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Absolute Pressure - The existing gauge pressure plus atmospheric pressure measured from absolute zero.

Aftercooler - 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 dessicant bed.

Area - The surface exposed to the flow of a fluid.

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.

Bar - A unit of pressure One (1)bar = 14.5 PSI.

Capacity - 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.

Coalescing filter - A specific type of filter that not only works to remove dust and particals from the air, but also oil from the air lines.

Compression Efficiency - Ratio of the theoretical to the actual work required to compress air. Takes into account slip leakage and frictional losses.

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) - The volume of air that is able to pass through an opening in one minute.

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.

Desiccant - A material having a large proportion of surface pores, capable of attracting and removing water vapor 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%.

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 measured in PSI (pounds per square inch).

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

Dessicant- A material used with air compressors and air dryers that helps to remove water vapor from the air.

Drain Cock- Used to release accumulated moisture from the compressor tank (Also see drain valve)

Drain Valve- A drain valve works to remove surplus liquid from the compressor. There are manual, float and electronic types of drain valves.

Dryer- Compressor tanks are made of steel, when water gets into the tank, the tank can rust thus reducing the life of the compressor. A dryer dries the air in the tank, reducing the amount of water collected.

Duty Cycle- Amount of time that a compressor can operate at full load over a thirty minute time  period.

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

Filter Efficiency- The rate of which a filter can remove particles from an air flow.

Free Air - Air at atmospheric conditions. Be careful with this term because altitude, barometric pressure and temperature will vary. This term may not mean air at identical conditions.

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

Gas - 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).

GPH - Gallons per hour.

GPM - Gallons per minute.

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) 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.

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

Inlet Temperature - Temperature at the inlet flange of the compressor or inlet filter.

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

Kick-in Pressure- Factory set low pressure point of the pressure switch that starts the compressor to re-pressurize the tank to a higher pressure

Kick-out Pressure- Factory set high pressure point of the pressure switch that stops the compressor from increasing the pressure in the tank above a certain level

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

Load Factor - The ratio of the average actual compressor output to the maximum rated output of the compressor for a defined 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.

Moisture Separator - A device designed to collect and remove moisture from the air during the cooling process.

Multi-Stage Compressors -  Compressors having two or more stages operating in series.

Oil free compressor- Air compressor that has no oil inserted into the compression chamber for lubrication, cooling or sealing. Typically used in the medical and food processing field

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.

Point of use- An outlet in a building used to connect tools or equipment to the air compressor system.

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.

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 Switch- An instrument which senses a change in pressure and automatically adds or decreases air in the tank.

PSID (P) - Pounds per square inch differential.

PSIG - Pounds per square inch gauge = PSIA minus atmospheric pressure (14.696).

Pounds Per Square Inch (PSI)- A unit of measurement referring to the pressure applied on one square inch of an object's surface.

Purging- The elimination of undesired gas or liquid from a 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.

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.

Rotary Screw Compressor- A compressor which utilizes two intermeshing helical rotors to trap a volume of air, then compress it to a higher pressure. Rotary screw compressors can be run at lower temperatures for 24 hours per day, 365 days a year.

Safety valve- A valve which limits fluid pressure by releasing some of the pressurized liquid or gas. a.k.a. Pressure Relief Valve

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

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

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.

Unloaded Horsepower - The power that is consumed to overcome the frictional losses when operating in an unloaded condition.

Vacuum Pumps- A vacuum pump is a compressor that operates with an intake pressure below atmospheric and discharge pressure.

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

VISCOSITY - A measure of the internal friction or the resistance of a fluid to flow. The standard unit of measure is poise or SSU.

Volumetric Efficiency - The ratio of the actual quantity of air delivered to the actual capacity of the compressor.