Hello from North East Ohio

Welcome Jim.

I think 3/16" is a very tight ID for #14. Mine are wrapped at 3/8" ID which makes for 1/2"OD. My 14g kanthal A1 was abot .21ohm/ft so I do agree that 45ft would put you at about 25a and 6kw with 240vac. You do want 3:1 coil stretch but preferably not more than 5:1. I have a Kanthal design handbook that has some guidelines for wound coils. See attached.

That's a good question. Pumpable was really too runny and difficult to make it stay where you put it so it had to be applied thin and dry over night. It took a long time to build up thickness. Moldable is probably easier in that respect but more difficult to make conform to the mold. I do really like the low mass furnace, but that stuff is pretty expensive for something that large and if it isnt a lift off furnace, it's pretty fragile compared to other refractories and wont like tool contact.

I had an approach using (2600F) ceramic fiber board where you would cut annular discs, machine coil shelf into each layer and stack them to the internal height. You would have 4 seperate coils, one for each layer. You run each end of each layer out of the furnace then connect them in series, if you need the resistance from the total length. Kilns usually use this approach. The advantage of this is you only replace a smaller section if one fails and it can ease construction. It woul dbe similar (low) mass and quite a bit less expensive than pumpable/moldable. I'm not quite certain hold well it would machine and I'm absolutely sure you wouldnt want to do it in your shop. I'd do it outside, where a respirator, and cahnge clothes and shower afterward. A long blade ina jig saw and finished with a drum/spindle sander would probably work well. The shelf could be cut with a razor knife by hand. If you're interested you can start a thread and I'll contribute.

I've had dense castable refractory, Insulating Fire Brick, and the Moldable CF. I have threads for all three incarnations I can point you at. The latter is definitely my favorite but the IFB was a close second, and IFB is pretty easy to work. There's a reason why kilns use it. It's in between the mass of the moldable and dense refractory. It makes for a very practical electric furnace that is easy to build in a weekend.

Best,
Kelly

 

I'm surprised there is no spring back at that ID. It's at the low end of the recommended coil OD/Wire d, (top of page 9) but you are probably ok if it fits your power and preferred lay out. I bet it's very stiff to stretch. Pages 8-12 are the meat of it for coil design. Worth reviewing those 5 pages. Just keep in mid, they're just recommendations for max life. Though the temp in the vicinity of the coil is likely a couple hundred F higher, I have my furnace set point at 1800F which is plenty for aluminum and well under Kanthal limits. You much more likely to splash aluminum on the lement than cook it.

The rating on the moldable pumpable CF is only 2300F and mine has been inservice with with bright orange coil contact for 3 years now.

The reason I recommend 2600F over 2300F CFB is because it's denser and more durable. The stuff under the skin tends to be less bonded.....the 2600F is better for cutting features like the coil shelves. A 10" Bore furnace is a nice size. Being a lift off, mine can handle an A20 which is a fairly good size crucible and at the upper end of what I want to handle free hand. I use an A10 for most stuff and the furnace could be a little smaller for that. Four courses of 2" thick board with coil shelvescut in the top so the coils are captured by the next layer, followed by what ever you need on top without coils for needed (keep element in lower 2/3 of furnace) height would be a good starting point.

Best,
Kelly

 

I have two seperate 13.5 ohm coils in my furnace that are 4kw each for 8kw total. Each coil is 63ft of 14g Kanthal. The surface area of each coil is 63ft x (12in/ft) x Pi x .062" = 147in2.

4000w/147in2 = 27w/in2 coil surface load.

Best,
Kelly

 

A long time ago I started to write a build thread on the subject. You motivated me to complete it. It's now in the Furnaces and Their Construction subforum. Might be some useful tidbits in it for you.

Best,
Kelly