Carrier handbook friction loss

[alex1987]

Hi,

I found this formula in Carrier Handbook,
delta p = 0.03*f*(L/d^1.22)*(V/1000)^1.82

delta p = friction loss (in. wg)
f = interior surface roughness (0.9 for galvanized duct)
L = length of duct (ft)
d = duct diameter (in.), equivalent diameter for rectangular ductwork
V = air velocity (fpm)

Here is the problem, I could not find the interior surface roughness, f for concrete duct at anywhere else. May I know where does this formula derived from? I only can find Darcy, Colebrook, and Altshul-Tsal Equations on the internet. I need the interior surface roughness, f for concrete duct.

Thanks

[Williamsma]

Here is what I know

1. smooth cement 1

2. ordinary concrete 1-3.33

3. coarse concrete 1-16.7

This info is posted on the the engineering tool box web site under roughness and surface coefficients of ventilation ducts.

[alex1987]

Thanks but this absolute roughness cannot be used in this formula as Carrier had simplified it. The roughness of galvanized duct given by Carrier (0.9) is different from the table in engineeringtoolbox (0.0005). I just wonder if anyone has the list of roughness from Carrier which can be applied into this formula

[bobberger1084]

Concrete duct
We had a 40% loss in a brick shaft for a bathroom exhaust system. we had an air balancing company use a balometer that was the result of thier report.
The engineer selected a fan at .25. As it turns out the measured staic pressure at the measured air flow was 1.66"