Ward kiln conversion for heat treating.

[GT Edgeworks]

Hey guys.

So a discussion was had in the new member area and some asked for details on my heat treating oven conversion.

Now I'm not a sparky so all my work was done at my own risk and everything is advice only. If you are not confident I suggest you have a sparky do your wiring If you decide to have a go.

So I was heat treating by eye with color and a magnet in a gas forge. It works and it's fine. However I wanted to start working with some more complex alloys that needed a bit of precision.

So I bought a front opening ward kiln. Now the potentiometer type controller is not suitable. So I got an inkbird PID on eBay and went about swearing at the kiln for half a day while I worked out how to wire it up.

Now she works a treat! I get very consistent results from multiple steel types.

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[BobL]

Good job on the PIDing.

That looks like a good size kin for only 8A/240V, what temperature can you get it up to?

I'll be interested to know what kind of element lifetimes you get. The kilns we had at work that were only being used on an occasional basis seemed to be always burning out or breaking elements. Do you know of any elements that don't do that or don't break as often?

I built a 2 torch mains powered gas furnace a few years ago but haven't used it much and it takes up a lot of space. It now needs a reline but on my back veranda I have an elderly 240/20A kiln that work chucked out about 5 years ago because it kept breaking elements and was thinking I might get it working to replace the gas powered unit, provided of course I could get elements that lasted.

[GT Edgeworks]

I haven't had to replace the element as yet and I use it weekly. I've had it up to 1080deg for doing stainless heat treats with no issues. Just takes it a while to get there

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[BobL]

I haven't had to replace the element as yet and I use it weekly. I've had it up to 1080deg for doing stainless heat treats with no issues. Just takes it a while to get there

Thanks - how many weeks? and what's "a little while"

[GT Edgeworks]

Well I've been running this element for the past 2 years and I did not replace it when I bought the machine second hand. So who knows how old the coils are! And to get to 1080 perhaps an hour or so. It gets to 815deg in about 30min that's where I am usually heat treating.

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[Torchgirl]

GT, when you say the ‘potentiometer type controller’ wasn’t suitable, was it because it was broken? Missing? Why wasn’t it suitable? I’ve just bought a kiln the same as bit of a project.
Thanks ☄️

[NedsHead]

GT, when you say the ‘potentiometer type controller’ wasn’t suitable, was it because it was broken? Missing? Why wasn’t it suitable? I’ve just bought a kiln the same as bit of a project.
Thanks ☄️

He probably means it's not ideal for controlled ramping up/down and soak temperatures for heat treating,

what are you using your kiln for?

[GT Edgeworks]

That's correct. Pyrometers have a place. But they are not stable enough to heat treat steel accuracly. A PID is set temp and walk away. Pyrometers mixed with potentiometers require attendance until you get used to your kiln. They are also not as energy efficient as a PID. The PID will feather the power supply to keep temp up. The potentiometer style runs at whatever percentage power full time. Where as the PID will get to temp and only potentially use 1% of the normal amperage to keep it there. There is probably math to work it out better. I've done several kilns since writing the original post now.

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[Torchgirl]

He probably means it's not ideal for controlled ramping up/down and soak temperatures for heat treating,

what are you using your kiln for?

Thanks, NedsHead

I want to use it for annealing torchworked glass beads, firing metal clays like silver, and mucking around with glass. But being able to heat treat other metal appeals too.

I have a bigger Tetlow kiln for clay. But it’s overkill for annealing small things.

[BobL]

A potentiometer wouldn't be run at % power at all times on a kiln because potentiometers cannot handle typical high kiln currents.
Potentiometers are used in some kiln current control circuits together with TC feedback to switch a relay, that in turn switches the kiln element on and off to regulate the kin temperature, which is the same as what PID controlled kilns do.
Potentiometers used in conjunction with a $5 micro controller and TC to monitor the temperature can achieve surprising thermal stability and circuits like these also use similar amounts of power to what a PID does
Last year I set up a thermal device with these controls and was able to achieve +/- ~1.5% temperature stability without trying too hard.

However, PID control offers other significant advantages
For example, PIDs are much better at controlling temp in low mass, poorly insulated devices.
Last year I set up a PID control for a home made tempering oven for the knife makers at the local mens shed.
The oven was just a 300 x 300 x 150 mm steel box made of 6 mm thick steel plate with no insulation but it was able to achieve a temp stability of +/- 0.5% .
In a well insulated, higher mass device, like a kiln , even greater temperature stability (0.2 - 0.1%, which translates to 1º at 1000ºC) could be achieved.
The other major advantage is that PIDs can be easily programmed to ramp up/down/hold for specific times.

[nigelpearson]

TorchGirl, welcome to the forums.


Bob, I assume the old kilns use a big "rheostat" (i.e. heavy duty wire-wound thing),
or a large multi-tap step down transformer,
instead of a potentiometer (small carbon film volume control type thing)?




Mind you, the only kiln I remember is the old one from school art classroom.
I think it was a bi-metallic strip thermostat,
with an auxilliary thermometer gauge poked in one corner :roll:

[BobL]

TorchGirl, welcome to the forums.
Bob, I assume the old kilns use a big "rheostat" (i.e. heavy duty wire-wound thing),
or a large multi-tap step down transformer,
instead of a potentiometer (small carbon film volume control type thing)?

Yeah bimetallic strip would have been widely used but the better ones would have used used a voltage comparator circuit not unlike I described in a previous post.

Here is a 55 year old muffle furnace (sans the front door which I do have somewhere) I'm attempting to resurrect- you might recognise the all SS trolley I welded up for it a couple of months back.
I've been physically incapacitated and have only just managed to get help lifting the furnace onto the trolley which also allows me to look at its internals.

It has only 3 doodads on the front.
3=switch, 1=indicator lamp, and 2 = red sliding/lockable knob attached to the temp gauge - this sets the temp.

393952

Underneath it looks like the photo below.
3 = switch, 2 - a large relay, and 1 is a box of analog electronics that is basically a voltage comparator.
The black circular thing on the lower left of the back of the comparator box is made of glass and looks like an electronic valve.
The pale green shrouded wire is the thermocouple (TC) lead.

The red knob on the front of the furnace sets a referencevoltage (calibrated to a temperature) inside the voltage comprator box.
The TC (amplified) signal is compared to the reference voltage every X seconds and if the TC signal is too low turn on relay, if it's too high turn it off.
This would have been quite an expensive furnace in it its day.
393953
You can see another


I'm having a hard time replacing a couple of the broken furnace elements as they are a uncommon size.

Apparently it has not been in use for about 20 years so I'm not holding my breath that it will work.
If it works, I have an optical pyrometer that I can use to check it to +/- 5ºC at 1000ºC.
If it doesn't work or hold temperature properly I will convert to PID control.

[Neil317]

Something to check:
Some older equipment will have asbestos covered high temperature wiring. The wires will have a coarse slightly uneven braiding compared to fibreglass.
Take care.
Neil

[BobL]

Something to check:
Some older equipment will have asbestos covered high temperature wiring. The wires will have a coarse slightly uneven braiding compared to fibreglass.
Take care.
Neil

Yep mine also has 2 hard asbestos sheets inside it and it also contained a badly fraying blanket of the stuff which has been replaced with regular furnace insulation (that stuff is not good for you either). Wearing PPE I vacuumed the areas in and around the friable stuff using an old vacuum cleaner which sin ce been damaged beyond repair and disposed off.

[GT Edgeworks]

Just for reference. Ward kilns NEVER contained refractory with asbestos in it. The wiring I'm unsure but the chap in Adelaide that built 16000 ward kilns in his career that does elements now told me that they never used asbestos bricks or blanket.

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[familyguy]

The 'potentiometer' used in most kilns is actually a simmerstat - and was commonly used in domestic electric cookers, simmerstats operate by varying the time on vs time off ratio. Back in the early 70's I took a year or so off uni and worked as lab tech in a senior college, I was called to 'service' the kilns in the art rooms a few times - mostly to replace elements but once to replace a simmerstat on a small benchtop kiln used for enameling jewelry, being so long ago my memory is a bit hazy but I believe the larger kilns used a mechanical cutoff where a small piece of pottery held a latch open - once the required temp had been reached - determined by the properties of the small piece of pottery - the piece collapsed allowing the latch to swing down, hit a switch and shut off the power - it was quite crude and often used in conjunction with a slump cone - a small cone shaped piece of pottery was put into the kiln along the bits being fired - once the cone collapsed the temp had been reached - a visual confirmation of correct temperature, if you were within earshot of the kiln you could hear the latch dropping and hitting the switch and could go and have a quick look to see if the cone had collapsed - a quick google search showed these things are still in use they are called pyrometric bars and cones.

[BobL]

The 'potentiometer' used in most kilns is actually a simmerstat - and was commonly used in domestic electric cookers, simmerstats operate by varying the time on vs time off ratio.

That's the way many of our larger lab hotplates worked. In the 1980's smaller ones started being controlled (not very well) by conventional incandescent lamp dimmers.

In 1970 one of the simmerstat hotplates was inside a PVC fume hood and its simmerstat short circuited went to full power and melted through the PVC fume hood into a cupboard underneath containing organics (ethanol, chloroform, ether etc) the resulting fire and fume damage was about $200,000 which was lot of money back then. The blackened simmerstat mounted on a polished wooden plinth is still in the staff trophy room or at least it was when I was last in there.