Standard Cubic Feet Per Minute - Actual Cubic Feet Per Minute

Actual Cubic Feet Per Minute

Actual cubic foot per minute (ACFM) is the volume of gas flowing anywhere in a system, independent of its temperature and pressure. If the system were moving a gas at exactly the "standard" condition, then ACFM would equal SCFM. Unfortunately, this usually is not the case as the most important change between these two definitions is the pressure. To move a gas, a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of gas, it is compressed. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume.

SCF and ACF for any gas are related in accordance with the combined gas law:

Defining standard conditions by the subscript 1 and actual conditions by the subscript 2, then:

where is in absolute pressure units and is in absolute temperature units (i.e., either kelvins or degrees Rankine).

To be very precise when the gas is air, then the above equation should include correcting for the difference between the relative humidity of the air at the standard and the actual temperature and pressure conditions. In most cases of engineering design, the humidity correction for air is often quite small and hence often ignored.