Following the procedure adopted in section 9.1, let a liquid be introduced into a vessel in which there is initially a vacuum, and let the walls of the container be maintained at a constant temperature. The liquid at once evaporates, and in the process its latent heat of evaporation is abstracted from the sides of the vessel. The resultant cooling effect is the starting point of the refrigeration cycle to be examined in this chapter, just as it was in the beginning of the vapour compression cycle considered in chapter 9.
As the liquid evaporates the pressure inside the vessel rises until it eventually reaches the saturation vapour pressure for the temperature under consideration. Thereafter, evaporation ceases and the cooling effect at the walls of the vessel is not maintained by the continued introduction of refrigerant. The latter merely remains in a liquid state and accumulates in the bottom of the container. To render the cooling process continuous it is necessary, as we have already seen earlier, to provide a means of removing the refrigerant vapour as fast as it forms. In the vapour compression cycle this removal is accomplished by connecting the evaporator to the suction side of a pump. A similar result may be obtained by connecting the evaporator to another vessel containing a substance capable of absorbing the vapour. Thus, if the refrigerant were water, a hygroscopic material such as lithium bromide could be used in the absorber. The substance used for this purpose is termed the ‘absorbent’.
In order to obtain closed cycles for both refrigerant and absorbent the next stage in the process must be the release of the absorbed refrigerant at a convenient pressure for its subsequent liquefaction in a condenser. This is accomplished in the ‘generator’, where heat is applied to the absorbent-refrigerant solution and the refrigerant is driven off as a vapour.
The absorber and generator together take the place of the compressor in the vapour — compression cycle. So far as the refrigerant is concerned, the rest of the absorption cycle is similar to the compression cycle, i. e. the vapour is liquefied in the condenser and brought into the evaporator through an expansion valve or an orifice. As for the absorbent, on leaving the generator it is, of course, returned to the absorber for another cycle.
In an absorption refrigeration system cooling water is required for both the condenser and the absorber.
The principal advantages of the absorption cycle over other refrigeration systems are that it can operate with low-grade energy in the form of heat, indirectly as steam or high temperature hot water, or directly as gas, oil, hot exhaust gases, or solar heat. Furthermore, it has few moving parts. Theoretically, only a single pump is required, that needed for conveying the absorbent-refrigerant solution from the low-pressure absorber to the comparatively high-pressure generator. In practice, two more pumps are frequently used, one to recirculate solution over cooling coils in the absorber and another to recirculate the refrigerant over chilled water coils in the evaporator. The basic single effect absorptionrefrigeration cycle without these refinements is illustrated in Figure 14.1.
Fig. 14.1 A single effect absorption-refrigeration cycle using lithium bromide as the absorbent and
Water as the refrigerant.
Posted in Engineering Fifth Edition