Heat Treatment Oven (with separate control cabinet)

[hemmjo]

Thanks for the input. Your log is very informative. I just ordered some proper connectors. I am building this furnace using as many of the old parts I have been collecting over the years as possible. Trying to use up the old parts I have salvaged.

I just began some preliminary testing of my furnace. It heated to 1200˚f in about 10 minutes, then I shut it off. The high setting draws about 24amps at 240v, about 5700 watts. There is also a LOW circuit which should draw 6 amps at 240v, about 1400watts, but the old contactor I used in that circuit was faulty so I do not have data for that yet. I am going to pause testing until the low circuit is working and the thermocouple connector installed.

From your build log, the 1400 watts in the 3.5 liter chamber works out to about 410 watts per liter, seems about right. Live and learn, the 5700 watts is over 1600 watts per liter.

I shut it down very soon after I removed the vent cap and peeked inside.



I have another question. My thermocouple has a nice braided steel shielded cable. But it is longer than necessary. Should I cut that cable then use the cut off piece inside the control box from the jack to the controller. Guessing the conductors in that nice cable are special also.

[Dr.Al]

I have another question. My thermocouple has a nice braided steel shielded cable. But it is longer than necessary. Should I cut that cable then use the cut off piece inside the control box from the jack to the controller. Guessing the conductors in that nice cable are special also.

I'm most definitely not an expert, but I think that would be okay to use inside the control box. Just make sure you maintain the polarity (i.e. use the same wire as + on inside and outside) as the two wires will be different materials to each other.

[hemmjo]

Dr. Al, I have another question. In short, did you have to heat your Kanthal A to form your elements?

Background;

During testing, I burned out my elements. Live and learn. I had 4 elements, each 10Ω with 2 pairs wired in series for 20Ω, 240v, 12A 2880W each. 5760W total stuffed into my little furnace. The elements were 25ft (7.60m) of 18ga (1mm) Nichrome 80. The 4 elements were dictated by the grooves already cut into the fire brick I recovered from an old ceramic kiln.



I had my control box wired with high and low circuits. Low operated at 120V for a total output of 1440 watts. While this worked and came up to temperature in about 30 minutes with an empty chamber, when it came to actually melting aluminum, it took a long time to provide that addition heat to transition to liquid. It was painfully slow to add additional material to the crucible, then bring that up to temperature, add more, wait repeat in order to get a near full crucible. (I have several large barrels filled with crushed aluminum cans I have been collecting for this)

Knowing the high setting risked burning out the elements, but also knowing i had to remake them anyway, I tried it. It was wonderful. I could just keep putting feeding cans into the furnace, until it started "buzzing", like angry bees, after about 30 minutes. Then, stopped heating.

As formed


Installed


As removed


Shorted, welded, burned out


Failure analysis shows I did not have enough space between turns. The elements expanded, shorted, burned through, welded back together, repeating until they could not take anymore. I tried to stuff too much wire into not enough space. I guess if you don't push limits, you don't know where they are.

My intended solution is 4 elements, 5Ω each, connected in series for 20Ω, 240V, 12A 2880W on high. Low would be 120V, 5A, 720W.

While investigating Kanthal A for the new elements, I found a site (https://shop.machinemfg.com/nichrome...he-difference/) with a nice write up comparing Nichrome to Kanthal.

From the site, I see this comparison, "Kanthal holds its shape well, making it easier to create consistent coils, though it requires higher temperatures to become malleable. Nichrome is easier to bend and shape at room temperature, making it preferred for intricate coil designs and applications requiring precise shaping."

I have the elements from the old kiln that donated firebrick, but they are very brittle. I had assumed that was from repeated firings over many years.

While the gauge is much too large for my furnace, I am curious, did you have to heat your Kanthal A to form your elements?

Thanks for your detailed build log. I have added an over temperature cut off power in case the SSR fails as you mention. I had no idea that could be an issue.

[Dr.Al]

Hi @hemmjo .

I had a somewhat variable spacing between the coils but it was of the order of 5 mm at a guess. You definitely need enough to stop it sagging and closing up: the way I stretched them out was to hold them vertically and gradually move a blowtorch down the length (which heated them up enough to anneal and then they sagged under their own weight). I'd originally planned to make a chamber that could be oriented horizontally for heat treating or vertically for casting, but it quickly became clear that the element would sag if it wasn't supported along its entire length and that made a dual-orientation chamber impractical.

I used a much thicker gauge wire than you I think (which also gives more chance of resilience and longevity). According to my look-up table, 18 gauge wire is 1 mm diameter, whereas I used 1.4 mm wire so 40% thicker.

Regarding heating to form the elements, Kanthal A1 work hardens. That means that it starts off soft but as you form it into the coil it becomes hardened. You can get a certain amount of coiling done before it hardens and then you have to heat it up to get it to be workable again. In my case I managed to form the coils without doing any annealing, but then had to heat the coil up to be able to form the ends and also to be able to stretch the coils out.

If I did it again, I think I'd form it at a wider spacing in the first place, although that would limit the maximum length of each individual coil section.

I've heard that one way of spacing the coils and leaving them in an annealed state is to stretch the coil along a concrete (or other heat-resistant and non-conductive) floor and then pass current through the coil to cause it to heat up enough to anneal itself.

[hemmjo]

Thanks, that is good information. I was limited to 18 gauge nichrome due to lack of experience and trying to get the resistance up and the current down.

Giving a good look at Kanthal, the higher resistance will let me with 16ga, 1.3mm. I should be able to turn my new elements with very close to the proper space, to eliminate the need to stretch them much at all. Then I may have to open up the grooves in the firebrick a little.

Found this online wire gauge chart. https://www.rapidtables.com/calc/wir...uge-chart.html

[tonyfoale]

I was saved the trouble of making an oven as planned. Santa was kind enough to send me a bought one. 200 x 200 x 300mm.
In initial testing I set the controller to 100 C and let it stabilise for 2 hours. I then put various thermocouples that I had in through the peep hole on the door and let them stabilise. I consistently got 112/113 degregardless of the depth of insertion. 12/13 deg difference to the control sensor at 100 C is unacceptable, especially for steel tempering.
Therefore I am thinking of adding a fan to even out the temperature. I am thinking about mounting a motor on the outside of the door with a shaft passing through the peep hole to a fan inside. Has anyone here had experience with forced circulation in benchtop size ovens?

[ductape]

I was saved the trouble of making an oven as planned. Santa was kind enough to send me a bought one. 200 x 200 x 300mm.
In initial testing I set the controller to 100 C and let it stabilise for 2 hours. I then put various thermocouples that I had in through the peep hole on the door and let them stabilise. I consistently got 112/113 degregardless of the depth of insertion. 12/13 deg difference to the control sensor at 100 C is unacceptable, especially for steel tempering.
Therefore I am thinking of adding a fan to even out the temperature. I am thinking about mounting a motor on the outside of the door with a shaft passing through the peep hole to a fan inside. Has anyone here had experience with forced circulation in benchtop size ovens?

Some of those ovens have a calibration adjustment in the controller. Not sure if that model does.