Absolute Pressure
Total pressure measured from zero (perfect vacuum).
Actual cubic feet per minute (acfm)
Flow rate of air at actual operating pressure and temperature.
A heat exchanger used for cooling air discharged from a compressor. Resulting condensate may be removed by a moisture separator following the aftercooler.
Atmospheric Pressure
The measured ambient pressure for a specific location and altitude.
Automatic Sequencer
A device that operates compressors in sequence according to a programmed schedule.
Brake Horsepower
Horsepower delivered to the output shaft of a motor or engine, or the horsepower required at the compressor shaft to perform work.
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 absolute discharge pressure to the absolute inlet pressure.
Cubic Feet per Minute (cfm)
Volumetric air flow rate per minute converted back to ambient conditions.
The series of steps that a compressor with unloading performs 1) fully loaded; 2) modulating (for compressors with modulating control); 3) unloaded; and 4) idle.
Cycle Time
Amount of time for a compressor to complete one cycle.
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 percent.
Discharge Pressure
The pressure at the discharge connection of a compressor.
Discharge Temperature
The temperature at the discharge flange of the compressor.
Devices for separating and removing particulate matter, moisture or entrained lubricant from air.
Full Load
Air compressor operation at full speed with a fully open inlet and discharge delivering maximum air flow.
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.
The removal of heat from air or gas between compressor stages.
An unintended loss of compressed air to ambient conditions.
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.
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. These may be reciprocating or rotating.
Force per unit area, measured in pounds per square inch (psi) or kilo Pascals (kPa).
Pressure Drop
Loss of pressure in a compressed air system or component due to friction or restriction.
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.
The order in which compressors are brought online.
A series of steps in the compression of air or a gas. Most plant industrial compressors have 2 stages.
Standard cfm
Flow of free air measured and converted to a standard set of reference conditions (14.5 psia, 68°F, and 0 percent relative humidity).
Start/Stop Control
A system in which air supply is matched to demand by the starting and stopping of the unit.
Variable Speed Drive
An electronic device to vary the speed of a motor enabling it to achieve energy savings at low flows.
Unload/No load
Compressor operation in which no air is delivered because the intake is closed or modified not to allow inlet air to be trapped.

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