Inlet Cubic Feet Per Minute

Understanding your air compressor terms

CFM or Cubic Feet Per Minute of air flow is a term that is often confusing in compressed air systems. It is also seen as SCFM, ACFM, and ICFM. So what are the differences?

SCFM or Standard Cubic Feet Per Minute is referenced to cubic foot of air at standard pressure, temperature, and relative humidity. In most cases, SCFM is based on 14.7 PSIA, 68°F, and 36% relative humidity. By these specific parameters, the density of a cubic foot of air is fixed. The mass flow of compressed air is therefore clearly defined.

ACFM or Actual Cubic Feet Per Minute is volume air that is constantly changing due to atmospheric conditions and/or compressed or expanded by air compressors or vacuum pumps. As temperature increases, the volume expands. As pressure increased the volume decreases. And vice-versa.

P1V1=P2V2 Boyle’s Law, with constant temperature, absolute volume changes with pressure

V1/T1=V2/T2 Charles’ Law, with constant pressure, volume changes with temperature.

ICFM or Inlet cubic feet per minute is a unit adopted by equipment manufactures as it relates to a volume air that can be displaced by the equipment. The actual mass of the inlet air varies with atmospheric conditions. The actual discharge volume varies with pressure and temperature.

If the inlet conditions happen to be at 14.7 PSIA, 68°F, and 36% relative humidity (standard conditions), a 1000 ICFM compressor at 100PSIG would produce 1000SCFM at the discharge. But when you convert that to ACFM (14.7psia X 1000CFM)/114.7psia = 128ACFM at the discharge (assuming constant temperature). From atmospheric pressure, you have compressed the air 7.8 times to 100PSIG.

As often misunderstood, this compressor example does not discharge 1000CFM but rather takes it in the inlet.