Advice on clay-based J-tube & Bell system

Hi all.

Thank you for adding me to the forum!

I am currently 3 months into a year living as much as I can off the land, foraging for all my food, plus as much of my living needs as possible (hut, clothes etc). See Nicola's Year on the Land Podcast webpage

I plan to build a stove and chimney into my hut (which has wattle and daub walls & a grass roof) asap, as the British winter is pretty wet and cold enough!

I am limited in materials to what I can reasonably collect around me- so clay, sawdust, grass. Only beach sand which I gather isn’t useful for building.

I’ve put together a stove design which I hope use for cooking and heating. I’ve tried my best to understand the principles involved but am definitely unaware of some of them! Hence, I’d love comments/suggestions from ye wise ones. The stove and chimney represent a massive amount of work (collecting/processing materials, building) and I am relying on them for getting through the winter- so I need a design highly likely to work. I also need to get cracking as I’ll need a couple of months for the whole thing to dry out.

The 1st two pics are schematic diagrams of the front & plan views. Apologies for the quality, I was in my hut without a ruler. The 3rd pic depicts the proposed materials & the 4th an alternative shape to the bell.

J-tube

I was planning a 5”diam round j-tube made with clay and sawdust. I can prefire this in my updraft kiln. The dimension of the tubes are 1:2:4, based on the centre lines.

Above the riser, there’s a hole cut for a pot (lorena style) plus a shaping so I can place a thick (2”) piece of granite over the hole when not cooking. (see bottom pf pic 1 for detail on this).  

Bell

The heat flow is then channelled down an insulated (clay/sawdust) 5” channel down onto a bell, to extract heat from the gases. I was thinking of making the bell from 40% earth, 40% clay & 20% grass. The grass for strength, the earth as thermal mass (would it be?!).

I’ve estimated the bell to need an ISA of 34 square feet. This is based on Peter’s calculations for 6/7/8” systems. I had to estimate as I couldn’t understand how to calculate the ISA for 5”s. Happy to be shown how!

Pic 4 gives an ideal shape for the bell, based on the hut space (small!) & the materials- this would be low, narrow & long. My understanding though is that tall and narrow is better for a bell?

I also wondered if I put a door into the bell, I could use it for cleaning plus access to bake in the bell? I’m not sure that the materials I have would allow regular access without crumbling.

Chimney

I am currently making some 5” clay pipe sections for the upper parts of the chimney, which will be fired. These will make the parts of the chimney which could get wet (through the thatch, outside).

I was thinking of making the lower parts of the chimney from a clay/grass mix. But would it be better for this section to be insulated with sawdust too?

Of course, I could make more pipes with the clay/sawdust mix, so the whole chimney is insulated, if this would be better.

Configuration

At the bottom of pic 2, I suggest a different layout for the components. I don’t think this makes any difference to the suitability of the design...

Materials

Am I using what I can get to it’s best use?  

Perhaps the walls will need to be thicker? I’m balancing the work to gather with the system being strong enough to last 9 months.

1. Front view  

https://imgur.com/KUux5xL

2. Plan view

https://imgur.com/IPRBvBh

3. Materials

https://imgur.com/5MrSLBH

4. Bell shape

https://imgur.com/fkSDb3W

Hi  Nicola and welcome to Permies!
Sounds like a fun and labor intensive year you signed up for.

What a challenge to build something rockety out of materials from your land (no metal I understand?).

You could embed the photos by using the "img"-button above the text. "". That would make it easier for people to see your design.

Let me try to answer and comment on some of your questions and your design:

the ratio of feed/burn tunnel/riser can be 1/1,5/3 enabling you to make a deeper feed, which means longer firewood.

Are you sure granite can withstand the heat just above the riser? It could reach over 500°C if I remember correctly. Some stones trap mositure and can even explode I think.

without the typical drum expect much less draft. I'm actually asking myself why the hot gases would even start to go down into the bell?! Definetely add an opening at the base of the chimney for cold starts/priming.

I'm sure you'll get plenty of ideas and help from the experts here.

Thank you Benjamin for your welcome and reply. I've tried to embed the pics- hopefully it'll work...

Yep, no metals, solely clay, earth, grass and sawdust.

I don't think granite will explode as it's a metamorphic rock. It might crack with the extreme heat though.

I thought the gases would go into the bell driven by the draft created by the riser....

I'll think about how to make an access point at the base of the chimney for priming.

Lots to think about and learn!

Hi Nicola - welcome to permies and thanks for sharing your fascinating project - I'll check out your podcast later! I've taken the liberty of embedding your materials scheme here to make it easier for people to reference.



I'm not a 'rocket scientist' but agree with Benjamin that without the steel barrel the gases will have less reason to go down into the bell and the exhaust could stall. My thought is that the exit doesn't have to  be at the bottom - just have the hole through slightly below the cooking surface, the hot air will rise to the top of the bell and stratify exiting at the bottom where it will be coolest. My understanding is that the main reason for the barrel is to give quick heat into the room.
Using the bell as an oven is an interesting idea. I don't see why it shouldn't work, although it will take some experimenting with.  (Ooh - think tandoori oven!)

Another thought, which if you have access to a kiln, you may wish to experiment with - cooking on a clay surface. Somewhere we have a thread where a massive clay bowl is used as the cooking surface over a fire. Maybe in the caribbean?

I think the main issue is balancing the temperature losses so that the fire burns cleanly and the chimney still draws in the UK winter temperatures. I'm sure you'll get some more advice as people get online - good luck!!

I agree with Nancy - my first thought on seeing the drawing was that there is no reason to force the gases to go to the bottom of the bell from the J-tube. My J-tube riser is entirely inside my bell and works great. Of course for cooking on, you would want a flat surface/hole just above the riser. A bell could easily be exactly that tall, so that the whole top is flat.

For sizing, the numbers Peter gives at batchrocket.eu are for batch boxes. A J-tube of the same system size will be distinctly less powerful, so can support a smaller bell ISA. I have found that my 8" J-tube heats a bell sized for a 6" batch box with little heat left over, so calculating for a batch box 3/4 the size of your J-tube may be in the right ballpark.

Several years ago I built a rocket stove entirely from cob except for a steel plate cooktop. Nothing was pre-fired; the core fired itself enough to be stable, and if you have halfway decent clay for your cob, yours should too. Beach sand may not be ideal for cob, but should work well enough for non-critical structural applications. In building medieval-style wood-fired pottery kilns from cob for many years, I have found that dried grass clippings work excellently for this. Nothing organic will remain after firing, of course, but the grass gives good wet strength yet leaves a smooth enough surface texture where you want that. Using grass-rich cob for the outer layer of the core will give some insulation, enough for the core to work properly.

You mention a 5" J-tube; You may have heard that 6" is the minimum recommended size for these as smaller ones start to run into surface-to-volume issues and are more finicky if they work at all. 5" may still work, but I would recommend going to 6", as it will give you more reliability. 6" will also give considerably more heat at the riser top, which will make cooking easier. You can always move the pot farther from the heat, but it's hard to get more heat if you find you need it.

If you have any other questions about cob rockets or kilns I will be happy to elaborate.

Thank you Nancy for embedding the picture- I'm not sure what I'm getting wrong on that.

And thanks Glenn for you feedback.

So perhaps I can either have a hole going straight from the cook top into the bell, or the J-tube going straight into the bell then cooking on the top of the bell.

What mix did you use for your cob rocket?

I was thinking a 5" system as I had read this was a good size. Probably in some of the Aprovecho liturature

My local clay deposits are direct glacial till, with around equal parts clay, silt, sand and gravel/rock... in other words, perfect cob material as is with the addition of straw or grass. What is the character of the clay you have available? I would probably not use any non-clay "earth" as you indicated in your mix. You can probably have as much as 2/3 sand, maybe more, in your cob. You need to test bricks of different compositions to see what works best.

Your bell shape image shows just an oval, which is accurate for a fairly thin-walled construction. A rectangular top profile would not work without reinforcement. I think you need to make something like a beehive shape, with a smooth arch internally. The external top can be flattened off for practicality.  Thick edges would have considerably slower heat transfer; my bell is 2 1/2" of brick plus 6" or so of cob, which takes a couple of hours to start feeling warm after a cold start. The corners, even though rounded, never get much warmer than the room unless I am burning for several hours at a time. A mostly roundish bell will have few thick corners to impede heat transfer. You might want to build your bell with a cob pillar in the middle to support the center of the roof. This will add to the heat storage capacity, and even increase the bell ISA a bit for the same external size. You just want to make sure there is plenty of free space for airflow.

A cooking rocket stove, which is what Aprovecho focuses on, may work fine at 5". A rocket mass heater introduces complications of flow path restrictions and greater heat extraction, which change the equations from those simple cooking purposes.

That's really helpful thank you. Shame I've already made some 5" chimney pipes! But better to know at this stage.

My 8" J-tube system connects to a "temporary" (for eight years now) 6" steel chimney that goes out the wall and up to above the roof. I have constant draft even stone cold, and it draws strongly when burning with no hiccups. You may be able to use your 5" chimney sections at the top end of your flow path, as the gases will have cooled significantly by then and will take lower volume than when they were hot, so may not slow significantly in the smaller end.

So I went back to the drawing board incorporating all the advice offered.

I've been trying to design a simple system with a 6" J tube going straight into a bell, with a cooktop on top of the bell. I am having problems however with the resultant height.

From the bottom up I would have:

6" of insulative materials inc. burn tunnel > 36" riser > 8" gap > 3" thickness of bell top

=53" high

Which is too high for a cook top!

What could I do differently to make a more usable system?

I cannot dig into the floor of the hut as we are in a very wet field with multiple shifting springs which look for holes to come through!

If I changed to an L tube, would I be able to have a shorter riser?

Is there something completely different I could do that I haven't thought of?!

Hi Nicola,
if you can't put the core in the floor, why not raise the floor around the cook top?
Like a step that goes around the whole cook top that you can step on and do your cooking.

Thank you for the suggestion. I'm living in a small mud hut without space to build up in that way.

Maybe you can crunch the numbers a bit. A shorter feed allows a shorter riser. So making the feed 10” (25cm) instead of 12 let’s you drop the riser to 30”.
What’s the 8” space above the riser? Maybe you can make that a bit less too.
Also the insulation under the core you might reduce a bit.
Maybe saving 10” in total. 43” is still to high?

43" is definitely more user friendly.

I originally planned to go 9":18":36"  for the feed:burn tunnel:riser

f I shortened the riser to 30", would I be looking at 7.5:15:30"? Would this still work well with a 6" system?

Nicola,
I would go with 1:1,5:3 dimensions. Thomas caught a too long burn tunnel in my design (https://permies.com/t/263398/RMH-build#2616537).

So 10:15:30" would be a possible set of dimensions.
I can't see why it would not work.
The higher the riser, the more draft. But since yours goes into a bell and then out the chimney I don't suppose you need a whole lot of draft.

Thanks Benjamin. I think I will go with your suggestion.

Your build looks great. How's it going?

Unfortunately stopped at the moment. Postponed to probably January.
I’m trying to find someone in the area for a project this winter, ideally a public space which would serve as a good publicity.
If none of it works out I’ll spend my time building one or two cottage rockets.