Smallest batch box feasible

It seems the smallest practical J-tube is a 5 inch design.

Is a batch box equally limited by scaleing to a small size.

This line of questioning is inspired by: A close friend of mine installed a wood burning stove in his van (he lives in a Van in Wyoming).
It works well except his coals die a bit before he wakes up, so he’s awful cold in the morning.
It would be interesting to try and replace it with
A a particularly small RMH.

Thanks

I think that depends on how you are going to define practical or feasible. What is the ultimate goal of the heater? 4" can be done. I've seen smaller, but the combustion efficiency in terms of clean burn were very low. I've looked into this a little as well for a trailer camper build I want to do. I don't recall the details of how I landed there, but 4" was the smallest I was going to go. 4.5" was going to be better if I could swing it. A double shoebox might be an option instead of the j tube.

I slapped together a 3" batch box core the other night and I gotta say it thoroughly kicked  the butt of the 4" j-tube core I had previously built.

Both for burn stability and output.

Unfortunately my expimental refractory is not quite up to snuff yet.

I'm seriously considering a 2.5" batch box for my next trial if the patches and repairs don't hold up on the 3 incher.

This is really interesting Alley!

It seems like the batch box is associated with building a really big stove, however you recent experiment seems to indicate the batch box has the most potential of known RMH designs for a micro RMH.

Could also be lucky with that try.
A successful 2.5" I guess would clinch it.

Guys, well, is anything under 4 inch batch, interesting? The firebox is about 6 wide, 8.5 high and 11 deep. That's two small log in there and that's it. About 5 kilos, 11 pounds of wood. May be.

During the Innovator's gathering in Montana Erica Wisner tried a 2" batch box core entirely built of insulating firebricks. The thing needed a longer riser in order to get it going. And it could only be run with nothing more than chop sticks size fuel. Conclusions: there should be a real fire in there, not a smoldering pair of small logs. The fire can't be scaled down that far and insulation is an issue in the sense it shouldn't be scaled down as well. Smaller burn chambers are disadvanteous due to the unfavourable volume / wall surface area ratio.

I've built a number of 4" batchbox rockets  myself and in my opinion it could be done. But you need to keep feeding it in order to achieve a clean burn longer than some minutes.

Peter, what then is the smallest practical batch box? 4.5, 5 or 6 inches? Or can I get a small propane flame at the top of the riser to incinerate any residual smoke?

I've now done 3 burns on a 2.5 inch batch box built pretty close to standard dimensions but with a 24" tall riser. I doubt a Testo would be very happy with it but it for the most part it works. I have little doubt it runs better than a J tube of the same system size or a bit bigger. As Satamax indicated the actual utility of such a small core is suspect but it was fun to play with. One would need to develop a pretty strict fuel stacking and lighting procedure to not have it be a source of frustration.

The peak temperature at top of riser was about around 550*C without over fueling it.

Hi Alley

Can you post a video of this thing in action?  Are you getting flame coming out of the riser?

The flame never climbed out of the riser unless it was overfuelled ie. adding fuel too soon to a bed of coals.

Making movie past midnight at the behest of some stranger on the internet - Check! :)

I'm rushing out the door, so hopefully I didn't do or say anything too embarrassing.
Only have windows movie maker right now so sorry I couldn't crop/zoom in on the riser flame.

Fortunately only 300ml of beer was spilled, refill was close at hand.

Enjoy!

https://youtu.be/YCdZa-fFigU

That's amazing!