Carnot refrigerator

Fig. 1. Schematic representation of (a) Carnot heat engine and (b) Carnot refrigerator used as a heat pump.

Carnot Refrigerator and Heat Pump Basic Vapor Refrigeration Cycle Actual Vapor Refrigeration Cycle Basic Vapor Heat Pump Cycle Actual Vapor Heat Pump Cycle Working Fluids for Vapor Refrigeration and Heat Pump Systems Cascade and Multistaged Vapor Refrigerators... 

If the Carnot cycle for a heat engine is carried out in the reverse direction, the result will be either a Carnot heat pump or a Carnot refrigerator. Such a cycle is shown in Fig. 1.5. Using the same graphical explanation that was used in the Carnot heat engine, the heat added from the low-temperature reservoir at Tl is area 1-4-5-6-1 g4i is the amount of heat added to the Carnot cycle from a low-temperature thermal reservoir. 

Referring to Fig. 1.5, the system undergoes a Carnot heat pump or Carnot refrigerator cycle in the following manner ... 

The Carnot cycle is a reversible cycle. Reversing the cycle will also reverse the directions of heat and work interactions. The reversed Carnot heat engine cycles are Carnot refrigeration and heat pump cycles. Therefore, a reversed Carnot vapor heat engine is either a Carnot vapor refrigerator or a Carnot vapor heat pump, depending on the function of the cycle. 

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

Figure 6.1 Carnot refrigerator or Carnot heat pump. 

Review Problems 6.1 Carnot Refrigerator and Heat Pump... 

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

Wu, C., Maximum obtainable specific cooling load of a Carnot refrigerator. 

Chen, L. and Wu, C., Performance of an endo-reversible Carnot refrigerator. 

Chen, L. and Wu, C., The influence of heat transfer law on the endo-reversible Carnot refrigerator. Journal of the Institute of Energy, 69(480), 96-100, 1996. Wu, C., General performance characteristics of a finite-speed Carnot refrigerator. 

C corresponds to an ideal Carnot refrigerator (Sidebar 4.4), which removes heat qc from the cold reservoir and expels heat qh at the high-temperature reservoir, with input of work w from the surroundings. 

Type I. Figure 1 illustrates reversible refrigeration machines of Type I. This machine receives an amount of heat q reversibly at temperature Tc and discharges it reversibly at a higher temperature, Ta. By the first principle of thermodynamics the necessary energy input, w, must also be discharged reversibly at Ta. The coefficient of performance of this machine is the same as that of a Carnot refrigeration machine. Thus we have... 

The following diagram shows the operation cycle of a Carnot refrigerator. The refrigerator operates between 25°C(7 2) and -10°C(7 1) and step 2 involves heat absorption of500J. 

Let engine E drive the Carnot engine backward as a Carnot refrigerator, as shown schematically in Fig. 5.1. For the engine/refrigerator combination, the net heat extracted from the cold reservoir is... 

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 work required by a Carnot refrigerator absorbing heat at temperatu Tc and rejecting heat at the temperature of the surroundings, here designat Ts, follows from Eqs. (9.2) and (9.3) ... 

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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