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Re = (5 x Dia x Den x Vel) / 0.0438

Where:

 

 Re        = Reynolds number
 Dia        = Duct diameter, in.
 Den        = Air density, lb./cu. ft.
 Vel        = Air velocity, fpm
 0.0438        = Inverse of fluid dynamic viscosity, ft-h/lbm

 

Ref:        ASHRAE 1985 Fund. - Equation 22, Page 3.4