CPD Programme
90
85
70.1
80
57.8
47.4
, °C
75
Solution temperature,
38.6
70
65
31.2
60
25.0
55
90
95
10 0 10 5
11 0
11 5
120 19.9
80
85
°C
15.7
50 75
45
70
12.3
65
40 60
9.58
35
55 7.38
50
5.62
30 45
4.24
25
40
35 3.17
20
30
V
apour pressure, kPa
25
2.34
15
20
1.70
Saturation temperature of pure water 10
15
C
rystallization 1.23
5
10
0.87
0
5 ˚C
0.61
10 20 30 40 50 55 59 60 67 68 69 70
Lithium bromide in solution, % by mass
Figure 2 Temperature/ pressure/concentration data for lithium bromide solution
> • Waste heat is available (eg, exhaust steam); chilled water at say 7C. If the evaporator • The heat input at the generator will be the
• A low cost source of heat is available (eg, temperature starts at 7C, its vapour pressure heat source selected; and
landfill gas, geothermal); as pure water is 1.0kPa and for equilibrium • The heat rejected from the condenser
• An existing site has an electrical load limit between the evaporator and absorber to be produces the condensation of the refrigerant
that would be expensive to upgrade; achieved, water would have to evaporate (water).
• A site is particularly sensitive to noise and/ in the evaporator and condense into the The only electrical input is for circulating
or vibration; and solution in the absorber. Provided that a fresh pumps (see figure 3) and control valves.
• Solar energy can be harnessed. supply of lithium bromide solution at 25C is Note that the removal of heat from the
continuously available, the process could go absorber and condenser can be by ambient
Absorption cycle theory on indefinitely and the water in the evaporator air in small absorption units and are
As well as a refrigerant, an absorption system would evaporate until its vapour pressure available as air cooled, air cooling units
needs an absorbent solution and various pairs drops to about 0.87kPa, and a temperature up to about 80kW cooling duty. Perhaps
of fluids are available. For air conditioning of 5C, cooling the chilled water. The lithium the most common absorption application
applications operating with evaporating bromide solution acts like a compressor since its conception has been in domestic
temperatures above 0C, lithium bromide in drawing off “refrigerant vapour”, in this refrigeration, where a system has been
solution is the absorbent, while water is the case water, from the evaporator, causing developed that has no electrical requirement
refrigerant. Below 0C, the most common the pressure and saturation temperature to and the system is driven by gas. In building
pairing is water as the absorbent and ammonia reduce to the required cooling temperature. services applications it is more common to
as the refrigerant. In this article, only the air This process is the basic principle by which find large capacity absorption plant, chilling
conditioning case is considered. the absorption cycle operates. water and rejecting heat through water
Lithium bromide is a solid salt crystal that To complete the cycle shown in Figure 1, the cooled condensers and absorbers by cooling
readily absorbs water vapour (it is used to ‘weak’ solution in the absorber is pumped to tower/ dry cooler water, passing through the
keep electronic equipment like cameras free a generator, where external heat is applied to absorber first, then the condenser.
of moisture), eventually becoming a liquid boil off or vaporise the water from the solution. For maximum heat exchange contact in
solution of lithium bromide and water. This This results in the water (refrigerant) vapour the evaporator, the refrigerant pump sprays
solution exerts a water vapour pressure that leaving the generator and being condensed refrigerant water over the chilled water tubing
is a function of the solution temperature and in a water or air cooled condenser, back to – similarly in the absorber, where solution
concentration. These temperature/pressure/ a liquid. Its pressure is then reduced before is sprayed over the heat rejection tubing.
concentration properties are shown in Figure feeding back into the evaporator to continue The heat exchanger improves the efficiency
2 for lithium bromide/water solutions. the cooling process. Meanwhile, the now between absorption and generation.
For example, a lithium bromide solution ‘strong’ solution in the generator is fed back It should be noted that heat rejection from
at 50 per cent concentration and 25C would to the absorber, also reducing in pressure as it absorption systems will be greater than
have a vapour pressure of 0.87kPa. This is goes and continuing the absorption process. that for an equivalent vapour compression
a typical condition for the solution in the The energy flows in Figure 1 indicate: system, because of the cooling required in
absorber vessel in Figure 1. Connected to the • The cooling duty heat input is to the the absorber – about 2.5 times the cooling
absorber is the evaporator, containing water evaporator; and capacity, for air conditioning applications,
as refrigerant, which we would like to be at a • Heat is generated by the absorption process which means larger heat rejection
saturation temperature of 5C for producing and this heat has to be removed. equipment.
56 CIBSE Journal November 2009 www.cibsejournal.com
CIBSEnov09 pp55-58 cpd.indd 56 10/23/09 1:19:56 PM
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