What is the duct velocity (in FPM) in a 6 in. duct with an airflow of 100 cfm?

Question

I know the answer but I am trying to understand how to convert between CFM and FPM. Please explain and work this problem in an easy way for me to use.

Answer 1

To get from CFM to FPM you have to divide the CFM by the cross-sectional area of the duct.

For a circular duct, the cross-sectional area is like a circle, and the area is: pi * r-squared.

6 inches is 1/2 a foot
pi * (1/2)^2 = 3.141 * 1/4 = 0.785 square feet (a little over 3/4 square foot)

If the airflow is 100 CFM you divide that by the area to get FPM

100 / 0.785 = 127 FPM.


IF the duct is square, then the area is 1/2 * 1/2 or 1/4 square foot (0.25 square foot).
The FPM is 100 / 0.25 = 400 FPM.
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Answer 2

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RE:
What is the duct velocity (in FPM) in a 6 in. duct with an airflow of 100 cfm?
I know the answer but I am trying to understand how to convert between CFM and FPM. Please explain and work this problem in an easy way for me to use.

Answer 3

cfm=cubic feet/minute = ft^3/min
fpm = ft/min

to get fpm from cfm, you divide by the cross-sectional area of the duct at the point of interest. I assume that the duct is round with a diameter of 6".

fpm = cfm / area = 100/[(3.14)*(6/2/12)^2]

fpm = 100/(3.14*.25^2) = 509.3 ft/min

Answer 4

velocity= flow (CFM)/cross section area of duct in in^2*144
V= 100/(3.14/4*(6^2)*144= 503.5FPM if duct is circular

V=100/(6*6)*144 =400 FPM if duct is regular

Answer 5

You didn't specify if the duct was square or round. But the way you determine linear velocity is to divide volumetric flow by area of the duct. So cubict feet per minute divide by square feet equals feet per minute.