- 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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