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Flow resistance for standard flexible duct?

Flow resistance for standard flexible duct?

Flow resistance for standard flexible duct?

(OP)
I have this Wheel-O-Duct sizing and I need to know what the "in-H20" I should use.  I am using standard 8,9,10,12,16, and 20" flexible duct sizes.  Should I use .08?  I know to shoot for 700 fpm velocity.

Thanks!

RE: Flow resistance for standard flexible duct?

The SMACNA HVAC Systems Duct Design Manual 1990 Fig 14-3 shows a chart of correction factors with velocity for various duct diameter (5" to 100") & with new vs medium rough vs rough with flexible ductwork classified as rough.
At 700 FPM, correction factors (apply to new duct in.wg pd/100 ft)are:
5" dia = 2.08
8" dia= 1.95
12" dia = 1.85
20" dia = 1.78
40" dia = 1.74
At 2000 FPM,correction factor for 12" dia = 1.94.

RE: Flow resistance for standard flexible duct?

(OP)
Thanks very much.  What's the adjustment if I am using, say, only 25 ft?  Would it simply be 1/4 of the 100ft length?

RE: Flow resistance for standard flexible duct?

You should use equivalent length, allowing for elbows. For 90 degree smooth elbow equv length = 9 X D with Diameter D & Length L in Ft. Don't forget pressure drop of equipment, terminal devices, entrance loss, exit loss.

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