Airflow Based on SP and Duct Size

[Tigerdawg]

It's been years since I've done any fluid analysis in depth. Is there a quick and dirty method to estimating SCFM when you have the SP and the duct size? I'm dealing with SP's in the range of 15-50 inches of water and duct sizes from 4-18 inches round.

Barring a quick estimate, about how close can you come if you neglect compressibility effects?

 

[lilliput1]

You can use the following formula for duct friction:

Pd in inches wg/100 ft of duct = (0.109136 x (CFM)^1.9)/ (d)^5.02

Where d = equivalent duct diameter in inches

For rectangular duct with side 1 = a & side 2 = b

d = 1.30 x ((a x b)^0.625) / (a + b)^0.25)

Thus for 2000 CFM in 24" x 12" duct:
d = 18.2805

Pd = 0.0944 in wg /100 ft of duct

This formula is a bit different from that in the Carrier manual but it fits the Chart values in SMACNA better.