Sergei_Eisenstein
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Registered: 13-12-2004
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Hybrid vacuum pumps
I may be able to acquire a second hand hybrid vacuum pump. These pumps combine a classical rotary vane with a diaphragm vacuum pump. I have been told
you can use these pumps without a cold trap, without having to change the oil every other day and running the risk of corroding the rotary vane pump
(the diaphragm pump stops most of the vapours). I would appreciate it if somebody with experience with this type of pumps could comment on the
practical side of it. I'm a confused man and no cold traps would be just great 
Brandtech RC6: http://www.brandtech.com/prodpage.asp?prodid=RC6%20Chemistry...
damnant quod non intelligunt
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hinz
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I wouldn't trust the membrane/diaphragm pump to stop all vapors, because as I understand it , the pump is basically a double stage one, first the
diaphragm and then the rotary vane pump. Since the pump can achieve 4x10-4 mbar and membrane pumps can normally achieve only about 150 mbar due the
dead volume in the pump, the rotary vane pump might pull the the vapors out of the membrane pump.
Maybe they're using some other traps like activated carbon, pottasium hydroxide for acids or sillica gel for humidity, but I don't think so.
But if you need a really resistant pump, there are some PTFE membrane pumps. For vacuum destillation or vacuum filtration 10-150 mbar should be enough
in most cases.
http://www.buchi-analytical.com/Vacuum-Pump-V-1000.1874.0.ht...
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Sergei_Eisenstein
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Perhaps I might have been a bit too optimistic... The membrane pump should minimize contact between the rotary vane pump's oil and solvent
vapors. As I understood it, there should be a seperate collector for the vapors as well. My old pump died, sadly enough, after 14 years of semper
fidelis. I understood the hybrid pump mechanism as "no cold trap needed", perhaps because of my wishful thinking.
damnant quod non intelligunt
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hinz
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I've read once aggain the description, I missunderstood something, the membrane pump is behind the rotary vane pump and holds the oil
of the rotary vane pump under vacuum, so less corrosive liquid is absorbed in the oil. I don't know how senseful this is, because high boiling
(corrosive) liquids might be sucked into the first pump but the membrane pump doesn't have an as high vacuum as the rotary vane pump, so the vapor
pressure of the liquid isn't reached aggain and the liquid is trapped inthe rotary vane pump anyway.
For really nasty liquids, it might bestill the best to use a good old glass water jet pump
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