Vacumn Boxes

this is a square box James Lurch made some years ago for aluminising telescope mirrors.
Its a good indication of how thick the chamber has to be to prevent distortion from the vacuum.
Admittedly his system goes a lot lower than you need but it does show a square unit is ok.
 
I have a large vacuum system I use in my wood shop for doing veneer layups and bending.

I make forms with 3/4" MDF ribs on 3" - 4" centers and they are not sufficient for more than a handful of uses.
It is really hard to wrap your brain around how much pressure is exerted at high vacuum. I have had what I though where really solid bullet proof bending forms made from voidless plywood collapse violently under vacuum. It is pretty scary.

I would not consider aluminum at all for the size you are talking about. 24" is just a huge span for a vacuum chamber at the pressures you are talking about. It would be very dangerous if the box collapsed because the aluminum might send off shrapnel as well.

I would weld up a torsion box of 1/2" steel with ribs crossing each other every 4" to 6" depending on the rib height. 4" on center for 3/4" ribs or 6" on center for 1 1/2" ribs. The ribs can go on the outside. Might be able to make the ribs 1/2" and the plates from 3/8" stock. I would just bolt the door on rather than mess around with latches, the door should be at least 1/2" around the edge where the bolts go through or it will deform over time from un-even pressure when bolting up.

Do not forget that the pump will need to run continuously until every last molecule of water is evaporated, and every last bit of trapped gas is exhausted. This can take quite a while with some stuff, and requires a pretty large pump to keep up with the off-gassing that occurs.

Don't waste your time on the systems sold to wood workers. They are seriously over priced and under powered. You want an HVAC pump that is designed to run near continuously, can pull a hard vacuum, and hold that vacuum when turned off with out leaking a bunch of crap back into the chamber (small cheap systems will backflow and fill the chamber with debris). In other words, you need a pump that will cost many hundreds of dollars. These pumps are designed to run for hours at a time while hooked up to an AC system, and are more resistant to corrosive gasses than cheaper pumps.

Do you really need 25" of vacuum? You might want to consider building something that runs a higher pressure and running it for longer. It will probably cut the cost down to a third of building something big. For what it is worth, I have a 15cfm pump on my system. I can do full 4x8 foot veneer layups. I feel my system is underpowered.

Lastly, make sure you double filter the line between the pump and the chamber. I did not filter this line well enough and ruined the seals in my pump and had it leak oil back into a one of a kind pair of matched desktops. It took me two weeks of soaking with ZEP brake cleaner and blotting to get the oil up so I could finish the job. Fortunately I was able to repair the pump.

-Josh
 
I've got a T-30 IR vac pump I'm not using. It's good for 29" HG and 60 cfm. Recommended with a 2 hp motor. Any help?

I'd also recommend a molecular sieve to protect the pump.
 
I haven't seen anyone requiring more than 25Hg to 29" Hg to infuse wood. The CFM, I wouldn't think needs to be too high for my purpose as I am not going to be in any particular hurry, I don't think, to reach that 29"HG, but obviously I have much to learn about this process. I had planned on putting a trap between the tank and pump since I won't be able to visualize the air bubbles and control the rate. The molecular sieve is a good idea as well. Just between the trap and pump? Something tells me I may start out smaller than I had planned just based on cost. Live and learn. Thanks everyone.
 
Ron can you not get some 20 " diameter 1 " thick walled steel pipe with faced /ground up ends and make simple ends from 1" thick steel plate with the seal set in them for your vacuum chamber . A cylinder will have a lot more strength than a cube type object . If the end caps appear too thin its going to be easy double up on them so you have two plates of 1" thick at each end . Such a cylinder can be easily mounted solidly on a heavy weight castor'd trolley to move it around .

Re :-
Getting your vacuum .
A decent petrol engine develops a maximum depression of 17 to 21 inches of mercury in the inlet manifold if it is timed correctly and is not a charged air system . So tapping into the inlet manifold of a simple naturally aspired engine may be a cheap & easy way of getting it , so long as the tapping point is made with a gas shut off tap ( USA + gas faucet ?? ). I seem to recall that a naturally aspired diesel engine develops a slightly reduced vacuum

The engine will run a bit different whilst you're developing the vacuum but so long as it is gentle and not jerky it should be OK for a few minutes run at a time . Perhaps if you are worried about the sucking out of an explosive gas from your vacuum chamber & putting it into your engine you could add anti flashback valves from gas welding lines . However if you're just sucking out clean air , then first closing up the chamber and then filing with the soup mix there will be no problems or the need for the anti flash back valves .

You may also develop the vacuum in a clean vessel such as a an old large Co2 cylinder and then transfer the vacuum via a steel pipe set up without having the flooding chamber connected to your engine
 
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Well, in my search for a vacuum chamber I am now wondering why I couldn't just use a 35 to 60 gal vertical compressor tank. Cut the top off and make a flat lid with the gages and valves threaded into it. The bottom drain port would be handy for draining the resin. I am wondering about the amount of rust it would have in it initially. Strong enough for 30Hg? What do you guys think?
 
If you visit your local scrap metal yard, they likely have discarded propane tanks.
Might be a easier source unless you already have a dead compressor. The propane tanks come in all manner of sizes.
 
Good call on the compressor tank. I would cut the top off, then weld flanges to either side of the cut, machine an o-ring groove on the face of the lower flange, then use it like that. Avoid using a flat door unless you are able to use a heavy enough section thickness to avoid distortion during use.

Also note that even though you are looking for a 30Hg vacuum, a vacum is still only the equivalent of about 15psi.
 
One has to realize that once you get down to a centimeter of vacuum, the increase in force on the walls changes very little. the force due to a rough vacuum of 10mm of mercury differs from that due to a high vacuum of sub microns of mercury by about 1.5%. (760mm of mercury is standard atmospheric pressure)

It is a different story regarding leaks and maximum attainable vacuum though. An insignificant leak at 10mm will totally swamp a high vacuum pump. Any welds will have to be impeccable.

I worked with vacuum systems for six years, pulling air out of uncured epoxy composites. We used Varian rotary vane pumps and were able to get down to about .01mm of mercury. At home, I have a Precision 20 l./min. 5 micron rotary vane pump which I have used for degassing epoxy mixes. While a lower vacuum will work for vacuum pressing or clamping, a higher vacuum is need to effectively pull air out of a material prior to replacement with your stabilizer. Pulling down to 25" will still leave 20% of the air behind.
 
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