PUMPS

In the picture is shown a turbomolceular pump placed on a mechanical pump. The turbomolecular pump is connected to the reaction chamber, but it requires also a mechanical pump. The turbomolecular can create very low pressures, but it cannot pump at atmospheric pressure, thus it requires a lower end pressure, which is insured by the mechanical pump. This combination is frequently used in Clean Rooms.

There are two types of pumps:

• pumps that physically remove the molecules from the chamber:
• mechanical pump (from 1 atm to 1 mTorrs) - it captures the gas, compresses it and expulses it outside; problems with oil contamination (oil is used for the moving parts, but it is not desired in a reaction chamber)
• turbomolecular pump (from 100 mTorr to 10-9 Torr) - transfers momentum from spinning turbines (very high speed -100000 rpm) to gas particles and the particles are easier removed
• oil diffusion pump (from 100 mTorr to 10-9 Torr) - transfers momentum from a jet of hot vapor oil to gas particles
• pumps that adsorb the molecules on a surface
  • cryopump (from 0.1 mTorr to 10-11 Torr) - molecules condense on a very cold surface (cooled with liquid He); when the surface is full, it is removed from the chamer and heated to let the gas particles evaporate