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100 micron Vacuum drying dielectric oil...

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Patrick

Tue Feb 01 2011, 07:36AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I need to vacuum dry some vegatable oil to keep its dielectric strength up to its max, for a 60 kv project I have coming up. I have done some research, here on the forum and on google, but its been awhile since Ive done vacuum stuff. I would like to ask some others who might have some vacuum experience to volunteer any good ideas and anything to watch out for so I dont die in a burst vessel explosion, or such.

My plan:
--To establish a 100 micron vacuum on about 1 quart of oil in a schedule 40 well built vacuum chamber.
--Then hold that 100 microns for 6 hours or so, put the whole thing in a safe place in case of bursting issues.
--Then let the vacuum go and immediatley use the oil in the final device.
--Hopefully the plastics I depend on are suffciently water impermeable for 1-2 years at a time.

Am I missing anything super-duper important? I dont think I will heat the oil, but if any physics/chemistry buffs can confirm my thinking on this next point:

first: if a liqiud is warm/hot, then solids will be more willing to dissolve and stay in solution while warm/hot.
But dissolved gases will escape more easilly.

second: if a liquid is cool/cold, then solids will be less willing to dissolve and stay in solution while cool/cold.
But dissolved gases will be more willing to stay in solution, due to slower kinetic motion.

Is the above stated right, or am I totally off? If I am right then, which phase would polar water molecules be in, (liquid or vapor) while partially dissolved in a highly non-polar fluid like oil? Am I assured that when the vacuum is drawn that the water will be vapor and definately leave in a hurry?

Ash Small

Tue Feb 01 2011, 08:13AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Shedule 40 pipe should be ok. It is good for 140 PSI and you won't exceed 15 PSI.

Water vapour is notoriously difficult to remove from vacuum systems. I'd imagine it would take a lot longer than six hours to remove all the water from the oil.

'Bake-out' under vacuum is often used to remove water vapour from vacuum systems, usually at temperature of between 150-400 degrees C.

I think it's probably a case of 'trial and error' in this case.

Patrick

Tue Feb 01 2011, 08:19AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I will consider what you have said, Ive learned that trace water reduces dielectric strength of oils by 50% so you can lose quite a bit. Most oils are good for 500v/mil, I use 300v/mil, and double the distance inside the liquid filled case. But now its time to learn oil vacuum drying.

Carl Pugh

Tue Feb 01 2011, 01:15PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

Oils are fun.
Did the following test:
Put transformer oil in two metal containers.
Put piece of Acrylic plastic in one metal container under the oil.
Applied vacuum.
After a while the oil in the container without the plastic became very calm.
The Acrylic plastic in the container never quit giving off gas. (Acrylic dissolves under a good vacuum?)

One thing that you might be careful of is that
Oil that has all the dissolved air removed has a high breakdown voltage.
Oil that does not have any of the air removed has a high breakdown voltage.
Oil that has some of the air removed has a much lower breakdown voltage.

Patrick

Tue Feb 01 2011, 03:13PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carl Pugh wrote ...

Oils are fun.
Did the following test:
Put transformer oil in two metal containers.
Put piece of Acrylic plastic in one metal container under the oil.
Applied vacuum.
After a while the oil in the container without the plastic became very calm.
The Acrylic plastic in the container never quit giving off gas. (Acrylic dissolves under a good vacuum?)

whoa! wierd!?

Carl Pugh wrote ...

One thing that you might be careful of is that
Oil that has all the dissolved air removed has a high breakdown voltage.
Oil that does not have any of the air removed has a high breakdown voltage.
Oil that has some of the air removed has a much lower breakdown voltage.

Really!? i have notbeen told this, strictly speaking its the water vapor i want out not nesscarily the air.

Antonio

Tue Feb 01 2011, 04:05PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

You will need professional equipment to make vacuum below a few mm of Hg. Anyway, all liquid insulators are very poor insulators when compared to solid insulators or to plain air. They can be used only on relatively low-impedance applications where a leakage of many uA is not a problem.

Patrick

Tue Feb 01 2011, 05:22PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Antonio wrote ...

You will need professional equipment to make vacuum below a few mm of Hg. Anyway, all liquid insulators are very poor insulators when compared to solid insulators or to plain air. They can be used only on relatively low-impedance applications where a leakage of many uA is not a problem.

Really!? chit! I was going to buy a 100 micron spec'ed rotary vane vacuum pump, single or double stage, and leave the pump continuosly pumping for 6-12 hours to hold the vacuum durring the degassing operation.

Ash Small

Tue Feb 01 2011, 05:49PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

100 micron should easily be obtainable with a rotary vane pump. (especially a new one).

I do think though, that a diffusion pump would achieve better results in this application than a mechanical pump alone, but you'd probably need to consider things like the vapour pressure of the oil being dried/de-gassed.

Mattski

Tue Feb 01 2011, 06:49PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

I would think that heating the oil speeds things up, it makes everything more mobile so it takes less time for a given water molecule to reach the surface of the oil, and when it does it is more likely to leave the oil because it has more energy. I don't know about oil specifically, but usually you bake stuff under vacuum to outgas it and prepare for vacuum use.

On a similar note, more surface area on the oil is good, and if you can maybe use a magnetic stirrer that might also speed things up depending on how much oil you're talking about.

GluD

Tue Feb 01 2011, 07:38PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

I agree with Mattski about heating the oil to outgas. As far as I know that is indeed what the professionals do to outgas things before vacuum use.

I'm not sure the magnetic stirrer is a good idea though, couldnt that also "blend in" new gas from the enviorment?

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