Molecular drag pumps

Molecular drag pumps vary from turbo pumps in that the momentum is transferred to the gas molecules not by blades but by a rapidly rotating solid surface. The stator is, in this case, a fixed surface very close to the rotor (see Fig. 1.19). 

The molecules being pumped are dragged along the surface, hence the term molecular drag pump. These surfaces can take the form of a rotating spiralled drum (Holweck) or slotted rotor discs (Gaede). 

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Fig. 1.19. Cross-section of a molecular drag pump (courtesy of Alcatel). 

A further reason to employ UHV techniques is the high ionization and fragmentation cross sections for hydrocarbons H, C, CH, etc ion peaks can be particularly troublesome at the higher photon energies. One should therefore use well-trapped turbomolecular pumps or turbomolecular pumps with magnetic levitation, backed by a molecular drag pump, which itself is backed by an oil-free diaphragm pump. 

The molecular pump functions in a fashion analogous to a diffusion pump. Instead of a molecule being diffused to the pump and then given a preferred direction by a momentum transfer from an oil vapour stream, the principle of the molecular drag pump is based on the directional velocity imparted to gss molecules which strike a fast moving surface. The overlapping of the directional velocity component with the thermal velocity component determines the overall velocity and the direction in which the particles are thrust away. If there is a second surface opposite the first, this process is repeated there. The undirected thermal movement of the gas molecules prior to collision with the moving surface is thus turned into a directed movement of the gas stream. 

When gas flows over a surface there is frictional drag on the surface, which produces a velocity gradient near the surface. This frictional drag reduces the flow of fluids on the surface in a direction counter to the gas flow (wall creep). This frictional drag is also used in the molecular drag pump to give gas molecules a directional flow. 

The molecular drag pump uses a high velocity surface to drag the gas in a given direction. The molecular drag element can be in the form of a disk (Gaede-type) or a cylinder with a... 

In many instances its use is being supplanted by the superclean combination of a hybrid turbomolecular/molecular drag pump backed by a diaphragm pump. 

Frictional drag (vacuum technology) The deceleration force applied to a moving surface by a gaseous environment in contact with the surface. See also Molecular drag pump. 

Molecular drag pump (vacuum technology) A kinetic vacuum pump in which velocity is imparted to the gas molecule by contact with a high velocity surface. See also Vacuum pump. 

Pump package (vacuum technology) A combination of pumps designed to work together in sequence. Examples Diffusion pump, roots blower, oil-sealed mechanical vane pump turbopump, molecular-drag pump, diaphragm pump. Also called a pumping stack. 

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