ACFM
Actual Cubic Feet per Minute (ACFM) is the volume of gas flowing anywhere in a system independent of its density. If the system were moving air 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 air a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of air, it gets smaller. When a vacuum is applied to a standard cubic foot of air, it expands. The volume of air after it is pressurized or rarified is referred to as its “actual” volume.
CFM is often a confusing term because it has no single definition that applies to all instances. In the most basic sense, CFM means cubic feet per minute. Sounds simple enough right? Unfortunately, air is a compressible gas. To further confuse the issue, a centrifugal fan is a constant CFM device or a constant volume device. This means that, provided the fan speed remains constant, a centrifugal fan will pump a constant volume of air. This is not the same as pumping a constant mass of air. Again, the fan will pump the same volume, though not mass, at any other air density. This means that the air velocity in a system is the same even though mass flow rate through the fan is not.
