How I am making an electric furnace

Discussion in 'Furnaces and their construction' started by Scott K., Sep 17, 2017.

  1. Scott K.

    Scott K. Copper

    If all goes well, I should have my first casting attemp mid next week.
     
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Can't wait too see :p:p
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    Me neither, your electric furnace look really slick!

    Jeff
     
  4. cactusdreams

    cactusdreams Copper Banner Member

    Don't say attempt. You WILL have your first casting mid next week.
     
    Last edited: Oct 26, 2017
  5. Jason

    Jason Gold

    That thing is bad ass. Much better than the 2nd brand new samsung microwave I own. I'm looking forward to seeing this sucker glowing red! I take it you are planning on conducting it's first real heat up to temp OUTSIDE right?
     
  6. Scott K.

    Scott K. Copper

    Thanks for all the encouragement.

    And yes, it will be outside.
     
  7. Scott K.

    Scott K. Copper

    image.jpg Well, I just did a test run of the oven. The controller apperantly has a max limit of 1200F. I was hoping to get to 1300. Hopefully 1200 is enough. It took about 50min to get to 1200. Does this sound about right, or slow?

    I got a lot of smoke from the vent around 800 and up. Think that was the galvanized duct? Other that that, only a few minor little puffs of smoke hear and there. Could have been paint. Or stickers, grease from the microwave I didn't get completely removed.

    The side of the oven that houses the electrics never got warm at all. The top barely got warm. The door got too hot to touch by 800F though. The back and other side got very warm. If I build another it'll have three inches of ceramic all the way around.

    I'll do an actual burn out tomorrow. And a pour.

    I'll let you know how it goes.
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I can't recall what you want to melt. Is this just a burn out oven? or do you plan to melt aluminum? If so you're going to need a higher temp and need to be at least a couple hundred degrees above your target pour temp. I use 1800F oven control point for aluminum.

    Seems a bit long for only 1200F, especially for a wool insulated chamber since the wool has very low mass, but depends on your input power too. Do you now what it is? You mentioned smoke out the vent. If there is smoke coming out the vent there is (cold) air entering the oven.

    Best,
    Kelly
     
  9. Scott K.

    Scott K. Copper

    This is just a burn out oven. And it's running on 120V and pulls under 15amps.

    Once it cooled down, I opened the door and was shocked at what I saw. Most of the wool is black. After peeking under the wool, I found the paint completely scorched and peeled. So I believe the smoke, and the blackness of the wool, is from the burned paint.

    I also found a one of the steel wires completely melted. How does a stainless steel wire melt at 1200F? And another one seemed broken. Also, it looks like the heating element melted halfway through one of the ceramic isolators. Again, how is that possible at only 1200F?

    I checked the coil, and it is still about 7ohms, so the coil is good. But it looks bad at the melted isolator.

    image.jpg image.jpg

    I will still try the burnout tomorrow, but by the looks of it, I will need to do a rebuild.

    So I'm thinking it would have been much better to build this from scratch instead of using an existing appliance. I should also use the proper element hanger wire, instead of the stainless steel wire. But I still don't understand how it melted into the ceramic isolator. What else could I use to support the element? Any ideas?

    Thanks,
     
  10. DavidF

    DavidF Administrator Staff Member Banner Member

    I didnt see any smoke from my hose so all is good. It takes my kiln from cold to 700C about 2 hours....
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    It's very practical given it's the upper end of range for a common household 115vac circuit but at ~1.7kw fairly low power. Still, in that volume I think it should easily achieve faster warm up and higher temps.

    Is the painted steel surface behind 2" of wool or is sandwiched between an inch of wool on each side? If the latter, not surprised but I wouldn't be too alarmed about the wool discoloration. It will happen from burn out too.

    Keep in mind that 1200F is the indicated temp at point of measurement. Where are you measuring? What is the sensor and how is it mounted? It could be much hotter in the oven if the sensor is sinked to cold surface or in leak or conductive path. The coils themselves can/will be much hotter, especially if they are confined in grooves and that's why they are de-rated for that kind of mounting. The coil spacing is also a factor. If it looks especially bad at one location any chance of going to ground at/near there?

    Maybe you had some localized alloying going on. Are you sure of the wire composition? Since I wound my own resistive coils I just used short pieces of the same (Kanthal) wire. The biggest thing in oven design is to have a cascade of refractory in the thermally circuit that gets to manageable temps for the metal structure without (thermally) short circuits and even as few air leak paths as possible. Even though it's nice to put you hand on the outside of the oven you can reach a point of diminishing returns as far as insulation. I suspect you may be conductive heat loss through your metal structure.

    I'd tend to agree about the purpose built versus repurposed enclosure, depending upon approach and required compromises. I have an inch of IFB and 2" of wool in my furnace and I can touch most of the shell with my bare hand when the interior is 1800F. It just has primer on it and I haven't lost paint anywhere. -Stay with it.

    Best,
    Kelly
     
  12. Scott K.

    Scott K. Copper

    The painted surface is between 1 1/4" wool one side (hot side), and 2" of wool on the other side. Yes, I know that's backwards, but that is what fit.

    I'm using a K type thermocouple mounted on the side between the two ends of the element.

    I think the oven could get hotter, it's just the controller won't let me put in more than 1200.

    The wire I used is .061 SS safety wire (for saftying aircraft hardware). I used it because I have heard of people using it for an element, so I thought it would handle the heat ok. And I had a roll of it.

    I'm thinking about using firebrick next time, but if anyone has an idea for supporting the element, let me know.

    Thanks again,
     
  13. Jason

    Jason Gold

    uggghhh.. black charry mess. Not good.

    What the hell are you safetying on an airplane that requires .061??? The biggest stuff we use is .041 and you ain't breaking that stuff.
     
  14. Scott K.

    Scott K. Copper


    Oops, my bad. I meant .041, not .061. I used to work at a coorporate jet service center, and yea, .041 is the biggest we used.
     
  15. Scott K.

    Scott K. Copper

    I originally did not use IFB because of the cost. And because I didn't know how to keep them in place. Would it work to bolt them in? I'm thinking something like a SS lag bolt from the back side.

    I'm leaning towards getting rid of the ceramic rings and wire, and resting the element in a groove in a fire brick.
     
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'd recommend keeping any metallic structural support for refractory isolated in a region shielded from heat. Better to just use good refractory materials. IMO the best schemes bind the IFB externally in compression like a big band clamp and the brick joints are mitered...this is essentially the technique used in Kilns. I used 3000F mortar to glue the bricks and thus far have had no problem with that and this is common in commercial stationary ovens, stacks, flues, etc. I cut the coil grooves afterward with a router bit but it can be done before assembly too. e best coil retention scheme I have seen has a groove in which the opening is slightly reduced to less than the diameter of the coil when the next stack of bricks is applied...that way the coil cant grow out of the groove. I used a wire hook through the IFB wall to anchor the coils.

    Best,
    Kelly
     
  17. Scott K.

    Scott K. Copper

    Thanks Kelly. Strictly for the point of saving a dime, I'm wanting to use the bricks sparingly. Instead of a solid wall of bricks, I was thinking of using a couple of bricks strictly to hold the element in place. Then pack the ceramic wool between the bricks. This is mainly because I'm cheap, and have already spent more than I wanted. I don't want to waste the wool I bought, and the bricks are fairly expensive.

    My only concern is keeping the bricks in place. I could use element hanging wire. Or mortar.

    I don't know. I have a few ideas. I would just use refractory cement and cast the chamber, but I also need to be able to move it in and out of the garage. I'm afraid the refractory cement would make it too heavy.
     
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Both IFB and ceramic wool are very effective insulators and are light. Downside is they are relatively fragile. You, might consider a solid IFB floor because it's a stable surface on which to place things yet good insulator. You might also coat the floor IFBs with mortar for durability. The 9x4.5x3 bricks (typically ~$3 each) can be easily cut to 1.5" thickness to save $. If you are considering investing in IFB you might want to give some further thought to continuing with the repurposed metal structure because IFB can be a freestanding structure. Open top i smore common and practical then side load but there are plenty of examples of both. Also, colloidal silica is used to rigidize ceramic wool. -Food for thought.

    Best,
    Kelly
     
  19. DavidF

    DavidF Administrator Staff Member Banner Member

  20. Scott K.

    Scott K. Copper

    Both good points, thanks
     

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