minimum rocket stove size?

[ringoism]

Eric here, formerly initiating the all-metal rocket feasibility discussion...

Finally had my design worked out and expecting it would work well, but regrettably the only local fabricator willing to take on the project is quoting really too high a price... granted metal prices are up (I reside in India) and he was planning to make it from scratch; but not being of local origin, the so-called "skin-tax" (sliding scale based on complexion) might be factoring in there somewhere, too.

Anyway, this all has me back to scouring the scrapyards for salvageable materials suitable for construction - since I will probably try and borrow an oxy-fuel setup and do the fabrication myself - but these materials may not always be of theoretically ideal dimensions. Moreover, I'm re-thinking the question of (system) size (I'm an E.F. Schumacher fan, incidentally).

As I mentioned earlier, I'm trying to heat a very small space (no more than 100-200sqft), in a not terribly cold region (lower Himalayas @ 7,500ft), with which honestly something like 2-4 kW's output would probably suffice. Locals over-size their box stoves and have to open the doors / windows all the time to cool off, or else burn very lazy fires unsuitable to the stove size - they also tell me it's good to use wet/green wood so it doesn't burn too fast / get too hot!!! To me, there's not much point in wasting fuel like that.

To the point: how small (i.e., system size) can a rocket stove be and still work? Flue pipes here are standard at 3-1/2" but can be made any size. Is there a theoretical minimum for the basic heat riser/ burn tunnel CSA (cross-sectional area or diameter)? I've got some heavy 3" steel pipe (2-3/4" ID) I thought to use for the former... 3" square channel also available. I've got 4" & 5" available, too, but I think I'm grossly oversizing (re: heat output) at that point. Any thoughts / experience out there? I'm thinking of the turbulence along flow surfaces, wondering whether that creates a theoretical minimum.

I've seen the so-called "pocket rockets" on U-Tube, etc - but mainly the sort without a heat-riser, that sort of sucks the air / loaded wood in through a tapered sleeve in top to a larger combustion chamber below, and then gases go straight up out the flue. Their fires seem to get hot enough (they use them here locally with a water-jacket for heating H2O), but would seem to lack efficiency on account of how much heat is escaping directly out without being re-directed through any radiating surfaces (i.e., pulled back down within an outer drum like "conventional" rockets).

Any advice greatly appreciated.

Thanks,
Eric

[Donkey]

I don't tend to build 'em small..
Though both Peterberg and Rectifier (in their respective threads in experiment area) seem to be having quite good results with theirs.
Both are around the 4" mark. Both are VERY cool.

[peterberg]

You could try the 3" square channel as system size throughout. That size has a csa of 9 square inch, which is about 43% less than my own experiments. The riser has to be overly tall, I'm afraid. The one I am using is 3' long from the bottom of the tunnel. For the 3" square I'd estimate the length would be adequate at 2.5'.
Don't be mistaken about a small rocket, though. The fire is tiny, heat production is nowhere near the same as a healthy running box stove.

Heavy steel pipe isn't the way forward, too much mass in the steel.

[beckyladakh]

I've been really enjoying this new (to me) rocket stove idea and reading this forum just tonight. But Eric's post prompted me to join. I'm also in the Himalayas, but quite a bit higher, and one of the things I've been thinking about is that for combustion to happen efficiently here we might need larger pipes for the air intake and combustion and everything.

Also, being in India and especially in a remote place, an additional issue will be that none of the commercial or sort of pure products like vermiculite, perlite, pure silica sand, etc., would be available here, so if we make one of these stoves here we'll want to find some other heat-resistant but insulating material. I guess the wood ash thing sounds worth a try.

For a bread oven, we're currently using a metal oven chamber embedded in the top of an open fireplace made of adobe and cob, but we've got a lot of cracking and smoke leakage.

[Donkey]

Wood ash by itself compresses over time and becomes progressively LESS insulative. 'Course, when it's fresh wood ash works REALLY well. I used it for a time in my stove at home, worked great, then after a while I started wondering what was wrong with the stove...

I've from time to time fooled with clay/sawdust, clay/ash and other natural material combos. I can't vouch for their durability but so far, the alternatives I've tried have worked in their short term, experimental setting(s).
Have you checked out some of the threads in the Materials section here? IMHO, some kind of casting made of natural clay soil, wood ash and biomass fiber/filler could be a winner.

[rectifier]

My little stove consumes around 30kBTU in fuel, which translates to ~9kW. Of course I'm not sure how much of the heat I get out, but it's a lot. It is capable of heating a shop that is roughly 5000 square feet with 30 foot ceilings, admittedly it doesn't heat it that hot but it is warm enough to keep the tools from sticking to our hands. I'm sure in a 100 square foot area with any insulation at all, it would be sauna conditions.

It has a 4" square firebox and a 5" round riser (which is only about a foot tall in the insulated portion). Exhaust is 3", so I don't follow any 'system size' as is typical. As the fuel I use is by nature super clean burning, I don't need a taller riser to get complete combustion - however, it would benefit from a taller riser to provide a more stable draft. I found that below a certain power level the flame can drop out of the short riser and the riser draft will 'shut off', causing the power to decrease until the stove goes out.

[ringoism]

Nov 23, 2011 11:28:55 GMT -8 beckyladakh said:

I've been really enjoying this new (to me) rocket stove idea and reading this forum just tonight. But Eric's post prompted me to join. I'm also in the Himalayas, but quite a bit higher, and one of the things I've been thinking about is that for combustion to happen efficiently here we might need larger pipes for the air intake and combustion and everything.

Becky: If you've been around Ladakh awhile, it's pretty likely we have mutual acquaintances... possible we even would've met at some point (name sounds familiar) - been out there several times since relocating here around seven years ago. I'm "just over the pass" (several hundred km to the south in H.P., that is) around the place famous for...eh...dope (and apples)... not that I'm involved in either. With fuel-wood being relatively scarce (excepting that imported from outside) out where you are, I tend to think that some of the passive solar solutions already being done might have the most potential for basic heating - of course, some of that depends upon house design / orientation / location, which in populated areas and on rental status, especially, is not always going to be ideal. Sounds like you're thinking of cooking with it, though.

Mmmm... maybe bigger system / pipes, yeah... I know my bike runs rich (low on oxygen) out there whenever I've gone. So I do have to re-jet it (reduce the petrol in the mixture) to compensate. So more air volume per unit of fuel, yes. What size are the flue pipes on the standard Ladakhi / Lahauli box stoves? Been to Spiti / Kinnaur, too, at similar altitudes, but can't remember exactly what they were using - I seem to remember on a couple occasions seeing larger (5"?) diameter pipes that were actually sections of municipal power poles (midnight stove parts supply?). Of course, the stoves are bigger and the place a lot colder, too, so if the pipes are bigger, there could be other reasons.

There's a lot of NGO's out there - wonder if one of them could be convinced to provide a little grant for some improved stove research.

Re: insulation, I'm kind of wondering whether if I used a double-walled heat-riser pipe, I might be able to get away with glass-fiber insulation ("glass wool" here) applied outside of that. I've been advised in this forum that it would melt if used in the typical way, but the melting point seems to be in the region of 1800F or so and at any rate it doesn't burn so isn't dangerous, and maybe that air gap would be enough to ensure I never actually saw those temperatures. You can find it in India, both new (Chandigarh, etc) and salvaged from around the tanks in discarded hot-water-heaters ("geysers"). Other idea I had is that if the wood-ash simply settles over time, then having the whole insulated heat-riser sort of detachable from the system (i.e., the burn tunnel below) by means of a sleeve-fit (it would have to be round), would allow it to be taken out and inverted / pounded on / shaken up a bit from time to time to restore the basic insulative properties. I've got a couple air-tanks salvaged from old trucks that I'm thinking of using - insert my heat-riser pipe through end-to-end and weld both ends, fill wood ash in through a funnel into a threaded port in the side which would then be closed with a pipe plug. I have a feeling it could work.

Regards,
-Eric

[ringoism]

Nov 23, 2011 21:45:08 GMT -8 rectifier said:

My little stove consumes around 30kBTU in fuel, which translates to ~9kW. Of course I'm not sure how much of the heat I get out, but it's a lot. It is capable of heating a shop that is roughly 5000 square feet with 30 foot ceilings, admittedly it doesn't heat it that hot but it is warm enough to keep the tools from sticking to our hands. I'm sure in a 100 square foot area with any insulation at all, it would be sauna conditions.

It has a 4" square firebox and a 5" round riser (which is only about a foot tall in the insulated portion). Exhaust is 3", so I don't follow any 'system size' as is typical. As the fuel I use is by nature super clean burning, I don't need a taller riser to get complete combustion - however, it would benefit from a taller riser to provide a more stable draft. I found that below a certain power level the flame can drop out of the short riser and the riser draft will 'shut off', causing the power to decrease until the stove goes out.

Do you not also have a flue pipe that exits higher, out the roof or whatever? I would've thought that with something like that at the end, sufficient draft would be maintained throughout the system... BUT then...

I did a little experiment the other day, sort of (not exactly) a little rocket-conversion of the local box stoves, and had the same problem, maybe - the stove ran great when I started it (had paper, wood, a little kerosene down a tapered feed tube) for maybe ten minutes - nice healthy roar and not much smoke up top - but when there was only 4-5 small-diameter pieces of wood left the fire became smaller, and after awhile the flame finally went out and wouldn't restart. This happened a couple times. Is this simply a function of the flame being too small to maintain enough heat / updraft in the flue? Didn't get smoke-back out the intake, though. My 3-1/2" flue is only about 8' tall. On the positive side, the heat output seemed about right while it was burning, and it didn't seem to require much wood at all to produce it.

Pertinent question: Do most / all rocket designs provide for fresh air intake BELOW or ACROSS the wood / fuel (assuming it's loaded vertically), rather than it simply being drawn in from the top of the feed tube? As it was sort of a borrowed stove, I didn't want to make holes in it to provide side / updraft, so was depending on whatever air came in past the sticks I'd put in there vertically. I have a feeling there's a problem with that. On the other hand, it really did seem to run well when there was lighter fuel in there. So I don't know...

Do you have any photos / drawings of your design posted anywhere? Didn't readily find it in the experiments section. Wondering how you attached a 5" round riser to a 4" square channel, etc... and overall dimensions. As I mentioned in my original thread, I'd like to keep this stove low overall height (on account of social factors), and a 1ft. high insulated section sounds interesting (how high is the riser overall?). Pretty amazing that you can "heat" (to whatever degree) such a huge place with such a small unit.

-Eric

[rectifier]

There are pics in the 'riley pellet burner' thread but there is not much detail on the rocket part of the stove, it has focused mainly on the burner, the rocket part being just part of the test setup for the burner. There are no good drawings because the whole thing is being tuned every day. When I'm happy I'll make a good drawing of it.

The 4" square/5" round section: the firebox is horizontal, then it is bolted on with a flange to a vertical piece of 4" that acts as an ash pit and the base of the heat riser. That piece is welded to the base of the 'barrel' with a 4" square hole zip cut to allow the fire in. Inside the barrel, the 5" round stovepipe was placed on the base over the hole and tack welded/stove cemented down, and 7" round stovepipe and perlite insulation around it. The only real reason for this was that the 4" square is too heavy to serve as a good heat riser, and 5" round is closer in area than 4" round.

The riser is about 16" overall, if you count a bit of the square tubing outside the 'barrel'.

Part of the reason I can heat such a big area with a little stove is that the stove being cast iron can be run really, really hot without damage. I run the top of the riser very close to the top of the stove and the side gaps are small, causing a lot of heat extraction in a small space. In the morning when the heat is cranked it's not uncommon to see temperatures above 900F on the top plate... a lot of radiant heat pours out of it and you can feel it 20' away if you hold up your hand. Also the side fins help it dissipate convective heat rapidly. It really amazes me that it can be 900F on the top and only 300F on the exhaust pipe which is 2' of gas flow away!

The flue exits on the roof but being only 3" and 200F after the aluminum flex section (more heat extraction), doesn't pull that hard. Most of the draft is generated in the riser when the flame is stretched out inside it. A pellet gasifier like mine is very dependent on draft to set power level. As such, it will either run away (flame is hot enough to draw more draft, increasing power, increasing draft etc...) or taper off (flame is not enough to keep the riser hot, less draft, less power, less draft etc...) of course, tapering off is much safer and more useful, so I set the throttle that way.

With standard wood (not pellets), you have to control the mixture by feeding more wood. The draft pulls in air, if there is not enough wood to burn and keep it hot, the excess air will cool the riser and it will weaken. I would suggest packing more wood in. How small is 'small diameter' and how big is your firebox?

Most side-feed rockets provide air below the wood. Most J-tube RMH draw it down over the wood. Some J-tubes have external air intakes allowing the feed tube to be closed off, but this is more of a consideration for smoke in the house in case of a backdraft.

[beckyladakh]

Thank you all for your suggestions.

Ringoism, yes, we're already doing all our space heating with solar -- It's well below freezing outside at night these days but our houses are still warm. After many years of experience, we really would only possibly want backup heat for January, though so far we've gone without backup heat just to make sure that we use our solar heating properly. Our houses in Jan are similar to normal Ladakhi houses in October, when people start thinking about starting up their heating stoves (bokharis). Our solar heating is all just earthen buildings for thermal mass (heat storage) and south facing windows and seasonally attached greenhouses. Very effective and cosy.

For our place, a school, I was thinking of the rocket stove only to replace the open fireplace that heats our bread oven, to reduce the fuel used in that. But then, we ourselves or family members may be building private homes in the next year or two, and I'm thinking about these rocket stoves as backup for Jan, when even solar heated houses can get chilly. Also we've got lots of thin wood from annual pruning, and some of my relatives use it for cooking in winter, on a normal local wood and dung burning cook-stove.

When I told my engineer/tinkerer/inventor here about rocket stoves, the insulation problem and the wood ash issue, he said maybe we could use wood ash, and make it possible to take the rocket apart and replace the insulation once a year or so. But I wonder whether that would cause a lot of sooty dust through the room. Recently he was experimenting with insulating earth plasters -- he mixed baking soda with some nice strong clay we found locally, and it dried to an attractive bubbly clay substance, but maybe the bubbles would expand and crack in the heat.

The common bokhari pipes here are not from utility poles, they are thin metal, I think usually 4 to 5 inches dia. If we do a rocket stove and go with say, 6 inches, should we have the same 6" dia for all pipes? I mean the one from the feed chamber to the rocket, the internal cylinder of the rocket, the horizontal pipe under the cob bench, and the final vertical riser to the roof?

[Donkey]

All channels, metal, firebrick or otherwise, square, round octagonal, etc., need to have the same cross sectional area.
Pick a "stove size" (usually determined by the chimney pipe you got) and stick with it throughout is the rule.

[ringoism]

Nov 26, 2011 22:50:16 GMT -8 beckyladakh said:

Thank you all for your suggestions.

Our solar heating is all just earthen buildings for thermal mass (heat storage) and south facing windows and seasonally attached greenhouses. Very effective and cosy.

For our place, a school, I was thinking of the rocket stove only to replace the open fireplace that heats our bread oven, to reduce the fuel used in that.

my engineer/tinkerer/inventor here...was experimenting with insulating earth plasters -- he mixed baking soda with some nice strong clay we found locally, and it dried to an attractive bubbly clay substance, but maybe the bubbles would expand and crack in the heat.

When I first went to Ladakh, I thought, "These must be much smarter people than in our valley." Huh? South-facing windows? Naw, we put our stairways on the south here, to block all that nasty winter sun I guess... In all fairness, in the old days things made a little more sense. Lots of thermal mass / cosy mud plasters / dead air space in the walls (quite earthquake-proof designs, too, mind you), the livestock downstairs emitting large amounts of body heat upwards, and at any rate, plenty of wood to burn in their little tin box stoves. But now the concrete invasion is well under way and forests nothing like what they were. It is certainly true that the amount of massive wood used in the old structures is unfeasible today. Anyway, I digress...

I've eaten bread baked in solar cookers (in Lahaul), and it was as good as any other.

Let me know if something works out well with your insulation ideas. As for me, a few days back I met a young metalsmith who seems relatively interested / motivated / skilled, so I might have him do most of the work on my design after all. Will post here if/when it is completed and running.

[ringoism]

Nov 26, 2011 17:03:04 GMT -8 rectifier said:

With standard wood (not pellets), you have to control the mixture by feeding more wood. The draft pulls in air, if there is not enough wood to burn and keep it hot, the excess air will cool the riser and it will weaken. I would suggest packing more wood in. How small is 'small diameter' and how big is your firebox?

Most side-feed rockets provide air below the wood. Most J-tube RMH draw it down over the wood. Some J-tubes have external air intakes allowing the feed tube to be closed off, but this is more of a consideration for smoke in the house in case of a backdraft.

Thanks for the explanations. And I'll look forward to seeing your design, too. My experimental, temporary-type firebox was I guess a square section about 6x6" and a foot long, with a tapered (wood/air) inlet in the top that goes from around 4" down to maybe 2-1/2" diameter. Really roared at first. With the inlet being small, by the time I dump more wood in there (generally 1" diameter or smaller), I think I limit my air supply. My idea was to be able to load longer, smaller sticks vertically and have them sort of auto-feed (being as lazy as I am, the reduced work in cutting it all up into small pieces also suits me better).

I was actually using one of the local box stoves for this, which are pretty low / flat - and put a divider down the middle to kind of create a u-shaped path for the flame to travel in before heading up the stack. This probably makes no sense without a drawing, sorry.

[rectifier]

I think I can understand, it's much like one of the early stoves I ran the pellet rocket burner on. This sounds like it does not have an insulated riser, correct? This will result in draft weakening rapidly as the updraft and downdraft sections equilibrate in temperature. You cannot have a downdraft section without a hotter, insulated updraft section to drive it. Read the 'drive behind the drive' thread (experiments and results) for discussion about this.

2 1/2" diameter is too small for a rocket stove throat. You don't want the fire to burn downwards in the feed (requires a massive draft), you want it to burn sideways pulled off the sticks. It sounds like your tapered design will not allow the fuel to fall to the base of the firebox, which will result in the fire burning downwards in the feed and the flame having to cram through the 2 1/2" hole. The velocity will get very high and it will make a loud roar, but it's actually being restricted. You want the fuel tips in the high velocity zone, not just the flame.

[ringoism]

Thanks Rectifier. I actually had thought the insulation was "simply" to make the fire hotter for more complete combustion - I hadn't realized the effect on draft, though I'd always planned to insulate in the final design anyway.

Yeah, I know my sizes were all out of whack - in a way, I just wanted to know the limits of how flexible the whole sizing thing was, whether it would more or less burn even if everything was "wrong" excepting the fundamental concept - and these were the materials on hand that I could sort of set up in twenty minutes or so with no equipment.

Actually the flame was not burning up in the feed tube at all, the downdraft was enough to keep the fire moving sideways, so long as it was a big enough flame. I re-ran it again a few days ago (upon our first snowfall) with slight modification to the shape / orientation of my burn tunnel, and a slightly larger feed tube, and it did well for several hours this time. I ran more fuel in it than before (re: your earlier post), i.e., around 4-5 long, straight sticks of around 3/4" to 1" diameter at a time. I must say, I really did like the way the "gravity auto-feed" worked out - just had to jiggle the sticks once in awhile to knock off the burned-out tips and provide a little more fresh fuel - and sometimes they just did everything entirely on their own. It seemed a lot easier and cleaner than opening a door/lid and re-loading a firebox on a conventional stove, as far as I'm concerned. I was actually set up in such a way that the feed-tube could be adjusted up/down in relation to the burn tunnel, i.e., in terms of how much of the fuel stick tips would be exposed / available to burn in the bottom of the combustion area. Tried anywhere from around 1-1/2" to maybe 3-1/2" and though higher should have made the flame bigger (more fuel available at any given moment), it didn't seem to be making any huge difference in heat output or flame stability - maybe due to the too-small diameter and limited airflow. But either way, unfortunately, after several hours of running fairly well, it again rather suddenly started burning slow and again went out... strange. Maybe a problem with my wood? I did notice that there were virtually no hot coals being left in or around the burn area, whereas when I burn the same wood in a more conventional manner (box stove), I get plenty. Which means, I think, that it's going to be a little less forgiving of less-than-ideal wood - because it's only the tips themselves that are burning, and if you've got a couple "bad" pieces of wood in there out of four and you need a big enough flame to keep the draft going, then it's probably going to have a pretty dramatic effect. The rocket concept is certainly a whole different animal... and I see that it has potential. I think the insulated riser will probably make a huge difference for me.

Many Thanks for your inputs. I'll get a proper stove made soon, God willing.