Diffusion pump
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Diffusion pumps are a type of vacuum pump designed to achieve better vacuum pressures than possible by use of mechanical pumps alone. They use a high speed jet of fluid to direct residual gas molecules in the pump throat down into the bottom of the pump and out the exhaust. The high speed jet is generated by boiling the fluid (typically silicone oil) and directing the vapor through a multistage jet assembly. Often several jets are used in series to enhance the pumping action. The outside of the diffusion pump is cooled using either air flow or a water line. As the vapor jet impacts the outer cooled shell of the diffusion pump the gas entrained in the jet flow coalesces, carrying the entrained pumped gases into the base of the pump where the gas pressure is increased and pumped by the secondary mechanical or rough pump from the diffusion pump outlet.
Unlike mechanical pumps, diffusion pumps have no moving parts and as a result are quite durable and reliable. They can function over pressures ranges of 10-8 to 1 pascals. They are driven only by convection and thus have a very low efficiency. Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 10 pascals.
One major disadvantage of diffusion pumps is the tendency to backstream oil into the vacuum chamber. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits. Due to backstreaming, diffusion pumps are not suitable for use with highly sensitive analytical equipment or other applications which require an extremely clean vacuum environment. Often cold traps and baffles are used to minimize backstreaming, although this results in some loss of pumping ability.
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[edit] Compressed-air vacuum pumps
One class of diffusion vacuum pumps is the multistage compressed-air driven ejector. It is very popular in applications where objects are moved around using suction cups and vacuum lines.
[edit] Steam ejectors
The steam ejector is a popular form of diffusion pump for vacuum distillation and freeze-drying. A jet of steam entrains the vapour that must be removed from the vacuum chamber. Steam ejectors can have a single or multiple stages, with and without condensers in between the stages.
