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Draw P-H and T-S diagram when the vapours are superheated at the end of compression and with under cooling of liquid

Subject:- Refrigeration and Air Conditioning

Topic:- Vapor Compression Refrigeration System

Difficulty:- Low

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in actual refrigeration cycles, the temperature of the heat sink wll be several degrees lower than the condensing temperature to facilitate heat transfer. hence it is possible to cool the refrigerant liquid in the condenser to a few degrees lower than the condensing temperature by adding extra area for heat transfer. in such a case the exit condition of the condenser will be in the subcooled liquid region. hence this process is known ass subcooling. similarly the temperature of heat source will be a few degrees higher than the evaporator temperature, hence the vapour at the exit of the evaporator can be superheated bu a few degrees. if the superheating of refrigerant takes place fue to heat transfer with the refrigerated space (low temperature heat source) then it is called as useful superheating as it increases the refrigeration effect. On the other hand it is possible for the refrigerant vapour to become superheated by exchanging heat with the surroundings as it flows through the connecting pipelines. Such a superheating is called useless superheating as it does not increase refrigeration effect.

fig. 1(a) P-h diagram with and without subccoling (b)T-s diagram with and without subcooling.

fig 1.(a) and (b) shows the VCRS cycle without and with subcooling on P-h and T-s coordinates. It can be seen from the T-s diagram that without subcooling the throtling loss is equal to the hatched are b-4'-4-c whereas with subcooling the throttling loss is given by the area a-4"-4'-b. thus the refrigeration effect increases by an amount equal to $h_4-h_{4'}=h_3-h_{3'}$

fig 2: Effect of superheat on specific refrigeration effect and work of compression (a) on p-h diagram (b) on T-s diagram.

fig 2 shows the VCRS cycle with superheating on P-h and T-s coordinates. As shown in figure, with useful superheating, the refrigeration effect, specific volume at the inlet to the compressor and work done of compression increase. whether the volumic refrigeration effect (ratio of refrigeration effect by specific volume at compressor inlet) and COP increase or not depends upon the relative increase in refrigeration effect and work of compression, which in turn depends upon the nature of the refrigerant used. the temperature of refrigerant at the exit of the compressor increases with superheat as the isentropes in the vapour region gradually diverge.