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Ceramic / refractory material for combustion baffle

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Nofossil, Oct 5, 2007.

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  1. Nofossil

    Nofossil Moderator Emeritus

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    I want to build a different combustion chamber baffle for my EKO 25. The insert that comes with the unit is a large block of firebrick with a trough in the top. It's hard to clean ashes from around it, and it doesn't maximize hot gas flow along the walls. I've built a much better design out of 8" square flue tile, but no good deed goes unpunished. It disintegrated from thermal stress. I glued it back together with refractory cement, and coated the whole assembly with fiberglass cloth and refractory cement. Same result. I then built the same thing out of 1/2" steel from snow plow blades and firebrick. Ate right through the steel and cracked the firebrick. Tried again with low-density firebrick, and melted some broken glass bottles on top of the steel to protect it. Kind of works, but ugly.

    The structure that I want to build is essentially an 'H' beam about 10" high by 8" wide by 20" long. At present, two courses of firebrick make the sides, and the steel plate makes the web. It's just 'stacked in place'. It's set up so that the secondary combustion happens towards the front of the top channel. Gas flow is front-to-back in the top channel. The back end is sealed, and the back of the web is cut away so that the gas then flows down to the bottom channel, where it then flows back to the front. It then exits to the sides and flows back to the hx at the back end of the boiler. The serpentine path means that all the gas flows along the full length of the boiler walls, and ashes are really easy to clean out.

    What materials are available out there for building a structure like this that can withstand 2200 degrees? I've seen what I think is a ceramic based board about 1/2" thick - that might be ideal. I can't find sources accessible to non-industrial buyers.

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  2. Highbeam

    Highbeam Minister of Fire

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    My Hearthstone stove uses a ceramic fiberboard type material for the secondary baffle. It's maybe 1/4" thick and lightweight, appears fragile. The board is removable and replacable.
  3. Nofossil

    Nofossil Moderator Emeritus

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    Cool - any idea where I can buy a sheet of the material? Fragile is OK - it doesn't have to withstand mechanical stress, just thermal abuse.
  4. Corie

    Corie Minister of Fire

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    Thermal Ceramics makes a "kaowool" board that would work. Not easy to find this stuff in small quantities or even find it where joe public can buy a piece. I had a hell of a time getting some last year.
  5. Highbeam

    Highbeam Minister of Fire

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    No idea where to buy a sheet. Heck, I couldn't even find micore in my state. It was actually suggested by my wife that I buy a spare baffle for my stove from HS to have on hand just in case I break the one in there now. It might take a while to be delivered and the board is right there where a chunk of wood could pop up and hit it.
  6. KeithO

    KeithO Minister of Fire

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    Unifrax corp Niagra Falls NY
    http://www.unifrax.com/web/UnifraxH...18268E0BC11D4A47852569570068EF11?OpenDocument

    Remember that there is airflow. Not the high velocity pulsating stuff found in automotive exhaust, but still airflow, which may tend to carry the board material away over time. Speak to those folks at Unifrax about the suitability of their board and they will tell you if it is right for your application. They do sell to the little guy, which is a rare thing these days.

    You can also try Thermal Ceramics: http://www.thermalceramics.com/Products/ByCategory.asp

    They have everything from fiber to casteable refractory and proper alumina firebrick etc. If you actually try the real alumina firebrick you will find it is totally incomparable to the vermiculite "junk" that is consumer grade.
  7. webbie

    webbie Seasoned Moderator Staff Member

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    A little bit of anything can be purchased from McMaster-Carr.......

    Is this a component of a gasification system? If so, almost 2000 degrees is possible. Thin stainless covering refractory might be an option.

    At Tarm, the gasification bricks were made one by one, in wooden molds using poured refractory cement with stainless fibers in it. I assume that is also how a lot of other similar products are made.

    But, off the cuff, I like the idea of thin...bent or rounded stainless steel as a cover to a fragile ceramic board. Might give you the best of both worlds...then again, Keitho and others here are REAL engineers. I just play one in my shop.
  8. KeithO

    KeithO Minister of Fire

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    The only material that is going to put up with 2000F service for any reasonable amount of time is alumina (Al2O3) or fused silica (artificial quarz glass like used in the space shuttle windows) Stainless in an oxidising atmosphere will be gone in no time. Inconel in the right grade may last a while (jet engine parts) but it is hard to get, expensive and hard to work and weld. Just so you know, I use all of those materials at work in our lab, so I'm not joking. We test samples of the mat that we use for mounting the substrates of emission control systems up to 1000 deg C in our lab equipment every day of the week. Some of those materials are 98% alumina too because that is all that will survive the application.

    The link I provided for the Thermal Ceramics site shows Alumina firebrick to be about $32/brick compared to $5-6 each for "regular" firebrick. Its a matter of how much do you need and what are the alternatives. Alumina brick will go on in that application forever, so you will certainly get your money's worth.

    In case you want to give Inconel a go, try these guys: http://www.hightempmetals.com/techdata/hitempInconel706data.php Info on Inconel 706
    Materials thay have in inventory in that grade: http://www.hightempmetals.com/hightemp/hitemp07.php#A020

    They will cut to order and will sell any amount to anyone and I find them as reasonable as anyone else considering the exotic nature of the material. GE and Williams etc will of course get better deals....
  9. webbie

    webbie Seasoned Moderator Staff Member

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    1500 is probably more the norm.....but I once used a temp sensor in the top of catalytic converter (in an early stove). It generally stayed at 1400 or so until one day when I put a small piece of heavy pine (probably southern yellow) from our barn floor into the stove. This stuff had been seasoned since 1898. That probe hit over 1800 degrees F. Of course, this is just the gas stream inside the honeycomb, and no materials of the stove ever reached anything much over 1000....still not a temp that makes regular plate steel happy.

    Stainless is used in many applications where it glows a very bright red on a full-time basis - I would assume 1200-1400? Probably more toward the lower end of that.

    All in all, your best bet is to use factory replacements....or, get yourself an inexpensive temperature monitor thingy and see what the actual temps you are dealing with are. As Keith says, 2200 or 2000 are near impossible to deal with, but that might be just an advertised temp or the temp of a flame somewhere in the middle of the air...not that material temp your boiler sees inside.
  10. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Ceramic fiber board:

    eBay item number 160100314489

    1 x 24 x 36 fifty bucks plus shipping.
  11. Nofossil

    Nofossil Moderator Emeritus

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    Thanks for all the suggestions. This is in fact a gasification application, and peak temperatures appear to be in the 2200 degree F range. I've attached an instrumented run from last year - the combustion temp is divided by 10 to get it to fit on the graph. The combustion probe is an Omega Inconel jacketed thermocouple, and all the other channels are glass encapsulated thermistors. Looks like I'll be doing some combination of alumina firebrick and ceramic board. I'll post pictures when it's done.

    Attached Files:

  12. KeithO

    KeithO Minister of Fire

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    Just so you know, most of the ceramic board materials are alumina silicate. Silicate is basically glass. Unless the alumina content is at least 80% or more, it loses its mechanical properties fast at any temperature over 600 deg C. 600 C is enough for many "conventional" combustion processes (like some of the OWB modification projects to get a secondary burn). Alumina board is generally called alumina and not ceramic, because they don't want to be compared to the cheap stuff.

    If you get the alumina bricks, ask them if they have alumina board, more than likely they do. Otherwise, you will be rebuilding after a relatively short time and fired refractory is always more difficult to handle without damage compared to the "green" state when it is delivered. If you do the board, think about cutting it into shorter pieces to allow for expansion and also to allow sections to be replaced without tearing up the whole thing. It will generally also get you a higher yield from the board you buy.

    Best of luck...
  13. webbie

    webbie Seasoned Moderator Staff Member

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    I know I may sound like I am repeating myself, but the combustion temps can differ vastly from the material temps - example would be the temp at the tip of the candle flame and the temp of the wax. So the real temp you need is exactly in the board or bricks that are being used. Any way you look at it, you need some good material!
  14. Nofossil

    Nofossil Moderator Emeritus

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    Thanks. Someone provided a link to an eBay vendor who has 'ceramic' board that they claim is good to 2300 F. They also have different grades of alumina bricks rated a various temps from 2000 to 2600F. I'll ask the alumina question about their ceramic board.

    I was planning on building the structure as a 'stack of blocks'. Sounds like that will meet your suggestion. Of course, the proof will come in another month or so....
  15. KeithO

    KeithO Minister of Fire

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    Craig:
    If the insulator forms a barrier to a cold surface, then obviously there will be a big gradiant across the material and the average temperature will be lower. What Nofossil is proposing is a labrynth combustion chamber with the gas flowing over 1 side in 1 direction and then back over the other side in the opposite direction. What that means in practical terms is that the baffle will see the full combustion temperature since it is "immersed" and is not going to cooled by any means other than direct radiation. Thats a pretty nasty application and he has already shown that ceramic (flue liner) can't take it. The gradient BTW is also no piece of cake for a rigid material like firebrick, since the differential expansion on the hot vs the cold side can easily cause it to crack. For felt type materials that have minimal stiffness this problem doesn't exist.

  16. Nofossil

    Nofossil Moderator Emeritus

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    Yeah, it's a little tough to know for sure. In this case, there is a 'blowtorch' effect and the flame/combustion fills the upper chamber of my baffle pretty well. I put my inconel probe on the surface of the bottom panel of the combustion chamber and measured 2200F. It's certainly hot enough to melt glass....
  17. Nofossil

    Nofossil Moderator Emeritus

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    Exactly right. However in Craig's defense, the temperature of the return path (bottom side of the same panel) seems to peak out at around 1600 F. The floor and walls of the top section are incandescently hot, but by the time the gases make the turn and come back, they've cooled considerably. Of course, this adds to the thermal gradient problem.
  18. Nofossil

    Nofossil Moderator Emeritus

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    New alumina brick / ceramic wool board labyrinth ready to go!

    Many thanks to KeithO for the technical information and BrotherBart for noticing a useful item on eBay. I've built my new labyrinth and it's ready to go. Of course, it's almost 70 outside. I'll post results when it gets cold enough for another fire, but I'm very optimistic.

    The alumina bricks and the ceramic board are rated for 2300F. The bricks are cool - they're very light, and can be cut easily on my beater bandsaw with a wood blade. The ceramic board is very fragile - you can easily gouge it with a fingernail. It's almost like lightly compressed wood pulp. It also cuts easily on the bandsaw. I was able to attach the ceramic board to the bricks with stainless steel drywall screws. Almost like driving them into styrofoam. The screws don't have to support any load at all - they just keep the vertical sheet from tipping over.

    In operation, the combustion nozzle empties into the top channel. The hot gases flow toward the right end, then downwards through the opening. They then flow back to the left end. At that point, they'll split and flow back along each side, around the vertical 1" baffle. and out.

    In the photo of the remains of the old baffle, you can easily see the badly eroded plates of hardened plow blade steel as well as the melted glass slag, broken firebrick, and a pile of gravel that used to be a 'C' shaped section of flue tile.

    Attached Files:

  19. Eric Johnson

    Eric Johnson Mod Emeritus

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    That's awesome, nofossil. How much did it cost in materials, and is there any danger of the ss screws disintegrating? I'm curious to see how it works out for you.

    BTW, are you familiar with the new BioMax boilers that New Horizon is selling? Like the Atmos, it has a curved refractory chamber and what looks like a curved (on the top) firebox. Looks like an Atmos knockoff to me. Presumably there's a good reason for the shape, but I'm not in a position to say what that would be, other than facilitating the heat distribution in the chamber.
  20. Nofossil

    Nofossil Moderator Emeritus

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    The ceramic board was about $50 delivered, and a box of 12 firebricks was about $45. I used a bit less than half the sheet of ceramic board, and I have one firebrick left over.

    The SS screws are on the back side - almost the coldest place in the whole structure. I'm hoping they hold up. They don't have to do much - just keep the end panel from tipping away from the firebricks. I tried to glue the panel to the bricks with refractory cement, but no joy. The refractory cement definitely does not stick to the ceramic board.

    Haven't seen the BioMax. I'd be interested to see if it performs better in any way.
  21. Eric Johnson

    Eric Johnson Mod Emeritus

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    One of our members, rreinhart, just bought one, I believe. He said he considered both and concluded that the Max had a slightly better feature set. I hope he lets us know more about it once he gets it installed. Or before. Not having a working model has never stopped me from pontificating.
  22. Gooserider

    Gooserider Mod Emeritus

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    Very cool thread, but seems more appropriate to the Boiler Room, so I have moved it accordingly.

    Hope the new baffle works well!

    Gooserider
  23. Nofossil

    Nofossil Moderator Emeritus

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    I wondered how long it would take to get moved. I agree, by the way.

    Policy / acceptable conduct question for you and/or Eric and/or Craig:

    Over the past few years, I've put together a site that has a bunch of information, pictures, notes, and data from my personal experience in doing the wood / solar / heat storage thing. It's purely informational, not commercial. I'm egotistical enough to think that it's a useful resource, but I don't want to cross any written or unwritten boundaries between being helpful versus shameless self-promotion (Look how smart I am!).

    I suspect it's also of interest to only a small subset of members. I realize that 'roll your own' control systems are a pretty small minority.

    For these reasons, I haven't posted a link to it in the 'Links to Articles and Resources' topic and I haven't started a thread to discuss what I've done. I have on occasion linked to a picture or a page within a thread when appropriate.

    The site is http://www.nofossil.org/. Have I found the right balance? Too self-effacing? Too something-or-other?
  24. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think it's a great resource, 'fos. Lots of good information and some insight into the man behind the avatar.

    Ken "slowzuki" McFarland has a nice one, too.

    http://www.onthefarm.ca
  25. Gooserider

    Gooserider Mod Emeritus

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    Threads that are active tend to get moved sooner :coolsmirk: I look mostly at new posts, and activity brings the thread up onto my radar - inactive threads tend to get buried and if I don't see them I won't do anything about them unless asked.

    As to the link question, I actually did look at some of your site, and *I* think that it is perfectly reasonable to post a link to a personal detailed site like yours, both as a general resource and to point to relevant subsections when talking about what you've done - it saves repeatedly typing the same long answers if you can say "I did thus and such, see <link> for details" I would add it to the L&R;thread, otherwise I'd say you've hit about the right balance. Craig sets the rules, but seems to me like even commercial activity is OK as long as it's kept low key - I would say the perfect example is Mike from Englander - it's no secret that he wants to sell stoves, and I hope his postings here helps him do so, but he doesn't make a big deal about it and doesn't say an Englander stove is the solution to everybody's needs...

    I would even say that a link to a commercial site isn't objectionable if it's relevant - for instance a list of boiler makers, and other useful parts manufacturers with links that lead to "meaty" stuff like technical references, spec sheets and so forth could be useful. A link to a dealers sales site (unless it has a really good technical reference section like Tom Oyen's shop does) is less valuable...

    Just my opinion, I figure Eric is more the chief boilermaker. :)

    Gooserider
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