Diffusion pump
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Diffusion pumps are a type of vacuum pump designed to achieve higher vacuum (lower pressure) than possible by use of mechanical pumps alone. They use a high speed jet of vapor 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 a low vapor pressure 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 working fluid condenses, is recovered, and directed back to the boiler. The pumped gases continue flowing to the base of the pump at increased pressure, flowing out through the diffusion pump outlet, where the pressure is increased to ambient, and exhausted by the secondary mechanical "rough pump". Note that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from laminar, to supersonic and molecular.
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.
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[edit] Further reading
Hablanian, M. H. [1983] (1994). Diffusion Pumps : Performance and Operation, 2nd ed., AVS Monograph Series, New York, NY: American Vacuum Society. ISBN 1-56396-384-1.
