Some air compressor specs include the compressor's capacity, pressure, tank size and type. Capacity is the amount of air a compressor can pump out; it is expressed in cubic feet per minute, or CFM. Pressure describes the pressure the air compressor uses, and most compressors operate at between 7 and 30 bar. Air compressors come in two main types: reciprocating and rotary screw compressors.
A user finds the necessary capacity by adding up the air requirements of all the compressed air tools she intends to use at one time. A user should only purchase a compressor that supplies just above the minimum capacity the tools require. While a compressor with a very small capacity cannot operate a large number of tools, a compressor with too much capacity for a given application may be a waste of money.
Like capacity, compressed air tools require a certain amount of pressure. However, unlike capacity requirements, pressure requirements are not cumulative for multiple tools. Therefore, a user can operate as many tools using 7 bar as she wants, as long as the compressor has adequate capacity.
Reciprocating compressors are available in both high and low-pressure variants, but these compressors always have relatively low capacity. Rotary screw compressors are always low pressure, but they typically have much higher capacity than reciprocating models.