Isentropic expansion refrigerator

FIGURE 5 (a) Schematic for combined isenthalpic and isentropic expansion refrigerator (b) temperature-entropy diagram for cycle. 

It is not uncommon to combine the isentropic and isenthalpic expansions to allow the formation of liquid in the refrigerator. This is done because of the technical difficulties associated with forming liquid in the engine. The Claude cycle is an example of a combination of these methods and is shown in Fig. 5a along with the corresponding temperature-entropy diagram (Fig. 5b). 

The effectiveness of a refrigeration cycle is measured by its coefficient of performance. For given values of Tc and TH, the highest possible value is attained by the Carnot refrigerator. The vapor-compression cycle with reversible compression and expansion approaches this upper limit. A vapor-compression cycle with expansion in a throttle valve has a somewhat lower value, and this is reduced further when compression is not isentropic. The following example provides an indication of the magnitudes of coefficients of performance. 

The Carnot refrigeration cycle is reversible and consists of adiabatic (isentropic due to reversible character) compression (1-2), isothermal rejection of heat (2-3), adiabatic expansion (3-4) and isothermal addition of heat (4-1). The temperature-entropy diagram is shown in Fig. 11-70. The Carnot cycle is an unattainable ideal which serves as a standard of comparison and it provides a convenient guide to the temperatures that should be maintained to achieve maximum effectiveness. 

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