Actual Cubic Feet per Minute

Manufacturers of blowers and compressors usually state their volumetic capacity as Cubic Feet per minute (CFM) or Actual Cubic Feet per Minute (ACFM). Since the volumetric capacity refers to the volume of air or other gas at the inlet to the unit, it is also often referred to as Inlet CFM (ICFM). All three terms (CFM, ACFM or ICFM) are used interchangeably to mean basically the same thing.

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SCFM

Main article: SCFM

Standard Cubic Feet per Minute (SCFM) is a volumetric flow-rate corrected to a set of "standardized" conditions of pressure, temperature, and sometimes relative humidity. The standard conditions are often defined as some pressure (e.g., 14.7 psia), some temperature (e.g., 68°F) and some relative humidity (e.g., 36% or 0 %) depending on the "standard" used. (See Standard conditions for temperature and pressure).

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 or other gas, it gets smaller. When a vacuum is applied to a standard cubic foot of air or gas , it expands. The volume of air or gas after it is pressurized or rarefied 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.

Centrifugal Fan Ratings

Ratings found in centrifugal-fan performance tables and curves are based on standard air (SCFM). Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot, with the barometric pressure at sea level of 29.92 inches of mercury and a temperature of 70°F. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both 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.

If a centrifugal fan is to operate at a non-standard density, then corrections must be made to static pressure and brake horsepower. At higher than standard elevation (sea level) and higher than standard temperature (70°F), air density is lower than standard density (0.075 pounds per cubic foot). Centrifugal fans that are specified for continuous operation at higher temperatures need to be selected taking into account air density corrections. Again, a centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures.

If, for example, a centrifugal fan moves 1,000 CFM at 70ºF it will also move 1,000 CFM at 200ºF. The air volume delivered by the centrifugal fan is not affected by density. However, since the 200°F air weighs much less than the 70ºF air, the centrifugal fan will create less static pressure and will require less brake horsepower. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and brake horsepower. 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. Since 200°F air weighs only 80% of 70°F air, the centrifugal fan will create less pressure. To get the actual pressure required at 200°F, the designer would have to multiply the pressure at standard conditions by an air density correction factor of 1.25 (i.e., 1.0 / 0.80) to get the system to operate correctly. To get the actual horsepower at 200°F, the designer would have to divide the brake horsepower at standard conditions by the air density correction factor.

Air Movement and Control Association (AMCA)

The centrifugal fan performance tables provide the fan RPM and brake horsepower requirements for the given CFM and static pressure at standard air density (0.075 pounds per cubic foot). When the centrifugal fan performance is not at standard conditions, the performance must be converted to standard conditions before entering the performance tables. Centrifugal fans rated by the Air Movement and Control Association (AMCA) are tested in laboratories with test setups that simulate installations that are typical for that type of fan. Usually they are tested and rated as one of four standard installation types as designated in AMCA Standard 210.

AMCA Standard 210 defines uniform methods for conducting laboratory tests on housed fans to determine airflow rate, pressure, power and efficiency, at a given speed of rotation. The purpose of AMCA Standard 210 is to define exact procedures and conditions of fan testing so that ratings provided by various manufacturers are on the same basis and may be compared. For this reason, fans must be rated in SCFM.

See also

• SCFM
• Standard cubic foot
• MMSCFD
• Gas compressors
• Centrifugal fans
• Combined gas law