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The chemical process by which a hygroscopic desiccant, having a high affinity with water, melts and becomes a liquid by absorbing the condensed moisture.

Actual Capacity

Quantity of air or gas actually compressed and delivered to the discharge system at rated speed and under rated conditions.

After Cooler

A heat exchanger used for cooling air discharged from a compressor. Resulting condensate may be removed by a moisture separator following the after cooler.

Air Compressor Automatic Drain Valve

Require no operator attention and automatically purge water, oil, dust, rust, metals and other debris to increase performance and decrease downtime.

Air Compressor Bypass

Diverting the pump’s compressed air to atmosphere.

Air Compressor CFM

Volume or rate of air flow the compressor will deliver in Cubic Feet per Minute.

Air Compressor Cut in PSI

Air receiver PSI at which the pump starts or goes out of bypass.

Air Compressor Cut out PSI

Air receiver PSI at which the pump stops or goes in bypass.

Air Compressor Discharge Line

Tubing, pipe or hose from the pump outlet to the air system.

Air Compressor Discharge Line

Tubing, pipe or hose from the pump outlet to the air system.

Air Compressor Displacement

The rate at which dead air is replaced by compressed air.

Air Compressor Light Duty

These units are designed for jobs that require lower PSI and CFM ratings.

Air Compressor Lubricator

A lubricator ensures proper lubrication of internal moving parts of pneumatic tools.

Air Compressor Manifold

All ports are at equal pressure.

Air Compressor Medium Duty

These units are designed for jobs that require average levels of PSI and CFM.

Air Compressor Stages

Dividing the total pressure among two or more cylinders by feeding the exhaust from the first cylinder (low pressure) to the inlet of the next cylinder (high pressure).

Air Compressor Two Stage

Compression is accomplished in two steps, passing through a low pressure cylinder through a cooling coil to a second stage high pressure cylinder for final compression.

Air Compressor Unloader

As the compressor cuts out, the unloader relieves pressure from the machine. This ensures that the compressor will not be loaded when the motor/engine starts.

Air Compressor Volumetric Efficiency

The actual volume capacity compared to the piston displacement. The efficiency of the pump drops as the pressure goes up.

Air Compressor Water Drain

Drains moisture from the air receiver. This should be done after each use to reduce corrosion to the tank.

Air Compressor Working Pressure

The maximum safe operation pressure.

Air Dryer

A device that removes moisture from compressed air. Typically accomplished by cooling the air through refrigerant or a desiccant bed.

Air End

Compression chamber on a rotary screw compressor where air is compressed.

Air Filter

Filters out moisture, dust, rust, etc.

Air Receiver

The air receiver is simply the air storage tank.

Air Regulator

Allows the user to adjust the PSI on the compressor.

Air-cooled compressors

Atmospheric air is circulated to cool the unit and /or the compressed air.

Air/Oil Separator

Filter designed to remove oil from compressed air in a rotary screw compressor.

Ambient air

The air around or surrounding you.

Amp Rating

The measure of strength of an electric current.

Anti-wear Agent

An additive that minimizes wear caused by metal-to-metal contact during conditions of mild boundary lubrication.

Approach temperature

Temperature above ambient.

Atmospheric Pressure

The measured ambient pressure for a specific location and altitude in PSI (pounds per square inch).

Auto Start/Stop

An air compressor with this feature automatically starts and stops when required, making it more economical than a constant run unit.

Auto-ignition Temperature

Minimum temperature at which a combustible fluid will burst into flame without an extraneous ignition source.

Automatic Sequencer

A device which operates compressors in sequence according to a programmed schedule.

Bearing Corrosion

Chemical attack on bearing metal or on one of the metals in a bearing alloy caused by acids evolved during chemical deterioration of the oil.

Belt Drive

The Pump is powered by a belt wrapped around the motor.


Increase air pressure (usually four times inlet PSI).


The amount of air flow delivered under specific conditions, usually expressed in cubic feet per minute (CFM).

Capacity Gauge

A gauge that measures air flow as a percentage of capacity, used in rotary screw compressors.

Centrifugal compressors

Compression of air/gas through turning impellers.


Cubic feet per minute.

Check Valve

A valve which permits flow in only one direction.

Chicago Fitting (crows foot)

1/4 turn fitting that can connect hose to a compressor or extend the length of hose used to flow gases.

Air Compressor Discharge Line

Tubing, pipe or hose from the pump outlet to the air system.

Clearance Pocket

An auxiliary volume that may be opened to the clearance space to reduce the volumetric efficiency of a reciprocating compressor.


A factor expressing the deviation of a gas from the laws of thermodynamics.

Compression Ratio

The ratio of the absolute discharge pressure to the absolute inlet pressure.

Compression, Adiabatic

Compression in which no heat is transferred to or from the gas during the compression process.

Compression, Isothermal

Compression is which the temperature of the gas remains constant.

Compression, Polytrophic

Compression in which the relationship between the pressure and the volume is expressed by the equation PVn is a constant.


Liquid discharged from compressor and /or air treatment equipment.

Constant Run

An air compressor with this feature will not stop automatically and will therefore run regardless of whether it is needed or not.

Constant Speed Control

A system in which the compressor is run continuously and matches air supply to air demand by varying compressor load.


The chemical change in the mechanical elements caused by the interaction of fluid or contaminants or both, more specifically related to chemical changes in metals.

Critical Pressure

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

Crude Oil

Naturally occurring hydrocarbon fluid that contains small amounts of nitrogen, oxygen, and sulfur derivatives and other impurities.

Cubic Feet per Minute (CFM)

A measurement of the rate of flow of gas leaving the air compressor.

Cut-In/Cut-Out Pressure

Respectively, the minimum and maximum discharge pressures at which the compressor will switch from unload to load operation (cut in) or from load to unload (cut out).


The series of steps that a compressor with unloading performs; 1) fully loaded, 2) modulating, 3) unloaded, 4) idle

Cycle Time

Amount of time for a compressor to complete one cycle.

Cylinder Double Acting

A cylinder in which pressure can be applied to the movable piston rod in either direction.

Cylinder Single Acting

A cylinder in which pressure can be applied to the piston in only one direction.

Decibel (db)

A measurement of noise.

Degree of Intercooling

The difference in air or gas temperature between the outlet of the intercooler and the inlet of the compressor.


Melting and becoming a liquid by absorbing moisture.


Flow of air at specific conditions required at a point or by the overall facility.


Test time required for a specified oil-water emulsion to break, ASTM D 1401.


A material having a large proportion of surface pores, capable of attracting and removing water vapor from the air.


An additive in lubricants generally combined with (and confused with) dispersant additives.

Dew Point

The temperature at which moisture in the air will begin to condense if the air is cooled at constant pressure. At this point the relative humidity is 100%.

Dew point suppression

Temperature below ambient.


A stationary element between the stages of a multi-stage centrifugal compressor.

Diaphragm Cooling

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


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

Direct Drive

The pump is powered directly by the motor.

Discharge Pressure

Air pressure produced at a particular point in the system under specific conditions measured in PSI (pounds per square inch).

Discharge Temperature

The temperature at the discharge flange of the compressor.


The volume swept out by the piston or rotor(s) per unit of time, normally expressed in cfm.

Drip-Proof Electric Motors

These motors will have all of its cooling vents situated so that any dust or moisture will not be able to contaminate the innards of the motor.

Dynamic Type Compressors

Compressors in which air or gas is compressed by the mechanical action of rotating impellers imparting velocity and pressure to a continuously flowing medium.


Any reference to efficiency must be accompanied by a qualifying statement which identifies the efficiency under consideration, as in the following definitions of efficiency:

Efficiency, Compression

Ratio of theoretical power to power actually imparted to the air or gas delivered by the compressor.

Efficiency, Isothermal

Ratio of the theoretical work (as calculated on a isothermal basis) to the actual work transferred to a gas during compression.

Efficiency, Mechanical

Ratio of power imparted to the air or gas to brake horsepower (bhp).

Efficiency, Polytrophic

Ratio of the polytrophic compression energy transferred to the gas, to the actual energy transferred to the gas.

Efficiency, Volumetric

Ratio of actual capacity to piston displacement.

Electric Air Compressor

A compressor driven by an electric motor.

Electric Motor Efficiency

How efficiently the motor turns electrical energy into mechanical energy.

Electric Motor Enclosure

The housing around the motor available in drip-proof, totally enclosed, and explosion proof.

Electric Motor Frame

The configuration on the baseplate of the motor.

Electric Motor Multi-Speed

Multi-speed motors are designed for two or more definite rates of speed remaining practically constant regardless of load changes.

Electric Motor Service Factor

The amount of additional horsepower over and above a motor’s listed rating that it can deliver when necessary as a protection against overload.

Electric Motor Synchronous Speed

The speed at which a motor will operate if the rotor turns at exactly the speed of the magnetic field that drives the rotor.


A mechanical mixture of two mutually insoluble liquids (such as oil and water).


A compound generally formed by the reaction of an alcohol with an organic acid. Esters were among the earliest types of synthetic lube oils; they are still widely used in this application.


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


Devices for separating and removing particulate matter, moisture or entrained lubricant from air.

Flange Connection

The means of connecting a compressor inlet or discharge connection to piping by means of bolted rims (flanges).

Flash Point

Minimum temperature of a petroleum product or other combustible fluid at which vapour is produced at a rate sufficient to yield a combustible mixture.

Foam Inhibitor

An additive which causes foam to dissipate more rapidly.


May occur when a liquid is intimately mixed with air.

Efficiency, Volumetric

Ratio of actual capacity to piston displacement.

Free Air

Air at atmospheric conditions at any specified location.


Air compressor operation at full speed with a fully open inlet and discharge delivering maximum air flow.


One of the three basic phases of matter. While air is a gas, in pneumatics the term gas normally is applied to gases other than air.

Gauge Pressure

The pressure determined by most instruments and gauges, usually expressed in psig. Barometric pressure must be considered to obtain true or absolute pressure (PSIG).

High-pressure air

Compressed air above 150 PSI

Horsepower (HP)

A measurement of power output. Combined with the CFM, this gives an idea of performance.

Horsepower, Brake

Horsepower delivered to the output shaft of a motor or engine, or the horsepower required at the compressor shaft to perform work.

Horsepower, Indicated

The horsepower calculated from compressor indicator diagrams. The term applies only to displacement type compressors.

Horsepower, Theoretical or Ideal

The horsepower required to isothermally compress the air or gas delivered by the compressor at specified conditions.

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.

Humidity, Specific

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


The time lag in responding to a demand for air from a pressure regulator.


The part of the rotating element of a dynamic compressor which imparts energy to the flowing medium by means of centrifugal force. It consists of a number of blades which rotate with the shaft.

Indicated Power

Power as calculated from compressor indicator diagrams.

Indicator Card

A pressure-volume diagram for a compressor or engine cylinder, produced by direct measurement by a device called an indicator.


A curved inlet section of an impeller.

Inlet cfm (icfm)

Cfm flowing through the compressor inlet filter or inlet valve under rated conditions.

Inlet Pressure

The actual pressure at the inlet flange of the compressor typically measure in PSIG.

Instrument-quality (IQ) air

Treated compressed air from an oil-flooded compressor (after-cooled, filtered).


The intercooler cools the compressed air as it travels from one stage to the next higher stage.


The removal of heat from air or gas between compressor stages.

Intercooling, Degree of

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

Intercooling, Perfect

When the temperature of the air or gas leaving the intercooler is equal to the temperature of the air or gas entering the inlet of the compressor.


An unintended loss of compressed air to ambient conditions.

Liquid Piston Compressor

A compressor in which a vaned rotor revolves in an elliptical stator, with the spaces between the rotor and stator sealed by a ring of liquid rotating with the impeller.

Load Factor

Ratio of average compressor load to the maximum rated compressor load over a given period of time.

Load Time

Time period from when a compressor loads until it unloads.

Load/Unload Control

Control method that allows the compressor to run at full-load or at no load while the driver remains at a constant speed.

Low Oil Shutdown

An air compressor with this feature will shut down when the oil level is low, preventing any damage to the unit.

Low-pressure air

Compressed air 150 PSI or lower.


A device which adds controlled or metered amounts of lubricant into a pneumatic system.


A moderate load-carrying ability over and above that indicated by an oils viscosity.


Measurement used to define particulates in the air stream (one micron = one millionth of a meter, about one eighth the thickness one human hair).

Modulating Control

System which adapts to varying demand by throttling the compressor inlet proportionally to the demand.

Motor Bearings

Support both ends of the motor shaft allowing it to rotate smoothly with a minimum of wear and friction.

Multi-Casing Compressor

Two or more compressors, each with a separate casing, driven by a single driver, forming a single unit.

Multi-Stage Axial Compressor

A dynamic compressor having two or more rows of rotating elements operating in series on a single rotor and in a single casing.

Multi-Stage Centrifugal Compressor

A dynamic compressor having two or more impellers operating in series in a single casing.

Multi-Stage Compressors

Compressors having two or more stages operating in series.

Neut Number

Short for neutralization number: the specific quantity of reagent required to “neutralize” the acidity or alkalinity of a lube oil sample.

Oil Free

A unit described as ‘oil-free’ does not require oil to run, greatly reducing maintenance costs.

Oil Lubricated

An ‘oil-lubricated’ air compressor keeps the pump cooler, allowing it to run for longer.

Oil/Water Separator

Device used to remove oil from air compressor condensation.

Oil-flooded air

Compressed air produced by a compressor that utilizes lubricating oil in the compression cycle.

Oil-free air

Compressed air produced by a compressor with no oil present in the compression cycle.


A form of chemical deterioration to which petroleum products like most other organic materials are subject.

Oxidation Inhibitor

Chemical added in small quantities to a petroleum product to increase its oxidation resistance and, hence, to lengthen its service or storage life.


Any solid material, such as dirt, rust, weld fines, pollen, etc., that is in the air stream.

Perfect Intercooling

The condition when the temperature of air leaving the intercooler equals the temperature of air at the compressor intake.

Performance Curve

Usually a plot of discharge pressure versus inlet capacity and shaft horsepower versus inlet capacity.


Term applicable to crude oil and the hydrocarbon products and materials that are derived from it.

Piston Displacement

The volume swept by the piston; for multistage compressors, the piston displacement of the first stage is the overall piston displacement of the entire unit.

Pneumatic Tools

Tools that operate by air pressure.

Positive Displacement Compressors

Compressors in which successive volumes of air or gas are confined within a closed space and the space mechanically reduced, resulting in compression.

Pour Point

A widely used low-temperature flow indicator and is 5 degrees above the temperature to which a normally liquid petroleum product maintains fluidity.

Power, Theoretical (Polytrophic)

The mechanical power required to compress poly tropically and to deliver, through the specified range of pressures, the gas delivered by the compressor.


Parts per million, measurement of the oil present in compressed air.


Force per unit area, measured in pounds per square inch (PSI).

Pressure Dew Point

For a given pressure, the temperature at which water will begin to condense out of air.

Pressure Drop

Loss of pressure in a compressed air system or component due to friction or restriction.

Pressure Range

Difference between minimum and maximum pressures for an air compressor. Also called cut in-cut out or load-no load pressure range.

Pressure Rise

The difference between discharge pressure and intake pressure.

Pressure Switch

A pressure switch has preset cut in/cut out pressure points to control engine/motor.

Pressure, Absolute

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

Pressure, Discharge

The pressure at the discharge connection of a compressor.

Pressure, Intake

The absolute total pressure at the inlet connection of a compressor.

Pressure, Static

The pressure measured in a flowing stream in such a manner that the velocity of the stream has no effect on the measurement.

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.


Pounds per square inch (unit for pressure of compressed air).


The mechanical device that compresses air within the unit.


Manufacturer of rotary screw and reciprocating air compressors.

Quick Disconnect

A coupling which can quickly join or separate a fluid line without the use of tools or special devices.

Recovery Time

The time from cut in to cut out at no load.

Rated Capacity

Volume rate of air flow at rated pressure at a specific point.

Rated Flow

The maximum flow that the power supply system is capable of maintaining at a specific operating pressure.

Rated Pressure

The operating pressure at which compressor performance is measured.


A vessel or tank used for storage of gas under pressure. In a large compressed air system there may be primary and secondary receivers.

Reciprocating Compressor

Compressor in which the compressing element is a piston having a reciprocating motion in a cylinder.

Relative Humidity

The ratio of the partial pressure of a vapor to the vapor saturation pressure at the dry bulb temperature of a mixture.

Required Capacity

Cubic feet per minute (cfm) of air required at the inlet to the distribution system.

Rotary compressors

Compression is produced by the positive action of rotating elements.


The rotating element of a compressor.

Rust Inhibitor

A lubricant additive for protecting ferrous (iron & steel) components from rusting caused by water contamination or other harmful materials from oil degradation.

Safety Relief Valve

A normally closed valve that opens when the pressure is greater than the valve rating.

Single Stage

Compression from initial to final pressure is completed in a single step or piston stroke.


Standard cubic feet per minute.

Screw Type Compressor

This compressor has two intermeshing helical rotors enclosed in a housing with an inlet port at one end and a discharge port at the other end.


Devices used to separate and minimize leakage between areas of unequal pressure.


The order in which compressors are brought online.

Service Pressure

The range of pressure in the pressure tank during the pumping cycle, usually expressed in pounds per square inch gauge.

Shaft Seal

A device mounted on the drive shaft between the impeller or airend and the inside pump housing which creates an air tight chamber within the housing.


The part by which energy is transmitted from the prime mover through the elements mounted on it, to the air or gas being compressed.

Shutoff Valve

A valve which operates fully open or fully closed.

Sole Plate

A pad, usually metallic and embedded in concrete, on which the compressor and driver are mounted.

Specific gravity

The ratio of the specific weight of air or gas to that of dry air at the same pressure and temperature.

Specific Humidity

The weight of water vapor in an air-vapor mixture per pound of dry air.

Specific Power

A measure of air compressor efficiency, usually in the form of bhp/100 acfm.

Specific Weight

Weight of air or gas per unit volume.


The speed of a compressor refers to the number of revolutions per minute (rpm) of the compressor drive shaft or rotor shaft.


A series of steps in the compression of air or a gas.

Standard air

Air at a temperature of 68F, 14.70 PSI atmospheric pressure, and relative humidity of 36% (per ASME). In the gas industry, temperature is 60F.

Standard compressed air

Untreated compressed air from an oil-flooded system (not after-cooled or filtered).

Start/Stop Control

A system in which air supply is matched to demand by the starting and stopping of the unit.

Stationary Air Compressor

A unit that cannot be moved & is usually electric driven.


A phenomenon in centrifugal compressors where a reduced flow rate results in a flow reversal and unstable operation.

Surge Limit

The capacity in a dynamic compressor below which operation becomes unstable.

Temperature Rise Ratio

The ratio of the computed isentropic temperature rise to the measured total temperature rise during compression.

Temperature, Absolute

The temperature of air or gas measured from absolute zero. It is the Fahrenheit temperature plus 459.6 and is known as the Rankin temperature.

Temperature, Discharge

The total temperature at the discharge connection of the compressor.

Temperature, Inlet

The total temperature at the inlet connection of the compressor.

Temperature, Static

The actual temperature of a moving gas stream. It is the temperature indicated by a thermometer moving in the stream and at the same velocity.

Temperature, Total

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

Theoretical Power

The power required to compress a gas isothermally through a specified range of pressures.


A torsional moment or couple. This term typically refers to the driving couple of a machine or motor.

Total Package Input Power

The total electrical power input to a compressor, including drive motor, belt losses, cooling fan motors, VSD or other controls, etc.

Totally Enclosed Electric Motors

These motors will be used in applications similar to drip-proof motors and are recommended for any outdoor use.

Two-stage compressors

Two-compression chambers (initial to intermediate) with one air discharge.

Unit Type Compressors

Compressors of 30 bhp or less, generally combined with all components required for operation.


(No load) Compressor operation in which no air is delivered due to the intake being closed or modified not to allow inlet air to be trapped.

Vacuum Pumps

Compressors which operate with an intake pressure below atmospheric pressure and which discharge to atmospheric pressure or slightly higher.


Devices with passages for directing flow into alternate paths or to prevent flow.

Vapour Pressure

Measure of liquid’s volatility.


A measure of the internal friction or the resistance of a fluid to flow.

Viscosity Index (V.I.)

Measure of the rate of change of viscosity with temperature.


Voltage is the measurement of electrical force. It is the force that delivers current when electricity is applied to an electrical device.


Stationary, spiral shaped passage which converts velocity head to pressure in a flowing stream of air or gas.

Water-Cooled Compressor

Compressors cooled by water circulated through jackets surrounding cylinders or casings and/or heat exchangers between and after stages.

Working Pressure

The pressure which overcomes the resistance of the working device.


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