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Post by Donkey on Mar 21, 2012 15:43:02 GMT -8
Here's a little six incher built into a tiny cob cottage. The space inside is something like 100 square feet. This little guy does fine. Attachments:
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Post by Donkey on Mar 21, 2012 16:09:35 GMT -8
I know, this area is for finishes and finished stoves, but... Here's a picture of the guts. This little thing has tight tolerances inside.. I had to open it up and carve out the cob below the barrel 3 or 4 times to get it right. That and a Peter-Channel and it works great. Attachments:
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Post by peterberg on Mar 25, 2012 2:19:08 GMT -8
Donkey, Now it's clear what you meant by a bell-like construction in the mud hut stove! It should work even better when the intake and exit openings are both at floor level. Preferably, the exit side a bit lower than the intake. In that way the heat will be trapped more efficiently in this bell-like shape and the end temp will be lower.
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Post by Donkey on Mar 25, 2012 9:05:58 GMT -8
Yeah.. You noticed.. I never intended for it to work like that, never thought of it. I only noticed AFTER it was done and burning, sitting on the stove will let you know how it operates. I think that I'll start building RMH benches more like bells in the future, with the pipes at or under floor level (or no pipes at all) and bell chambers along the bench.
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hpmer
Full Member
Posts: 240
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Post by hpmer on Mar 25, 2012 9:45:22 GMT -8
Donkey, am I correct in seeing that you don't have a 90* pipe connector for the turn, but rather the gas path comes and goes in that end compartment, otherwise known as a bell?
If so, that would seem to make construction easier and much less expensive since the connectors are the biggest cost of the pipe run.
And, if no pipe at all, that would help with the transition from the barrel. One could make it large enough without worrying about connecting it back to the pipe run.
I wonder, though, if that would limit the bench length to something shorter than with pipes, as more heat would get stored by the benches if they are constructed as long bells.
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Post by Donkey on Mar 25, 2012 9:57:37 GMT -8
That's right. Also, ALL of the pipes and other metal bits came out of the local dump and the bricks were a gift from my neighbor. That stove cost me exactly $0.00 to build. Just time..
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Post by Donkey on Mar 25, 2012 10:00:03 GMT -8
It's also a WHOLE LOT easier to clean. One port gets me into both runs of the bench without any weird bends.
I guess I should also point out that the thing was made with adobe bricks.. (cob, formed into bricks and dried in that shape) All (or most) of the drying time was done before I even started building the stove. Assembly was a snap! Dip the bricks in clay slip and stick 'em together. Any weird spots or bends or places where the bricks won't fit, you just cob. This made it SOOO easy to do, the stove was assembled in a single day. We had a fire in it that evening.
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Post by Donkey on Mar 25, 2012 10:14:25 GMT -8
And, if no pipe at all, that would help with the transition from the barrel. One could make it large enough without worrying about connecting it back to the pipe run. Yes, Yes!! Beware though, conventional wisdom has cob as being "too porous" to use this way. Folks are carefully heat taping their pipes (buried in cob) to avoid CO leaking. 'Course, I've never actually heard of the benches leaking.. Aren't bricks porous too, what about the joints in brick and other types of masonry?? Folks don't worry about that much, though I'm sure there is some kind of rule about that... Oddly enough, cob HAS NO joints. I imagine it would.. I've no idea what the limit would be (peterberg probably does, or could figure it out). The benches would probably need to be shorter, but they would heat up more evenly. No more "hotter by the barrel" syndrome, or less of it anyway.
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hpmer
Full Member
Posts: 240
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Post by hpmer on Mar 27, 2012 14:18:32 GMT -8
I think that I'll start building RMH benches more like bells in the future, with the pipes at or under floor level (or no pipes at all) and bell chambers along the bench. If you were to build it without the pipes, but rather the bench as just an empty void/bell, are there any rules of thumb as to size, either minimum or maximum for the interior or exterior?
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Post by peterberg on Mar 28, 2012 8:58:29 GMT -8
I've no idea what the limit would be (peterberg probably does, or could figure it out). The benches would probably need to be shorter, but they would heat up more evenly. Figured it out... All the ducting in a reasonably length of bench equals a certain surface area inside all the pipes. The bell or bells, or the whole hollow bench should be the same inside surface area as the ducting. That is to say, the ceiling and walls of it. Not counting the floor, that will be the very last to take up heat and is neglectable. For example: in an 8" system the ductwork could be as long as 30 ft. The internal surface area would be roughly 63 sq.ft. The pipes in a bench won't transfer much heat to the bottom of it, so you have to deduct some surface area. Estimated to be a quarter of the total surface, that would be 47.25 sq.ft. A bell construction has a lower friction compared to pipe, so 50 sq.ft. will be the maximum I'd think. A tunnel of say, 1 1/2 ft wide and high inside should therefore be no longer than 11 ft. Obviously, there's less mass in there so you have to fatten up the walls and ceiling of the tunnel. @donkey, in this way you should be able to compare your mud hut stove to one with a fully ducted bench.
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Post by nk14zp on Mar 28, 2012 11:20:46 GMT -8
Are you more worried about cracks?
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Post by Donkey on Mar 28, 2012 12:06:26 GMT -8
I'm NOT worried about cracks. That's a function of the mix you are using and how well you build the thing. I'm confident in my mixes here. I've been making cob from my soils for over 12 years and know them well. In other places, I would have to do a lot of testing to acquire the same confidence. I know how to go about that. There are rules to follow with ANY building method. Cob and brick laying follow some of the same rules, mainly in expansion joints. Allow for expansion of different materials, exposed to different heats.. That's the main deal. Besides that, don't use sand that has a crystalline structure in high temp mixes. No quartz or crystalline silica. Crystals expand and contract differentially when heated. Mixes with crystalline sands will tear themselves apart over time.
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Post by matthewwalker on Mar 28, 2012 13:12:42 GMT -8
Very, very cool Donkey! I can picture a lot of uses for this type of configuration.
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Post by Donkey on Mar 28, 2012 14:40:36 GMT -8
Peterberg said:
OK. That makes a lot of sense, sort of obvious now that you've said it. One thing though, in a pipe you get a lot of laminar flow which reduces heat transfer. The dynamics in a bell are different. Wouldn't there be better transference to a bell? Do we need to take this into account? What would be the "fudge factor" for that?
I dunno, mine's sort of halfway between, maybe not the best model to compare.. Good point though.
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Post by Donkey on Mar 28, 2012 14:46:49 GMT -8
Well.. I started this thread to provoke a discussion of plasters.. Anyway at this point, it belongs over here.
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