Ratio of Free Air to Compressed Air

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Free air is air at ambient conditions at a specific location where 

• ambient temperature
• moisture content
• barometric pressure

are stated.  

Compression Ratio

Compression Ratio is the ratio between Discharge Pressure Absolute and Suction Pressure Absolute.

The compression ratio of free air - to compressed air, is indicated in the diagram below.

Compression Ratio - Pressure (psi)

• 1 psi = 6.9 kPa = 0.069 bar
• 1 ncfm = 0.5 nl/s

Example - Converting a Compressed Air Volume to a Free Air Volume

A compressed air volume flow of 10 acfm at 100 psig must be multiplied with a compression ratio approx. 8 to find the free air flow at atmospheric pressure.

• in this case approx. 80 scfm

Compression Ratio - Pressure (bar)

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