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P 1 – pressure of gas which intakes to cylinder of the first stage;
P n - pressure of gas which are expelled from last stage.

5.5 Rotary and Turbo Compressors
Rotary or turbo-compressors deal with larger flow rates of air than reciprocating
compressors but usually at lower delivery pressures. Rotary compressors can be driven
by high speed electric motors, steam turbines, and internal combustion engines. They
are usually multi-stage machines of the centrifugal or axial-flow types.
In centrifugal compressors a number of impellers are mounted on a common rotor in
a robust casing. Air from the atmosphere enters the eye of the first impeller it then
acquires kinetic energy from the rotating impellers. The air is directed from the
periphery of the impeller into stationary diffuser vanes which are designed to convert
the kinetic energy of the gas to increased pressure. The gas is directed inwards to the
eye of the next impeller and the process is repeated as it passes through each stage the
pressure being progressively increased.
In the axial-flow compressor, the air is compressed while continuing its original
direction of flow. The rotor has fixed blades that force the air rearward much like an
aircraft propeller. In front of the first rotor stage are the inlet guide vanes. These vanes
direct the intake air toward the first set of rotor blades. Directly behind each rotor
stage is a stator. The stator directs the air rearward to the next rotor stage. Each
consecutive pair of rotor and stator blades constitutes a pressure stage.
Higher duty rotary compressors are usually provided with water cooling with
intercoolers.

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