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Post by DCish on Nov 28, 2014 7:05:20 GMT -8
BACKGROUND In the "Pre-port tube secondary air arrangement" thread (http://donkey32.proboards.com/thread/1433/pre-port-tube-secondary-arrangement), Walker discusses the idea of having slow burn in the primary combustion chamber of a batch box to: 1) have more sustained heat output over a longer period of time, and 2) low air velocity to minimize the amount of fine ash swept into the exhaust stream as particulate emissions. He mentions how modern box stove manufacturers are using this approach in their stoves. I looked up the combustion efficiency of my current box stove (Hearthstone Heritage), and it is 81%. My wife LOVES this stove, and is reluctant to give it up for a rocket. Having heated exclusively with this stove for all of last year, it quickly became apparent that it is only efficient when the afterburner is fully engaged. The afterburner is only reliably operating when exhaust temps are in the 500-700 range, sending *lots* of waste heat up the flue. IDEA What if I keep the stove intact and build a bell bench to the side of it, and once it is up to operating temperature, close a damper that routes flue gasses through the bench? Any reason to think that this wouldn't work?  |
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Post by satamax on Nov 28, 2014 8:50:20 GMT -8
Why not, but isn't your afterburner relying on air speed to work  Shed less hot gases into the chimney, = less speed! |
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Post by matthewwalker on Nov 28, 2014 9:43:48 GMT -8
I think you might be able to get away with it. I'd be inclined to build a brick bell that was more upright so the inlet T could dump right into it, thereby hopefully limiting restriction. You could test this pretty easily using a 55g drum as the bell before committing to a large brick build.
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Post by thickstrings on Nov 28, 2014 11:14:06 GMT -8
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Post by DCish on Nov 28, 2014 12:37:34 GMT -8
Why not, but isn't your afterburner relying on air speed to work Shed less hot gases into the chimney, = less speed! Well, I have a 30 ft (9 meter) chimney that gives more draft than the stove can handle without overrunning, so once it is up to speed I generally run it with the damper completely closed (damper still allows 20% flow), and it still maintains at least 500-600F, so I think I have some cushion. |
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Post by DCish on Nov 28, 2014 12:42:53 GMT -8
I think you might be able to get away with it. I'd be inclined to build a brick bell that was more upright so the inlet T could dump right into it, thereby hopefully limiting restriction. You could test this pretty easily using a 55g drum as the bell before committing to a large brick build. Nice ideas. I'm hoping to keep the lbs/sq ft low, but I probably can go up that high on the basement slab without problems (I'll have to check what sort of load the slab can handle). If that works it would mean less stove pipe purchased too, which is a nice $ saver. I like the barrel test idea. I'll have to see if I can drum one up  |
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Post by DCish on Nov 28, 2014 13:09:22 GMT -8
Here's what this guy did........http://www.handprintpress.com/featured/greenhouse-heater/ The stove had to be insulated in order to have enough heat to pump the system... This is great! That stove is higher above the bench than mine would be, and it worked, so I am reassured that it would be possible to run a setup like this. I was worried that it might be difficult to get it started going down into the bench then back up. But I suspect that if that one worked, my scenario would for sure. In studying ASTM E1602 that regulates masonry heaters, I noticed that there is a 20% permanent gap in some of the heater designs. I recall reading elsewhere that it provides a bypass at startup, but once gas velocity increases it goes relatively unused, the bulk of gasses passing through the main channel. I'm hoping that my damper with 20% closure will operate in the same manner. Fingers crossed. Anybody have any experience with this? |
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Post by Daryl on Nov 28, 2014 15:09:10 GMT -8
www.handprintpress.com/featured/greenhouse-heater -- Looks like this guy is using a plain ol' stove box. I agree with Max. The fancier the stove, the more it will take away from the flow. Another thought to consider, possible temp flux from the flue pipes. You have the hot gases going into uninsulated metal pipe then brick mass. The different thermal masses used as materials may screw up the drafting of gases. Try it. Good experiment. Besides, of course I adore mass so I will never say, "NO, Don't do it!" Lol.
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Post by patamos on Nov 29, 2014 7:21:57 GMT -8
Yes, i have been considering this too. The topic recently arose in a post with Peter vdBerg mentioning that it is done often in Holland.
A friend has just installed a big ol heavy wood stove (1/4" plate steel all around) in his basement and This stove box is 28" deep by 18" wide and 20" tall. Exit out the back roof. It really roars when the door is left open. We are considering adding mass to this as an alternative to a full rocket batchbox
Two factors come to mind. If your stove is designed to titrate smaller burns, then the stove body is likely designed to radiate quickly and thoroughly throuhg the metal body. Therefore it may be less suited to higher operating temps - particularly when insulated as in Max Edelson's example.
The other question/factor is how to construct the bell so that weight is well distributed on the slab. And at what point is a double skin no longer necessary.
I look forward to hearing how your experiment goes
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Post by Daryl on Nov 29, 2014 8:09:09 GMT -8
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Post by DCish on Nov 29, 2014 18:40:44 GMT -8
The topic recently arose in a post with Peter vdBerg mentioning that it is done often in Holland. Huh, guess I must have missed that one, if you happen to remember where to find it, I'd be interested in looking it over. Two factors come to mind. If your stove is designed to titrate smaller burns, then the stove body is likely designed to radiate quickly and thoroughly throuhg the metal body. Therefore it may be less suited to higher operating temps - particularly when insulated as in Max Edelson's example. Yes, I thought about adding mass to the stove proper, but it is a soapstone stove, and the manual cautions against over-firing (firing until the top stone measures over 600 degrees). Insulating would definitely quickly result in over-firing and likely damage. That said, I don't mind the quick heat that regular firing gives me to warm the house, what I am shooting for is harvesting the heat that is wasted out the chimney, hence routing the flue into a bell rather than simply adding mass to the existing stove. The other question/factor is how to construct the bell so that weight is well distributed on the slab. And at what point is a double skin no longer necessary. Yeah, I'm asking around the neighborhood to see if anyone else has any knowledge about weight limits. One guy has a heavy lathe in his basement and hasn't had any problems. I suspect that keeping it low and long will be fine. As for double-skinning, my plan was to make a two-brick-thick wall for plenty of heat accumulation as the stove chews through a full load of wood, then have the bench be releasing heat as the stove is dying back. The plan is to interlock the bricks and use thin fireclay mortar. However, if anyone thinks that I should use a double skin with the inner dry-stacked and an expansion gap, do let me know. My thought was that the entering gasses will have already given up some heat, and that double-skinning would not be needed, but I'd love to hear feedback from an experienced person on this. I look forward to hearing how your experiment goes Thanks! I picked up some brick from a friend tonight, so that's a start. I'm short on time and expect it to be a slow build, but will post updates as it goes along. |
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Post by DCish on Nov 29, 2014 18:56:19 GMT -8
And how much mass is needed? Newer and antique metal stoves covered in soapstone pieces claim to radiate for hours compared to basic models. Yeah, this has some truth, there is about a 15 minute lag time between what happens inside the box and what is transferred to the outer surface. Still, compared to a mass heater it holds woefully little heat, and since it is a box stove, it still pumps insane amounts of heat out the chimney without post-combustion harvesting opportunities. Maybe a smaller bell surrounding the stove will do? It would look like a chimney insert or something similar to the masonry towers seen on that link I provided. That would be one idea, but it concentrates the weight in one spot, which I want to avoid. And I hope for this to be an iterative build -- bench first, then as my skills with outdoor prototypes improve (I have a PvdB batch box with S-portal shaping up nicely in the yard, will post that in a new thread when I finally get it up and running), I can remove the stove and build a rocket core that can use the same bench. Since by manufacturer's specs this stove can only ever hope to reach 81% combustion efficiency under optimal firing conditions (which can be a bit finicky to achieve), even if I get this bench thing functioning well to harvest heat from the flue, I'm loath to give up on the idea of having a full rocket in the house . |
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Post by ericvw on Nov 30, 2014 8:49:34 GMT -8
Hi DCish,
I've been following this thread and am interested in your results when you get there.... Had a thought or two I'd like to share-
You will insulate the inlet flue to the bench, no? And do you plan on using a standard damper as your diverter? If so, they do not seal completely so some exhaust will not bypass, and would this hurt your goal? Just throwing some ideas out there. Maybe you already have these things covered!
Can't wait to see a few pics,
Eric VW
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Post by DCish on Nov 30, 2014 15:44:40 GMT -8
Hi DCish, You will insulate the inlet flue to the bench, no? Yes, I plan to use double-wall pipe. Not cheap, but very effective, and it'll mate with the double-wall I already have on the stove now. Hi DCish, And do you plan on using a standard damper as your diverter? If so, they do not seal completely so some exhaust will not bypass, and would this hurt your goal? I'm still thinking this one through, any feedback is appreciated. On the one hand I currently burn it with the damper fully closed (20% opening remaining when closed), so I know it'd always have a cushion to not throttle and stall the stove. The question is, will it behave the same way as the 20% "gas slot" at the top of a zig-zag flue on a masonry heater, where it lets enough through to get running, but once up to speed the larger volume of gasses use the larger pathways? If that doesn't work I think I could put some ceramic wool over some or all of it to seal it better. Thanks for your interest! I think I can guarantee that it'll be a slow build (I have a long commute by train, and two small children, so more time to theorize than to actually build), but I'll be sure to post progress. Today I took advantage of the beautiful weather and the relative down time of the holiday weekend to lug home more bricks. I have 225 or so now, certainly good enough to start mocking up the thing and making final plans. But who knows how much time I'll be able to devote between now and the next holiday. |
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Post by DCish on Dec 2, 2014 20:10:26 GMT -8
Ok, so here are some of the bricks. I'm thinking to fill the holes, probably with a sand-heavy clay-sand mix, to add to the thermal mass. Thoughts?  |
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Shed less hot gases into the chimney, = less speed! 
