Replacing an element with Kanthal for 1150 Celsius hold

[babiels]

Hi,

Please bear with me - I am talking about a furnace and melting metal, but the electrics and equipment are almost identical to many kilns used by potters so I'm hoping someone here can help!

So after 6 months or so of playing around with DIY electric furnaces I decided I needed something professionally made and reliable for my fledgling foundry startup. I bought a 10kg electric resistance furnace from a Chinese company on Ali baba. It cost $1200 including two spare crucibles and shipping from china to the UK. I guess I also paid duty on that but don’t recall how much that came to. I use it for melting about 4kg of bronze at a time, which takes about 45 mins, holding at 1150 Celsius for 15 mins or so.

Anyway, the furnace coil burnt out on the third firing. I took it out and saw where the coil had burned and melted some of the fibreglass around it. So I contacted the seller and after a lot of wrangling and a long delay (about 3 months) I got my replacement coil, nicely wrapped around a ceramic tube with (I hope) fibreglass string strung in-between the coils to keep them apart. Plonked it in and wired it up as instructed, carefully packed the fibreglass back in and did a test run. Worked beautifully.

Three firings later, the exact same thing happened. Now the seller is understandably reluctant to send another replacement or refund, and I am lamenting a waste of $1200 on what I thought would be a wise investment. I am not keen on the idea of buying a new coil from this manufacturer as the same thing is likely to happen again.

I am no expert, but I’m thinking that while this has been advertised as capable of melting gold and bronze at temps up to 1150 Celsius, maybe they are using a nichrome-type wire that is only rated to 1200 celclius and is quickly worn down by having to hold for 15 mins near to the top of its range.

It has a decent PID and thermocouple setup, an ammeter, 32A 240V circuit breaker and nice big relay. Other than that it is just some sheet metal boxes, some fibreglass, the ceramic tube and a coil with string. All to say that I am comfortable rewiring these things given their simplicity, however I have some questions for anyone who knows more than me.

Details:

Brand new, the coil tested at 13 Ohms, drawing about 16A from my 240V power supply. It was 2.6mm in diameter, but can’t tell the length or resistance per m. It’s a single element so annoyingly cannot be replaced in sections. All the components are rated for 32A, as is my power supply

Questions:

1. I plan to replace the coil with Kanthal A1 wire which is rated to 1400 Celsius. Hopefully that will do better?
2. I’m going to use a thinner gauge wire of the same resistance to keep costs down, but hopefully the higher temperature rating should mitigate some of the breakage risk?
3. As I’m going with a new coil with thinner wire, is there any benefit to increasing the resistance to reduce the overall current draw?
4. Does anyone else have experience of a 1200 ceclius kiln/furnace working with 13 gauge (1.8mm diameter) Kanthal wire?
5. Does anyone have any other theories for why the coils are breaking?
6. Does anyone have any better ideas?

Ideally I’d just buy a better furnace, but they’re expensive and I can’t find any 1-3k options anywhere by Ali Baba.

 

Thanks!

 

 

 

[Bill Kielb]

So to give a little of what I am aware of. You need the wattage rated to make rated temperature so your resistance should be a baseline. . A smaller diameter wire generally will fail faster than the larger as every firing causes the wire to thin out so to speak. So smaller diameter with better wire likely will fail fairly quickly or at least is not better than the same better wire at the original diameter or larger diameter. I don’t know if it’s worth the risk of making it smaller in diameter (wire size) then the original.

Loading is a thing and there are acceptable loading amounts which affects longevity. So basically If I can produce 10kw of power with 10 feet of wire it will be loaded twice as much as producing that same 10kw of power with 20 feet of wire. Google the Kanthal wire element guide for some design perspective on this.

Final thought - post a picture of the failed element and it will likely spark some good observations here. Maybe a silicon carbide element makes more sense in your application.

[Mark C.]

Its not fiberglass but ceramic fiber as fiberglass melts at low temps.Ceramic fiber comes in many densities and thickness and temps.

The thicker elements as Bill said will last longer work better. I would stick to the same size element no mater the cost-you can have it made (not sure about UK places but any kiln place will know)

[neilestrick]

If you're certain that the old wire was nichrome, I would do some research on Kanthal vs nichrome to see if the same size wire heats the same before deciding that rolling the element the same is the way to go. Make the element according to Kanthal specs, using the thickest wire you can within those specs.