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Subject: Vacuum pumps

Vacuum pumps

From: Walter Henry <whenry>
Date: Sunday, April 12, 1992
The following appeared in sci.chem and is reposted here without the
knowledge or consent of the authors.

  Article: 6441 of sci.chem
  From: mcdonald [at] aries__scs__uiuc__edu (Doug McDonald)
  Subject: High vacuum questions
  Date: 9 Apr 92

  In article <8APR92.17150106 [at] uwpg02__uwinnipeg__ca>
    chem [at] uwpg02__uwinnipeg__ca writes:
  >Hi. What are some methods used by chemists to create high vacuums? If
  >a vacuum is created in a glass container, wouldn't the atmospheric
  >pressure (external) smash the glass ? Finally, how does a vacuum pump
  >work ? Thanks for any info. MC
  >Internet:  CHEM [at] UWPG02__UWINNIPEG__CA

  What a question!!!!

  "Low vacuum" is created by sucking with a mechanical vacuum pump or,
  amazingly enough, condensing air out in the pores of a zeolite by
  cooling it with liquid nitrogen. This gets you to about 1/1000 or
  1/100,000 atmosphere.

  Mechanical vacuum pumps work in the obvious mechanical manners -
  various valves and pistons or rotors. The common ones work more or
  less like car motors - some quite like the rotary RX7 in reverse.
  There are also some that work like aquarium air pumps in reverse.

  To get high vacuum (10e-9 atmosphere down to 10e-15 atmosphere) there
  are many many different methods.

  1) diffusion pumps -- these have a boiler which boils oil (or mercury)
     and sends it up a tube at the top of which a sort of "hat" (that is
     the technical term :-) )  turns the flow dowm. The oil molecules
     hit the air molecules and push them down and out the bottom into
     the line to the mechanical pump. The oil then hits a cold wall,
     condenses, and flows back into the boiler.

  2) Turbo pumps - several fans in series, running at 40,000 to 100,000

  3) Ion pumps - powerful electric and magnetic fields create a thin
     plasma that ionizes molecules which hit walls of, usually,
     titanium, sputtering out Ti atoms, which hit the ions and
     eventually drive them to the walls, where more Ti atoms bury them.
     Oxygen and nitrogen and many organic molecules also directly react
     with the fresh Ti coated walls and form involatile compounds.

  4) Ti sublimation pumps - Ti is boiled off a filiament and works like
     the latter part of 3).

  5) Cryopumps - things stick to cold surfaces, either liquid nitrogen,
     liquid helium, or a surface cooled to about 12K by a helium
     refrigerator. These are helped along by using zeolites - the vapor
     pressure of something stuck in their pores is lower than that of
     the bulk material.

  As to smashing glass containers, consider light bulbs or CRTs!!!
  Ordinary round bottom flasks are specifically rated to take a high
  vacuum - organic chemists use them for vacuum distillations!

  Doug McDonald

                  Conservation DistList Instance 5:49
                 Distributed: Wednesday, April 15, 1992
                        Message Id: cdl-5-49-010
Received on Sunday, 12 April, 1992

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