Refrigeration Systems

1. Simple Vapor-
Absorption
Refrigeration System

5. Simple Vapor Absorption Refrigeration System
@Point 1
-low temperature and low pressure refrigerant
vapour from evaporator at state 1 enters the
absorber and is absorbed by solution weak in
refrigerant (state 8) .
The heat of absorption (Qa
) is rejected to an external
heat sink at T∞
.

6. Simple Vapor Absorption Refrigeration System
@Point 2
-The solution, rich in refrigerant is pumped to the
generator pressure (Pg
) by the solution pump (state 3).
The pressurized solution gets heated up sensibly as it
flows through the solution heat exchanger by extracting
heat from hot solution coming from generator (state 4).

7. @Point 5
-Heat is supplied to this solution from an external heat
source in the generator (Qg
at Tg
), as a result refrigerant
vapour is generated (absorbent may also boil to give off
vapour in case of ammonia-water systems) at state 5.
This high-pressure refrigerant vapour condenses in the
condenser by rejecting heat of condensation to the external
heat sink (Qc
at T∞
) and leaves the condenser as a high
pressure liquid (state 9).
Simple Vapor Absorption Refrigeration System

8. @Point 10
- the high pressure refrigerant liquid is throttled in the
expansion device to evaporator pressure Pe
from where it
enters the evaporator, extracts heat from low temperature
heat source (Qe
at Te
) and leaves the evaporator as vapour at
state 1, completing a cycle.
Simple Vapor Absorption Refrigeration System

9. @Point 6
- The hot solution that is weak in refrigerant (state 6)
leaves the generator at high temperature and is cooled
sensibly by rejecting heat to the solution going to the
generator in the solution heat exchanger (state 7).
Then it is throttled to the evaporator pressure in the throttle
valve (state 8), from where it enters the absorber to complete
the cycle.
Simple Vapor Absorption Refrigeration System

11. COP for Ideal Vapor Absorption Refrigeration
System

12. • Absorption system requires a relatively large amount of low-
grade thermal energy at generator temperature to generate
refrigerant vapour from the solution in generator. Thus while
the energy input is in the form of mechanical energy in vapour
compression refrigeration systems, it is mainly in the form of
thermal energy in case of absorption systems.
COP for Ideal Vapor Absorption Refrigeration
System

13. • The solution pump work is often negligible compared to the
generator heat input. Thus the COPs for compression and
absorption systems are given by:
COP for Ideal Vapor Absorption Refrigeration
System

14. • Thus absorption systems are advantageous where a large
quantity of low-grade thermal energy is available freely at
required temperature. However, it will be seen that for the
refrigeration and heat rejection temperatures, the COP of
vapour compression refrigeration system will be much higher
than the COP of an absorption system as a high grade
mechanical energy is used in the former, while a low-grade
thermal energy is used in the latter.
COP for Ideal Vapor Absorption Refrigeration
System

15. • However, comparing these systems based on COPs is not fully
justified, as mechanical energy is more expensive than
thermal energy. Hence, sometimes the second law (or
exergetic) efficiency is used to compare different refrigeration
systems. It is seen that the second law (or exergetic) efficiency
of absorption system is of the same order as that of a
compression system.
COP for Ideal Vapor Absorption Refrigeration
System

16. Maximum COP for Ideal Vapor Absorption Refrigeration
System

17. From first law of thermodynamics,
Where
Qe
is the heat transferred to the absorption system at
evaporator temperature Te
,
Qg
is the heat transferred to the generator of the absorption
system at temperature Tg
,
Qa+c
is the heat transferred from the absorber and condenser of
the absorption system at temperature To
and
Wp
is the work input to the solution pump.
Maximum COP for Ideal Vapor Absorption Refrigeration
System

18. • If we assume that heat rejection at the absorber and
condenser takes place at same external heat sink temperature
To
, then a vapour absorption refrigeration system operates
between three temperature levels, Tg
, To
and Te
.
Maximum COP for Ideal Vapor Absorption Refrigeration
System

19. The maximum possible COP of an ideal VARS system is given by:
Maximum COP for Ideal Vapor Absorption Refrigeration
System

20. Thus the ideal COP is only a function of operating temperatures
similar to Carnot system. It can be seen from the above
expression that the ideal COP of VARS system is equal to the
product of efficiency of a Carnot heat engine operating between
Tg
and To
and COP of a Carnot refrigeration system operating
between To
and Te
,
Maximum COP for Ideal Vapor Absorption Refrigeration
System

21. Thus an ideal vapour absorption refrigeration system can be
considered to be a combined system consisting of a Carnot heat
engine and a Carnot refrigerator as shown in Fig.14.4. Thus the
COP of an ideal VARS increases as generator temperature (Tg
)
and evaporator temperature (Te
) increase and heat rejection
temperature (To
) decreases. However, the COP of actual VARS
will be much less than that of an ideal VARS due to various
internal and external irreversibilities present in actual systems.
Maximum COP for Ideal Vapor Absorption Refrigeration
System

22. Example of VARS
• 1. Hydrogen enters the
pipe with liquid
ammonia (or lithium
bromide solution)
2. Ammonia and
hydrogen enter the
inner compartment of
the refrigerator. An
increase in volume
causes a decrease in
the partial pressure of
the liquid ammonia.
The ammonia
evaporates, requiring
energy to overcome
the ΔHVap. The required
energy is drawn from
the interior of the
refrigerator, thus
cooling it.

23. Example of VARS
• 3. Ammonia and
hydrogen return from
the inner
compartment,
ammonia returns to
absorber and dissolves
in water. Hydrogen is
free to rise upwards.
4. Ammonia gas
condensation (passive
cooling).
5. Hot ammonia (gas).
6. Heat insulation and
distillation of ammonia
gas from water.
7. Heat source
(electric).
8. Absorber vessel
(water and ammonia
solution).

24. • 1. Hydrogen enters the pipe with liquid ammonia (or
lithium bromide solution)
2. Ammonia and hydrogen enter the inner compartment
of the refrigerator. An increase in volume causes a
decrease in the partial pressure of the liquid ammonia.
The ammonia evaporates, requiring energy to overcome
the ΔHVap. The required energy is drawn from the interior
of the refrigerator, thus cooling it.
3. Ammonia and hydrogen return from the inner
compartment, ammonia returns to absorber and dissolves
in water. Hydrogen is free to rise upwards.
4. Ammonia gas condensation (passive cooling).
5. Hot ammonia (gas).
6. Heat insulation and distillation of ammonia gas from
water.
7. Heat source (electric).
8. Absorber vessel (water and ammonia solution).

25. Practical Vapor Absorption Refrigeration System
• A practical VARS has 3 additional parts :
• ANALYSER
• RECTIFIER
• HEAT EXCHANGER

26. Practical Vapor – Absorption Refrigeration System
• Analyser
-When ammonia is vaporized in the generator some water
is also vaporized ,and flow into the condenser along with
ammonia.
Thus the ammonia refrigerant leaving the generator
carries appreciable amount of water vapor. If this water
vapor is allowed to be carried to the evaporator, the
capacity of the refrigeration system would reduce. The
water vapor from ammonia refrigerant is removed by
analyzer and the rectifier.

27. Practical Vapor – Absorption Refrigeration System
• The analyzer is a sort of distillation column that is
located at the top of the generator. The analyzer consists
of number of plates positioned horizontally. When the
ammonia refrigerant along with the water vapor
particles enters the analyzer, the solution is cooled. Since
water has higher saturation temperature, water vapor
gets condensed into the water particles that drip down
into the generator. The ammonia refrigerant in the
gaseous state continues to rise up and it moves to the
rectifier.

28. Practical Vapor – Absorption Refrigeration System
• Rectifier - In case the water vapour are not completely
removed in the analyser, a closed type of vapour cooler called
rectifier is also known as dehydrator is used, it may be of
water cooled.
• The rectifier is a sort of the heat exchanger cooled by the
water, which is also used for cooling the condenser. Due to
cooling the remaining water vapor mixed with the ammonia
refrigerant also gets condensed along with some particles of
ammonia. This weak solution of water and ammonia drains
down to the analyzer and then to the generator

29. Practical Vapor – Absorption Refrigeration System
• Heat Exchanger -The heat exchanger provided between
the pump and the generator which is used to cool the
weak hot solution returning from the generator to
absorber.

30. Practical Vapor – Absorption Refrigeration System

31. Practical vapor – absorption refrigeration system
Generator: The strong solution
of ammonia refrigerant and
water absorbent are heated by
the external source of heat
such as steam or hot water. It
can also be heated by other
sources like natural gas, electric
heater, waste exhaust heat etc.

32. Practical vapor absorption refrigeration system
Generator - Due to heating the
refrigerant ammonia gets
vaporized and it leaves the
generator. However, since
water has strong affinity for
ammonia and its vaporization
point is quite low some water
particles also get carried away
with ammonia refrigerant, so it
is important to pass this
refrigerant through analyzer.

33. Vapour Absorption
Refrigeration Systems
Based On Water-Lithium Bromide
Pair

34. VARS based on H2O – LiBr Pair
• Vapour absorption refrigeration systems using water-lithium
bromide pair are extensively used in large capacity air
conditioning systems.
• In these systems water is used as refrigerant and a solution of
lithium bromide in water is used as absorbent.
• Since water is used as refrigerant, using these systems it is not
possible to provide refrigeration at sub-zero temperatures.
Hence it is used only in applications requiring refrigeration at
temperatures above 0o
C.

35. • Hence these systems are used for air conditioning
applications. The analysis of this system is relatively easy as
the vapour generated in the generator is almost pure
refrigerant (water), unlike ammonia-water systems where
both ammonia and water vapour are generated in the
generator.
VARS based on H2O – LiBr Pair

39. Steady flow analysis of Water-Lithium Bromide
Systems
A steady flow analysis of the system is carried out with the
following assumptions:
i. Steady state and steady flow
ii. Changes in potential and kinetic energies across each
component are negligible
iii. No pressure drops due to friction
iv. Only pure refrigerant boils in the generator.
The nomenclature followed is:
m= mass flow rate of refrigerant, kg/s
mss= mass flow rate of strong solution (rich in LiBr), kg/s
mws= mass flow rate of weak solution (weak in LiBr), kg/s

40. Circulation ratio (λ)
-defined as the ratio of
strong solution
flow rate to
refrigerant
flow rate.
It is given by:
λ = mss/m

41. @Condenser
m1 = m2 = m3
Qc = m(h1 – h2)
Pc = Psat (TC)
where TC is the
condenser
temperature

42. @Expansion valve (refrigerant):
m2 = m3= m
h2 = h3
@Evaporator:
m3 = m4 = m
QE = m(h4 – h3)
PE = PSAT(TE)
where TEis the
evaporator
temperature

43. @Absorber
From total mass balance:
m + mss = mws
but mss = λm,
mws = (1+λ)m
QA=mh4+ λmh10
- (1+λ)mh5

44. @Solution pump
m5 = m6 = mws
Wp = mws(h6-h5)
=(1+λ)m(h6-h5)
Even though
the solution pump
work is small it is
still required in the
selection of
suitable pump.

45. @Generator
m7 = m8 +m1
Heat input to the
generator is given by:
QG=mh1+λmh8
-(1+λ)mh7

46. @Solution heat exchanger
m6 = m7 = mws
m8 = m9 = mss
heat transfer
rate in the solution
heat exchanger, Q
is given by:
QHX = (1+λ)m(h7-h6)
=λm(h8-h9)

47. @Solution expansion valve
m9 = m10 = mws
h9 = h10
The COP of the
system is given by:

48. Sample Problem in Simple VARS
9. The operating temperatures of a single stage vapour
absorption refrigeration system are: generator: 90o
C; condenser
and absorber: 40o
C; evaporator: 0o
C. The system has a
refrigeration capacity of 100 kW and the heat input to the
system is 160 kW. The solution pump work is negligible.
•a) Find the COP of the system and the total heat rejection rate
from the system.
•b) An inventor claims that by improving the design of all the
components of the system he could reduce the heat input to the
system to 80 kW while keeping the refrigeration capacity and
operating temperatures same as before. Examine the validity of
the claim.

49. • Solution:
Sample Problem in Simple VARS