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Post by joselb on May 2, 2008 3:32:43 GMT -8
Hi
Anyone has experience with smaller exaust tubes? For example for a standard rocket, 6" riser, the book mentions "...K is the exhaust, the horizontal flue that carries hot gas through your floor, heated bench, bed etc. This should be at least the size of the heat riser (8" to 10" diameter)..."
Besides to increase a lot the installation costs, these 8"/10" tubes are difficult to install, manage, eats a lot of space, curves are huge, etc.
The ideal would be 4" tubes to spread the heat.
So, if all tubes after the burner (after the barrel) could be 4", that would be great! Even if I have to do a higher chimney, it would pay!
I can imagine that the hot gases, going faster into the tube, would transfer less heat, but the real question here is: in practice this will "clog" the gases exit?
Thanks
Jose
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Post by Donkey on May 2, 2008 11:31:57 GMT -8
The total cross sectional area of exhaust MUST be the same or greater, NOT less than that of the heat riser. A higher chimney can not make up for the difference. If you must use 4" tubing, add up the areas of all and be sure they are the same or more..
An eight inch heat riser has an area of (PI radius squared) 4 X 4 X 3.14 or 50.24 square inches. Four inch piping has an area of 12.56 square inches. This means you will need 4, 4inch pipes to equal the area of a single 8 inch pipe. It may indeed spread the heat faster, though I doubt that you will save any money on it..
If you need to save money, you can go to the dump or local scrap yard and pull out enough used pipe to do the job. You should probably do this anyway as a matter of habit. Why buy new and expensive stuff (both monetarily and environmentally) to do a job that can be done with free stuff that will only get wasted? Just check the pieces carefully to insure they are in good condition and get on with it.
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Post by joselb on May 3, 2008 13:01:15 GMT -8
Thanks Donkey, that are not good news.
In practice, what can I expect if I use a 4" tube, a high chimney and a 8" riser? And if I decrease the riser to 6"?
In Brazil, as it's not so cold (i have a home in mountains-about 8-10 degree Celsius), there is no tradition in heating, so it's difficult to get used parts. Just to give you an idea, a one meter 6" tube (3.3 foot) is about $25, and each curve is about the same price.
And about a riser with 5" (6.25x3.14)= 19.62, with 4"tubing(12.556)?
I'm asking about your practical experience, as theoretically we could compensate the smaller diameter with higher gas speed on the narrower tubes = higher chimney... even if I loose some eficiency ...
this would not be similar to the experiences you are doing now about bleeding edges? I mean, a bit smaller diameter at riser end?
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Post by Donkey on May 5, 2008 9:10:25 GMT -8
In my "bleeding edge" post, the tapered heat riser features a sudden widening at the bottom of the heat riser, then tapers back to the original (minimum) size.
In my experience so far, any constriction anywhere will cause the stove to back up or smoke into the room. What I've seen is, as the flow is (gradually) constricted, first you will see a noticeable slowdown in flow, flame will creep up the wood in the wrong direction.. As the pipe is constricted more, it will reach a point where the stove begins to pulse, as if it were breathing. First it will flow the right direction for a moment then flow reverses and will puff smoke out the feed tube, then it will reverse itself again, cycling back and forth. If the pipe is constricted further, smoke and flame will then stream in the wrong direction steadily.
Sorry, but you NEED the flow throughout to remain. If you start with six inch, or eight inch, or whatever, you need to finish with it as well.
Perhaps you could find a different way to provide the heat channel.. ?? perhaps a brick or tile channel. old corrugated roofing could be wired into a tube then cobbed over. Dunno..
I ALWAYS go to the local dump/scrap yard first with the question in my mind. Usually something presents itself. Ask yourself what is the essential function and what could be found/constructed to do the job. Better not to buy stuff if you can help it. Don't be afraid to innovate a little or use different materials in novel ways.
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Post by joselb on May 8, 2008 14:08:58 GMT -8
Hi Donkey,
Thanks a lot to share your experience with all of us, I really will take then, BUT sorry, one question is still in my mind: on all that you described the "height" of the chimney was "higher"? I mean, a very high chimney would not prevent against "wrong direction" or "pulsing"? Besides cost, deal with 8"pipes I believe could run from a bit complex to a nightmare, what do you think?
Thanks
Jose
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Post by Donkey on May 9, 2008 20:42:37 GMT -8
As far as I can tell, a taller chimney will NOT make up for inadequate pipe sizing. I wouldnt trust it in any case. You could experiment with it, though its safer to keep to the rule. I have had little trouble (yet) with 8 inch pipe. Just be sure there is ample room for it all to fit, especially if you plan to double the pipe back on itself or something like that. If I understand you correctly, you want to stay away from 8 inch pipe because of its cost. Understandable.. As Ive said before, there are other things to try.. Just BE SURE of your sizing, if you start with one size (volume), you will NEED to finish with it. You could possibly use 4, 4 inch chimneys with an 8 inch heat riser.  Remember, when you double pipe diameter, the volume is QUADRUPLED! |
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Post by Donkey on May 10, 2008 0:32:59 GMT -8
... You could just make your stove 4 inches throughout.. Depending on the size of the space you plan to heat it may be a viable option. I would give it a rather long heat-riser pipe and insulate it extra - extra..  A possible problem spot would be at the feed chamber. Being so narrow, your wood would need to be rather small. It would tend to burn quickly which would require lots of fiddling to keep it burning well. Perhaps smaller chunks of wood in a hopper to facilitate feeding. ?? Also, the burn tunnel would get blocked often with ash, and air flow might suffer if the hopper is too tight. Maybe, some kind of fire grate with a drop chamber/air intake. |
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Post by martin on May 10, 2008 13:32:43 GMT -8
I have a little experience with a 4" rocket stove. (Actually 3 1/2" x 3 1/2") They do need to be tended every 10 to 15 minutes. The ashes must be cleaned out each time you start it. You have to watch that the embers don't build up and choke the entrance to the burn tunnel, or it will not draw properly. It does burn cleanly, hot for its size, and you can run it all day on about a 5 gallon bucket size of wood, about 1" to 1 1/2" size wood, about 12" to 14 " long.
Could the burn tunnel and riser be constructed to match the area of two 4" pipes? (25.1 square inches) A six inch riser would be 28.27 sq. in. If it were slightly smaller, say, made from brick so you could make it any size you want, then you should be able to make it match.
Donkey, would this be a workable option?
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Post by martin on May 10, 2008 13:38:18 GMT -8
P.S. It seems to me that with the 4" pipes they would have to be the same length & same amount of turns. Simply put, they would have to be matched bookends to work evenly. If it's not exactly matched, I don't think it will work properly.
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Post by Donkey on May 10, 2008 16:59:34 GMT -8
I think this is a workable idea. .. To answer your question Martin, Yes. In theory, why not. You may be right about the pipes being matched, bend for bend. I would think that they should at least end completely level with each other. A good question would be if one leg could travel further, have more bends, etc. if its chimney were appropriately taller. IT could take quite a bit of trial and error and/or perhaps some various maths to figure out... (Trial and error would be my tack ) Getting an even draw may pose some problems. Worth a try in any case. |
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