My Attempt at an IFB Domed Furnace Lid

Discussion in 'Furnaces and their construction' started by Melterskelter, May 6, 2019.

  1. Melterskelter

    Melterskelter Silver Banner Member

    The right angle I am thinking of is not intended as a setup tool like those widely and cheaply available from tool discounters. Mine would be intended for intended for metrology. So, it would be lighter in construction and narrower and taller than a setup/work-holding angle plate.

  2. oldironfarmer

    oldironfarmer Silver Banner Member

    I didn't follow. The first picture shows trapezoidal brick and the others show triangular. It would save a lot of cutting for a furnace small enough if yo made all triangular then cut off the tips to get a flue.

    In my case, with a 22 inch diameter arch, I couldn't use triangles but might cut both sides of each brick all the way to avoid the shorts. They have not been a problem for me yet.
  3. Melterskelter

    Melterskelter Silver Banner Member

    OIF, you might have been able to avoid shorts by making long side cuts the same on all the bricks, but I do not see any benefit. In fact, the way the small triangles seated against the larger bricks in mine (copying your design) really locked up the all of the bricks nicely as the short pieces wedge securely into place. Because the bricks naturally have a high friction coefficient one against the other, that dome is rock (or brick) solid and nearly air-tight.

  4. oldironfarmer

    oldironfarmer Silver Banner Member

    I like to think my arch is as solid as a brick crucible house.

    My small triangles act as keys as well. No way they're ever falling through, they're just too Dolly Parton on top.
  5. Melterskelter

    Melterskelter Silver Banner Member

    Sounds to me that if your lid collapses it will be T.U. ;-)

    Don’t know whether to sign this one,
  6. oldironfarmer

    oldironfarmer Silver Banner Member

    I see you're milking it for all it's worth.
  7. PatJ

    PatJ Silver Banner Member

    Your cuts are pretty much flawless, so I would say there is little chance of the short bricks falling out, but in general, I would say the shorter bricks are going to be more prone to falling out.
    What science/knowledge do I base this idea on?
    None at all; it is sheer conjecture on my part, and could absolutely be completely false.
    But given my limited experience with insulating fire brick vs iron temperatures, my guess is that the short bricks will fail/fall out first, and that is just a hunch/guess and not anything based on science.

    Is one design better than the other?
    Clearly the diagonally cut brick is limited by the diameter of the lid, as melter mentions.
    As with burners and most other foundry equipment, everyone has their favorites, and often it becomes a Chevy/Ford/Chrysler Hemi argument.
    (Side note: Which is better? A Chevy, Ford or Chrysler Hemi racing engine? Clearly Chevy RULES !, but having said that, I have seen more than a few Hemi's wipe a Chevy's assets).

    Would I use a short brick design if I were making a larger diameter furnace lid?
    Probably not, but I don't have a better design for a larger diameter lid either, so there may not be a better solution.

    And as I mentioned above, my bricks rest on top of 1" of Mizzou and another 1" of ceramic blanket, so part of what they do is insulate, and part of what they do is isolate the ceramic blanket and prevent damage.
    So your bricks are arrayed in a structural configuration (there is nothing under them I presume?), and mine are not.
    So I think we may be comparing apples to oranges.

    As tight as your cuts are, and if your lid shell does not distort (sometimes the lid metal shell can overheat if flame gets deflected on it; mine did), and if you don't overheat your bricks, then I think your lid will never have a problem.

    If the bricks are not rated for a high enough temperature, chances are they will fail regardless of the configuration.

    I am assuming you will be melting iron with this lid, and the combustion gas temperature at the top of the furnace is quite high, so whatever is up there will get a serious test.

    All speculation and conjecture.
    Let us know how this lid works out.

    Last edited: May 15, 2019
  8. PatJ

    PatJ Silver Banner Member

    I am having to scramble to find my internet slang dictionary to keep up here.

  9. Melterskelter

    Melterskelter Silver Banner Member

    There is no way the short bricks can fall out or through. As you likely noticed, they are cut tapered not just front to back but top to bottom with the top being significantly wider than the bottom much like the keystone of an arch. All of the bricks, long and short, are cut that way. This design was figured out few thousand years go and a few examples are still standing. I am pretty sure I could stand on it without any risk of damage.

  10. PatJ

    PatJ Silver Banner Member

    But I have seen insulating fire bricks do strange things at 3,820 F (light fuel oil flame temperature).

    I guess the old large iron furnaces had domed lids, but I would guess that was hard fire brick, not soft fire brick (I am assuming you are using soft fire brick).

    If your lid design turns out to be successful, I will be the first (or second, or whatever) to adopt it.

    I tend to be on the skeptical side since I don't think the bricks are rated anywhere near the temperature that they could see with an oil burner.

    Fire it up and lets see how it goes.
    You know the saying: "The proof is in the pudding".

    And another thing that is causing me bias is that I seem to recall scavenger making at least two lightweight furnace lids, and failed completely after just one melt, and the other lasted a bit longer (I think his third design worked well), but I don't recall ever seeing bare insulating fire bricks stand up to iron/oil burner temperatures, so some of this opinion is based on scavenger's experiences with his multiple lid designs/failures.

    Last edited: May 15, 2019
  11. Melterskelter

    Melterskelter Silver Banner Member

    I will not attest to an IFB lid having a long lifespan though OIF has subjected his to a lot of heat and some iron melts with good results. I will not be firing these bricks bare. They will have satanite on them or maybe bubble alumina---I am vacillating on the BA. I had pretty good results from my wool and satanite lid and I think could have been much better had it been designed with a chimney in mind and a better vent arrangement. As it was, the lid had relatively poor load -bearing strength as the metal shell was to thin and the vent liner did not manage exhaust well and allowed it to get leak through the wool, etc,etc. Brick is decidedly tougher than wool and I hope will hold up well enough. I am going slowly on construction the steel aspects of the lid in hops of managing some of these aspects better. there are so many factors to consider and the muffling chimney considerably complicates matters as there is not completely unrestricted vertical venting of gases but rather swirling of hot gas around the vent and some mild back pressure that encourages hot gases to try to heat the area around the vent inside and out.

    I do not expect to get 100's of melts out of it though. But we will see what happens. I am just trying as hard as I can to avoid a massive furnace and am willing to make a few mistakes along the way.

    That said, good fitting of the bricks will help them as one thing I have clearly observed is that if gas can flow through gaps in refractory, it will eat refractory away. On the other hand if there is a dead-end gap in refractory which does not allow a flow of gas through the gap, then the refractory is not nearly as likely to break down. So, that is why I spent some time fitting the bricks quite closely and why I will use wool to plug any potential gaps.

    I also think the rated temperature of refractory materials assume exposure to oxygen containing atmospheres. I think the melting point of those refractories is significantly higher than their nominal rating. This is well demonstrated in the walls of my furnace which are only wool coated with satanite and have held up to many many firing to a white heat directly impinged upon by my oil burner. The wool has not melted under the satanite. But, bare areas would have melted away the first firing. And then consider that a drop of iron resting on satanite and wool will eat a hole in the satanite and wool in a firing or two. This whole adventure is quite interesting and challenging especially since exactly what a "temperature rating" of a material is is not clear in my mind, but I take it to be a relative comparison of resistance to heat and air but then throw in an actively combusting atmosphere or a highly reactive element like iron or eliminate it and the whole equation changes.

    oldironfarmer likes this.
  12. PatJ

    PatJ Silver Banner Member

    I think the coating is going to be very important to the brick life, as you mention with the ceramic blanket.
    I use ITC100 a lot with Mizzou, and it stops most of the degradation that occurs without it.

    I saw a furnace today with a vent out the back, so perhaps one day you can consider building a furnace with a horizontal back discharge, and perhaps use a horizontal muffler that is cooled by a fan or leaf blower running on lower than maximum speed.

    This may also solve some of your lid problems, since the lid would be totally independent of the muffler and discharge.

    I have no idea if my 1" thick Mizzou domed lid is going to hold up.
    Only time will tell.
    I am new at the iron game, so still in the curve.

    As I understand it (not very well), the temperature rating of refractory and crucibles is the temperature that they normally operate at (check me on that).
    So if that is true, then I would guess that you could operate a material at a higher rating if you trade off life of the material (similar to oil-filled power transformers).

    I will be interested in what is happening with these furnaces/materials in about a year from now when we may have some solid data to compare.

  13. oldironfarmer

    oldironfarmer Silver Banner Member

    A key aspect of a structural brick arch is to keep the outside uninsulated so there is a good temperature gradient across the brick. My brick gets to a couple hundred degrees on the top. I think you would find if you add a little insulation to the outside of the brick it will fail much quicker. The actual inside temperature of the brick will be perhaps a 100F lower than the gas inside the furnace due to the heat escaping through the brick.

    On sealing cracks with wool, I chose to smear a little mortar (Satanite) on the outside of the cracks. The mortar cracks but a crack a few thousandths wide will not pass appreciable gas and the mortar will seal cracks too small to get wool in.

    On temperature rating of materials, remember the ratings are usually for continuous service, or even intermittent service which is practically continuous compared to a home furnace. A year is about 8,766 hours, for continuous service. Intermittent service would be 2,000 hours to 4,000 hours, maybe in 24 to 72 hour periods. Excursions into excessive temperatures of a few hours at a time are generally acceptable and what we see in a home furnace are excursions in minutes. If you spend an hour bringing your melt up to 2,600F you are only at 3,000F in the center of the box for ten minutes at the best. The refractory system is still warming up and stabilizing when you cut the fire and may not have reached 2,600F the melt is at. Then you open the box, shock the refractory with cold air, make your pour and if you reload for another pour the system is way back down under 2,000F when you relight. It reheats quickly but you'll probably still not get the refractory to it's highest level for over ten minutes. If you do one pour every day for a year, at ten minutes per pour, the refractory is at temperature for 60 hours in that year. The thermal shock is a larger load than the time at temperature which is why stable materials like Missou, brick, wool, and Satanite are used. Many good castable refractories do not hold up well to the shock but are great for continuous service. Most industrial furnaces have a strictly prescribed warm up and cool down ramp rate to protect the refractory and they watch the rate, don't just cut the fire. And never ever open a hot box. Until I became acquainted with home foundry work I would not open my pottery kiln hot due to my industrial experience.

    On sealing the seam between the arch and the sidewall, my refractory rope has worked well. You might find some big wood stove door sealing rope and run a few rounds at the outer edge. It will be cool out there if there is no leakage. I put rope down, used it a couple of times to seat the arch and rope, then put rigidizer on the rope to avoid the airborne particles. I painted my metal yellow so I could see if it turned brown. It never turned brown but is getting black from oil fumes escaping. If your wall is rough, seat the rope in wet mud and set the arch down to press high areas down into the mud. A gas tight seal is imperative.
  14. Melterskelter

    Melterskelter Silver Banner Member

    OK, vacillation is over. I ordered some bubble alumina from High Temperature Tools. I hate to plunk the 130 bucks for 20 pounds. But, if I don't try it out, I will forever wonder. And I will be doing my part for "research" on this forum. ;-)

    Besides the cost as an inhibiting factor, the wait time is also a drag as I am enthused to fire this new lid/furnace up. But, I will just have to hold my horses as they say.

  15. oldironfarmer

    oldironfarmer Silver Banner Member

    Great!! Will it take over a pound?

    But the big question, can't it be put on fired brick? We really need to see some heat.
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    When Andy built his IFB furnace, one of the features I liked the most was the loose piece lid because of it's ability to allow movement without fracture. Thermal shock and big temp swings are just a fact of life with the lid of a hobby furnace. Based upon my IFB furnace, I'd hazard a guess to say if you mortared the pieces together there would be a stress relieving fracture somewhere after the first melt.

    When it comes to coating IFB, I would say do not coat unless you need to. You need to because of insufficient refractory and resistance to chemical attack. Time at temperature is certainly a factor but I think IFB mechanical properties degrade quickly when exposed above there working temp. The thing is, that only occurs very near the surface as the temp gradient quickly reduces the temperature as you progress deeper into the brick.

    Coating IFB with mortar makes the surface strong but the mortar shrinks when it cures imparting stress on the IFB. It then expands a lot more then the IFB when heated because of the temperature gradient across the wall and that causes fractures at the interface with IFB and sluffing/flaking off of the mortar surface with repeated cycles. Easy to repair but it does cause some attrition of the IFB with each cycle.

    I would think the bubble alumina would be a better coating because it's more insulating than Satanite and other high alumina mortars. That would mean a much steeper temp gradient across the surface coating and since the BA is a bunch of little spheres, you may just get a series of localized micro cracking instead of larger spalts. I wouldn't glue the bricks together unless it was just a little dot here and there to hold it together so you easily could turn it upside down to spread the coating.

    BA is expensive stuff but I would like to see you try it. I know what mortar on IFB does.

  17. Melterskelter

    Melterskelter Silver Banner Member

    There is one way to answer that question. It is intended to coat wool, so I strongly suspect it will adhere to IFB. But, we shall see.

  18. PatJ

    PatJ Silver Banner Member

    ITC100 is mixed two parts ITC to one part water, and sprayed on in a thin slurry.
    If applied that way, it will not crack or spall off.
    If it is applied thicker like a puddy it will crack and fall off.

    ITC does a good job of protecting the refractory, but as an aside, it also does a good job of reflecting the heat back into the furnace since it becomes very luminescent at high temperatures.

  19. PatJ

    PatJ Silver Banner Member

    I am not sure I would agree with that.
    Just from observation, which is not scientific at all, my refractory gets extremely hot within 5 minutes of starting the burner, and with melter's low mass, his refractory will be very hot even sooner.
    When I say "extremely hot", I can only guess at that that temperature is, but if the flue gas can be 3,820 F (the flame temperature of light fuel oil and air), then I suspect the top of the furnace can approach that temperature.

    The bottom of the furnace is definitely cooler just because of the introduction of a large amount of ambient temperature air, but the top and middle of the furnace get quite hot, and my guess is the differential temperature required to melt iron in a crucible is fairly high, ie: 3,830 F - 2,800 F, or about 1,000 F.

    I have tried using temperature sensors on the outside of my refractory shell at various places vertically, but unfortunately just melted those, so the outside of my refractory is hotter than perhaps 2,250 F.

    Just guessing at all of this.
    At these temperatures it is difficult to get real measurements, and so about the best we can do is speculate, and try to derive temperatures from observed results.

    I have seen the heat in my furnace radiate through 1" of Mizzou, 1" of ceramic blanket and through a 2,600F insulating fire brick, and get the stainless shell red hot in spots on the outside of my furnace.
    Oil burners run really hot.
    Last edited: May 16, 2019
  20. oldironfarmer

    oldironfarmer Silver Banner Member

    Boy I agree with that, the joints should not be mortared in the traditional sense. I put a little mortar smeared on the top of wide cracks to seal them. That way they crack along the thin mortar layer but you still bet a gas seal.



    You can see the fine cracks running along the seams.
    We have had different experiences here. I have coated brick with Satanite and had very good results, even in areas of flame impingement. I believe the type of mortar can make a big difference. I remember seeing your experience with mortar on brick and wondering why it was different than mine.

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