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

Let the controversy begin!!
409 RAM
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.

SCFM TO ACFM


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: 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:

ACFM FOR 1000 SCFM

Do the math and the answer is 1,101.7 acfm.

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 1,169 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.

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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