Terms used across sizing, safety and running costs
Free air delivery (FAD)
The volume of air a compressor actually delivers per minute, measured back at atmospheric pressure and temperature, at the working pressure the compressor is running at. A compressor rated at 280 l/min (9.9 cfm) FAD at 8 bar delivers that much air once it has been drawn in, compressed and returned to normal conditions, which is the figure that tells you what a tool will actually get.
Displacement
The volume swept by the piston or screw element each minute before losses from leakage past valves or seals are taken out, always a higher number than free air delivery. A compressor listed at 356 l/min (12.6 cfm) displacement might only deliver around 280 l/min (9.9 cfm) of free air at 8 bar once those losses are accounted for.
Bar-litres
A receiver's capacity in litres multiplied by its maximum allowable pressure in bar, the figure that decides whether it needs a written scheme of examination. A 24-litre receiver rated to 10 bar has a bar-litre product of 240, which sits under the 250 bar-litre threshold set in the Pressure Systems Safety Regulations 2000.
Written scheme of examination
The documented inspection programme required for a pressure system above the 250 bar-litre threshold, setting out what gets examined, by whom and how often. A receiver crossing that threshold needs its scheme drawn up before it goes into service.
Duty cycle
The proportion of time a compressor runs under load, usually given as running time per hour or as a percentage. A compressor rated for a lower duty cycle is built to run under load for only part of each hour, with the rest left for it to cool and recover.
Pressure dew point
The temperature at which moisture in compressed air starts to condense out, measured at the system's working pressure. A dryer that brings the pressure dew point below the coldest temperature the pipework reaches keeps water from condensing out further down the line.
Competent person
Someone with the knowledge and practical experience to examine a pressure system safely, a status defined in the Pressure Systems Safety Regulations 2000 rather than a title anyone can take on. The written scheme names who is expected to carry out each examination, and that person has to meet this standard before the examination counts.
ISO 8573-1
The international standard that sorts compressed air into quality classes according to how much particle, water and oil contamination it contains. A spray booth feeding an air-fed hood needs air held to a far tighter class than a line just running an impact wrench or a sander.
Non-domestic electricity price
The unit price for business electricity, published each quarter by the Department for Energy Security and Net Zero, used to turn a compressor's power draw into an annual running cost. Multiplying a compressor's motor power by its running hours and this rate turns a kilowatt figure on a spec sheet into a pound figure on a bill.
Receiver
The pressure vessel that stores compressed air between the compressor and the tools drawing on it, smoothing out demand peaks and cutting how often the compressor has to start. A receiver sized against the actual peak demand lets two tools run together in a bodyshop without the pressure at the gun dropping mid-job.
Breathing air
Compressed air treated and tested to a standard fit for a person to breathe through an air-fed hood, distinct from the air supplied to tools running off the same compressor. Spraying isocyanate paints through an air-fed hood calls for air tested against set limits for carbon monoxide, carbon dioxide, oil and water.
