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machine that decreases the volume of air or other gas by the application of pressure. Compressor types range from the simple hand pumppump,
device to lift, transfer, or increase the pressure of a fluid (gas or liquid) or to create a vacuum in an enclosed space by the removal of a gas (see vacuum pumps under vacuum).
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 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 aircompressed air,
air whose volume has been decreased by the application of pressure. Air is compressed by various devices, including the simple hand pump and the reciprocating, rotary, centrifugal, and axial-flow compressors.
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 exerts an expansive force that can be used as a source of power to operate pneumatic toolspneumatic tool
, instrument activated by air pressure. Pneumatic tools are designed around three basic devices: the air cylinder, the vane motor, and the sprayer. The air cylinder contains a piston that is pushed the length of the cylinder by compressed air and returned by air
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 or to control such devices as air brakesbrake,
in technology, device to slow or stop the motion of a mechanism or vehicle. Types
Friction Brakes

Friction brakes, the most common kind, operate on the principle that friction can be used to convert the mechanical energy of a moving object into
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. Air under compression can be stored in closed cylinders to provide an continuous or as-needed supply of pressurized air.



a device for compressing air or another gas and supplying it under pressure. The pressure ratio in a compressor is more than 3. Blast blowers are used for delivering air with a pressure increase of less than 2–3 times, and ventilators are used at heads of 10 kilonewtons per sq m (kN/m2), or 1,000 mm H2O. Compressors, which came into use in the mid-19th century, were first built in Russia in the early 20th century.

The principles of the theory of centrifugal machines were set forth by L. Euler, and the theory of axial compressors and ventilators originated from the work of N. E. Zhukovskii and S. A. Chaplygin.

Compressors are classified according to their principle of operation and basic design features (reciprocating, rotary, centrifugal, axial-flow, and ejector-type), the type of compressed gas (air, oxygen, and so on), the pressure pn created (low pressure, 0.3–1.0 MN/m2; medium pressure, up to 10 MN/m2; high pressure, above 10 MN/m2), and capacity (the volume Vc of gas taken in or compressed per unit time, usually in m3/min). Compressors are also characterized by the speed of rotation n and power consumption N.

A reciprocating compressor consists of a cylinder and piston; it also has inlet and delivery valves, which are usually located in the cylinder head. Most reciprocating compressors have a crank and connecting-rod assembly with a crankshaft for imparting reciprocating motion to the piston. Reciprocating compressors may be of the single-cylinder and multicylinder types, with vertical, horizontal, V-shaped, or W-shaped cylinder arrangement, single or double action (in which the piston operates on both sides), and single-stage or multistage compression.

The operation of a single-stage reciprocating air compressor is as follows (see Figure 1): upon rotation of the crankshaft, the connecting rod imparts backward motion to the piston. Simultaneously, rarefaction develops in the cylinder because of the increased volume between the piston head and the cylinder head, and atmospheric air, overcoming through its own pressure the resistance of the spring retaining the inlet valve, opens the valve and enters the cylinder through the air intake and filter. During the reverse piston stroke the air will be compressed, and then, when its pressure exceeds the pressure in the discharge nozzle by an amount sufficient to overcome the resistance of the spring holding the delivery valve to the seat, the air opens the delivery valve and enters the piping.

Figure 1. Diagram of a reciprocating compressor: (1) crankshaft, (2) connecting rod, (3) piston, (4) cylinder, (5) cylinder head, (6) pressure piping, (7) delivery valve, (8) air intake, (9) inlet valve, (10) pipe for supplying cooling water

When a gas is compressed in the compressor, its temperature is increased considerably. To prevent spontaneous combustion of the lubricant, the compressor is equipped with water cooling (with a pipe for water supply) or air cooling. In this way, the air compression process will approximate an isothermal process (with constant temperature), which is theoretically most advantageous. The single-stage compressor, based on safety conditions and the economy of its operation, is best suited for use with a pressure ratio under compression of β = 7–8. Multistage compressors, which make possible the generation of extremely high gas pressures (above 10 MN/m2) by alternating the compression with intermittent cooling, are used for greater compression. Reciprocating compressors are usually equipped with automatic capacity control that depends on the flow rate of the compressed gas to maintain a constant pressure in the pressure piping. Several control methods exist; the simplest of them is regulation of changes in the rate of shaft rotation.

Rotary compressors have one or more rotors, which may be of various designs. Considerable use is made of rotary (sliding-vane) compressors (Figure 2), which have a rotor with slots into which the vanes fit freely. The rotor is located eccentrically in the cylinder of the casing. Upon clockwise rotation of the rotor,

Figure 2. Diagram of a rotary vane-type compressor: (1) air inlet aperture, (2) rotor, (3) vane, (4) casing, (5) cooler, (6) and (7) cooling water-supply and discharge pipes

spaces bounded by the vanes, as well as the surfaces of the rotor and cylinder casing, will increase in the left half of the compressor, thus causing the inlet of gas through the aperture. In the right half of the compressor the volume of the spaces decreases and the gas within them is compressed and then delivered from the compressor to the cooler or directly into the pressure piping. The casing of rotary compressors is cooled by water, which is supplied and discharged through pipes. The pressure ratio in a single stage of the rotary vane-type compressor is usually 3–6. Two-stage rotary vane-type compressors with intermittent gas cooling provide pressures up to 1.5 MN/m2.

The principles of operation of rotary and reciprocating compressors are basically similar; they differ only in that all processes in the reciprocating compressor take place in the same place (the cylinder) but at a different time (hence the necessity of providing valves), whereas in the rotary compressor inlet and delivery occur simultaneously but in different places, separated by the rotor vanes. Other types of rotary-compressor designs are known, including screw compressors, which have two rotors in the form of screw propellers. Rotary liquid-piston vacuum pumps are used for the removal of air to create rarefaction in a space. The capacity of a rotary compressor is usually regulated by changing the frequency of rotation of the rotor.

A centrifugal compressor consists of a casing and a rotor (Figure 3), which has a shaft with symmetrically placed impellers. A six-stage centrifugal compressor is divided into three sections and is equipped with two intermittent coolers from which the gas enters two channels. During operation of the centrifugal compressor, rotary motion is imparted to the particles of gas between the impeller blades, as a result of which centrifugal force acts on them. Under the action of this force the gas is moved from the compressor axis to the periphery of the

Figure 3. Diagram of a centrifugal compressor: (1) shaft; (2), (6), (8), (9), (10), and (11) impellers; (3) and (7) annular diffusers; (4) return guide channel; (5) stator; (12) and (13) channels for delivering gas from coolers; (14) gas inlet channel

Table 1. Types of compressors and their characteristics
 Limiting parametersArea of use
Reciprocating ...............Vc = 2–5 m3/min pn = 0.3–200.0 MN/m2 (in the laboratory, up to 7,000 MN/m2) n = 60–1,000 rpm N to 5,500 kWChemical industry, refrigeration plants, pneumatic system supply, garage facilities
Rotary ...............Vc = 0.5–300.0
m3/min pm = 0.3–1.5 MN/m2n = 300–3,000 rpm N to 1,100 kW
Chemical industry, blasting in certain metallurgical furnaces
Centrifugal ...............Vc = 10–2,000 m3/min
pn = 0.2–1.2 (less frequently, up to 3) MN/m2n = 1,500–10,000 (up to 30,000) rpm N to 4,400 kW (for aviation compressors, up to tens of thousands of kilowatts)
Central compressor stations in metallurgy, machine building, mining, and petroleum refining
Axial-flow ...............Vc = 100–20,000 m3/min pn = 0.2–0.6 MN/m2n = 2,500–20,000 rpm N to 11,000 kW (for aviation compressors, up to 70,000 kW)Blast-furnace plants and steel foundries; super-charging of piston engines, gas-turbine installations, and jet aircraft engines

impeller, where it is compressed and acquires velocity. The compression continues in the annular diffuser because of decrease in velocity of the gas (the transformation of kinetic energy into potential energy). After this, the gas enters another stage of the compressor along a return guide channel.

The creation of high gas pressure ratios in one stage (more than 25–30 and, in industrial compressors, 8–12) is limited mainly by the ultimate strength of the impellers, which make possible tip speeds as high as 280–500 m/sec. An important feature of centrifugal and axial compressors is the ratio of the pressure of the compressed gas, power consumption, and the efficiency to the capacity (output) of the compressor. The nature of these ratios for each type of compressor, as plotted on graphs, is called the performance characteristics.

The operation of centrifugal compressors is controlled by various methods, including changing the speed of rotation of the rotor and throttling the gas on the inlet side.

An axial-flow compressor (Figure 4) has a rotor that usually consists of several rows of blades. Rows of stator blades are located on the inside wall of the casing. The gas is sucked in through one channel and discharged through another. A single stage of the axial compressor consists of a row of rotor blades

Figure 4. Diagram of an axial-flow compressor: (1) channel for supplying compressed gas, (2) casing, (3) gas inlet channel, (4) rotor, (5) stator blades, (6) rotor blades

and a row of stator blades. During operation of the compressor, the rotating rotor blades exert a force on the gas particles between the blades, forcing them to be compressed and to move parallel to the compressor axis (hence the name “axial-flow compressor”) and to rotate. The network of fixed stator blades mainly provides the change in the direction of the gas particles’ velocity required for efficient operation of the next stage. In some types of axial-flow compressors an additional pressure increase also occurs between the stator blades as a result of a decrease in the velocity of the gas. The pressure ratio for a single stage of an axial-flow compressor is usually 1.2–1.3 (that is, it is considerably lower than that of centrifugal compressors), but the axial-flow type has the highest efficiency of all types of compressors.

The ratio of pressure, power consumption, and efficiency to capacity for certain constant speeds of rotation of the rotor at the same temperature of the gas being compressed is represented in the form of performance characteristics. Axial-flow compressors are controlled in the same manner as centrifugal compressors. Axial-flow compressors are used in gas-turbine installations.

The technical efficiency of axial-flow compressors, as well as the rotary, centrifugal, and reciprocating types, is evaluated according to their mechanical efficiency and certain relative parameters that indicate the extent to which the actual gas compression process approaches the theoretically most desirable process under given conditons.

Ejector-type compressors are similar in design and principle of operation to jet pumps. They include ejectors for the inlet or delivery of gas or a vapor-gas mixture. Such compressors provide a higher compression ratio than jet pumps. Water vapor is often used as the working medium.

The main compressor types, along with their characteristics and uses, are shown in Table 1.


Sherstiuk, A. N. Kompressory. Moscow-Leningrad, 1959.
Ris, V. F. Tsentrobezhnye kompressornye mashiny, 2nd ed. Moscow-Leningrad, 1964.
Frenkel’, M. I. Porshnevye kompressory, 3rd ed. Leningrad, 1969. Tsentrobezhnye kompressornye mashiny. Moscow, 1969.



(computer science)
A routine or program that reduces the number of binary digits needed to represent data or information.
The part of a compandor that is used to compress the intensity range of signals at the transmitting or recording end of a circuit.
(mechanical engineering)
A machine used for increasing the pressure of a gas or vapor. Also known as compression machine.


A machine that increases the pressure of a gas or vapor (typically air), or mixture of gases and vapors. The pressure of the fluid is increased by reducing the fluid specific volume during passage of the fluid through the compressor. When compared with centrifugal or axial-flow fans on the basis of discharge pressure, compressors are generally classed as high-pressure and fans as low-pressure machines.

Compressors are used to increase the pressure of a wide variety of gases and vapors for a multitude of purposes. A common application is the air compressor used to supply high-pressure air for conveying, paint spraying, tire inflating, cleaning, pneumatic tools, and rock drills. The refrigeration compressor is used to compress the gas formed in the evaporator. Other applications of compressors include chemical processing, gas transmission, gas turbines, and construction. See Gas turbine, Refrigeration

Compressor displacement is the volume displaced by the compressing element per unit of time and is usually expressed in cubic feet per minute (cfm). Where the fluid being compressed flows in series through more than one separate compressing element (as a cylinder), the displacement of the compressor equals that of the first element. Compressor capacity is the actual quantity of fluid compressed and delivered, expressed in cubic feet per minute at the conditions of total temperature, total pressure, and composition prevailing at the compressor inlet. The capacity is always expressed in terms of air or gas at intake (ambient) conditions rather than in terms of arbitrarily selected standard conditions.

Air compressors often have their displacement and capacity expressed in terms of free air. Free air is air at atmospheric conditions at any specific location. Since the altitude, barometer, and temperature may vary from one location to another, this term does not mean air under uniform or standard conditions. Standard air is at 68°F (20°C), 14.7 lb/in.2 (101.3 kilopascals absolute pressure), and a relative humidity of 36%. Gas industries usually consider 60°F (15.6°C) air as standard.

Compressors can be classified as reciprocating, rotary, jet, centrifugal, or axial-flow, depending on the mechanical means used to produce compression of the fluid, or as positive-displacement or dynamic-type, depending on how the mechanical elements act on the fluid to be compressed. Positive-displacement compressors confine successive volumes of fluid within a closed space in which the pressure of the fluid is increased as the volume of the closed space is decreased. Dynamic-type compressors use rotating vanes or impellers to impart velocity and pressure to the fluid.


A machine for compressing air or other gases which is a basic component in some refrigeration systems; draws vaporized refrigerant from the evaporator at a relatively low pressure, compresses it, and then discharges it to a condenser.


Two types of compressors used in aero-engines.
A rotor that draws air into the engine and is driven by a turbine. A compressor must provide the required pressure rise; the compression must be affected with the least possible loss; it must be aerodynamically stable over the operating range of RPM; and the tip speed must not approach too close to sonic speed. The two types of compressors are axial flow and centrifugal flow.


1. any reciprocating or rotating device that compresses a gas
2. the part of a gas turbine that compresses the air before it enters the combustion chambers
3. any muscle that causes compression of any part or structure
4. a medical instrument for holding down a part of the body
5. an electronic device for reducing the variation in signal amplitude in a transmission system


(1) A device that diminishes the range between the strongest and weakest transmission signals. See compandor.

(2) A routine or program that compresses data. See data compression.