Multistage Compression System:

Whereas a gas is compressed from a provided pressure to a higher pressure in one stage, this takes more work for compressing the similar mass and requires a larger swept volume of the compressor than while more than one stage is utilized. If the compression ratio is very high, then the compressor can stop delivery of the fluid. Solitary stage compressors are then designed with the compression ratio approximate 7 to 9. But, for high capacity refrigeration systems, even at this pressure ratio, multistage compression is utilized.

Multistage compression results in advanced volumetric efficiency. It means that the compressor capacity has amplified as compared to a single stage compressor with the similar stroke & clearance volume.

The functioning of a multistage compression system is described with the help of the following T-s diagram (fig 10 (a) & (b)). The refrigerant from the evaporator on state 1 is compressed through the low pressure (LP) compressor to the flash intercooler. The vapour is desuperheated from state 2 to state 3. It joint with the vapour separated through throttling from state 5-6 and it increased mass of vapour is now delivered to the high pressure (HP) compressor. The compressed vapour through the HP compressor is delivered at state 4. Now this vapour is condensed in the condenser before being throttled. At the intermediate pressure pi the liquid is again throttled to the evaporator pressure. The heat transfer to the evaporator is responsible to the vaporization of the refrigerant, therefore completing the cycle.