Formula Colebrook-White is used to evaluate the coefficient of friction losses

For all values of Reynolds number

l is the coefficient losses regular

Re is the Reynolds number

e = roughness (average size of features on the wall)

D is the diameter of duct

MECAFLUX uses this equation for R e > 10 5

Colebrook' diagram (Moody) giving the coefficient friction pressure loss (left ordinate), according Reynolds number (abscissa) and relative roughness (right ordinate)