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CHEMICAL ENGINEERING
3.13.3. Multistage compressors
Single-stage compressors can only be used for low pressure ratios. At high pressure ratios,
the temperature rise will be too high for efficient operation.
To cope with the need for high pressure generation, the compression is split into a
number of separate stages, with intercoolers between each stage. The interstage pressures
are normally selected to give equal work in each stage.
For a two-stage compressor the interstage pressure is given by:

P i D P 1 ð P 2 3.39
where P i is the intermediate-stage pressure.

Example 3.12
3
Estimate the power required to compress 1000 m /h air from ambient conditions to
2
700 kN/m gauge, using a two-stage reciprocating compressor with an intercooler.

Solution

2
Take the inlet pressure, P 1 , as 1 atmosphere D 101.33 kN/m , absolute.
2
Outlet pressure, P 2 , D 700 C 101.33 D 801.33 kN/m , absolute.
For equal work in each stage the intermediate pressure, P i ,

5
5
5
D 1.0133 ð 10 ð 8.0133 ð 10 D 2.8495 ð 10 N/m 2
For air, take ratio of the specific heats, ,tobe 1.4.
For equal work in each stage the total work will be twice that in the first stage.
Ž
Take the inlet temperature to be 20 C, At that temperature the specific volume is
given by
29 293 3
v 1 D ð D 1.39 m /kg
22.4 273
1.4 1 /1.4
1.4 2.8495
5
Work done, W D 2 ð 1.0133 ð 10 ð 1.39 ð 1
1.4 1 1.0133
D 338,844 J/kg D 339 kJ/kg
From Figure 3.7, for a compression ratio of 2.85 the efficiency is approximately 84%. So
work required

D 339/0.84 D 404 kJ/kg
1000
Mass flow-rate D D 0.2 kg/s
1.39 ð 3600
Power required D 404 ð 0.2 D 80 kW