Building an Oven...

[AppleNation]

Anyone have any experience creating an oven?

constant 80 degrees celcious is required. More than that would be nice.

[marshmat]

80 C is fairly easy to reach. The prepregs I used to work with needed 120 C; our ovens had 3" aluminum/Roxul walls and could reach this, albeit slowly.

Do you have access to Roxul mineral wool insulation, or something similar? It's very cheap, fireproof, non-conductive and doesn't make you itchy.

The hard part is the supporting structure. If you can build a sound frame that won't collapse on top of your mould and vacuum bag, you're most of the way there. Cover it with whatever sheet metal you have lying around, pile on the Roxul, and (if you're feeling fancy) put a sheet metal or plywood skin over that to finish it off.

The 250W heat lamps sold for bathrooms make wonderful heating elements. Now and then, the police forces have to dispose of marijuana grow-op equipment they've confiscated, and the 1000 W bulbs the drug growers use make wonderful curing lamps if you can get your hands on a few. On our old ovens, 800 to 1000 watts of lamp per cubic metre of oven, with 3" of Roxul, would get us to 80 C in 40 minutes or so, and 120 C in less than two hours. You'll need a thermostat (driven from a thermocouple taped to the part) to drive relays to switch them off at the proper temperature. (Oh, and make sure you run each bank of lamps from a different extension cord to a different circuit- 4 kW will most certainly blow a breaker if they're all on one plug!) A small fan or two (motors outside the oven, driving a long shaft to the blades inside) keeps the temperature fairly uniform.

[AppleNation]

Thanks.

You're a legend.

Too bad you're in canada, I'd offer you a job !

How expensive to create this "oven" would you guess?

[marshmat]

The last one I used had been built from scrap 15 years earlier by former members of the solar car team. I don't think they spent more than a few hundred bucks on it, plus paying for an ESA inspector to verify that it was wired up safely.

A 40 square foot bag of 5-6" Roxul is about $30, that's about 17 pound Sterling to you Brits. The rest of the oven can be built from scrap metal for next to nothing, if you have a way to get scrap heating ducts and such. If you use plywood for the outer casing, it'll be less likely to burn your workers than if the whole thing is metal. The only expensive parts are the thermostat and relays to keep the temperature stable, and these have to meet your local electrical codes.

[AppleNation]

Brilliant.

Thanks again.

Off now to day-dream about ovens.

Merry Xmas.

[AppleNation]

And what if this oven needs to be up to 60 feet long 20 foot wide and 16 feet high?!

hmmm...

[TeddyDiver]

Quite large sauna...but not too difficult..

[AppleNation]

Thanks Teddy.

Just wondering whether I should hire a big shed and turn the whole shed into an oven... at least then i do not have to build a structure.....

[TeddyDiver]

How about a big tent and few layers cheapo chinese tars etc to give a bit of thermal isolation. For warming a couple of industrial heaters. Tents and heaters as rentals?
I have myself a tent 17'x40'x10'. with one additional plastic layer and with a regular (with wood) owen I get it 20C above the ambient.. so reckon you might need 3 additional plast or tar layers and about 30kw to get 60C warmer than outside..

[AppleNation]

hmm... possible I guess....

Thanks.

[marshmat]

Getting the oven to 60 C above ambient will take quite a lot more power than you might expect. You're going to need some form of thermal insulation, be it mineral wool, fibreglass batts, etc. If you're using a conventional structure I'd suggest insulating on the inside, so the structural members themselves remain on the cool side of the walls.
Also consider air leakage. On a huge oven like this, you'll need to make sure it can let a bit of air in and out so that it doesn't become pressurized as it heats up. But air leakage wastes energy and makes it hard to reach high temperatures, so put some thought into how to build it as tightly as possible. Not a big issue on small ovens but when you get to this size, the heat loss could be large.

[Jimbo1490]

I have built several curing ovens over the years, the most recent was 18" X 18" X 45' for curing masts. It could do ~90C. This particular oven was built of standard ductboard. This stuff proved a pretty good insulator, and externally the oven was only about 35C in use. I had previously used a type of cored building material from Hexcell that consisted of sheets of aluminum bonded to treated cardboard honeycomb, about 3" thick. This stuff had phenomenal insulating properties, but I don't think Hexcell makes it anymore.

I selected ductboard for this oven since it would be suspended from the cieling on ropes and counterweights while laying up the mast, then lowered into place when needed.

I used a high temp blower (a stock item at Grainger) recirculating from a 'heater box' to the oven. The heat box used heavy gauge NiCr wire coils scavenged from an old AC unit, about 4000 watts worth. The hot air was distributed via a duct (standard aluminum downspout material) which ran the length of the oven. I used an 'off the shelf' PID process controller which read from 8 thermocouples to keep tab on the temps, times, ramping, etc. Even had data logging and alarms. You'll need to use a big relay(s) to turn the heat on and off since the controller will only be able to switch a couple of amps. The whole setup worked very well and could keep temps within +/- 1*C.

Jimbo

[Sam III]

Jimbo,

What is 'standard ductboard'?

Thanks

Sam

[MarboMan]

Once used an old U Haul truck box. Removed wood paneling on inside. Lined with duct board, then finished with 15 guage sheet metal from local shed builder.

Made doors from same sheet metal and duct board. Used a propane powered salamander. Was able to reach 280 F in about 20 minutes and hold as long as needed.

[Jimbo1490]

This is duct board.

Using special tools, it can be cut and formed into orthogonal boxes and tubes, and there are tapes and sealants designed specifically for bonding it together. It's fairly light and rigid, too so it's a good choice for a 'drop down' type oven enclosure as I last built. It's made of foil covered compressed fiberglass insulation, reinforced with strands of structural fiberglass. Since it was intended for carrying air for heating and cooling, it was designed to have a pretty high 'R' value.

Jimbo