Use of CF Batts as flue liners in ceramic kiln chimney

[decrepitoldguy]

I'm building a high-fire, cross draft wood fired ceramic kiln. The basis for design comes from Fred Olsen's "The Kiln Book". Throughout the building process I've often referred to the various threads on this site to help me decide on building techniques and materials choices. Thank you to all for the critical help that the information provided!

The kiln from the bourry box through the first 6' of the chimney is constructed of second hand high duty firebrick (hot face) with cheeper low duty firebrick used as backer. I'll insulate the whole affair with a fire clay/perlite/water glass top coat later.

My final design problem relates to the 12' extension of the chimney above the brick. I want to keep the weight of the assembly low, so I've decided to use 30 gallon drums with the ends removed and then welded together to form a thick carbon steel pipe. I need to line this pipe with refractory to keep it safe from melting (worst case) and heat induced degradation of the steel. I expect to operate the kiln in the 2300 - 2400°F range, though I have no idea of what temperatures to expect in the chimney. My first concept was to simply line the interior of the pipe with 3" of homemade FB, but that would be extremely labor intensive. The idea that I am now considering is to line the pipe with 1",8# CF Batts coated with ITC 100HT & ITC 296a.

Does anyone know how this assembly might hold up to scouring/expansion cracking?

My assumption is that the scouring/thermal expansion in my chimney would be similar to what you folks see in your rocket stove builds, so I'm hopeful of profiting from your experience.

The chimney's final I.D. needs to be 12" (excluding tapering considerations), so if the proposed assembly works, l will probably add a couple of inches of cheeper batt (rock wool?) in the space between the cold side of the CF batt and the chimney to space the assembly to fit the final diameter.

I've done a pretty thorough search of the boards for information specific to this problem, but didn't find anything regarding scouring of the batts or expansion cracking of the ITC coatings . Please share any posts that I missed, and anything else (experiences or new ideas) that might help me. Thanks.

[masonryrocketstove]

Mar 29, 2023 13:31:19 GMT -8 decrepitoldguy said:

My final design problem relates to the 12' extension of the chimney above the brick. I want to keep the weight of the assembly low, so I've decided to use 30 gallon drums with the ends removed and then welded together to form a thick carbon steel pipe. I need to line this pipe with refractory to keep it safe from melting (worst case) and heat induced degradation of the steel. I expect to operate the kiln in the 2300 - 2400°F range, though I have no idea of what temperatures to expect in the chimney. My first concept was to simply line the interior of the pipe with 3" of homemade FB, but that would be extremely labor intensive.

Bourry Boxes need a consistently strong draft to maintain downward pull.. Olsen says 1 ft of chimney for every 3 feet of horizontal pull + 3 ft of chimney for every foot of downward pull... and a chimney cross section 10 times the fire grate space.. So with a 12" chimney that's a grate of 1131 square inches, or roughly 2 ft x 4 ft bourry box. Sounds like a good sized kiln, those stacked barrels are gonna be pretty tall and may need to have additional support structure to keep from falling over with horizontal wind loads hitting it from the sides. Staked guy wires might do it to keep it lighter, but framing would be sturdier..

Are you only doing oxidation firing, without planning on ever wanting to do reduction firing? Reason I ask is for inclusion of dampers, which will be harder in the barrels to implement than in stacked brick. Or harder to change or modify after building. A lot of wood-fired kilns have removable bricks in the chimney for additional air inlet to afterburn any smoke from reduction firing. That would be hard to do in a ceramic fiber lined barrel, but you could add an afterburner to the top of the stack above the barrels. But that also makes it taller. I guess you could maybe build the damper system into a chimney base approximately the same height as the kiln, with the barrels only adding a bit of height for the draw.. but I think that approach might need some angle bar framing to keep from crushing the bricks in the lower section, because of how thin the barrel walls are.. and all that weight transferring downward from the bottom barrel's rim to the bricks they're resting on.

The idea that I am now considering is to line the pipe with 1",8# CF Batts coated with ITC 100HT & ITC 296a.

The 296a is the more insulating coating, and it would do well over the 100HT base layer in the body of the kiln to keep internal temps high, but I don't think you'll need that extra heat-reflecting dual coat in the chimney. Chimney temps tend to be lower and that's not a problem.. so I think you're right to consider the gas velocity and erosion more than trying to keep temps up in the stack.

An economical route would be to use a pressurized sprayer to force kaolin slurry into the ceramic fiber. Then bisque fire your chimney sections to rigidize them when the kaolin vitrifies. Then you could use the 100HT as the top coat, assemble the stack, and let it fire in place as you use the kiln. That approach is basically like using the ceramic fiber as a skeleton to make it into homemade insulating firebrick / flue liner.. but without all the labor involved in shaping, drying, and firing hundreds of rectangular brick units and then having to lay them all up.

Have you tried looking for ideas over on the CeramicArtsDaily community? They've got a lot of helpful content and people there, too.

[decrepitoldguy]

Nice job of reverse engineering - you nailed the bourry box dimensions! As for the chimney support system, the chimney is "pinned" at the base of the drum-built section where it connects to the 6' tall x 3' deep x 6' wide brick portion of the chimney. I'm going to bolt it to an angle iron framework that reinforces/contains the brick portion. Six feet up from there the drum portion gets support (not pinned for thermal expansion) to prevent any lateral movement, as it passes through the roof of the kiln shed. The remaining 6.5' to 9' of chimney_that protrudes above the roof is probably OK without additional support, but I can always guy anchor it with cable and springs back to the roof as warranted.

[decrepitoldguy]

My thoughts on the ITC coatings:

1. Chimney fires produce temps greater than 2000°F and often involve creosote . . .
2. Tall chimneys are more subject to creosote production due to lower exit gas temperatures.
3. ITC coatings will focus IR heat in towards the center of the flue, which will result in higher gas temperatures, which will produce cleaner burns (less smoke), and which should also produce higher chimney gas velocities for a given chimney height (velocity being a function of volume in this case).

Hence my concern about scouring gas velocities in the chimney and ITC's ability to resist scour.

The design is a dynamic process, and I have revised it again. I can modularize the chimney by drum, where the drums are bolted together as discreet units, each containing 1" CF batt over 2" mineral wool over a stainless steel mesh that just fits inside the drum.
This should enable me to change out/repair/re-coat the chimney with out a major rebuild.

I just need more information about scour before I can lock this design down and go back to building the kiln. Im going to call ITC tomorrow and see if I can get an engineer on the phone . . . Not an easy thing to do these days.

Thanks for the input!

[masonryrocketstove]

Oh my bad, I misread that first part of your post about the 6' brick section. Sorry about missing that. I gotcha now.

If you do get ahold of somebody over there at ITC, I'm really curious to know what they say, too. Keep us posted? Seems like useful knowledge to have for all kinds of builds, stoves or kilns alike.


On the scouring or erosion question.. just a semi-educated guess: I'm thinking you'll probably need to rigidize the ceramic fiber in one way or another. 100HT can be used for rigidizing, but the company highly recommends a pressurized sprayer and diluting the coating so you can drive it into the fibers. If I remember right, it seems like they even had a spray pressure table on the website a few years ago to adjust for the density of the fiber and the penetration depth of the 100HT. If you just brush it on, the coating stays on the surface, and its important not to make a solid coating layer more than 2 or 3 mm thick 'cause its more likely to crack. The pressure spray method coats the individual fibers inside the batt and leaves the insulating air gap pockets between them. That apparently avoids the cracking problem from having a thick solid layer. Has something to do with zirconia phase shifts and an expansion-contraction hysteresis loop between high and low temps

Reason I'm thinking rigidizing will probably be needed: ceramic fiber kilns usually compress the fiber in the main body or ware chamber by about 30% during anchoring, before applying a coating. Compaction of the fiber batt tends to make the bond interface between coating and fibers more dense and sturdy, which helps prevent it from separating from the wall if you don't use a pressure sprayer.

Without compressing / anchoring the fiber or rigidizing it in some way, I'd be concerned about sagging, cracking, or tear-out.. just from the weight of the coating layer wanting to pull downward with gravity in a tall stack. The hard coating would basically be hanging from soft/ loose/ flexible ceramic fiber.. plus the chance of the mineral wool backer also shifting and settling downward with gravity, time, and heat cycling.. That might put inward pressure on the hard coating lower down in he stack and crack it.. Then the higher gas velocity in the chimney (compared to the ware chamber) would be more likely to scour / erode the exposed fiber in and around the crack. With woodsmoke, that could quickly lead to the coating tearing away. Potentially even obstructing the chimney flow.

If it were me, I'd probably opt for 2 inches of ceramic fiber lining and rigidizing them.. 2 inches may be overkill, but it would act as extra insurance policy against potential cracks making their way through the ceramic fiber to the mineral wool layer and causing insulation melting or chimney failure.

1. Chimney fires produce temps greater than 2000°F and often involve creosote . . .
2. Tall chimneys are more subject to creosote production due to lower exit gas temperatures.
3. ITC coatings will focus IR heat in towards the center of the flue, which will result in higher gas temperatures, which will produce cleaner burns (less smoke), and which should also produce higher chimney gas velocities for a given chimney height (velocity being a function of volume in this case).

You make very good points and this could very well be right.

At least in theory, a high-fire kiln's exit temperatures should still be well above the temp where vaporized tars / creosote would condense on the chimney walls. Also in theory, the limiting factor to a kiln's clean burn is the amount of available oxygen, rather than the temperature. Kiln flue gas temps are often hot enough to re-ignite or "afterburn" some of the smoke when it exits the top of the stack.. or if the exhaust gasses encounter more tertiary air inlets midway up the stack. ..At least that's the theory. ..under ideal conditions. ..which sometimes seem to be the exception rather than the rule. lol 

[decrepitoldguy]

According to the ITC application instructions found on their website, "Apply ITC-100 HT or other ITC products according to instructions. Fire thoroughly (ITC recommends two or three times) before applying ITC-296A."

I'm tentatively planning to build the drum inserts (SS mesh cylinder cage with a dozen ceramic buttons and high temp wire fasteners holding the mineral wool and CF batts to the cage), then attaching (screws & washers) the insert inside the drum, then spray coating the inside with 100 Ht (perhaps over another rigidizer), then fully assembling the drums together (w/ bolts, nuts, washers and steel straps), then bisque firing the kiln a couple of times, then dismantling the chimney and spray coating the hot side w/ 296a, then performing the final chimney assembly, and then fine tune the chimney height as required to adjust kiln velocity (per Olsen).

I still need to figure out the inspection and maintenance schedule (more questions for ITC).

I've also included a passive damper above the kiln shelf damper (in the brick section) which will not only serve to provide back pressure, but should also provide oxygen to (hopefully) help with continued combustion of the (drum) chimney gasses during reduction.

I will continue to post my progress on this thread, maybe even with pictures if I can figure out how it's done. Again, thanks for the suggestions/considerations!