compressor, machine that decreases the volume of air or other gas by the application of pressure. Compressor types range from the simple hand pump and the piston-equipped compressor used to inflate tires to machines that use a rotating, bladed element to achieve compression. The four basic types of compressors are reciprocating, rotary screw, centrifugal, and axial-flow. These are further classified by the number of compression stages, the cooling method (air, water, or oil), the drive method (e.g., engine, motor, steam, gasoline, or diesel), and lubrication.

The most popular type is the reciprocating (or piston-and-cylinder) compressor, which is useful for supplying small amounts of a gas at relatively high pressures. In this type of compressor, a piston is driven within a cylinder; the gas is drawn in through an inlet valve on the suction stroke of the piston and is compressed and driven through another valve on the return stroke. Reciprocating compressors are either single- or double-acting. In single-acting machines the compression takes place on only one side of the piston; double-acting machines use both sides of the cylinder for compression. Multiple cylinder arrangements are common. The rotary-screw compressor uses two meshed rotating helical rotors within a casing to force the gas into a smaller space. Advantages of this type of compressor include smooth, pulse-free gas output with high output volume. The centrifugal compressor consists of a rotating impeller mounted in a casing and revolving at high speed. This causes a gas that is continuously admitted near the center of rotation to experience an outward flow and a pressure increase due to centrifugal action. Centrifugal compressors are particularly suited for compressing large volumes of gas to moderate pressures; they produce a smooth discharge of the compressed gas. In an axial-flow compressor, the gas flows over a set of airfoils spinning on a shaft in a tapered tube. These draw in gas at one end, compress it, and output it at the other end. Axial-flow compressors are used in jet aircraft engines and gas turbines.

Air is the most frequently compressed gas, although natural gas, oxygen, and nitrogen are also often compressed. Compressed air exerts an expansive force that can be used as a source of power to operate pneumatic tools or to control such devices as air brakes. Air under compression can be stored in closed cylinders to provide an continuous or as-needed supply of pressurized air.

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