Hardening furnace HERAEUS KR 260, anyone? (long & confusing)

Hi
Took home a HERAEUS hardening (type KR 260) furnace for 100 EUR. Inside dimensions: 260 * 150 * 450 mm. Type-plate says 20 Amps.
Only problem is, one of the heating elements is burnt. I thought that it will be a piece of cake to repair it. Dumb assumption!
Now I tried to find out by some "logic" what the right resistance would be. It has 5 heating elements: Bottom: infinite (burnt) Top: 24.2 Ohms Side left: 12.2 Ohms Side right 1: 8.2 Ohms Side right 2: 4.5 Ohms
Side right 1 and 2 is quite strange. Adding the two, I get 12.7 Ohms, so this would be equal to side left. But why two separate elements? Seems the 4.5 Ohms are going into the temperature-switch and is heating it. Uh!? OK, maybe to fool the sensor for pre-heating?
Now as the thing is 3-phase, it is making sense that they switch together left and right (-> 24 Ohms) and have it on one phase, top on one phase and bottom on one phase and thus I guess bottom should be 24 Ohms too.
But it ain't that simple! There is a row of connectors that go to the elements, has some bridges and receives 4 lines from a magic switch / relays combination.
Drawing the elements as resistors, I have:
bott left right 1 right 2 top +----+ +----+ +-----+ +-----+ +----+ +--| xx |----| 12 |--+--| 8.2 |--+--| 4.5 |----| 24 |----+ | +----+ +----+ | +-----+ | +-----+ +----+ | | | | | c1 c2 c3 c4
c1 .. c4 are the wires coming from that "magic switch and relays". Interesting (maybe) to note is, that c1's cable has a different color than the rest. xx is the unknown resistor. Calculating from the remains of the broken resistor, I get 30 Ohm for the bottom-element.
From the drawing, there is no way to get an even load on the phases.
The magic switch has 5 positions: Off pre-heating 600°C 800°C 1150°C
Maybe this switch can be ignored for now, because I assume it works. Also, it's hard to find out how he switches, because there is that relay in between that doesn't work without power (that I don't want to try out right now).
Anyhow, the -for me- un-understandable grouping of resistors and the completely uneven load on the phases (OK, it doesn't matter; but it is not desireable) gives me the impression, that someone rewired the oven (with some abuse on beer-tokens?)
OK, now to the core question: Does anyone have some insights how resistors (or better: heating-capacity) is distributed in furnaces. I wouldn't wonder if most heating capacity is on the ground and least on the top.
HERAEUS quit the production of heat-treating furnaces and doesn't seem to answer. Yes, I googled but only found that this type is sold for about 1500 EUR used.
Nick
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assumption!
would be.

Nick,
I have a 12 KW (originally 415 v 3 phase) kiln that has three elements (left side, right side and base) each simply connected as a 'star' connection. I have re-wired them in parallel to run off 240v single phase. But it is interesting to note that when I did this I also measured odd differences in the resistances (can't remember the figures, it's a few years back) BUT I worked out that the elements had the same length of wire and the wire diameters were the same (at the ends) so the resistances should have been the same. My conclusion was that the elements had differentially oxidised and infact some bits were thinner than others. In your case at least one element has died so perhaps this is a factor in your problem.
AWEM
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Andrew Mawson wrote:

Forgot to say: Mine has 8kW.
Gentlemen! Having read all your tips -thanks a lot-, I decided to replace the missing heating-element with a light-bulb and meassure the voltages between c1 .. c4 and against N.
Side-note: Even 60 Watt-bulbs get **verrrrry** bright at 400V. :-) Some don't survive very long. Next time I'll use two 500 W in series, but I only had one at hand.
OK, drawing voltages across the resistors I found out how and when they heat with what voltages. And now it is making sense (but it ain't logical).
Preheating has the highest power dissipation (and I guessed the lowest). c1 is connected to L1, c2 to L2 and c4 to L3. Knowing the resistance between c2 and c4 (36 Ohms -> 4kW) and the power dissipation (8kW) I get the resistance between c1 and c2. As one resistor is known to be 12 Ohm the missing one has to be 24 Ohm (to get the 36 Ohm).
The other stages partially work with 230V or 400V and even a mix. No matter how they are connected, left side, right side and bottom are allways heated. The top element is left out most of the time or just gets 230V (at 1150°C) (with the exception of pre-heating).
Ah, and a note to the resistance-change at higher temperatures: They only increase from cold to (very) hot at 3..10%. Kanthal (http://www.kanthal.com ) has some nice stuff for that and the data-sheets.
Only thing missing is finding the right wire and a dealer for it, bending my coils to fit, disassemble the oven and get the new coil in ... Only!
Anyhow, the kiln is making a nice humming sound, even with the one element missing.
Thanks again! Nick
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Hi Nick
It doesn't make much sense to me either but a quick look at the figures suggests that the resistors can't all be connected to the phases using the C1 to C4 connections as this would draw more current than 20A (unless I've got my sums wrong).
I think you'll have to try it under power - you could just try and see what temperatures it achieves on each setting before you start diving in with probes.
I'd have another go at contacting the manufacturers - they must have a circuit diagram.
Good luck
Russell
Nick Mueller wrote:

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figures
using
see
diving in

a
Russell,
Don't forget that the hot resistance of the Nichrome or Kanthal wire is very much more than the cold, and presumably Nick is measuring cold.
AWEM

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Thanks Andrew
I had forgotten.
Now I know why I was doubting my calculations.
Russell
Andrew Mawson wrote:

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Nick Mueller wrote:

Sounds like you discovered why it was so cheap. Faced with such a dilemma I would be close to the point where I rip out the 'magic switch and relays' and install something I understand.
Although an element on the bottom sounds good for heating purposes, it sounds bad for risk of electrical shorts. Presumably these elements are not exposed.
I have just ordered a 'bargain' used kiln to help me heat treat small odds and sods, but it has no temperature control so I have been looking around to find whats out there. There is quite a business in small kilns for firing pottery and lower temperature small kilns for enamelling. The hobby companies might be more approachable than industrial kiln manufacturers. One company I found who sell insulation, elements and controllers is http://www.scarvapottery.com - I have not used them but they look like the sort of people who could provide advice - as they have stuff to sell you (like a replacement element). I expect they can answer questions about placement of kiln elements, oxidation of elements and the different types. There are also books on build-your own kiln.
At the moment I am waiting for the postman to deliver kiln, and PID controller with output for solid state relays. The SSRs should allow fine power control by switching part way through every cycle of the AC, this should reduce thermal cycling of the elements. This new-ish sort of power control would mean you don't need the strange pre-heat setup you appear to have Of course this is all theory at the moment - but in the words of Baldrick "I have a cunning plan".
Steve Richardson
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Steve wrote:

Yes, I knew that the bottom element was kaputt. And I thought about completely rewiring the thing and investing in a handfull of SSRs, a real temperature-controller, ... But then, it no longer would have been a bargain.

The elements are in grooves, but can/could be touched. But not, if you lay something flat over them. Will go for some kind of table to put into the oven. They do exist.

The pre-heat setting is for heating up quickly. It has the highest power consumption (I guessed wrong). With the SSRs, you are right. If it is up and running, I might replace the mercury switch with an SSR. The mercury switch does some damping, because it takes some time to cool down and close the relay again. This way, they reduced the on/off cycles. A SSR would be more gentle to the elements and could have a higher switching-frequency. I'll see how close it keeps the temperature.
Nick
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Nick Mueller wrote:
[burnt heating element]

Found a "service"-company. Some sell-out of Heraeus. But they want so sell me a complete base plate, together with the wire. Maybe they do stock it, maybe not. No matter how, they don't call back. They can keep it!
Kanthal doesn't sell the wire in small quantities and I don't intend to buy a 100kg coil at maybe 40.- EUR / kg. At least they gave me a tip who might sell me something.
OK, finally found one wanting to sell me the 55m "Kanthal AF" needed to get it working again. Nearly 90.- EUR for 1.3 kg of wire. 8-/ Kanthal AF is: 22% Cr, 5.3% Al, rest Fe The chrome is the slight plus++ to the price-tag.
Nick
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Nick Mueller wrote:

Could you tell us who you got it from please?
Thanks
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Peter Fairbrother


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Peter Fairbrother wrote:

Sure! Someone not so far away from Munich: <http://www.kittec.de
He doesn't list the wire on his page, but, as he is a manufacturer of furnaces, he has a collection of wire on stock. You have to call / eMail him to find out.
BTW: I got the temperature-indicator working again. No amplifier for a Pt-Rh-Pt element. Very delicate. Have to re-calibrate it when all is up and running. But at least it is moving again and shows kind of right values.
Nick
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of
eMail
Pt-Rh-Pt
running.
Nick,
I have a Pt / PtRh panel mounted temperature indicator that you can have for the postage if it's any good to you. Came off my induction furnace but I've replaced it with a type K one as it was only measuring water temp and a type K element and display was cheaper than a Pt / PtRh element !
AWEM
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Andrew Mawson wrote:

Thanks for the kind offer! If the original doesn't work properly, I'll come back. I'd like to use the original, because of the antique look. I have a testo type K hand-held measuring device, so I can cross-check (up to 1000°C). I have lend the testo to someone who claimed it doesn't work: "If I stick it into boiling water, the temperature never goes above 98°C". :-)))
Nick
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Nick Mueller wrote:

OK, it's up and running again. Had to get it out of the way. So I finally started winding the elements today (see my posting "Winding a 5.5m long spring ... ;-)"), assembled them into the carrier, assembled all the rest, connected the element, made a safety-check for isolation and fired it up. All that with a gas-mask on. That damned thing is isolated with asbestos! :-(
OK, 550°C in 1/2 hour. I'll have a look later how long it takes to reach 1000°C. It's gonna make me poor and the electricity works rich. Heat-treating will be something for the weekend or the night, when electricity is cheaper.
Nick
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