Identification and Function

An air compressor is a machine that decreases the volume and increases the pressure of a quantity of air by mechanical means. Air thus compressed possesses great potential energy, because when the external pressure is removed, the air expands rapidly. The controlled expansive force of compressed air is used in many ways and provides the motive force for air motors and tools, including pneumatic hammers, air drills, sandblasting machines, and paint sprayers.

How air compressors operate

How air compressors operate

Types of compressors

Air compressors are of two general types: reciprocating and rotating.

Reciprocating Compressor

In a reciprocating, or displacement, compressor (Fig. 1), which is used to produce high pressures, the air is compressed by the action of a piston in a cylinder. When the piston moves to the right, air flows into the cylinder through the intake valve; when the piston moves to the left, the air is compressed and forced through an output-control valve into a reservoir or storage tank.

Rotating Compressor

A rotating air compressor (Fig. 2), used for low and medium pressures, usually consists of a bladed wheel or impeller that spins inside a closed circular housing. Air is drawn in at the center of the wheel and accelerated by the centrifugal force of the spinning blades. The energy of the moving air is then converted into pressure in the diffuser, and the compressed air is forced out through a narrow passage to the storage tank.


As air is compressed it is also heated. Air molecules tend to collide more often with each other in a smaller space, and the energy produced by these collisions is evident as heat. This heat is undesirable in the compression process, so the air may be cooled on the way to the reservoir by circulating air or water. For high-pressure compressed air, several stages of compression may be employed, with the air being further compressed in each cylinder and cooled before each stage.