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Post by Solomon on Feb 18, 2021 12:19:59 GMT -8
Hey all, I've been farting around here for a few months talking about stuff of which I know not (like most newbies do) but now I have been deflowered. I built and tested my first CFB core yesterday and it it went straight from awesome to scary. Here it is, 2300 degree ceramic fiber board, based on Matt Walker's 6" core but scaled up, and using a 2" thick 4 foot CF riser sleeve as a heat riser. I placed a 1300C (2372F) thermocouple six inches up from the riser/core interface and sticking about 4" into the gas stream. Yesterday, I cut it all out, put it together and fired it up. No hiccups, everything worked perfectly. I also placed a copper wire about 56" down the riser to check temperature through melting. Everything was going great, burning out the binder in the board, and the temps continued to rise. Then it approached and passed 2300 degrees and that's when I started getting concerned because my materials are all rated at right around that. This was burning dry but not particularly well cured small split wood and some dry 2x4s and 2x6s. At the end of the burn it was hitting 2300 degrees burning ONLY TWO 2X6s! Here is the peak I measured. This is a screen cap from the video. 2446.7 was the last number the thermocouple read before going all -----. On the video, the temperature is still rising at a rate of 2-3 degrees every half second. So, though I don't have numbers, I would say it approached 2500 degrees before backing off. Later during cool down, the riser made loud cracking noises. Disassembly showed cracks in the lower riser section. Cracks appear to propagate about half the thickness of the riser. Copper melted up to just below the edge of the top of the burn tunnel. Copper melts at 1984F. A small nodule of copper dripped down and made a pile (though not a puddle). Now the major problem, every piece of the internal layer of CFB warped and cupped. You can see in the picture down the riser above that the back of the burn tunnel piece curved into the flow path. This does not seem to have affected the draw or output of the core at all. The feed tube is still likely the most constricted point in the system. The outer layer of the CFB seems to have had little issue. After burning for several hours, there was maybe half a cup total of ash. Firebricks were unaffected, and were still at 200 degrees four hours later when I took them out and hauled everything inside to avoid rain. Even though the thermocouple went well outside specifications, it seems to have survived unscathed. I named this core the Aslan Core. Points to the first person who figures out why.
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Post by gadget on Feb 18, 2021 20:04:14 GMT -8
Sorry to see your failure on this core. This is very interesting, thanks for posting. The CF riser cracking is really interesting. I've seen plenty of CF crumble but never crack like it was a monolithic structure. Seems the temperature gradient was to much stress? I would just swap it out for a 5 minute riser and it will be fine next time.
I'm wondering if your CF Board has an unusual binder, perhaps flipping the warped pieces around to heat the opposite side and refiring will straiten them out and you will be fine going forward??
It seems there are lots of CF products that claim a certain rating but are in fact much lower grade. I'm not saying its the case with your boards but just a thought. Temp rating usually has to do with percentage of alumina, more alumina = higher temp rating due to less expansion and contraction when cycling. Its usually not a melt rating but rather cycle life at elevated temps. Its the heating a cooling cycles that breaks down ceramics.
Most common CF products binder typically is fumed silica and does not burn but some CF comes with a temp "shipping" binder to help keep the shape during shipping that burns out first firing. Some fumed silica also uses glycol for freeze protection and that what you may have seen burn out.
I'm curious what the last rectangular plate is. My hats off to you for doing a build, lots of people dream but never actually go for it. I hope you don't feel down about this, I have had many failures in my builds but they where all worth the knowledge gained. You can only do some much on paper, building is where you really learn. I can't wait to see your next build. Post the video if you get a chance.
Oddly, some of the best ceramics I have used have been cheap home made refractor from my back yard clay.
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Post by martyn on Feb 19, 2021 1:05:26 GMT -8
Looks like you had some fun, I have never used or even seen ceramic fibre tube.
Moisture is the bugbear of all refractory materials, it could be you superheated the tube the board and the brick while it contained even a low amount of moister. Moister can be easily sucked in by damp or humid air. Did you see any steam? Did you measure the heat on the outside of the box and riser? The fact your stone table cracked suggest there was a lot of heat on the outside... another sign of moister in the product, maybe even in the table.
Having said that I guess there is only one inch of board under the brick and that will still get very hot on the outside.
If you have any offcuts you can use them like batons under the base to make an air gap between the table top and the board.
You may find that the ceramic board becomes very brittle after one or two firings as will be much more delicate to handle!
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Post by Solomon on Feb 19, 2021 12:19:13 GMT -8
Moisture is the bugbear of all refractory materials, it could be you superheated the tube the board and the brick while it contained even a low amount of moister. Moister can be easily sucked in by damp or humid air. Did you see any steam? Did you measure the heat on the outside of the box and riser? The fact your stone table cracked suggest there was a lot of heat on the outside... another sign of moister in the product, maybe even in the table. Having said that I guess there is only one inch of board under the brick and that will still get very hot on the outside. If you have any offcuts you can use them like batons under the base to make an air gap between the table top and the board. You may find that the ceramic board becomes very brittle after one or two firings as will be much more delicate to handle! There was no moisture. This was all protected the entire time in my posession. Insulation merely slows heat transfer. I'm told it takes 600 degrees to burn out the binders. The exterior does not quite reach that without further insulation. I put some extra pieces on the side to try to get it up to temperature. The stone on the bottom acted like an extra layer allowing heat to build up to the point it eventually cracked. It was just an old piece of something I found when I moved into this house. I did check with an infrared thermometer. The exterior of the heat riser measured around 250 degrees, you could still touch it for about 3 seconds. I think the problem with the heat riser was the massive temperature differential between inside and outside. The infrared showed 2000 degrees at the top of the riser. There was two inches of CFB all around. I have some 2" on hand. I could maybe do something with that. But thicker insulation doesn't better withstand high temperatures, it actually creates them. High temp spec insulation does withstands higher temperatures. Depending on how the RMH throttles this performance, I may need to upgrade with higher spec CFB. And pre-baked also.
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Post by Solomon on Feb 19, 2021 12:22:36 GMT -8
This clearly shows the ridiculousness of all of you believe in expensive commercial things. How so? Doesn't seem clear to me. Higher performance materials do higher performance. A classic brick core would never get this hot, despite some unverified claims.
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Post by Deleted on Feb 20, 2021 7:01:28 GMT -8
This clearly shows the ridiculousness of all of you believe in expensive commercial things. How so? Doesn't seem clear to me. Higher performance materials do higher performance. A classic brick core would never get this hot, despite some unverified claims. LOL. CFB that deserves this name cannot break in this way.
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Post by Solomon on Feb 20, 2021 9:12:03 GMT -8
Okay, I gave you a chance to be nice. Blocked.
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Post by Orange on Feb 20, 2021 11:38:38 GMT -8
Thanks for the info, now we know much higher temps can be reached. I've heard here that coating with waterglass helps against bending, maybe screwing them together also helps.
And CFB max temp rating is only for 15min or so and there should be max temp rating for continous use.
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Post by josephcrawley on Feb 20, 2021 12:39:42 GMT -8
This clearly shows the ridiculousness of all of you believe in expensive commercial things. How so? Doesn't seem clear to me. Higher performance materials do higher performance. A classic brick core would never get this hot, despite some unverified claims. I would be curious to see what max temp you could get in a brick core. If you have enough brick and don't mind doing the testing.
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Post by Solomon on Feb 20, 2021 13:21:06 GMT -8
How so? Doesn't seem clear to me. Higher performance materials do higher performance. A classic brick core would never get this hot, despite some unverified claims. I would be curious to see what max temp you could get in a brick core. If you have enough brick and don't mind doing the testing. I don't have the brick, but it doesn't cost that much. I could throw together a core and test it. I'd need to get a masonry bit too. But the more difficult part would be insulating it. Maybe I could just buy a bag of loose vermiculite and build a temporary container around the core and test it that way.
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Post by gadget on Feb 21, 2021 7:01:12 GMT -8
Okay, I gave you a chance to be nice. Blocked. looks like more then your core had a melt down
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Post by Orange on Feb 21, 2021 9:47:53 GMT -8
But the more difficult part would be insulating it. Maybe I could just buy a bag of loose vermiculite and build a temporary container around the core and test it that way. BB doesn't require insulation, for example Batchblock has none. And I remember Peter said somewhere insulation is preferred after the first port.
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Post by Solomon on Feb 21, 2021 11:16:11 GMT -8
But the more difficult part would be insulating it. Maybe I could just buy a bag of loose vermiculite and build a temporary container around the core and test it that way. BB doesn't require insulation, for example Batchblock has none. And I remember Peter said somewhere insulation is preferred after the first port. Right, but you're not going to get the temps without the insulation. Two inches of CFB and 2 inches of firebrick are entirely different animals.
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Post by josephcrawley on Feb 21, 2021 13:15:10 GMT -8
BB doesn't require insulation, for example Batchblock has none. And I remember Peter said somewhere insulation is preferred after the first port. Right, but you're not going to get the temps without the insulation. Two inches of CFB and 2 inches of firebrick are entirely different animals. I would love to see the results of with and without insulation fired till the temp plateaus.
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Post by gadget on Feb 21, 2021 20:58:26 GMT -8
BB doesn't require insulation, for example Batchblock has none. And I remember Peter said somewhere insulation is preferred after the first port. Right, but you're not going to get the temps without the insulation. Two inches of CFB and 2 inches of firebrick are entirely different animals. So, going for maximum NOx output?
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