Hi,

sorry for this question, but have been trying to find info on how to calculate the internals for a certain size tank for days, but just cannot find anything.
The Excel sheet that is linked on this forum does not come close to what I have.
Can you please point me to a link on how to calculate it?

What I have is a propane tank (1100mm (43") tall - tank only, 500mm (20") inner diameter)

The only internals dimensions i could find that come close to what I have is for a 38"x12" tank.
http://www.alt-nrg.org/rocket.html

The stove I am build is for a green house with an aquaponics system (5500L/1453gallon).
The water is at 5C/41F at the moment and the fish are not feeding.
Will add a copper coil to heat water.

Thanks in advance.

There is not a "calculation" of J-tube dimensions for a given tank/barrel size; rather, you can figure how large a system (internal diameter) will fit inside a given barrel, or what minimum barrel dimensions you need for a given J-tube system.

If you are starting from the barrel, find the inside dimensions and subtract the air gap needed for downward flow, generally a minimum of 1 1/2 to 2 inches times 2, which gives the maximum outer diameter of your insulated heat riser. Compare that with the diameter of system you are considering and see how much is left for insulation around the riser (divide by 2 to get actual thickness).

20" ID - (2" x 2) = 16" max OD riser. A 6" system built with a cast cylindrical riser would leave 10" divided by 2 or up to 5" for riser structure plus insulation, which is plenty. If you are using firebrick for the riser, you need a 6" square inside, which will leave less thickness for external insulation. Firebrick splits (4 1/2" x 9" x 1 1/4" thick) are best for a riser, which would give an approximately 9" outside square or 8 1/2" if you can cut all the bricks to exact dimension of 7 1/4" long. A 9" square is about 13 1/2" across the corners, so you could only have 1 1/2" of insulation at the corners. It could be much thicker, up to 3", along the flats, though, compensating for the thinner corner insulation.

The actual J-tube interior dimensions and proportions can be made to fit your tank/barrel height, as long as you keep the general ratio of 1:2:4 for feed tube, burn tunnel and riser. More detailed advice would depend on your plans for the overall system, and how you want to heat. The ZeroFossilFuel plans on that site are for a stove, not a mass heater. A stove heats only when burning, while a mass heater stores heat for long slow delivery lasting for hours after the fire is out. Do you want to heat the fish tank directly through your coil, or a reservoir tank that will be pumped through the fish tank as needed to keep its temperature appropriate?

Someone recently (last year) built a 6" RMH which had circulating flue channels under the fish tank, and reported that it worked well. He did use a fan to draw combustion air at a constant rate, which makes a system dependent on constant electricity supply

Another point that needs to be emphasized is that using a copper coil to transfer heat from a RMH can be dangerous, as it will develop external temperatures several hundred degrees above boiling and may cause the water in the coil to flash to steam and explode. There have been safe designs developed for water heating, involving a largish tank open to the atmosphere which cannot explode, and coils in the tank to transfer heat to the storage or use system. As water under pressure has a higher boiling point than open water, the coil can never boil or explode even if the tank boils. Search for Matt Walker's water heating experiments, along with others.

Thanks, that was exactly what I was looking for.

There is only one question left - what inner dimension of the j-tube to use.
Same dimension all the way through, or smaller feed tube and burn chamber (ZeroFossilFuel).
Read about the smaller burn chamber several times, as it uses the venturi principle of speeding up flow and creating vacuum.
Using it to aerate my fish tanks and it works well.

This will only be a stove not a mass heater, as in the summer time it has to go, to make room for plants.
I was going to divert some of the water after the filter to run through the copper tube and back into the sump.
The pipe will be open at one end. do you still think this could still burst the pipe?

@Glenn
Maybe we should put your post into a PDF and make it a sticky at the top of the rocket stove thread?
Am sure I am not the only one scratching my head and trying to sort through all the info, where a lot contradicts itself.
I could just whack the stove together and see, but copying from someone's experience saves so much time and money LOL

The standard advice (from people who have done a lot of research and building) is that the system cross section should remain the same throughout, with the burn tunnel being maybe slightly smaller if there has to be any variance. A square section has about the same capacity as a circular section of the same width, as the corners cause turbulent drag and detract from efficient flow. You don't need a constricted burn tunnel for the venturi effect (which would be very slight without an exaggerated constriction), but you do want it to have the sharp angle with the riser to give turbulent mixing of the hot gases.

As you do not want masonry mass, the only way to store heat and not have wild swings over the day will be a water reservoir. Without mass around a flue, the only heat-gathering opportunity will be the barrel exterior. A copper coil would either not gather enough of the heat, or would gather so much that steam bursts are a likely hazard. A long enough length of tube would be susceptible to steam burst even if it is open at the end, as the water may not be able to exit fast enough for the steam expansion.

I suggest the best way to collect heat would be to make another tank surrounding your propane tank, say 8" or so larger in diameter, open topped and sealed to the propane tank at the bottom. Being non-pressurized and in a greenhouse, I think a galvanzed sheetmetal construction with seams bolted or riveted together and caulked with silicone would do fine. Size it to hold enough water for your heating needs, and submerge your circulating coil in it and run that to your fish tank. You could insulate the tank outside and add a loose cover if desired. This will collect a lot of the heat as the gases flow down around the barrel on the way to a manifold that leads to the chimney. Having a smaller outside diameter to the riser would allow the gases to move slower on the way down and give up more heat. A perlite/clay cast riser can be 2" thick, or 10" OD for a 6" ID riser. Packing the perlite/clay between two sections of stovepipe works well; the inner stovepipe will corrode and disintegrate, but the outer one will keep the riser sound. You can use two pieces of 5" snap-together stovepipe, or a 6" and a 4", to make the 10" diameter outer pipe. I have done exactly that and it worked fine. Look around this forum for descriptions of mixing perlite-clay.

You talk about wanting to move the system outside in summer to make room for plants, but I think that would be a lot of work and introduce risks of damaging the system. The footprint is small enough that I think if you have any container growing possibilities, you could set containers on top of the system (design it to be flat-topped) and just pretend it is a table.

Great, thanks.
Will get started on it tomorrow!!

Hey Glenn,

I'm a newbie on the forum. Thanks for answering the question re: the aquaponics inquiry. Your response however, triggers a couple questions in my mind since I'm trying to get my mind around the principles of rocket stoves. Does the area around the j-tube structure have to be in the neighborhood of 2", or what space does that need to be in relation to the j-tube structure? Ex., if I have a 6" OD j-tube, then 2" or 3" of vermiculite, mortar, etc. around the j-tube, could I then have more than 2" between it and the outside of the barrel or whatever I come up with to use for the unit. I'm thinking more like a conventional looking wood stove, maybe even plate welded up in the shape of a square, with the exhaust exiting at the bottom like the rocket stove typically has. The stove would be used as heating unit in our house. I just don't want the 'mass' look of barrel, masonry couch, etc., just a stove to heat about 1200 sq. ft.

Thanks in advance for your response, David

I've been away and offline for 2 1/2 weeks, so couldn't answer earlier. The 2" space around the heat riser and inside the barrel is a general minimum. The more space you have, the slower the gases will travel in the barrel, and the more time it will have to radiate heat from it. If you want direct or immediate heat, this is a good thing. You can make the barrel pretty much any size or shape you want, as long as it is able to radiate.