As per general gas laws, if the pressure increases temperature also increases. For example : if the exit pressure of compressor is 5 bar in a single acting compressor, the compressor air temperature can rise to over 200 and the motor power needed to drive the compressor rises. Therefore single stage compressors are not used for high pressures. Multistage compressors are used when high pressures are required, because better cooling between stages can effectively increase the efficiency and reduce the input power requirements.
Single stage machines compress the air to pressure of about 6 bars and in exceptional cases to 10 bars, two stage machines normally discharge pressure up to 15 bars. Discharge pressures in the range of 250 bars can be obtained with high pressure reciprocating compressor of three and four stages.
In single stage compressor, entire compression of air takes place in single stroke of the piston. In multi stage compressor, compression takes in stages. For maximum compressor efficiency, it is desirable to cool air after one stage using inter- stage cooler. In two stage compressor, initial compression takes place in the low pressure cylinder. Air from this stage (low pressure cylinder) is passed through the inter cooler to reduce the temperature. Then the cooled air is compressed in the high pressure cylinder.
Figure shows the two stage (inline type) reciprocating air compressor. When the prime mover connected to crank shaft rotates, crank rotates and the piston in the first stage reciprocates. It sucks the air through the suction filter and inlet valve. The air, compressed to a certain degree passes from the left cylinder to right cylinder through the intermediate cooler. The compression ratio in the first stage is determined by the degree of cooling required.
The air is first compressed in the LP cylinder to intermediate pressure p2. It is then passed through an intercooler where air is cooled at constant pressure before it is compressed in the HP cylinder. If the air is cooled back to initial temperature, then the inter cooling is said to be perfect. The PV diagram for two stage compressor is shown in the figure.