Building a vacuum box

[ports394]

I work with some samples that are dissolved in ethanol. Currently, we plate them, and evaporate overnight in a fume hood, and then bake them using a standard convection lab oven. It takes about 24 hours and really slows me down.

And buying an explosion proof vacuum oven is ridiculously expensive. I've been quoted $40-50,000 for "table top" units.

So, I figure, I'm a mechanical engineer... I could just build me one.

But I've never designed a vacuum chamber. How do I go about it? I know the myth of "just build it thicker" is a bad.

Ideally, what I'd like is a square box, with 4 walls, and a bottom that's 12" x 12" x 12". The top would be a 14" square, with a gasket that could be laid on top of the box, closing it. and I've had 2 ports on the side of the box, one for a vacuum break valve, and another for the vacuum line to hook up to.

My vacuum pump is a 3-4 CFM air operated vacuum pump. It's capable of 25" of Hg vacuum.

I can externally heat the box with an intrinsically safe hot plate, or steam source.

I'd appreciate some help, or a lot of help, with attacking this project.

Thanks everyone.

 

[Ron]

Structurally, vacuum is just pressure...in the opposite direction. Reinforce the box as necessary for the required vacuum. In most cases, it doesn't matter if you get a little "oil canning", since I doubt that you will cycle the vacuum enough to present a fatigue problem.

As for the rest of it, make sure you accommodate the explosion issue. If there is a chance of explosion within the chamber, provide pressure relief for the explosion. Make sure all lids are secured for vacuum AND for pressure, since an explosion will be a quick reversal.

The evacuation line must also consider explosion possibilities as will the vacuum pump itself (should be rated for flammable or explosive materials).

Find a spherical shaped device that clamps together in the middle and convert it.

Good luck.

 

[MikeHalloran]

So ... where does the ethanol go, on the exhaust side of the vacuum pump?



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

If you look around, you'll see that most vacuum chambers are either cylindrical or spherical or both, and there's a good reason for it, since those shapes are best able to resist pressure differentials. That's why submarines are basically rounded cylinders. That's one reason why soft drinks come in cylindrical shapes.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ports394]

I've been reading some papers on designing a vacuum chamber, and yes, the box is the least rigid form for a vacuum chamber. I'll look around for something cylindrical that I could convert.

My plan was just to buy 12" square peices of 1/4" or 3/8" carbon steel plates, because they can withstand 30k-60kpsi.

Mike, good question. The ethanol would go through the (compressed air powered vacuum pump) pump, out the exhaust, which is connected to a fume hood. My fume hood is massively overpowered. It's internal space is 40" wide, and the XP rated blower on it pulls 900 CFM. Even with the sash totally open, you feel a nice breeze.

As for the vacuum pump, it is intrinsically safe. I bought it from McMaster about 2 weeks ago, and it's been one of my greatest purchases from there. It's just like a vacuum pump that you hook up to a faucet, expect it uses compressed air for the fluid. The whole thing is basically the size of a fat sharpie marker.

And as far as an explosion, my plan is to run it inside the fume hood, with the sash closed. That should contain the explosion. And I'm looking at 3-4 trays with 15 mL of 100% ethanol in each. Basically 1 shot of everclear worth of liquid. Not the largest explosion or fire.

 

[MiketheEngineer]

Maybe I am all wet here - but what about just blowing on the samples with a heavy duty fan??  May be not as fast - but much faster than current method and less chance of explosion.  Just blow/suck it into the fume hood.

 

[ports394]

Mike, my samples are in aluminum sample weight boats that weigh about 1-4 grams each, depending on the size.

And we're drying a dissolved solid which forms powders and flakes in the pan. You'd end up blowing the pan away, or the product out.

 

[ports394]

Mike, I'm not being a smartass, but I don't know if you already know this.

Pure ethanol boils around 185F. So if I heat my sample pans on a hot plate, I've gotta be really careful not to exceed that. A water bath isn't acceptable because of splashing and contaminating the sample.

I'm working with a compound that is heat sensitive. It effects the dried product quality.

If I lower the atmospheric pressure above the ethanol with a vacuum box. I could get my boiling point at 140-150F.

So i could pretty quickly dry it, and not expose my sample to high heat, or long heat.


On a side note, I see a lot of people online building vacuum chambers from PVC. But they really don't say how low of a vacuum they reach. Any ideas on the compression strength of 6" pvc. Compression into the pvc from the side, not along the pipe/tubes main axis. I tried searching it last night and could only find the minimum water pressure which was 130-220 psi depending on schedule 40 or 80.

 

[MikeHalloran]

>>>My plan was just to buy 12" square peices of 1/4" or 3/8" carbon steel plates, because they can withstand 30k-60kpsi.<<< The numbers quoted are yield stress, not pressure. Luckily, the external pressure can only amount to 15-ish psi max, but it can develop a pretty decent stress in a thin plate. You should be able to roughly analyze each plate using equations from Roark.

I have seen rectangular vacuum boxes a bit bigger than your 12" cube; I'm not sure if they were obtained commercially, or custom built from 1/4" steel plate with 1" Lexan tops and flanges at the free edges. I do remember some difficulty with the flat gasket oozing into the chamber, and I also remember application of vacuum grease making the problem much worse, and I think there was a personal injury associated with the top being launched sideways somehow, so be careful.

This is a little smaller than what you intended to build, but it's cheap, and it's done:

http://www.artmolds.com/product_details.cfm?product_id=221&page=0&cat_name=Studio Accessories

Mike Halloran
Pembroke Pines, FL, USA

 

[BAretired]

The easiest way to make the box is to start with a round hollow section with 16" diameter and cap it with a plate top and bottom. A local steel fabricator could make it for you for a heck of a lot less than forty or fifty grand.

BA

 

[Compositepro]

Wow, you have really blown your problem out of proportion. How could it possibly take 24 hours to evaporate 15 grams of ethanol? I bet you are covering your dish. Leave the dish uncovered in the fume hood so there is a gentle draft over the dish. Then if you want to speed-up drying, put a 250 watt heat lamp a foot or two above the dish.

 

[ports394]

Compositepro, realistically it's between 18-20 hours. My lab only runs an 8-5 shift. So no one is there at 2am when the sample is "dry".

The sample is a solution, not a suspension. Some of the solids have the ethanol bound to them. Takes a bit longer to get rid of it.

The proof of this is the inconsistency in sample results from other lab equipment when the samples are not dried long enough.

 

[Compositepro]

What you are doing is a standard laboratory procedure. Move your question over to one of the chemical engineering forums. You do not need to build a vacuum chamber and ask structural design questions. By the way, Mason jars are vacuum chambers for food storage.

 

[ports394]

Mike, thanks for the heads up on the PVC vacuum chamber. I was working on something similar in my notebook, but was struggling to find the right data for 12" PVC.

Compositepro, I think I let this discussion drift too far from the original question. I wanted help with designing a vacuum box. I wasn't seeking advice or opinion on laboratory procedure.

I'm an engineer, and have a BSME, and EIT on the wall to prove it. I would love to build the simpler lab equipment, than buy it. Especially if I am learning in the process.