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Standard Cubic Feet Per Minute

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Standard Cubic Feet Per Minute (SCFM) is a term used to denote the volumetric flow rate of a gas (in the United States customary units) corrected to "standardized" conditions of temperature, pressure and relative humidity, thus representing a precise mass flow rate. However, great care must be taken, as the "standard" conditions vary between definitions and should therefore always be checked. Worldwide, the "standard" condition for pressure is variously defined as an absolute pressure of 101,325 pascals, 1.0 bar (i.e., 100,000 pascals), 14.73 psi, or 14.696 psi and the "standard" temperature is variously defined as 60 °F, 0 °C, 15°C, 20°C or 25°C. The relative humidity is also included in some definitions of standard conditions. There is, in fact, no universally accepted set of standard or normalized conditions (see Reference conditions of gas temperature and pressure).

In Europe and many other countries, the standard temperature is most commonly defined as 0°C (but not always). In the United States, the standard temperature is most commonly defined as 60°F or 70°F (but again not always). A variation in standard temperature can result in a significant volumetric variation for the same mass flow rate. For example, a mass flow rate of 1,000 kg/hr of air at 1 atmosphere of absolute pressure is 455 SCFM when defined at 0°C (32°F) but 481 SCFM when defined at 60°F (15.56°C).

To avoid confusion, it is necessary to clearly state the temperature and pressure reference conditions whenever expressing a gas volume or gas volumetric flow rate.

Relation of "standard" and "normal" conditions

In countries using the SI metric system of units, the term normal cubic metre (Nm3) is very often used to denote gas volumes at some standard or normalized condition. Again, as noted above, there is no universally accepted set of standard or normal conditions.

In the SI metric system of units, the molar volume (Vm) of any ideal gas is:

Vm = 22.414 m3/kmol at 0 °C and 101.325 kPa absolute pressure
Vm = 22.711 m3/kmol at 0 °C and 100 kPa absolute pressure

In the United States customary units, the molar volume of any ideal gas is:

Vm = 379.48 ft3/lb-mol at 60 °F and 14.696 psia
Vm = 378.61 ft3/lb-mol at 60 °F and 14.73 psia

Thus, comparing normal conditions defined as 0 °C and 101.325 kPa absolute pressure to standard conditions defined as 60 °F and 14.696 psia:

1 Nm3 = 37.325 SCF

and comparing normal conditions defined as 0 °C and 100 kPa absolute pressure to standard conditions defined as 60 °F and 14.73psia:

1 Nm3 = 36.725 SCF

There are many other such comparisons depending on what temperatures and pressures are used to define normal and standard conditions.

ACFM

ACFM (actual cubic feet per minute) is the volume of gas flowing anywhere in a system, independent of its temperature and pressure.

SCF and ACF (for any ideal gas) are related in accordance with the combined gas law:

For non-ideal gases, the following equation may be used:

where is the gas volume, is the absolute pressure, is the absolute temperature (i.e., either kelvins or degrees Rankine) and is the gas compressibility factor.

Defining any set of standard conditions (or reference conditions) by the subscript and actual conditions by the subscript , then for ideal gases:

and for non-ideal gases:

To be precise when the gas is air, then the above equations should include a correction for the difference between the relative humidity of the air at the standard and the actual temperature and pressure conditions.[1] In most cases of engineering design, the humidity correction for air is quite small and hence often ignored.

Other notational usage

In the SI system, the notation for 103 is k (kilo) and the symbol for 106 is M (mega).

However, when expressing large gas volumes in the United States, the notation for 103 is most commonly M (a thousand) and the notation for 106 is MM (a million).

References

  1. SCFM (Standard CFM) vs. ACFM (Actual CFM) (Specifically for air flows only)