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Glossary of Terms

Air Compressor Glossary of Terms

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Absolute Pressure - Total pressure measured from zero.

Absolute Temperature - The temperature of a body referred to the absolute zero at which point the volume of an ideal gas theoretically becomes zero. (Fahrenheit scale is minus 459.67 degrees / Celsius scale is minus 273.15 degrees.

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

Adsorption - The process by which a desiccant with a highly porous surface attracts and removes the moisture from compressed air. The desiccant is capable of being regenerated.

Actual Capacity - Quantity of gas actually compressed and delivered to the discharge system at rated speed and under rated conditions. Also called Free Air Delivered (FAD).

Air Receiver - See Receiver.

Aftercooler - 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 which operates compressors in sequence according to a programmed schedule. 


Brake Horsepower (bhp) - Horsepower delivered to the output shaft of a motor or engine, or the horsepower required at the compressor shaft to perform work. 


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

Capacity, Actual - The actual volume flow rate of air or gas compressed and delivered from a compressor running at its rated operating conditions of speed, pressures, and temperatures. Actual capacity is generally expressed in actual cubic feet per minute (acfm) at conditions prevailing at the compressor inlet.

Capacity Gauge - A gauge that measures air flow as a percentage of capacity, used in rotary screw compressors as an estimator during modulation controls.

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

Compression, Isothermal - Compression in which the temperature of the gas remains constant.

Compression Ratio - The ratio of the absolute discharge pressure to the absolute 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.

Cfm, Free Air - Cfm or air delivered to a certain point at a certain condition, converted back to ambient conditions.

Cfm, Actual [Actual Cfm](acfm) - Flow rate of air at a certain point at a certain condition at that point.

Cfm, Inlet [Inlet Cfm] - Cfm flowing through the compressor inlet filter or inlet valve under rated conditions.

Cfm, Standard [Standard Cfm] - Flow of free air measured and converted to a standard set of reference conditions (14.5 psia, 68 degrees F, and 0% relative humidity).

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).

Cycle - 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. 


Degree of Intercooling - Difference in air or gas temperature between the outlet of the intercooler and the inlet of the compressor.

Deliquescent - Melting and becoming a liquid by absorbing moisture.

Desiccant - An adsorption type material having a large proportion of surface pores, capable of attracting and removing water vapor from the air. Industry standards are activated alumina, silica gel and molecular sieves.

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%.

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

Discharge Pressure - Air pressure produced at a particular point in the system under specific conditions.

Discharge Temperature - The temperature at the discharge flange of the compressor.


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 an 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, Volumetric - Ratio of actual capacity to piston displacement.


Free Air - Air at atmospheric conditions at any specified location, unaffected by the compressor.

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.


Horsepower, Brake - The horsepower input to the compressor shaft, or more generally to any driven machine shaft.

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. 


Indicated Power - Power as calculated from compressor-indicator diagrams.

Inlet Pressure - The actual pressure at the inlet flange of the compressor.

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


Leak - An unintended loss of compressed air to ambient conditions.

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.


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


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

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.

Pressure - 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. Resistance to flow. Defined as the difference in pressure upstream and downstream.

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


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

Rated Pressure - The operating pressure at which compressor performance is measured.

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

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

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


Sequence - The order in which compressors are brought online.

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 or acfm/bhp.

Specific Weight - Weight of air or gas per unit volume.

Standard Air - The Compressed Air & Gas Institute and PNEUROP have adopted the definition used in ISO standards. This is air at 14.5 psia (1 bar), 68° F (20° C) and dry (0% relative humidity).

Start/Stop Control - A system in which air supply is matched to demand by the starting and stopping of the unit.

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


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 Rankine temperature. In the metric system, the absolute temperature is the Centigrade temperature plus 273 and is known as the Kelvin 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 Rise Ratio - The ratio of the computed isentropic temperature rise to the measured total temperature rise during compression. For a perfect gas, this is equal to the ratio to the isentropic enthalpy rise to the actual enthalpy rise.

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.

Torque - 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, cooling fan, motors, controls, etc.


Unload - (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.


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