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

volume Volume is a measure of regions in 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) ...

tric flow. It is commonly used by manufacturers of blowers and compressors.CAGI Performance Verification Data Sheets
. 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. Since the volumetric capacity refers to the volume of

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

or other

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

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 ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

. 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 conditio ...

(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 eve ...

. To move air, either a positive pressure or a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

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 and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

of gas that passes through a certain point because gas is compressible. 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 ...

, 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 perpendicular to the axis of rotation of the fan. Centrifugal fans often contain a ducted fan, ducted housing to direct outgoing air in a specific direction or ...

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) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...

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 The cubic foot (symbol ft3 or cu ft)
, . is an atmospheric pressure Atmospheric pressure, also known as air pressure or 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 1,013. ...

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

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 In fluid mechanics the term static pressure refers to a term in Bernoulli's equation written words as ''static pressure + dynamic pressure = total pressure''. Since pressure measurements at any single point in a fluid always give the 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 t ...

. 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 at a given point and time. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmosph ...

. 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 (geography), ''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 equipotenti ...

, 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 r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 de ...

(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 science, scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as s ...

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 of transfer of electrical energy within a electric circuit, circuit. Its SI unit is the watt, the general unit of power (physics), power, defined as one joule per second. Standard prefixes apply to watts as with oth ...

and energy 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.

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