Recommendation? Help with heat treat oven

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kick1121

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Feb 17, 2013
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Hello all I just finished my heat treat oven and used it for the first time today it is running on 220v It heated up way to quick got to 1300 deg f in about 5 minutes my pid was displaying hh I don't know what that means I heard an ark turned it off let it cool and found my coils melted into the brick and the brick melted also
specs
13 ohms for 2 coils combined 22 g canthal
Twidec mv100 pid 40 amp
40 amp ssr
220 v
1300c thermal couple k type
I was trying to get to 1560 f to heat treat a 52100 blade I have at 1300f it displayed hh but I think it got way hotter as it melted the brick any ideas as to what happened. Thanks
 
I think 22 gauge might be a bit thin. There is a Kanthal handbook here that shows max. surface loads for various applications. For 20 gauge wire (don't have the specs for 22 gauge), I estimate your coil would be somewhere above 200 W/in^2. This is an order of magnitude higher than the surface loads recommended in the handbook.

I don't know how that would explain the fast heat up time, though. Did you wire the coils in series or parallel, and what was the resistance of each coil? Also, what is the volume of the chamber?
 
22ga Kanthal seems very thin indeed. 13 Ohms at 220v seems to give about 3.7 kW if my calculation is correct: pretty high by most standards.

I would suspect that the element burnt out. Do you still have 13 Ohms across it?

My guess would be that the surface loading was so high that the element exceeded the melting temperature of the IFB and melted it where it touched, probably quite briefly before the element burned out?

I have tended to find that the "better" IFBs for HT ovens are those with the lowest density and highest insulation value. Over here, it means the most expensive readily available grade23 IFBs (JM23, which are Italian IFBs from the same company as the US-made K23 and having broadly similar specs). They also seem to be the most prone to melting once the rated temperature is significantly exceeded.

The thinnest Kanthal wire I have ever used for elements was 16 AWG. With hindsight, it was probably good enough for a hobby oven, but there were 3 element failures out of the first 5 ovens. All of the failures were on ovens used by guys who make knives as a full-time or part-time job, so they were the result of fairly heavy use. I used 1.6mm (14 AWG) Kanthal A1 for the more recent ovens and it seems much more reliable.

The other thing that may have failed is the SSR. A 40A SSR needs a heatsink capable of dissipating the heat generated by the actual current it is passing. A lot of the extruded heatsinks can cope with about 15A max. Putting a 40A SSR on a heatsink that can only cope with 15A means that it is effectively a 15A SSR. Over here, we have 13A mains sockets and the plugs are fused 13A. I have still had 25A SSRs on 15A heatsinks fail.
 
kick1121 said:
Hello all I just finished my heat treat oven and used it for the first time today it is running on 220v It heated up way to quick got to 1300 deg f in about 5 minutes my pid was displaying hh I don't know what that means I heard an ark turned it off let it cool and found my coils melted into the brick and the brick melted also
specs
13 ohms for 2 coils combined 22 g canthal
Twidec mv100 pid 40 amp
40 amp ssr
220 v
1300c thermal couple k type
I was trying to get to 1560 f to heat treat a 52100 blade I have at 1300f it displayed hh but I think it got way hotter as it melted the brick any ideas as to what happened. Thanks
Click to expand...

We need pictures.
How big is your oven? How is it insulated?
22ga is WAY WAY WAY to fine for that kind of surface load. To give you an example my oven uses 13ga coiled elaments and at 240v I’m sitting around 4000-4500w. Takes about a half hr to hit a good usable temp that’s stable. 13ga is .072 and 22ga is .025 thick where did you get your information to use 22ga? To give you a comparison I have a total of 43,587min of oven on time and 25,135min of element on time. That means those elaments have been exposed to over 726hrs of heat and thy still look about as good as new.
 
Good advice above. I had a failure in my diy oven where the wire clips holding the coils were too far apart. The coils dropped and sagged till they shorted out on the next coil. Didn't burn through though.

Make sure your pid is on degrees f. I replaced my pid once and it was only reaching 1600 degrees or so when I was going for 1980. Realized it was in degrees c
 
I believe the brick is k26 I got my info from a video on YouTube about the wire size each coil measured at about 26 ohms which gave me around 13 ohms together inner dimensions of the oven are 4 1/2 x 4 1/2 x 13 long is there a good/ cheap pid you guys would recommend or some good info on what the different settings do im pretty sure it reached the desired temp but it didn't keep it there it just kept going up also I wanted to show pictures but I don't know how I'm planning on making some new coils so what gauge should I use thanks again
 
That was one long sentence there!

With two 26 Ohm coils in parallel, the power density is still too high with 22 gauge wire. With 16 gauge, you would be around 25 W/in^2, which is within the recommended range IIRC, but it has been a while since I built my oven.

The interior volume of your oven is very low, about 0.15 cu.ft. That explains why it heated up so fast. There is a pretty detailed DIY heat treat oven article by Dan Comeau here that lists the power per cubic foot of interior volume for commercial heat treat ovens, and yours is about five times higher than the average commercial oven. The article also has a coil calculator spreadsheet that might be useful. The spreadsheet does not calculate coil surface load, but that is easy to add.

I think it is not as easy as picking a different gauge size and trying again. With larger wires, the resistance per unit length drops, so the coil will have to be longer or a larger diameter, or you have to reduce the power. It is a bit difficult to make a recommendation without seeing exactly what you have, but I would probably run it on 120V and shoot for around 1,200W (about 8kW/cu.ft). With 16 gauge kanthal that will require about 37 feet of wire for a surface load of 17 W/in^2 and a coil with a 1/4" ID would be about 2 feet long before you stretch it to just over 4 feet to fit in two channels on each side wall. That is similar to what I built last year (I have a bit larger volume and wattage), and it has worked well for me. If you want to run on 240V, you could get about the same surface load with a 20 gauge wire at around 60 feet length at the same power. Or you could use 93 feet of 18 gauge for half the surface load, but you would have to fit 8 feet of stretched coil into the walls somehow. I would encourage you to check out Dan's article and do your own calculations. That way, you can pick what works best for your situation.
 
If your elements are still intact, you could try connecting them in series instead. That would bring down the power by a factor of 4 and give a corresponding reduction in surface loading. It's not a solution to the problem, but it might allow you to run for long enough to determine whether there are any other shortcomings that should be addressed at the same time.
 
When I built my oven I followed this write up

http://dcknives.blogspot.com/p/electric-forge.html

Dan's site is a wealth of knowledge with fantastic writeups. He used 12 gauge for his oven. IIRC I used 14 gauge. As the wire gauge goes down you need a longer coil to reach the desired ohms as the resistance drops. That's why my oven was built with a single coil instead of 2 in parallel. I didn't have enough room for two in parallel. If your current design is based on 22g wire you may be short on space. Especially if you want to run parallel coils. There's always flexibility though in how far to stretch out the coil after coiling.
 
Same question, pictures? Let’s see your build.
 
Sorry guys I still can't figure out how to post pictures. I made a new single coil out of 316 ss twisted 26 gauge reading 16 ohms and switched to 120 v now the oven won't get up to temp should I lower the resistance or go back to 220v ? It took about a half-hour to go from 900 to 975 and is approaching 1000 even slower.
 
Are your temperatures in Fahrenheit or Celsius? If they are Fahrenheit, something is wrong.

If you have a Kill-A-Watt or similar device, plug it into that and check the actual draw. You are nominally at 900W. I would try to get the resistance to about 12 Ohm to start for your small chamber. With that, you should get to 1,500°F in around half an hour, give or take ten minutes.

Like JT said, let's see some pictures. You can upload them to imgur, there is a thread in the stickies with instructions.
 
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