Absorption Refrigeration Cycle

Figure 5 illustrates a typical absorption refrigeration system. Once again, heat is picked up from the surroundings by the boiling of the low-pressure refrigerant liquid. As the vapor exits the evaporator, it contacts the absorbent and dissolves. The solution is pumped around to the boiler, where it is heated. The refrigerant boils off from the stillliquid absorbent and is transferred to the condenser, where it loses heat and is liquefied. The liquid refrigerant is stored in a reservoir until required, when it passes through an expansion valve into the evaporator. The absorption at low temperature and pressure, heating, and boiling off of the vapor at high temperature and pressure are the equiv-




Heat exchanger

Expansion valve



Absorption chamber

Figure 5. An absorption refrigeration system, schematic.

aient of the mechanical compression step. In each case, the refrigerant is taken from the Tu P1 condition to the T2, P2 condition. The ammonia absorption refrigeration system requires some additional components because the absorbent fluid—water—is itself volatile; therefore, there is a need to separate a significant amount of water vapor from the ammonia vapor exiting from the boiler. This is achieved by using a distillation column.

For the absorption system, the special components are the absorption chamber, the boiler, and the heat exchangers that transfer the heat in the returning "stripped" absorbent to the refrigerant-loaded absorbent being pumped to the boiler. As mentioned earlier, for an ammonia-water system, a distillation coil is required to separate water from ammonia in the exiting vapor stream from the boiler. The evaporator and condenser can be similar to those of the vapor compression systems, although special modifications may be required to deal with residual water in the ammonia-water system.

To compare the capabilities of different refrigeration systems, some definitions are appropriate.

Ton of refrigeration. The standard unit of refrigeration capacity is known as the ton of refrigeration. It is the

Table 2. Properties of Some Important Refrigerants

Vapor pressure (MPa)

Temperature (°C)


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