Absolute pressure. Total pressure measured from absolute zero.
Aftercooler. Heat exchanger for cooling air discharged from air compressors.
Atmospheric pressure. Pressure above absolute zero at a specific location and altitude.
Brake horsepower (bhp). Horsepower delivered to the output shaft of the drive motor. Unit of comparison between motors. Total package bhp is the sum of all motor shaft outputs, including compressor and cooling fans.
Capacity. The amount of air flow delivered or required under some specific conditions. May be in acfm, scfm, etc.
Cubic feet of air per minute (cfm). Volume rate of air flow.
Cubic feet of air per minute, free air (cfm free air). Cfm of air delivered to some specific point and converted back to ambient (free air) conditions.
Actual cubic feet per minute (acfm). Flow rate of air measured at some reference point and based on conditions at that reference point.
Inlet cubic feet per minute (icfm). Cfm flowing through the compressor inlet filter or inlet valve under rated conditions.
Standard cubic feet per minute (scfm). Flow of free air measured at some reference point and converted to a standard set of reference conditions (e.g., 14.7 psia, 60( F, and 0% relative humidity.)
Demand. Flow of air under specific conditions required at a particular point.
Discharge pressure, rated. Air pressure produced at a rated reference point.
Discharge pressure, required. Air pressure required at the system inlet.
Displacement. Amount of air (in cfm) displaced by the compressor piston under no load, discharging directly to the atmosphere.
Dual control. Load/unload control system that maximizes compressor efficiency by matching air delivery and air demand. Compressor is normally operated at full load or idle, and is stopped and restarted automatically depending on demand.
Free air. Air at ambient conditions of temperature, humidity, and atmospheric pressure at any specific location.
Inlet pressure. The total pressure at the inlet flange of the compressor.
Load factor. Ratio of the average compressor load to the maximum rated compressor load during a given period of time.
Modulating control. System will run the compressor at varying loads to accommodate demand variations. Running a compressor at less than full load results in a drop in compressor efficiency and thus an increase in operating costs.
Pressure. Force per unit area.
Pounds per square inch (psi). Force per unit area exerted by compressed air.
Pounds per square inch absolute (psia). Pressure above absolute vacuum. Atmospheric pressure is stated in psia.
Pounds per square inch gauge (psig). Pressure at some reference point as measured with a gauge and dependent on atmospheric pressure.
Pounds per square inch differential (psid). Pressure difference between two points.
Pressure dew point. Temperature at which water will begin to condense out of air at a given pressure.
Pressure drop. Loss of pressure in a compressed air system due to friction or restriction.
Receiver tank. Tank used for storage of air discharged from a compressor.