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) and the American Society of Mechanical Engineers (ASME) standards, which are 14.7 PSIA, 68°F and 36% relative humidity (RH). This converts to a density of 0.075 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:

P

P

P

RH

RH

PV

PV

T

T

Temp°Ft |
PressPSIAp |
Temp°Ft |
PressPSIAp |
Temp°Ft |
PressPSIAp |
Temp°Ft |
PressPSIAp |
Temp°Ft |
PressPSIAp |
Temp°Ft |
PressPSIAp |

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: | 1,050 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 (14.7 psia, 36% 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

As you can see, if flow is not corrected for actual conditions, you would miss your requirement by 10.2%. 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

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.

If you would like a printable version of this article, please click here.

If you would like us to convert a flow or verify a selection, please click here to send us an email.

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