tnhrbtnhb - 23-2-2007 at 23:15
I am trying to construct an apparatus that involves a vacuum chamber, and I need some help finding or constructing a suitable chamber. I got the
pump working (an air conditioner compresso){ hooked it up, and wouldn't you know it, the pail's wall collapses at 300 torr!
It is a 29L HDPE pail originally intended for winemaking and Ih had my doubts about it but couldn't find anything else suitable in other respects....
All I need is something that opens entirely or mostly at the top, can withstand a vacuum of 10 torr, and holds more than 25 litres. Is that too
much to ask? 
So, I need ideas - all are welcome. Actually the plastic pail seems to not have suffered any damage, so if anyone has any bright ideas on how to
reinforce it, that would be great too!
[Edited on 24-2-2007 by tnhrbtnhb]
[Edited on 24-2-2007 by tnhrbtnhb]
[Edited on 24-2-2007 by tnhrbtnhb]
Levi - 24-2-2007 at 01:29
Glass reinforced plastic comes to mind. I'm not sure how well it would bond to the outside of the container, though. I expect that it could be made
to work but it is a bitch of a health hazard to work with as I understand it (never done it).
Mix the epoxy and wrap with fibre glass. Respirator is probably necessary so check the instructions if you intend to go that route. Very strong
stuff.
12AX7 - 24-2-2007 at 09:54
Wrap? Don't you mean line!?
At the very least, secure some braces (preferrably metal) inside, much as a wine cask has bracing on the outside. Don't forget the bottom, the most
vulnerable portion...
Tim
roamingnome - 24-2-2007 at 10:02
gorrilla glue, or urethane glue is what it is..
that stuff keeps impressing me for strength
make a laminate
its sets up slow.. but put it to the test...
Magpie - 24-2-2007 at 15:11
This might do it...
http://www.pressurecooker-outlet.com/925.htm
but it won't be on the cheap.
ciscosdad - 25-2-2007 at 14:46
This project reminds me of a method I've seen online of mixing and curing (amateur) Composite Rocket propellent.
The vacuum mixing chamber was constructed of 25mm plywood. The shape was generally cubic and approximately 400mm on a side. The joints were all
epoxied and braced inside with smaller pieces of wood. The top was removable, with a suitable gasket and a small viewing window included. I guess the
window could be an any (or all) sides.
10 Torr is not a high vacuum, but the gross forces are still close to the same as a .01 or whatever Torr. You just don't have the same outgassing or
small leak problems as with High Vacuum. The builder did comment that the sides bow inwards noticably when the vacuum was applied.
Please keep us informed.
There are interesting electrical discharge effects in vacuums of this order (if that is of any interest to you).
not_important - 26-2-2007 at 01:50
Get a heavy-walled metal shipping drum, large enough to hold the 25 liter pail. You may need to reenforce the drum on its interior, particularly the
bottom and lid. Gasketing of the lid will be needed, but such gaskets are sold.
Put the pail in the drum, seal the drum, and evacuate. Run any needed tubing through the drum lid down to the pail, if you've got to be able to add to
the contents while under reduced pressure.
HDPE isn't that strong, 3 mm thick pieces can easily be bent by hand.
tnhrbtnhb - 26-2-2007 at 15:23
Thanks for the advice, but what I really need to know is: Where do I get something like that? And the gasket? I have found a place giving away 55
gallon fuel drums, but they only have a small opening at the top for pouring fuel out. If I could get a lid for them somehow...... As for the
strength of HDPE, I was originaly thinking it was pretty strong, because I knew dyneema was polyethylene, but yeah, it's not actually that strong....
it's tensile strength is about 2.54 ksi, so about one fifth or fourth of steel :-(. So even if this pail is 3mm thick (and that's probably an
overestimate) it's not going to hold, is it?...... What thickness of regular boring carbon steel will withstand a vacuum on one side, the cross
section of the peice being a circle with a 20 cm radius?
[Edited on 26-2-2007 by tnhrbtnhb]
Magpie - 26-2-2007 at 21:17
A regular cylindrical vessel with a welded flange at the opening that would bolt to a blind flange could be made from regular carbon steel pipe. I
was going to suggest that but did not know if welding something up like this would be too expensive. This then would be a cylindrical vessel
stiffened at the bottom (by the circular bottom plate) and at the top by the bolted blind flange.
For the above vessel, using V=25 liters and r = 20 cm, then I can give you a minimum wall thickness based on the nomograph of figure 24-8 in Perry's
Chemical Engineers' Handbook, 3rd edition:
If diameter = 40cm then height = 19.9cm. If we assume a wall thickness of 0.09" (2.3mm), and a maximum temperature of 300F(149C), then from the
nomograph the maximum external pressure = 15.7psi (812mmHg.) Since 1 atm = 760mmHg this wall thickness is somewhat more than enough.
In my standard piping handbook the smallest wall thickness for 16" (40.6cm) diameter pipe is 0.165" (4.2mm). It therefore appears that any standard
pipe would be acceptable for full vacuum in the above configuration.
Now, if you choose to use a steel drum, the above calculation may not hold unless the opening is suitably stiffened to prevent buckling.
I hope this helps, and I would not at all mind someone checking my calculations and use of the figure.
Twospoons - 26-2-2007 at 21:53
A 9kg propane cylinder would just about do it. Cut in half, weld on a flange, and bolt it back together.
Pressure Cooker
MadHatter - 26-2-2007 at 22:01
I like Magpie's idea of using a pressure cooker. My 21-quart model is contructed of a very
thick stainless-steel. Even my mother's old 10-quart aluminum model could easily
withstand vacuum. But Magpie's right - you won't get them cheap !
Magpie, I went to the link you posted. At 27 LBS and made of aluminum that thing must be
very thick-walled !
[Edited on 2007/2/27 by MadHatter]