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Air Compressor Guide

Air Pressure

This can be stated in metric units, i.e. bar, or in imperial units, i.e. psi (pounds per square inch). 1 bar pressure is equivalent to 14.5 psi.

Air Flow

Measurement of air is complicated as air is compressible. One method is to measure the volume of air being sucked into the pump at atmospheric pressure. This is known as the piston displacement.

F.A.D. (Free Air Delivery)

This is always less than the piston displacement. It is best to think of it as the flow of compressed air being delivered from the pump to the air receiver tank. i.e. the volume per minute is now less as it has now been compressed.

Size of compressor

When purchasing a compressor to operate for example an air wrench, the compressor F.A.D. figure should be compared against the air requirements of the tool. This is less critical if there is a large air receiver (air tank).

Maximum Pressure

Air output from compressors normally fluctuate from maximum pressure (when the motor switches off) to a lower pressure (when the motor switches back on). Typically, the lower pressure may be about 30 psi lower than the air compressor rating.

Air Supply

When using compressors, be aware of possible condensation as air expands. Consider whether an air dryer is required. Always filter the air, and add lubrication if operating air tools, or air driven pumps. Also fit an air regulator.


Some typical uses :- Airbrushing by artists - real airbrushing, not on a computer ! Inflation of tyres in garages and forecourts. Spray painting of vehicles, or industrial spray painting. Shotblasting of industrial machinery prior to painting. Power supply to air-driven pumps. Air supply for CNC machines. For air operated logic and control systems in machinery. Engine driven compressors for mobile maintenance vans. Engine driven compressors for mobile tyre fitters. Operation of nail guns and staplers by joiners. Air supply to plasma cutting machines, to blow molten metal from the cut. Operation of air tools in garages and workshops, e.g. sanders, wrenches, chisels etc.

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