Multistage Compression | Advantages of Multistage Compression

Multistage Compression

As the pressure of the air increases, its temperature rises. It is essential to reduce the air temperature to avoid damage of compressor and other mechanical elements. The multistage compressor with intercooler in-between is shown in Figure . It is used to reduce the temperature of compressed air during the compression stages. The inter-cooling reduces the volume of air which used to increase due to heat. The compressed air from the first stage enters the intercooler where it is cooled. This air is given as input to the second stage where it is compressed again. The multistage compressor can develop a pressure of around 50 bar. 

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multi stage compressor
multi stage compressor

Multistage compression refers to the compression process completed in more than one stage i.e., a part of compression occurs in one cylinder and subsequently compressed air is sent to subsequent cylinders for further compression. In case it is desired to increase the compression ratio of compressor then multi-stage compression becomes inevitable. If we look at the expression for volumetric efficiency then it shows that the volumetric efficiency decreases with increase in pressure ratio. This aspect can also be explained using
p-V representation shown in Figure

Effect of Intercooling on compression Works :

effect of intercooling on the compression works
effect of intercooling on the compression works

A multi-stage compressor is one in which there are several cylinders of different diameters. The intake of air in the first stage gets compressed and then it is passed over a cooler to achieve a temperature very close to ambient air. This cooled air is passed to the intermediate stage where it is again getting compressed and heated. This air is again passed over a cooler to achieve a temperature as close to ambient as possible. Then this compressed air is passed to the final or the third stage of the air compressor where it is compressed to the required

Advantages of multistage compression are:

1) Reduced work of compression per kg of refrigerant

2) COP increases

3) Wall thickness of L.P. cylinder is reduced, since it has to withstand lower pressures. This makes compressor lighter and cheaper.

4) Heat removal during condensation is reduced hence size of condenser is reduced.

5) Volumetric efficiency of compressor increases due to reduced pressure ratio in each stage.

6) Temperature at end of compression would be less. As a result lubrication would be effective. Hence, compressor life increases.

7) Leakages past the piston are reduced

8) Operating cost is reduced

9) It gives more uniform torque, hence size of flywheel is reduced.

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