The Carnot Refrigerator

If a reversible heat engine operates so as to produce a positive amount of work in the surroundings, then a positive amount of heat is extracted from the hot reservoir and heat 

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A schematic diagram of the Carnot refrigerator or Carnot heat pump is illustrated in Fig. 6.1. 

Why is the Carnot refrigeration cycle executed within the saturation dome not a realistic model for refrigeration cycles ... 

The word refrigeration implies the maintenance of a temperature below that of the surroundings. This requires continuous absorption of heat at a low temperature level, usually accomplished by evaporation of a liquid in a steady-state flow process. The vapor formed may be returned to its original liquid state for reevaporation in either of two ways. Most commonly, it is simply compressed and then condensed. Alternatively, it may be absorbed by a liquid of low volatility, from which it is subsequently evaporated at higher pressure. Before treating these practical refrigeration cycles, we consider the Carnot refrigerator, which provides a standard of comparison. 

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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