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.