Compressed Air vs. Free Air - Compression Ratio

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 = p_d / p_s                                (1)

where

CR = compression ratio

p_d = discharge absolute pressure (bar abs, psia)

p_s = suction absolute pressure (bar abs, psia)

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

• Compression Ratio - Piston Engines

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

q_F = CR q_C                                   (2)

where

q_F = free air flow (m³/s, cfm)

q_C = 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.

q_F = 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.

q_C = (1 m³/s) / 11

             = 0.091 m³/s