Who's tried a small batchbox? How long is a burn?

[stoker]

How small can a batchbox be and work well?

I've spent a good while searching the forum (and also on Permies) but there's hardly anything about batchboxes smaller than six-inch system-size.

The batchbox dimensions-table in the reference-library goes down to 3" or 75mm, but has anyone tried one that small, and did it work? I get the impression that all the experimenting to get the right proportions was done on 6" (maybe some 8") so we don't know if the same proportions are good for small sizes, or if a somewhat different shape would work better.

Also we know that once a batchbox system is built, you can give it a full or half load of wood and the burn-time will be about the same (about 45-90 minutes depending on the rest of the system, and on how small the wood is split).

But how does that burn time vary with system-size? Does anyone know? I'd like to work out burn rates: weight of wood divided by burn time.

From scouring this forum for numbers, it seems that for a J rocket the weight of wood burned per hour (and therefore the power) scales with the square of system-size, so for example a 6" system has 36/16 times the power of a 4" system. No-one has published the numbers that I would need to work out how power scales with system-size for batch-rockets though.

[satamax]

I think burn time varies with system size, but not that much.

12cm, with a box for a 10cm

www.permies.com/t/35569/rocket-stoves/Range-retrofit

There's another thread here too, in which i might have gone a bit more into details.

[peterberg]

The smallest batch box system I tried is 100 mm (4") riser diameter and works remarkably well. All of those have been built last year: one in Warchau, one in The Hague and one in Montana, that last one is called the "Fat Rabbit". All of those were built from vermiculite board so the little core heated up very fast but probably wouldn't survive many winters.

The load of a 6" system is about 6 kg, it scales up and down by the volume of the firebox. Since the scaling method is linked to the diameter of the riser all of the 4" system's dimensions are 66% of a 6" version. It's possible to calculate the volumes but it's much easier to multiply all three dimensions 0.66 x 0.66 x 0.66 = 0.287 times the volume of the 6" version. Take the 6 kg fuel of the 6" core and multiply that by 0.287 and you've got about the load what a 4" core can take.

An 8" core's volume is 2.35 times the volume of a 6" core so the average load jumps up to 14 kg of fuel. Both the smallest core and the biggest core here runs roughly 40 to 60 minutes on one load, it has something to do with the physics involved. Erica Wisner tried a 2" batch rocket last year in Montana but it turned out to be very difficult to scale the fire down that small.

[stoker]

Thank you very much, both of you!

Satamax, I had seen your range retrofit but hadn't spotted specific dimensions or burn-times.
Peter, yes, obviously a 4" will have 8/27 the volume (and load) of a 6", and that same fraction of the power if burn-times are constant (and an 8" will have 64/27 the volume and power of a 6").

So, 4" works well, 2" doesn't, and 3" might be on the limit.

It just struck me that if power goes as the cube of system-size then, since CSA of each part goes as the square, gas speed must be proportional to system-size... which means the time for gas to flow up the riser is the same in all sizes.

If I'm right about J-rockets having power proportional to the square of system-size then there must be a size below which J-rockets are more powerful than batch-rockets. If a 6" batch burns about 6kg/hour, and a 4" J about 1.1kg/hour, then that point is at 2.475" system-size, but anything that small won't be much use. (Or if the 4" J burns 1.333333kg/hour then the crossover is at 3" exactly.)

And the vermiculite makes sense: a small core is going to need to be especially well-insulated, and for easy starting the core mustn't have too much heat capacity. Maybe something like vermiculite with a thin layer of something tougher to protect the areas the wood will bash and scrape.

[ronyon]

 About the Jtubes potentially  being more powerful than the batch boxes at a smaller scale, even 4 inch J ihas been said to  to be rather difficult to build and operate successfully, and smaller ones  are deemed marvels indeed so it would seem a moot point.

[patamos]

4" Js also require near constant tending. This is okay for cooking applications, but not for mass heating.

[patamos]

Peter, in a 4" batch box would you recommend a P-channel or bottom air channel with wider inlet than exit?
I have been away from the boards for some time, so have surely missed some of your information

[peterberg]

For simplisity, mount a p-channel. The configuration with floor channel isn't tested thorougly in a 4" system (yet).

[patamos]

Thank you

[colinsaengdara]

Stoker, I built a 3.125 inch system size batchbox rocket per the Peterberg dimensions in the x,y plane. But I extended the depth by 4.5 inches to allow for more reasonable fuel length. It was the core of a cookstove so needed to be able to light it and walk away rather than tend it like a j-tube. The reason I went down to a 3.125 inch system size was because I was working backward from heat riser height. I'm going from memory when I say I think it was a 35 inch riser.

It burned for approximately 45 minutes from full to embers, same or only slightly less than the larger sizes. It was made with castable refractory and performed brilliantly. It put out a tremendous amount of heat. Riser height was still much too low by my standards. Flames extended approximately 24 inches above the riser in full burn. Lengthening the burn chamber and lighting from the front probably volatilized more gasses, more quickly than is ideal. In this case, because it's used in a cook stove, the flames are heating a fire brick top. I was planning modifications to eliminate them hitting such a heat sink knowing this would cause creosote buildup.

My job took me out of the country on short notice, so I never got around to the disassembly and rebuild. Some details are at Matt's site, I don't think I posted here because everything was still experimental at that time.

www.permsteading.com/viewtopic.php?f=6&t=467

If you have any questions I'll do my best to answer from memory.

C

[invention1]

Jun 9, 2016 17:13:25 GMT -8 patamos said:

4" Js also require near constant tending. This is okay for cooking applications, but not for mass heating.

True, unless you are using a pellet feeder!  

[nixsee]

I'm just arriving here now, looking to probably make a 3" system for cooking as I'm limited by riser height. Has anyone made any progress with these small designs? What about running it in open-mode, with no door or channels? I'm trying to make it as simple and cheap as possible to help extremely poor people in the developing world. Thanks!

[roelof]

Hi nixsee, some years ago I spend a lot of time figuring out a version of the batchrocket that was as small and as cheap as possible. Like yours, my intentions were to make it available to poor people. It resulted in a 4" version, you can find it on my website roquetinho.eu.

One year ago I made an even smaller 3"-version, with just a piece of glass instead of a door (it's on youtube). For the second winter now it's keeping me warm in my 13 m2 caravan. But it does have some issues. I have to use an electric blower to heat up the pipe, in order to get an initial draft when I start up the stove. And although cleaner than any traditional woodstove, it doesn't get as clean as a batchrocket is supposed to be, due to the volume of the fire that doesn't get large enough to reach 1100 ºC.

So I guess a 4" system is the smallest possible batchrocket, if you want the clean properties. My next project is an even simpler to build 4,5" (115mm) stove, if you're interested I can send you the Sketchup-file.

Roelof

[nixsee]

Thanks very much roelof! You have done some beautiful work on those! Mine will be much more... artisanal. I expect it can be built for under $10, obviously sacrificing some performance. I don't care at all if it can reach 1200 - I've made a couple basic L tube stoves in a bucket that reach 700, and it's far more than enough to eliminate smoke and save people 3x the wood. I'll leave the rocket scientist stoves to you fine folks!

Moreover, this is for cooking rather than heating, so temperatures like that are likely undesirable! I suspect I'm going to have to alter it in order to make it less powerful - I'll try using larger pieces of wood first, however.

So, while your 3" version is fancier than I am looking for, it is encouraging to see that it can work well at that size! I will start there and maybe even try a 2.5" version later!

I'd love to see the simpler stove that you're working on - are you able to send me the file through a private message, or even just post a link publicly here?

Thanks again for the reply!

[roelof]

Hi nixsee, I overlooked that you wanted your stove for cooking, sorry! Mine is for heating (only). But you could just build the core and make a support on top of the riser for cooking. I tried that with a 3" L-tube once, and it worked well (see picture).

Materials for my upcoming 115 mm stove will cost around €200,- (in the Netherlands, where I live). You can download the .skp-file here.

Roelof