When a centrifugal fan is specified for a given CFM and static pressure at conditions other than standard, an air density correction factor must be applied to select the proper size fan to meet the new condition. However, since the 200☏ air weighs much less than the 70☏ air, the centrifugal fan will create less static pressure and will require less brake horsepower. The air volume delivered by the centrifugal fan is not affected by density. If, for example, a centrifugal fan moves 1,000 CFM at 70☏ it will also move 1,000 CFM at 200☏. Again, a centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures. Centrifugal fans that are specified for continuous operation at higher temperatures need to be selected taking into account air density corrections. At higher than standard elevation, usually sea level, and higher than standard temperature (70☏), air density is lower than standard density (0.075 pounds per cubic foot). If a centrifugal fan is to operate at a non-standard density, then corrections must be made to static pressure and brake horsepower. The volume of air will not be affected in a given system because a fan will move the same amount of air regardless of the air density. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and brake horsepower. Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot, with the atmospheric pressure at sea level of 29.92 inches of mercury and a temperature of 70☏. Ratings found in centrifugal fan performance tables and curves are based on standard cubic feet per minute (SCFM). However, there is not an agreed upon definition for what constitutes normal conditions.įree air delivery (FAD) indicates delivered air, referred to as 'free air', at inlet conditions. Normal cubic feet per minute (NCFM) refers to air at normal conditions, such as 14.7 psia, 68☏, 36% relative humidity. (See Standard conditions for temperature and pressure). The standard conditions are often defined as 14.7 psia, temperature 70☏ and relative humidity 0%, but the conditions may vary depending on the 'standard' used. Standard cubic feet per minute (SCFM) is a volumetric flow-rate corrected to a set of 'standardized' conditions of pressure, temperature, and relative humidity. Main article: Standard cubic feet per minute This means that the air velocity in a system is fixed even though mass flow rate through the fan is not. For instance, a centrifugal fan is a constant CFM device or a constant volume device, meaning that, at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than a constant mass.

If the pressure is doubled, then, for an ideal gas, the mass of the gas that passes by will also be double for the same rate of flow in cubic feet per minute. The term cubic feet per minute (CFM) is ambiguous when it comes to the mass of gas that passes through a certain point because gas is compressible. The volume of gas after it is pressurized or rarefied is referred to as its 'actual' volume. When a vacuum is applied to a standard cubic foot of gas, it expands. When positive pressure is applied to a standard cubic foot of air or other gas, it gets smaller. To move air, either a positive pressure or a vacuum must be created. However, this usually is not the case as the most important change between these two definitions is the pressure. If the system were moving air at exactly the 'standard' condition, then ACFM would equal Standard cubic feet per minute (SCFM). Since the volumetric capacity refers to the volume of air or other gas at the inlet to the unit, it is often referred to as 'inlet cubic feet per minute' (ICFM).Īctual cubic feet per minute is the volume of gas and air flowing anywhere in a system independent of its density. This is the actual gas delivery with reference to inlet conditions, whereas cubic foot per minute (CFM) is an unqualified term and should only be used in general and never accepted as a specific definition without explanation. It is commonly used by manufacturers of blowers and compressors.

JSTOR ( April 2007) ( Learn how and when to remove this template message)Īctual cubic feet per minute ( ACFM) is a unit of volumetric flow.Unsourced material may be challenged and removed.įind sources: 'Actual cubic feet per minute' – news Please help improve this article by adding citations to reliable sources.

Feet per meeter