Air - SCFM versus ACFM and ICFM

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) (520 ^o R)
• 0% Relative Humidity (RH)

Europeans normally use one ata and 0 ^oC as SCFM.

Note that the The Compressed Air & Gas Institute and PNEUROP have adopted the definition used in ISO standards with dry (0% relative humidity) air at 14.5 psia (1 bar) and 68 ^oF (20 ^oC).

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 [P _std / (P _act - P _sat Φ)](T _act / T _std )                                      (1)

where

ACFM = Actual Cubic Feet per Minute

SCFM = Standard Cubic Feet per Minute

P _std = standard absolute air pressure (psia)

P _act = absolute pressure at the actual level (psia)

P _sat = saturation pressure at the actual temperature (psi)

Φ = Actual relative humidity

T _act = Actual ambient air temperature ( ^o R)

T _std = Standard temperature ( ^o R)

Note!

P _sat Φ ƒ < P _act

Related Mobile Apps from The Engineering ToolBox

• SCFM - ACFM App

- free apps for offline use on mobile devices.

Online SCFM - ACFM Calculator

The calculator below can used to calculate ACFM:

SCFM

Standard absolute air pressure - P _std - (psia)

Actual absolute air pressure - P _act - (psia)

Saturation pressure at the actual temperature - P _sat - (psia)

Actual relative humidity - Φ

Actual ambient air temperature - T _act - ( ^o R)

Standard air temperature - T _std - ( ^o R)

• temperature converter
• pressure converter

• Make a Shortcut to this Calculator on Your Home Screen?

Example - SCFM to ACFM

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

• elevation 5000 feet (1500 m) - atmospheric pressure P _act = 12.23 psia
• temperature 80 ^oF - absolute temperature T _act = 540 ^o R
• saturation pressure P _sat = 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 ^o R ⁾ / (520 ^o R))

= 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 (P _act / P_f ) (T_f / T _act )                                               (2)

where

ICFM = Inlet Cubic Feet per Minute

P_f = Pressure after filter or inlet equipment (psia)

T_f = Temperature after filter or inlet equipment ( ^o R)

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

Air Volume Conversion Factors

Air volume conversion factors related to air with temperature 70°F (21.1°C) :

Air volume conversion factors related to air with temperature 70°F (21.1°C)

Air Temperature		Factor
°F	°C
0	-17.2	1.152
10	-12.2	1.128
20	-6.7	1.104
30	-1.1	1.082
40	4.4	1.060
50	10.0	1.039
60	15.6	1.019
70	21.1	1.000
80	26.7	0.981
90	32.2	0.964
100	37.8	0.946
110	43.3	0.930
120	48.9	0.914
130	54.4	0.898
140	60.0	0.883
150	65.6	0.869
160	71.1	0.855
170	76.7	0.841