Subject:- Refrigeration and Air Conditioning

Topic:- Vapor Compression Refrigeration System

Difficulty:- Medium

The compression process as shown in figure involves a wet compression of the vapor from state 1' to dry saturated vapor at 2’. It is called wet compression. Wet compression is highly undesirable as the compressor now has to deal with two different fluid phases. Besides, the liquid droplets present in the vapor would now react with the lubricant in the compressor which is highly undesirable.

It is a usual practice to admit slightly superheated vapor before the beginning of compression to avoid the possibility of wet compression. Wet compression is undesirable as there may be accumulation of liquid inside the cylinder, which in turn will wash away the lubricant resulting in severe mechanical difficulties. Thus, to avoid this, a 5 to 20 K superheat of the refrigerant is always desirable.

Another way of achieving dry compression in Carnot refrigeration cycle is to have two compressors – one isentropic and one isothermal as shown in Figure. The refrigeration system with dry compression consists of one isentropic compression process (1-2) from evaporator pressure ‘Pe’ to an intermediate pressure ‘Pi’ and temperature ‘Tc’, followed by an isothermal compression process (2-3) from the intermediate pressure ‘Pi’ to the condenser pressure ‘Pc’.