Air compressors come in all shapes and sizes, but there are broadly two methods of compression, which are known as positive-displacement and negative-displacement. Positive displacement is a form of compression in which air is forced into a chamber whose volume reduces as compression takes place. Piston-driven air compressors use this principle – as do rotary screw compressors, where two helical screws guide air into the chamber and then compress the air as the screws turn. Vane compressors – another type of positive-displacement compressor – use a slotted rotor with varied blade placement to guide air into a chamber and compress it prior to use.
The other method of compression, known as negative-displacement, is used in centrifugal air compressors (which are designed mostly for very large applications). These compressors use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air. Broadly speaking, most air compressors are either reciprocating piston type, rotary vane or rotary screw.