I am looking for recommendations on making a fairly good-sized rectangular electic heat treat oven. I will be the only one using it. So, it does not have to be built to withstand abuse or be built to be completely idiot proof (maybe on the idiot part). First my questions and then some background. 1)I am thinking that I would like to use rigidized or Satanite-coated ceramic wool for most of the floor, walls and top. How exactly to mount shop-wound heating coils is one question I have not got fully in mind. I realize it is far easier to use IFB with gooves for wires. But that would be pretty expensive for the furnace I want to build. So, what is a good strategy for holding the coils in place? 2)The optimal resistance wire to choose? Canthal vs nichrome vs other? Background basic design parameters: Maximum temperature needed 1750F. Most of the time operating at 1150. Inside dimensions: 52"L x 14"W x 9"H Rectangular shape with the floor portion staionary and the sides and lid being one piece and hinged to swing open. Heating wires located in the two long walls presumably. The floor to have cross ribs of brick (Probably faced with hard brick for durability) spaced every 12 inches to support castings to be heat treated. The castings are long and narrow in shape and weigh a max of 50 pounds each with a max anticipated load of 200 pounds. Power supply 220V AC 20 or 40 amp. Programmable temperature controller and thermocouple. Support frame and facing of 1 x 1" angle and thin-guage sheet metal 2 or 3" of ceramic wool insulation. I appreciate any help or insights. Denis
Wrap the coils on a ceramic/mullite, Al2O3/etc rod. Hang the rod on similar insulating posts that penetrate the wool and mount to exterior wall. For that temp range, I wouldn't think there would be much preference for FeCrAl over NiCr wire, but, NiCr has more hot strength and will droop less than Kanthal. That probably doesn't matter if you use a wrapped rod but it may if you just string the coils between support posts. The resistance can vary quite a bit between the two so that may also affect your choice, but in general heavier gauges are more durable but require much more length to get the require resistance. How closely do you want to control temperature? I presume that will be for iron and steels? I ask because you can get rather dramatic difference from floor to ceiling or parts that are close to the radiating coils. When I t6 heat treat aluminum, I use radiation shield (a metal bucket). It's not needed for T5. Placement of elements is easy when they are on wrapped rods. We used to have a kiln at work that circulated internal air.....and had great temp uniformity. Many schemes just have a fan on and extended shaft of poor conducting material. I saw one furnace that had the heating element in a tube with the fan at one end. The fan ran continuously and the coil fired as needed. The air was distributed through baffles. For my larger furnace I've toyed with the idea of stainless tube ejectors for circulation......like small Reil burners but using compressed air that gets preheated through a small coil on it's way to the nozzle. A little circulation can make a big difference in uniformity, but if you don't need close control, simpler is better. -Just some ideas. Best, Kelly
That is a terrific idea on the mullite tube. Never would have thought of that. Where is a good place online to buy it? I have found a spot or two, but figure you have figured out what sources are better than others. I think that evening out heat would generally be a good idea. I like the idea of a fan blade (SS) on a non-conduction shaft to stir the air. Seems like having a setup where the coils were drawing power only 25% or less of the time would also help even out the heat. Thank you for the very helpful thoughts. Denis
The only Mullite tube I ever bought was a thermocouple sheath.....so no preferred source, but the stuff can be pretty pricey and you most commonly find them at scientific supply houses. Being the consummate tight wad that I am, I have an idea you might consider. Buy a plastic or cardboard tube the desired diameter, vibe/fill it with dense castable refractory, and just burn the tube off when you fire it. You can make it any size, length, shape you can cast. The shaft on my degassing lance is about an inch and it cast easily. Have a look half way down the first post here. http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/ Here's where I buy my resistive wire. They have Mullite tubes too but $20 for 12" length. https://euclids.com/collections/element-wire Being that well insulated, it will have a duty cycle of <25% after it reaches set point and the contents stabilize. Also, for your wool walls, if you want 3" thickness, I'd suggest you buy 1/2" wool and fold it into 3" pleats. You can then use metallic anchors half way into the pleats because the temp will be low there. This is how big walk-in/drive in ovens are made. It will be stronger, more durable, and less friable that way. I wouldn't use mortar on the surface though. Just buy Colloidal silica and spray in on to stabilize the surface. My local refractory source has a drive in oven like this to cure the big iron and steel ladles they manufacture except their walls are 6" thick made form pleated 1-2" thick wool. The floor is concrete and the load with fork trucks, then install wool on pallets like tiling the floor.......pretty slick and in expensive way to may a very large oven. Best, Kelly
Some good ideas, Kelly. With respect to the oven where they tile the floor with wool after loading, are the ladles hanging so that the floor is clear? How do they access the floor of a loaded oven? I think I will be casting rods of dense castable rather than buying mullite tube. I will likely put a piece of nichrome down the center of it in case it cracks. I will not want the nichrome wire to be stuck to the castable though as differential expansion might cause cracking. That way the broken segments with still be strung like long beads on the wire and the heating element should not just fall to the floor. Mullite would be nice, but is very expensive as you say. I will also probably cast some sort of dense standoffs to support the rods. I'll probably use a looped piece of Nichrome wire in the cener of the standoff and twist the wire to the angle iron frame to secure the standoff to the external frame. Thinkin... But it is starting to come together in my mind. Once this thing is done it will save me a bunch of trips to the heat treater 60 miles away. Plus, I should be able to cure Blu-Ram furnace parts in it when it comes time to reline my furnace. Denis
No, They built a steel pallet with fork skids that holds layers of loose IFB. Top coarse is dense brick. The ladles just sit on the pallet. The ones I saw were about 6ft tall and 4ft in diameter but they make much larger. They don't once it's loaded. The pallets are all the same size and they have a grid etched on the concrete floor. They load the furnace from the back wall to the front. The entire front opens with two hinged doors. The room is roughly 20-25ft on each side. It was built free standing in warehouse space. That's probably a good idea. Might be a good application for those stainless refractory needles or chopped ceramic fiber. Agreed, I wouldn't imbed the wire in the castable. Some muffle ovens have the resistance wire cast into replaceable refractory plates but according to the Kanthal design handbook, the more surrounded the heating element is, the more they derate it, and it doesn't even read on the case of being completely encased. You might want to take a gander at that design guide. Page 9 has the comparison for free air, grooves, wound tubes, etc. When I designed my furnace, I designed to the max furnace wall surface loading. There is also a limit to the surface loading of the heating element. The only time I bumped into that was when I modeled thinner gauge wire to reduce the overall length at a given power level. There are some other rules of thumb stated in the guide too, like using a stretch between 3x-5x of solid wound length, and a few other tidbits. Other than that it's just using the length of wire require to achieve your resistance/power target. The resistance wire source I listed gives a four decimal place resistance per foot for your wire and it has always been bang on -You'll have fun with it. Let me know when you get to the controller phase of your planning. https://www.kanthal.com/globalasset...-elements/resistance-alloys_s-ka041-b-eng.pdf Best, Kelly
