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Post by pinhead on Dec 6, 2012 11:16:01 GMT -8
Thanks!
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Post by pinhead on Dec 5, 2012 14:14:26 GMT -8
Anyone interested in a math problem?
I have a 6" diameter feed tube and am trying to decide how to get the required 5% area for the P-channel...
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Post by pinhead on Dec 5, 2012 13:57:47 GMT -8
You dream flow contour transitions.. Yessssss.
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Post by pinhead on Dec 5, 2012 10:42:02 GMT -8
Whenever you see sticks laying in a pile you instantly think "wasted fuel!"
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Post by pinhead on Nov 30, 2012 9:42:13 GMT -8
Okay.. they go out through the chimney... I get that... but dont all the gases just have a big opportunity to just hang out in the system and look for leaks? Surprisingly, it doesn't. I've measured the atmospheric pressure inside the bell and the chimney, and both where negative throughout 99% of the burn. In other words, only in the first minutes of the burn there will be a slight over pressure inside. The rest of the time there will be a small but consistently vacuum. No smoke is coming out possible leaks, but cool air is sucked in instead. Conveniently, don't you think? I'd like to add that the bell in this configuration will be in a state of vacuum only when the exhaust inside the chimney is warmer than outside air -- in other words, when the chimney is aiding with the "stack effect." Luckily, as peterburg said, that's 99% of the time.
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Post by pinhead on Nov 30, 2012 7:58:00 GMT -8
I would do it a bit differently.
The heat being pulled away from the stack will reduce the stack effect.
You want the most heat inside the stack as possible, and the coolest air outside the stack as possible. This increased temperature differential will assure a strong draft.
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Post by pinhead on Nov 30, 2012 6:50:36 GMT -8
Here's a drawing that I did... Doesn't really explain much but may help you visualize the "bell" system... EDIT: Donkey's modification to my drawing shows a more optimal design than what I'm running right now.
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Post by pinhead on Nov 30, 2012 6:31:47 GMT -8
pinhead, I have a grate in my build for ash and ember removal. If you get a lot of coals in your feed tube, all that radiating heat ignites the rest of the fuel supply. That's what I'm thinking is the "problem" -- I can feel the radiating heat all the way up to the roof directly above the feed tunnel even after all of the wood has burned down to coals. I did notice, though, that the problem is greatly reduced when burning dense wood such as hedge or even cedar. Burn-back is also reduced if i burn a big log -- maybe three inches across in a six-inch system -- surrounded by the smaller sticks to take up the extra space. This keeps the air velocity high enough to cool the wood that isn't yet down in the feed throat, while also burning enough fuel to keep the BTUs up. I got a window sealed up last night (was a series of slats with three-inch gaps between each of them so for all intents and purposes, the window was "open") and it looks like I may not need a bigger system, after all. There's still a lot of leaks in the crumbling rock walls and I was able to work without a jacket last night. This thing may work out yet!
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Post by pinhead on Nov 29, 2012 12:55:46 GMT -8
Both ends of the T's were capped and the outside was covered with the clay mixture -- there was no secondary air feed. Last night I completely filled the void under the feed tunnel with ash-clay. Next time I pull the barrel I intend on filling the chamber under the heat riser, too.
The metal of the inside duct has already started to burn out. That's why I used furnace cement and vermiculite between the six-inch and eight-inch ducting; I can pull the metal shards out, leaving the concrete form intact.
The bell is made of two halves of a fifty-five gallon drum.
The transition from the barrel to the bell is as wide as the half-barrel and goes all the way around the eight-inch ducting. I'll measure it when I pull the barrel, but I would guess each side of the burn tunnel transition, when viewed from the top, is 3"x16" or so for a total area of 96in^2 feeding into the entire width and height of an open half-barrel.
Viewing the transition from the half-barrel side, you'd see the open end of the half-barrel with an eight-inch circle blocked. The rest of it is open. This would calculate out to (226in^2 - 3.14*4*4), or about 175.75in^2 or slightly less.
I'll try to get some pics and more accurate measurements.
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Post by pinhead on Nov 29, 2012 11:18:34 GMT -8
I've been delaying starting a thread about my RMH since until last night it's been pretty temperamental. It was hard to start and smoked back unless the fuel was positioned perfectly. It burned decently warm once it was going, but wouldn't run continually without repositioning the wood to effectively stuff more fuel into the throat of the feed; otherwise it would either smoke back or burn out.
This changed last night when I made a very small modification that made a HUGE difference.
I had originally built the burn tunnel and heat riser with two six-inch HVAC duct Tees attached end-to-end. The heat riser went on one of them and the fuel was fed into the other.
The 6-inch ducting is surrounded by eight-inch ducting with a slightly insulative vermiculite-furnace cement mixture between the two. This whole contraption was surrounded by a much more insulative clay-grass-perlite mixture.
The problem I was having was due to the open "chamber" that was created by the tee directly beneath the fuel feed. This allowed heat to build up and air to swirl around, eventually finding it's way out the top and into the room.
I filled this chamber (see the accompanied drawing) with a clay-ash mixture which seems to be a pretty good insulator thus far. This eliminated all of the problems that i was having.
I think I should have went with an 8-inch system, though.
I haven't taken any pics of the thing yet since I haven't yet added the mass to the half-barrel bells.
Hopefully once I get the stove finished up it'll work better and I'll be able to begin sealing the old rock shop up.
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Post by pinhead on Nov 28, 2012 6:06:16 GMT -8
You have probably already gathered this, but I think it won't hurt to repeat.
If you connect them at the top the temperature will be pretty even between the two.
If you connect them at the bottom the first one will be much warmer than the second one.
I set it up as two bells (second configuration) because my second bell is in the corner of the shop close to the chimney where I don't need a whole lot of heat. The first bell, being closer to the middle of my tiny shop, puts out more usable heat.
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Post by pinhead on Nov 27, 2012 11:14:05 GMT -8
I used the half-barrel system for my mass, in a dual-bell configuration (the first half-barrel feeding into the second at the bottom of the two, and the exhaust exiting at the bottom of the second). The first barrel gets a lot hotter than the second and the exhaust is barely above room temperature.
I definitely consider the barrel configuration a success, as the natural convection within the bell makes for a very efficient heat exchange system.
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Post by pinhead on Nov 1, 2012 10:24:05 GMT -8
I haven't cobbed over the barrels yet so they're probably radiating more heat than they will when finished.
From my understanding of a bell, the outlet/exhaust should be close to or on the floor of the bell. This allows convection to occur within the bell, exhaling only the coldest exhaust.
I did this with my system and it definitely makes a big difference; the half-barrel right next to the outlet is warm and the exhaust pipe is cool.
Do you feel that the outlet/chimney on your bell bench is contributing to the overall flow of the stove once the bench is warmed up? How tall is your chimney and what is it's diameter? Is it insulated?
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Post by pinhead on Nov 1, 2012 9:58:59 GMT -8
Nice setup!
I used your half-barrel/bell idea in the RMH I'm building for my small workshop.
Even with just a full barrel (two halves end-to-end) the exhaust from my 6-inch system is barely warm to the touch.
What heat riser diameter did you use to keep such a large bench warm?
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Post by pinhead on Sept 6, 2012 10:15:09 GMT -8
Nedreck's last reply was Apr 2011. Very unfortunate.
Calling Nedreck: If you're out there, throw us a bone!!
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