Small Kiln Auxillary Melting Furnace.

There's about a 5% increase in resistance from cold to hot for NiCr (Nichrome 80/20) and FeCrAl (Kanthal) at he high end of the operating range.

Also MS, as you are thinking about coil design and electrical calcs, I think I saw you mention 220v. In the US these days 110/220vac is 120/240vac. Small detail but will make ~9% difference in Ohm's any law calcs.

Best,
Kelly

 

On my furnace, I have two spiral coils that nest like a double lead thread then both make 3 trips around the 10" bore. For your kiln, I'd wind separate 6 coils, but with any decent wire diameter, probably you'll have to connect at least three of them (maybe all depending upon wire gauge) in series to get the resistance high enough. I think I'd shoot for 9kw total.

The big benefit to 6 separate coils is if you splash metal on a coil, it's just a small piece to replace. Double or triple up[ the wire diameters and run them out the kiln wall in all the same location, then you can connect them in combinations of series or parallel however you want.

The Kanthal design guide says the furnace wall loading can be up to 4.6kw/ft2. Even at 9kw yours would only be about 2kw/ft2.

I'd use 14 gauge if you could and still get enough resistance. 3/8" OD is tight for that gauge but still permissible (OD should be at least 3x wire diameter). Mine are 1/2" coil diameter and easily wound, but it's all a matter getting enough length/resistance in the grooves you have. I'm sure you can with all 6 in series, maybe even three in series and the two sets of three in parallel. When you come up with the coil with the right resistance and dimensions, you need to check to insure you are under the max coil surface loading limit.

It's all worth doing if you want something you can push to the high temp limits and still get good life, all described in the Kanthal design handbook.

Best,
Kelly

 

Looked like your first pictured showed six grooves but only 4 of them populated........but was looking on my phone. With 4 grooves of the dimensions you indicated earlier, I get the total groove distance to be about 7.5" x 6 x 4 grooves or 180". The tightest you can space coils on .062" (14ga) wire would be .186. So that means 967 coils. 967 x .375" diameter x Pi = 1140" or 95ft of wire. Each of my 14ga coils were about 63ft, 13.5ohms or 4kw with 240vac, so the best you would do with 14 ga in your kiln grooves is only ~6kw. If you drop to 12ga, that would get you 9.5kw.

Edit: You can shorten the 14ga for higher power but it would require stretching it beyond the recommended limit of 5x solid length. 12 ga is just right at about 4x stretch for your total groove length and .375 diameter coil.

The maximum element surface loading for Kanthal is 39w/in2. A 95ft 3/8" OD 12ga coil has ~290in2 of surface area. So for 9kw the surface loading would be 31w/in2 so you would be ok with 12ga Kanthal wire. In fact, 12ga would allow up to 11kw.

This is the source I've used. If you drill into the links you can see details on both.

https://euclids.com/pages/kiln-elements

This is 12ga Kanthal. They sell it by the pound. If you drill into the links it will tell you the length per lb of wire for 12ga is about 54ft/lb.

https://euclids.com/collections/element-wire/products/1lb-kanthal-a-1-wire-12g

They also have 13 ga which would probably also get you to 9kw

https://euclids.com/products/1lb-ka...35853&pr_ref_pid=5133804568621&pr_seq=uniform

Double check me on my calcs. I'm on vacation in the sun and have had a few.

Best,
Kelly

 

I'd suggest to order enough to make an extra coil or two and also account for needing at least a 6" straight lead on each end of each coil, and each of those tripled up to reduce resistance at the interface with your conductor. In other words that in itself is an additional 12ft (3ft x 4coils). I cant remember if they sell in fractional pounds....

For 8kw you should be in great shape with your 50a circuit. At 8kw you'll be 32a and resistive loads are pretty tame.

Then there's that heat treating furnace your contemplating....

Best,
Kelly

 

For 8kw with 240vac = 33amps. The coil would need to be 7.2 ohms total resistance for that power (cold). 12ga Kantal A1 is .134ohms/ft so you would need 53.8ft of active coil. The resistance will increase with temp by up to 5% so if you want 8kw hot, you would reduce the total length by about 5% or about 51ft.

You have about 180" of total groove length (7.5" x 6 x 4, probably a little more since this was the interior dimension of the kiln not the groove). Every .375"D coil is 1.18" of wire. so you would have 519 coils. When you wind that in solid length (no space between coils), it would be 519 x .081" or 42" solid length. Since you have 180" of groove length the coil stretch is 4.2x which is right about in the middle of the recommended range of 3x-5x.

This all would require each of the four separate coils to be connected in series.

But, the surface loading of the coils will be pushing it. 51ft of 12 ga has 156in2 of surface area. 8000/156 = 51w/in2 which is above the recommended max of 39. You'd have to iterate to see what length coil converges on 39w/in2 but I'm guessing it would reduce the max power to around 7kw. It's not terribly out of bounce, but if you decided to observe this limit you'd simply wind a longer 12ga coil (about 61ft) to up the resistance and it would be stretched 3.6x solid length.

12ga Kanthal A1 is about 63ft/lb. I mentioned tripling up 6" lads on each end of the coils adds another 12ft. Coincidentally that requires 63ft total, but I'd by some extra.

That concludes my morning mental exercise to shake out the cob webs....

Best,
Kelly

 

Caribbean

I’ll run the numbers on 13ga and wind a 14 gage sample at 3/8” OD when I get back.

Best,
Kelly

 

MS, I'm back in the frozen tundra and took a look at you heating elements.

I grabbed my coil winder and winding 14ga on a 3/8" OD is no problem.



Here it is at solid wound length...



....and here stretch 3x solid wound length



...and for comparison, next to a 1/2" OD coil.



As for winding a coil for your kiln at 8kw, I took a little longer look at that. Besides getting the right resistance there are recommended maximum wall and element surface loading limits. The wall loading for your kiln is no problem, but I've concluded you just don't have enough groove length (~15ft) to fit an 8kw coil within the recommended guidelines, namely the element surface loading. To get enough surface area on the coil you need to use smaller gauge (larger wire) because the resistance per foot goes down and the surface area per foot goes up as does the wire length for a given resistance.

It's also recommended that the coil be stretched at least 3x but at most 5x of solid length. When I design my coils I made this simple spreadsheet. Here are the parameters for kiln at 8kw with Kanthal A-1. All exceed the surface loading limit, but 12ga isn't too bad and it will fit your grooves in the recommended range of stretch.



If I reduce the power to 7kw, everything falls within the recommended limits.



Now, I don't have any 12ga Kanthal to try but based upon how the 14ga wound, I'd say it could be wound on your diameter, but it would be a stiff bugger. It would take some effort to stretch it and I suspect you'd have to encourage it into the hoops on the OD of your grooves. My grooves accept 1/2" OD coils and even this makes a difference compared to 3/8" because you can get 33% more total wire length in the same length groove.

I don't know how big of a deal operating at 49w/in2 vs 39w/in2 really is. It may just mean 200hrs of life at the limit versus 300hrs.....or it may fail sooner. I'd suspect it's not drastic. You'd be out $50 and have to wind a 7kw coil. If you were just going to melt aluminum probably not a big concern but at 2300f and bronze......?

-My 2 cents

Best,
Kelly

 

That sample I wound was .063" wire on .25" mandrel and measured .390" OD, so about .015" for 14 ga. It may be lessened with more winding tension. It may increase slightly with 12ga vs 14ga.

Here's link to the design guide.

https://www.kanthal.com/globalasset...-elements/resistance-alloys_s-ka041-b-eng.pdf

I have a pdf that has friendlier units and formatting I can email you (PM me if you want it). Both have decent common sense guidance and rules of thumb. Here are a couple relevant charts on surface loading and life versus operating temp.



My coils are 26w/in2 surface loading which is favorable for very long life at 1800F where I'm operating them but something worth noting is you only operate at the max temp for a (small?) percentage of operating time, so my guess is you can temporarily push them to higher temps with less significant affect compared to continuously operating them there.

Best,
Kelly