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Post by Donkey on Aug 19, 2011 12:35:24 GMT -8
I just taught a rocket stove workshop and was reminded once again of one VERY important trick that is NOT IN THE BOOK. The space below the barrel, where exhaust is gathered and directed into the bench needs to be LARGER THAN YOU THINK! The math won't really help you either, it's bigger than than the numbers indicate. In fact, if you hit the numbers exactly, you'll have a tempermental stove on yer hands. This is the NUMBER ONE failure point of builders with rocket stoves. It's where I look first in troubleshooting. This public service announcement was brought to you by your friendly neighborhood Donkey..  |
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Post by canyon on Aug 19, 2011 16:58:58 GMT -8
Thanks for that Donkey!
Could you share some more specific details as to what you've run into and how things were adjusted? Things like "on a six inch system with a barrel heat riser cover x" gap between barrel and heat riser back drafted with roughly x diameter bottom opening but worked when increased to x diameter etc. If you don't have time it is understood but some specifics would be great if you could. Thanks!
Canyon
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Post by peterberg on Aug 20, 2011 10:28:27 GMT -8
The space below the barrel, where exhaust is gathered and directed into the bench needs to be LARGER THAN YOU THINK! Very true, Donkey. I've attended a rocket mass heater workshop in the Netherlands last week and we've had a heated discussion about this very same point. I'd cut out the opening at a little bit larger than system size, and the instructor of the workshop insisted it was far too small. This is what I've learned this week: There's a difference between a stream opening and a stream profile. The main culprit is the gap between insulation canister and barrel. When you're only looking at the opening in the side of the barrel, it's easy to think you have to make the opening the same as system size. This is utterly wrong, because the gases are not streaming straight out of the opening. Instead, it is coming from left and right, and from the top. Moreover, in the corners two streams can't pass there at the same time, so you have to compensate for that. Maths can help here. Start with system size area, divide it by the gap between inner and outer barrel, add twice the size of the gap for the top corners and you've got the length of the stream profile. The profile length consists of the top rim and sides of the opening. For example: system size of 8" equals a little bit more than 50" square. The gap is, say, 2" wide, which will get us at 25", adding the gap twice will give us 29". Presuming the opening is one foot wide, the height need to be half of 17", which is 8.5 inches. The open area in this instance will be about 90" square. The resulting opening in the side of the barrel will look enormous, nevertheless this is absolutely the correct method. The recommended gap is smaller than in my example, so the profile length will be even larger. P.S. The top rim is rounded, so you have to measure it along the contour, not as a straight line. |
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Post by peterberg on Aug 20, 2011 23:57:41 GMT -8
For better understanding of the stream profile phenomenon.
The imaginairy ring on top of the heat riser is exactly such a profile. It's very handy to calculate it like this: divide the cross sectional area of the heat riser by its circumference. Pi R² : Pi D= height of ring.
For example, an 8 inch system via Pi R² will give you 50 square inch, the circumference is Pi D, a little bit more than 25 inches. Fifty inches divided by 25 inches is exactly 2 inches for the gap between end riser and the top of the barrel. Mark however, this is the minimum size for such a system according to the book, page 36.
The same method goes for a square heat riser, by the way.
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Post by Donkey on Aug 30, 2011 7:49:13 GMT -8
Thanks for that Donkey! Could you share some more specific details as to what you've run into and how things were adjusted? Things like "on a six inch system with a barrel heat riser cover x" gap between barrel and heat riser back drafted with roughly x diameter bottom opening but worked when increased to x diameter etc. If you don't have time it is understood but some specifics would be great if you could. Thanks! Canyon I can't give you numbers.. Oddly enough, I hardly (ever) do the numbers. My strategy is to fire the stove and run it AS I BUILD. The gap at the top of the heat riser can be found (then) by simply moving the barrel up and down till it sounds "right" and wedging up to that point. The space below the barrel can be discovered in the same way.. After a while, you just develop a "feel" for it.. That's my methodology anyway and it works well (for me).. So to fix 'em, I'll pop off the barrel and look. More often than not, you can see the deficit clear as day. The space down there was hardly described in the book AT ALL and folks (including experienced stovers) have no idea of it's importance. You'll see things like shallow, narrow trenches or no space at all. I've seen (some images posted here) the bottom of the barrel itself cut to fit the stovepipe with the barrel sitting on a flat (closed) surface. In all these cases (except the pipe off the barrel itself trick) digging a deep, wide channel down there (and pulling out bricks, rocks and other filler in the way) tends to be the fix. |
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roy
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Post by roy on Sept 27, 2011 9:39:31 GMT -8
can you post some pictures of what changes need to be made. i hope to start on my RMH in the next couple of months and i hope to do it from the book. no time or money for a workshop, just youtube and this wonderful forum.
thanks
roy
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Post by canyon on Sept 29, 2011 17:30:55 GMT -8
I really need to express my gratitude to both Donkey and Peterberg for these posts above! Since those posts I have built two rockets (one 6 inch and one 8 inch) and am inspired by the results using Peterberg's shared formula for the opening and increasing the ash pit size from previous experiences. I am looking forward to revisiting an earlier project in particular and redoing these areas and comparing. I am convinced that is the reason for sluggishness on that particular one where we didn't pay attention to this phenomenon. Thank you gentlemen!  |
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Post by peterberg on Sept 30, 2011 6:22:23 GMT -8
Just for the people who'd rather have a drawing instead of 100 words. The first drawing is the situation I've described above: an 8" system, 55 gallon barrel, gap between the insulation canister and barrel 1.5". The opening to the ash pit is through the side of the barrel. Length of the stream profile would be 50"/1.5"+2*gap=a little more than 3'. It's obviously, the ash pit will be huge, because you'll need to take it down (like a funnel) to the 8" of the bench.  The opening is 8" high and 20" long, measured along the barrel contour. The next situation is with a large round hole in the side of the barrel. The circumference length need to be a bit more than 33", provided all around the hole the gases can stream through. The diameter of the hole need to be (50"/1.5")/Pi, which is 10.6". And it looks like this:  It's not needed to compensate for the corners, because there aren't any. However, the bottom of the hole has to be at least 2" away from the rim of the barrel. Otherwise, the circle has to be wider to compensate for the loss of 1/5 of the circumference. The diameter of the circle would be 12.7" when lower or even crossing the rim. In this situation the ash pit funnel is higher than wide, providing a convenient ash trap. The third situation is like the barrel is above it's ash pit, and a large proportion of the circumference between insulation canister and barrel is open. Because the opening is nearly a third of the circumference, the barrel will become unstable. A solution could be to divide the opening in two, with a brick in the middle to support the barrel.  In this case the barrel will be very uniform in temperature, with a very wide and a relatively shallow ash pit. It's up to the builder what configuration to use. During the build, it could become clear the ash pit would become too wide. In that case, it's an alternative to widen the gap to, say, 2". This will shorten the stream profile length somewhat, from 33.3" to 25". |
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roy
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Post by roy on Oct 1, 2011 23:42:43 GMT -8
peterberg,
the pictures explain it all. in my head i has option 3 in mind, so im relived im on the right track. many thanks. if you have any more pictures of this area of the stove which isnt shown in the book i would be grateful.
cheers
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Post by hippiebuilder on Oct 5, 2011 19:34:52 GMT -8
Hello fellow rocket stove enthusiasts... I am Jeremy, and I have purchased the RMH book by Ianto Evans and I plan to have it be the Primary heating system in my house.... I have a front room that has a dirt floor that will solve the problem of supporting 12,000 pounds... I have read the book, prepared the jobsite and built a small RMH in my backyard to see the Physics working for myself.... I live in Upstate NY and the cold is coming quickly (36 degrees tonight already). I plan on locating materials this week and begining construction... I dont have any questions just yet but I wanted to introduce myself and let everyone know what I'm doing.... Im sure to run into a few questions and I will be sure to post some picture.... Thank you to all you experts out there for sharing your knowledge!
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Post by hippiebuilder on Oct 21, 2011 8:08:01 GMT -8
So Im having the same problem as Roy only it seems like he understood it better... I have my core built and Im trying to figure out how to connect the bottom of the 55 gallon barrel (the ring opening as pictured above as option 3) and the 8" horizontal pipe... I will be trying the beehive brick method and using fire mortar and cob to shape and seal this space but the idea of its shape, size, stream profile, and how to install a cleanout, all seem unclear to me.... any advice or pictures would be greatly appreciated!
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Post by jgreen on Oct 21, 2011 21:05:57 GMT -8
hippiebuilder - I thought I'd respond to your request for pictures regarding this "transition area" since I have some from a stove I built a couple years ago. It's been the primary & only heat source in the mobile home I put it in and its in its third winter of solid service. I uploaded the best/clearest pictures to my picasa account with the link below. I would have added diagram information to the pics but my photo editing software isn't installed and don't feel like doing that right now. I tried to add captions that would describe important details. Here is the link to the picasa album: picasaweb.google.com/seedballs/HJRocketStove_DetailsHope that helps you visualize one way to do it. I'm sure there are plenty of other ways to make it work. The size and shape of this area on this particular stove worked out well based on consistent positive feedback from the owners of the stove. This stove used 7" flue pipe and a rectangular combustion tunnel & heat riser with dimensions around 5.5"x7" (if my memory is correct)... -Jay |
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Post by canyon on Oct 22, 2011 13:52:12 GMT -8
Jgreen, maybe it is just me but I couldn't get your link to work! |
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Post by jgreen on Oct 22, 2011 16:50:08 GMT -8
Canyon -- you're right. I forgot to unlock the album so that people who aren't me can see it. It should be viewable now. Thanks for pointing that out...
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Post by hippiebuilder on Oct 25, 2011 9:27:35 GMT -8
Hey thanks for the help Jay, much appreciated it... I think Im overkilling this one a little... Im doing this with mortared firebrick and a steel pipe riser... I cant believe yours is mostly just cob and it doesnt crack or leak smoke...Also I thought you werent supposed to use cob with straw that close to the pipes or the heat....
Your stove looks great and the photos definitely helped...
Very Encouraging!
I'll be posting photos soon too...
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