Connecting a Pump to High Vac System

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
bk8509a
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Connecting a Pump to High Vac System

Post by bk8509a » Wed Dec 09, 2009 1:56 am

I've been checking out the Vacuum FAQs and I cant find anything about how people are connecting mechanical pumps such as a two stage rotary pump to a steel vacuum chamber. My chamber is going to be pretty standard in design, CF hemispheres with a bunch of 2.75 inch half nipples. What kind of port should I have welded on my chamber when I purchase it in order to connect a pump to it? Do different pumps have different ways of connecting to a vacuum chamber?

Thanks in advance,

BK

PS: Sorry if this is explained in detail somewhere, I've been searching FAQ's and haven't found much of anything

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Re: Connecting a Pump to High Vac System

Post by Tyler Christensen » Wed Dec 09, 2009 2:08 am

Typically you would either run a metal bellows hose to the pump or a rubber reinforced tube. The rubber tube could be hooked into a 2.75 or KF - hose adapter and clamped on with a hose clamp (http://vacuumshopper.stores.yahoo.net/stainhosad.html). A metal bellows would require plumbing at the pump end to most likely convert conflat to NPT depending on the pump specifics

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Re: Connecting a Pump to High Vac System

Post by Nicker » Wed Dec 09, 2009 4:59 am

Pretty easy.

First step, figure out what fitting is on the roughing pump (KF16,KF25).
-page 1-60 of Lesker 9th edt. for KF sizing if you do not know.
( viewtopic.php?f=10&t=3809#p24448 )
Second step, KF(size) connected to bellows hose KF(size) to KF(size). (page 1-133)
-Don't forget the KF Centering rings and clamps for sealing. (page 1-69)

Third step, KF(size) to CF 2.75, mounted to your chamber. (page 1-105)




But if I were you I would do Roughing pump to Diffusion pump, Diffusion to chamber.

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Re: Connecting a Pump to High Vac System

Post by John Futter » Wed Dec 09, 2009 7:27 am

Nicker
If you are going to advise please get the terminology right!
A mechanical pump behind whatever high vacuum pump is a backing pump ie it is backing up the high vacuum pump

Roughing pumps are what they say--- they are there to produce a rough vacuum that is within the range of a high vacuum pump to operate to continue to a higher vacuum.

Some systems use separate mechanical pumps to do both functions-- but most use valving to isolate the backing line for a short time so the same mechanical pump can be used to rough out the space above the high vacuum pump and then be returned via the valving to the usual backing function.
High vacuum pumps (turbos and diffusion) require a backing pressure of around 1 by ten to the minus ten millibar --- above this level they stall with internal contamination and contamination of the high vacuum space a real possibility.
I realise that you are saying to rough through the high vacuum pump but you forgot to say that if you use this technique the high vacuum pump must not have been enabled yet

ie diff pump cold / turbo not started.

one problem of roughing through the system is backflow of mechanical oil pump vapour into the high vacuum pump innards and also the chamber being evacuated. If you are going to use the same pump for roughing and backing then a foreline trap of activated alumina in the line to the mechanical pump is a must to prevent contamination of the rest of the system.

Brian the web is font of knowledge on vacuum technology as is this site, not all is covered in the FAQ's a proper read of the Vacuum technology forum should answer all your questions. If the web scares you there are countless texts on vacuum technology --search for " high vacuum techniques" Most here on this site have done this before asking vacuum 101 questions.
A good place to start with knowledge is the commercial vacuum suppliers such as Edwards, Varian, Phieffer, Leybold, Alcatel, and further research from system suppliers such as AMAT , and vacuum parts suppliers Kurt Lesker, (sorry Cindy Mental block), MDC, Vacuum generators, and a few others

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Chris Bradley
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Re: Connecting a Pump to High Vac System

Post by Chris Bradley » Wed Dec 09, 2009 12:49 pm

John Futter wrote:
> High vacuum pumps (turbos and diffusion) require a backing pressure of around 1 by ten to the minus ten millibar --- above this level they stall with internal contamination and contamination of the high vacuum space a real possibility.
I hope you mean 10^-1 mbar, for a backing pressure!?



> most use valving to isolate the backing line for a short time so the same mechanical pump can be used to rough out the space above the high vacuum pump and then be returned via the valving to the usual backing function.
I was under the impression that you merely had to connect the things up in serial as a turbo and/or diffusion pump are just "through-volumes" when not running? I guess this is very relevant if one needs ultra-clean UHV turbo internals to stay that way. I'll take the advise as I have a few electromechanical valves I can rig up to be "A OR B". But if you run two valves like that, why does it make much of a different when backstreaming up the 'other' pipe can simply then run on into the 'top' of the turbo pump, rather than the 'bottom'?

This has become a bit more relevant to me as I appear to have acquired some turbomolecular bits to help push my tufnol/glass-cylinder arrangmement into unit microns. (I'm currently able to get 20microns out of it with an E2M2, which I think is doing pretty good seeing as the chamber is 0.5cu.ft. while the pump only has a 1.6CFM flow rate. But it takes a long time to pump down once it's into 10's of microns and I'm presuming a quick blast of a turbo pump will help out here.)

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Richard Hull
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Re: Connecting a Pump to High Vac System

Post by Richard Hull » Wed Dec 09, 2009 3:31 pm

Chris is correct. The bypass valving is just not needed at all in fusor work as we never go to high scientific vacuums. They can simply be serial volumes. There is zero need to hit 10e-6 torr in any fusion chamber. 10e-4 torr is more than adequate and 10e-5 is super. You are going to back fill at flowing 10e-2 toor, anyway.

Only two valves are needed in any fusor system. Both must act to simply isolate the diff or turbo pump.

I start by closing all valves and mechanically pump the foreline to about 10 microns. (takes about 1 minute on my system. I then open the valve to the diff pump and due to its fine valving and seals, it pumps to 10 micons in under 1 minute. I next open the valve to the chamber and it pumps to about 10 microns in 4 minutes, but once it crosses 50 microns (less that 40 seconds), I turn on the diff pump heater and cooling fan.

Within a total of 20 minutes from air, I am at a fusor pressure of 5X10e-5 torr and can let in Deuterium against a restricted valving off of the diff pump to a flowing D2 pressure of 10e-2 torr.

I have operated like this for 10 years now. I have had the same fusor IV online without tear down for 5 years. So the above works just great.

For the purest or for anyone with only one vacuum system that is used for truly HV work that also services a fusor, a foreline trap would be a must.

For most here, they have never used or seen a vacuum system. What they assemble will be solely used for a fusor system. Thus, there is no need for a foreline trap.

In my earliest systems through Fusor III, I used the micromaze foreline trap as my high vacuum pump!!! Old posts will show that fusion is quite possible exhausting to only slightly sub micron vacuums (6x10e-4 torr), afforded by a rigorously maintained micromaze. It is a bit wasteful of D2 gas, but so is normal running with flowing D2 and a diff or turbo pump.

All the above has been expounded upon many times over the years by me in many postings.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Doug Coulter
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Re: Connecting a Pump to High Vac System

Post by Doug Coulter » Wed Dec 09, 2009 3:45 pm

Chris,

I believe e-1 mbar is the right number there, some things will take a little more, perhaps 10x e-1, probably a finger fumble. At least those are the numbers I and all my vacuum books use. Turbo-drag combo pumps are so tolerant the roughing pump can be a diaphragm pump, and that's what's in Pfeiffer's little leak tester pump station! Kind of cute, completely oil free, and quite reliable in my experience. I have a big piston two stage dry pump on my big chamber, because, hey, it's really big and otherwise it would take too long to rough.

For a turbo, which isn't spinning, you don't need to have the extra valving, as yes, you can just "pull through" it while bringing the chamber down. When it's time to bring it back up to room pressure, you'd like to slow the turbo gently by venting between it and the roughing pump, so as not to burn off the rotor tips by letting air, or whatever purer gas mix, on the high vac side while it's supersonic.
And if your turbos are anything like mine, you will want to have a way to "brake" them as they take forever to spin down from bearing friction alone - hours in HV. They spin up pretty fast, going the other way, however, and my controller, which also controls the roughing pump, sequences fine simply by limiting the drive power to the turbo -- it doesn't start getting fast until the roughing pump has done most of the roughing and so stays out of supersonic shock waves on the rotor tips during that process.
Eg, the motor alone can't burn out a turbo rotor, but once spun up, the flywheel energy in one surely can and it can make an expensive death rattle you'll never forget. There can be a pretty dangerous amount of mechanical energy stored in a flywheel going 800-900 rotations per second!

For a diff pump, the issues are very different. When they are hot, you just can't let that hot oil be exposed to much air at all -- fire! Even if not that extreme, most oils will decompose under those conditions. And, they take quite a while to cool the boiler when turned off -- so much so that the fancier ones have separate water cooling plumbing to do that if quick shutoff is needed. That part of course needs some valves so as not to water cool the boiler when running.

If you want to be able to cycle from air to vac to air to vac more than about once a day, you need a way to valve off the diff pump entirely (eg valves on both ends of it) while you bring your chamber up to air to do things in it, then rough the chamber with the roughing pump while the diff pump is still valved out of the system, then finally open the diff pump valve to the chamber and to the roughing pump - that way, you can leave the diff pump hot the whole time and be ready for it run right away again. This has to be done with some care, as a quick air inrush through a diff pump will deposit its expensive oil in the roughing pump -- and since I've not yet seen one with a sight glass, I suspect what happens next will be confusing and waste some time as well as that money. So a diff pump system needs at a minimum 3 valves to work quickly when you are testing things and need to cycle the chamber a lot. One is roughing pump to chamber, one is roughing pump to diff pump output, and the last is diff pump to chamber. The first two can be fairly cheap ball valves, even, that last needs to be a big one, the kind of thing you try to find on ebay rather than buy new. We are using a gate valve there, full bore size, in our diff pump system. This, along with a light dimmer on the boiler, can be used to "throttle" the diff pump if you want to let gas flow through the system, but not waste a ton of gas keeping a certain pressure in there. Even smart guys make mistakes, and they are costly here if you're using good diff pump oil, so we made checklists on the operations and their orders to keep air off hot diff pumps, and avoid quick rush throughs. So far, we've been careful enough and not had accidents.

The turbo is better that way, just turn it on and off, and manipulate a leak valve (we use a tiny needle valve there, cheap) to bring the system up and down. It's faster and more convenient to cycle often, which is probably its main attraction in fusor kinds of work -- ours also give much better ultimate vacuum, which does help any outgassing go a little quicker when cycling, and gives us more confidence we're running only in the gas we let in on purpose. Most turbos can throttle by setting a lower rotor speed in the controller. For a fusor, running it helps the tank outgass better than almost anything else ever could -- those hot ions and electrons hitting the tank walls eject adsorbed gasses with alacrity. It does a faster and more thorough job than a couple of kw of quartz heaters in there unless those are on for longer than my patience allows. In a non fusor system, it'd almost be worth it to put a small fusor like thing in there for helping with that.

When I'm on a vicious cycling loop, like trying several things a day, I use welding argon to bring up the tank, only have the door open a little while for whatever change I make, then right back down. This speeds up the next pumpdown by a factor of several, as the argon doesn't stick to the tank walls like the more reactive shop air molecules and water do, and the tank then outgasses quick.
Why guess when you can know? Measure!

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Re: Connecting a Pump to High Vac System

Post by David Rosignoli » Wed Dec 09, 2009 4:57 pm

Richard,

So, when you first open the valve to the main chamber, the inner chamber is at 1atm, and the foreline is in the microns level? Doesn't that expose the diffusion pump to a higher pressure than intended? Is there an advantage to doing it in your sequence versus connecting the mechanical pump to the main chamber and pumping it down first, then valving in the diff pump, by connecting it to the foreline? Or have I misunderstood you.

Dave

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Doug Coulter
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Re: Connecting a Pump to High Vac System

Post by Doug Coulter » Wed Dec 09, 2009 5:22 pm

Richard's method does and will work just dandy for the operational type of thing he's doing -- he's not in and out of his fusor several times a day -- he just runs it, and his system is fine for that, and simpler and cheaper in the bargain.

If you want to cycle a lot, to try a lot of things, then the more valve system is better with diff pumps.
Diff pumps don't mind atmosphere when they are cool, some mind a heck of a lot when hot.
So, if you don't mind waiting, you don't need the valves.
Why guess when you can know? Measure!

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Re: Connecting a Pump to High Vac System

Post by Richard Hull » Wed Dec 09, 2009 8:05 pm

I just posted a full FAQ in this forum on the startup-operation and shutdown of a fusor using either a diff or turbo pump. All is made manifest there.

Dave did misunderstand. The entire system is pumped to 10 microns with the mechanical pump before any turbo or diff pump is turned on. Again, refer to the FAQ.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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