Electric Furnace

[ozzie34231]

Hi All,
I'm posting here because you guys seem to know a lot about furnaces.
I want to do some crude heat treating, not casting.
I have a ceramic kiln, 1000 deg. C thermocouple/controller.
I want to get to 1650 F quicker than the present elec elements provide for.
Can I use old household oven elements to do this? Will they stand that heat?
If not what and where can I find such elements?

Ozzie

[awemawson]

Ozzie,

Most domestic oven heaters will not take that sort of heat - they are only designed to be in an environment of up to about 250 deg C iirc.

Kiln elements are made of Kanthal usually, and there are several grades for differing upper temperatures, with the top end having exotic prices, but 1000 deg C isn't too taxing. The elements survive by having aluminium as one of the alloying constituants, and the aluminium oxide migrates to the surface of the metal, giving a protective coating. The elements need to be physically supported at those temperatures or they will sag. Most pottery suppply houses can provide kiln elements as a pre-wound 'spring' which is pressed into a suitable recess in the kiln lining for support.

Beware of raising your kilns temperature too rapidly, you may have problems with solid refractory insulators spalling

AWEM

[ozzie34231]

Cool Andrew,
Thanks for the quick advise.
Couple questions, if I may. What is spalling?
The small kiln I have is made from several circles of seven bricks, total about 7 or 8 inches deep total. There is a spiral of two circles of elements inside, setup for 115 V.
I have a couple 220V 50A outlets in my garage and the controller I have, which I bought seperately, is setup for 220,(thermocouple,relay and digital box). I've asked a few pottery suppliers to figure what I need to do what I want and they seem reluctant, and never come up with an answer.
I'd like a setup that would come up to temp in an hour or less. Is that practical, doable, etc.
Can you take a wild guess as to what I need in the way of wire size, length, and guage, and most impotant a supplier or two.
Many Thanks,
Ozzie

[awemawson]

Ozzie,

Spalling: If you heat something up by radiant heat (like your element) then the face of the object heats faster than the body. Consequently the heated outer layer will expand more than the body. If this happens too fast or too vigerously then the shear forces set up by the thermal expansion flake off the outer layer. Same effect happens in brickwork that gets waterlogged and then freezes. The expanding water (now ice) flakes off the outer layers and is a common sight below the damp proof course of brickwork in cold wet climate (like here in the UK for instance).

Heating up in an hour is probably about the limit dependant on the type of insulating brick. If you can measure the current draw from your furnace then you can calculate the power rating of the element (amps x volts = watts) however this is probably marked on the makers label. Now you don't say how long it takes to get to 1000 deg C, but lets assume 2 hours. If you can get a second element of the same power rating installed (hence doubling the power input) you will very approximately halve the time to your one hour target. Now if your second element is also rated at 115v then you can wire it in series with the original to run off your 220/230 v supply.

Another point to remember is that electrical insulation degrades at elevated temperatures so be careful what you use to mount the elements, and if you are not absolutely happy about what you are doing get professional advice.

(example - normal window glass is virtually non conductive of electricity at room temperatures, however if you warm it to about 650 deg C it will pass enough current to keep itself glowing orange !)

AWEM