Air compressors are generally made out of stainless steel, cast iron or aluminum and can be powered by gas or electricity. Portable or mini air compressors are often housed in plastic and use rechargeable batteries. Compressor components include an air tank, hose, motor and pressure gauge as well as various fittings. Some air compressors have handles or anti-vibration feet. A critical safety feature that all compressors should have, including used or refurbished models, is a relief valve that opens if the pressure exceeds the maximum level.
Air compressors are used in many different industries and consequently, used compressors are widely available. The most common application for air compressors is to power pneumatic tools such as nail guns and jackhammers. Compressed air is up to eight times as dense as regular air, so tires, tubes and air tanks are filled using air compressors in order to increase their capacity. Air compressors are used industrially to circulate and purify air; oilless air compressors are characterized by their ability to compress air without contaminating it with oil. They are also used for commercial cleaning and drying.
Used air compressors are similar in operation to pumps. They both increase the pressure on a liquid or gas to convey them through a system. However, fluids are relatively incompressible, and therefore the transportation industry only utilizes gases like air to be pressurized and used for power. Used air compressors are either positive displacement or dynamic compressors. They are identified by how they condense the air. In positive displacement compressors, a space or chamber is filled with air and then collapses.
The compressed air is emptied from the chamber and stored in the tank. Dynamic compressors have a rotating component that spins rapidly, accelerating the air. When it slows, the air molecules collide with the rotor and each other, and the air becomes denser. In the storage tank, the pressure is maintained because the crowded air molecules move faster than they would under normal conditions. The air rushes towards an open valve in an attempt to expand to its regular size, while hoses and nozzles channel and direct the air into a useful application.
Within the two categories of used air compressors there are many variations and designs, including centrifugal, mixed-flow, axial-flow, reciprocating, rotary, screw, scroll and diaphragm compressors. Used air compressors can be purchased with confidence if approved by the American Society of Mechanical Engineers (ASME). They certify the quality and safety of compressors and in some states only allow the purchase and rental of ASME-approved machines.