Building a flat rectangular vacuum chamber

[bobbybloom]

I'm uneducated in Physics so I'm not sure where to begin with this.

I want to build a vacuum chamber that is rectangular and fairly flat. It will measure 48"X96"X8"

I plan on constructing it from clear polycarbonate sheet on one face (would clear acrylic be a better choice?) and aluminum sheet on the other. The narrow sides will be either polycarbonate sheet or some sort of epoxy resin.

I know my chamber will be dealing with ~14 psi coming at it from all sides, and my common sense tells me it will need to be reinforced with supports (I'm picturing something like rigid thick walled polycarbonate tubes spaced every 12") internally to hold up to the pressure.

If anyone could advise me how to go about determining things such as how thick of walls I will need, and how thick of internal supports are needed, and how often to space them, etc I would be much appreciative.

 

[AlephZero]

Just a quick sanity check:

48 in x 96 in x 15 psi = 69,000 lb = about 31 tons of force on the flat sides.

If you can design something out of polycarbonate that won't break if you drive a 30 ton truck over it, go right ahead. Otherwise, it's time for a rethink. Most big vacuum tanks have curved walls and not flat ones - for a good reason.

 

[bobbybloom]

I understand the cylindrical shape is superior, but if you could indulge my insanity for a moment...suppose I were to attempt such an absurd construction.

How do I go about calculating how thick it would it need to be? And what property of the sheet is it testing? Is it the tensile strength? Flexural(?) strength?

Also, would it not be slightly different than rolling a 31 ton truck over it as the weight is distributed evenly across the entire surface?

 

[sophiecentaur]

I understand the cylindrical shape is superior, but if you could indulge my insanity for a moment...suppose I were to attempt such an absurd construction.

How do I go about calculating how thick it would it need to be? And what property of the sheet is it testing? Is it the tensile strength? Flexural(?) strength?

Also, would it not be slightly different than rolling a 31 ton truck over it as the weight is distributed evenly across the entire surface?

Aleph Zero is not too wrong but the details (i.e. the resulting deformed shape0 would be different)
Your problem sounds very much like designing a floor in a building. If you define the amount of flexing you can tolerate then, for a given material modulus and span, you can calculate how thick the sheet /floor needs to be. Having pillars in the middle will make life much easier (as in Multi-storey car parks).
If you put your particular query on the Mechanical Engineering PF Forum, you may well find the right guy to help you.

PS why not do it in steel with a few small windows in? A much easier proposition, cheaper and relatively straightforward to construct.

 

[bobbybloom]

Thank you for the help and reality check, I will post the question in the Engineering Forum for further assistance.

 

[AlephZero]

A quick play with a plate calculator on the Web suggests you would need about a 1.5 inch thick steel plate for the 48x96 panels to keep the deflections down to something reasonable (i.e. about 0.1 inches in the center).

Polycarbonate is about 100 times more flexible than steel, so that would increase the thickness to about 7 inches without any reinforcements.

Compare that with a 96in diameter circular tank, where (ignoring the possibility of buckling under external pressure!) steel about 0.03 inches thick would withstand 15psi quite easily.

 

[bobbybloom]

That sounds like a very handy calculator. Could you provide me with a link? I can't seem to find it.

 

[AlephZero]

Google for "plate stress calculator".

I used the efunda one that comes up first, but it wants you to register after a few calculations. And it only calculates displacements not stresses. You can probably find a better one.

 

[bobbybloom]

Thanks for the help. I've been toying with the efunda as well as another beam calculator and now I'm getting a better idea of the forces that would apply.

If the structure were supported by internal pillars every 12 inches, would that change the effective dimensions for calculation of deflection to L 12 and W 12, or would it be L 96 and W 12 in this case?

 

[bobbybloom]

If I were to use a multi-walled polycarbonate or acrylic sheet for this application, how would that affect my calculations. The multi-walled sheets are built with a cell like structure in various thicknesses. Could I simply use the thickness of the multi-walled sheet as though it were a solid single walled sheet of the same thickness? Or would it be substantially weaker?

 

[sophiecentaur]

If I were to use a multi-walled polycarbonate or acrylic sheet for this application, how would that affect my calculations. The multi-walled sheets are built with a cell like structure in various thicknesses. Could I simply use the thickness of the multi-walled sheet as though it were a solid single walled sheet of the same thickness? Or would it be substantially weaker?

The manufacturer could help you there, I think. Strength of a complex structure would not be easy to predict (unless you were in the business) but they are used for roofs so there must be lots of data.

 

[AlephZero]

If the structure were supported by internal pillars every 12 inches, would that change the effective dimensions for calculation of deflection to L 12 and W 12, or would it be L 96 and W 12 in this case?

This is a different situation because you are reacting all the load at one point (the pillar) rather than distributing it around the edges of the sheet. As an order of magnitude, one square foot of air pressure is a force of about 1 ton.

I'm not a civil engineer, but you might be able to find some information on designing floors of a building supported by an array of columns that would read across to your design.

If I were to use a multi-walled polycarbonate or acrylic sheet for this application, how would that affect my calculations. The multi-walled sheets are built with a cell like structure in various thicknesses. Could I simply use the thickness of the multi-walled sheet as though it were a solid single walled sheet of the same thickness? Or would it be substantially weaker?

The cell structure would be weaker than a solid sheet of the same thickness, but much stiffer than a solid sheet of the same mass, which is often a useful property.

The mechanical properities of cell structures are quite sensitive to exactly how the structure is bonded together, etc. The manufacturers should be able to supply the technical data you need to design using them. If not, there isn't really an alternative to measuring the properties yourself. There are ways to make theoretical estimates of the properties, but they aren't of much practical use unless you know how accurate they are - catch 22!