Pressure drop in empty tubes

Normally, the pressure drop in a tubular reactor is small compared with the operating pressure and in this case, the fluid can be treated as incompressible. Then the pressure drop in a tube of length L and diameter d is given by 

At high Reynolds numbers (Re 2500), the surface roughness is an important parameter and must be allowed for in the calculations. Friction factor charts [53] include curves relating to various values of the relative roughness, that is the ratio of the mean height of surface roughness to the tube diameter. 

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