Well, if I were still need to use this equation to calculate the friction loss, do you know what is the interior surface roughness? I don't think your 3.0mm rough finish is suitable for this equation, please correct me if I'm wrong.
Thanks
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...
