Flow resistance for standard flexible duct?

[HighPotential]

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!

 

[lilliput1]

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&quot & 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.

 

[HighPotential]

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?

 

[lilliput1]

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.