Air Compressors

Guide to Compressed Air

Displacement (Volume)

This term is used to decide the theoretical volume of a compressor, ie the swept volume of a cylinder multiplied by the number of compressions in a minute and expressed in cubic feet per minute (CFM).
This figure should not be used in calculating the size of compressor required, as it bears no relationship to the free air figure that you may require.

Free Air Delivered

This measurement is the volume of air taken into a compressor and therefore describes more accurately the volume of air available for use.
Expressed as CFM/FAD at a given pressure.

Remember: Displacement is the theory and CFM/FAD is the actual volume available.

Single Stage Compressors

One or more cylinders producing the final pressure in one compression.
Normal maximum pressure 150 psi. g.

Two Stage Compressors

Air is compressed to approximately 30 psi. g. cooled then compressed to final pressure in the second stage.
Normal maximum pressure 200 psi. g.

NB: Two stage provides more air for less energy.

Choosing the right compressor

Three-phase compressors are more efficient producers of compressed air than single-phase equivalent units, so where a three-phase supply is available, the best option is the three-phase compressor.
Single-phase compressors up to 2.5hp can operate from a 240V 13 Amp power supply, with the exception of the new 3 hp Low Current models. 3.0 hp and above must operate from a 240V 30 Amp supply.
Wherever possible choose a larger compressor than you require at present to allow for expansion.
Compressors with cast iron cylinders running slow, offer a much extended service life.

Quick guide to CFM/FAD output (approx) of existing compressor:
Multiply motor hp by 3.3 = output in CFM/FAD
Multiply motor kW by 4.5 = output in CFM/FAD
Multiply motor kW by 2.1 = output in L/sec/FAD

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