Used air compressors are machines that increase the pressure of the air by reducing its volume. Large volumes of air are confined in a closed space resulting in higher pressure. A piston within a cylinder compresses the air, creating the pressure. Reciprocating air compressors are available in single or double stage styles, used and new. Single style is generally used for lower pressure, under 100 PSIG. Double style used air compressors are often used for higher pressures up to 250 PSI.