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Help Determining Minimum Thickness of Vacuum Chamber

  1. Oct 31, 2014 #1
    Hello everyone, not sure if this is the right forum to post to, but I hope you can help me out. I am not a math person and am completely dumbfounded when it comes to engineering. I believe my request should be pretty straightforward for a person with the right knowledge. Basically, I'm thinking of building a small vacuum chamber from an anodized aluminum square cake pan. The dimensions are 8"x8"x4", 14 gauge. I plan on putting a 1/2" polycarbonate lid on the chamber. My only fear is the pan not being thick enough, risking implosion. Also, this is to be used only for low/rough vacuum(max vacuum of around -29.5 inhg. So does this sound plausible?
     
  2. jcsd
  3. Oct 31, 2014 #2

    Danger

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    Strangely enough, I would actually be more concerned with the finish of the pan than the structural integrity. It might well be strong enough to survive a full vacuum, but if the plexi lid doesn't sit flat enough against the rim for a total seal, you're cooked. Given that cake pans aren't in quite the same price range as Cadillacs, I say just run a bead of silicone gasket sealant around the edge, slap on the lid, and see if the wings fall off.
     
  4. Oct 31, 2014 #3
    Thank you for your reply. I guess I hadn't put too much thought into the bonding, but I was assuming it would be alright. The description states that there is a 1/2" flange around the perimeter and from the looks of the pan it seems pretty flat. I have some neoprene sheet gasket material I was planning to cut to size; think that'd work? Another container I have been contemplating is a 8-10" round x 4" tall cake pan, but it is only 16 gauge. I know cylinders are much stronger that square containers for a vacuum chamber, but without knowing the physics behind it, I feel it is too thin. But it would be perfect if they would work without fear of implosion. Here's a link of the square pan I am considering for reference(hope outside links aren't against policy)https://www.amazon.com/Fat-Daddios-...d_sim_k_1?ie=UTF8&refRID=050MK1838Q8AZF85Z779
     
    Last edited by a moderator: May 7, 2017
  5. Oct 31, 2014 #4

    Danger

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    :DD (Sorry about laughing so loud, but...)
    Other than the brand name, I see nothing wrong with the basic product.

    The gasket material that you mention should absolutely be adequate. It managed to prevent leaks in my 440 Roadrunner valve covers when nothing else could, including massive amounts of the aforementioned silicone in conjunction with pricey paper and/or felt gaskets.
    For a definitive answer before the fact you'll need to consult a structural engineer or at least a specifications table. At that price, though, I'd seriously just hook it up and go for it. If the thing implodes, just toss it out and claim the loss under "kitchen expenses".
     
  6. Nov 5, 2014 #5
    Well a perfect vacuum is 14.7 p.s.i. @ sea level. Easy math area x pressure = force.

    Flat surfaces aren't the best to handle forces generated by fluid pressure. The pressure will cause a lot of movement at geometric transition points and they will eventually fatigue and fail. Aluminum has really poor fatigue life, compounding the problem if you will cycle it a lot.

    That's why most aerosol cans and such had curved surfaces (so stress is tensile and compression not bending and shear stress). I designed a vacuum chamber for a guy a long time ago who wanted to de-gas polyurethane prior to pouring it into a mold. I used schedule 40 pipe (overkill but cheap) and welded a dome on the bottom and a flange with an o ring groove machined into it on it's face at the top.Added a thick acrylic cover so he could see the degassing occur, and a volume chamber connected via a 3 way valve so he could accumulate vacuum between cycles thus reducing the pump capacity required. It's been running daily for over 20 years.

    If you really want inexpensive for a small capacity, why not just connect a series of used aerosol cans connected to a manifold made from plastic plumbing components? As was pointed out, you can use caulking as a sealant.
     
  7. Nov 5, 2014 #6

    Baluncore

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    You are really building a chamber to survive 15 psi of external atmospheric pressure. The fundamental strategy is to avoid flat surfaces.

    The body could be a 4” length of 8” diameter round pipe, maybe plastic or metallic. Thinner curved walls will perform like thicker flat wall. Both ends should be domed outwards if possible, but 1/2” thick flat polycarbonate could be used.

    When a vacuum is drawn, the tube will reduce it's diameter slightly while the ends will be pushed in slightly by atmospheric pressure. That may result in a cyclic movement at the seal between the ends and the body. Seal movement can be reduced by machining a step in the end-plates that fits inside the body tube.
     
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