hard firebrick batchbox core w ITC100?

greetings all:)

I've been studying up on design and materials and am wondering if anyone has tried this approach to building a batchbox...

I understand that one of the key theories of RMH operation is a well insulated core, but for reasons of durability, toughness, and longevity I am inclined to build my batchbox
interior using hard firebrick rather than a more insulating material.

I want to build a fairly large box (roughly 5750cu") that will be loaded multiple times/day.
For the floor and walls I would use a thin hard firebrick, maybe 1.25", so there will be some mass to heat, but the bricks would be wrapped in a blanket of ceramic fiber, and coated internally with ITC100.  
My thinking is that, yes, the mass of the harder bricks does absorb some of the heat during the initial firing, but once a fire has been going for a bit, the extra heat in the thermal mass will help with keeping the whole thing going as new fuel is added at varying points through the day, and the ITC will in theory also reflect much of the heat back into the combustion zone.

The roof of the box (and the riser) would be either soft IFB, cast insulating refractory, or CFB... (probably IFB).

Thoughts?

My thoughts are ….. it seems like you want to build a batch box in the same way as the majority are built?
Perhaps the most popular method is dry stacking full fire bricks on edge but split bricks with a supporting backing would be fine.

Hi Dewy;
Welcome to Permies!
I second everything Fox had to say.
Both of my batch boxes are full bricks, that are dry-stacked & wrapped by super wool.
What size Batch are you wanting?
7" or 8"?  As long as Peter's sizes are followed you can go larger.
The larger the box the larger span for a roof.
I think you will need to cast one or switch to a kiln shelf, heavy firebrick tile, or cast iron.
The riser would be best with 2600F insulating firebricks or a five-minute riser with super wool.

ok, thanks.  I didn't realize most people were building with hard firebrick... I thought soft IFB or cast insulating refractory were the more highly recommended materials.
(I was initially thinking of casting the box w Kast-o-lite, but I don't currently have any way to properly control the curing/firing process.)

Yes, I'm thinking of using thin hard brick on edge, maybe with a little thin clay "mortar", and was thinking to use some angle-iron and all-thread to fabricate an external support structure to keep them clamped together.

I've seen that some of these bricks will both expand and contract depending on the heat, so am concerned of structural collapse and or cracking of bricks but not sure what can be done about that... if the steel is expanding while the bricks are contracting then there won't be much force holding them together.
Conversely, if the bricks expand while metal is still cool and tight, that pressure may break bricks.  

Was also thinking of maybe trying to work up an external framework of some sort of castable and/or maybe something like nichrome wire or a mesh (something like: https://mybuildingsupplystore.com/collections/ceramics/products/armormesh) embedded in a thin layer of castable or some sort of high temp cement to hold it all together.

For the roof, I was thinking of maybe trying to drill aligning holes through IFB and run some stainless rod though them to hold them up, though I have some doubts about that working, so yeah, maybe more likely CFB or kiln shelf roof.

I could also attempt to drill holes in the sides and ends of the wall bricks and cut some stainless rod and sort of "pin" them together, kinda like IKEA cabinets if you know what I mean... So long as the holes are slightly larger than the OD of the pins so the metal doesn't expand enough to split the bricks apart...

I'm still working on dimensions, but if I stack regular 9 x 4.5 x 2.5 IFB's in a single brick square pattern for the riser, the internal area across would be 6.5x6.5 so 42.25sq".

If I build the box as big as I'd like to, I'd use 12 bricks per side, which would result in 27" x 18" (assuming 9x4.5 bricks), and the front and back walls would be 18" x 18".
I'd use the #'s Peter established for the air-flow/port/riser dimensions and rough ISA of a bell.  

Which brings up a question that's probably answered in here somewhere if I could find it, but my thinking is that the internal volume of the box itself is a less crucial dimension (within reason) relative to the more "functional" dimensions of the ratio of combustion air inlet, port size, and area of riser.  Does that make any sense?

Hi Dewy, Whatever size you build, you must stick to Peters spec's, bricks are cut to size with a wet cutter so they can be used in any size box.
You need a buffer between different materials ie … bricks, CFM, steel.

Dewy Pike wrote:Which brings up a question that's probably answered in here somewhere if I could find it, but my thinking is that the internal volume of the box itself is a less crucial dimension (within reason) relative to the more "functional" dimensions of the ratio of combustion air inlet, port size, and area of riser.  Does that make any sense?

According to what you are mentioning, the riser is 6.5 inch square. A square is comparable to a circle as far as aerodynamics in a vertical tube are concerned. Provided the diameter of the circle is the same as one of the sides of the square.
Your system is a 6.5" one, so the proposed firebox is far too large. Height and wdth of the firebox should be roughly 14"x 9.4", 18"x 18' is far too large. I fully expect the thing to smoke like mad with a box like that filled with cord wood and an undersized afterburner behind it. Dimensions of the firebox are paired with the afterburner function's capacity, so the volume of the box and its dimensions are as crucial as all the others, they form a fixed set. There's one exception, though, depth of the firebox could be 25% oversized without compromising complete combustion.
As Fox James mentioned, the design is really tight, meddling with proportions won't pay off.

Of course you are entitled to do what you see fit, it's your house, your heater.

thank you all very much:)  I am going to work at scaling down the box size and scale up the riser size to as close to those listed on https://batchrocket.eu/en/building#material as I can, though I am curious if there's maybe still a little room for further experimentation.

I've been heating with regular old fashioned wood stoves for over 30 years now, and know how to build and maintain a very hot and usually at least mostly smokeless fire (at least minimal visible smoke, though I know the Testo would show otherwise:)... So my goal here is maybe not quite as lofty as to attain the levels of perfection that some of you have achieved, more to just improve hopefully at least a little bit over a regular old woodstove.

Also, questions for Peter regarding the reasoning for the ratios... You mentioned that a half size load of wood burns in about the same amount of time as a full size load of wood.  That was the statement that caused me to think that a maybe a somewhat larger box could be built, so long as the proper air flow proportions were in place.

Can you explain why the structural size ratios of the heater are more important than the quantity and quality of fuel present?  
In theory it seems fairly clear that if there's too much space and too much fuel relative to the burn rate, the combustion space and fuel just won't get hot enough fast enough for an efficient burn.  But say for example, to your point, an oversized box was stacked with crisscrossed, very fine, very dry, very fast burning fuel... A fire built like that has the potential to generate a huge amount of heat in a very short time, so the limiting factor would be the capacity for airflow, which is not entirely determined by the precise volume of the container through which everything is flowing.  Does that make any sense?  (Clearly no one builds fires like that in real life, but just for the sake of trying to grasp some of the physics...)

It's also such a different strategy/methodology to shoot to burn the entire load of fuel all at once with no regulation of the fire other than through the established design ratios, in contrast to how a standard wood stove is operated... opening or throttling supply air relative to temperature, quantity and quality of fuel, desired burn time, etc...

I guess I just have questions around why the size ratios need to be so precise when there are so many other (I would think significant) variables that exist even if the size ratios are "perfect", most especially like how the fuel is stacked, what size it is, what type of wood it is, how dry it is, the thermal qualities of materials used in construction (soft IFB's vs hard bricks vs kiln shelves for example) etc etc...  All these and (lots) more will have effects over the state of the fire over time, right?  I suppose the theory is that the design ratios will still be optimal regardless of all these other "external" variables?

(I'm still reading through https://batchrocket.eu/en/ and other sites, so apologies if this is already answered somewhere in there)

Peter van den Berg wrote:

Dewy Pike wrote:Which brings up a question that's probably answered in here somewhere if I could find it, but my thinking is that the internal volume of the box itself is a less crucial dimension (within reason) relative to the more "functional" dimensions of the ratio of combustion air inlet, port size, and area of riser.  Does that make any sense?

According to what you are mentioning, the riser is 6.5 inch square. A square is comparable to a circle as far as aerodynamics in a vertical tube are concerned. Provided the diameter of the circle is the same as one of the sides of the square.
Your system is a 6.5" one, so the proposed firebox is far too large. Height and wdth of the firebox should be roughly 14"x 9.4", 18"x 18' is far too large. I fully expect the thing to smoke like mad with a box like that filled with cord wood and an undersized afterburner behind it. Dimensions of the firebox are paired with the afterburner function's capacity, so the volume of the box and its dimensions are as crucial as all the others, they form a fixed set. There's one exception, though, depth of the firebox could be 25% oversized without compromising complete combustion.
As Fox James mentioned, the design is really tight, meddling with proportions won't pay off.

Of course you are entitled to do what you see fit, it's your house, your heater.

Thanks:)  By buffer you mean maybe something like a layer of ceramic fiber or maybe some sort of hi-temp caulk or RTV between the brick and angle-iron, for example?  It also occurred to me I could maybe use springs in the clamping structure to allow for expansion/contraction, but could you say more about what you mean by buffer, what types of materials, etc?  Sort of like a gasket in engines for example, to allow for the different rates of movement but still maintain a seal?  I've been looking at lots of different materials but am still unsure what's best depending on application...

Fox James wrote:Hi Dewy, Whatever size you build, you must stick to Peters spec's, bricks are cut to size with a wet cutter so they can be used in any size box.
You need a buffer between different materials ie … bricks, CFM, steel.

I sell a nice 1/8" thick superwool gasket material that I used between my bricks and metal.
Superwool blanket can be used.
Cob mortar can also be used in some places.

Hi Dewy, expansion is a real issue when building rocket stoves and you really need to think hard to avoid bigger issues.
Different materials expand at different rates ie … placing steel inside concrete will never work as the steel will expand at a faster rate and cause the concrete to crack.

That is why dry staking fire brick can be an easy option, most fire bricks are very regularised with accurate and smooth surfaces, so it is possible to dry stack them together.

The big issue with a batch box is the metal door to brick interface as it is difficult to get a rigid  fixing and also allow movement!

One way is to avoid fixing anything to the actual core but, instead attach the door to whatever is containing the core ie .. the bell.
As the bell wont be getting as hot as the core it wont expand as much and can be made more rigid by cementing the bricks togher.

As an example … you could dry stack a core, cover the core with ceramic fibre mat and build another brick box around the ceramic fibre.
In the outer brick box you can now fit the door frame with a flexible gasket and keep your fingers crossed it will work.

Once you have a design and show us a picture of what you want, it will be easier to discuss and problem solve any issues.