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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 pressure and delivered to the air receiver after cooling
sufficiently in an after-cooler. When air at high pressure is required, multi-staged
compression is more efficient than using a single stage compressor. Also single stage
compressors delivering high pressures result in high gas temperatures which effect the
lubrication and increase the risk of burning.
Advantages of multi-stage compression:
- the work done in compressing the air is reduced, thus power can be saved;
- prevents mechanical problems as the air temperature is controlled;
- the suction and delivery valves remain in cleaner condition as the temperature and
vaporization of lubricating oil is less;
-the machine is smaller and better balanced;
- effects from moisture can be handled better, by draining at each stage;
- compression approaches near isothermal;
- compression ratio at each stage is lower when compared to a single-stage machine;
- light moving parts usually made of aluminum, thus less cost and better maintenance.

Refer to the diagram of a multi-stage compressor (Fig.5.6), where it is evident that
the work done by the compressor is less when compared to a single-stage machine for
same delivery pressure. It is required to compress air from P 1 to P 3. The diagram below
I
shows the curve for single stage compression P 1-1-2-4 -P 3. The curve for ideal
isothermal compression is also shown 1-3-5.
The area enclosed by the curves indicates the
work done per cycle and it is clear that the work
done in the ideal isothermal process is far less
than that done in the single stage compression.
Assume a two stage compressor process is
used. The air is compressed from P 1 to P 2 (1-2)
and the air is transferred into a receiver and
cooled to its original temperature (2-3) and the
air is then transferred from the receiver to a
second cylinder and compressed to P 3 (3-4). The
Fig. 5.6 –A two stage compressor process overall process is represented by curve P 1-1-2-3-
4-P 3. The cooling brings the process closer toward the ideal isothermal (constant
temperature) curve. The saving in work done per cycle is identified by the shaded area.
The pressure ratio for every stage of multi-stage compressor equals

P
x  Z n ,
P
1
where Z- number of stage;
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