Double walled vacuum chamber

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tbogusiak
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Double walled vacuum chamber

Post by tbogusiak » Sun Jan 27, 2013 8:31 am

We're all aware of the problems of scale with our vacuum chambers, as size goes up, price shoots through the roof due to the quantity of stainless steel and the size of the flanges required. I was wondering, what if a double walled chamber was used instead?

The outside shell would be made of a cheaper material such as mild steel or aluminium with a thin walled chamber inside made of stainless steel, the outside chamber is connected to the roughing pump while the inside chamber is connected to a diffusion or turbo pump. Connections are welded to the inside chamber and bolted to the outside chamber to hold the whole assembly together.

I was thinking that a setup like this would allow the majority of the chamber to be made out of cheaper, less vacuum-friendly material while the inside and feedthrough tubes are still stainless steel. I have not given any thought to cooling the inside chamber yet, and I'm not sure about how to seal the inside halves so as to avoid leaks from the outer low vacuum side to the inner high vacuum side. I've attached a model of what I'm proposing.

Keen to hear your opinions and ideas.
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double wall chamber.png

Tyler Christensen
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Re: Double walled vacuum chamber

Post by Tyler Christensen » Sun Jan 27, 2013 8:44 am

In principle I don't see any issues with this. In practicality, I see quite a few issues:


1. You can't get Aluminum or mild steel flanges of this size or type, so you'd have to custom fabricate them. This would cost a lot or require access to some pretty big machine tools to make those big flanges yourself.

2. You would have to assemble the inner shell. This requires all the tubes to be welded to multiple surfaces (also leading to the issue of sealing the inner chamber). This assembly would require huge efforts to manufacture. Even if the project were DIY with sufficient shop access, you'd take up so much time that you may as well have just bought the stainless parts for an entire chamber.

3. You can't weld stainless to aluminum, or even mild steel (at least not to vacuum reliability).

4. Maintenance would be vastly more difficult. There would be tons of hidden surfaces inside that couldn't be accessed. What if some contaminant gets in between the two hemispheres? No way to clean that out thoroughly short of full submersion in nasty chemicals.

5. As you mentioned, it'd be a thermal issue for a fusor. You'd probably have to water cool the inner chamber, or have lots of thermal standoffs moving heat out which would make assembly even harder.


Those points aside, if you make it, post some pictures because it would be some fantastic metallic eye candy if built well!

Jerry Biehler
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Re: Double walled vacuum chamber

Post by Jerry Biehler » Sun Jan 27, 2013 9:21 am

The material is not really the expensive part. It is all the stuff you add to it like flanges and paying a highly skilled welder to put it together and then helium leak check it. That is where the money is.

Your design would probably cost 2-4 times as much as a simple stainless sphere.

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tbogusiak
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Re: Double walled vacuum chamber

Post by tbogusiak » Sun Jan 27, 2013 9:34 am

Good points about cleaning, I hadn't thought much about that. The idea was that the outside shell would be at maybe only 10-2 torr (or whatever the roughing pump can handle). The connections between the tubes and the outside shell can be o-rings or even glued on with JB weld, as there will only be a low grade vacuum between the two puter halves, hopefully it shouldn't need much cleaning.

The outside halves would be sealed together with an o-ring so it shouldn't need much in the way of flanges, a few clamps should be sufficient to hold it together, the vacuum-atmosphere pressure differential should do the rest.

I admit that building a sphere would require a decent machine shop, but the concept could be applied to a box shaped assembly so the chamber could be made out of a set of metal plates bolted together, as the outside only needs to hold off a low vacuum, it could be sealed with some silastic or something.

The welds on the feedthroughs on the internal halves should be the only welds in the whole build (aside from the ones to the conflat connections), however, I don't know how to make a high vacuum seal between the two internal halves to prevent leaks from the low vacuum side.

A design like this would likely only be economical with chamber sizes beyond 300mm, the model has a 400mm I.D.

I know some people with a good workshop so when I have some more money to buy the materials, I'll have a go building a prototype and post the pics (might be a while though)

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Re: Double walled vacuum chamber

Post by Jerry Biehler » Sun Jan 27, 2013 11:06 am

It would probably end up being a virtual leak nightmare.

And getting the inner half to seal would be difficult to do. You would have to weld everything together and then surface grind the flanges to make sure they are perfectly flat and parallel which will take a pretty good sized grinder. And then as things heat up who knows how much and what will move.

As a fabricator and a machinist I can tell you right now this thing will cost a small fortune and in the end you are trying to come up with a solution to a problem that does not exist.

If you need a big chamber there is no reason to stick with spherical. Others on this board have made very well performing fusors with cylindrical chambers. And these are much more common on the surplus market.

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