Where the line contains elbows or other curves (such as in right-angle valves), these can be taken into account by assuming a greater effective length l_(eff) of the line. This can be estimated as follows:
l(eff) = l_(axial) + 1.33 ⋅ Θ/180° ⋅ d
Where
l_(axial) : axial length of the line (in cm)
l_(eff) : Effective length of the line (in cm)
d : Inside diameter of the line (in cm)
θ : Angle of the elbow (degrees of angle)
Axial length
The technical data in the Leybold catalog states the conductance values for vapor barriers, cold traps, adsorption traps and valves for the molecular flow range. At higher pressures, e.g. in the Knudsen and laminar flow ranges, valves will have about the same conductance values as pipes of
corresponding nominal diameters and axial lengths. In regard to right-angle valves the conductance calculation for an elbow must be applied.
In the case of dust filters which are used to protect gas ballast pumps and roots pumps, the percentage restriction value for the various pressure levels are listed in the catalog. Other components, namely the condensate separators and condensers, are designed so that they will not reduce pumping speed to any appreciable extent.
The following may be used as a rule of thumb for dimensioning vacuum lines: The lines should be as short and as wide as possible. They must exhibit at least the same cross-section as the intake port at the pump. If particular circumstances prevent shortening the suction line, then it is
advisable, whenever this is justifiable from the engineering and economic points of view, to include a roots pump in the suction line. This then acts as a gas entrainment pump which reduces line impedance.