Vapour absorption refers to the process by which a vapor is absorbed by another substance. This phenomenon has applications across various fields, from chemistry to environmental science and engineering. The process typically involves a gas (or vapor) being taken up by a liquid or solid material. Let's delve a bit deeper into this concept. In chemistry, vapour absorption often occurs in the context of solutions. For example, the absorption of gases like carbon dioxide into water forms carbonic acid. This is a key process in the carbon cycle and affects ocean chemistry. Vapour absorption is a crucial part of many industrial processes, especially in the realm of HVAC (heating, ventilation, and air conditioning) systems. These systems use vapour absorption to cool spaces. Vapour absorption is a fundamental process that plays a crucial role in various scientific, engineering, and industrial applications. This comprehensive exploration will cover the principles underlying vapour absorption, the different processes involved, and its wide-ranging applications across different fields. 1. Introduction to Vapour Absorption Vapour absorption refers to the process where a vapour (or gas) is taken up by a liquid or a solid material. This absorption can occur under specific conditions of temperature and pressure, leading to changes in the physical or chemical properties of the substances involved. Understanding the mechanisms behind vapour absorption is essential for leveraging its applications effectively. 2. Principles of Vapour Absorption The absorption of vapours by liquids or solids follows principles rooted in thermodynamics and chemical equilibrium. The driving force for absorption often involves the affinity between the vapour and the absorbent material. Key principles include: Henry's Law: This law states that at a constant temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the ambient. Applications: Vapour absorption processes find extensive use in various industries and scientific fields: HVAC Systems: Absorption chillers use vapour absorption to generate cooling. This technology is particularly useful in areas where waste heat or other energy sources can drive the absorption process. Environmental Engineering: Vapour absorption is employed in processes like gas scrubbing to remove pollutants from industrial emissions. Chemical Processing: Vapour absorption plays a role in chemical separations, such as removing specific gases from mixtures. Renewable Energy: Absorption systems can be integrated with renewable energy sources for efficient energy storage and utilization. 5. Challenges and Future Developments: Despite its utility, vapour absorption processes have challenges such as energy intensity and material compatibility. Future developments aim to enhance efficiency, reduce environmental impact, and explore novel applications in emerging field.

1. Refrigeration and air
conditioning
Dr. Najam Ul Hassan Shah
Lecture # 8

2. Vapor Absorption
Refrigeration Systems
(Chapter 17)
Part - 2
2

3. Basic Absorption Refrigeration Unit
• The Basic Absorption Cycle - The condenser, expansion
valve and the evaporator are similar as in a standard vapor-
compression cycle. The compression operation is now
provided by the assembly in the left-half of the diagram.

4. COP of Ideal Absorption Cycle
( )
( )
r
a
s
a
s
r
Gen
evep
T
T
T
T
T
T
Q
Q
COP
−
−
=
=
It can be noted from
above equation (and
Figure) :
As Ts increases → COP
increases
As Tr increases → COP
increases
As Ta increases → COP
decreases
Also see Ex. 17.1 and
equations 17.2 – 17.4
Ts
Tr
Ta

5. Absorption Refrigeration System
S.# Refrigerant Absorber Absorber
State
1 Ammonia Water Liquid
2 Ammonia Sodiumthiocynate Solid
3 Ammonia Lithium nitrate Solid
4 Ammonia Calcium chloride Solid
5 Water Lithium bromide Solid
6 Water Lithium chloride Solid
7 Methylene
chloride
Dimethyl ether or tetra
ethylene glycol
Liquid
Refrigerant-absorber pairs

6. Temperature-Pressure-Concentration Properties
Lithium Bromide (LiBr) Water Absorption Cycle
LiBr is a solid salt crystal. When exposing it to water vapor (in
absorber tank) it will absorb the water vapor and become a liquid
solution
• If two vessels were connected as shown in the figure,
one vessel containing LiBr-water solution and the other
pure water, each liquid would exert a water-vapor
pressure that is a function of the solution temperature
and the concentration of the solution.

7. Temperature-Pressure-Concentration Properties
Lithium Bromide (LiBr) Water Absorption Cycle
• At equilibrium the water-vapor pressure exerted by the two
liquids would be equal , reaching a T-P-C point state
having Tw = 40 C, Tsolution = 80 C and concentration of
solution 59% LiBr.
• Many such combinations Tw = f ( Tsolution , LiBr %, Pvapor) are
possible for a given solution and can be plotted on graphs;
like Fig 17.5 let see it

8. Temperature-Pressure-Concentration Properties

9. Temperature-pressure concentration diagram:
LiBr-water solutions
• Concentration is the abscissa of the graph and water-
vapor pressure could be considered as the ordinate on
the vertical scale on the right
• The saturation temperature of pure water corresponding
to these vapor pressures is shown as the ordinate on the
left
• The chart applies to saturated conditions where the
solution is in equilibrium with water vapor
Temperature-Pressure-Concentration Properties

10. Temperature-pressure concentration diagram:
LiBr-water solutions
• If the temperature of pure water is 40 °C, the vapor
pressure the liquid exerts is 7.38 kPa, which can be
determined from the opposite side of vertical scale.
• A LiBr solution with a concentration of 59 % &
temperature of 80°C also develops a water-vapor
pressure of 7.38 kPa
• If the solution had a concentration of 54% & temperature
of 70°C the water-vapor pressure would again be 7.38
kPa
Temperature-Pressure-Concentration Properties

11. Ex.2 :Temperature-Pressure-Concentration Properties
The pressure in vessel A is 4.24 kPa and it is 1.22 kPa in
vessel B. The surrounding temp is 30C. If the valve between
vessels is opened. Initially due to pressure difference water
vapor will flow from vessel A to vessel B, and this vapor will
be absorbed by the solution in vessel B

12. Ex.2 :Temperature-Pressure-Concentration Properties
If the concentration and
temperature of vessel B
are maintained constant
at 50 % and 30oC,
respectively. Then at
equilibrium, the pressure
in the entire system
(vessels A and B) will be
1.22 kPa (equilibrium
pressure of 50 % LiBr
solution at 30oC).
And temperature of water
in vessel A will lower to
the saturation
temperature
corresponding to 1.22
kPa, which is equal to
about 10C, as shown in
the figure

13. Ex.2 :Temperature-Pressure-Concentration Properties
Since the temperature of A is lower than surroundings, cooling
effect will be produces ; Qc
Now for the above process to continue, there should always be
pure water in vessel A, and vessel B must be maintained
always at 50 percent concentration and 30oC

14. Ex.2 :Temperature-Pressure-Concentration Properties
• In the example discussed the system is a closed system with finite
sized reservoirs
• Thus, gradually the amount of water in A decreases and the solution in
B becomes diluted with water. As a result, the system pressure and
temperature of water in A increase with time
• Hence the refrigeration effect at A reduces gradually due to the
reduced temperature difference between the surroundings and water.
Thus, refrigeration produced by systems using only two vessels is
intermittent in nature.

15. ME - 437 : Refrigeration & Air
Conditioning
Ex.2 :Temperature-Pressure-Concentration Properties
• In these systems, after a period, the refrigeration process
has to be stopped and both the vessels A and B have to be
brought back to their original condition. This requires
removal of water absorbed in B and adding it back to vessel
A in liquid form, i.e., a process of regeneration as shown in
Fig. below

16. Calculation of Mass Flow Rate and Enthalpy
Example 17.2 : Compute the rate flow of refrigerant (water) through the
condenser and evaporator in the cycle shown in Figure below if the pump
delivers 0.6 kg/s and the following temperatures prevail: generator, 100C;
condenser, 40C; evaporator. 10C; and absorber, 30C.

17. Calculation of Mass Flow Rates
• The mass flow rate abortion unit can be determined by
knowing the concentrations of the LiBr in the solution
• Two different pressures exist in the system: a high pressure
prevails in the generator and condenser, while the low
pressure prevails in the absorber and evaporator
• Knowing that the pressure in condenser & generator must
be same and at condenser inlet (station 3), saturated pure
water enters from generator at the condensing temperature
(40 C) gives us High Pressure of cycle. i.e
• At 40°C → (from fig 17,5 ) , Pgen = 7.38 kPa
• Similarly, evaporator an absorber pressure are same and at
evaporator operating at 10 C gives (Fig 17.5) the value of
low pressure;
• At 10°C → (from fig 17,5 ) , Pabs = 1.23 Kpa

18. Calculation of Mass Flow Rates
• At 40°C → (from fig 17,5 ) , Pgen = 7.38 kPa
• At 10°C → (from fig 17,5 ) , Pabs = 1.23 Kpa
• Knowing Low and high pressures, the concentration of LiBr
in absorber (Station1) and generator ( Station 2) can be
established from Fig 17.5 , see extract of Fig 17.5 below
X1 = 0.5 or 50%
X2 = 0.664 or
66.4%

19. Knowing and applying mass balance across generator
give us following two equations
Total mass flow balance :
LiBr Solution Mass flow :
Calculation of Mass Flow Rates
s
Kg
m /
148
.
0
3 =
•
s
kg
m /
6
.
0
1 =
•
6
.
0
1
3
2 =
=
+
•
•
•
m
m
m
2
2
1
1 x
m
x
m
•
•
=
s
kg
m /
452
.
0
2 =
•
( ) ( )
664
.
0
50
.
0
6
.
0 2
•
=m
Solving these two equations,
you get the values of mass
flow rates

20. Enthalpy of LiBr Solutions
• For thermal calculations on the absorption cycle, enthalpy
data must be available for the working substances at all
crucial positions in the cycle
• Water in liquid and vapor forms flows in and out of the
condenser & evaporator, so enthalpies at these points can
be determined from a table of properties of water
• In the generator and absorber, LiBr-water solutions exist for
which enthalpy is a function of both - solution temperature
and concentration

21. Absorption Refrigeration System

22. Calculation of Enthalpy, Q and COP
Ex. 17.3 : For absorption system of previous example, compute qg, qc, qa,
qe & COP.
SOLUTION:
Data from Previous Example:
m1 = 0.6 Kg/s, m2 = 0.452 Kg/s, m3 = m4 = m5 = 0.148 Kg/s,
X1 =0.5 and X2 =0.664
Enthalpy of Solution from Fig. 17.8
at 30 °C and x1 of 50% → h1 = -168 kJ/kg
At 100 °C and x2 of 66.4% → h2 = -52 kJ/kg
Enthalpy of water are found from Table A.1 as:
h3 = hvapor @ 100 C = 2676 KJ/Kg
h4 = hliquid @ 40 C = 167.5 KJ/Kg
h5 = hvapor @ 10 C = 2520 KJ/Kg

23. Absorption Refrigeration System

24. Absorption Refrigeration System

25. Calculation of Enthalpy, Q and COP
KW
h
m
h
m
h
m
Q
g 3
473
1
1
2
2
3
3 .
=
−
+
=
•
•
•
KW
h
m
h
m
Q
c 2
.
371
4
4
3
3 =
−
=
•
•
KW
h
m
h
m
h
m
Q
a 3
.
450
1
1
5
5
2
2 =
−
+
=
•
•
•
KW
h
m
h
m
Q
e 2
.
384
4
4
5
5 =
−
=
•
•
Ex. 17.3 : For absorption system of previous example, compute qg, qc, qa,
qe & COP.
736
.
0
6
.
476
2
.
348 =
=
=
g
e
Q
Q
COP
Now from Energy Balance, heat transfer rates are established as:

26. Absorption cycle with heat exchanger

27. Absorption cycle with heat exchanger
• By addition of heat exchanger, the COP increases
• The heat-exchanger transfers heat between the two streams
of solutions
• It heats the cool solution from the absorber on its way to the
generator and cools the solution returning from the generator
to the absorber

28. Example 17.4
A water-LiBr absorption refrigeration system is shown (see
figure). The temperature at point 2 is 52 °C. The mass flow rate
delivered by the solution pump is 0.6 kg/s. What are the rates of
energy transfer at each of the components and the COP of this
cycle? Also, what is the temperature at state 4?

29. Example 17.4

30. Example 17.4

31. Example 17.4

32. Example 17.4
The solution of Ex. Show that Qa reduces while Qg reduces
slightly and increases COP to 0.783 (from 0.736)

33. Example 17.4

34. • Crystallization occurs when the solution state is on right side of
crystallization line on P-T-C diagram
• Dropping into this region formation of a slash or solidification of
Solution, which can block the flow in a pipe and stop the operation of
absorption unit
• Crystallization must be avoiding by proper designing
• See Example 17.5
Crystallization

35. Example 17.5
In the system shown in Fig. 17-9, the ambient wet bulb
temperature decreases so that the temperature of the cooling
water drops, which also reduce the condensing temperature to 34
°C. All other temperatures specified on Fig. 17-9 remain
unchanged. Is there a danger of crystallization?

36. Example 17.5

37. Example 17.5

38. Example 17.5

39. Example 17.5

40. Example 17.5

41. Capacity Control ; It reduces the reduction to a desired level. Without
capacity control a given system yields maximum refrigeration. The
control can be achieved by following three methods :
▪ Reducing flow rate delivered by pump at Staion1
▪ Reducing generator Temperature
▪ Increasing the condensing Temperature
Capacity Control

42. Example 17.6

43. Example 17.6

44. Many Other Systems including combination with other cycles and
different combination of refrigerant and absorber. Few commonly
used systems are listed here:
Other Systems
▪ Double Effect Units
▪ Combined absorption
and vapor
compression system
▪ Aqua Ammonia
System

45. Double-effect units

46. Combined absorption and vapor compression system

47. Example 17.7

48. Example 17.7

49. Example 17.7

50. Aqua-Ammonia System
• In aqua-ammonia absorption system, water is used as an
absorbent while ammonia is used as a refrigerant
• Dissolution of NH3 into water is exothermic and inversely
proportional to temperature.
• The system consists of all the components i.e., generator,
absorber, condenser, evaporator, and heat exchanger----
plus a rectifier & analyzer.
• The work input to the pump is usually very small, and the
COP of absorption refrigeration systems is defined as
gen
L
in
p
gen
L
abs
Q
Q
W
Q
Q
input
Work
effect
Cooling
COP

+
=
=
,

51. Aqua-Ammonia System

52. Aqua-Ammonia System

53. Aqua-Ammonia System
• Additional components as refrigerant vapors released at
generator contains water vapor as well
• Normally aqua-ammonia system operate at evaporating
temperature below 0 °C
• If large amounts of water vapors are present in the
evaporator, chance that they may get converted to ice &
block the lines
• So, to remove as much water vapor as possible, the vapors
driven off from the generator first flows through the
rectifier, which is a direct-cooled heat exchanger
• In the rectifier, the vapors from the generator first flow
counter-current to the incommoding strong solution from
the absorber
• Next the solution passes through the analyzer which is a
water-cooled heat exchanger, condensing some water rich
liquids which drains back to the rectifier

54. Solar heating based absorption chiller

55. • Ideally fits into the concept of Integrated Energy Systems
such as Cogeneration involving combined generation, heat,
refrigeration and power (CHRP Plants) on various fuels like
bio-mas, coal, Natural Gas, Heavy Oil, Solar, geothermal,
etc.
• Excellent for waste heat utilization
• Earns carbon credits, reduces taxes, promotes sustainable
development.
• Uses best eco-friendly refrigerant – ammonia
• Wide operational range + 5 C to – 55 C
• Low maintenance cost – no moving parts
• Can operate well for over 25 years
Advantages of ammonia systems

56. Comparison: Vapor compression and absorption
systems

57. Thanks
Any Questions…???