Building a Low Mass Resistive Electric Furnace

Discussion in 'Furnaces and their construction' started by Al2O3, Dec 8, 2022.

  1. Al2O3

    Al2O3 Administrator Staff Member

    Most forum members know I built and use an aluminum duty, lift off, resistive electric furnace. For the work I do, I really like it. I get asked quite a bit about the build on my YT channel so thought I’d start this thread to pass on some suggestions for those considering such a build. My furnace has a lot things that most wouldn’t bother with. I’ll take more of a minimalist approach for this thread, will break this up into sections, and if there is enough interest, hopefully update and refine as time permits. I’ll also attach links containing many supporting pictures and videos.

    So why low mass? Most people have limited power available and electric furnaces tend to be challenged on warm up and melt times compared to fuel fired furnaces, so low refractory mass helps a lot. It also makes the furnace more portable for the hobbyist.

    Why a lift off furnace? A guided mechanism that lifts the furnace to expose the crucible allows the crucible to be snatched and poured with one handling tool. In my case, an open ring shank.

    Lift off also allow lower clearance from the crucible to the furnace wall since there does not need to be clearance for tongs. This in turn helps minimize furnace size and mass, prevents damage from tool contact, and also places the heating elements in close proximity to the crucible. This gives the crucible an extremely good look at the radiating heating elements and improves heat transfer and melt times.

    You could also choose to drop the floor of the furnace instead of lift the body. The build advantage is it would keep the electrical power cord to the center body stationary, but the disadvantage is safety in having a traveling platform with a crucible of molten metal moving on it, so I don’t recommend it.

    The incarnations of my furnace have used dense refractory, Insulating Fire Brick (IFB), dense refractory, and Molded Ceramic Fiber (MCF) refractory construction. The latter is by far my favorite, with IFB a close second. You can read about these in the links below and those in my signature.

    Before designing the furnace, you should decide how large of a crucible you want to accommodate and how much power you have available as both of these will influence your design. As a reference, my furnace is 8kw, 10” bore, and can accommodate up to an A20 crucible, which is a fairly large crucible. I use an A10 most of the time and it can melt 10lbs of aluminum in about 30 minutes. Once the internal furnace dimensions are established, the heating elements can be designed
    Last edited: Dec 8, 2022
  2. Al2O3

    Al2O3 Administrator Staff Member

    Heating Elements

    There are several popular resistive wire manufactured for heating coils. The most common are NiCr 80/20 and Kanthal A1(FeCrAl). The latter has higher operating temps but less hot strength compared to NiCR. For a melting furnace, since you already have to build in coil shelves, you should opt for the higher melt temps. As opposed to one continuous coil/heating element, a layered construction has some advantages. I won’t go into the design details here but instead attach the Kanthal design hand book below which describes how to construct the coil for best life and performance. Pages 8-12 are the meat of it. You may need to iterate a few times to converge on your parameters.

    There will be a temptation to use small diameter wire to achieve the necessary resistance. Small diameter resistance wire will be much more fragile. My elements are 14ga. Smaller furnaces may be able to use 16ga or even 18ga. Afterward, I’d strongly suggest you make your own coils as opposed to buy them. You’ll be able to get exactly what you want to fit your furnace and power requirements and will always be able to make replacements if necessary. Making a coil winder is surprisingly easy. All you need is a cordless drill, a metal rod the ID of your coil, and a simple fixture to guide the wire onto the rod. Here’s a link to mine.

    I have replaced several coils but not because they failed due to overheating. Once, I carelessly splashed aluminum on the coil while adding/dropping a small piece of aluminum into the melt. The small dot of aluminum happened to land on the coil instead of the furnace wall. When this happens, the aluminum will alloy with the coil, reduce it’s melting temp, and quickly fail. I broke the other coil trying to stretch it back into size to fit back in the coil shelf. This happens because when the Kanthal coil cycles it expands a lot and then contracts a lot. No matter how you initially fit it, it will always have a small diameter when it cools, often enough to pull itself out of a simple groove after a number of cycles. Kanthal becomes brittle after the first and subsequent heating cycles and will break if stretched. That’s why a sloped coil groove/shelf with slightly larger diameter is the best design.

    The other thing you will need to design your heating element is a basic understanding of Ohm’s law.

    Power = Voltage x Current or P = V x I

    Voltage = Current x Resistance. V = I x R

    You can start with your desired power level, solve for current, and decide if you can accommodate that current. Or, you can choose your available current and voltage and determine your maximum power. Once you know your current and voltage you can solve for resistance in the second equation. Once you have resistance, you can determine the total length of resistive wire and use the parameters in the design handbook to design your heating element. If that makes you eyes cross, I’m sure a forum member would be willing to help if you create a separate post.

    If you can, use higher voltage. In the US, using 240vac vs 120vac will allow for smaller resistive wire or 4x the power. If you only have 120vac available, you should build a smaller furnace because you will be limited in power. At best, residential 120vac circuits are 20amps. Resistive loads are pretty tame but even so, you should design for 75%-80% of that current for continuous duty. 120v x 15amps = 1800watts.

    -See attached handbook.

    Attached Files:

  3. Al2O3

    Al2O3 Administrator Staff Member

    Furnace Body Design

    I built my low mass furnace using moldable/pumpable ceramic fiber (MCF). It was fun, but quite time consuming.

    The pumpable variety is a bit runny requiring many thinner layers with overnight drying times for each. The moldable stuff is more difficult to conform to features like coil grooves/shelve. Applied thick it won’t dry very quickly and both will shrink more if applied thick. It can be cured quickly with heat if you have a big oven and the pattern can take the heat. It’s also quite expensive.

    Ceramic Fiber Board is a good alternative because it is less expensive per ft3 than MCF, and can be bought in sheets which can ease construction. The following design approach can use either CFB or IFB. CFB is in the 15lb/ft3 and IFB (K-23-K26 variety) about 30lb/ft3. Either are suitable but MCF/CFB will have higher insulating values and with IFB, the center body can get heavy for a 10” bore furnace, which can require a more substantial body frame and lifting mechanisms. For small furnaces the differences aren’t as noticeable

    So how do you package this? The heart of the build and how well the furnace works as a melting furnace will be the central body. You can build the furnace as a monolithic block with one long heating element, but I’d suggest building it in layers or wall segments with multiple elements. It will ease construction and make it more rebuildable and serviceable. The furnace layers do not have to be annular, but it does minimize both thermal mass and lifting mass. However, a hexagonal or octagonal shape is a close second and may be easier for you to build and make efficient use of material. In either case, whether CFB, or IFB, you’ll want make the outer metal shell function like a large band clamp to secure the refractory. It also needs a metallic floor/shelf to support the refractory as the body lifts. You can make this from scratch if you are a metal fabrication skills or in the case of the round exterior, use a steel barrel of suitable dimension. If your furnace body is round a barrel is a good start since cutting a hole in the bottom would incorporate a built in shelf for the refractory to rest upon. It can then be slit up the side for claping. Below are several examples.

    Furnace Body Features.JPG

    IFB & CFB can be easily cut and shaped with hand tools. You can also use power tools, however, both are aluminosilicates and are hazardous to breath and can cause lung injury. Power tools will be fast but they will also send large amounts of material airborne. This will coat everything in your shop that will become airborne many times thereafter. Vacuuming will make this worse. For sure, do this outside, and for either material, where a respirator and PPE, and change/wash your clothes and shower immediately afterward. If you work with hand tools, it will make a mess but far less airborne, provided you carefully brush up the fines and not vacuum them.

    A band saw/saber saw can be used for non linear cuts followed by hand or drum sanding. A back saw or chop saw for straight cuts. For IFB, a rotary rasp and fixture can be used for cutting the coil shelves. CFB can get a bit ragged, and a Razor knife works best for cutting coil shelves. It pays to practice before cutting on a lot of stock. Both materials are very abrasive so don’t use your favorite blades. Consider whatever you use as expendable.

    The furnace base can be bowl shaped but there is really not much of a reason to make it anything other than flat. I’ve never had a crucible fail or major spill but it can be built as a bowl with drain hole. MCF/CFB are completely unwetted by aluminum whereas molten aluminum will attack IFB. By far, the simplest and lowest mass approach to the lid is just using a solid piece of CFB. Perhaps 2” thick. It will be low/mass, light, and easily handled by hand if you attach a metal handle.

    Most IFBs are 9 x 5 x 2” or 2.5”. If you have a small crucible that would fit within and hexagon or octagon with a 5” side and 9” tall, it’s hard to beat how simple the build is with IFB and K23-K26 is sufficient light and very easy to work. Even though it would not be layered construction, you can still use multiple coils and easily cut coil shelves across each brick. The same construction could be used with CFB at even lower mass. For smaller furnaces this may actually be and easier build and more robust method of construction as layers can become thin and fragile.
    Last edited: Dec 8, 2022
  4. Al2O3

    Al2O3 Administrator Staff Member

    Lifting Mechanism

    You can get elaborate as you like here, I did. I’ll propose the simplest, which is very practical for smaller furnaces. It’s a guided lift with a counterbalance. If you keep the furnace body small and light, you may not need counter balance but if you add the counterbalance, you’ll find lifting very easy and with the right weight, will stay in place where lifted. At 10” bore, you’ll want a counterbalance. It can easily be added. Also see links in my signature for my furnaces.

    Lifting Mechanism.JPG
    Last edited: Dec 8, 2022
  5. Al2O3

    Al2O3 Administrator Staff Member

    Electrical Controller:

    Again you can get elaborate as you want here. I have a lot more instrumentation and safety mechanisms than most builds. I’ll list the minimalist’s approach here but the disclaimer is your dealing with a real shock hazard and I’d strongly encourage you build in features that prevent the coils from being energized whenever the furnace is open, whether they be manual or automated.

    You need a PiD controller, Solid State Relay (SSR), with heat sink, and K-Type Thermocouple (TC). I also recommend a multi-pole contactor. These things are inexpensive and all over eBay and Amazon. Perhaps I can add a schematic with time, but, most people simply use a SSR with a PiD control, with temperature feedback from TC. I place the TC in the top middle of the furnace lid. I like the Inconel sheathed TCs because they are compact and easy to insert and remove. A caution when using US 240vac single phase power. Most people just use the SSR to open one leg of the power to the coil because it’s simple and only requires one SSR. This works fine for control purposes but it also leaves one side of the coil at 120vac to ground. This is why I suggest using a multi-pole contactor to completely de-energize the circuit when you open the furnace. I did not take the minimalist approach.

    I set my control temp to 1800F. It’s somewhat arbitrary but in my IFB furnace, it didn’t matter what the set point was, because the aluminum charge would always lagged the furnace temp by a couple hundred degrees and the furnace never reach the set point before the aluminum was at pouring temp. On my MCF furnace, it does achieve 1800F before the metal reaches pouring temp, but not long before the melt occurs, so a higher set point still would not make much difference in melt times and the lower temps will make your furnace and elements live a long happy life. You don’t need to use a ramp and soak PiD. In fact, they are a PITA for melting only, but they can be useful if you intend to use your furnace for heat treating.
    Last edited: Dec 8, 2022
  6. Al2O3

    Al2O3 Administrator Staff Member

    Feel free to post questions and comments about this content here. If you have specific questions about a build of your own those may be best handled in a dedicated thread of your own. If so, please do post a limk to your build.

    Last edited: Dec 8, 2022
  7. Al2O3

    Al2O3 Administrator Staff Member

    Place holder
  8. Al2O3

    Al2O3 Administrator Staff Member

    Place holder
  9. garyhlucas

    garyhlucas Silver

    Good write up. You mention a main contactor, and here is why that is a really good idea. The mostly like failure mode for SSR is ON. Bought couple of chinese ones to power the heat bed on PLC controlled 3D printer. Was waiting for the bed to cool down and suddenly realized it was way hot! The SSR shorted and continued to feed power. Two SSRs are therefore safer as you need both for 240volt power. However that isn't safer with respecto electrocuting you touching the element still energized from one end.. I'd always switch power off with a mechanical contactor so you know the coils really are off.

    Chinese PID controller off Ebay. Don't bother we tried them way off by different amounts any every temperature. The one from Automation Direct work well, as do their contactors, SSRs etc. For those of you electrically challenged they have excellet tech support and good manuals to boot.
  10. Al2O3

    Al2O3 Administrator Staff Member

    Pretty sure that's what I said, but I'll make it clearer if need be. My comment was people (like many of the YouTuber "how to guys") don't do that, and worse, the way they (USA based DIYs) wire their circuit, it's always energized to ground if their furnace is plugged in. I assume this is because the so called eBay "kits" they get from the auction sites come with one SSR and have circuit that shows such, but that ciruit expects Euro style 220vac, which is one load leg. The tip off should be the "220" refernce because that is the operating voltage in the EU and many other places in the world but the USA is actually 240vac these days (two 120 legs 180 degrees out of phase), but people still call it 220/110 as a carry over from the old days. I know you know this Gary but for sake of the thread......I'm posting here.

    I have power lamps and volt/current/power meter and observing them all is part of my procedure before opening the furnace. The power meter was an inexpensive import ($15 at the time), but also it's quite useful for health monitoring the heating elements......I can see the intial current and how it changes as the furnace heats due to the changing resistance of the resitive wire element.

    I suppose like all things eBay, it's hit and miss. It's going on 7 years ago now since I built my furnace. I actually bought my PiD and SSRs from Auber Instrument, but make no mistake, they are Chinese imports. My primary motivation for buying from them was they were only marginally more expensive but had good manuals online that I could view before purchase. I have two furnaces with 4 SSRs, and at least 400 operating hours between them........never had an SSR failure. They are sized with plenty of margin, and very well (thermally) attached to generous heat sinks. I like Automation Direct for sure.....great site and broad prodcut offering.

    As far as accuracy, it's as accurate as I can measure with a K-type. I have two elements in my furnace. With both of them active, the empty furnace will cycle within 4-5F of an 1800set point and half that with one coil active, just due to the thermal mass of the element. It's all kind of moot and of little value for a melting furnace though, but of more value for heat treating. Good enough for who/what it's for.

  11. CastawayJunk

    CastawayJunk Copper

    I repaired two commercial electric muffle furnaces, one had a half dead SSR, the other a bad main contactor. The bad main contactor means no power, no heat but safe.
    The half dead SSR would turn on fine, but when off it still conducted in one direction. Took a while to decipher that because it only did it like 60% of the time. I wired the indicator lamp directly to the output, not the sense leads, that way any accidental energize will turn on the light, regardless of what the controller is doing.
    Noticed the light going from full bright to half bright and figured it out lol.

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