In this system, air is taken into the compressor from atmosphere and compressed and then the hot compressed air is cooled in heat exchanger up to the atmospheric temperature. The cooled air is then expanded in an expander. The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion. The low temperature air coming out from the expander enters into the evaporator and absorbs the heat. The cycle is repeated again and again. The working of reversed Brayton cycle is represented on P−V and T−S diagrams in Figure 8.3.
Figure 8.3 Reversed Brayton Cycle
Process 1−2: Suction of air into the compressor.
Process 2−3: Isentropic compression of air by the compressor.
Process 3−4: Discharge of high pressure air from the compressor into the heat exchanger. (The reduction in volume of air from V3 to V4 is due to the cooling of air in the heat exchanger).
Process 4−1: Isentropic expansion of air in the expander.
Process 1−2: Absorption of heat from the evaporator at constant pressure and suction of air into the compressor.
Work done per kg of air for the isentropic compression process 2−3 is given by,
Work developed per kg of air for the isentropic expansion process 4−1 is given by,
Net refrigerating effect per kg of air is given by,
For perfect inter-cooling, the required condition is T4 = T2
For isentropic compression process 2−3 and for expansion process 5−6, we have
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