Vacuum chamber thickness

[UmarIAwan]

Hi Guys,

Backstory: Im trying to design a vacuum chamber for large scale chemical vapor deposition of Aluminium mirrors - for telescopes. Im planning to use this vacuum chamber for coating telescope mirrors, in the scales of 0.3m^2. Because I Intend to use this for cold wall graphene cvd as well, Im going to be inserting a variety of valves, electrical inlets, etc, into this vacuum chamber. I estimate this setup will have internal dimensions: 1200mm*1100mm*150mm. Im using Aluminium 6061-T6 over Stainless Steel 404 because of its higher yield strength and cheaper prices. Also, I intend for this vacuum chamber to be able to handle pressures near ultra high vacuum chambers i.e. pressures down to 1*10^-8 torr.

Problem: I dont know the thickness of my aluminium plates required to handle this vacuum pressure. I am familiar with the weld types required to prevent gas contamination, and to prevent the plates from callapsing, as well as the fact that at the this size of the vacuum chamber, I would need a lot of strengthening ribs.

My Question to you: What should be the plate thickness to prevent it from caving in at the centers? Do you think theres some sides that should have thicker plates than the others? Is it possible that I can save more money by using strengthening rib of cheaper material in the centers of the plate to decrease costs? If so, please recommend to me the cheapest way to design this vacuum chamber that can afford a safety factor of x2 - or whatever it is that you thing is nessecary.

 

[EdStainless]

Chambers are usually cylinders with domed ends on them. This gives the best structural conditions.
You might want to reconsider materials, strength isn't the limiting factor. Rigidity (or stiffness) is far more important for external pressure applications and 304 SS has twice the modulus that Al does.
There are many design guides for vessels with external pressure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[TGS4]

I would recommend securing the services of a mechanical engineer skilled in these matters. There's a lot more to it than can be explained over the anonymous internet.

 

[UmarIAwan]

Thank you guys for your Input,

EdStainless[and any one else who can answer my following questions], I found this page and though I didnt understand the calculations required, I did manage to take valuable information on the designing of the vacuum chamber from this website.
According to your information, I ran some stress test simulations on solidworks, and yes you are right, 304 SS is more than twice as resistant to deforming under external pressure loads as Al 6061-T6.

I decided to make the end walls of my vacuum chamber cylindrical; with dimensions 1100mm*1100mm*152.4mm. At the edges of this cube are the cylidrical walls of internal radius 2.5 cm, capped with hollow spherical corners at edges, and supporting ribs 1 1 inch high and 1 inch wide as can be seen in Image 1[

. All walls for this vacuum chamber are 0.5 inch thick.

I then ran some simulation studies with using external pressure 101000 n/m^2, (meaning vacuum pressure was zero and external pressure was atmospheric pressure)

and 303000 n/m^2 (3X safety factor)

. The deformation at worst for 101000 n/m^2 was 1.12 cm, and the deformation for 3X safety factor at worst was 16.72 cm.

My question for you is:
1) Can I trust this solidworks model ignoring human error during welding etc? I dont know the math, I am just blindly trusting solidworks found someone who could put the map for deformation on paper?
2) If I can trust these readings, should I go ahead with it, considering that 3X factor deformed more than the thickness of vacuum chamber?
3) What are ways In which I can decrease the material cost of the 304 SS used? Preferably involving thinning of the chamber walls?

 

[pdiculous963]

It is very concerning to see you are just blindly trusting the software. As TGS4 noted, you should find someone who is experienced in designing standard and non-standard pressure components (many of us do this for a living). Running linear elastic stress analysis in Solidworks is not the appropriate method to check for buckling. I would NOT go ahead with it. Find someone who understands the failure modes involved and can design for them.

 

[EdStainless]

Buckling is how this could fail, so any dimensional irregularities could be fatal.
People who design these use huge safety factors to allow for fabrication.
Hire an expert.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[UmarIAwan]

pdiculous963, I see your point and looking up the topic of non linear stress has convinced me that you are right. Though this is a personaly funded build and I dont believe I could afford to hire an expert, please suggests me the method of hiring an expert and give me a quote of what you would charge to design this vacuum Chamber. Also, I found an option in Solidworks simultation study for non-linear stress analysis. Might this give me a proper estimate of the thickness I need for this vacuum chamber?