Types of Air Compressors

There are two major classifications of air compressors: positive displacement compressors and dynamic comprssors. A reciprocating compressor is an example of a positive displacement compressor. In this type of compressor, successive volume of air are trapped in a closed space, and the prssure is increased as the piston moves toward the top of cylinder and reduces the size of the closed space. Reciprocating air compressor have good energy efficiency characteristics at both part-load and full-load. It is more difficult to capture the waste heat from a reciprocating compressor than from a rotary screw of centrifugal air compressor sience the pistons in the reciprociating unit are exposed to the open air around the machine.

The rotary screw compressor is another example of a positive displacement air compressor. In this unit, air enters the inlet and is trapped between mating male and female rotors and compressed to the required dischange pressure. Rotary screw compressors have excellent efficiencies at full-load conditions, and average efficiencies at part-load conditions. Heat recovery is easiest from the rotary screw compressors sience the entrie compressor selection is enclosed.

The centrifugal air compressor is an example of a dynamic compressor, where air is compressed by the dynamic action of rotating impellers or vanes imparting velocity and pressure to the air. Almost all large air compressor (greater than 300 CFM and 50-psig) are centrifugal compressors. The efficiency of these centrifugal air compressors is lower than either the reciprocating or rotary screw models, so smaller air compressors are almost never the centrifugal type. Heat recovery is also somewhat difficult from these compressors.