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Post by muddome on Apr 15, 2012 20:37:12 GMT -8
The good news: My brickwork and riser seem to be done well. The stove rockets very loud, there is no smoke and hardly any ash after burning lots of wood. It gets up to speed almost right away.
The bad news: When I place the heat exchange barrel over the insulation barrel, everything comes to a grinding halt. The rocket noise is gone and the fire burns very lazy. No smoke is backing up, but the draw is definitely reduced.
I'm confused. It's a 7" system. The outer barrel diameter is 22.5 inches (A=397.6) and the inner one's diameter = 19.5" (A=298.6 to the outer edges of the ribs)
So I'm going to have to make a new insulation container. One of my options is a hot water tank with a 16" diameter. Another is to make one out of some roofing tin but I'm not sure it would be thick enough to hold up over time. The water tank would leave about a 3.25 inch gap. I'm wondering if this would be too much?
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Post by canyon on Apr 15, 2012 21:14:38 GMT -8
3.25 inch gap is great. Be sure you have extra area for the exit into your ash pit. Peterberg set us up with those calcs in Donkeys thread called something like "friendly neighborhood reminder" if I am even close in memory.
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Post by Donkey on Apr 16, 2012 7:16:23 GMT -8
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Post by canyon on Apr 16, 2012 7:56:40 GMT -8
Thats it, thanks Donkey!
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Post by muddome on Apr 17, 2012 14:52:34 GMT -8
Thanks canyon and Donkey. I will be going ahead with the water tank then giving me a 3.5" gap. At first I thought that would be less than ideal but after trying a gap of 1.5" and the system slowing right down I knew there had to be something better. My confusion came from the book's suggestion that I was on the right track with 1.5". I will be using option 2 in the helpful hint thread for my opening to the bell. Will have to wait till tomorrow to get rolling again. I had to go to town to get more cut-off disks for the grinder and that was a 5 hour round trip. BTW as I was trying to figure out other options, I tried just connecting a pipe to the top of the riser. I thought perhaps no exchange barrel would work well. It did not. The duct work got very hot quickly but there was a real lack of draw through the burn tunnel.
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Post by muddome on Apr 18, 2012 18:02:21 GMT -8
The three inch gap is making a big difference. I managed to cut the tank just right too for a nice snug fit over the bricks of the burn tunnel. The rocketing is still a little less than without the exchange barrel but I'm so far quite encouraged.
I'm wondering what other people's experience is. Are you getting the same rocket draw with the barrel on as you do with just the riser? Or should I expect it to be less?
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Apr 18, 2012 23:58:45 GMT -8
My stove did the opposite - a 2.5" gap didn't work as well as the 1.5" gap we ended up with after various trials.
I suspect the design/size of the exit into the ash pit/bench will determine the optimal gap.
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Post by Donkey on Apr 19, 2012 8:17:15 GMT -8
I suspect the design/size of the exit into the ash pit/bench will determine the optimal gap. Bingo!
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Post by muddome on Apr 19, 2012 17:37:49 GMT -8
So is there a great way to figure this out the other direction? I'm already locked in for the insulation, barrel, gap. I've re-read the helpful hint thread dozens of times and understand it pretty good now, I think. Except for one thing. On my 7" system with a 3 inch gap my stream profile works out to about 19 inches. So the 3 sides of of my cut out (option 1) have to equal 19". But that could be 3.5+12+3.5 (area = 42) or 4+11+4 (area=44) or 5+9+5 (area =45).... What's the best? The one that works out to the largest area?
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Post by Donkey on Apr 19, 2012 19:28:56 GMT -8
Whatever the NUMBER is, shoot a little larger. This will make it easier to do and give you more wiggle room for mistakes. I've noticed that a LARGER volume down there (larger even than the numbers suggest) seems to improve draw. I don't know why it is, but it seems to be so. In any case, too big harms nothing, too small does.
As to shape, try to visualize the smoothest transition from one volume to the next.. This probably doesn't matter much as long as it's big enough.. So yeah, go for the one with the largest area.
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Post by muddome on Apr 19, 2012 20:01:51 GMT -8
Thanks Donkey. I'll go a bit bigger than the numbers work out. Also, I'm tempted to go with an arc shaped cutout. That might help with transition.
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Post by Rinchen on Apr 20, 2012 0:07:37 GMT -8
I have quite a large volume in the transition area of about 60 liters/16 gallons. The top of this area gets hot (175 Fahrenheit, sometimes more) after about one or two hours even though its about 4 inches thick (2.5inch concrete+1.5inch clay/sand). Much hotter then the rest of the bench after it. The draw is always excellent when up to temperature, I never get smokeback even with long planks of softwood. I think a larger volume here (and in my case also a large barrel gap top and sides) does tap a lot of the heat, making less available for heating the bench. I need to run my system on top performance to avoid condensation. I have about 26 feet of 8 inch duct in my bench.
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Post by nk14zp on Apr 20, 2012 12:12:57 GMT -8
Any pics of your stove rinchen ?
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Post by woodburner on Apr 21, 2012 2:08:59 GMT -8
If you have a large gap, the gases will get through the stove too fast and not have time to cool to make the pump as effective. If you use a smaller gap, the gas has tim to cool, the pump will probably work better, the combustion will be more complete, and your wood usage will decrease. That's why a 1.5" gap is used so often.
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Post by Donkey on Apr 21, 2012 18:10:16 GMT -8
woodburner, I think you mean the gap at the top of the barrel, between it and the heat riser. And yes, you can tune that for different characteristics. The gap at the bottom of the barrel is what's in question here. In this place, bigger (within reason) is better. 'Course, what we're learning is that if you make it TOO large, it gets rather hot in that spot.
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