ACFM (Actual Cubic Feet per Minute) is the actual rate of flow of compressed air delivered by the compressor, but expressed in terms of prevailing inlet air conditions. This is also known as Free Air Delivered (FAD).
SCFM (Standard Cubic Feet per Minute) is the measurement of air flow measured at standard conditions.
Since there are different standards in different disciplines, always make sure you understand which standard is being used when discussing compressed air flow. The standard agreed to by the compressed air industry represented by the Compressed Air and Gas Institute is:
68 deg. F at 0% Relative Humidity and 14.5 psig (1 bar)
The geographic location of the compressed air system plays an important role when determining the volume of air that is required. This is the ACFM. There are three location specific components that affect the cfm that is delivered by a compressor. They include heat, humidity and temperature.
To convert the required scfm to the flow that will be required at a specific geographical location (ACFM) this formula can be used:
ACFM = SCFM x (14.5 - 0)/(P1-VPm) x (460 +T1)/(460+68)
Where:
P1 = Absolute Inlet Pressure (psia)
T1 = Inlet Temperature deg. F
VPm = Vapor Pressure of moisture in inlet air (or the Vapor Pressure of the saturated air at inlet temp x Relative Humidity)
Thus, if the Absolute pressure is 14.2 psia, the vapor pressure of the inlet air is zero and the inlet temperature is 35F, 1 scfm is equivalent to:
ACFM = 1 x (14..5-0)/(14.2 - 0) x (460+35)/(460+68) = 1.02 x 0.9375 = 0.96 cubic feet
Thus compressors at altitude typically do not put out as much air as compressors at sea level.