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Compressed Air vs. Free Air - Compression Ratio

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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 based on the Ideal Gas Law and is the ratio between Discharge Pressure Absolute and Suction Pressure Absolute.

CR = pd / ps                                (1)


CR = compression ratio

pd = discharge absolute pressure (bar abs, psia)

ps  = suction absolute pressure (bar abs, psia)

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

Converting Compressed Air Volume Flow to Free Air Volume Flow

A compressed air volume flow can be converted to a free air volume flow by using the equation

qF = CR qC                            (2)


qF = free air flow (m3/s, cfm)

qC = compressed air flow (m3/s, cfm)

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

A compressed air volume flow of 10 acfm (actual cfm) at 100 psig must be multiplied with Compression Ratio approximately 8 to estimate the volume of free air at atmospheric pressure.

qF = 8 (10 acfm) 

    = 80 scfm  (standard cfm)

Compression Ratio - Pressure (bar)

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

A free air volume flow of 1 m3/s is compressed to 10 bar (gauge) and must be divided with Compression Ratio approximately 11 to estimate the volume of compressed air.

qC = (1 m3/s) / 11

             = 0.091 m3/s

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