In a rotary air compressor, the air is entrapped between two sets of engaging surface and the pressure of air is increased by squeezing action or back flow of the air. rotary air compressor is driven either by an electric motor or an engine. The maximum delivery pressure is 10 bar only and the maximum free air discharge is as high as 3000 m³ / min. The speed of rotary air compressor is high as compared to reciprocating air compressor.

Compressor capacity is the volume of air delivered by the compressor, and is expressed in m³/min or m³/s.

Free air delivery is the actual volume delivered by a compressor when reduced to the normal temperature and pressure conditions. The capacity of a compressor is generally given in terms of free air delivery.