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Airflow Based on SP and Duct Size

Airflow Based on SP and Duct Size

Airflow Based on SP and Duct Size

(OP)
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?

RE: Airflow Based on SP and Duct Size

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.

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