Creating a heated Vacuum chamber.

chayced

Not sure whether this would be better in materials or mechanical.

I'm building a vacuum chamber which is going to house silica gel, I've got a prototype made out of PVC, but I'm afraid of running it up to 180-200 deg F to regenerate the silica. For the large scale I'm gonna need to build a chamber to house around 40 gallons of silica gel, I will have a better estimate once I play around with the prototype some more. Oh and I plan on using multiple tubes rather than a single pressure vessel.

I was wondering if anyone has any experience with heating CPVC. I know it will do better by looking at the pressure temperature tables, but is this a reasonable solution? Is there something better?
Thanks.

mgb_phys

If the vacuum is good enough the walls shouldn't get hot, especially if you line them with aluminium foil - remember thermos flasks!

Assuming you put the heater coils directly on the containers of silica gell and are carefull to insulate the tanks from the walls, ie. use fibreglass standoffs. But 40 gallons seems quite large, are you sure about the thickness of the walls to survive the pressure load?

chayced

Insulating the walls is a great idea, but that leaves me with finding a way to standoff the liner from the vessel. Darn that thermal conduction. Foam won't work due to the vacuum. Darn that thermal conduction. Any suggestions? Or should a foil liner fly for these temperatures?

If nobody has any experience I'm gonna go create some.

mgb_phys

Fibreglass is easiest, cheap, simple to cut, drill and glue and has low thermal conductivity.
Normal way is to build a couple of rings that fit the inside diameter of the tube with thin arms to hold the tray. You can get vacuum tight feedthroughs for the power to the heater but unless you want superhigh vac you can just use a waterproof connector and lots of epoxy!
Be careful about the insulation on the heater wire. Without any air to conduct the heat away the wires will heat up a lot more, it's best to overrate the cables.

40 gallons is about the size of an oil drum and you need a vacuum jacket around that, plus you really want the silica spread in a thin layer to effectively dry it. You might have to make a series of stacked trays with 2-3 inch deep layers of silica on them.

Foil will help keep the walls cool and keep the heat in, plus you want the walls to be clean. Aluminium foil held on with double sided sticky tape is common in vacuum dewars - you might have to make pin holes in the foil to let out any air caught behind it.

ps. Don't worry about creating a really high vacuum, you will need to run the pump continually to draw the water vapour out anyway, a rotary backing pump shoudl be fine.