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

  • ambient temperature
  • moisture content
  • barometric pressure

are stated.
[h=Compression Ratio]3[/h] 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.
[h=Compression Ratio - Pressure (psi)]3[/h]

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

[h=Example - Converting a Compressed Air Volume to a Free Air Volume]4[/h] 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

[h=Compression Ratio - Pressure (bar)]3[/h]