Magpie
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vacuum chamber
I often see procedures that call for vacuum drying. Not wanting to buy or fabricate a vacuum chamber I've been wondering if a small pressure cooker
might work.
Having all the parts readily available for a test I tried pulling a vacuum on my 4-quart (~4 liter) aluminum pressure cooker that I normally use as a
steam generator. To my delight it sealed right up and the vacuum reading was climbing on the gauge when I stopped. The only reason I stopped was
that I didn't want to take a chance on ruining the thermal safety relief valve. And I hadn't made any provisions for safety in case of an implosion.
If I can find another used pressure cooker I will remove the safety relief valve and plug the hole. Then with proper shielding I will give it a full
vacuum test.
The present lid gasket is rubber. To use this as a heated vacuum chamber (>100C) I would have to find (or fabricate) a gasket that could withstand
the required temperature. Perhaps a graphite gasket would be a good choice.
Does anyone see why this won't work, is unsafe, or otherwise not a good idea?
The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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Quote: Originally posted by Magpie  | | The present lid gasket is rubber. To use this as a heated vacuum chamber (>100C) I would have to find (or fabricate) a gasket that could withstand
the required temperature. |
There are RTV silicone gasket materials that are rated for engine manifold use, so
you'll have no problem forming a gasket.
The only downside is that you won't have a visual inspection. On the other hand, I suspect that you could do just fine with an internal webcam and
some LED illumination. That's cheaper these days than a sight glass.
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johansen
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Are you sure the thermal relief can't hold a vacuum? The ones i've seen would require a few hundred psi before they were pushed through the lid. (1 cm
dia.) Perhaps you meant the heat would melt it.
There's virtually no chance of it imploding. I could drop one in Puget Sound when i'm back home in a few months if you're that curious. (its 7-900
feet deep in various areas)
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Magpie
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One application I have in mind specifies a temperature of 320C for 6 hours. That might be a severe test for RTV. But it would be a good material
choice for T greater than 100C but less than 320C, I would guess.
I'm thinking that after removing the thermal safety relief disc the remaining port would be a good place to fix a thermometer, either glass or TC.
I would heat the chamber on a hotplate. The product to be dried would be contained in a removable pan supported a few inches above the vessel bottom
using non-heat conducting support legs.
It would be nice to be able to see the drying material, and that would be the advantage of glass. But I'm not sure how necessary this is.
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| Quote: Originally posted by johansen | Are you sure the thermal relief can't hold a vacuum? The ones i've seen would require a few hundred psi before they were pushed through the lid. (1 cm
dia.) Perhaps you meant the heat would melt it.
There's virtually no chance of it imploding. I could drop one in Puget Sound when i'm back home in a few months if you're that curious. (its 7-900
feet deep in various areas) |
No, I'm not at all sure. But I want to keep the integrity of my present pressure cooker for use as a steam generator. When I find another used
pressure cooker I'll have to remove it for thermal reasons.
Thanks for the offer of the test, however. 
[Edited on 27-6-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Mr. Wizard
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A pressure vessel, weather a railroad car or a smaller tank such as a pressure cooker are designed to hold pressure, which is a stretching or tension
on it's outer surface. If the container is cylindrical or spherical, as most are it can withstand much more pressure than compression. An aluminum or
stainless steel container such as a pressure cooker will be made to handle it's rated pressure, but it may not handle a vacuum without being crunched
by the atmospheric pressure, which will be a compression inward. The pressure cooker may also be designed to leak inward to prevent a vacuum from
being formed, which would lock on the lid.
This wouldn't always be dangerous to test on a small metal vessel, but it would ruin it. For larger vessels the 'pucker factor' would be larger.
http://www.neatorama.com/2008/04/23/railroad-tank-implosion/
[Edited on 27-6-2011 by Mr. Wizard]
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unionised
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Is it just me who thought
"The product to be dried would be contained in a removable pan supported a few inches above the vessel bottom using non-heat conducting support legs.
"
And insulated by a vacuum so it doesn't heat up?
Also, if you heat Al to 320C it will lose a lot of its strength.
[Edited on 27-6-11 by unionised]
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Magpie
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@Mr Wizard: Yes, I know that vacuum can collapse a vessel not designed for same. I have seen a very large tank collapsed by its transfer pump when
someone sealed the vent. I am relying on the relative small volume of the pressure cooker and its thick walls. A test will tell.
@unionized: I'm assuming that the primary mode of heat conduction would be radiation.
[Edited on 27-6-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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albqbrian
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For the seal...
In our town, 500,000 folks; there was one main company that sold gasket material. The owner was always willing to sell (or occasionally give)me small
pieces for my rocket schemes. He'd cut the stuff to what ever size I needed. A great resource.
He had a bunch of old asbestos based stuff that he pretty much gave away as no commercial (that is regulated) company could use the stuff. You could
try to find something like that.
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Panache
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cut one from annealled copper sheet, or cut it then anneal it, you'd want to be able to torque down the lid on it quite well, but once heating begins
the copper expansion will make a great fit in future heating cycles, have guide points so its always fitted exactly in the same position
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Magpie
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At one time copper sheet was reasonably priced. For a typical pressure cooker you might need a ring 12" (30cm) in diameter, and that might be fairly
expensive. But a sheet of graphite that size might not be cheap either.
Pressure cookers typically seal through a cam action consisting of only ~30 degrees of lid rotation. I don't know if that would generate the kind of
force that copper would require. But that would depend on the slope of the cam too. The normal pressure cooker gasket is rubber and the design may
give a better seal as the pressure increases. With a vacuum it might give a poorer seal.
The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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Perhaps. But the the ΔP for vacuum is only 1 atm,
which just isn't very high. And for chemical work, the acceptable vacuum inside is rather high, so leaks that would be large for, say, a mass
spectrometer, are small in molar terms. FWIW, copper seals are used in UHV (ultra-high vacuum) work, but the geometry is different. They're designed
to fit two-level flanges with a sharp transition. They deform in the process, sealing tight to all surface variations in the flange. They're also only
usable once per mounting.
Another sealing technique I've heard in passing uses a tin-coated loop of spring-tempered metal. The tin is malleable enough that it seals right up.
The tin is electroplated, as I recall, since just melting it on yields too much surface variation.
As for fabrication, it's occurred to me that you might try a product called COPPRclay. It's sold into the jewelry trade, but it's essentially a copper powder in a binder. The firing first burns out the binder, then sinters
the copper. It's fired in aggressive reduction: in a bed of activated charcoal. There's a firing schedule for enameled piece, which presumably makes a
closed-cell foam in the finished pieces. That may be good enough. If not, then plating on a goodly layer of tin should do the rest.
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johansen
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I would start a new thread but my brother was unable to recover but 700 grams of the glass left over from one of these bottles.
He sent it down to 285 feet of water before it imploded, 1 atm air inside.
On a separate occasion last week he pulled a vacuum on it, and sent it down to 85 feet, the bottom of the harbor.
285 feet of water makes about 8.3 atm.
On two other occasions last month he did a pressure test, two bottles burst at 85 and 80 psi.
Side wall thickness for all three bottles was on the order of 3 mm.
Personally i have been using them for vacuum chambers for years, on three occasions I personally have fired a .17 caliber bb gun through them with a
hard vacuum, then duct taped over the holes and repeated the shot a few times.
Next up is a drop test, any recommendations? I think a 3 foot fall onto tile would be catestrophic.
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peach
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I've had quite a few pressure cookers. I don't think the lids on mine even cam down. Rather, I think the twist simply locks it in place and the seal
is formed by the gasket it's self trying to squeeze out through the gap as the pressure rises.
I agree with unionised's point about using aluminium at that kind of temperature under vacuum. I think it may be pushing it, particularly if you ever
up the heat going in to try and get the pan it's self, inside, to dry better. I'd go straight for stainless, which tends to go through it's curie
style loss of strength at about 700C from memory.
With regards to the seal, you are encountering precisely the same problem as the guys who designed the blackbird, in that they needed to find some
polymer that could get extremely hot yet also remain flexible and durable. They never did, so the plane always leaked fuel when sat on the runway and
would only seal when it warmed up in flight. To their knowledge, when discussing it decades later, there was still no such seal. I'm not aware of one
either.
I'm curious to know, I may have missed it, what scheming you are up to on what's to be dried. 
You mentioned supporting the pan off the pressure cooker's base, which seemed odd to me since it would heat up so much better if it were in contact
with the base. However, further thinking inspired the following, brought to you by Gimp;
Fix a cooker's heating element to the base of the drying pan. Fix ceramic insulator stand offs to keep this off the base and walls of the cooker.
Attach a thermocouple to the pan. Run these out through feed throughs in the lid. Attach a thermocouple to the lid it's self, near the gasket.
Possible additional embodiments include the use of foil to provide radiant heat shields.
The pan will get hot, quickly. The vacuum and stand offs will prevent convection and conduction from transferring that heat to the walls. The only
remaining mode of transmission is radiation. I suspect it'd be quite easy to keep the lid below the service temperature for engine type sealants like
this; you can now also cool the walls without cooling the mode of transmitting heat to the internal pan. Using a thermocouple directly on the drying
pan is going to save you a lot of guessing. Dependant on what it is you will be drying, aluminium is also likely to be a bad idea due it's reactivity,
you may find it is corroding over time if anything remotely acidic or basic is coming off the drying pan. You should also consider the possibility of
the thing being dried expanding out of the pan, or splattering onto the hot aluminium walls.
SR71's before taking off;
{edit} Yes, the alien's name is Brian.
[Edited on 8-8-2011 by peach]
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Magpie
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| Quote: Originally posted by peach | I've had quite a few pressure cookers. I'm not sure the lids on mine even cam down. Rather, I think the twist simply locks it in place and the seal is
formed by the seal it's self trying to squeeze out through the gap as the pressure rises.
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I think you are right.
| Quote: Originally posted by peach |
With regards to the seal, you are encountering precisely the same problem as the guys who designed the blackbird, in that they needed something to get
extremely hot yet also remain flexible. They never did, so the plane always leaked when sat on the runway and would only seal up when it warmed up in
flight. To their knowledge, when discussing it decades later, there was still no such seal. I'm not aware of one either.
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Graphite is flexible, and will take the heat, of course. But I'm skeptical that it could be made to seal a pressure cooker in a vacuum application,
because, as you say, normally sealing is provided by the gasket being pushed into place. However, my limited testing described above showed that a
vacuum rapidly formed with the normally used rubber gasket.
When making dyes/intermediates vacuum pan drying is often specified. There are other applications also.
| Quote: Originally posted by peach |
You mentioned supporting the pan off the pressure cooker's base, which seemed odd to me since it would heat up so much better if it were in contact
with the base.
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Pressure cookers commonly come supplied with offsets. This, I imagine, is to prevent localized overheating of glass jars. I would want one for the
same reason, ie, preventing localized overheating.
[Edited on 8-8-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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MeSynth
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What about rotovaps? I haven't seen one for sale EVER.
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Panache
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hey was thinking about this again yesterday, is there a reason gold isn't used more for gaskets its so very maleable and inert?
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watson.fawkes
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| Quote: Originally posted by Panache | | hey was thinking about this again yesterday, is there a reason gold isn't used more for gaskets its so very maleable and inert?
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Cost. There are cheaper ways to get the same result, even when labor costs are taken into account.
Hell, if it weren't for cost, I'd have a gold frying pan. Better conductor of heat than copper, more inert than stainless.
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peach
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There goes one. If no one bids, you could probably get it for even less.
| Quote: Originally posted by Panache | | hey was thinking about this again yesterday, is there a reason gold isn't used more for gaskets its so very maleable and inert?
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Indium wire is sometimes used for high vacuum seals, since it can be formed into different shapes rather than relying on the premade ISO circles.
If only there was someway to make a paper drying pan, fill it up with recycled bachelor chow and the disposable lifestyle would be complete. Based on
how often other people want to use my frying pan for me, it could be solid ultra pure platinum and never go anywhere.
A piece of crappy wire that needs to go in the bin?
Nah, it's quite expensive.
[Edited on 18-8-2011 by peach]
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