jrl
Junior Member
Posts: 101
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Post by jrl on Jun 20, 2014 7:27:13 GMT -8
I'm working on a design to build a rocket heater with a fire brick J tube and riser directly in my fireplace. Rather than use a barrel as a vessel for radiating heat and then exhausting to my masonry chimney (no ducting through cobb or anything in this setup), I'd like to use the fireplace itself as the radiating vessel. By this I mean that the front of the fireplace would be plated up with steel like a fireplace insert would be. From the front you would see only the feed tube and the steel facing and maybe an access door or cleanout port on the front of the feed tube. Inside the fireplace (already lined with fire brick), the riser would blow hot exhaust into the fireplace chamber. The damper up in the fireplace would be replaced with a steel sheet. On one side of the fireplace it would be walled up with an opening near the bottom to basically make the fireplace a bell that can only exhaust under the side wall. The little side chamber would exhaust up the chimney flue.
A picture is worth a 1000 words, and I drew a crude drawing in MS Paint, but the attachment feature is not working. I get "Error: This forum has exceeded its attachment space limit. Your file cannot be uploaded." I will try to post it later.
What do you think the viability of this type of system is? My observations are that the whole, all-masonry fireplace would act as big thermal mass like any fireplace. The steel front facing would radiate heat into the room. Adding a blower tube in and out of the chamber would convect more heat into the room. The side wall with bottom exhaust port would make the whole fireplace chamber a big bell.
Do you think I should still put some sort of barrel or tank over the riser to get that 2" gap concept, and just leave the bottom of it open so it exhausts into the fireplace chamber below the barrel?
The masonry chimney has good draft and should always create negative pressure in the system (theoretically), but I don't want this thing back puffing into the living space if the wind blows or something. Do you think sealing the front facing will be sufficient to keep the CO and exhaust within the fireplace?
My objectives are:
- Utilize the fireplace (currently never used)
- Add supplimental heat for spring and fall in my living room (I burn anthracite coal in a big stove on a bigger hearth downstairs in the winter)
- Achieve complete combustion of material via rocket
- Use scrap wood and tree branches from some trees felled last year, nicely seasoned and dry now, but will rot in a couple years if I don't use it.
My constraints are:
- Money - I only want to spend a few hundred.
- Space - The fireplace is small (36 W up front x 24 W in back x 30 tall x 21 deep). This prohibits the use of a large barrel and would make for a really small CSA and riser to fit the space if I can't use the chamber itself as the barrel. My using the fireplace as the vessel, I can put a 6 inch system in there.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 20, 2014 7:31:21 GMT -8
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Post by matthewwalker on Jun 20, 2014 7:40:54 GMT -8
I like it. I think it could work well without the barrel, just as you propose. Cue Max posting his fireplace sketchup design in 3, 2, 1....
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Post by satamax on Jun 20, 2014 8:37:36 GMT -8
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Post by matthewwalker on Jun 20, 2014 8:50:35 GMT -8
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 20, 2014 10:13:40 GMT -8
Thanks for the info. By plunger tube you mean something like a piece of stove pipe extending down from a flange int the damper plate to near the bottom of the fireplace (instead of building a wall to do that)? I read it's supposed to be 4 times the size of the exhuast of the J-Tube. For a 6 inch J-Tube, I'd need a pipe a little over 12". Is that accurate? Would two 6-inch pipes suffice? Also, I'm getting confused about ISA calculations for the bell. If my whole fireplace is the bell, then I need to add up the surface area of all walls, floor, and ceiling to determine the ISA of the chamber. Is that correct? And then how would I know if a 5 or 6 inch J-Tube will support that size? What would be the effect if the chamber being too big or too small? Less/more draft? I read the system could stall, but I don't see how that's possible with the constant negative pressure provided by the chimney itself. Can you point me to a resource on ISA and why it's relevant here?
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Post by satamax on Jun 20, 2014 10:46:32 GMT -8
Ouch, point you out to a resource? donkey32.proboards.com Well, in a bell ISA, you don't account for the floor of the bell. But you add up all other surfaces. 4m² ISA for a 6 inch J tube is good, 6m² ISA for a 6 inch batch box. Your plunger tube or flue, or chimney doesn't need to be any bigger than system size. Usualy it's the heat riser which is 3 to 4 times the lengh of the feed tube. That's what you might have read. Or may be about the barrel to flue transition area. If you use your damper as a bell top, insulate it above, so you don't loose too much heat. The advantage with a plunger, is that you can raise the bottom from the floor ( by cutting it of having a sleeve which rises up ) to adjust your isa, and stalling point. Start at 3 inches from the floor i'd say. And see what happens.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 20, 2014 11:37:06 GMT -8
Sorry, I meant an article or other thread that explains the relationships to ISA in a bell, so that I could do the research without making you explain everything to me. Can you tell me how raising or lowering the plunger tube affects system stall and how it relates to the ISA of the bell?
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Post by satamax on Jun 20, 2014 13:01:45 GMT -8
Well, when the hot gases enter the "chimney" (plunger tube in this case.) they don't go any further down, only cooler gases go lower (well, theere's still a temp gradient, but that's trivial) So you could consider the portion of the walls lower than the entry point of the chimney, as non working surface. It doesn't extract heat from the hot gases. So if you raise the tube, ISA decrease, and so does the heat extraction, And if you lower the tube, it will increase the ISA and the heat extraction.
If the bell is still hot, there will be less heat extraction from the gases, hence more heat left in the gases, to power the stack effect of the chimney. So it could be intresting to have an adjustable plunger tube. Plus, to start the draft, it's better to light a piece of newspaper or denatured alcohol into the bell underneath the "chimney" , before you start your fire. Hence the need for a door.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 20, 2014 16:45:50 GMT -8
Well, when the hot gases enter the "chimney" (plunger tube in this case.) they don't go any further down, only cooler gases go lower (well, theere's still a temp gradient, but that's trivial) So you could consider the portion of the walls lower than the entry point of the chimney, as non working surface. It doesn't extract heat from the hot gases. So if you raise the tube, ISA decrease, and so does the heat extraction, And if you lower the tube, it will increase the ISA and the heat extraction. If the bell is still hot, there will be less heat extraction from the gases, hence more heat left in the gases, to power the stack effect of the chimney. So it could be intresting to have an adjustable plunger tube. Plus, to start the draft, it's better to light a piece of newspaper or denatured alcohol into the bell underneath the "chimney" , before you start your fire. Hence the need for a door.
Sweet! Thanks for the explanation.
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Post by satamax on Jun 20, 2014 21:53:52 GMT -8
JRL, have you read the Overhelmed thread by Natasha? Pretty much the same question!
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 23, 2014 11:49:58 GMT -8
I have a question about the J-Tube design. From a few tests I've run outdoors with a brick structure (not mortared), I've observed that the burn tunnel needs to be a smaller dimension (in CSA) than the heat riser in order to get any sideways burn/rocket action. When the tunnel was around the same CSA as the riser, I couldn't get it to draft at all. When I restricted the burn tunnel to around 66% of the heat riser CSA I got great draft and rocket effect. I've read "the book" and I haven't found anythign that says what the optimal ratio of burn tunnel CSA to riser CSA is. How much smaller in CSA should the burn tunnel be than the heat riser? There must be a point where it becomes too small to be effective.
Also in "the book" it says that the burn tunnel should be as short as possible, but all the pictures I see of RMH designs show these really long burn tunnels. And, all of Peterburg's dimensions are related to batch box design. How long should the burn tunnel be on a standard J-Tube. I ask because I'd like to make it short, really short, like 2-4 inches because of my space constraints on this project. Does having a longer burn tunnel add anything to the system? I would think they are only longer because everyone's trying to fit a barrel dead center on the heat riser and they need the space.
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Post by satamax on Jun 23, 2014 16:03:30 GMT -8
Hi JRL, well, not mortared rockets usualy don't work that well. Did you insulate it? Force the draft at first, because the bricks weren't hot? Basicaly, the restriction of the feed tube is given by the sticks of wood usualy, and the rocket is constant CSA, i mean in normal design. Some have a narower burn tunnel, but it's usualy marginal. There's one thing thought, because of the way most of the people lay bricks, the feed tube, burn tunnel and heat riser are sometimes of diferent sectional shape. Rockets, in my experience, don't like this much. For example squarish feed tube and heat riser, and a low flat rectangular shape for the burn tunnel. That's no good. For the dimensions here you go donkey32.proboards.com/thread/1170/tube-dimensionsTwo to four inches burn tunnel is possible. Thought my prototype using about 4 didn't work well. I'm not absolutely sure, but i think in this case, you need to lenghten the riser, to have a longer "insulated" burning time, but that's just gut feeling.
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jrl
Junior Member
Posts: 101
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Post by jrl on Jun 24, 2014 5:54:54 GMT -8
Hi JRL, well, not mortared rockets usualy don't work that well. Did you insulate it? Force the draft at first, because the bricks weren't hot? Basicaly, the restriction of the feed tube is given by the sticks of wood usualy, and the rocket is constant CSA, i mean in normal design. Some have a narower burn tunnel, but it's usualy marginal. There's one thing thought, because of the way most of the people lay bricks, the feed tube, burn tunnel and heat riser are sometimes of diferent sectional shape. Rockets, in my experience, don't like this much. For example squarish feed tube and heat riser, and a low flat rectangular shape for the burn tunnel. That's no good. For the dimensions here you go donkey32.proboards.com/thread/1170/tube-dimensionsTwo to four inches burn tunnel is possible. Thought my prototype using about 4 didn't work well. I'm not absolutely sure, but i think in this case, you need to lenghten the riser, to have a longer "insulated" burning time, but that's just gut feeling. Ok, I will try a full 6 inch throughout system tonight again and compare. Maybe there was an obstruction or something last time. Last night I sealed up the feed tube, burn tunnel, and first part of the riser by skim-coating it with mud. This made a big difference in performance obviously. Do you know what kind of difference insulating the core has on performance? Everyone says to insulate it, and I plan to, but how does a non-insulated j-tube and riser compare to an insulated one? Are flames going to start shooting out the riser or something? Right now the flames make it about 2-4 inches into the riser, there's no smoke, and the exhaust is hot, but certainly doesn't feel like 2400 degrees or anything. How far should I expect the flames to travel into and up the riser?
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Post by satamax on Jun 24, 2014 9:37:22 GMT -8
Hi JRL, well, not mortared rockets usualy don't work that well. Did you insulate it? Force the draft at first, because the bricks weren't hot? Basicaly, the restriction of the feed tube is given by the sticks of wood usualy, and the rocket is constant CSA, i mean in normal design. Some have a narower burn tunnel, but it's usualy marginal. There's one thing thought, because of the way most of the people lay bricks, the feed tube, burn tunnel and heat riser are sometimes of diferent sectional shape. Rockets, in my experience, don't like this much. For example squarish feed tube and heat riser, and a low flat rectangular shape for the burn tunnel. That's no good. For the dimensions here you go donkey32.proboards.com/thread/1170/tube-dimensionsTwo to four inches burn tunnel is possible. Thought my prototype using about 4 didn't work well. I'm not absolutely sure, but i think in this case, you need to lenghten the riser, to have a longer "insulated" burning time, but that's just gut feeling. Ok, I will try a full 6 inch throughout system tonight again and compare. Maybe there was an obstruction or something last time. Last night I sealed up the feed tube, burn tunnel, and first part of the riser by skim-coating it with mud. This made a big difference in performance obviously. Do you know what kind of difference insulating the core has on performance? Everyone says to insulate it, and I plan to, but how does a non-insulated j-tube and riser compare to an insulated one? Are flames going to start shooting out the riser or something? Right now the flames make it about 2-4 inches into the riser, there's no smoke, and the exhaust is hot, but certainly doesn't feel like 2400 degrees or anything. How far should I expect the flames to travel into and up the riser? www.youtube.com/watch?v=dM-GbmlVGXo
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