This system consist of (1) Evaporator (2) compressor (3) condenser and (4) expansion device. In vapour compression refrigerator, the refrigerant is used, which is circulated in system in which it alternately evaporates (liquid to vapour) and condenses (vapour to liquid) thus it undergoes a change of phase. In the evaporation it absorbs the latent heat from the space to be cooled. In the condensing or cooling, it rejects heat to atmosphere. Functions of main parts of vapour compression system ( Follow Fig.1 ) Compressor Function of compressor is to remove the vapour from the evaporator and increases its pressure and temperature up to it can be condensed in the condenser. Pressure of refrigerant coming from compressor should be such that the saturation temperature of vapour (corresponding to this pressure of vapour) is higher than the temperature of cooling medium in condenser, so that high pressure vapour can reject heat to cooling medium in the condenser. Condenser The function of condenser is to facilitate a heat transfer surface through which heat transfer takes place from the hot refrigerant vapour to the atmosphere. Expansion valve or device The function of expansion valve is to meter the proper amount of liquid refrigerant and reduces pressure of liquid refrigerant entering the evaporator.Hence liquid will vaporize in the evaporator at the desired low temperature and absorb heat from the space. Evaporator An evaporator provides a heat transfer surface through which low temperature liquid refrigerant can absorb heat from space and it vaporized. Four main processes in vapour compression system ( Follow Fig.2 & Fig. 3 ) Process 1-2 Inlet of compressor (at point 1), low pressure and low temperature vapour enters the compressor. Compressor compresses the vapour at high temperature and pressure. The condition of refrigerant at exit to compressor (at point 2) is high pressure and high temperature vapour. Process 2-3 High pressure, high temperature vapour coming from compressor condenses in the condenser by the rejecting heat to cooling medium. Cooling medium is usually air or water.The condition of refrigerant at exit to condenser (at point 3) is low temperature saturated liquid. Process 3-4 The saturated liquid coming from condenser passes through expansion device (throttling valve) where pressure of saturated liquid decreases from condenser pressure to evaporator pressure.The condition of refrigerant after throttling is low temperature and low pressure liquid. Process 4-1 Liquid refrigerant coming from expansion device enters into evaporator where it absorbs latent heat of evaporation from space to be cooled (refrigerator compartment). Due to absorption of heat liquid refrigerant converted into saturated vapor or superheated vapour at low pressure and low temperature. Again this vapour enters into compressor and the cycle is repeated.
