Monday, April 23, 2012

Gas Transfer Pumps

(OLD)

Vacuum pumps may be divided into two broad categories: gas transfer pumps and entrapment pumps. Gas

transfer pumps remove the gas molecules from the pumped volume and convey them to the ambient in one or

more stages compression. Gas transfer pumps remove gas permanently.

Gas trensfer pumps may be further subdivided into positive - displacement and kinetic vacuum pumps. Rotary

mechanical and Roots pumps are important examples of the positive - displacement variety. Diffusion and

turbomolecular pumps are the outstanding examples of kinetic vacuum pumps. Among the entrapment pumps

commonly employed are the adsorption, sputter-ion, and cryogenic pumps. Each pump is used singly or in

combinations. Pumps do not remove the gas molecules by exerting an attractive pull on them. Yhe moleculesare

unaware that pumps exist. Rather, the action of pumps is to limit, interfere with, or alter natural molecular motion.

We start this brief survey of some of the more important pumps with the positive - displacement types.

(M., Ohring, The Materials Science of Thin Films, p.62)

(NEW / BETTER)

The gas transfer pumps transfer the gas from inside of the vacuum system and exhaust it to outside. The entrapment pumps trap the gas within the vacuum system by freesing the gas on a cold surface, physiochemically trapping it in a porous solid such as graphite, or chemically reacting it with a solid to reduce the vapor pressure to some insignificant level. The entrapped gas will eventually saturate or fill the capacity of its trap and the system will have to be emptied or regenerated.

(Ronald R. Willey, Practical Production of Optical Thin Films, p.11 )