FAQ / Glossary / Compressor Ratings Article

Adiabatic compression. See, Compression adiabatic

Aftercooler. Heat exchangers for cooling air or gas discharged from compressors. They provide the most effective means of removing moisture from compressed air and gases,

Air bearings. See Gas bearings.

Air-cooled compressors. These are machines cooled by atmospheric air circulated around the cylinders or casings. 

Base plate. A metallic structure on which a compressor or other machine is mounted.

Boolean logic. The logic of binary systems, such as control systems in which all operations my be reduced to on/off, open closed, or some similar dichotomous basis.

Capacity. Capacity of a compressor is the full rated volume of flow of gas compressed and delivered at conditions of total temperature, total pressure, and composition prevailing at the compressor inlet. It sometimes means actual flow rate, rather than rated volume of flow.

Capacity, actual. Quantity of gas actually compressed and delivered to the discharge system at rated speed of the machine and under rated pressure conditions. Actual capacity is usually expressed in cubic feet per minute (cfm) at that stage inlet gas conditions.

Casing. The pressure containing stationary element that encloses the rotor and associated internal components of a compressor, and it includes integral inlet and discharge corrections (nozzles).

Check valve. A valve that permits flow in one direction only.

Clearance. The maximum cylinder volume on a working side of the piston minus the piston displacement volume per stroke It is usually expressed as a percentage of the displace volume.

Clearance pocket. An auxiliary volume that may be opened to the clearance space to increase the clearance, usually temporarily, to reduce the volumetric efficiency of the compressor

Compressibility. A factor expressing the deviation of a gas from the laws of hydraulics. (See also Super compressibility)

Compression, adiabatic. This type of compression is effected when no heat is transferred to or from the gas during the compression process

Compression, isothermal. isothermal compression is a compression in which the temperature of the gas remains constant. For perfect gases, it is represented by the equation PV is a constant, if the process is reversible,

Compression, polytropic. Compression in which the relationship between the pre~ sum and the volume is expressed by the equation Pv is a constant.

Compression ratio. The ratio of the absolute discharge press= to the absolute inlet pressure

Critical pressure. The limiting value of saturation pressure as the saturation temperature approaches the critical temperature.

Critical temperature. The highest temperature at which well defined liquid and vapor states exist. It is sometimes defined as the highest temperature at which it is possible to liquefy a gas by pressure alone.

Diaphragm. A stationary element between the stages of a multistage centrifugal compressor, It may include guide vanes for directing the flowing medium to the impeller of the succeeding stage. In conjunction with an adjacent diaphragm, it forms the diffuser surrounding the impeller.

Diaphragm routing. A method of removing heat from the flowing medium by circulation of a coolant in passages built into the diaphragm.

Diffuser. A stationary passage surrounding an impeller, in which velocity pressure imparted to the flowing' medium by the impeller is converted into static pressure

Digital controls, see Logic controls.

Displacement. Displacement or a compressor is the piston volume swept out Per unit time; it is usually expressed in cubic feet per minute.

Dynamic Type compressors. Machines in which air or gas is compressed by the mechanical action of routing vanes or impellers imparting velocity and pressure to the flowing medium.

Efficiency. Any reference to efficiency of a dynamic type compressor must be accompanied by a qualifying statement which identifies the efficiency under consideration, as in the following definitions.

Efficiency, compression. Ratio of calculated isentropic work requirement to actual thermodynamic work requirement within the cylinder, the Inner as determined from the cylinder indicator card.

Efficiency, isothermal, Ratio of the work calculated on an isothermal basis to the actual work transferred to the gas during compression.

Efficiency, mechanical. Ratio of thermodynamic work requirement in the cylinder (a shown by die indicator card) to actual brake horsepower requirement.

Efficiency, polytropic. Ratio of the polytropic compression energy transferred to the gas no the actual energy transferred to the gas,

Efficiency, volumetric. Ratio of actual capacity to piston displacement, stated as a percentage.

Exhauster. This is a term sometimes applied to a compressor in which the inlet pressure is less than atmospheric pressure.

Expanders. Turbines or engines in which a gas expands, doing work, and undergoing a drop in temperature. Use of the term usually implies that the drop in temperature is the principle objective. The orifice in a refrigeration system also performs this function, but the expander performs it nearly isentropically, and is thus more effective in cryogenic systems.

Filters are devices for separating and removing dust and dirt front air before it enters a compress.

Flange connection. The flange connection (inlet or discharge) is a means of connecting the casing to the inlet or discharge piping by means of bolted rims (flanges).

Fluidics The general subject of instruments and controls dependent upon low rate flow of air or gas at low pressure as the operating medium. These usually have no moving Parts,

Free air. Air at atmospheric conditions at any specific location. Because the altitude, barometer, and temperature may vary at different localities and at different times, it follows that this term does not mean air under identical or standard conditions.

Gas. While from a physical point of view a gas is one of the three basic phases of matter, and thus air is a gas, a special meaning is assigned in pneumatics practice. The term gas refers to any gas other than air.

Gas bearings are load carrying machine elements permitting some degree of motion in which the lubricant is air or some other gas.

Guide vane. A stationary element that may be adjustable and which directs the flowing medium to the inlet of an impeller.

Head, adiabatic. The energy in four pounds required to compress adiabatically and to deliver one pound of a given gas from one pressure level to another.

Head, polytropic. The energy in foot pounds required to compress polytropically and to transfer one pound of a given gas from one pressure level to another.

Horsepower, brake. The horsepower input to the compressor shaft, mature generally to any drive, machine shaft.

Horsepower, indicated. The horsepower calculated from compressor indicator diagrams. The term applies only to displacement type

Horsepower, theoretical, or ideal. The horsepower required to compress adiabatically the air or gas delivered by the compressor through the specified range of pressures.

 Humidity, specific. The weight of water vapor in an air vapor mixture per pound of dry air.

Humidity, relative. The relative humidity of a gas (or air) vapor mixture is the ratio of the partial pressure of the vapor to the vapor saturation pressure at the dry bulb temperature of the mixture.

Impeller. The part of the totaling element of a dynamic compressor that impacts energy to the flowing medium by means of centrifugal force. It consists of a number of blades mounted so as to rotate with the shaft.

Indicator card. A pressure volume diagram for a compressor or engine cylinder produced by direct measurement made by the use of a device called an indicator.

Inducer. A curved inlet section on an impeller

Intercoolers. Heat exchangers for removing the heat of compression between stages of a compressor They usually condense and remove a considerable amount of moisture as well.

Intercooling. The removal of heat from the air or gas between stages or stage groups.

Intercooling degree of. The difference in air or gas temperatures between the inlet of  the compressor and the outlet of the intercooler.

Intercooling, perfect. Perfect intercooling exists when the temperature of the air leaving the intercoolers equals the temperature of the air at the compressor intake.

Isentropic compression. An adiabatic compression with an increase in entropy; a reversible‑adiabatic compression.

Isothermal compression. See Compression, isothertnal.

Liquid piston. A liquid‑piston compressor is a rotary compressor in which a vaned motor revolves in an elliptical casing, with the rotor spaces sealed by a ring of liquid rotating with it inside the casing.

Land factor. Ratio of the average compressor load during a given period of time to the maximum rated load of the compressor. It applies also to air tools, where it is the product of the work factor times the time factor.

Logic controls. Control devices the operation of which may be reduced to binary operation such as on off, 0-1 or open closed They are also referred to as digital controls.

Multicasing compressor. When two or more compressors, each with a separate casing, are driven by a single motor or turbine, the combined unit is called a multicasing compressor.

Multistage axial compressor.  A machine having two or more rows of muting vanes operating in series on a single rotor and in a single casing.

Multistage centrifugal compressor. A machine having two or more impellers operating in series on a single shaft and in single casing.

Multistage compressors. Machines employing two or more stages.

Overhung type centrifugal compressor. A single-inlet compressor with the impeller or impellers mounted on an extended shaft of the driver (i.e., one in which the compressor has no shaft of its own).

  Pedestal type centrifugal compressor. A single‑inlet compressor with the impeller or impellers mounted on a shaft supported by two bearings in a pedestal, with the driver coupled to the compressor shaft.

  Performance curve. This curve is a plot of expected operating characteristics (e.g., discharge pressure versus inlet capacity, shaft horsepower versus inlet capacity).

Piston displacement. Net volume actually displaced by the compressor piston at rated machine speed, generally expressed in cubic feet per minute, (usually cfm). For multistage compressors, the piston displacement of the first stage only is commonly stated as that of the entire machine.

Polytropic compression. See Compression, polytropic.

Polytropic head. See Head, polytropic.

Positive displacement compressors. Compressors in which successive volumes of air or gas are confined within a closed space, and compressed. They may be either totaling or reciprocating.

Power wheel.  A built in mechanical device to recover a portion of the power consumed by a constant speed centrifugal compressor when operating at reduced capacity or reduced pressure rise, or both.

Power, theoretical (polytropic).  The mechanical power required to compress polytropically and to deliver, through the specified range of pressures, the gas delivered by the compressor.

Pressure, absolute.  The total pressure measured from absolute zero, (i.e., from an absolute vacuum).

Pressure, critical. See Critical pressure.

Pressure, discharge.  The absolute total pressure of the discharge flange of a compressor

Pressure, intake.  The absolute total pressure at the inlet flange of a compressor.

Pressure ratio. See Compression ratio.

Pressure rise. The difference between the discharge pressure and the intake pressure.

Pressure, static. The pressure measured in a flowing stream (liquid or gas) in such a manner that no effect on the measurement is produced by the velocity of the stream.

Pressure, total. The pressure that would be produced by stopping a moving stream of liquid or gas. It is the pressure measured by an impact tube.

Pressure, velocity. The total pressure minus the static pressure in an air or gas stream,

  Receivers. Tanks used for the storage of air discharged from compressors. They serve also to damp discharge line pulsations.

Reciprocating compressors.  Machines in which the compression element is a piston having a reciprocating motion in a cylinder.

Reduced pressure. Ratio of the pressure of a gas to its critical pressure, in like units.

Reheaters. Heat exchangers for raising the temperature of compressed air in increase its volume.

Reynold's number. A dimensionless flow parameter, (Jnr/m), in which J if is a significant dimension, often a diameter, n is the fluid velocity, r is mass density, and m, is dynamic viscosity, all in consistent units.

Rotor. The rotating element of a machine and, in the case of a compressor is composed of the impeller (or impellers) and shaft, and may include shaft sleeves, and a thrust balancing device.

Rotary compressors. Machines in which compression is effected by the positive action of rotating elements.

Rotary, sliding vane compressors. Machines in which axial vanes slide radially in an eccentrically mounted rotor.

Rotary, two impeller positive displacement compressors. Machines in which two mating lobed impellers revolve within a cylinder or casing and are prevented from making contact with each other by timing gears mounted outside the cylinder.

Rotary, liquid piston compressors. Machines in which water or other liquids are used, usually in a single rotating element, to displace the air or gas handled.

Seals. Devices used between rotating and stationary parts to separate, and minimize leakage between, areas of unequal pressures.

Shaft. The part of the rotating element on which the rotating parts are mounted and by means of which energy is transmitted from the prime mover.

Shaft sleeves. Devices that may be used to position the impeller or to protect the shaft.

Single stage compressors. Machines in which air or gas is compressed in each cylinder or casing from initial intake pressure to final discharge pressure.

  Single-stage centrifugal compressors. Machines having only one impeller.

  Sole plate. A metallic pad, usually embedded in concrete, on which the compressor feet are mounted.

Specific gravity. This property is the ratio of the specific weight of air or gas to that of dry air at the same pressure and temperature.

Specific heat. The quantity of heat required to raise the temperature of a unit weight of a substance by one degree (whether Celsius or Fahrenheit must be stated).

Specific weight. This property of a gas is the weight of air or gas per unit volume. Unless otherwise specified, it refers in compressor practice to the weight per unit volume at conditions of total temperature, total pressure and composition which prevail at the compressor intake.

Speed. The speed of a compressor refers to the number of revolutions per minute of the compressor shaft.

Stability or percentage stability. Stability is 100 minus the "surge limit" at rated discharge pressure, where the surge limit in percentage of rated capacity.

Stages. Steps in the compression of a gas. In the case of reciprocating compressors each stage usually requires a separate cylinder, and in dynamic compressors, each requires a separate rotor disc; in either case, the gas is passed through the stages in succession,

Standard air.  Air at a temperature of 68 °F, a pressure of 14.70 psia, and a relative humidity of 36 % (0.0750 density). This is in agreement with definitions adopted by ASME, but in the gas industries the temperature of "standard air" is usually given as 60 °F.

Supercompressibility, factor. This is a factor expressing deviation of a gas from perfect gas laws. See also compressibility.

Surge limit (pumping or pulsation point). In a dynamic compressor, surge limit is the capacity below which the compressor operation becomes unstable.

  Temperature,absolute. Absolute temperature equals the Fahrenheit temperature plus 459.6. It is referred to as the Rankine temperature. In the metric systems, absolute temperature equals the Celsius temperature plus 273 and is known as the Kelvin temperature.

Temperature, critical.  See Critical temperature.

Temperature, discharge. The total temperature at the discharge flange of the compressor.

Temperature, Intake. 'The total temperature at the intake flange of the compressor

Temperature rise ratio. This is the ratio of the computed isentropric, temperature rise to the measured total temperature ride during compression. For a perfect gas, this is equal to the ratio of the isentropic enthalpy rise to the actual endralpy rise.

Temperature, static. The actual temperature, of a moving gas stream. It is the temperature indicated by a thermometer moving in the stream with the same velocity as the stream.

Temperature, total. The temperature which would be measured at the stagnation point if a gas stream were stopped, with adiabatic compression front the flow condition to the stagnation pressure.

Thrust balancing device (balance piston or drum). The part of the rotating element that counteracts any inherent thrust developed by the impellers.

Torque. Torsional moment or couple. It usually refers to the driving couple of a machine or motor.

Turbine. A prime mover in which a stream of fluid, such as water, stream or gas, provides the impulse to drive a bladed rotor.

Two stage compressors. Machines in which air or gas is compressed from initial pressure to an intermediate pressure in one or more cylinders or casings.

Unit type compressors. Compressors of 20 hp or less capacity, generally combined with all the components required to put them into operation.

Vacuum Pumps.  Compressors that operate with an intake pressure below atmospheric and discharge pressure usually atmospheric and slightly higher.

Valves.  Devices with passages for directing flow into alternate paths.

Volute.  A stationary, spirally shaped passage that converts velocity head to pressure.

Water-cooled compressors.  These are machines cooled by water circulated through jackets surrounding the cylinders or casings.