My First Furnace Design/Construction

Discussion in 'Furnaces and their construction' started by Dazz, Jan 31, 2020.

  1. Dazz

    Dazz Copper

    I have made an initial cut at designing an iron casting furnace. I am aiming for a conventional design fitted with a pressured oil burner (no compressor = much less $$). I will fit a silencer on the exhaust and air inlet. If the neighbours complain (likely) I will sell this and look at an electric option.

    Internal dims are 320mm dia, 460mm depth. Mainly driven by the materials I can buy here (small range). I can buy bricks 230mm x 115mm x 75mm plus ceramic wool. I am told that the bricks are soft enough to hand cut. The bricks are manufactured from pure Mulite and Alumina powder

    The drawings only show the brick work. No point in going any further unless that is right.
    It is really important to me to minimise the range of materials ( = reduced cost) so please don't tell me I should use castable as well.

    I have elected to make the lid from bricks that will be held together with stainless steel pins and a steel hoop. The cone shape will lock them in place without castable.

    I expect it would be very difficult to make an oblique hole for the burner so I have squared up the entry.

    Any feedback/improvements welcomed.


    Attached Files:

    Last edited: Jan 31, 2020
  2. joe yard

    joe yard Silver

    The design looks nice. If $ is a major decider on materials. I would just use a home made refractory behind the bricks to save the cost of the wool and spend just a minimal amount on Satinite to hard face the bricks. If you do not put something on the bricks they will decay rapidly. Even a 1/4" coating of Satinite or similar will increase brick life several times over non coated bricks.
    Petee716 likes this.
  3. It looks pretty good so far, So you'll have a sheet metal skin to constrain the bricks I'd imagine?. If burning waste oil/diesel/kerosene you may need extra height under the crucible for a pre-combustion chamber but height could be added later for this should it prove necessary.
  4. Dazz

    Dazz Copper

    It might be a good thing to do, but unfortunately Satinite isn't available here.

  5. Dazz

    Dazz Copper

    I am planning on a polished stainless steel skin to reflect heat back into the furnace and to reduce the radiated heat from the skin.

  6. Dazz

    Dazz Copper

    What about lengthening the fire brick burner tuyeres so the flame has time to ignite before it slams into the side of the furnace.
    I think this could work well with a swirler with annular air flow, which might help keep the spray off the bricks will adding to the combustion.

    Just a thought.

  7. PatJ

    PatJ Silver Banner Member

    I have heard of people using insulating fire bricks (IFB's) with an oil burner, with some sort of a protective coating.

    I made a furnace from 2,600 F IFB's, and did not use a coating, and the oil burner caused them all to crumble the first time I used the furnace.

    I built an entire furnace out of IFB's, and it lasted exactly one melt with an oil burner.

  8. I built my furnace with that in mind, so it has a 10cm diameter stainless steel tuyere to allow up to a 2.5cm lining of refractory cement. The main issue I had and still have is getting a narrow angle nozzle with a fine enough spray so that the fuel doesn't hit the tuyere sides. This still happens in my current furnace with it's doughnut swirl chamber until the refractory gets hot enough to flash vaporize the fuel. I run the fuel nozzles about 10 cm back in the tuyere now, much deeper and I get fuel puddling out the bottom of the furnace.
    Last edited: Feb 1, 2020
  9. Dazz

    Dazz Copper

    OK. I will ask my supplier if their bricks are compatible with diesel/oil.
    Has anyone else had a similar experience with IFBs?
    Did the furnace exceed the IFB temp limit?

    Last edited: Feb 1, 2020
  10. Dazz

    Dazz Copper

    Big furnace burners have a parabolic like shape after the burner to keep fuel off the refractory.


    Attached Files:

  11. No direct experience with IFB but I have noticed they are used a lot in electric resistance heated pottery furnaces. Is there any issues with using a calcium aluminate based castable refractory?. I quite like the stuff: $48 for a 25Kg bag and it forms a 60MPa concrete (80 MPa before firing) which is three times stronger than normal concrete. I have seen a gas fired furnace capable of melting monel at 1350 deg C with a wall thickness of 70mm castable and no other insulation behind it, admittedly the outside of the furnace reached 360 deg C but the wall refractory was cast in the mid 1980's with only a new plinth every year or so due to crumbling, so it's durable.

    A 25mm wall thickness hollow cylinder with stainless outside skin to confine it mechanically (304 stainless is good for 1400 deg C) and then 50-75mm of Kaowool type insulation would still keep it simple and it would be resistant to oil flames but not iron slag (not much is). such a wall thickness would ensure rapid heating and minimize concrete use too.
    Last edited: Feb 1, 2020
  12. Petee716

    Petee716 Silver Banner Member

    That burner design shows peripheral air that has to be pressurized somehow presumably with the same source as you're using for your primary combustion air. That would be an interesting design indeed.
    There are lots of design calculators that would produce templates so you could accurately cut or mold in your tuyere at the appropriate angle. Entering straight in will only aggravate the issues being mentioned above. Regardless of the entry angle, you can estimate the actual target region on your bore wall and design your furnace with repair or replacement of that consumable material in mind.
    _3D_ over at Alloy Avenue seems to be having ok luck so far with his y-tong expanded insulation bricks but I don't think he has really given it a hard trial with oil. I have very little experience with insulating fire bricks in other applications, but I think we're asking more from them than they're capable of here, at least without a coating.

  13. Dazz

    Dazz Copper

    I don't know if it is available here, and if it is, it won't be for the price you indicated.
    The thing I like about bricks is that if one fails, I can replace it. If one gets hotter than the others, it won't start a crack through the whole furnace.
    Also, my understanding is the ceramic IFBs are low mass, low specific heat = faster melts.

    If I tried to cast a hot surface inside the completed IFB furnace, that will make the construction a whole lot more complicated and difficult.
    If the castable is too thin, cracks are more likely. If too thick, I will need to increase the diameter of the IFBs lining (= more $$$ + castable $$$ + time).

    IF I can find a source of suitable castable, I would apply a layer to each IFB before assembly. That would retain the benefits of bricks and avoid the complexity of casting inside a cylinder of bricks. It will reduce the problem of differential thermal expansion between the brick and the hard face. I could then do a much thinner layer of hard face( 10mm-20mm?) and quality control would be a lot easier. I have not seen any furnace where hard face is applied to individual IFBs before assembly, so it could be a bad idea.

    I am also looking at options to retain the bricks. I have seen steel straps used somewhere but I am leaning toward using a light rolled SS skin to mechanically hold the bricks. I would bolt the skin at the seam to enable disassembly/maintenance. I also intend to use loose SS dowels to mechanically link the bricks. Cheap and easy to do.

    One of my design aims is to make the design modular. If one part fails, I want to be able to fix that part without replacing the whole furnace.
    360 deg C is way too hot for me. That translates to a lot of energy escaping that isn't available to heat the melt. I am thinking of hardface?, IFBs, ceramic wool, fibreglass wool (free, 200 deg C limit.) and then a polished stainless steel skin.

    I am also looking at making the outer skin practically air tight to minimise super hot air flow between the bricks and into the insulation. It will also allow me to experiment with pressurizing so air flows between the bricks into the furnace.

    Last edited: Feb 1, 2020
  14. Dazz

    Dazz Copper

    Domestic burners use a swirl disc with peripheral air flow as shown in the photo. No separate air supply. The reason I am looking at industrial burners is because I know they will be applying the best available technology/methods.
  15. joe yard

    joe yard Silver

    I have had several furnaces over the years. All but one was over 20 years ago. The new furnace was a collection of what was on hand. I started with an unknown 4 piece factory made insulating brick lining and went from there\. I expanding on the bricks by putting refractory between them to increase chamber size.
    Long story short. The first firing showed very bad cracking of the brick with some pieces falling off. The refractory between the bricks was not noticeable damaged. I coated the damaged brick with between 1/4" and ½" of refractory and things have held up very well since then. I have not tried to melt cast iron yet but I am confident it will be fine.
    My burner is a China made knock off of a Delron siphon nozzle using waste motor oil.
  16. It's best to phone around and talk to the castable refractory supplier : in my case I found a supplier selling dense castable refractory for boiler furnace relining that sold an identical product to the foundry supply company for 60% of the price. Both are in Brisbane, Australia so I have to ship it 1800 kilometres to get it here. The only supplier of the IFB is the same foundry supply company and they want around $11 per brick plus 10% tax. So for the cost of four bricks, I can buy a 25 Kg bag of 1650 deg C refractory. When I rang the supplier to order it, he told me he'd just shipped a pallet to my city for a sugar mill and if I'd been a day earlier he could have shipped the two bags for free. I paid $87 per bag delivered and am happy to have it for that price as the other supplier was $90 or so per bag plus delivery.

    Refractories are going to crack anyway, it's just a given due to the thermal cycling, mine developed a full length crack with a loud bang on the first firing but the stainless steel container keeps it all together a bit like a cathedral arch wedges individual stones together: the cracked concrete refractory can't go inwards as it's wedged and it can't go outwards as the stainless cylinder retains it. It just looks like a fine hairline crack at room temperatures. I'm experiencing about a 15 minute warm up time for the furnace with 6.5cm/50Kg of dense castable backed by silica wool: you can see when it gets incandescent and starts to get really hot.

    I'd be inclined to not use the bricks at all, just cast a dense concrete liner in a stainless stainless skin.... it will crack like all concrete does but it's not a problem due to geometry and the retaining skin. This will get very hot but several centimetres of of high temperature silica fibre wool over the stainless skin will provide the insulation.

    This is what I currently use: it's a 6cm wall thickness or so of dense castable in a stainless beer keg with 75mm of silica wool insulation and then a 0.6mm stainless skin over the top. It's similar to what you propose except it has castable substituted for brick as I had too much problem with the clay silica fire bricks intended for pizza oven use that cracked with every single firing until they fell apart. This is a classic beer keg furnace with extra silica wool insulation added as an afterthought as the outside proved to be too hot at 360 deg C. There is a bottom hatch for replacing the base refractory that cops the most wear and tear and the wool insulation is enclosed away from the refractory lining so the lining could be cut/broken out leaving the fibre insulation intact. All of the stainless fabrication is cut out with a grinder or tin snips and shaped by hand. The skin is retained by stainless sheet metal screws, all welding is stick welding with 316L rods. So it's similar to what you plan except using castable instead of insulating fire brick.

    furnace bore 2.jpg

    Mineral wool insulation.jpg

    finished insulation 1.jpg
    Last edited: Feb 1, 2020
    joe yard likes this.
  17. Dazz

    Dazz Copper

    Your furnace is very close to what I had in mind. I like the flange. I am going to steal that idea.

    IFBs appeal to me because they are relatively light and modular.

    So the message I am getting is that a castable liner without IFBs is the way to go but cracks regardless, so why not pre-crack it?
    What about casting tiles (say 25mm x 100mm x 150mm) and enclosing them in a SS skin. No ceramic bricks.
    Damaged and broken tiles can then be replaced as required.

    I also plan to the furnace on iron castors and add lifting eyes.

  18. Hi Dazz, if I was a master at precise fabrication and casting of concrete then individual tiles would be the way to go, my beer keg body has distortions from welding bits on that distort the cross section from a perfect circle to a slight egg shape next to the stainless lid rests are welded on. This meant casting in situ eliminated precise fit and join issues of tiled bricks with possible flat smooth gaps that allow flame wash past the bricks. With calcium aluminate castable from either Foseco or Refractech, if the instructions regarding water content are followed then it's a very durable material that can only be broken with difficulty or diamond grinder wheels. I know of a roughly 35 year old lining whose only problem is slag buildup reducing the bore diameter. I haven't looked at any cracks too closely, chunks haven't fallen out so the lining has integrity, if a lump did break free, I would gouge out the hole with a diamond bit in a plunge router or die grinder to let some fresh concrete key into the hole.

    The flange section of tube was slit and about a 10mm wide strip welded in to allow the 100mm stainless tube to be a slide fit into it for early burner experiments, normal run of the mill silicone rubber is used to get a good air tight seal on the flange face.

    My unit has about 5 x 25 Kg bags of cement in the extended body, base and lid but a thinner 30mm thick cylinder bore should save 1-1.5 bags of material. It's too heavy for casters on dirt so I use a large removalist trolley if I need to shift it but it lives in the backyard with a small corrugated colorbond iron lid to keep the rain out. If the four lid bolts and lid hinge is removed and tipped on it's side, it rolls around on a concrete floor easily.
    Last edited: Feb 1, 2020
  19. Dazz

    Dazz Copper

    5x25kg plus water is far heavier than I can use. If I could get down to <1 bag of castable, that would be OK.

    Joe Yard reported above good success with coated bricks. For me, this solution would require bricks and castable = >$

    with custom cast tiles, I could do a few things to make them fit. For example, the edges could have a tongue & groove to reduce flame blow by. The tiles could be trapezoid shaped to make it easy to wedge them together. The last tile in a row could be cast in situ to take up any slack.

    I would expect the tiles to move quite a bit as the stainless steel backing reacts to the heat.

    Maybe pop riveting with SS rivets might be better than welding. I have a MIG welder.

    Last edited: Feb 2, 2020
  20. My furnace is taller than average due to how it functions, so it's 1.5 beer kegs tall vs 1 keg for an average beer keg furnace. Also reducing the castable thickness to half of what I used is feasible, not to mention 66% of the height which gets you 5 x 0.5 x 0.66 = 1.66 bags of refractory. There's plenty of scope for weight reduction from my example as I didn't set out to use high temp silica wool insulation, it's an afterthought as I planned to use only dense castable.

    There's few places where pop rivets could be substituted for welding that I can think of, maybe with the thin sheet stainless parts, keep in mind that unless you buy some monel or stainless pop rivets, the common aluminium ones can't be used in hot areas. MIG and stick welding stainless is easier to do than mild steel in my experience and uses lower current too. Using common "found" objects like beer kegs will keep fabrication and material costs low too, also kegs are just thick enough to weld with thin 1.6mm stick welding rods.

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