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Converting scfm to acfm Equations and Calculator

Fluids Systems Design and Engineering

Flow Rate Measurement scfm (Standard Cubic Feet per Minute) and acfm (Actual Cubic Feet per Minute) Calculator

Converting scfm to acfm Equations and Calculator

The two types of flow rate measurement are mass flow measured by weight, often used in chemical process industries, and volumetric flow measured in cubic feet per minute (cfm) (liters per minute).

The flow rate measurement for vacuum is exactly the opposite of that for compressed air. Using ambient, standard free air at atmospheric pressure (referred to as standard cubic feet per minute [scfm] as a common reference, the ambient free air entering the vacuum pipe will expand to fill the vacuum in the piping system, creating a lower vacuum pressure. Standard air is dry (0 percent relative humidity), 14.7 psia (101.4 kPa), and 60°F (15.5°C).

The flow in a vacuum pipe is expanded cfm and represents the flow under actual vacuum conditions, referred to as actual cubic feet per minute (acfm), which is greater than scfm. Another phrase used to indicate acfm is inlet cubic feet per minute (icfm).

Converting scfm to acfm:

The following formula is used to convert scfm to acfm.

acfm = scfm · ( P_s / P )] · ( T + 459.67 ) / T_s

or

acfm = scfm ·E_ar · ( T + 459.67 ) / T_s

scfm = acfm / ( E_ar · ( T + 459.67 ) / T_s ) )

where:

P = Actual pressure for the scfm being converted, inches of mercury (In. Hg)
P_s = Standard absolute pressure (29.92 In. Hg)
T = Actual temperature for the scfm being converted, °F
T_s = Standard Air Temperture °R 520
E_ar = Expanded Air Ratio, see Table 1
459.67 = Conversion from °F to °R
acfm = Actual Cubic Feet per Minute
scfm = Standard Cubic Feet per Minute

For practical purposes, a numerical method for solving the above equation can be used if the temperature is 60°F (15.5°C). At that temperature, the second part of the equation becomes
unity. Table 1 gives the numerical values for 29.92/P. To find acfm, multiply the scfm by the value found in the table opposite the vacuum pressure (P).

A direct ratio for converting scfm to acfm (sL/s to aL/s) for various pressures is given in table 2. Multiply the scfm (sL/s) by the factor corresponding to the pressure in inches
of mercury abs (kPa abs) to find acfm (aL/s).

Expanded Air Ratio (E_ar), 29.92/P as a Function of Pressure, P (in. Hg)
Table 1

Direct Ratio for Converting scfm to acfm (sL/s to aL/s)
Expanded Air Pressure Ratio, 29.92 / P
as a Function of Pressure, P (in, Hg)

P	29.92 / P	P	29.92 / P
29.92	1.00	10.92	2.7399
28.92	1.0346	9.92	3.0161
27.92	1.0716	8.92	3.3542
26.92	1.1114	7.92	3.7777
25.92	1.1543	6.92	4.3236
24.92	1.2006	5.92	5.0540
23.92	1.2508	4.92	6.0813
22.92	1.3054	3.92	7.6326
21.92	1.3650	2.92	10.2465
20.92	1.4302	1.92	15.5833
19.92	1.5020	0.92	32.5217
18.92	1.5814	0.80	37.40
17.92	1.6696	0.70	42.0742
16.92	1.7683	0.60	49.8667
15.92	1.8794	0.50	59.84
14.92	2.0054	0.40	74.80
13.92	2.1494	0.30	99.7334
12.92	2.3158	0.20	149.60
11.92	2.5101	0.10	299.20

Direct Ratio for Converting scfm to acfm (sL/a to aL/s )
Table 2

Table 10-5 Factor for Flow Rate Reduction Due to Altitude

In. Hg	kPa Abs.	Factor
1	97.9	1.05
2	94.5	1.1
3	91.15	1.1
4	87.77	1.15
5	84.39	1.2
6	81.01	1.25
7	77.63	1.3
8	74.19	1.35
9	70.81	1.4
10	67.43	1.5
11	64.05	1.55
12	60.67	1.62
13	57.29	1.75
14	53.91	1.85
15	50.54	2.0
16	47.09	2.15
17	43.71	2.3
18	40.33	2.5
19	36.95	2.73
20	33.57	3
21	30.20	3.33
22	26.82	3.75
23	23.37	4.28
24	19.99	5
25	16.61	6
26	13.23	7.5
27	9.85	10
28	6.48	15
29	3.10	30
29.92	0	60

Reference: ASPE