in refrigeration under cooling supercharging

Q5 With reference to a vapour compression refrigeration plant

a)Explain why Each of following is desirable

i) undercooling of the refrigerant at the condenser outlet    02 marks

Undercoolingor sub cooling  is defined as the reduction in a refregerent below its saturation temperature.  undercooling is measured in °C. undercooling of the refrigerant is necessary to avoid vapour bubbles in the refrigerant ahead of the expansion valve. Vapour bubbles in the refrigerant reduce capacity in the expansion valve and thereby reduce liquid supply to the evaporator. undercolingcooling of 4-5K is adequate in most cases. that will increase the net refrigeration effect. 

ii).supercharging at the compressor suction        02 marks

 Expansion valve ensure that the refrigerant gas leaving the evaporator is superheated by 5 to 7 ^OC to ensure that maximum utilization of the latent  heat of evaporation at the cooling compartment also the super heat will ensure the gas entering the compressor will behave as a gas. Any condensation of the refrigerant at the compressor inlet will lead to excessive pressure build up inside the compressor. It will lead to lifting up of the safety head without compressing the refrigerant. Due to liquid entering the compressor cylinder Consequently it may cause damage to the compressor body. 

b) Describe with the aid of a sketch how a   heat exchanger could be incoperated in the circuit to enhance undercooling.

As shown in the diagram below refrigerant liquid leaving the condenser is further cooled by removing sensible heat, using an internal heat exchanger here the cooling medium is the superheated refrigerant gas returning from the evaporator.

internal heat exchanger

c) Explain the possible consequences of the refrigerant having a dryness fraction at the compression suction    02 marks

Wet compression cycle is not preferred since liquid droplets of refrigerant will damage the valves and moving parts of the compressor, liquid refrigerant carrying lubricating oil from compressor would adversely affect the heat transfer rates, has low volumetric and mechanical efficiency Effect of operating variables on performance of vapour compression cycle are super heating of suction vapour increase refrigerating effect,.