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Calculate Length of Restrictions
The volumetric flow through an empty pipe is determined by the Hagen-Poiseuille law:
F = volumetric flow
ID = internal diameter of the pipe
ΔP = pressure drop between inlet and outlet
L = the pipe length
η = the dynamic viscosity of the gas
Flow and pressure drops are dependent on length and diameter of the pipe.
If one pipe should be replaced by another one with the same "flow resistance", then the pipe flow and pressure drop have to be the same as well as the gas.
That means pipes of different dimensions are equal, if ID4/L = const. and the length of the second pipe equals the length of the first one times the ratio of the squares of the diameters: L2 = L1 · ID24 / ID14