Flip
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2.5 CFM Vacuum pump vs. 1.5 CFM
So which has the better functionality in distillation? I know that 1.5 would suit my needs, but there are 2.5's on the market for
cheaper. I was curious whether 2.5's are a pain in the ass to deal with, low boiling solvents just spontaneously dissapearing into thin air and the
like : ) Seriously though, I had some boil overs with the 1.5 so i'm wondering if a 2.5 is still manageable.
I was also wondering if anyone has had experience with implosions of roundbottoms and the like while operating a 2.5 cfm vacuum pump.
I always used to use a 1.5 cfm, but the old girl quit on me, and i've found a 2.5 for cheaper. I am hesistant, however, because the money I could
save wouldn't be worth the price of the glass that I could lose.
Also, anyone had experience with Gast vacuum pumps?... I'm not sure what most of them are rated.
My last question would have to do with the volume of the system to which the vacuum is applied. Not on the assumption that the system is perfectly
sealed, but accounting for the loss of vacuum that can occur in practice, does the size of the system generally have a noticeable impact on
the boiling point of a liquid? What kind of differences have you guys noticed, if any, for the same vacuum applied to the same liquids in
smaller/larger flasks and systems utilizing HVAC pumps as the vacuum source?
Thanks guys,
Flip
[Edited on 7/6/06 by Flip]
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Oxydro
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Why not just put a valve on the vacuum line, then you can choke the pump if 2.5 cfm is too much? For tthe glassware, it's the ultimate vacuum, not the
volume moved, that's important, so just put a gauge on it and make sure you don't let it get too low.
However, if a flask is able to take moderate vacuum, it's likely to take a hard vacuum. For instance, say you have a 100 mbar (absolute, that is)
vacuum. That's around 90% of a complete vacuum, and the remainder is unlikely to cause a flask that has lasted that long to fail.
I think that a larger flask would tend to boil slightly lower, because there is more space for nucleation (sp??) points and so it's more likely to
boil as soon as it reaches the boiling point. That though is just a guess and I have no experience etc to back that up, except that I've found test
tubes tend to superheat and boil violently.
"Our interest's on the dangerous side of things" -- Browning
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Magpie
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I don't have a vacuum pump so can't speak from experience. So I'm just offering my opinions:
1. If your system is tight then ultimate vacuum would seem to be a more important criteria than capacity (CFM) for selecting a pump.
2. If you tend to have leaks or have large equipment to evacuate then a larger capacity pump would seem to have an advantage.
I agree with Oxydro that vacuum can be controlled with a valve in the line (assuming some leakage and/or off-gassing upstream).
You could also install a tee in the vacuum line with a valve on the open branch. This would provide an inlet (bleed) for "controlled leakage," which
would also control your vacuum. It might prove easier on the pump also as it would have some air to move for cooling.
[Edited on 6-7-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Oxydro
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Magpie, I think Flip was talking about distillation: | Quote: | | So which has the better functionality in distillation? |
So ultimate vacuum rating of the pump is irrelevant so long as its below the vapor pressure of the least volitile distillate (at the desired
distillation temperature, of course).
Also, the boiling of whatever liquid is distilled will eliminate any need for a controlled leak - pump throttling and temp control can control the
pressure precisely.
Don't forget a gas inlet capillary to bleed air in for nucleation points to initiate boiling. Boiling stones are quickly "deactivated" in vacuum, and
lack of a gas inlet can cause bumping and make boil-overs more likely. Or, you can use a magnetic stirrer.
Edit:
| Quote: |
It might prove easier on the pump also as it would have some air to move for cooling. |
Most decent pumps can run inlet-capped indefinitely, I believe. For instance, you usually run a pump overnight to outgas the chamber if you need a
really good vacuum, and over most of that time, an absolutely tiny amount of gas is being moved.
[Edited on 7-7-2006 by Oxydro]
"Our interest's on the dangerous side of things" -- Browning
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