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Post by spmp09 on Jun 11, 2012 23:45:02 GMT -8
Gidday All, My flatmate and I have been playing with our RMH for a few months now, at one stage rebuilding ever few days, and we STILL dont have it right. After another attempt tonight, I have to throw my hands in the air and ask the experts, what the *()##! are we doing wrong. We based our original design on the utube video by the guys at permies (who talk about this stuff... all day long) www.youtube.com/watch?v=GkHOwmKyL7A with the smaller barrel and 6" piping. For a while we had fires that started really well once rebuilt, but then after running a while began to smoke and never worked right again. We traced this to using straight clay and the clay shrinking and cracking creating leaks between the burn chamber and the heat riser etc. We also had problems with oscillating beahviour, plenty of suck for a while then blow suck, blow suck... etc. We fixed this by enlarging the area underneath the barrel, and havnt had oscillation for a while. The problem currnetly is not enough draw, and if for some reason the fire works well, it will do so for an hour or so then start smoking and there seems to be nothing we can do about it. (Unlike before the fire will work again once cold and the ash emptied out) We reduced the burn chamber size, this helped. I just upsized it again this afternoon, but no difference. I also tried what had been suggested by donkey and that was to adjust the height of the barrel until we got maximum draw. The riser tube doesnt height and brickwork means we cannot go less than 2", but we ended up with about 3.5" gap by trial and error today. Images are in the zip file attached. Anyway, the dimensions so far: Heat riser: 6" inner tube 8" outer Filled with clay slip+polite. Cut similar to that on the youtube video, just without the false floor, so straight onto the bricks Burn chamber (before modifying today): 7.8" long 4.7" wide 3.5" high We built it off the floor with bricks supporting some galv. sheets, then brick base on top. The exhaust can escape the barrel on all sides, leading to an enlarged opening (under the barrel) on the exhaust pipe side. The exhaust pipe rises at a slight incline, and at this stage goes straight outside. The fire is constructed of one layer of bricks sealed with clay. no extra material around the outside. I remade the exhaust gas -> pipe bit to try and increase flow... it seems unlikely that there is a bottle neck there. We have made sure that the horizontal burn chamber is isolate from the barrel exit. Following the 'troubleshooting' section of 'the book' the only things I can think of are that the wood enterance is too wide or too high. gaaa! is it that we are in the southern hemisphere and the only ppl that have built one on this island? Or the way we hold our tongues? Everyone who hasnt built one has loads of fantastic suggestions. The most practivcal being that of using two barrels on top of each other.... Anyway, your advise is greatly appreciated. I cannot ingest much more smoke! Jasper
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Post by satamax on Jun 12, 2012 2:26:31 GMT -8
Drect answer, burn chamber feed tube too small.
6x6xPI/4 28,27sq in
3.5x 4.7 =16.45sq in, way too small imho. And seeing your pics , it seems that the feed tube is the same.
You could get away if your within 10%. Well it can work other ways, but you'd have to be lucky.
Yoour heat riser seems a bit short too. And your burn tunel needs to be insulated too.
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Post by spmp09 on Jun 12, 2012 20:08:12 GMT -8
Thanks satamax, We were going by the idea that the burn chamber was supposed to be heaps smaller than the riser tube etc. Currently the burn chamber is 4.5x4.7" = 21.15sq in. Is this close enough? What dimensions would you recommend for the burn chamber cross section? Is burn chamber length an issue.
Feed tube... what dimensions do you recommend? > than burn chamber?
How high should the heat riser be? ATM it is about 28" high.
How much insullation on the burn chamber? we are not noticing it getting too hot.
questions questions..... But thanks for the feedback.
Jasper
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Post by spmp09 on Jun 13, 2012 0:25:35 GMT -8
aaaah, Satamax, Will we find a cure for the seedpod trees and save the Molefa. Coincidence? Strange I dont believe in coincidence.... Just read this chapter to my girls tonight...
8)
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Post by satamax on Jun 13, 2012 10:24:42 GMT -8
Huh, sorry, whatcha yar talking about?
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Post by spmp09 on Jun 13, 2012 12:52:37 GMT -8
Huh, sorry, whatcha yar talking about? its obviously out of context. I have been reading Phillip Pullmans 'His Dark Materials' series to my girls, and we have just read a bit with a character called Satamax, hence I figured this was your pseudonym's namesake. I suppose I am wrong... I have read 'the book', ROcket mass heaters anyone can build, and have been following as best we can by proportion, hence I am asking for other assistance regarding proportions, based on experience. Cheers.
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Post by satamax on Jun 13, 2012 14:07:02 GMT -8
Hi Jasper. Huh, i didn't know about that series of books, i will try to find a copy. I'm a pratchett fan, so another Brit fantasy writer might apeal to me! Mate, sorry to say, but if you haven't sussed the part about the burn tunel section being the same or thereabouts as the heat riser and feed tube, and burn tunel being insulated as well as the heat riser from the book, you might be in need of re reading it better. And read most of the posts here and on permies wood stove forum. I admit Peterberg, Donkey, Canyon, Ernie have made discoveries since the book was printed, but it's still mostly valid.
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Post by spmp09 on Jun 15, 2012 14:44:30 GMT -8
Mate, sorry to say, but if you haven't sussed the part about the burn tunel section being the same or thereabouts as the heat riser and feed tube, and burn tunel being insulated as well as the heat riser from the book, you might be in need of re reading it better Maybee... We read and re read the book. Made and remade the fire, using many different proportions and dimenstions. We started as per the book, burn chamber half the length of the heat riser, and found that we got better results as it got shorter. We also noted that most examples of RMH's we could find images of looked like the burn chamber was much shorter than half the heat riser. We also read in numberous posts (and the book) that the burn chamber should be the smallest part of the system. Certainly insulatitng the burn chamber is something we have never considered, but it has never seemed hot enough to worry about. rarely has it ever been too hot to touch. I have also seen many posts talking about the fluid dynamics of the system, the volumetric differences based on temperature etc. that need to be taken into account. There is nothing in the book about this stuff and it seems like these are the critical factors. I have had other friends who have worked on commercial fires in the past talk of "the set of formula" outlining the riser height to cross section and feed tube volume etc. Again, not in the book. Hence I feel justified in asking on a forum, we started with the book, and experimented and experimented, but never finding the perfect combinations.. well maybee that is not quite right. We were pretty close when we started, but we were using clay instead of sand/cob so it would stop working as soon as the clay shrank... approximatly 2hours.. Unfortunatly cant remember what we did, hence I am mooving backwards to this point, starting by enlarging the burn chamber, but it wont fire at all! I will try again today. Anyway, any feedback, and links to posts that may _fully_ explain the math/theory esp. stream stuff is still greatly appreciated.
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Post by satamax on Jun 15, 2012 16:41:26 GMT -8
Hi Jasper. Well, in section, my aétempts were either weird, or normal. Weird with the cyclonic, and normal with the J tube rocket. Tho, both have a very elongated barrel and heat riser. Longer heat riser gives a better "stack effect" for whatever that means. As i understand it, in the insulated heat riser, hot gases reach a higher velocity, then the longer downwards travel make thoses cool more, so they get heavier and tend to go down, and not restrain the gaz flow too much. You can atain the same effect with a bigger diameter barrel too, less presure difference due to the height of teh barrel, but cooling the gases more due to a bigger heat exchange surface. Insulating the feed tube and burn tunel is meant to keep the fire as hot as possible. Not for protection purposes! Hence the use of cob, or clay perlite, or clay vermiculite mixtures. What i have gathered over here, is that narower higher burn tunel works better than flat and wide. The trip wire and Peter chanel seem to make the stove work better, tho i have no way to measure. donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=226www.permies.com/t/12203/stoves/make-your-Rocket-Stove-more
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Post by Donkey on Jun 18, 2012 8:53:38 GMT -8
spmp09, I am unable, for some reason, to download your images. I can attempt an answer based on your description and the conversation thus far. If I were you, I'd go back to the design straight out of the book, get that to work before trying anything different. Make the cross sections the same throughout the machine, burn tunnel is half heat riser height at MAXIMUM, though keeping it as short as possible is best. Insulate EVERYTHING including the feed, insulating the feed is not totally critical but helps. Make your heat riser as tall as is practical, 2 1/2 - 3 feet is good. Pay CAREFUL attention to the space under the barrel, downstream from the heat riser. This is the most common area for failure, it MUST be rather large. The "breathing in and out" symptom is usually an indication that this area is too tight. Tune the top of the barrel using my method.
Try all that, if yer STILL stuck at that point, shout back and we'll see what we can do.
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Post by spmp09 on Jun 25, 2012 16:20:40 GMT -8
Thanks heaps, I rebuilt it keeping with the dimension of the heat riser tube, being 145mm diameter, and put the heat riser on more bricks to raise it some more, and with the extra layer of bricks was able to 'bell out' the space underneath some more. So have ~760mm height in the heat riser. I will mix some clay sand and polite to insulate the feed tube area, and put some polite underneath. I still need to add a round thing with lid at the feed hole 8) I think all is working as it should now. Very minor smoke back, which occurs when there are too many coals constricting the flow.
Two questions: 1) When the book said that the entrance to the feed tube should be tight, should it be less than the area of the burn tunnel? It always seemed to work better when it was. 2) Donkey, can you give any ideas as to how you tune the barrel height? I know you have said much experience and you just know when it sounds right.... I have it set pretty much at as the book dictates, such that the surface area of the extrapolated inner heat riser tube is the same as the burn tunnel. 3) I made the burn tunnel as short as possible. As such it is the thickness of an the thin side of two bricks + the thickness of the heat riser insulation. SHould this be an issue.... It works, but if this was longer would it be better?
Attached are some pics.
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Post by satamax on Jun 26, 2012 10:08:53 GMT -8
Thanks heaps, I rebuilt it keeping with the dimension of the heat riser tube, being 145mm diameter, and put the heat riser on more bricks to raise it some more, and with the extra layer of bricks was able to 'bell out' the space underneath some more. So have ~760mm height in the heat riser. I will mix some clay sand and polite to insulate the feed tube area, and put some polite underneath. I still need to add a round thing with lid at the feed hole 8) I think all is working as it should now. Very minor smoke back, which occurs when there are too many coals constricting the flow. Two questions: 1) When the book said that the entrance to the feed tube should be tight, should it be less than the area of the burn tunnel? It always seemed to work better when it was.2) Donkey, can you give any ideas as to how you tune the barrel height? I know you have said much experience and you just know when it sounds right.... I have it set pretty much at as the book dictates, such that the surface area of the extrapolated inner heat riser tube is the same as the burn tunnel. 3) I made the burn tunnel as short as possible. As such it is the thickness of an the thin side of two bricks + the thickness of the heat riser insulation. SHould this be an issue.... It works, but if this was longer would it be better? Attached are some pics. For the round thing with a lid on the feed tube, i use a brick which is also covering more or less the feed tube when in use to regulate a bit. For the smokeback try a peter channel.
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Post by Donkey on Jun 26, 2012 10:44:57 GMT -8
Two questions: 1) When the book said that the entrance to the feed tube should be tight, should it be less than the area of the burn tunnel? It always seemed to work better when it was. Yes. Well when the firebox is chock full of wood, the mouth is (effectively) tighter than system size. As the thing burns down, you can use a brick to tighten the intake around the wood. The appropriate air intake size appears to be somewhere between 1/4 and 1/3 system size, most often 1/4.. You can make a cap that fits over the feed with a hole in it of this size if you wish... For me, the practical matter of cleaning the thing out and all that is such that I keep the feed at around system size. Bigger doesn't work well and smoke back pretty badly and smaller is difficult to clean out. Get the works good and hot first. Then place the barrel over the heat riser. Lower it till it's OBVIOUSLY choking the machine, then raise it again and repeat.. Listen carefully as you go, you will notice that it will pass through a "sweet spot" where the sound of the fire is best. Try it a few times till you notice the differences between a little too high and a little too low.. Burn tunnel should be as short as you can get away with. The only issue I can think of is that the cob around the barrel could be a little thin and fragile up front, which could crack and/or fail. You'll want a little fat around the barrel, just enough to be sure that the seal won't fail.
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Post by spmp09 on Jul 6, 2012 17:41:38 GMT -8
Heya experts, Unfortunately the smoke back, even though it is slight is creating a toxic environment in the room 8( (i.e it eventually fills up), and the draw has reduced such that with oily wood such as pinus radiata offcuts I am getting black smoke similar to coal smoke out the chimney (and a thin wisp of it back up the feed tunnel) Upon a friends recommendation from her research on Russian fires, we made the feed tube almost horizontal, with a slight upward incline. This was ok, harder to start, and fussier on how much fuel etc... still too much smoke back. I havnt tried the Peter channel yet, and will today. Looking at the horizontal feed fire, is it looking like the outlet of the P-Channel (enhancement mode field effect) is better at the end of the burn tunnel, at the union of with the heat riser. Has anyone done this in a standard RMH? I could do it with a piece of galv running along the underside of the top bricks, with the entrance at the feed tube.
still have a couple of queries:
Is my only option to make the riser HIGHER? Is there some other dynamic that is possibly wrong?
I cannot reduce the barrel/heat riser distance to the point the fire is choking, as such the minimum gap is ~1.5" with no improvement if higher. Could this be the problem.
Looking at some of Peterbergs experiments got me thinking, my issue is draw, but would a trip wire type thing, something to create turbulence at the burn chamber/heat riser interface help. Rationally I believe that I should get the simplest thing right first, which thus far I have failed to do 8(
In parallel I am experimenting with round fire bricks, made of compressed 'sawdust'. It looks like what I need is two tubes for them to travel down into the burn chamber, and extra piping for airflow, with a grill to allow air to circulate around and underneath them for burning to occur over the face of them. They work alright with the stock burn chamber, but need a bit of care to not clog the whole thing up as they dissasociate into a pile of sawdust. Again, due to poor draught, the room slowl fills with choking smoke 8( These fire bricks are really attractive to me as they are super cheap, and are potentially a way of loading the fire with a few hours worth of fuel
Anyway, should be fun... Should do these experiments in summer where failure doesnt equal being cold 8) Compared to the broken fire fitted to the house which puts out pitiful heat when filled with firebricks, the RMH is incredible! Its been great to have that comparison. (But boy does the internal fire draw and roar as all the heat goes up the chimney!)
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Post by peterberg on Jul 7, 2012 2:06:55 GMT -8
Is my only option to make the riser HIGHER? Is there some other dynamic that is possibly wrong? Your problem is lack of a stable draw, isn't it? What is your outside stack length and what is the height of the house? In case the house is higher than the outside stack, and this isn't insulated, this could be the source of your trouble. Even a relatively small opening somewhere in the uppermost region near the roof will result in being the house a better chimney than the chimney. The situation should be improve dramatically by lengthening the chimney to a level well above the roof. Leaving out insulation won't result in a stove "by the book", by the way. Moving the P-channel to the tunnel/riser interface won't work, this gizmo need to be in the spot where the best results are obtained. You trying to do it different over and over again, I would think it's not very sensible.
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