Moody Diagram
Useful diagram for estimating the relative duct roughness factor in calculating pressure losses in ducts.
It uses the Reynolds Number (Re) a dimensionless number which is obtained from
where 
is the density of the air at temp and pressures prevailing and 
is the Viscosity of the air. V is the velocity in the duct, d is the diameter of the duct.
Almost all LEV systems run with the Re above 2000 ie in the turbulent zone.
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