New from Delaware

Any specs or links to manufacturers website?

Best,
Kelly

 

If I recall correctly,small electric furnaces like that have a wound coil for a heating element and there is usually an alumina sleeve with a spiral groove the houses the coil, then than sleeve slides into the furnace such that the coil is captive and a thin alumina cylinder wall between the coil and furnace ID, like a muffle furnace is that correct? Could you show us a picture?

Making a coil should be straight forward and inexpensive. For 1500w at 120vac (in USA, not 110vac), you need 12.5 Ohm resistance coil. IF you measure the groove it must fit in, the wire diameter, diameter and number of coils, and total length is easily calculated.

Having the coil completely surrounded derates the operating temp considerably, but at 2100F max, Kanthal A1 or similar AlCrFe can probably still handle it, but that is a marginal temperature for melting brass and bronzes.......


Best,
Kelly

 

The first thing you need to do is tell us the dimensions of the groove the coil needs to fit into. Like the cross section and length of the groove.

Best,
Kelly

 

What's the outside diameter of the alumina cylinder that houses the heating element and how many total times does the element/coil encircle it? From the picture it looks like 12 or 13 wraps.

Best,
Kelly

 

Since the furnace is 1500w, by Ohm's law, the resistance for US 120vac would need to be 12.5 ohms. When I know the overall coil shelf dimensions I can back solve the wire and coil design required to suit.

Best,
Kelly

 

I've never owned one of those small restistive electric furnaces, but have built and operated one of my own design for seven years now, so the following is just my educated guess based upon that experience.

Neither the manual nor the links to replacement heating elements indicate if the wire is NiCr or FeCrAl but the upper operating limit of 1050C/1922F seems to suggest it is NiCr. However, all of my furnaces have one open side on the heating element groove exposing the element to the furnace interior whereas these furnaces have the element enclosed on all sides which makes the element operate at a higher local temperature than the furnace interior and derates the max operating temp compared to an open sided groove.

The other thing about the design of these small furnaces is they have significantly higher wall loading (watts/cm2 of furnace hot face) than is recommended in the Kanthal handbook which also increases the localized temperature of the element compared to the furnace temperature and further derates the max furnace operating temp. A properly designed open groove resistive electric furnace with a Kanthal A1 (FeCrAl) element could be as high as 2500F. Having the element packaged the way these firnaces do does make them more compact and decrease the chance of electrical shock so I suppose that's why they do so.

As far as the (high rate of?) failures with these furnaces go, it's possible a contributing factor is not following the manufacturer's directions to gradually increase the furnace temperature 200C at a time and allowing the furnace to stabilize before increasing the temperature. Just setting the furnace on the max setting and letting it go probably causes localized temp of the element to exceed its max operating limit. The manufactuer also advises allowing the furnace to cool after continuous use which probably means it heat soaks. But following the manufcaturers recommendations greatly slows melt times so I suspect folks tire of that. I don't know whether the PiD controller limits the upper temp setting but it's also possible folks exceed the recommended operating limit and try to melt brassess and bronzes. You need a well design resistive electric for bronze duty.

If the element in your furnace is/was NiCr instead of FeCrAl, then winding your own FeCrAl element may make it more reliable and have an increase upper operating limit.

I took a quick look at what a Kanthal A1 (FeCrAL) element might look like. Surprisingly it's similar to the (two) 4kw elements in my furnace because mine operate at 240vac instead of 120vac which yields 4x the power for a given resistance. For 1500w element and the size of your heating element groove, 14ga FeCrAl is about right. That wire is about .209 ohms/ft so for your 12.5 ohm element you would need ~60 feet of it! After you wind that wire to ~.25"OD and stretch it, it would fit your groove. But Kanthal A1 isn't cheap. It's about $55/lb and 1 lb =~100ft, and 1lb increments are usually the smallest increments in which you can buy large resistive wire.

For a given wire, NiCr is about 75% the resistance of FeCrAl so you need more of it and it is comparably priced so using that yields no economy. The links to the replacement elements you posted cost about the same as making your own. You might be able to source them offshore for less if you look hard at sources like Alibaba. So you could just press the easy button, buy a replacement element and operate the furnace more conservatively and per manufacturers recommendations. If you are just melting aluminum, there is no need for a higher set point than 1700-1800F.

.......Or buy and wind your own FeCrAl element and possibly have a more reliable higher performing furnace, that is of course unless the existing elements are already FeCrAl. I can link you to a how to wind your own.....pretty simple, but not as simple as clicking Buy It Now.

Best,
Kelly

 

Oh boy, no beat down for you. You are right about the 9.6 ohms. 12.5 is the amperage (not resistance) in the 1500w 120vac circuit with that resistance?

I should have had my coffee first. Power = v * i and v = i * R

But, that circuit with a 9.6 ohm resistance heating element would be 120vac x 12.5amps = 1500 volt-amps (watts in this case) not 1200 watts. In my discussion above that would mean you'd only need about 46ft of the that 14ga KA1 wire instead of 60 ft so you could make 2 coils if you purchased 1lb of wire makingthem about $25 each.

They're all 20 amp 120vac circuits in my home, so I assumed the 1500w furnace meant it really was just that. Resistive loads are pretty easy on circuits anyway compared to inductive loading.

Do you'd know the resistance/ft of your wire? Any resistance wire I buy very accurately states the resistance and that makes it very easy to design a coil to a total resistance figure (if I calculate it correctly )......thus power with known voltage.

Best,
Kelly