Quantitative Description of the Pumping Process

If the duct is assumed smooth, round, straight and sufficiently long (L d), the conductance for the viscous flows can be expressed as 

It is important to note that in the molecular flow regime the conductance depends on the cube of the diameter of the tube and the -1/2 power of the mole mass of the gas being pumped, but the conductance is independent of the pressure. 

For the parallel connection of the ducts, the total conductance (Q is the sum of all individual conductances and is given by [9] 

Because of the resistance to gas flow in the connection between a vacuum chamber and the pump, the effective pump volume speed (S) at the chamber is usually less than the volume flow rate of the pump (Sp) at its inlets. The relationship between S and Sp is given by [3] 

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