R&ac lecture 16

Lesson
16
Vapour Absorption
Refrigeration Systems
Based On Ammonia-
Water Pair
Version 1 ME, IIT Kharagpur 1

The specific objectives of this lesson are to:
1. Introduce ammonia-water based vapour absorption refrigeration systems (Section
16.1)
2. Discuss the properties of ammonia-water mixtures and introduce pressure-
temperature-concentration (p-T-ξ) and enthalpy-temperature-concentration (h-T-
ξ) charts (Section 16.2)
3. Analyze some basic steady flow processes using ammonia-water mixtures such as
adiabatic and non-adiabatic mixing, throttling of solution streams and the concept
of rectification (Section 16.3)
At the end of the lecture, the student should be able to:
1. Differentiate between water-lithium bromide and ammonia-water systems vis-à-
vis their properties
2. Explain the concepts of bubble point and dew point temperatures
3. Obtain thermodynamic properties of ammonia-water mixtures using p-T-ξ and h-
T-ξ charts
4. Analyze important steady flow processes involving binary mixtures
16.1. Introduction
In vapour absorption refrigeration systems based on ammonia-water pair,
ammonia is the refrigerant and water is the absorbent. These systems are more versatile
than systems based on water-lithium bromide as they can be used for both sub-zero
(refrigeration) as well above 0o
C (air conditioning) applications. However, these systems
are more complex in design and operation due to the smaller boiling point temperature
difference between the refrigerant and absorbent (about 133o
C). Due to the smaller
boiling point temperature difference the vapour generated in the generator consists of
both ammonia as well as water. If water is allowed to circulate with ammonia in the
refrigerant circuit, then:
i. Heat transfer in condenser and evaporator becomes non-isothermal
ii. Evaporator temperature increases
iii. Evaporation will not be complete
iv. Water may get accumulated in the evaporator leading to malfunctioning of the plant
iv. Circulation ratio increases
Since all the above effects are detrimental to the performance of the system, it is
necessary to minimize the concentration of water vapour in ammonia at the inlet to the
condenser. This requires additional components, namely a rectification column and a
dephlegmator between generator and absorber, which increases the design complexity
and cost and also reduces the system COP compared to water-lithium bromide system.

16.2. Properties of ammonia-water solutions
16.2.1. Composition
Similar to water-lithium bromide solutions, the composition of ammonia-water
solution is also expressed either in mass fraction (ξ) or mole fraction (x). However, for
ammonia-water solutions, the mass and mole fractions are defined in terms of ammonia.
For example the mass fraction ξ is defined as the ratio of mass of ammonia to the total
mass of solution, i.e.,
WA
A
mm
m
+
=ξ (16.1)
where mA and mW are the mass of ammonia and water in solution, respectively.
Similarly, the mole fraction of ammonia-water solution is defined as:
WA
A
nn
n
x
+
= (16.2)
where nA and nW are the number of moles of ammonia and water in solution,
respectively. The number of moles of ammonia and water can easily be obtained from
their respective masses in solution and molecular weights, thus;
W
W
W
A
A
A
M
m
nand;
M
m
n == (16.3)
where MA (= 17.0 kg/kmol) and MW (= 18.0 kg/kmol) are the molecular weights of
ammonia and water respectively.
16.2.2. Vapour pressure of ammonia-water solutions
Liquid ammonia and water are completely miscible in all proportions, hence can
form solutions of all concentrations from 0 to 1, at normal temperatures. The effect of
ammonia in water is to lower the vapour pressure of water, similarly the effect of water in
ammonia is to lower ammonia’s vapour pressure. Thus the total pressure over ammonia-
water solutions is made up of partial pressure of ammonia and partial pressure of water
vapour, and is always in between the saturation pressures of pure ammonia and water.
If Raoult’s law is applied to ammonia-water mixtures, then the total pressure at
any temperature, Ptotal is given by:
(16.4)WAtotal P)x1(xPP −+=

where x is the liquid phase mole fraction of ammonia, P and PA W are the saturation
pressures of pure ammonia and pure water at that temperature.
However, similar to water-lithium bromide solutions, ammonia-water solutions also
deviate from ideal solution behaviour predicted by Raoult’s law in a negative manner,
i.e., at a given temperature of the solution the actual vapour pressure will be less than that
predicted by Raoult’s law (activity coefficient is much smaller than 1.0).
For example, at a mass fraction of 0.4 and temperature of 40o
C, Raoult’s law
predicts a vapour pressure of 6.47 bar, whereas the measured vapour pressure is 3.029
bar.
The vapour pressure data of ammonia-water solutions is also available in the form
of Dühring and other P-T-ξ plots.
16.2.3. Composition of ammonia-water vapour
Since the vapour above ammonia-water liquid consists of both ammonia and
water vapour, it is essential to distinguish between the composition in liquid phase and
composition in vapour phase. The superscripts L and V will be used to distinguish
between liquid and vapour phase compositions. Thus ξL
stands for liquid phase mass
fraction and ξV
stands for vapour phase mass fraction. Though the vapour phase
composition, can be obtained by assuming ideal solution behaviour, it is observed that the
actual vapour composition deviates from that predicted by ideal mixture equations. Based
on experimental measurements, charts have been developed for obtaining composition of
ammonia-water mixture in vapour phase in equilibrium with a solution of ammonia and
water at different temperatures. Figure 16.1 shows the construction of such a chart using
which one can obtain the composition of mixture in vapour phase from known values of
liquid phase mass fraction (ξL
) and saturated temperature of pure ammonia or pressure.

16.2.4. Bubble point and dew point for ammonia-water mixtures
Figure 16.2 shows a cylinder containing mixture of ammonia and water. The
pressure on the mixture is maintained constant with the help of a free-floating piston with
fixed weights. Initially (State 1) the cylinder consists of subcooled solution of ammonia-
water mixture. Now heat is supplied to the system and the temperature of the solution is
increased steadily, the mass fraction of the solution remains constant at ξ1 initially. At a
certain temperature the first vapour bubble appears. The temperature at which the first
bubble appears is called as bubble point (=Tbubble) of the solution at that concentration and
pressure. Further heating results in increase in temperature and formation of more vapour
as shown in the figure (State 2). If heating is continued further, then the temperature
Mass fraction of ammonia in vapour,
ξL
P Tsat,NH3
V
ξ
Fig.16.1. Vapour-liquid equilibrium chart for ammonia-water solution

increases continuously, as more liquid is converted into vapour, and finally at a particular
temperature the last liquid droplet vaporizes. The temperature at which the last liquid
droplet evaporates is called as dew point temperature (Tdew). When heating is continued
further the mixture enters into superheated vapour state (State 3). It should be noted that
unlike pure fluids, the temperature of the ammonia-water mixture increases continuously
as the liquid undergoes vaporization. This is to say that the phase change process is
characterized by a temperature glide, which is the difference between the dew point and
bubble point temperatures. If this process is repeated with different initial concentrations
starting from 0 (pure water) to 1 (pure ammonia) and at the same pressure, different
values of bubble and dew points will be obtained. Of course when the concentration is 0
(pure water) or 1 (pure ammonia) the bubble and dew points coincide. Now if we plot the
temperatures (bubble point and dew point) against concentration and join all the bubble
points by a curve and all the dew points by another curve, then we would get the
equilibrium Temperature vs concentration curve for ammonia-water mixtures at that
pressure as shown in Fig.16.3. The loci of all the bubble points is called as bubble point
line and the loci of all the dew points is known as the dew point line. The bubble point
line is the saturated liquid line and the dew point line is the saturated vapour line for the
mixture at that pressure. The region between the bubble and dew point lines is the two-
phase region where both liquid and vapour coexist in equilibrium. Different bubble point
and dew point lines will be obtained if the experiment is carried out with different
pressures. For example, Figure 16.4 shows the bubble and dew point lines for two
different pressures, P and P1 2. The same results can also be obtained if one starts the
experiment initially with superheated vapour and then start cooling it. In this case, the
dew point is the temperature at which the first liquid droplet forms from the vapour and
the bubble point is the temperature at which the last vapour bubble condenses.
P
P
P
V
V
L L
Heat Heat Heat
(1) (2) (3)
Fig.16.2: A simple experiment illustrating the principle of bubble and dew points

T
ξ
TW,Sat
TA,Sat1
2
3
1
P = Constant
Superheated
vap
ξ1ξ2
L
ξ2
V
0
Dew Point line
L+V
our
Tbubble
Tdew
2V
2L Bubble Point line
Subcooled liquid
ξ1
V
ξ1
L
Fig.16.3: Equilibrium temperature-concentration curve for NH3-H2O at a constant pressure
T
ξ
P = P1
P = P2
P2 > P1
0
(pure H2O)
1
(pure NH3)
Fig.16.4: Bubble point and dew point curves at two different pressures

Now since the process is carried out in a closed system, the mass of both ammonia and
water will be conserved. The concentration of subcooled liquid will be same as the
concentration of superheated vapour. However, in the two-phase region in which the
saturated liquid exists in equilibrium with saturated vapour, the concentration of liquid
and vapour will be different. For example, at point 2 in Fig.16.3, the temperature of
saturated liquid and vapour will be same as they are in equilibrium, hence, the
concentration of liquid will be ξ2
L
(intersection of constant temperature line with
bubble point line) and that of vapour will be ξ2
V
(intersection of constant
temperature line with dew point line) as shown in the figure. Obviously the vapour
formed initially will be richer in the low boiling point substance (ammonia) and the liquid
remaining will be rich in high boiling point substance (water). For example, as shown in
Fig.16.3, the concentration of the first vapour bubble will be ξ1
V
and the concentration of
the last liquid droplet will be ξ1
L
.Since the total mass as well as mass of individual
components is always conserved, we can write mass balance for total mass (mtotal) and
ammonia (mA) mass at state 2 as:
V
2
L
2total mmm += (16.5)
total1
V
2
V
2
L
2
L
2A mmmm ξ=ξ+ξ= (16.6)
where and are the mass of liquid and vapour at state 2, respectively.L
2m
V
2m
From the above equations it can be easily shown that:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
ξ−ξ
ξ−ξ
= L
21
1
V
2
V
2
L
2
m
m
, or (16.7)
)(m)(m 1
V
2
V
2
L
21
L
2 ξ−ξ=ξ−ξ (16.8)
The above equation is called as the mixing rule or lever rule for the binary
mixtures such as ammonia and water. It implies that the fraction of liquid and vapour in
the two-phase mixture is inversely proportional to the distance between the mixture
condition 2 and the saturated liquid and vapour states 2L
and 2V
, respectively.
16.2.5. Enthalpy of ammonia-water mixtures
Liquid phase:
The enthalpy of ammonia-water solution in liquid phase, hL
is calculated in a
manner similar to that of water-lithium bromide solutions, i.e., by the equation:
mix
L
W
LL
A
LL
hh)1(h.h Δ+ξ−+ξ= (16.9)

where is the liquid phase mass fraction of ammonia, are liquid phase
enthalpies of pure ammonia and water respectively. Δh
L
W
L
A handhL
ξ
mix is the heat of mixing, which is
negative (exothermic) similar to water-lithium bromide mixtures.
Using the above equation one can calculate the specific enthalpy of ammonia-
water solutions at any concentration and temperature provided the heat of mixing is
known from measurements. Thus enthalpy charts for solution are plotted as a field of
isotherms against mass fraction by taking suitable reference values for enthalpy of
ammonia and water. Since pressure does not have a significant effect on liquid enthalpy
(except at critical point), normally pressure lines are not shown on typical solution
enthalpy charts. Also enthalpy of subcooled liquid is generally assumed to be equal to
the saturated enthalpy at that temperature without loss of much accuracy.
Vapour phase:
Evaluation of enthalpy of a mixture of vapours of ammonia and water is more
complicated compared to liquid phase enthalpy. This is due to the dependence of vapour
enthalpy on both temperature and pressure. However, to simplify the problem, it is
generally assumed that ammonia and water vapour mix without any heat of mixing. Then
the enthalpy of the vapour mixture, hV
is given by:
V
W
VV
A
VV
h)1(h.h ξ−+ξ= (16.10)
where is the vapour phase mass fraction of ammonia and re the
specific enthalpies of ammonia vapour and water vapour respectively at the temperature
of the mixture. However, since vapour enthalpies depend on temperature as well as
pressure, one has to evaluate the vapour enthalpy at suitable pressure, which is not equal
to the total pressure. An approximate, but practically useful method is to evaluate the
vapour enthalpies of ammonia and water at pressures, P
V
W
V
A handh aV
.ξ
A and PW given by:
totalW
totalA
P)y1(P
yPP
−=
=
(16.11)
where y is the vapour phase mole fraction of ammonia and Ptotal is the total pressure. It
should be noted that P and PA W are equal to the partial pressures of ammonia and water
only if they behave as ideal gases. However since ammonia and water vapour may not
approach the ideal gas behaviour at all temperatures and pressures, in general PA and PW
are not equal to the partial pressures. Using this method enthalpies of ammonia-water
mixtures in vapour phase have been obtained as functions of temperature and mass
fraction.
16.2.6. The complete enthalpy-composition diagram for ammonia-water mixtures:

Normally, charts of enthalpy-temperature-mass fraction are available which give
both liquid phase as well as vapour enthalpy of mixtures. Figure 16.5 shows one such
chart. Figure 16.6 shows the enthalpy-composition diagram at a constant pressure P. In
the figure point a represents the condition of saturated liquid mixture at a temperature T
with a liquid phase mass fraction of ξL
. The liquid phase enthalpy corresponding to this
condition is given by hL
. The composition and enthalpy of vapour mixture in equilibrium
with the liquid mixture at temperature T and pressure P are obtained by drawing a
vertical line from a upto the auxiliary line and then drawing a horizontal line to the right
from the intersection of the vertical line with the auxiliary line. The intersection of this
horizontal line with the dew point line a’ gives the vapour phase mass fraction ξV
and the
vapour phase enthalpy hV
as shown in the figure. The isotherm T in the two-phase region
is obtained by joining points a and a’ as shown in the figure. Point b in the figure lies in
the two-phase region. The specific enthalpy of this point hb is given by:
V
b
L
bb hh)1(h ψ+ψ−= (16.12)
where ψb is the quality or dryness fraction of the two-phase mixture at b. Since points a,
a’ and b are co-linear, the dryness fraction ψ is given by:b
LV
L
b
b
ξ−ξ
ξ−ξ
=ψ (16.13)
In actual enthalpy-composition diagrams the isotherms are not shown in two-phase
region as a different set of them exist for each pressure.
It is important to note that it is not possible to fix the state of the mixture
(subcooled, saturated, two-phase or superheated) just from temperature and mass fraction
alone, though one can calculate enthalpy of the mixture from temperature and mass
fraction. This is due to the reason that at a given mass fraction and temperature,
depending upon the pressure the point can be subcooled or saturated or superheated.
For example, a liquid mixture with a mass fraction of 0.4 and temperature of 80o
C has an
enthalpy of 210 kJ/kg, and it will be in subcooled condition if the pressure is 4.29 bar and
saturated if the pressure is 8.75 bar.

Fig.16.5: h-T-ξ chart for ammonia-water solution

hfg,A
hfg,W
Determination of temperature of mixture in two-phase region:
A trial-and-error method has to be used to determine the temperature of a point in
two-phase region if its enthalpy, liquid phase mass fraction and pressure are known. The
trial-and-error method can be graphical or numerical. Figure 16.7 shows a graphical
method for finding the temperature of point x in the two-phase region which is at a
known pressure Px, liquid phase mass fraction ξx and enthalpy hx. To start with, point a’
is obtained as shown in the figure by drawing a vertical line from point x upto the
auxiliary line and then drawing a horizontal line from the intersection point a” upto the
dew point line, the intersection of which gives a’. Then a straight line a’-x-a is drawn as
shown. Next point b’ is obtained by drawing a vertical line upto the auxiliary line and
then drawing a horizontal line from b” upto the dew point line to get b’. Then line b’-x-b
is drawn passing through x. This procedure is repeated until convergence is obtained.
Numerically the temperature can be obtained from the equation, which needs to
be satisfied for each end of the isotherm passing through x, i.e.,
L
x
L
x
x
V
x
V
hhhh
ξ−ξ
−
=
ξ−ξ
−
(16.14)
To start with guess values of hL
and ξL
are assumed by taking some point on the bubble
point line. Then saturated vapour properties hV
and ξV
are obtained from the enthalpy-
composition charts using the guess values of hL
and ξL
. Then using the above equation,composition charts using the guess values of hL
and ξL
. Then using the above equation,
ξ
ξL
ξV
T
T
h
hL
hV
Dew point line
Auxiliary line
0 1
a
a’
b
ξb
hb
P = Constant
Bubble point line
Fig.16.6: Enthalpy-composition diagram of NH3-H2O at a constant pressure P

L L
new values of h and ξ are obtained. Then these new values are used to obtain next set
of hV V
and ξ . This procedure is repeated till the values converge. Once the converged
values of hL L
and ξ are obtained then the temperature is read from the enthalpy-
composition chart.
6.3. Basic steady-flow processes with binary mixtures
) Adiabatic mixing of two streams:
ξ
h
0 1
a
a’
x
ξx
hx
Px = Constant
b’
b
a”
b”
Fig.16.7: A graphical method for finding temperature of liquid-vapour mixture
1
a When two streams of ammonia-water solutions are
(16.15)
mm ξ+ξ
rom the above equations, the mass fraction and enthalpy of the mixture at 3 are given
mixed adiabatically as shown in Fig.16.8, one can write mass and energy balance
equations as:
321 mmm =+
33m ξ= (16.16)2211
32211 hmhmhm =+ 3 (16.17)
F
by:

( )12
3
2
13
m
m
ξ−ξ+ξ=ξ (16.18)
( )12
3
2
13 hh
m
m
hh −+= (16.19)
two-ph
1
2
3
Adiabatic mixing
Chamber
1
1
3
2
0
h
ξ ξ1
ξ
3 ξ2
h1
h3
h2
m2/m3
m1/m3
Fig.16.8: Adiabatic mixing of two solution streams
Figure 16.9 shows the adiabatic mixing process with the mixture state 3 lying in
ase region on the enthalpy-composition diagram. The mixture state in two-phase
region implies that some vaporization has occurred during adiabatic mixing of the two
inlet streams 1 and 2. The enthalpy and composition of the two-phase mixture at 3 can be
obtained by using the equations given above. However, since this is in two-phase region,
the mixture consists of saturated liquid and vapour. The dryness fraction and temperature
of the mixture (T3) have to be obtained by trial-and-error method by applying mixing
rules. The fraction of the vapour in the mixture at 3 is then given by:
L3V3
L33
m
m
L
3
V
3
L
33
3
V
3
=
ξ−ξ
ξ−ξ
= (16.20)
) Mixing of two streams with heat transfer:b The process of mixing of two streams with
written as:
heat transfer takes place in absorber and generator of absorption refrigeration systems.
For example, Fig.16.10 shows the mixing of saturated refrigerant vapour (state 1) with
saturated solution of refrigerant-absorbent (state 2) in the absorber. The resulting mixture
is a solution that is rich in refrigerant (state 3). Since the process is exothermic, heat (Q)
is released during this process. Mass and energy balance equations for this process can be

ξ
h
ξ1ξ3ξ20 1
h2
h1
h3
T3
T3
2
3
1
3V
3L
Fig.16.9: Adiabatic mixing of two streams on h-T-ξ diagram

(16.21)321 mmm =+
332211 mmm ξ=ξ+ξ (16.22)
Qhmhmhm 332211 +=+ (16.23)
From the above equations, the enthalpy of the mixture at 3 is given by:
( )
3
12
3
2
13
m
Q
hh
m
m
hh −−+= (16.24)
Thus with heat transfer from the mixing chamber, the exit state lies at a vertical
distance of (Q/m3) below the state which would result without heat transfer (point 3’).
The exit point would lie above the state without heat transfer if heat is transferred to the
mixing chamber.
c) Throttling process: Throttling or isenthalpic expansion of ammonia-water solution
takes place in the solution expansion valve of the absorption refrigeration system. Figure
16.11 shows the throttling process on enthalpy-composition diagram. Since both mass
and energy are conserved during this process, and there is neither work nor heat transfer,
we obtain:
(16.25)21 ξ=ξ
21 hh = (16.26)
h
ξ3=ξ3’0 1
3’
’
2
1
3
Q/m3
ξ
1
2
3
Absorber
Q
Fig.16.10: Mixing of two streams with heat transfer

Hence the inlet and outlet states, points 1 and 2 are identical on enthalpy-composition diagram as
shown in the figure. However, as there is possibility of vapour generation due to flashing, the
exit condition may be a mixture of saturated liquid and vapour at the outlet pressure P2 then the
exit temperature T will be much lower than the inlet temperature T2 1. Taking point 2 as in the
two-phase region corresponding to the outlet pressure P2, one can get the vapour fraction and exit
temperature T by trial-end-error method as discussed earlier.2
d) Heating and cooling process – concept of rectification: Figure 16.12 shows an
arrangement wherein an initially subcooled solution (state 1) is heated in a heat
exchanger A (HX A) in such a way that the exit condition 2 lies in the two-phase region.
This two-phase mixture then flows into an adiabatic separator (SEP A) where the
saturated liquid (state 3) and saturated vapour (state 4) are separated. The saturated
vapour at state 4 is then cooled to state 5 in another heat exchanger B (HX B) by rejecting
heat 4Q5. The resulting two-phase mixture is then fed to another adiabatic separator B
(SEP B), where again the saturated liquid (state 6) and saturated vapour (state 7) are
separated. It is assumed that the entire process takes place at a constant pressure and is a
steady-flow process.
1 2
1,2
h1=h2
P2, Dew point line
P2, Bubble point line
P1
T1
T2
T2=TL=TV
V
L
ξ1 = ξ
h
Fig.16.11: Throttling of ammonia-water solution
2
ξ

HX (A)
HX (B)
SEP (A)
SEP (B)
Saturated liquid, 3
Saturated liquid, 6
Vapour, 7
1 2
4
5
1
2
3
4
5
7
6
1Q2/m1
4Q5/m4
P=Constant
1Q2
4Q5
L
L
V
V
Isotherms
Fig.16.12: Heating and cooling of NH3-H2O solution – concept of rectification
Now mass and energy balances are applied to each of the components as shown below:
Heat exchanger A:
Mass balance:
21 mm = (16.27)
21 ξ=ξ (16.28)
Energy balance:
)hh(mQ 12121 −= (16.29)
Separator A:
Mass balance:
432 mmm += (16.30)
443322 mmm ξ+ξ=ξ (16.31)
Energy balance:

443322 hmhmhm += (16.32)
from the above equations:
34length
24length
hh
hh
m
m
34
24
34
24
2
3
−
−
=
−
−
=
ξ−ξ
ξ−ξ
= (16.33)
34length
32length
hh
hh
m
m
34
32
34
32
2
4
−
−
=
−
−
=
ξ−ξ
ξ−ξ
= (16.34)
Similar equations can be obtained for heat exchanger B and separator B. The entire
process is also shown on enthalpy-composition diagram in Fig.16.12.
It may be noted that from the above arrangement consisting of heating, cooling
and separation, one finally obtains a vapour at state 7 that is rich in ammonia. That is the
combination of heat exchangers with separators is equivalent to the process of
rectification. Heat exchanger A plays the role of generator, while heat exchanger B plays
the role of dephlegmator. To improve the process of rectification in actual vapour
absorption refrigeration systems, a rectifying column is introduced between the generator
and dephlegmator. In the rectifying column, the vapour from the separator A comes in
contact with the saturated liquid coming from separator B. As a result, there will be heat
and mass transfer between the vapour and liquid and finally the vapour comes out at a
much higher concentration of ammonia.
The practical ammonia-water based vapour absorption refrigeration system
incorporating rectifying column and dephlegmator in addition to the basic components
will be discussed in the next lesson.
Questions and Answers:
1. Presence of water vapour in the refrigerant circuit of a NH -H3 2O system:
a) Decreases evaporator temperature
b) Increases evaporator temperature
c) Increases circulation ratio
d) Leads to non-isothermal heat transfer in evaporator and condenser
Ans. b), c) and d)
2. Compared to H O-LiBr systems, a NH -H O system:2 3 2
a) Requires additional components due to the requirement of rectification
b) Yields higher COP
c) Yields lower COP
d) Increases design complexity and system cost

Ans. a), c) and d)
3. Which of the following statements regarding the definition of concentration are TRUE:
a) A strong solution of H O-LiBr implies a solution rich in refrigerant2
b) A strong solution of H O-LiBr implies a solution weak in refrigerant2
c) A strong solution of NH -H O implies a solution rich in refrigerant3 2
-H O implies a solution weak in refrigerantd) A strong solution of NH3 2
Ans. b) and c)
4. Which of the following statements regarding NH -H O solution are TRUE:3 2
a) The bubble point temperature is always higher than dew point temperature
b) The bubble point temperature is always lower than dew point temperature
c) At a given pressure, the bubble point and dew point temperatures are higher than the
saturation temperature of NH but lower than the saturation temperature of H O3 2
d) At a given pressure, the bubble point and dew point temperatures are lower than the
saturation temperature of NH but higher than the saturation temperature of H O3 2
Ans.: b) and c)
5. For NH -H O solution at equilibrium, which of the following statements are FALSE:3 2
a) The concentration of liquid phase is lower than the concentration of vapour phase
b) The enthalpy of subcooled solution is a function of temperature and pressure
c) The enthalpy of superheated vapour is a function of temperature only
d) The state of the mixture can be uniquely determined by temperature and concentration
Ans.: b) and d)
6. When a binary solution of NH -H O is throttled adiabatically:3 2
a) Temperature always remains constant
b) Temperature may decrease
c) Temperature may increase
d) Enthalpy always remains constant
Ans.: b) and d)
O is at a temperature of 40o
7. A binary mixture of NH - H3 2 C and a liquid phase mole
fraction x of 0.5. Find the vapour pressure of the solution, if the activity coefficient of the
solution is 0.65. The saturation pressures of ammonia and water at 40o
C are 1557 kPa and
7.375 kPa, respectively.

Ans.: From Raoult’s law, the vapour pressure is given by:
kPa19.782P).x1(P.xP OH,satNH,satRaoult,v 23
=−+=
Using the definition of activity coefficient, a; the actual vapour pressure P is given by:v
(Ans.)kPa42.50819.782X65.0P.aP Raoult,vact,v ===
8. A binary vapour mixture consisting of ammonia and water is at a mole fraction of 0.9
and 10o
C. If the partial pressures of ammonia and water vapour in the mixture are 616.25
kPa and 1.227 kPa, respectively; and the specific vapour enthalpies of ammonia and
water are 1471.57 kJ/kg and 2519.9 kJ/kg, respectively, find a) the vapour pressure of the
mixture, and b) the specific enthalpy of the mixture.
Ans.:
a) Assume the vapour mixture to behave as a mixture of ideal gases, then the total
pressure of the mixture P is given by:v
kPa75.554P).y1(P.yP OHNHv 23
=−+= (Ans.)
b) The mass fraction of the mixture ξV
is given by:
WA
A
WWAA
AA
WA
AV
n18n17
n17
M.nM.n
M.n
mm
m
+
=
+
=
+
=ξ
Since the mole fraction of the vapour mixture is 0.9 ⇒ nA = 9 nW
Substituting this in the expression for mass fraction, we find that ξV
= 0.895
Again assuming the vapour mixture to behave as a mixture of ideal gases; the enthalpy of
the mixture is given by:
hV
= ξV
.hA + (1-ξV
)hW = 1581.64 kJ/kg (Ans.)
9. Find the dryness fraction (quality) and specific enthalpy of the two-phase (liquid &
vapour) of ammonia-water mixture using the following data:
Liquid phase mass fraction, ξL
= 0.30
Vapour phase mass fraction, ξV
= 0.87
Mass fraction of 2-phase mixture, ξ = 0.50
Specific enthalpy of saturated liquid, hL
= 340 kJ/kg
Specific enthalpy of saturated vapour, hV
= 1640 kJ/kg

Ans.:
351.0
mm
m
LV
L
LV
V
=
ξ−ξ
ξ−ξ
=
+
=ψ (Ans.)Dryness fraction,
Enthalpy of the two-phase mixture is given by:
kg/kJ3.796hh)1(h VL
=ψ+ψ−= (Ans.)
9. Two solution streams are mixed in a steady flow device. A heat transfer rate of 24 kW
takes place from the device. Find the exit concentration and enthalpy using the data given
below:
Stream 1: Mass flow rate, m = 0.1 kg/s1
Concentration, ξ1 = 0.7
Enthalpy, h = 110 kJ/kg1
Stream 2: Mass flow rate, m = 0.3 kg/s2
Concentration, ξ2 = 0.4
Enthalpy, h = 250 kJ/kg2
Ans.:
From mass balance of solution and ammonia, the exit concentration is given by ξ :3
475.0
)mm(
)mm(
21
2211
3 =
+
ξ+ξ
=ξ (Ans.)
From energy balance of solution and ammonia, the exit concentration is given by h :3
[ ] kg/kJ155
)mm(
Q)hmhm(
h
21
2211
3 =
+
−+
= (Ans.)

R&ac lecture 16

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