Actual cubic feet per minute is a unit of volumetric capacity. It is commonly
used by manufacturers of blowers and compressors. This is the actual gas
delivery with reference to inlet conditions, whereas cubic foot per
minute 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 or other gas at the inlet to the
unit, it is often referred to as "inlet cubic feet per minute".
Actual cubic feet per minute 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 Standard cubic feet per
minute. However, this usually is not the case as the most important change
between these two definitions is the pressure. To move air, either 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 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 is ambiguous when it comes to the mass of
gas that passes through a certain point because gas is compressible. 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. 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. 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 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
60°F and relative humidity 0%, but the conditions may vary depending on the
"standard" used.. Normal cubic feet per minute
Normal cubic feet per minute refers to air at normal conditions, such as 14.7
psia, 68°F, 36% relative humidity. However, there is no agreed upon
definition for what constitutes normal conditions.
Free air delivery Free air delivery 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. 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°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, usually sea level,
and higher than standard temperature, air density is lower than standard
density. 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 1.25 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 fan rating
The centrifugal fan performance tables provide the fan revolutions per minute
and brake horsepower requirements for the given CFM and static pressure at
standard air density. 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 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, electric power 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. External links
Xchanger Inc, webpage Calculator for SCFM, NM3/hr, lb/hr, kg/hr, ACFM & M3/hr
gas flows. ACFM versus SCFM for ASME AG-1 HEPA
Filters SCFM versus ACFM
SCFM versus ACFM What is SCFM?
SCFM, ACFM, ICFM...What Does it Mean? What is the difference between ACFM and
SCFM? SCFM vs. ACFM
SCFM vs ACFM Gas Density
Properties of Atmosphere ACFM versus SCFM versus ICFM
Temperature and Altitude Affect Fan Selection
ACFM to SCFM vs ICFM Calculations SCFM VS ACFM
Good write up on the math See also
Cubic feet per minute Standard cubic feet per minute
Standard cubic foot Million standard cubic feet per day
Gas compressor Centrifugal fan
Combined gas law