Compressed Air Guide

Compressed Air Guide
There are a number of terms used in the compressed air industry, below are a few of the most used and a brief explanation of what they mean.
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 in to 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.


This is the way force, ie power, in the compressed air system is expressed and is measured in either pounds
per square inch (psi) or bar. Pressure is important because to have too low a pressure would cause the equipment not to work correctly, too high a pressure would at best rapidly wear out the equipment, at worst it could make the equipment lethal.

Power Supply

Single Phase.
Standard supply for domestic and light industry 220/240 volts. • 5 Amp light circuit Not suitable for equipment
• 13 Amp ring main Max 2.5 hp std compressor Max 3.0 hp for LC compressors • 45 Amp cooker/shower Max 3.5 hp compressor

Three Phase

Main electrical supply to industry. • 380/440 Volts Any size of compressor
 Main Benefits of Three Phase
• Approx 2/3 cost saving over single phase • Stable supply • Longer motor life
Main Benefit of Single Phase
• Excellent second-hand resale value

Compressor Motor to Volume Ratio

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

Choosing the Right Compressor

Three-phase rotary screw 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 2.5 hp 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.

Cost Of Air Leaks

Hissing Sid is at work in most companies you visit. Hissing Sid is a length of air hose which has become the family ‘air’ loom, it must be, because this hose is costing its owner a small fortune and yet to suggest replacement would be a cardinal sin. So what does Hissing Sid cost to run???

Air leak size CFM lost Energy
0.75mm dia.   1.6 cfm   300W
1.5mm dia.     6.5 cfm   1100W
A typical garage working a 7 day week (service station) multiplied by 365 days -
= 1,365,000 cubic feet per year!!! Or = 6,205kW per year!!! Or = £550.00 per year!!!
Repairing air leaks  makes economic sense.