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SCFM versus ACFM and ICFM

Actual air compressor capacity (ACFM) versus standard air capacity (SCFM) and inlet air capacity (ICFM)

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SCFM - Standard Cubic Feet per Minute

It is common to rate compressed air consumption in Standard Cubic Feet per Minute - SCFM.

The SCFM - Standard Cubic Feet per Minute - determines the weight of air to fixed or "Standard" conditions. There are several definitions of SCFM. The most common used in the United States is with the "sea-level" properties:

  • 14.696 Pounds per Square Inch (psia)
  • 60 Degrees Fahrenheit (oF) (520oR)
  • 0% Relative Humidity (RH)

Europeans normally use one ata and 0 oC as SCFM.

ACFM - Actual Cubic Feet per Minute

Real life "actual conditions" are seldom "standard conditions". When

  • pressure is applied a volume of air - it gets smaller
  • vacuum is applied to a volume of air - it expands

The actual air volume flow is often termed ACFM - Actual Cubic Feet per Minute.

The Actual Cubic Feet per Minute - ACFM - depends on the

  • pressure
  • temperature
  • humidity

of the actual air.

The conversion from SCFM to ACFM can be expressed

ACFM = SCFM [Pstd / (Pact - Psat Φ)](Tact / Tstd)                                      (1)

where

ACFM = Actual Cubic Feet per Minute

SCFM = Standard Cubic Feet per Minute

Pstd = standard absolute air pressure (psia)

Pact = absolute pressure at the actual level (psia)

Psat = saturation pressure at the actual temperature (psi)

Φ = Actual relative humidity

Tact = Actual ambient air temperature (oR)

Tstd = Standard temperature (oR)

Note!

Psat Φ < Pact

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Online SCFM - ACFM Calculator

The calculator below can used to calculate ACFM:

SCFM

 standard absolute air pressure - Pstd - (psia)

actual absolute air pressure - Pact - (psia)

 saturation pressure at the actual temperature - Psat - (psia)

Actual relative humidity - Φ

Actual ambient air temperature - Tact - (oR)

Standard air temperature  - Tstd - (oR)


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Example - SCFM to ACFM

The actual CFM of a compressor operating at "non-standard" conditions like

  • elevation 5000 feet (1500 m) - atmospheric pressure Pact = 12.23 psia
  • temperature 80oF - absolute temperature Tact = 540oR
  • saturation pressure Psat = 0.5069 psia
  • relative humidity Φ = 80%
  • demand: 100 SCFM

can be calculated as

ACFM = (100 SCFM) [(14.7 psia) / ((12.23 psia) - (0.5069 psia) (80 / 100))]((540 oR) / (520 oR))

    = 129.1  

ICFM - Inlet Cubic Feet per Minute

Inlet Cubic Feet per Minute - ICFM - is used by compressor vendors to establish conditions in front of additional equipment like inlet filter, blower or booster.

When air passes through the filter there will be a pressure drop. The conversion from ACFM to ICFM can be expressed as

ICFM = ACFM (Pact / Pf) (Tf / Tact)                                               (2)

where

ICFM = Inlet Cubic Feet per Minute

Pf = Pressure after filter or inlet equipment (psia)

Tf = Temperature after filter or inlet equipment (oR)

Note!

The Ideal Gas Law is accurate only at relatively low pressures and high temperatures. To account for the deviation from the ideal situation, another factor is included. It is called the Gas Compressibility Factor, or Z-factor. This correction factor depends on the pressure and temperature for each gas considered.

The True Gas Law, or the Non-Ideal Gas Law, becomes:

P V = Z n R T  (3)

where

Z = Gas Compressibility Factor

n = number of moles of gas present

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