feed water pump, the deaerators
should be mounted 8 to 9 m above the pump axis.
Closed or Surface Heaters: Such heaters consist of closed shell in which there
are tubes or coils through which either steam or water is circulated. Usually, the water is
circulated through the tubes and the steam and water may flow either in same direction
or in opposite direction. The closed heaters c‖ c of vertical or horizontal type. Figure
shows a vertical closed heater. A horizontal closed heater is just similar to a surface
condenser
In closed water heaters, the feed water can never be heated to the temperature of
steam. To maintain a high overall heat transfer for the heater, the water velocity should

260
be high but pumping costs will limit the velocity. A balance will result in water
velocities of about I to 2.5 rn/s
Arrangement of Feed Water Heaters
A power plant steam cycle contains typically three low pressure feed water
heaters and two or three high pressure ones. The steam cycle normally contains at least
one open feed water heater (other heaters are closed type) that operates at a pressure
slightly higher than atmospheric pressure. This heater is called the ‗Deaerating heater‘
discussed before. Steam for 1.p. heaters is bled from Lp. turbine and that for h.p. heaters
and deaerator, from i.p. and h.p. turbines. Figure shows the feed water heaters
arrangement for a typical 120 MW turbine system. The condensed bled steam in the
heater shells is usually drained to the next lower heater through a trap (T). Thedrain
from high pressure heater 3 (H.P.3) will get cascaded to H.P.2. H.P.2 would cascade
through a trap into H.P. 1 and H.P. 1 would cascade into the deaerator. No.2 low
pressure heater (L.P.2) would cascade into No.1 low pressure heater (L.P.1) again
through a trap and No. I low pressure heater would be drained through a drain cooler
(D.C.) to the condenser. The purpose of low pressure drain cooler is to recover some of
the sensible heat in the drain water before it is drained 10 the condenser where it will be
wastefully cooled to vacuum temperature. A ―steam trap‖ (T) is a float valve and when
the water levl reaches a given point, the float opens the valve and throttles the water to
the next heater, which is at the lower pressure. It is clear in the Fig., that all the h.p.
heaters are placed between the first stage boiler feed pump, as also known as booster
pump (B.P.) and the main boiler feed pump (B.F.P.). The low pressure heaters and
deaerator are placed between the condensate extraction pump (C.E.P.) and the booster
pump.

261
In modern steam power plants, the bled steam to the last three heaters (h.p). is
usually superheated and is at very high temperatures. Normally, the superheat of the
steam cannot be utilized in the heaters because the temperature in the heater shell is
determined by the pressure. During the process of condensation, the prevailing
temperature corresponds to the pressure. To utilise the superheat in the steam,
desuperheating heaters can be incorporated in the system. This would result in increased
feed water temperature as compared to that obtained by condensation alone, Fig. 11 .l2.
The superheated bled steam is first passed through smaller desuperheating heaters
(D.S.H.). Desuperheating the bled steam gives a feed water temperature rise of 3.5°C for
No. 2 heater and this means 3.5°C less feed heating for No.3 heater. Similarly, the 4°C
feed temperature rise across No. 3 desuperheating heater results in decrease in amount of
heat to be added in the boiler. However, it also means a decrease of 4°C in the terminal
temperature difference for H.P.3 and so on. The purpose of the flash boxes is to prevent
the flashing drain water from damaging the heater tube nest as it probably would if the
pressure was broken down at the inlet of the heater shell. The pressure drop of the drain
water which liberates the flash steam occurs in the flash box, which is specially designed
to withstand the erosive condition. This arrangement results in some loss of available
energy because heat tapped off H.P.3 finds its way as flash steam into H.P2. Thus, part
of the feed heating in H.P.2 is being done by steam from H.P.3 which is wasteful of
available energy

262
A better arrangement would be for each heater to have its own drain- cooler
where part of the sensible heat in the drain from each heater would be utilised to
increase feed water temperature, Figure
EVAPORATORS
Evaporators are used to produce distilled water for make up in steam power
plants. Steam is used as the heating medium and the vapours produced may be
condensed to give a supply of pure feed water. These vapours can either be condensed in
feed water heaters by the feed water or in Separate evaporator condensers using feed
water as the cooling water. There are two

263
main types of evaporators: -
1. Film or flash type.
2. Submerged type.
Film or Flash Type:
In this type, there are tubes or coils through which the steam is passing. Raw
water is sprayed by means of nozzles on the surface these tubes and some of the raw
water will be convereted into vapours. These vapors leave the evaporator and are
condensed to give a pure and distilled water for boiler
Submerged Type.
In this type the tubes through which the steam is passing, are submerged in raw
water, gig. 11.15. The vapours rising from the raw water are collected and condensed to
give a supply of pure make up feed water. Due to the continuous evaporation of raw
water, concentration of impurities goes on increasing. So the raw water should be
blowed down periodically. Scale accumulated on the surface of the tubes will retard heat
transfer and it is removed by draining the raw water from the shell and then spraying the
tubes with cold water while tubes are kept hot due to flow of steam. The scale is cracked
off andls washed away by the spray.

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