You are here: Home  >  FAQ  >  Compressors

Compressors

Why are some compressors rated in free air delivered and some in displacement?
How does pressure affect my compressed air system?
What is a single stage compressor?
What is a two stage compressor?
What is the difference between the single and three phase power supply?
How much air does my air tool use?
How do I choose the right compressor?
Compressor size guide
What is the cost of air leaks?
How do I select the correct bore of hose?
Where did the pressure go? - pressure drop
What does PSSR stand for and is it important?


Why are some compressors rated in free air delivered and some in displacement?

In simple terms, displacement is the theoretical output figure without taking into account heat expansion, friction losses and general inefficiencies. Free air delivered is the actual amount of air available for use at the output of the compressor. For a fuller explanation see: http://www.britishcompressedairsociety.co.uk/downloads/terminology.doc

How does pressure affect my compressed air system?

This is the way force, i.e. 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.

See the technical information chart below for the correct pressure for individual pieces of equipment.

What is a single stage compressor?

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

What is a two stage compressor?

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

Note: two stage provides more air for less energy.

What is the difference between the single and three phase 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

Main benefit of a single phase compressor

  • Excellent second-hand resale value

Three phase - main electrical supply to industry.

  • 380/440 volts - any size of compressor

Main benefits of a three phase compressor

  • Approximately 2/3 cost saving over single phase
  • Stable supply
  • Longer motor life

How much air does my air tool use?

Click here to see the Air Equipment Consumption Guide

How do I choose 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 2.5 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.

  1. Bodyshop - using the Air Equipment Consumption Guide, add all the equipment consumptions together and divide by two. The resulting figure is the minimum free air you require.
  2. Workshop - using the Air Equipment Consumption Guide, add all the equipment consumptions together and divide by three. The resulting figure is the minimum free air you require.

Note: for calculation purposes always use free air delivered figures.

Compressor size guide

a) Ask questions

  1. What is the air to be used for?
  2. What is the maximum pressure required (see consumption guide)
  3. What electricity supply is available (single/three phase)
  4. What size compressor is currently in use? (see quick calculation guide below)
  5. How well does existing compressor cope?
  6. What are the future plans for additional staff and equipment?

b) Complete the following list

  1. Number of tools and type
  2. Number of users
  3. Air consumption of largest tool/equipment using air

c) Complete survey form

d) Select compressor from catalogue (use only free air figures)

Note: 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

What is the cost of air leaks?

Hissing Sid is at work in most companies you visit. Hissing Side 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 a replacement would be a cardinal sin. So what does Hissing Sid cost to run?

Air leak size: 0.75mm dia =  CFM lost: 1.6 = Energy: 300W
Air leak size: 1.50mm dia = CFM lost: 6.5 = Energy: 1100W

Cost:
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!!!

Message: Clearly a replacement hose and couplings makes economic sense.

How do I select the correct bore of hose?

An air tool needs the following:

  1. Correct size of compressor to ensure sufficient air available
  2. Correct size of air hose to ensure sufficient air available
  3. Correct pressure at tool (see chart)
  4. Correct type of lubricant (not engine oil)
  5. Clean dry air!!! (use filters, regulator, dryer, and lubricant)

Recommended air hose sizes:

Hose size Uses Max flow CFM
1/4" tyre inflator, air brush 5
5/16" std spray guns, 3/8" drill, ratchet wrench 15
3/8" HVLP/LVLP spray guns, 1/2" & 3/4" wrenches, sanders 25
1/2" 1" impact wrenches 50

Note: Always keep hose lengths as short as practical. e.g. 1/2" impact wrench with 20 metres of 1/4" bore hose will develop less than 40% of its available power!

Where did the pressure go? - pressure drop

A 10 metre length of air hose flowing 15 CFM at 100 psi. g. will lose the following pressure:
1/4" bore loses 30 psi. g.
5/16" bore loses 12 psi. g.
1/2" bore loses 1 psi. g.

What does PSSR stand for and is it important?

The Pressure System Safety Regulations 2000.

You are here: Home  >  FAQ  >  Compressors

Registered in England & Wales. Registration Number 177707