U.S. Department of Energy - Energy Efficiency and Renewable Energy

Federal Energy Management Program – Operations and Maintenance

Types of Air Compressors

There are two general types of air compressors—positive displacement and centrifugal.

Positive Displacement Compressors

Rotary screw compressor—Click image to enlarge and view reference.

Rotary screw compressor—The main element of the rotary screw compressor is made up of two close clearance helical-lobe rotors that turn in synchronous mesh. As the rotors revolve, the gas is forced into a decreasing inter-lobe cavity until it reaches the discharge port. In lubricated units, the male rotor drives the female and oil is injected into the cylinder serving as both a lubricant and coolant, and also as an oil seal to reduce back slippage. On non-lubricated types, timing gears are used to drive the rotors and multistaging is necessary to prevent gas temperatures from going too high.

Single acting two-stage compressor—Click image to enlarge and view reference.

Reciprocating compressor—A reciprocating compressor is made up of a cylinder and a piston. Compression is accomplished by the change in volume as the piston moves toward the "top" end of the cylinder. This compression may be oil-lubricated or, in some cases, it may require little or no lubrication in the cylinder.

The cylinder in the reciprocating machines may be air cooled or water cooled. Water cooling is used on the larger units. This cooling action is very important to increase compressor life and to keep maintenance and repairs low.

Multiple stage compressors have a minimum of two pistons. The first compresses the gas to an intermediate pressure. After the first stage compression, intercooling of the gas occurs before entering the second stage. Two stage units allow for more efficient and cooler operating levels, which increase compressor life.

Centrifugal Compressor

Four-stage centrifugal compressor—Click image to enlarge and view reference.

The compression action is accomplished when the gas enters the center of rotation and is accelerated as it flows in an outward direction. This gas velocity is then transferred into a pressure rise. Part of the pressure rise occurs in the rotor and part in a stationary element called the diffuser. The rotating element can either have forward curved blades, radial blades, or backward blades.

The centrifugal compressor will usually have more than one stage of compression with intercooling between each stage. One of the drawbacks of this machine is its inability to deliver part-load flow at overall efficiencies as high as other types of compressors. Many people consider the centrifugal machine a base-load machine.