Actual cubic feet per minute (ACFM) is a unit of

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The de ...

tric flow. It is commonly used by manufacturers of blowers and

compressors A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

. This is the actual gas delivery with reference to inlet conditions, whereas

cubic foot per minute Cubic may refer to: Science and mathematics * Cube (algebra), "cubic" measurement * Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex ** Cubic crystal system, a crystal system ...

(CFM) is an unqualified term and should only be used in general and never accepted as a specific definition without explanation. Since the volumetric capacity refers to the volume of

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

or other

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

at the inlet to the unit, it is often referred to as "inlet cubic feet per minute" (ICFM). Actual cubic feet per minute is the volume of gas and air flowing anywhere in a system independent of its

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

. If the system were moving air at exactly the "standard" condition, then ACFM would equal

Standard cubic feet per minute Standard cubic feet per minute (SCFM) is the molar flow rate of a gas expressed as a volumetric flow at a "standardized" temperature and pressure thus representing a fixed number of moles of gas regardless of composition and actual flow conditions. ...

(SCFM). However, this usually is not the case as the most important change between these two definitions is the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

. To move air, either a positive pressure or a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

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 gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume. The term cubic feet per minute (CFM) is ambiguous when it comes to the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

of gas that passes through a certain point because gas is

compressible In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...

. If the pressure is doubled, then, for an

ideal gas An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is a ...

, the mass of the gas that passes by will also be double for the same rate of flow in cubic feet per minute. For instance, a

centrifugal fan A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a ...

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. This means that the air velocity in a system is fixed even though mass flow rate through the fan is not.

Standard cubic feet per minute

Standard cubic feet per minute (SCFM) is a volumetric flow-rate corrected to a set of "standardized" conditions of pressure, temperature, and relative humidity. The standard conditions are often defined as 14.7 psia, temperature 70°F and relative humidity 0%, but the conditions may vary depending on the "standard" used. (See

Standard conditions for temperature and pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union o ...

Normal cubic feet per minute

Normal cubic feet per minute (NCFM) refers to air at normal conditions, such as 14.7 psia, 68°F, 36% relative humidity. However, there is not an agreed upon definition for what constitutes normal conditions.

Free air delivery

Free air delivery (FAD) indicates delivered air, referred to as "free air", at inlet conditions.

Centrifugal fan rating

Ratings found in centrifugal fan performance tables and curves are based on standard cubic feet per minute (SCFM). Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per

cubic foot Cubic may refer to: Science and mathematics * Cube (algebra), "cubic" measurement * Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex ** Cubic crystal system, a crystal system ...

, with the

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

of 29.92 inches of

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

and a temperature of 70°F. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both

static pressure In fluid mechanics the term static pressure has several uses: * In the design and operation of aircraft, ''static pressure'' is the air pressure in the aircraft's static pressure system. * In fluid dynamics, many authors use the term ''static pres ...

and brake

horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the ...

. 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 The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...

. 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 The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...

, usually 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 70°F it will also move 1,000 CFM at 200°F. The air volume delivered by the centrifugal fan is not affected by density. However, since the 200°F air weighs much less than the 70°F air, the centrifugal fan will create less static pressure and will require less 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 0.8 (i.e., 0.8/ 1.0) to get the system to operate correctly. To get the actual horsepower at 200°F, the designer would have to multiply the brake horsepower at standard conditions by the air density correction factor.

Air Movement and Control Association fan rating

The centrifugal fan performance tables provide the fan

revolutions per minute Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

(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 A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicia ...

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,

electric power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions o ...

and

energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a ra ...

, 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.

References

External links

Xchanger Inc, webpage
Calculator for SCFM, NM3/hr, lb/hr, kg/hr, ACFM & M3/hr gas flows.

What is the difference between ACFM and SCFM?

ACFM versus SCFM versus ICFM

Temperature and Altitude Affect Fan SelectionACFM to SCFM vs ICFM CalculationsSCFM VS ACFMGood write up on the math
{{DEFAULTSORT:Cubic feet per minute Units of flow