Diffusion pump



Diffusion pumps use a high speed jet of vapor to direct gas momentum transfer pump. The diffusion pump is enjoying widespread use both in industrial and research applications.

Oil diffusion pumps

The oil diffusion pump is operated with an oil with low vapor pressure. Its purpose is to achieve higher vacuum (lower pressure) than possible by use of positive displacement pumps alone. Although its use has been mainly associated within the high vacuum range (down to 10-9 mbar), diffusion pumps today can produce pressures approaching 10-10 mbar when properly used with modern fluids and accessories. The features that make the diffusion pump attractive for high and ultra-high vacuum use are its high pumping speed for all gases and low cost per unit pumping speed when compared with other types of pump used in the same vacuum range. Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 0.1 mbar.

The high speed jet is generated by boiling the fluid and directing the vapor through a jet assembly. Note that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from molecular. 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 they are compressed to to ambient pressure by the secondary mechanical forepump and exhausted.

Unlike convection and thus have a very low energy efficiency.

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.

Steam ejectors

Main article: Injector

The steam ejector is a popular form of diffusion pump for vacuum condensers in between the stages.

Compressed-air ejectors

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.

References

  1. ^ GAEDE, 1915. Ann. der Phys., 46, 357.
  2. ^ D. G. Avery and R. Witty. Diffusion pumps: a critical discussion of existing theories. Proc. Phys. Soc. 59 (1947) p. 1016-1030. Accessed Oct 21, 2007 at IOP electronic journals

See also

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. 

 
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