SCFM –vs- ACFM

There is a great deal of confusion between the terms SCFM and ACFM.

Lets first start off with the definitions:

• SCFM - “Standard” Cubic Feet Per Minute - This is the volume of air a pump is able to move at a “standardized” pressure, temperature, and relative humidity. In most cases, the “standard” ambient conditions defined by the CAGI (the Compressed Air and Gas Institute) are 14.7 psig atmospheric pressure, 68 degrees Fahrenheit, and 0% relative humidity
• ACFM – “Actual” Cubic Feet per Minute – This is the “actual” airflow (uncorrected) that is measured at the air inlet of the pump. This is a realistic representation of the actual air flow that was moved during the pumping process.

So, theoretically, if you were moving air at exactly 14.7 psig, it was a perfect 68 degree day, and there was no humidity (you are in the desert) these two standards to reference a flow rate would be identical. Unfortunately, this usually is not the case as the most important change between these two definitions is the pressure. While moving air, you are either creating a pressure or a vacuum. When pressure is applied to a standard cubic foot of air, it gets smaller. When vacuum is applied, it expands. The volume of air after it is pressurized or rarified is referred to as its “actual” volume

As a rule of thumb, an SCFM result will be lower than an ACFM result during a vacuum application, and a SCFM result will be higher than an ACFM result during a pressure application

How this effects our calculations: The standard EPA formula for calculating mass removal rates for hydrocarbons requires an ACFM (Actual Cubic Feet per Minute) flow result. If you were to use an SCFM result in the EPA formula (unless you were at the exact “standard” parameters of a SCFM calculation) would provide you with an inaccurate result.

CONVERSION CHART - ACFM/SCFM

 Vacuum Vacuum Vacuum Level Conversion Level Conversion Level Conversion (Gauge) Factor (Gauge) Factor (Gauge) Factor 1" 1.03 11" 1.58 21" 3.33 2" 1.07 12" 1.67 22" 3.75 3" 1.11 13" 1.76 23" 4.28 4" 1.15 14" 1.88 24" 5.00 5" 1.20 15" 2.00 25" 6.00 6" 1.25 16" 2.14 26" 7.63 7" 1.30 17" 2.31 27" 10.00 8" 1.36 18" 2.50 28" 15.00 9" 1.43 19" 2.73 29" 30.00 10" 1.50 20" 3.00

To find ACFM - Multiply SCFM by conversion factor

EXAMPLE:

• 99 SCFM @ 19" Hg 99 SCFM @ 25" Hg
• 99 x 2.73 = 270.27 ACFM 99 x 6 = 594 ACFM

To find SCFM - Divide ACFM by conversion factor

EXAMPLE:

• 270.27 ACFM @ 19" Hg 594 ACFM @ 25" Hg
• 270.27 divide by 2.73 = 99 594 divide by 6 = 99
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