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# Compressed Air vs. Free Air - Compression Ratio

## The ratio of compressed air pressure to free air pressure.

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.

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## Citation

• The Engineering ToolBox (2008). Compressed Air vs. Free Air - Compression Ratio. [online] Available at: https://www.engineeringtoolbox.com/free-compressed-air-d_1190.html [Accessed Day Month Year].

Modify the access date according your visit.

10.2.10