Vacuum Tanks

[Horst]

Is it reasonable to use air storage tanks (11 gal. 125 psi; new from Harbor Freight) as vacuum holding tanks (29" +)?

[Fasttrack]

You shouldn't have a problem as far as the tank structure is concerned, but vacuum can be a funny beast. If the tank is painted or coated with oil or zinc or some material to prevent rust, you may have difficulty generating the desired vacuum as these materials gas off - they can also be hell on your vacuum pump. An air holding vessel like that is impractical for scientific vacuum, but at 29" Hg (or about 20 torr) I think you'd be just fine. Rough vacuum pumps are pretty forgiving about pumping contaminants.

[George_Race]

A small particle filter in the suction line BEFORE the vacuum pump may be helpful to keep anything from getting to pump from the tank.
George

[macona]

If he was going down to sub millitorr out gassing would be an issue. But 29" is nothing. A normal air tank ought to be fine.

Building a vacuum forming machine?

[rkepler]

I wouldn't be surprised to hear that the tank collapses with a vacuum inside - the tanks are made to resist internal pressure and not external pressure (on 15 psi but still...)

[ckelloug]

I was talking to a Hydrogen tank engineer a few weeks ago. He was telling me about the equations for a vacuum jacket hydrogen tank. I didn't go verify what he was telling me but he said that its the thickness and modulus of the material rather than the thickness and strength of the material that are are the factors in the design equation for vacuum vessels. Apparently, with vacuum, tanks fail by buckling instability rather than tensile or compressive failure of the material.

In another random conversation, I was talking to a chemical engineer a couple weeks ago had worked on boilers and she told me that when a boiler fails because it cooled off and developed a vacuum inside of it, they call the failure beer-canning.

Back to your regularly scheduled programming.

[Horst]

I wouldn't be surprised to hear that the tank collapses with a vacuum inside - the tanks are made to resist internal pressure and not external pressure (on 15 psi but still...)

That is what I was wondering about. I'll bet others have tried this for vacuum chucks, vacu-forms, etc. Is there anyone who has collapsed an air tank this way? If the tank did fail would it pose a safety risk?

[rkepler]

I know it's not the same but...

https://www.youtube.com/watch?v=h5l0K62n8B4

[Rosco-P]

or: http://www.youtube.com/watch?v=b5mbj5iV3H4&feature=plcp

[darryl]

I have a 100 lb propane tank that I converted into a vacuum tank. Not had any problems, but I recognize the potential for it to collapse. Seems likely that it would begin in the straight center section, so if that is of any real length it might be prudent to put a ring around it there to help keep it round. I've used my tank standing alone, but now have it placed in a corner where the workbench top is fitted over it. I did my best to cut the hole round and as small in diameter as I could and still fit it down onto the tank. Don't know how much that will help, but it makes me feel a bit better about it anyway.

I've used the 20 lb tanks for vacuum, and for two reasons I don't think they would collapse. One is that the center section is not very long before the ends start curving, and second is the way they are constructed puts some extra metal around the center. It's two halves nested together before welding, so essentially it has a thicker band around the center.

I wonder about air compressor tanks- maybe the material is a little thinner- ? Possibly. I know that a propane tank can withstand over 1000 psi, since I've done that test myself. I have my doubts about the little air tanks in this regard-