Heat treat oven questions

J

joedhiggins

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May 31, 2016
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485
My muffle furnace recently had an element burn out. With the cost of a new muffle being nearly that of building a kiln/oven from scratch, I have decided to build an upgraded oven (may still fix the muffle, but it is just not big enough). I have installed a bunch of 220/50 outlets in my shop and am planning on using one of these for power.

I was planning on using probably at least 30 of those 50 amps for the element, though I could easily bump that up, I am worried about a few things.

First, my internal volume is 668 cubic inches. I haven't seen many ovens this small with that much juice, and I worry that I could somehow be overpowering it. I cant really think of how this would be a problem, but it doesn't seem that other kilns/ovens use that watt-to-volume ratio. Maybe the rapid heating might make the IFB more prone to cracking? Not sure, does anyone think this is an issue.

Second, having always used a muffle, do I need to be concerned about radiant heat overheating the blade? should I sort out some type of muffle to shield the blade? Maybe just use toolwrap for everything, or is this not a concern.

Also, I am using k26 IFB for the build. Should I consider coating with ITC, satinite or any other?

Finally, for the door, does anyone have any particular hinging recommendations? I was thinking about a bottom mounted hinge (fabricated from bent round bar) and counterbalance. Is this door type inconvenient, do people prefer side hinged doors?

I am trying to avoid the double hinged door like on my thermolyne a1500 because it would prevent the easy addition of a latch if I ever find the need (maybe not a concern)?

Any thoughts are appreciated.

Joe
 
I don’t forsee any particular problem with such a high heat input, assuming the temperature control system can cope, but nor do I see any particular advantage. The big names (Paragon, Evenheat) do things in a certain way because it works. Unless you have a specific application for which it doesn’t work, there does not seem to be a lot of point in deviating from accepted practice.

Controlling the powerful elements without temperature overshoot can be difficult. The difficulties are not insurmountable, but some process control experience is helpful. Since you felt the need to ask, it seems unlikely that you have a background in control and itwould seem sensible to stick with around 15A at 220V.

I am in England, where domestic power sockets are rated for 13A at 230V. I have built ovens using JM23 IFBs (European-made equivalents to the K23s) to give a 7” x 6” x 27” chamber on this 3kW supply. The ovens will easily reach 1300 degC, 2372degF, which is the upper limit of the type N thermocouple that I use.

I think K26 IFBs are poorer insulators than K23s. Unless you really need the higher temperature rating, the K23s are probably a better choice.

No coating on the IFBs is necessary. It does not (to me) even seem likely to be beneficial.

I have always side-hinged the doors. The first one or two had over-centre catches, which needed both hands to close and made it difficult to treat multiple blades. Don’t make that mistake. I came up with a door catch system that only needed one hand for the next one.
 
Thanks for the info. I am using an off the shelf PID which doesn't appear to allow the sort of programming that wood be required for the high heat. I used the k26s because I already had a bunch laying around from building/relining forges. I may wrap the whole firebox in insuwool if you think the difference in R value would be that significant with the 26s vs the 23s.

I will target a 3k watt range and if it isn't cutting it (I am sure it will), I will just get a higher gauge kanthal and rewind some coils.

One more question. I imagine I should double/triple wrap the element lead wires (at least it looks like many do this) to keep the heat down as it exits the brick. Alternatively, I can just tig weld some heavier gauge lead wires on the end of the coils (this seems like a better option). Either way, is the thermal conductivity of the kanthal a1 sufficiently low that I can use an insulating rubber grommet to prevent the leads from shorting to the steel box, or should I look for a higher temp insulator. Alternatively, I did buy some 500C rated wire that I can run into the channel through the firebrick, but it seems more elegant to have the element leads terminate external of the oven wall.
 
Yet another question:

How hot on average are the outside of these boxes getting. My muffle got warm to the touch, but it had very thick insulation and I never got it much past 1600f.

I think I did my math correct (thermodynamics was a very long time ago), and I figured at max firing temp (1300c) that I should have something in the neighborhood of 600 Watts at the skin of the box, and an outside surface area just under 1 square meter. Turning that into a temperature with any degree of accuracy requires math that I can't really remember so I cheated, used an online calculator and got about 200 F in still air.

Does this number jive with what folks observe from their HT furnaces?

Also, I really thought I was done with thermo for the rest of my life.
 
They can get very hot unless well insulated. An oven of just firebrick will get hot at 1500F and very hot ( burn you) at 1950F. Adding a 1" blanket of insulating wool around the brick core will make a much more efficient ... and cooler ... oven. Make a simple frame from thn wall angle iron to hold the bricks tight. Weld it together. Wrap ths with the wool, and then put the sheet metal box outer shell around that.

It takes about twice the number of bricks, but placing them so as to make thicker walls will be much better. If the brick is 8X4X2, stack them with the 4" sides together.
IIRC, it isn't linear - a 4" thick wall will be more than twice as efficient as a 2" wall.

Some Tips:
Mortar the bricks together with satanite and powdered firebrick.

Assemble each wall in a knocked together 2X4 frame as you mortar them, and let dry. Drill three 1/4"starter holes through the wood at one end where you will run 1/8" thread-all rods through the bricks from end to end to keep them aligned . Pound one end of a piece of 1/8" round stock flat to be about .15" wide and grind it as a long spade bit. Chuck in a drill and drill through each starter hole ( you can also do this one brick at a time as you assemble the bricks). It will drill through the brick easily. Pull in and out as you go to clear the brick dust. When you finish the holes, take the brick wall out of the frame and run the threaded rod through, adding fender washers and nuts. This will keep the bricks together snug and allow easy assembly of each of the four walls. You make the two ends the same way.

Place the sides that will get elements on a firm surface ( or back in the assemble frame) and use a router and a guide board to cut the element channels. Do the same for the back if your arrangement will have elements there.

Cut 2"x2"thin gauge angle iron about 4" longer than the length of the sides, and place on the corners of the assembled sides. The extra should be sticking out the back to hold the rear wall. The angle irons should be flush at the front. Use tape to hold the assembly tight temporarily.

Take each assembled end and trim the brick so it inserts 1" into the chamber (you can fit both on the front). One will be the back, and will be mortared into the sides. The other will be the door.
Slide the rear wall into the angle sticking out and mortar in place. Trim the excess angle iron off.
Cut appropriate length 1" wide strips off thin stock ( .040") to fit the sides and weld the angle iron to each other all the way around on each end.
Weld four 1" wide strips on the front face to make a metal lip around the chamber entry.
Weld two strips across the back.
This will lock all the bricks snug from front to back.

Do all coil assembly at this point, leaving the wires sticking out the back s your plan dictates.
A metal box can be built and placed over the connection area on the back and screwed to the metal strps to hold it in place.

Wrap the whole thing in 1" kao-wool and make a shell from 24 gauge stainless steel. Use pop-rivets or sheet metal screws to close the shell.
Cover the back as seems best with 1" wool and sheet metal.

The door can stay just brick, but it will need an angle iron frame welded around it. Use a piano hinge to fit the door to the shell. You may have to shave a slight angle on the outside face of the inner brick lip on the door for it to close properly.
 
Thanks a bunch. I think I am gonna need to go buy some more firebrick, but that sounds like a box that will never really need relining.
 
Build it better than any commercial unit and it will last a couple lifetimes. maybe a coil of TC will need replacing, but the box will be there forever.
 
I have built several HT ovens using mainly 3" JM23 IFBs and the highest external surface temperature I have measured is 135 degC, 275 degF, after a couple of hours of test-running at 1300 degC (the upper limit of the type N thermocouple that I use). I don't skin mine with steel because I don't see a need to do so. I fully expected to need to when I built the first one and decided to see how hot it actually got before choosing a suitable material (candidates were Zinc-coated steel, Aluminum, Stainless steel, in increasing order of cost). I found the IFB does not transfer heat to skin at all well and I can press my hand firmly onto the outside for 10 seconds without suffering damage (though it's not exactly comfortable). I feel the risk of contact burns is low enough that no further measures are needed. If I skinned it with steel, the surface temperature would be close to the same, but heat transfer from a metal surface to bits of me would be MUCH higher.

I build without mortar, solely because I simply don't have the knack. I do make the frame very tight. When I have tried to use mortar, the only way I have been able to get it to squidge is to get the IFBs properly wet. If they are dry or merely damp, they suck the water out of the mortar too quickly for it to move and form a good join.

I also thought about an extra layer of Kaowool on the outside, but it seemed to be a lot of effort for little gain.

Doubled, twisted tails brought out through the IFB are fine. I used stainless allthread to go through the IFB on the first 2 or 3, but didn't feel happy with this. Then one of the elements failed and I felt the allthread was probably a contributing factor so went to the twisted tails. When I started using the twisted tails, I toyed with the idea of tripled twists, but did not find it to be necessary.

On most of the ovens I have built, I have used an angle-iron and flat frame to retain the IFBs and have extended the frame a couple of inches at the back to give a ventilated chamber for the wiring connections to the tails. I bend the twisted tails to a loop and clamp it between a couple of large-diameter washers on 6mm (1/4") stainless allthread. On the other end of the allthread, I do something similar with ring terminals on the power wiring. By using longish lengths of allthread, I can keep the cabling temperature low and get away with cheap cable. Most of the homebuilds I have seen connect the cabling closely onto the element tails and use expensive high-temperature cable.

On the last 2, I brought the connections out of the sides and used a bolt-on back, so that the 2 ovens could be bolted together to make one long one for swords. Coming out of the sides was quite a lot of extra work. I certainly would not do it again unless the ability to join 2 ovens was critical.

Although I always use IFBs for the floor and walls, cutting the element grooves with a router, I have used 2" ceramic fiber board for the roof and door on some of my ovens. Initially, it was a way of being lazy when I built a quick-and-dirty 42" oven: I could use a full brick for the floor, giving a 9" width, and full bricks for the sides (9" less the 3" thickness of the floor brick for 6" height). Using the board meant that having a 9" gap to bridge was no problem. It worked so well I have since used it on some 7" wide ovens as well. It also avoids the need to joint bricks in the door, which is nice.
 
Gents,

I just finished my build, and I want to say a warm thanks for all the advice. This thing works great, and compared to the Muffle I had been running heats much faster and has that bit of extra length that is SO nice. As always, you guys are great and I appreciate all the knowledge bouncing around in the noggins that hang out here.

Joe
 
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