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FF = [1 / (-2 x LOG((12RF/(3.7 x Dia)) + 2.51/(Re x FF0.5)))]2

Where:

 

 FF        = Friction factor
 LOG        = Stands for "Log to the base 10"
 RF        = Material roughness factor, ft.
 Dia        = Duct diameter, in.
 Re        = Reynolds number

 

Ref:        ASHRAE 1985 Fund. - Equation 22, Page 33.4