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Help engineering a basic vacuum reservoir.

Just a Sparky

Just a Sparky

Hot Rolled
Joined
May 2, 2020
Location
Minnesota
One of my upcoming projects will require a small vacuum reservoir. I'd like to build it from 303/316 round stock (O-ring gasketed end caps) and polycarbonate tube stock. How might I go about calculating the minimum wall thickness needed to construct a safe and reliable vacuum receiver?

Thanks.
 
The hoop stress is calculated from the formula P*D/2t, where P is the pressure in psi, D is the diameter in inches, and t is the wall thickness in inches. The pressure is -14.7 psi (a perfect vacuum). Also, there is a longitudinal stress exerted by the endcaps. For small tube diameters this is pretty small. For a large tube, it can get higher and needs to be figured into the total stress. Most polycarbonate tube material is good for about 9000 psi (material yield strength, not internal pressure) , you need to verify the published yield strength of the material in the actual tube you plan to buy, and you want a pretty healthy safety factor (the plastic can degrade with time). Also, you want to be careful about oil or solvent vapors getting into your reservoir. (Maybe you’ve seen filter drier plastic reservoirs crack after being exposed to oil vapors- most of these are made from polycarbonate. ) What diameter and length did you want to use for the reservoir? Your available space and the volume that you want for the reservoir pretty much drive that. If you have a rough idea, then all you’re really figuring out is what wall thickness you need. I suspect that most available wall thicknesses for a reasonable size tube will have adequate margin, and going a little thicker to get loads of safety margin won’t add much to the cost. This is a link to a calculator for hoop stress. Pressure Vessel , Thin Wall Hoop Stress Calculator Strength of Materials | Engineers Edge | www.engineersedge.com


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Just a Sparky said:
One of my upcoming projects will require a small vacuum reservoir. I'd like to build it from 303/316 round stock (O-ring gasketed end caps) and polycarbonate tube stock. How might I go about calculating the minimum wall thickness needed to construct a safe and reliable vacuum receiver?

Thanks.
Click to expand...

See prior cautions. Simply don't USE a "plastic" that is not already proven in long, tough, use. DIY is best in all-metal, rather.

Custom-built vacuum reservoir? Why? You the first guy in the history of mankind to need one?

Common item, parts channels, auto industry. Made in the multiple tens of millions. Every year. And about ten bucks, even on Amazon:

But one example of MANY:

Vacuum Reservoir | 47076 | Vacuum Storage Canister | Dorman Products

Size is an issue? Shape?

Next fastest and cheapest is to adopt one of the far more common pressure tanks. Most will do max 1 BAR, gage, negative direction just fine. Many liquid FUEL tanks will do so as well.

Available sizes are all over the lot, new and used. So is metal tubing.

Think aircraft breathing Oxygen, common air suspension reservoirs, LESS common "jumping" trick-car air tanks, heavy truck air brakes, air-horns, paint-pot pressure tanks, air compressors.... "etc".

Steel. Aluminium. Stainless steel.

Copper, even. See "header" or "expansion" tanks, water-heater safety. Cap a length of ignorant but costly (and durable) Copper DWV line.

Otherwise one can find treated, coated."whatever".

Fittings usually included.

Why re-invent a cheap wheel when you have OTHER s**t to do that is harder to "just buy" and needs YOUR best-efforts .... or doesn't get DONE at all?

Especially with "plastic" a known sub-optimal choice for low-maintenance longevity?

Leverage your time onto the "critical path".

More time cannot BE "bought".

2CW
 
As is so often the case in these inquiries, the size and shape as well as the pressure (negative re atmosphere) is necessary to make any sort of reasonable estimate. In a sense, there is no such thing as a vacuum, only a pressure lower than 14.7 PSI absolute. The sort of vacuum that powers your boosted brakes is one thing and the sort you encounter in things like X-ray tubes and atomic particle accelerators is quite another. When you get into micron levels, it is a trip through the looking glass, a different world, but I suspect that you are not going that far.

Bill
 
I have pulled a vacuum on an empty Freon tank to 10 microns using an Edwards vacuum pump and a silicon oil diffusion pump. It did not go MOOB. It's pretty easy to crush a thin walled metal can like a gallon of solvent comes in with just a partial vacuum, but you are not dealing with extraordinary forces here. Freon recovery systems pull some pretty good vacuums on the recovery tanks. The commercial tanks have a bit thicker wall then the ones Freon comes in but they are not crazy thick. You might consider one of those since they are readily available.
 
Size matters.

Let's ignore that for a moment...when I needed a vacuum pot, I looked around and found that there are a bunch of good sized pots sold. Who buys the bulk of these? Why, people who are interested in producing hashish in their own basements.

Thanks to those good folks, you can buy, on Ebay, a 2 or 3 gallon vac chamber for cheap. They sell them under the guise of 'epoxy degassing'. I'm sure a few people use them for that, but hash rules the land:



3 Gallon Vacuum Chamber Stainless Steel kit Essential Oils Degassing Urethane | eBay
 
GregSY said:
Size matters.

Let's ignore that for a moment...when I needed a vacuum pot, I looked around and found that there are a bunch of good sized pots sold. Who buys the bulk of these? Why, people who are interested in producing hashish in their own basements.

Thanks to those good folks, you can buy, on Ebay, a 2 or 3 gallon vac chamber for cheap. They sell them under the guise of 'epoxy degassing'. I'm sure a few people use them for that, but hash rules the land:



3 Gallon Vacuum Chamber Stainless Steel kit Essential Oils Degassing Urethane | eBay
Click to expand...


if you live in a 'legal' state you see these things sold out in the open occasionally.

Its basically a vacuum desiccator.

If you want to build a similar vacuum chamber, buy a 3 or so gallon aluminum pot (which is a bit thicker) at the kitchen supply warehouse, put a 3/4 polycarbonate 12 x 12" plate on top with a
1/4 silicone gasket. Will hold below 1 torr no problem and provides with a clear view of the process in the pot.


You can add electrical feedthroughs by tapping a 1/4 or so hole , putting
a screw through and sealing with RTV in the polycarbonate top. If you need motion, you can tap a swagelok ultratorr fitting into it and get either linear or rotation as needed.

The aluminium pot is can also be put in a water bath if you want to heat or cool it. This is a little easier and cheaper than welding up a custom vac chamber on a university budget.

Depending on the volume, I would look at making a vac reseviour out of .042 wall SS tubing and caps, its a pretty standard size for 1-2.5" vac chambers and you can get caps from mcmaster.
 
Pressure cooker. There are some good sized ones sold for home canners, and you can often find them used and cheap.

If you don’t need access, how about an expansion tank from a hot water heating system? Or a pressure tank from a well water system? Once the bladders leak they are tossed, but that shouldn’t bother you.
 
I did not read the other responses so I may be repeating.

Atmospheric pressure is only 14.7 PSI and that is the maximum amount of pressure that any vacuum chamber will ever see anywhere on this earth. Almost any thickness of any SS alloy that you can machine on a lathe or mill without being in danger of breaking through will certainly stand up to that pressure on the end cap of a small diameter, drum shaped reservoir. And if you are machining the end caps from one solid piece of SS, then there is little reason not to just leave them a bit on the thick side.

As for that plastic tubing, the manufacturer probably provides both pressure and vacuum numbers for it so just look it up.

You do not give any dimensions other than "small" so it is hard to back up that seat-of-the-pants statement but I think it is probably correct, at least up to the largest diameter polycarbonate tube (6" OD x 1/8" Thick) listed by McMaster which is rated at 50 PSI. All the other listed sizes have larger PSI ratings. I think you are safe using any of them.
 
Almost any glass jar in the supermarket, with it's original lid would make a good vacuum chamber. Heck, most of the plastic ones under the half gallon size also would. A Mason Jar with one of their lids would be great.

Such a low bar, no pun intended, and so many possibilities.



Strostkovy said:
Paint sprayer pots are dandy vacuum chambers.
Click to expand...
 
Failure mode of a vacuum tank will be buckling as in pressure from the outside. You also have to consider outside forces which could dent the cylinder and precipitate a buckling failure.. small tanks the size of a propane torch cylinder would be extremely robust for a vacuum chamber ..
The best, most material efficient shape would be a sphere. One of the worst I can think of is a beer can.

Turn your calculating search toward buckling calculations.
 
I'm looking at roughly 3"ID by 12" long, 1/8" wall poly tubing. McMaster lists it as suitable for use at 100PSI.

Catalogue number 9176T17.

Close enough?

Going with clear tubing and end-caps for cosmetic reasons and to save myself from external plumbing considerations. Just turn and drill whatever I need into the end caps and move on. No screwing around with copper lines, tee fittings or manifolds. One self-contained package all ready to go.
 
Rickyb said:
Failure mode of a vacuum tank will be buckling as in pressure from the outside. You also have to consider outside forces which could dent the cylinder and precipitate a buckling failure.. small tanks the size of a propane torch cylinder would be extremely robust for a vacuum chamber ..
The best, most material efficient shape would be a sphere. One of the worst I can think of is a beer can.

Turn your calculating search toward buckling calculations.
Click to expand...

Beer cans are actually a pretty okay shape; they are just incredibly thin.

A box would be worse, and an accordian, raisin, or flat pancake would be incredibly worse.

I did at one point intend on purchasing those cheap pancake compressor tanks for a vacuum application I ended up reworking.
 
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