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Refrigerator and Heat Pump

6.5.1 Refrigerator

A refrigerator is a device which operating in a cycle, maintains a body at a temperature lower than the atmosphere or surroundings.

Fig. 6.8. shows a cyclic refrigeration plant. Let a body be maintained at temperature

T2 which is lower than ambient temperature T1. Q2 is extracted from the body at T2. As surrounding is atmosphere at temperature T1 (> T2), there will be heat leakage Q2 to the body. Q2 is then absorbed by the refrigerant (working fluid) and gets evaporated in the evaporator E1. Q2 supplies the latent heat of evaporation. The body thus maintains temperature T2 lower than T1 (process 4 -1). The vapour is then compressed by C1 which absorbs work We (process 1 – 2). It is condensed in C2 rejecting heat Q1 to atmosphere (process 2 – 3). The condensate then expands adiabatically through turbine E2 producing work WE, also the temperature drops to a value lower than T 2 (process 3- 4) such that Q2 flows from the body to evaporate again the refrigerant. E1 – C1 – C2 - E2 cyclic flow process is termed refrigeration and the device is refrigerator.

Coefficient of performance (COP) is defined as

[ COP] Ref = Q2 / W net = Q2 / Q1 –Q2

Where                                  W net = WC – WE = Q1 – Q2


6.5.2 Heat Pump

Heat pump is a device which operates in a cycle and maintains a body at a temperature higher than the temperature of the atmosphere (surroundings).


[ COP] HP = Q2 / W net = Q2 / Q1 –Q2


Subtracting (6.1) from (6.2) we get

[COP]HP – [ COP]Ref = 1

Fig. 6.10 shows cyclic heat pump (C.H.P). The C.H.P continuously supply heat of amount Q1 to the body which maintains temperature at T1. W net work is done on the C.H.P.

W net = WC – WE