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 = pd / ps (1)
where
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)
where
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
Related Topics
• Gases and Compressed Air
Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.
Related Documents
Air Tools - Air Consumption
Air consumption of air tools.
Compressed Air - Pipe Line Pressure Drop Diagram
Calculate pressure drop in compressed air pipe lines with a nomograph diagram.
Compressed Air - Pipe Line Pressure Loss - Online Calculator with Metric and Imperial Units
Calculate pressure drop in compressed air pipe lines - metric and imperial units.
Compressed Air - Pressure Drop Diagrams, Metric Units
Pressure loss in compressed air pipe lines.
Compressed Air - Storage Volume
Calculate the storage volume of compressed air or other gases.
Compressed Air Pipe Line Capacity
Capacities of compressed air pipelines - pressure ranging 5 - 250 psi (0.5 - 17 bar).
Compressed Air Pipe Lines - Pressure Loss vs. Capacity and Operating Pressure
Pressure drop in compressed air pipe lines.
Compressed Air Quality
Compressed air is used for power tools, instrumentation, processing, breathing and medical purposes.
Compressed Air Receivers
Calculating air receivers capacity.
Horsepower required to Compress Air
Calculate horsepower required by air compressors.
Piston Engines - Compression Ratios
Cylinder volume and compression ratios in piston engines.
Pneumatic Piping - Pressure Loss vs. Air Flow
Pressure loss in pneumatic tubes - sizes ranging 5 - 36 mm.