# SCFM (Standard CFM) vs. ACFM (Actual CFM)

Let the controversy begin!!

I’ve been in the air and gas handling business since 1982 and I think this topic causes the most confusion of any I’ve seen in this business.

I will plagiarize the Roots SCFM vs ACFM Guidebook to offer the correct calculation for air.

In specifying blower performance, major problems occur in distinguishing ACFM from SCFM, and in correctly converting from one to the other. Some people even use SCFM and ACFM interchangeably.

SCFM is normally used to designate flow in terms of some base or reference pressure, temperature and relative humidity. Many standards are used, the most common being the Compressed Air & Gas Institute (CAGI) standards, which are 14.5 PSIA, 68°F and 0% relative humidity (RH). This converts to a density of 0.074 lbs/cu.ft. for air.

SCFM is usually established from a weight flow corresponding to some system requirement for oxygen. Therefore, if actual site conditions are different from the standard or reference conditions, corrections must be made to reflect the actual conditions of pressure, temperature and relative humidity (i.e. convert to ACFM). Blower performance calculations, including head (used for centrifugal compressors) and horsepower, are based on actual (not standard) conditions existing at the inlet and outlet connections of the blower.

These corrections must, therefore, be made to assure that the blower furnished will provide the proper amount of oxygen or other elements for the process to function properly.

The formula below is strictly for ambient air, if another gas is required, additional considerations are required.

where:
PS = Standard pressure (PSIA)
PB = Atmospheric pressure – barometer (PSIA)
PA = Actual pressure (PSIA)
RHS = Standard relative humidity
RHA = Actual relative humidity
PVS = Saturated vapor pressure of water at standard temperature (PSI)1
PVA = Saturated vapor pressure of water at actual temperature (PSI)1
TS = Standard temperature (°R) NOTE: °R =°F+460
TA = Actual temperature (°R)1: See vapor pressure chart >

 Temp °F t Press PSIA p Temp °F t Press PSIA p Temp °F t Press PSIA p Temp °F t Press PSIA p Temp °F t Press PSIA p Temp °F t Press PSIA p 32 .08859 47 .15909 63 .2850 79 .4909 95 .8162 111 1.3133 32.018 .08866 48 .16520 64 .2952 80 .5073 96 .8416 112 1.3516 33 .09223 49 .17151 65 .3057 81 .5241 97 .8677 113 1.3909 34 .09601 50 .17803 66 .3165 82 .5414 98 .8945 114 1.4311 35 .09992 51 .18477 67 .3276 83 .5593 99 .9220 115 1.4723 36 .10397 52 .19173 68 .3391 84 .5776 100 .9503 116 1.5145 37 .10816 53 .19892 69 .3510 85 .5964 101 .9792 117 1.5578 38 .11250 54 .20635 70 .3632 86 .6158 102 1.0090 118 1.6021 39 .11700 55 .2140 71 .3758 87 .6357 103 1.0395 119 1.6475 40 .12166 56 .2219 72 .3887 88 .6562 104 1.0708 120 1.6940 41 .12648 57 .2301 73 .4021 89 .6772 105 1.1029 121 1.7417 42 .13146 58 .2386 74 .4158 90 .6988 106 1.1359 122 1.7904 43 .13662 59 .2473 75 .4300 91 .7211 107 1.1697 123 1.8404 44 .14196 60 .2563 76 .4446 92 .7439 108 1.2044 124 1.8915 45 .14748 61 .2655 77 .4596 93 .7674 109 1.2399 125 1.9438 46 .15319 62 .2751 78 .4750 94 .7914 110 1.2763 126 1.9974

Let’s put the equation to the test with the following criteria

Location:  Atlanta, GA
Elevation: 960 feet above sea level
SCFM:  1,000
Ambient Temperature:  80°F
Relative Humidity: 70%
Inlet Pressure Drop: 0.3 psi (due to filter and silencer)
Standard Conditions: CAGI Standards (14.5 psia, 0% RH and 68°F)

We can use our elevation table to get our barometric pressure which can be interpolated as 14.18 psia. Let’s plug these numbers into our equation:

Do the math and the answer is 1,095.86 acfm.

As you can see, if flow is not corrected for actual conditions, you would miss your requirement by 9.6%. This would be a greater miss if all parameters stayed the same but we assume a 100°F. Based on a 100°F day, the flow would be 1,136 acfm.

A more detailed calculation is available in our Tech Talk Article titled “Volume and Mass Flow Calculations for Gases.” This article shows how you can convert mass flow to either SCFM or ACFM and volume into mass flow.