# Compressed Air

Air at any pressure greater than atmospheric pressure.

For a single stage compressor the power in Horse Power (HP) is given as:

Where

R = the ratio of the compressed air absolute pressure to the ambient air pressure.
P = atmospheric pressure [psi]

Using this equation: R = 3.2 (2.2 bar gauge pressure), P = 14.7 psi.

• Power = 140 Watts/litre/s at 2.2 bar.

There will be an additional loss due to the inefficiency of the electric motor and losses in the compressed air system.

According to the Carbon Trust the power required to compress air is as follows:

• Average 300 watts power required per litre/sec of compressed air.
• 2.2 Bar pressure differential

For a two stage compressor the power in Horse Power (HP) is given as:

Using this equation: R = 3.2 (2.2 bar gauge pressure), P = 14.7 psi.

• Power = 128 Watts/litre/s at 2.2 bar.

### Efficiency

It takes about 8 KW of compressor input power to provide 1 KW of power at an air tool.

This gives overall efficiency = 1/8 = 0.125 = 12.5%

If you include the electric motor efficiency, at best 90% then the overall efficiency drops to 11%

### Reference

Compressed air equipment A guide to equipment eligible for Enhanced Capital Allowances, Carbon Trust

Donald R. Wulfinghoff, "Compressed Air System Efficiency", Wulfinghoff Energy Services, Inc.

Subjects: Mechanical Engineering Tools