Bell/stratification chamber size question…

Hello all!

I’m currently working on my second RMH build. Last year I did a j-tube and now I’m working on a 6 in batch box. The bench/strat chamber will be an L-shape with firebox on one end and exhaust on the other. In an effort to have the burn chamber where we want it to land, my internal measurements would be a bit small to stay within the 57 ft2. (Contrary to my lovely, sage teacher and mentor, I prefer not to install a bypass 😊). The internal measurements I would like to do are-
12 inches tall
13 inches wide
(Length is approximately 114” on one leg and 90” on other)

This would create a small but long chamber… does anyone know if there would be any negative aspects to this shape?

Cheers and thanks!

Heather Arvensis wrote:I’m currently working on my second RMH build. Last year I did a j-tube and now I’m working on a 6 in batch box. The bench/strat chamber will be an L-shape with firebox on one end and exhaust on the other. In an effort to have the burn chamber where we want it to land, my internal measurements would be a bit small to stay within the 57 ft2. (Contrary to my lovely, sage teacher and mentor, I prefer not to install a bypass 😊). The internal measurements I would like to do are-
12 inches tall
13 inches wide
(Length is approximately 114” on one leg and 90” on other)
This would create a small but long chamber… does anyone know if there would be any negative aspects to this shape?

To be frank, it won't work at all. Just a quick calculation shows  that the Internal Surface Area would be close to 80 ft2. Far too large, according to the recommended values. The L-shape of the bench makes matters worse, lots of friction is expected, due to that sharp bend.
How do you plan to lead the gases from the combustion core into the bench, and what variant of core do you want to use?

Peter van den Berg wrote:

Heather Arvensis wrote:I’m currently working on my second RMH build. Last year I did a j-tube and now I’m working on a 6 in batch box. The bench/strat chamber will be an L-shape with firebox on one end and exhaust on the other. In an effort to have the burn chamber where we want it to land, my internal measurements would be a bit small to stay within the 57 ft2. (Contrary to my lovely, sage teacher and mentor, I prefer not to install a bypass 😊). The internal measurements I would like to do are-
12 inches tall
13 inches wide
(Length is approximately 114” on one leg and 90” on other)
This would create a small but long chamber… does anyone know if there would be any negative aspects to this shape?

To be frank, it won't work at all. Just a quick calculation shows  that the Internal Surface Area would be close to 80 ft2. Far too large, according to the recommended values. The L-shape of the bench makes matters worse, lots of friction is expected, due to that sharp bend.
How do you plan to lead the gases from the combustion core into the bench, and what variant of core do you want to use?

Hi Peter! I’m so grateful you chimed in!
I’ll include a rough layout photo for reference. I re-ran my numbers just now, twice. I’m getting 53.66 sq ft for the numbers in the photo. And 56.74 sq ft for the layout in original post. 🫣 Now, if my brick cutting today is any indication, I am totally bad at math… 😬
(I did subtract the floor… is that not ok??)

As for the L shape- I had no idea that would pose a problem! I got away with it for my j-tube but maybe because it goes from barrel to transition area to into Strat right next to corner… shoot. What if I made the corner wider (less cob there) to help relieve friction?
As for leading gases I was planning on going into barrel from riser and dump straight into bench with as wide and big a hole I can manage.
As for variant of core I’m not quite sure what you are asking… type of batch box? If so, I’m doing the basic, original design.
I sure do appreciate your expertise!
Cheers!

Heather Arvensis wrote:I’ll include a rough layout photo for reference. I re-ran my numbers just now, twice. I’m getting 53.66 sq ft for the numbers in the photo. And 56.74 sq ft for the layout in original post. 🫣 Now, if my brick cutting today is any indication, I am totally bad at math… 😬 (I did subtract the floor… is that not ok??)

Apologies, I recalculated and it seems your numbers are correct. Almost sure that I forgot there are 12 inches in a foot... The floor shouldn't be in the calculation, that's true. In case you are using a barrel, this should be in the calculation, no doubt about.

Heather Arvensis wrote:As for the L shape- I had no idea that would pose a problem! I got away with it for my j-tube but maybe because it goes from barrel to transition area to into Strat right next to corner… shoot. What if I made the corner wider (less cob there) to help relieve friction?

It would help a great deal if you made the corner wider, yes. A great deal wider, I'd say, with rounded corners. Especially the inner corner rounded off in a quarter circle that's as large as the internals of the bench itself.

Heather Arvensis wrote:As for leading gases I was planning on going into barrel from riser and dump straight into bench with as wide and big a hole I can manage.
As for variant of core I’m not quite sure what you are asking… type of batch box? If so, I’m doing the basic, original design.

Straight into bench with a large dump and hole is good. Since you are using the first-generation batchrocket design, you already mention you would need a barrel. Keep in mind that the space above the riser (commonly know as top gap) should be as large as the system's diameter. Batchrockets are very picky about friction, that's why. It would be best to provide a straight trajectory and/or lots a space.
I spotted another potential friction spot, though. In your plan view, the barrel is in front of the bench. When the hot gases are dumped into the bench, they are forced around a 90 degree corner immediatly after that. It would be much better to place the barrel beside the bench, so that the gases just stream lengthwise into the bench, unhindered.

Just a remark: there's another possibility. Although the bench won't be as large and has a voluminous heated backrest. This has been done using the latest incarnation of the batchrocket concept, the Shorty. Doing away with the barrel and feeding the bench directly. Have a look: https://permies.com/t/281596/Mass-bench-fed-modified-sidewinder
Worked like a charm, for this workshop all bricks of the bell were on edge to speed up the build. Best to have all bricks on flat, mass would double like that.

Thanks peter! We have been back at the drawing board trying to come up with a better design since putting the barrel at the end of the bench puts it too close to too many combustibles. It could be doable but a pain. So with all the potential problems of that original design we are trying to come up with a new design entirely. There are windows and doors everywhere… a tree trunk in the middle of the room… and the exhaust is already installed so lots of variables to work around. What if we did something like this quick, not to scale sketch? Since we are building mostly with cob I can do a bench wider at gas receiving end and narrower at the other end. I would maybe need a baffle or wall between barrel area and exhaust since this design would have barrel in front of exhaust?
(I’m not sure on exact measurements of width yet as that can be adjusted to hit the desired ISA. I would also need to get creative with supporting such a wide bench top span. I do have rebar, angle iron and bricks for towers!)
I do like the backrest bench design you did! We are still pondering that idea.

Thanks for the tip on including the barrel in the ISA… does that apply to jtube builds as well?

Cheers!

Heather Arvensis wrote:There are windows and doors everywhere… a tree trunk in the middle of the room… and the exhaust is already installed so lots of variables to work around. What if we did something like this quick, not to scale sketch?

Possible, this design has the better papers.

Heather Arvensis wrote:Since we are building mostly with cob I can do a bench wider at gas receiving end and narrower at the other end. I would maybe need a baffle or wall between barrel area and exhaust since this design would have barrel in front of exhaust?

Provided the following is met: the entire bench the same width throughout. There will be a warm gas stream along the ceiling of the bench and a colder one along the floor in the opposite direction. The less friction (again!) the better this will work. The division between colder and warmer gases isn't at the same height all the time, this varies quite a bit. Most people think like you: trying to "steer" the gases in the right direction. However, this could also be done another way, by making use of the temperature difference of these same gases. The warmer ones are against the seat, as long the forward velocity of the gases is slow, shortcutting to the low positioned exit won't occur. Feeding into the bench should be as high as the bench internals, and width as large as you can manage. I.e. not by a single piece of stove pipe.
Details how to construct this later.

Heather Arvensis wrote:(I’m not sure on exact measurements of width yet as that can be adjusted to hit the desired ISA. I would also need to get creative with supporting such a wide bench top span. I do have rebar, angle iron and bricks for towers!)
I do like the backrest bench design you did! We are still pondering that idea.

Seeing the space where the heater should be placed: the French bench, couch, divan or whatever opens up other possibilities. Placing it in front of one of the windows looks downright silly to me, because of the high backrest.
But what do you think of this: the combustion core in the middle of the room and the bench itself at right angles with the wall?
The door side of the firebox could be placed in any of three directions this way.
Plus the possibilty of a ceramic glass window on top of the riser.
The only French couch to date was a single-sided one, what about a shorter but double-sided one?
Maybe even placed at another angle instead of 90 degrees?

Heather Arvensis wrote:Thanks for the tip on including the barrel in the ISA… does that apply to jtube builds as well?

Yes, heat is absorbed and radiated out by the barrel, so it's part of the heat absorbing surface area.

ok, thanks!
I think I understand, mostly.

We have been toying with many new designs and want to run this one by you before breaking ground. (The following design would require the least amount of wall prep, wall protection or moving of electrical outlets.)
The dimensions in the photo are not set in stone, by any means…. I would also prefer to extend the length of the right side arm a bit if you think that would be ok for friction?
(I circled the exhaust with red.)

Cheers and many thanks!

Heather Arvensis wrote:The dimensions in the photo are not set in stone, by any means…. I would also prefer to extend the length of the right side arm a bit if you think that would be ok for friction?
(I circled the exhaust with red.)

The shape you are drawing now is doable, as long as it is within the guidelines for ISA. Lengthening the right side arm could result in a front corner that won't get warm at all, because the gases won't get past the corner. The exhaust into the bench should be still as wide and high as the bench internals, I can't picture how you would achieve that in this situation.