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Post by josephjcole on Dec 31, 2010 7:05:23 GMT -8
Well I've spent the last several evening drawing and redrawing different stove designs. I'm fairly space limited for my stove which will go in our pottery studio. I had originally planned on going with a small heat riser pushing the exhaust through pipes buried in our dirt floor. However after a little test digging in which I found out just how hard it is to dig out our floor (it's an old machine shed with a packed gravel floor which has been there for longer than I've been alive) I've switched my train of thought to a rocket bell combo. I tried to keep it as small as possible. Vertically small as well as this will go between my wife and my work space and I still want to be able to see her while we work/converse.
I'd also like to try and build the whole thing out of cob. Adding perlite where appropriate for insulation (burn tunnel, heat riser). Possibly some high temperature cement (Calcium Aluminate Cement) in the burn tunnel to protect against abrasion.
So I'm sure it's a case of build it and see, but none the less I thought I might get a little insight from people here if I posted my drawings. A quick explanation of the drawing: counter clock wise from the upper left hand corner you have the above view; side view showing burn tunnel and flue locations; the second bell with the flues from the first bell being on the outside and the exit to the metal chimney in the center; and finally dimensions which are approximate as I'm not that great as figuring out square footage of and area under arches.
Certainly this will be an experiment of sorts, however I also will eventually need heat so I'm hoping not to have to tear down the whole thing. With out tearing down the whole thing though I could easily change the second bell from a down draft to an updraft if there was not enough draw. Also the burn tunnel could be made smaller by adding cob through the stoking area. I tried to add a little extra mass right above the heat riser where I assume it will be hottest, certainly a little more could be added latter. For reference all the walls are 4.5 inches thick except the burn tunnel and heat riser which are 3 inches.
I hope this diagram is some what legible after some examination. I can add arrows showing flame/heat path if that would be helpful.
I would welcome any thoughts/comments.
We have a large pile of local clay on our property that we will use to make the stove, however it's covered by snow at the moment so actual construction probably won't start until spring.
thanks for looking
Joe
oh one square of graph paper is 3" in the diagram
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Post by Donkey on Dec 31, 2010 9:43:52 GMT -8
Cool idea!
Gonna take some thought, looks like you got a start on that..
I'd make a ton of adobe bricks and use them to build a large portion of your stove.
Freehand cob domes and arches are tricky. They sometimes require slow going, doing a little part and waiting for partial drying, etc. It's nice to be able to just stack up the easy stuff, have it (mostly) dry already and ready to take weight. Takes some of the frustration out of it.
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Post by josephjcole on Dec 31, 2010 14:11:02 GMT -8
I like the adobe brick idea. I had figured on laying a row of cob and then hitting it with the weed burner, before moving on. Despite lacking some of the thrill of using the weed burner, I think your idea is still probably better.
I feel pretty confidant about free handing the main dome structure. However I'll probably build arch forms for the burn tunnel and flues. Simply to keep the demensions exact if for no other reason.... also following a form always makes me feel better. I think some small pieces of plywood cut to the appropriate shape, with a large hole in the middle. If spaced close enough together in the burn tunnel they could be covered in cardboard and then cobbed over. Then burnt out when everything is finished... probably something similar for the flues.
Joe
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Post by Donkey on Dec 31, 2010 15:46:47 GMT -8
Arches can be made VERY quickly with adobes and cob.
Build a form and shim it up with something, right where you want it.
Build your arch with adobes. Dip 'em in clay slip and stick 'em together, narrow flat faces of the bricks should point at 90 degrees to the form going round. Fill the wedge shaped voids between the bricks with firm(ish) cob. Build from the outsides to the middle, the last brick can be carved into a keystone shape. Cob over everything and bingo! You can take the form down just about immediately if the bricks are well layed, let it go for a day or two if yer not sure. To pull the form, slip out the shims (usually this means bang 'em out with a hammer and rod), the form now has room to drop down and out.
Arches push out at the feet (bottom) as the top tries to come down so remember to buttress the arches (domes too) with plenty of weight to hold 'em in place.
You can practice doing it dry with adobes (or red brick) and a form. You can use tile, rocks, chunks of adobe or dried cob or even wood bits as temporary "mortar". It's a good way to kinda get the feel of it. If it's well layed, the form can come right out.
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Post by Donkey on Dec 31, 2010 16:01:03 GMT -8
Your shapes (in the drawing) look like catenary arches, which are the strongest shape for arches.
If you want a TRULY catenary shape, forms and templates can be lofted very easily.
Prop a chunk of plywood or some such up vertically, droop a heavy rope or chain in front of the board and pin its ends level to each other as far apart as the width that you want the arch, let hang between enough slack to define the height. The rope or chain will naturally hang in the shape of your catenary arch, just upside down. Trace and cut.
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Post by josephjcole on Dec 31, 2010 23:56:29 GMT -8
Having built several pottery kilns before I've pulled arch forms out of straight catenary arches before (the second and third chamber of our kiln are both straight catenary arches), however with a shape like the burn tunnel, where the widest part is in the middle I don't think there is an easy way to pull the arch form out in one piece. With kilns I've built in the past I've just crawled in and taken them apart with a reciprocating saw. The burn tunnel is obviously too small to get a saw in there though, so it seem like burning it out is the best bet.
I ended up with the shape of the main arch being a little straighter than a catenary and a little flatter on top. This is a common shape in Japanese kilns as the arch forms are made with bent bamboo which is inserted after part of the wall in already completed.
I started with a catenary shape but to keep the heat riser large enough in diameter I made it a little straighter down low. Any thoughts on the tapered heat riser? With the main arch tapering in it seems difficult to make a straight heat riser. With the top of the heat riser the dimension that I want it seems like it will work. However having never built a rocket stove before this is all speculation. Also any thoughts on the burn tunnel size. I made it to expand out quite a bit in the middle, hopefully to accommodate more wood. I'm hoping the rising ceiling will keep unburnt gases from collecting too much. Again, all just speculation...
thanks for the comments/advice
Joe
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Post by josephjcole on Jan 1, 2011 0:00:30 GMT -8
having just looked at my drawing the two arches don't match at all... sorry. The side view with the burn tunnel is the shape I want. The view of the second bell looks like a catenary... wasn't really thinking when I drew it apparently  Joe |
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Post by Donkey on Jan 2, 2011 19:06:30 GMT -8
I don't know about the tapered heat riser thing anymore.. I originally thought that it would improve the stoves, but so far in my couple little tests, it hasn't quite panned out. I'd say inconclusive up to this point.
If you've got a better way, I'm all ears (or eyes, as the media may afford)..
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Post by josephjcole on Jan 3, 2011 6:48:49 GMT -8
Hmmm... "results inconclusive" can I take that to mean that there was no discernible improvement? Does that also mean that they weren't really a disaster either. I feel like it should work even with the large "collection area" at the base of the heat riser.... but seeing as how this is one area of the stove that would be difficult at best to change after construction I would hate to find out I was wrong.
I guess another way to do it would be to have both bells under the same arch with a straight internal wall dividing the two bells. Then you could easily have a non tapered riser tube. The overall eggs shape of the resulting arch though would be more difficult to build with out an arch form....
I'm not sure what you mean by "if you've got a better idea I'm all ears". A better way to make the tapered heat riser work? I looked through your tapered heat riser thread. The dimensions seem pretty good. In wood fired pottery kilns I usually aim for the chimney to be 85-95% of the flue size. Though maybe doesn't apply here. Anyways.... nope no better ideas. Though it sounds like inconclusive isn't really a negative so maybe I'll leave my heat riser tapered, as that would make construction easier.
Any thoughts on the burn tunnel?
Thanks for sharing you experience/advice I really appreciate it.
Joe
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Post by Donkey on Jan 3, 2011 18:51:42 GMT -8
Hmmm... "results inconclusive" can I take that to mean that there was no discernible improvement? Does that also mean that they weren't really a disaster either. I feel like it should work even with the large "collection area" at the base of the heat riser.... but seeing as how this is one area of the stove that would be difficult at best to change after construction I would hate to find out I was wrong. Yeah.. Inconclusive. Not enough data, not enough time with it, not enough iterations, not a controlled enough environment.. Not a failure either, in that with the limited experiments I did in this, I didn't really notice much of a difference either way. And I've often wondered if an inverted taper (wide end at the top) wouldn't be better.. I can see arguments for both ideas. One thing that I CAN say, is that I've killed a stove (that is built a test that would not work properly at all) by making TOO LARGE a wide spot underneath the heat riser. The idea was to improve turbulence with a bluff body, a sudden widening directly under the heat riser. (At one point, I made this work, though not really well enough to warrant the extra headache.) Anyhow, one of the experiments in particular had an extra large volume and it wouldn't burn properly even without a barrel, just open to the sky. So, watch that. Yep. I'm still holding onto the theory that it should work... Just that somehow I screwed the pooch in my implementation. Really, my casting and testing method on that one left a WHOLE lot to be desired. Umm... Seems to me that your drawing looks good. It looks like one of those Japanese hillside, climbing kilns.. Wadder they called again??? I wonder if the upward slope wouldn't be difficult to stoke on the smaller scale.. ?? And, I'd advise that you build the burn tunnel/heat riser and TEST THE HECK out of it before building the enclosure over it. I dunno.. Maybe use a 50 gal barrel(or larger(or some such mockup-type chamber)) to test your working bits before you go beyond the "easy to take back" phase. |
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Post by josephjcole on Jan 5, 2011 4:44:46 GMT -8
. And I've often wondered if an inverted taper (wide end at the top) wouldn't be better.. I can see arguments for both ideas. I can easily see the argument for a tapper from wide base to narrow top. Increased turbulence at the base of the heat riser from the slightly larger base, tapering into increased velocity toward the top. I can't see the argument for the wide end at the top. It seems like you would loose your velocity right where you need it the most. Maybe this just goes to show my lack of understanding of rocket stoves though. I would be interested in hearing your ideas on the subject. Anagama? maybe the rise in the burn tunnel is more work than it's worth. I added the rise to help deal with unburnt gases and then added a flattening on the floor to keep embers from falling out, and possibly get more wood in. I'll give it some more thought. For testing maybe test first just the burn tunnel/heat riser with nothing over it. Then test with a barrel over the heat riser? One last question then I'll stop bugging you. I believe your stove is all cob. Do you ever get expansion cracks? We get them pretty bad on our kiln. You can see it here: 2.bp.blogspot.com/_jcqwZKMurKE/SmcguFwoBUI/AAAAAAAAA_c/gJa-kUyg2rY/s1600-h/DSC00659.jpgIt's pretty standard in kilns unless you use ceramic fiber between the firebrick and cob. As you can see later on in the firing it does emit some smoke from the firing. Not a problem when you are outside (we re-coat every year or two). I imagine it could be quite a problem in a small enclosed studio... any thoughts on cob stoves and expansion cracks. I usually think of largest expansions starting around 1200 degrees F. I certainly sounds like most rocket stoves are getting to at least 1400. Maybe I'm worried over nothing though..... Joe |
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Post by Donkey on Jan 6, 2011 8:32:29 GMT -8
I do have some cracking in my stove. Its up high, right at the junction between three (maybe 4) different materials.. Fortunately the cracks are more cosmetic than anything and they don't leak.. I should have built in expansion joints.
From your image, it seems like you could add more sand to the cob mix to keep the cracking down a bit. As long as it's the right kind of sand, or grog maybe instead of (or as well as) sand.
Yer right to worry some, You don't want to suck exhaust.. Getting a good cob mix that won't crack and leak will be important.
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Post by josephjcole on Dec 3, 2011 14:42:51 GMT -8
For whatever reason the summer passed me right by and I didn't give the stove a second thought... too warm I guess. Now that it's cooling off again I'm back at it. After running a couple of tests with different adobe bricks I decieded to build the stove out of used firebricks. We've already got the brick, but were considering saving them for other projects down the road. We figured there was a good chance the first version of our stove would need some tweaking, using firebricks gives us the option of easily tearing it down and rebuilding with nothing really lost.
I switched to a straight/level burn tunnel. Here's where things stand now, footers to raise the stove off of ground level to make stoking slightly easier, and burn tunnel completed.
Hopefully I'll make it far enough tomorrow to do a little testing to see how the burn tunnel/heat riser work.
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Post by josephjcole on Dec 4, 2011 17:12:07 GMT -8
. And, I'd advise that you build the burn tunnel/heat riser and TEST THE HECK out of it before building the enclosure over it. I dunno.. Maybe use a 50 gal barrel(or larger(or some such mockup-type chamber)) to test your working bits before you go beyond the "easy to take back" phase. I'm at a good point to test the heat riser... anything in particular I should be looking for? I'm assuming just start a small fire... let things dry out for bit, and then start pushing the temperature up, and see how it draws? Any advice would be much appreciated. thanks |
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Post by Donkey on Dec 4, 2011 17:41:23 GMT -8
You got it..
Run it a bit longer than you think you normally would. You want to see if (at some point) is stops working for any reason.
Theoretically, there may come a point where the temperatures in the heat riser and the chamber (barrel, whatever) approach the same measurement.. When that happens, the stove should stop working properly. You want to know when that will be, will it come early, late or not at all.
I'd say, just overdo it. Over stoke it, over heat it, cram the firebox too tightly, feed it too much paper... Generally do everything wrong, stress test it. If it does well, be happy! If it does not, see if an improvement can be made in the gizmo itself. When everything is said and done, you will know the limits in any case.
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