Other systems
The lithium bromide-water system is now almost the only one chosen for air conditioning applications. An alternative that has been used extensively in industrial work and, to a small extent, in air conditioning, is the system in which ammonia is the refrigerant and water the absorbent. Lower temperatures can be produced with this arrangement than are possible when water is used as a refrigerant, but there are difficulties. These arise because the absorbent vaporises in the generator as well as the refrigerant, and the system therefore has to be provided with a rectifier to condense and return as much of the absorbent as possible. A diagram of the system is shown in Figure 14.6.
Steam consumptions for water-ammonia systems generally exceed those attainable with lithium bromide-water systems and are in the region of 1.1 to 1.5 g s_1 kW-1.
Srivastava and Weames (1997) also report the results of work on vapour adsorption machines.
1. Show on a line diagram the essential components of a steam heated aqua-ammonia absorption refrigerating plant. Briefly outline the cycle of operation and describe the function of each component of the plant.
2. Make a neat sketch showing the layout of a lithium bromide-water absorption refrigeration plant using high-pressure hot water for the supply of energy to the generator. How may its capacity be most economically controlled? Show a suitable control system for this purpose in your diagram.
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Description Unit Concentration of LiBr, per cent by weight, in the absorber Concentration of LiBr, per cent by weight, in the generator Coefficient of performance Maximum possible coefficient of performance Specific heat of the liquid refrigerant kJ kg-1 K-1 Rate of heat removal at the absorber kW per kW of Refrigeration |
Ca
COP
COPmax c 1
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Hc |
Rate of heat removal at the condenser |
KW per kW of refrigeration |
|
Rate of heat addition at the generator |
KW per kW of refrigeration |
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K |
Enthalpy of the solution leaving the absorber |
KJ kg-1 |
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Hg |
Enthalpy of the solution leaving the generator |
KJ kg"1 |
|
Hc |
Enthalpy of the refrigerant liquid leaving the |
KJ kg-1 |
|
Condenser |
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|
Enthalpy of the refrigerant vapour leaving the |
KJ kg-1 |
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Evaporator |
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Hy g |
Enthalpy of the vapour leaving the generator |
KJ kg 1 |
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Mr |
Mass flow rate of refrigerant circulated |
K gs_1 per kW of refrigeration |
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«sa |
Mass flow rate of solution leaving the absorber |
K gs_1 per kW of refrigeration |
|
Msg |
Mass flow rate of solution leaving the generator |
K gs_1 per kW of refrigeration |
|
Pc |
Condensing pressure of the refrigerant |
KPa |
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Pt |
Evaporating pressure of the refrigerant |
KPa |
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<2g |
Heat supplied at the generator, in kJ kg-1 of refrigerant |
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Qr |
Refrigerating effect, in kJ kg-1 of refrigerant |
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T Ac |
273 + tc |
K |
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T |
273 + te |
K |
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T G |
273 + tg |
K |
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Tc |
Temperature of the refrigerant leaving the condenser |
°C |
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Te |
Temperature of the refrigerant leaving the evaporator |
°C |
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?a |
Temperature of the refrigerant in the absorber |
°c |
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H |
Temperature of the refrigerant in the generator |
°c |
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W |
Rate of work done |
KW per kW of refrigeration |
Posted in Air Conditioning Engineering