Glossary of Terms


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

Air Dryer

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

Atmospheric Pressure

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

Automatic Sequencer

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


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

Check Valve

A valve which permits flow in only one direction.

Compression Ratio

The ratio of the discharge pressure to the inlet pressure.

Constant Speed Control

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

Cubic Feet Per Minute (CFM)

Volumetric air flow rate.

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 (for compressors with modulating control), 3) unloaded, 4) idle.

Cycle Time

Amount of time for a compressor to complete one cycle.


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


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

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%.

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.


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


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).

Horsepower, Brake

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

Humidity, Relative

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

Inlet Pressure

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


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


An unintended loss of compressed air to ambient conditions.

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.

Modulating Control

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

Multi-Stage Compressors

Compressors having two or more stages operating in series.

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.


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.


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 vapour to the vapour saturation pressure at the dry bulb temperature of a mixture.

Specific gravity

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


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


(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.


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