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Thread: Comments on Furnace Design?

  1. #1

    Comments on Furnace Design?

    I'm working on building a new, larger furnace, capable of cast iron. My goal is to use a waste oil burner (Lionel's Brute) and I'd like a efficient, quick heating furnace. Please comment on my design, and point out any changes that should be considered.

    I'm planning on making it a 15" internal diameter, with a 3/4" to 1" hot face. The hot face would be backed with a ~1-1/2 layer of castable refractory mixed 1:3 with styrofoam for insulation. All of this would form a shape like myfordboy's furnace (i.e will have a extruding ring of solid castable sticking 2" out all the way around the top and bottom). The void between the top and bottom "extruded" sections, would be filled with 2" Kaowool.

    The one question I have is whether the 1:3 mix of castable/styrofoam will be suitable for cast iron temperatures, when behind a 3/4" hot face. Please comment, as I'm new to the higher temperature world.

    Benjamin

  2. #2
    Senior Member Tobho Mott's Avatar
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    Your plan will probably work, I just don't understand shelling out for real refractory then altering (/compromising?) it with diy ingredients. The styrofoam trick is usually used in homebrewed mixtures as far as I know. Not that people who've used it don't think it is great or anything... Why not use an insulating refractory instead for the backer layer, such as Cast-o-lite (might be spelled differently) or insulating firebricks or ceramic fiber blanket (or both, like SVSeeker on youtube). Hell, if you're gonna use a brute why even insulate at all, you could make a cheap sand + fireclay (or any clay probably, if you feel like digging it up) rammable mix for behind the hot face like (iirc) Petee's new furnace or one of Rasper's. For the record I use a hot shot like burner with a 1", 3200F dense refractory hot face with 2" of ceramic fiber blanket behind it and it has been a pleasure to melt in.

    Edit - I never melted iron but I believe dallen has used insulating castable refractory to do so.

    Edit 2 - no insulation would not actually end up more efficient and fast heating, I forgot you mentioned that requirement while I was replying. Adding the Styrofoam to the sand/clay mix would cover that. But I still think if you're willing to pay for all that refractory in your original plan, better to spend that $ on insulating refractory or CFB &/or IFB. IMO.

    Jeff
    Last edited by Tobho Mott; 08-01-2017 at 03:11 PM.
    Tobho had learned to work Valyrian steel at the forges of Qohor as a boy. Only a man who knew the spells could take old weapons and forge them anew.

    How I built my oil furnace | My Photo Album | My Videos

  3. #3
    Thanks for the commets Jeff. Here are my reasons for the refractory/foam mix is as follows. A thick hot face is not considered an asset to furnace efficiency, so I'd like to limit it to ~3/4". Some people just cast a 3/4" hot face and back it with ceramic fibre, but as this furnace will be movable I hesitate to have a 3/4" hot face "unsupported," as I think it could crack too easily given some jostling. Thus my idea was to have a 3/4" hotface backed by a more substantial layer of the foam/refractory mix, which is rigid, unlike the fibre blanket, and would give the hotface some support. Then behind this I would use the ceramic blanket, which I already have.

    So I guess the question comes down to whether or not mixing the refractory with foam will damage the integrity of the furnace, when brought to cast iron temps. As I had understood it, the foam melts out of the refractory on first firing, and you are left with a very porous (and thus insulating) layer of refractory. I would think then, that this shouldn't harm the integrity of the furnace. Anyone here with experience in this area?

    As an extra note, I already have most of the ingredients. I got the ceramic blanket free, and I got the 3300 deg dense refractory at an amazing price, far cheaper than I could buy insulating refractory for.

  4. #4
    Senior Member Tobho Mott's Avatar
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    Ah, I didnt realize you already had the dense stuff. Makes sense then. I think the sticky foundry tutorial thread has a section on diy refractories that may help you dial in a ratio that has the right balance of strength and insulation. There's another sticky just on diy refractory I believe. Dense castable is really tough stuff, I bet you will be fine.

    Jeff
    Tobho had learned to work Valyrian steel at the forges of Qohor as a boy. Only a man who knew the spells could take old weapons and forge them anew.

    How I built my oil furnace | My Photo Album | My Videos

  5. #5
    Any more commentst on this design, particularly the durability of the foam/castable mix?

  6. #6
    There is no need to make the base the same diameter as the top it only needs to be the same as the outside of the walls. This will save material and reduce the mass that has to heated.

  7. #7
    Myfordboy: when you say the "outside of the walls," I assume you refer to the outside of the hotface? My idea in making the base extend as far as the top was for bracing purposes, so that each would be supported by the exterior of the furnace...

  8. #8
    Yes hot face.
    When I built mine my first thought was to make the bottom the same as the top. I then realised it is not necessary to make the bottom the same.
    The refractory base sits on a ceramic sheet which is the same diameter as the top so the outer sheet can be fitted and this also insulates the bottom of the furnace.
    furnace exploded 3.JPG
    In the pic
    Green- outer casing
    Grey and blue- refractory
    orange- ceramic fibre
    yellow- ceramic sheet

  9. #9
    Thanks for the clarification; I wasn't planning on using the ceramic sheet, so that's why I changed the design so that the bottom would be the same as the top. If I can get a confirmation that a 2:1 or 3:1 mix of foam and castable is sturdy and durable, I'll put that under a 3/4" hotface for the bottom, basically to replace the ceramic sheet.

  10. #10
    Senior Member Tobho Mott's Avatar
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    I can't imagine how any kind of insulation would protect a dense castable hot face. Hot faces protect the insulation! Adding any foam to castable is sure to weaken it. Pure castable is pretty much bulletproof if you mix it suitably dry, and the hot face alone on a 15" bore will already be really heavy. You won't be playing catch with it or anything... Much easier to crack it with thermal shock than by rolling it around on a wheeled cart. Since you have lots of it and it was so cheap, why not build a smaller furnace to experiment with your idea on until you are used to working with molten metal? Then you can let us know if it worked! Probably best to start out with less crazy-hot metals than cast iron too though.

    None of our active members these days have tried anything like this that I can recall reading about. I imagine this is the reason nobody has spoken up to confirm or refute your plan's likelyhood of success.

    But years ago I think there were lots of fireclay and sand/grog with foam beads for insulation furnaces built and documented here. It's possible somebody tried this back then.

    Above, I already mentioned the sticky DiY Refractories thread - have you checked that out? Might well be that your answer is in there somewhere.

    Jeff
    Tobho had learned to work Valyrian steel at the forges of Qohor as a boy. Only a man who knew the spells could take old weapons and forge them anew.

    How I built my oil furnace | My Photo Album | My Videos

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