Refrigeration Cycles

The basic refrigeration cycle includes a circulating refrigerant that undergoes phase changes in which it absorbs or rejects heat. Thermal energy passes from a heat source into low-pressure liquid refrigerant, causing it to evaporate. Heat is then rejected from the refrigerant vapor as it condenses at high pressure and temperature. In vapor compression cycles, mechanical energy is input to provide the compression needed at the refrigerant condenser and a throttling device maintains low-pressure conditions at the evaporator. Absorption cycles are similar to vapor compression cycles systems, except instead of compression, they rely on application of heat and the strong chemical affinity of an absorbent for the refrigerant to drive the cycle.

In practical application, heat is transferred from the air-conditioned space or process heat source directly to the refrigerant or indirectly through a secondary cooling fluid such as brine or chilled water. The refrigerant gives up its heat directly to atmosphere or to a cooling medium such as condenser water, which then transfers the heat to a heat rejection devise such as an outdoor cooling tower.

In the ideal heat engine cycle described in Chapter 2, work is produced as heat passes from a higher to a lower temperature. In a refrigeration cycle, work is input to transfer heat from a lower to a higher temperature. The ideal refrigeration cycle can thus be represented by a reverse of the Carnot cycle used to model heat engine performance. Figure 33-1 is a temperature vs. entropy (Ts) diagram of the reversed Carnot cycle. In the reverse cycle, work is added to the system and heat is removed from the lower temperature region (TL). The energy rejected to the higher temperature region (TH) equals the work performed by the compressor plus the thermal energy removed from the heat source.

In actual operation of mechanical compression systems, the heat rejected from the cycle is equal to the refrigeration effect plus the driving energy to the compressor. In actual absorption cycles, the heat rejected from the cycle is equal to the refrigeration effect plus the generator input.

In Figure 33-1:

Qout = Heat rejected from the cycle = area 3-4-1-a-b-2-3

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