The idea of building a metal rocket stove was brewing in my brain since few months. Today i found some time and this is the result so far.
The pics show what i did today. Its all scrap metal, bought for 6 euros. The diameter is about 100 milimiters.
The process is still half way, but i decided to perform burning tests before closing the whole thing with two gas balons ( one can be seen on the pics).
The thing is, that at a certain point, the fire started to spread and rise up the feeding chamber.
As a humble rookie, i beg for your advices.
If needed, i can post better pics with more details and measures.
Thanks in advance, and my excuses for my poor english.
Greetings from portugal,
Hi Lewis, welcome to Permies, good to see another person tinkering.
What's happening with your stove is that as the burning wood heats up your infeed tube that tube starts a draft of it's own that is competing with the draft you want happening in your heat riser. Add in the secondary air inlet you have at the bottom and you've made a very convenient path for the air to flow through the fire and up the feed tube. You can either eliminate the air inlet and make it a true j-tube, or you can make an air-tight cap for the feed tube. It would also help to insulate between the heat riser and the feed tube to keep the feed tube cooler.
Also, be ready for some negative feedback on the metal design, there haven't been many built that have stood the test of time. You'll have a lot better long term results using ceramic based materials (fire-brick and the like).
"Instead of Pay It Forward I prefer Plant It Forward" ~Howard Story / "God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests and floods. But he cannot save them from fools." ~John Muir
I am aware that metal stoves dont last long. But for the moment i cant build a ceramic one. Besides, i need the stove to be mobile and rather small.
Guess that my next steps will be adding doors to both feed and intake tubes or increasing the distance between the feeding tube and the riser in order to minimize the heating of the feeding area. Hope it will help.
There are a couple of factors in why the fire climbs up the wood in the feed tube.
First, you have made the feed tube much too long; it should be no more than 1/4 to 1/3 of the height of the heat riser. This means the feed tube has a draw that competes with the heat riser.
Second, you have an open front cleanout and air supply opening. This when not closed will allow lots of air to enter at the bottom, and make no suction to pull air down the feed tube past the wood. Therefore the wood can burn up with no opposition.
The main issue, however, is that you have built the whole thing of steel, with no insulation. Do you have insulation planned? Without that, the combustion zone will never get up to complete combustion temperatures; with it, the steel in the heart of the burn tunnel and riser will be corroded by the heat and atmosphere, and fail before long.
(Written last night before Michael's response, but not sent until this morning...)
As long as you understand that longevity is not likely, that's okay. But insulation is essential, around all of the burn tunnel and heat riser. And you need to cut down the feed tube to less than 1/3 of the riser's length (1.4 is better), or you will always have a risk of the feed tube competing for draft and weakening the correct draft.
Lastly, when you put the gas bottle over the riser and force the exhaust to go down, you will lose a lot of draft, so you will need to connect a good chimney to the end.
The lenght of the feed tunnel being too long is easy to solve, i just need to cut it to the desirable lenght.
Insulation is part of the plan since the start, but i was just performing burning tests before proceding. I planned to insulate only the riser, now, i understand that the burning chamber needs insulation as well.
About the short life time of the metal, i have been thinking of how to solve this issue. Pouring refractory concrete around all the burning tunnel and about an inch up the riser could be a solution? When the steel eventually wears out, there would still be the concrete around it. Is it a nonsence or will it function?
Today i had just a couple of hours to work on the stove. All i did was adding the lenght of the burning chamber of 80 milimiters. The idea was to increase the distance between the feed tunnel and the riser and consequently reduce the temperature in the feed tunnel and avoid that wood burns in it.
Thanks for input Glenn.
Well, after a week of thinking and waiting for opinions, i had time to move forward with my project. Bought some refractory bricks and refractory cement and started to cover all the metal surface of the burner, feed tunel and riser tube. Still dont know if it makes any sense or not, guess that will have to try the thing to find out.
hey... nice project. it looks like you re making progress there i think with the fire bricks it will work a lot better. i am not sure if the insulation will be enough. and i am not sure if the system will be stable enought once the metal burns out. But you will see
what are you planning to use that stove for?
would this approach have worked better: use a clay and perlite mix. build a form around the lower end of feed-tube and burntunnel and fill it with the mix. fill the bottom of the barrel with the mix. then add a tube around the heat riser that has an inner diameter 3-4 inches greater than the outer diameter of the heat riser. fill the gap with the mix to insulate heat riser.
... i am just thinking and wondering
have you considered making an airtight door with a window for that (lower) air intake?
A perlite/clay mix would definitely work better as insulation, and can and should still be added around the firebrick cladding. The feed tube and burn tunnel do want the firebrick first, as that will be much more durable against the action of wood feeding and cleaning.
I saw a steel plate to close off the lower air supply, and that should be made more permanent (a cleanout there will be needed on a 4" system, as a hand cannot reach deep in to clean the way a 6" or 8" system allows.)
Thanks Tobias and Glenn
Doors to both feeding tunnel and air intake are planned since the first moment, but were not made yet.
About insulation around the burning chamber and heat riser, i thought about using mineral wool inside a large diameter tube. You guys write about perlite and clay. What kind of clay? Refractory? I live in a region where red clay is easy to find in nature, soils here are rich in it. Will it work if mixed with perlite?
The stove will be used (if it works) to warm a room of about 36 square meters. In this room i have a usual open fire place wich is absolutely useless. It burns tons of expensive firewood and gives barely no heat.
People who have used rockwool or mineral wool as insulation around the combustion zone have found that it gets too hot, and the wool starts to become brittle and crumble at the inner face. In the hottest parts it can even melt. Perlite and clay (just enough clay to hold the perlite particles together) will probably be more durable. Red clay should be adequate in this use, though it would not serve by itself as it likely has a lower softening point and would crack more than refractory clay.
That stove that you put together looks gigantic and very well build. You use it to warm your home?
I just didnt understood what is inside that huge rectangular tank. All the rest seems clear and not that complicated to build.
The materials you mention are easy to get here. Clay i could get as much as i want. Flat stones too, maybe not the ideal ones (hard limestone). The tools and the skills to use them, i own as well.
The question here is the space it would take... You mention a 500 liter tank, i am sure it wouldnt pass trough my front door. Besides, i live on a 4th floor, that fact also doesnt help. For these reasons i must keep my project as small as possible.
Will see how that "thing" that i built works (after i finish it of course). If the results are poor or if it will colapse within short time, then i shall consider something else, better planned and better buildt.
how are you planning to make the stovepipe when you have finished the oven you re building right now?
please consider your floor. when you build with clay and stones, it might become too heavy for your floor.
i don t think that your oven might collapse within short time. but you should put perlite and clay around your firebox and heat riser. you could make a metal sheeting with 4-5 cm space around it. like putting a bigger pipe around the heat riser and filling it (gently but firmly packed) with clay-perlite. so if/when the steel tube burns away, then your firebricks and the clay perlite would still stay in place.
A clay/perlite mix will be added all around the riser and burning chamber.
the next steps will be to cut the riser and feed tunnel to the desired lenght, preparing the second gas cylinder that will close the whole thing (i will be bolted together, not welded). Then i must decide about how low i shall cut the exaust opening from where a articulated pipe will lead the exaust up my exixting chimnee.
For the moment, i wouldnt use stones or too heavy materials.
some clay/stone might help by serving as a thermal mass to store energy. but that would depend on how stable your floor is and how/when you are needing the heat from the oven. like: would you need the room to stay warm for many hours after firing up the oven?
i do not know much about your climate and seasons.
Floor is armed concrete. Can hold tons. As pics show, its a small stove. This means not much mass to heat. Winter temperatures here go as low as 0 degrees celsius. The stove woulnt keep the room warm for many hours because of its small dimentions and low mass. But it will also warm faster once i fire it up.
thank you... what i thought of was to cob the oven in. to cob the base of it in solidly. but around "the bell" build cob walls with an airspace and opening (on top?) to have airflow. like you case that thing in cob but allow airflow for convective heat (quickly heating up the room) but with more mass to store some of the heat for a longer time
A cob then, is an idea to consider. For sure that some stored heat would be great. But for now,what really will keep me busy is finishing the stove and get it running. I am a bit concerned about i there will be enough draft to make it work properly without filling the room with smoke. By the way, is it possible to "force" the extraction by means of some sort of ventilator or blower?
The stove was fired up few times already and it always had good draft except once. One time, the flames instead of going to the riser, they came up the feeding tunnel. I was told that the feed tunnel was too long and that was the reason of the back fire. I have shorten it and problem seems to be solved. Anyway i still have to shorten the riser (shall leave 4 cm between the top of the dome), so, probably will be forced to shorten the feeding tunnel again.
What creates some confusion in my head is the fact that the exaust hole will be opened just a few cm's above the level of the burning chamber... this means that exaust gases must come down from the top of the dome to find their way out through the exaust opening... Probably its just my head complicating simple things)) Shall work some hours on it tommorow and post the results at evening.
i try to describe the effect (correct me where i m wrong).
in the heat riser the air gets really hot. so it expands and makes pressure. it the goes into the barrel, where it sheds the heat. the air contracts (looses volume), so it has an underpressure. but the air is still hot, so it will rise through the chimney. at the same time the hot air from heat riser will continue to push from the behind.
once you get your oven a bit heated up, this effect might lead to very good draft.
Today some progress was made on the stove.
First i must add that, once the durability of the stove is a bit short, i decided to keep the costs as low as possible using as far as possible only materials i had laying around my workshop. So, this morning i started by cutting off a section of both the feed tunnel and of the heat riser. The a metal sheet was added around the riser and (part of the) burning chamber. The gap between was filled with a mix of small gravel, sand and clay. Half of this mix was clay. Water was added too, in order to make it more compact.The dome (second gas cylinder is also ready. Tomorrow will be time to cut the exaust hole and add a section of pipe to connect the chimney tube. Here's some pics.
This morning some progress was made. The pics speak for them selves. The exaust opening was cut, a temporary chimney was installed and i could perform the first burning tests with the dome in place.
I was actually surprised but the draft increased when i closed the whole thing with the dome. Now still must think of a door for the air intake as well as some insulation on the outside.
Unfortunatelly, weekends are too short, and monday real life starts again((
thank you for the photos. nice to see that you re making progress.
your mixture is not that optimal. it will act more as a thermal mass than as an insulation. but if it works, it works. you re not planning to add 30 feet of vertical pipe, so your draft should be ok.
to insulate the burning-chamber/feed-tube you could use rockwool and a metal casing around it. or fill the casing loosely with perlite or some other mineralic grain stuff. i mean, if it works now without that, it wont get worse if you would use sub-optimal insulation.
but what i ve been thinking of: the paint of your gas-bottles. it might not be able to stand the temperatures and emit nasty fumes.
Tobias, like i mentioned before, i will try keep the costs as low as possible on this project. That is the reason of using gravel, sand and clay instead of perlite.
It seems a bit out of logic to start doing things correctly now that i know that i started wrong since the first momment. Lets just say that this project was like a teaching to me.
Anyway, the thing seems to be working. I left it burning this afternoon for about two hours with a small log of olivewood split in two. It burned the wood till the end almost. The exaust fumes are mostly water steam. That is a good sign of proper burning temperatures, i guess.
About that green pain, for sure it must be removed, i plan burn it away with a blow torch and repaint with high-temperature paint that lays in my shop since few years. In fact the paint on top of the dome started to make bubbles few minutes after the stove was burning.
Shall consider the mineral wool to insulate the feed and air tunnel, but why not cover it with the same mix of clay and gravel? at least it would add some thermal mass.
concerning insulation: if it works now, it should work in your flat. additional insulation might help a bit when other conditions are suboptimal or to get it to heat even quicker. but you have the firebricks, they should do the job. additional mass there might help. just test it out. clay is not that permament, you can change it with no big effort later.
Insulation i shall use around the feed tunnel and air intake. As well as doors. It should make some difference. But i see mineral wool as to fragile maybe. It gets pretty hot around the feed tunnel due to the proximity of the burning chamber.
You're right when you say that clay is not that permanent. In fact all in this stove is easy to dismantle and rebuild properly. Even the heat riser and burning chamber can be removed easy.
For the rest of the week i havent possibilty to make more progress. But my head will still be busy with it.
Thanks for all replies.
concerning the rock wool, there might be some information in other threads. i remember that people wrap it around the heat riser (from bricks) and fasten it with wires or mesh.
the problem is that when one uses this like around the feed tube and burn chamber, then it would be compressed. so it would loose the insulation value. it works best when it s loose and fluffy.
just thinking. you have clay which is a good thermal mass. when your oven is in place you could build small cob walls left and right next to your burn chamber and build them unto the barrel. leave 4-5 cm space. let the walls dry until they re stable. build them like 4-6 cm higher than the burn tunnel. fill the spaces with rock wool. put bars or sheets above it and cob it in.
it might take some time and some experiments to find out the right clay to sand ratio so it won t crack. but it could look really great.
I am following this build, keep the updates coming, good or bad. I am also using steel and concrete in my riser and burn chamber for my first rocket prototype, so I want to know how yours works out. I have burned mine for about a total of 8-10 hours, burning and testing about 2 hours at a time. I'm not having the best draw yet since I added concrete to my riser, but I'm working on an insulative liner for the riser to help
Hello Tobias, John,
Since the last time i wrote here, there werent major progresses in my stove. Real life comes first, work, family obligations, health issues kind of absorbed almost all my time.
Despite that, few more details were added. The feed tunnel and air intake received a double skin. Its mostly to make it look better, but the ceramic and the clay/sand behind it, handle the heat pretty well.
Every time i have the chance, i fire it up. Its tricky to fire it by usual methods (lots of smoke out), but using a small blow torch for a few seconds helps to create the needed draft to get it going. It burns smooth without big issues, just need to make sure the firewood doesnt get stuck in the feeding chamber. I kept it burning for long hours, 6 to 8 hours with hardwood(oak and olivewood). So far so good
Next month, due to professinal reasons, will be spent out of the country, so, no progress will take place before february.
A happy new year for all is my sincere wish.