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.