A vacuum pump is a device for creating, improving and/or maintaining a vacuum (an environment in which the pressure is below atmospheric pressure). Two basically distinct categories of vacuum pump may be considered: Gas Transfer Pumps and entrapment or capture pumps (Figure 1).
Gas transfer pumps can be subdivided into positive displacement pumps and kinetic pumps.
A positive displacement vacuum pump is a pump in which a volume filled with gas is cyclically isolated from the inlet, the gas being then transferred to an outlet. In most types of positive displacement pumps the gas is compressed before the discharge at the outlet. Two categories can be considered: reciprocating positive displacement pumps (e.g., piston pump) and rotary positive displacement pumps (e.g., liquid ring pump and sliding vane rotary pump).
Here an eccentrically placed rotor is made to turn tangentially to the fixed surface of the stator. Normally, two vanes slide in slots in the rotor and contact the internal wall of the stator. The rotating mechanism isolates the gas from the inlet, compresses and then expels it through an outlet valve.
A kinetic vacuum pump is a pump in which a momentum is imparted to the gas or the molecules in such a way that the gas is transferred continuously from the inlet to the outlet. Two categories can be considered: fluid entrainment pumps (e.g., vapor diffusion pump) and drag vacuum pumps (e.g., turbomolecular pump).
Here gas transport is achieved by a series of high velocity vapor jets (normally oil vapor is used) emerging from an assembly within the pump body. In normal operation a portion of any gas arriving at the inlet jet is entrained, compressed and transferred to the next stage.
This pump contains a rotor with inclined blades moving at high speed between corresponding blades in a stator. Gas molecules entering the inlet port acquire a velocity and preferred direction superimposed on their thermal velocity by repeated collisions with the fast moving rotor. Rotational speeds for small pumps can be up to 90,000 rev min−1.
A vacuum pump in which the molecules are retained by sorption; chemical combination or condensation on internal surfaces within the pump.
This makes use of the gettering principle, in which a cathode material (usually titanium) is vaporized, or sputtered, by bombardment with high velocity ions. The active gases are pumped by chemical combination with the sputtered titanium, the inert gases by ionization and burial in the cathode, and the light gases by diffusion into the cathode.
Operation is achieved by the condensation, freezing and/or sorption of gas at surfaces maintained at extremely low temperatures, thus removing them from the gas phase in the vacuum system.
International Standards Organization, Document Ref. 1SO3529/2-1981 Vacuum technology—Vocabulary—Part 2: Vacuum pumps and related terms.
Harris, N. S. (1989) Modern Vacuum Practice, McGraw-Hill, New York.
Harris, N. S. (1987) Vacuum and present-day problems for vacuum pumps, Phys. Bull, 38, 224-226.