Heat Treatment Oven (with separate control cabinet)

[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.