Does anyone know the proper plaster/cement type of material to cast electrical heating elements into? I started with a nice old heat treating oven that I received low cost because two of the elements were bad. It has the nice double doors in the front and heating elements on the floor, side, side, and ceiling. Each of the 4 original heating elements were kanthal wire cast in ceramic and each measured 7 ohms. I successfully made 2 new side elements by cutting grooves into lightweight firebrick and laced some new Kanthal coils through them. This worked great. When I went to install the ceramic cast heating elements for the floor and ceiling, the wire broke off of the floor element. Broke right up against the ceramic. I would like to make a new floor heating element to be a higher resistance, but I don't want the wire exposed like it is on the sides. ( I actually wish it was not exposed on the sides.) I cannot determine with confidence what type of plaster/cement/compound that I need to use to pour around my Kanthal electric element to make a solid usable floor. Furnace max temperature is 2200F (1200C). Sides are working good at 7Ohm each side (14 Ohm in series) /230VAC/16.4 Amps/3778 Watts. If I can pour a new floor heating element at 14 Ohms, it would have (when in series with a 7 Ohm Ceiling element) an 1694 Watt output with 11 Amps through it. Each heating element of this furnace is 9" x 18" x 0.75" thick.
I would think any of the dense castable refractories would work just fine but I've never thought cast-in elements were good idea. You can double or triple (twist) up the leads for more strength but the expansion rate of the wire and refractory differ and cause large stresses and breakage. The max operating temp of Kanthal varies depending upon how much of the element is exposed. The more it is confined, the more it is derated and you can't get more confined than cast in place. Muffle furnaces run out of room for elements and exposed elements in the floor obviously isn't a good option and it's hard to suspend one from the roof. As an alternative, you can cast the groove into the floor and ceiling plate to retain the coil and expose the other solid side to the furnace interior. Or you could do same with soft IFB and use a thinner dense fire brick to isolate those coils from the furnace interior. Best, Kelly
I know there are reasons not to have embedded elements, but I have a 90% complete furnace that would be that much better if the floor panel was embedded. This is why I was also going to use a higher resistance for this panel, so that it is doing less of the heavy lifting for the heating. Maybe I will need to do grooves in IFB and then have a cap over it. I just feel like this is still going to be prone to scale and debris finding it's way into the coils. It seems as capping over elements in air would drive a higher element temperature than having the elements embedded.
Not so sure about that. Dense castables are poor conductors. Hard to say whether they are lesser so than air. The Kanthal handbook doesn't speak to imbedded coils most certainly because they don't recommend it. Of course, it's hard to overlook the fact we're only talking about the failed imbedded elements. See attached. Best, Kelly
