jrl
Junior Member
Posts: 101
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Post by jrl on Jun 30, 2014 18:45:37 GMT -8
If i was doing something like this for myself, i would push the heat riser all the way to the left, and make the batch box cental. It will be a little more centered, but it's a rumford fireplace, so there are angled sidewalls and a back wall that curves/slopes from back to front. This should help project toward the front steel facing. I can't decide if the plunger should be towards the front or in the back along the sloped back wall. Been trying to think if it makes any difference.
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Post by satamax on Jun 30, 2014 20:24:16 GMT -8
It should be the furthest away from the heat riser as possible.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 1, 2014 10:35:33 GMT -8
Does anyone feel that a small barrel (or square welded box) over this 30" riser would accomplish the same or more radiation/convection to the living space than walling up the fireplace with steel and making the fireplace a bell? I keep coming back to the effectiveness of the system for space heating and wondering if I'm headed down the wrong path. Why do so many people use barrels over their risers for RMH rather than bell designs? Is it just the simplicity of design? Can someone convince me why this would be less effective? It's a smaller air space being heated, but it's in contact with hotter gasses than the front wall would be. The steel box could be made the trapezoidal shape of the fireplace (not depicted below) which would give me a big metal surface area. In the bell design, the fire brick walls of the fireplace are absorbing heat which gives you some mass to heat, but isn't it more important to radiate as much of the heat out of the gases as quickly as possible immediately after the riser? (losing my mind over a rocket stove! )
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Post by DCish on Jul 1, 2014 11:44:50 GMT -8
Nice drawing, I'm a sucker for graphics, even simple ones.
My 2¢: a metal radiator will get you quick heat. Charging a thermal mass will get you slower, more sustained heat. A combination (as in the classic model) will get you a bit of both, just depends on your goal. Because a rocket stove is optimized for complete combustion and thus always burns full throttle, storing heat is something that most folks choose to do. However, if all you want to do is pump massive quantities of heat into a room over a short period of time, a batch box with an all metal heat exchanger would definitely be the way to go.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 1, 2014 12:14:25 GMT -8
It should be the furthest away from the heat riser as possible. So if the plunger is on the opposite side from the riser, should it be towards the front metal facing or towards the back wall of the fireplace?
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Post by matthewwalker on Jul 1, 2014 14:34:55 GMT -8
if all you want to do is pump massive quantities of heat into a room over a short period of time, a batch box with an all metal heat exchanger would definitely be the way to go. This. I think the fireplace as bell is destined to give you very little heat into the space given your situation. I'd go for a lot of radiator myself.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 1, 2014 16:37:49 GMT -8
if all you want to do is pump massive quantities of heat into a room over a short period of time, a batch box with an all metal heat exchanger would definitely be the way to go. This. I think the fireplace as bell is destined to give you very little heat into the space given your situation. I'd go for a lot of radiator myself.
Can a batch box have a port depth > 2" ? That would let me disconnect the riser from the batchbox enough to get a radiator wall down around the bottom of the riser. Right now the batch box wall is also the riser wall to keep a 2" port depth per Peterberg dimensions.
Also to get enough surface area I'd have to have a couple ups and downs to wind the gases through. Maybe out into the living space above the batch box. Does anyone have any experience using a blower / heat exchanger on these things? Electric fan tunnel = more exchange in a smaller footprint.
More bricks cut now and taking form:
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Post by satamax on Jul 1, 2014 20:40:05 GMT -8
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 2, 2014 5:09:59 GMT -8
Negative, there is no space to add additional hearth in my living room. I have 18" of hearth in front of the fireplace and that's to maintain clearance to combustibles. I have a big hearth downstairs where I could potentially put a second rig, next to the coal stove. There is even an unused 6" port on the back of my coal stove that I could potentially use to exhaust into my coal stove making it a second heat exchanger. But, then I'm spending my time tending a batch box fire in the basement instead of spending time with the family up in my living room. Side note: My 4 year old son has been very concerned about what I'm doing in the fireplace. He's been upset that I may be blocking Santa Claus from getting down our chimney!
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 2, 2014 5:51:10 GMT -8
Here's another crude sketch for you guys. This is a blower concept. The blower fan sits outside the fireplace and some rectangular tubing sends air from the living space up and around the top of the riser, and then back out into the living space. Obviously my welds would have to be perfect for this to work. This would in theory extract heat from the hottest area and would help cool and stratisfy the gases within the bell. What do you think?
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Post by DCish on Jul 2, 2014 8:03:02 GMT -8
To reduce complexity you might consider jacking the stove up a couple of inches and creating a passage for air to flow under the core, then up behind the bell and out across the top. This would put air flowing directly over the top of the heat riser, the hottest point in the system. Natural convection would give you reasonable flow, which could be augmented by a low profile fan (e.g.,http://www.smgeneralstore.com/bk-blower-fan-bk.aspx) if desired. Or, if minimum riser height prohibits such a setup (i.e., if the riser must be taller than the entrance opening of the fireplace), you could have an air entrance channel on one side, flow up over the top, then down and out the other side, achieving a similar flow over the top of the core, but without the advantage of natural convection moving the air.
If you were to want to use natural convection but were worried about insufficient riser height, you may be the perfect candidate for Walker's "broken riser" concept -- essentially the idea is that if you have a well-drafting chimney, the insulated "riser" can be conceptualized as an "insulated afterburner / secondary burn area" rather than as a draft producer, and all or a portion of it could be horizontal. Perhaps a short horizontal run followed by a short riser (or vice versa -- short riser followed by a short horizontal run) would combine to give you the full length of insulated secondary burn area without going too tall.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 2, 2014 8:46:57 GMT -8
Wow, I do have significant chimney draft, and riser height is a major constraint. I'm reading that thread now.
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Post by satamax on Jul 2, 2014 8:46:55 GMT -8
Well JRL, you're going into unknown territory. I'm prety sure that the heat from the front metal plate should be plenty enough. And you will have the storage via the bricks. But it's your project, so you do whatever you want with it. But if you run into trouble with it, may be none of us will be able to help.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jul 2, 2014 9:46:25 GMT -8
To reduce complexity you might consider jacking the stove up a couple of inches and creating a passage for air to flow under the core, then up behind the bell and out across the top. This would put air flowing directly over the top of the heat riser, the hottest point in the system. Natural convection would give you reasonable flow, which could be augmented by a low profile fan (e.g.,http://www.smgeneralstore.com/bk-blower-fan-bk.aspx) if desired. Or, if minimum riser height prohibits such a setup (i.e., if the riser must be taller than the entrance opening of the fireplace), you could have an air entrance channel on one side, flow up over the top, then down and out the other side, achieving a similar flow over the top of the core, but without the advantage of natural convection moving the air. If you were to want to use natural convection but were worried about insufficient riser height, you may be the perfect candidate for Walker's "broken riser" concept -- essentially the idea is that if you have a well-drafting chimney, the insulated "riser" can be conceptualized as an "insulated afterburner / secondary burn area" rather than as a draft producer, and all or a portion of it could be horizontal. Perhaps a short horizontal run followed by a short riser (or vice versa -- short riser followed by a short horizontal run) would combine to give you the full length of insulated secondary burn area without going too tall. If a batch box riser can handle a short jog to the left before heading up, then the 'broken riser' concept would allow for a shorter riser in my confined space. And the fire would still linger in the insulated riser long enough to get complete combustion. It would rely on chimney draft, but wouldn't temperature difference between the insulated riser and the steel bell (design below) also drive the system? The shortened riser would allow for a radiator the shape of the fireplace, and within the radiator I could implement 3 chambers as bells, before exiting to the chimney. Might this work? None of this changes my cost of materials much, but might the exposed steel radiator get more heat into the room? Seems like since the fireplace is open in this design, convection would naturally pull cool air into the fireplace, up around the radiator, and back into the room. I don't know.
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Post by DCish on Jul 2, 2014 10:49:35 GMT -8
Donkey at one point posted the numbers regarding what factors influence draft (sorry, I can't search the forum when I'm away from home to give you the link), and as I recall, the effect of adding a few feet of chimney height was far more dramatic than the effect of the kinds of temperature differentials rocket stoves are able to achieve over the roughly 3-4 foot height of an insulated "riser" followed by a heat shedding "downer". So my guess is that, yes, you could get a bit of drive effect through the temperature differential, but it would be negligible compared to the chimney draft. However, it is possible that with less of the initial engine effect you may have to rely more on priming the chimney to light the stove than you would with a straight vertical riser, but that would only be known after the build, I'd think.
Based on your drawing, you would roughly double your radiator size if you are able to take it down to almost floor level. Stratification of gasses is more effective when there is more vertical height to stratify over, and a bell with larger volume means more dwell time during which heat can be exchanged. You could nix the baffles, jack up the core by the height of a brick or two to allow airflow under it, extend the bell (heat exchanger) down to the level of the bottom of the core, and extend your plunger tube down close to the bottom of the bell. This would give a taller bell for better stratification, more bell volume, more surface area, and fewer internal obstructions to restrict flow -- win-win-win-win, methinks.
I'm extrapolating based on lots of reading... any more experienced builders have some critical feedback?
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