burnout oven


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Posted by joewizard on December 29, 1999 at 17:27:21:

I noticed a burnout oven question. Since this post wont reply directly to it, but it may have some useful info for someone, ill post it as a new string.

I recently built a small, simple electric burnout oven. One cubic foot (7.5 gallons).

I enjoy building equipment as cheaply as possible. The 'plan' sort of evolved as I succeeded or failed in finding the materials that seemed suitable, so i can't provide a plan, as such.

Burnout ovens can be simple or complex, I suppose. The principle of the things seems to be to let a draft of air into a hot box, and out again, carrying away water vapor and other gasses, while maintaining some control over the temperature.

This project took a week and cost me about $150.

A 16"x16"x24" steel box, formerly housing electronic equipment was found at a scrap yard.

My search for a cheap heat-insulator in sheet form brought me to a hardware outlet. What caught my eye was the fire-resistant sheet rock. Tests on small pieces, stripped of the outer paper, proved it had good insulating properties, and quite a bit of hot strength (up to about 1400 F is my guess). Fire resistant sheetrock (most, if not all 5/8" stuff) also has figerglass strands for strength.

Inside [bottom, sides, back, ceiling] was lined with 3 layers of 5/8" sheetrock, having first dampened and peeled/rubbed off the paper from both sides of the cut-to-fit pieces. (something you dont get good at no matter how much practice you get)

The first side-back layer holds up the first ceiling layer..like a house of cards. Then 2 more layers for a total of about 2" thick all around.

Sides and back are layered with insulating firebrick (sawn down the middle to 1.25 x 4.5 x 9, thus saving a few brick) and hold, in grooves, the heating elements. Wires exit through the back.

The floor was then layed. Hard firebrick splits (1.25 thick, available at a brickyard at about 75cents each) for the floor. Trim with a grinder to fit . Drilled a hole for wax drainage near the floor center.

The ceiling is also hard brick splits. It rests on the side walls. It is arched to hold the brick up there. Grind a suitable bevel where necessary. Drilled a vent hole through the ceiling. Any larger voids are filled with vermiculite. Seams can be sealed with fireplace cement.

Door is 3 full sized insulating firebrick, in an angle-iron welded frame, hinged at the bottom. Filled in around the door with shaped pieces of insulating firebrick/mortar. Peep hole/vent in the door's center. The door fits close, but it's not air tight.

Inside dimensions are about 18" wide x 10 deep by 10 high. (The door opening is 13.5 x 9)

Heating elements were obtained from junkyard clothes driers, and cut, stretched, joined to the needed length-ohms.. (you gotta know a bit about ohms/volts/watts etc.) I need about 4000 watts to get this thing up to 1000 F in about an hour. Temperature controller is an old industrial model (scrap yard.. i collect junk ) but it was a 300 F max with a type J thermocouple...

so i added 3 - one ohm resistors in parallel across the thermocouple and rewrote the temperature scale. Output from this particular controller is 2 volt DC, which trips a solid state relay, which trips a contactor. Contactor came from a scrapped electric water heater.

Temperature control is very good, keeping things within about +/- 10 F at 1000F.

This (one cubic foot) oven is about as big an electric oven as i care to build. Gas is the way to go for larger sizes. I just found a brush-torch, propane, used by farmers to walk along and burn dry grass. $20 new. It is a venturi type burner, but it sure roars.

The sheetrock will shrink, and if over heated, probably fall to a powder. It cracked randomly, but is supported on the outside by the steel box, and within by the brick. The strength here is in the steel box and the brick. Careful fitting of the brick is important.

Further details about this oven are probably not important, because so much depends on exactly what materials one can gather together.

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