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.