Absolute Pressure
The pressure with reference to absolute zero, that is with no reference to an absolute vacuum: it equals the sum of the atmospheric pressure and the gauge pressure (ISO 3857/1)
Absolute Temperature
The temperature measured from absolute zero (ISO 3857/1).
Actuator
Any pneumatic device which applies power, for example a cylinder, motor or the device which provides an input signal to the component.
Adiabatic
Expansion or compression of a gas without loss or gain of heat content.
Aftercooling
The removal of heat from the air after compression is complete.
Ambient Temperature
The temperature of the environment in which the equipment is working.
Atmospheric Pressure
The absolute pressure of the atmosphere as measured at the place under consideration (ISO 3857/1).
Bar
The unit commonly used with compressed air. 1 bar = 105N/m2 = 105 Pascals
Closed Loop System
A system in which air exhausted from the actuators is returned as a closed pressurised circuit to the compressor inlet.
Compressor
A device which causes a gas to flow against a pressure; it converts mechanical force and motion into pneumatic fluid-power.
Compressor Capacity
The actual volume rate of flow compressed and delivered at the standard discharge point, at stated inlet conditions, usually expressed in terms of Free Air Delivered (ISO 3857/2).
Compressor Regulator
A device fitted to the compressor to control the output of the machine.
Condensate
The liquid formed from water vapour in the air because of a drop in the air temperature and/or an increase in pressure.
Dewpoint
The temperature at which the air is fully saturated with water vapour at the pressure prevailing.
Displacement
The volume displaced by the compression element of the first stage per unit of time.
Dryer
A device that reduces the water vapour content of the compressed air.
Filter
A device which removes foreign matter from the working medium.
Free Air
Air at the atmospheric conditions at the inlet point unaffected by the compressor; it is unusual for the output of an air compressor to be referred to the stated atmospheric conditions at the inlet, and the consumption of a tool or pneumatic cylinder to be expressed in terms of a standard reference atmosphere.
Fusible Plug
A device fitted in the hot discharge zone of a compressor for protection against high temperature.
Gauge Pressure
The pressure as measured with reference to atmospheric pressure; where no other indication is given, pressures expressed in 'bar' are assumed to be gauge pressures (ISO 3857/1)
Intercooling
The removal of heat from the air between stages in multistage compression.
Isothermal
Expansion or compression carried out without change of temperature.
Lubricator
A device which introduces a controlled quantity of lubricant into the working medium.
Multi-Stage Compression
Compression from initial to final pressure in two or more stages (steps) with cooling between each stage.
Overall Stage Pressure Ratio
The pressure ratio for any particular stage in a multi-stage compressor, the discharge pressure being measured after the intercooler (including separator), this is calculated using Absolute Pressures (ISO 3857/2).
Pipeline
The Pressure Systems Safety Regulations define a pipeline as a "pipe (or system of pipes) used for conveyance of a relevant fluid (e.g. compressed air) across the boundaries of premises, together with the values, pumps, compressors and similar works which are annexed to, or incorporated in, the course of the pipe or system". Avoid the use of "pipeline" when referring to the pipes within the factory premises which have their own compressor installation.
Pipework
A term that should not be used when referring to the pipes that convey compressed air. 'Pipework' has a special meaning in the context of the Pressure Systems Regulations i.e. A pipe or system of pipes with associated valves, pumps, compressors and other pressure containing components includes hoses but does not include the air receiver or any protective devices. Avoid the use of 'pipeline' also also as this also has a specific meaning in the Pressure Systems Safety Regulations.
Pressure Ratio
The ratio between the Absolute discharge pressure and the Absolute inlet pressure; (ISO 3857/2).
Pressure Regulator (Pressure Reducing Valve)
A device which reduces the line pressure and maintains it relatively constant despite changes in inlet pressure and outlet flow rate.
Pressure Relief Valve (Safety Valve)
A device which limits the maximum system pressure by exhausting the compressed air to atmosphere when the required back pressure is exceeded; also known as a Back Pressure Regulator.
Protective Device
A term employed in the Pressure Systems Safety Regulations and includes all the devices such as pressure gauges, temperature gauges and other items of measuring equipment, pressure relief valves, bursting discs, fusible plugs, excess temperature or pressure shut-down controls which enable system failure to be prevented by keeping it within safe operating limits.
Pulsation Dampener
A chamber fited at the inlet or discharge of a reciprocating compressor to remove pulsations and prevent resonance.
Receiver, Air
A pressure vessel in which compressed air is stored.
Relative Humidity
The ratio of the amount of water vapour actually contained in a volume of air at a specific temperature and pressure, to the maximum amount possible under these conditions; normally expressed as a percentage.
Ring Main
An air main which begins and ends at the compressor so that every outlet has two possible surces (routes) of supply.
Separator
A device which removes liquids from the compressed air.
Single-Stage Compression
Compression from initial to final pressure in a single stage (step).
Specific Power Consumption
The shaft power input per unit of compressor capacity (Joules/litre=kw.s/m3)(ISO 3857/2).
Standard Reference Atmosphere
The agreed atmosphere to which specification 1 bar values and test results determined in other atmospheres are corrected; Pneumatic fluid power uses 1 bar, 20°C and 65% RH; the compressor and the pneumatic tool industries also use 1 bar, 20°C and 65% RH (ISO 2787); the aerospace (ISO 2533) and petroleum (ISO 5024) industries both use 1013 mbar, 15°C and dry at mean sea level.
Volumetric Efficiency
The ratio of free air delivered to the displacement of a compressor.; this ratio is normally expressed as a percentage; condensates, may possibly be taken into account (ISO 3857/2).
Working Element
The component that transforms fluid energy into linear, rotary or limited movement (e.g. a pneumatic cylinder or pneumatic motor or semi rotary device).
