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How to Convert SCFM to ICFM

Discussions concerning pressurized air systems such as air compressors deal in terms of actual air compressor capacity (ACFM), standard air capacity (SCFM) and inlet air capacity (ICFM). The SCFM measures the amount of compressed air consumed. The standard conditions that apply for SCFM are those accepted for sea level conditions, which are zero relative humidity, 60 degrees Farenheit and 14.7 psia. The ACFM convert SCFM to real-world conditions and depend on the relative humidity, pressure and temperature of a location. As temperature and pressure change, so does the value of ACFM. ICFM is a measure of air compressor capacity that compressor vendors use for their products. This value measures the air conditions prior to entry into any equipment such as filters, boosters or blowers.

Things You'll Need

Barometer
Humidity Gauge
Thermometer
Air Pressure Reference
SCFM of system
Calculator

Instructions

- 1
  Use a barometer to measure the ambient air pressure and the pressure after the inlet. The units reported on a barometer for air pressure are mmHg or torr. Convert the measurement to a value with units that are required for the calculation, which is psia. By definition, 14.7 psia = 760 mmhg = 760 torr. Divide the barometer reading by the conversion factor of 760 / 14.7 = 51.7 to yield the air pressure in psia.
- 2
  Use a thermometer to measure the ambient temperature and the temperature after the inlet in degrees Fahrenheit. Convert the temperature to degrees R, reference to the point of absolute zero, which has a value of -460 degrees Fahrenheit. The temperature of 0 degrees Fahrenheit on the R scale is 460 degrees R. Add 460 to the reading of the thermometer in degrees F to yield the ambient temperature in degrees R.
- 3
  Use a humidity gauge to acquire the ambient humidity and look up the saturation pressure in an air pressure reference. Saturation pressure is dependent on humidity, temperature and air pressure.
- 4
  Convert your value of SCFM for the application of interest to ACFM. Use the following equation to solve for ACFM, ACFM = SCFM * [ Pstd / (Pact -- (Psat * phi))] / (Tact / Tstd). The variable are: Pstd = standard absolute air pressure (14.7 psia), Pact = absolute pressure at the actual level (psia), Psat = saturation pressure at actual temperature (psi), phi = relative humidity, Tact = ambient temperature (degrees R) and Tstd = standard temperature (520 degrees R). For example, assume that the SCFM is 100, the air pressure is 12.23 psia, saturation pressure is 0.5069 psia, relative humidity is 80 %, and actual temperature is 540 degrees R. Insert the values into the equations and calculate. ACFM = SCFM * [ Pstd / (Pact -- (Psat * phi))] * (Tact / Tstd) = 100 * [14.7 / (12.23 -- (0.5069 * 0.80))] * (540 / 520) = 100 * (14.7 / (12.23 -- 0.4055)) * 1.038 = 100 * (14.7 / 11.82) * 1.038 = 129.1. By definition of ACFM, the units must be in feet per minute.
- 5
  Convert ACFM to ICFM using the following formula, ACFM = ICFM (Pact / Pf) (Tf / Tact). Continuing the example, assume ACFM = 129.1, Pf = pressure after the auxiliary equipment (10.5 psia) and Tf = temperature after the auxiliary equipment (542 degrees R). All other variables have the values used previously. Rearrange the formula and solve for ICFM. ICFM = ACFM * (Tact / Tf) * (Pf / Pact). Insert the value of the variables and find the value of ICFM. ICFM = 129.1 * (542 / 540) * (12.23 / 10.5) = 129.1 * 1.004 * 1.165 = 151.0 feet per minute.

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