hanee
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Post by hanee on Nov 16, 2018 15:39:00 GMT -8
batchrocket.eu states that the firebox depth should be between base*4 and base*5.5.
Is there a reason to prefer one extreme or the other?
I've read in other threads that making the firebox deeper is less efficient (though I've also heard it would extend burn times, something I might actually need in my application) -- does this mean stretching it to base*5.5 or does it mean stretching it beyond these stated design limits? Also, is it the depth of the firebox itself that makes it more inefficient or is it the increased length of wood in proportion to the system size?
Also, I've read Peter state in other threads that fuel loaded into the firebox should allow for 2" of airspace both in front and behind (and that not leaving space will just result in a less efficient burn). Is this recommendation general or is it perhaps assuming an average 6-8" system's firebox? Does it apply to all system sizes or is it proportional to overall system size or depth?
(i.e. does both a 4" system with a minimum sized 11.6" deep firebox and an 8" system with a maximum sized 31.8" deep firebox ideally have the same 2" of airspace on both sides of the fuel load?)
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Post by peterberg on Nov 17, 2018 1:30:37 GMT -8
batchrocket.eu states that the firebox depth should be between base*4 and base*5.5. Is there a reason to prefer one extreme or the other? No particular reason, both should work as intended. Mark, however the stated extended size is 25% more, that would be base*5. My preference: the short version, easier to light on top/rear, less prone to shifting fuel in the port. I've read in other threads that making the firebox deeper is less efficient (though I've also heard it would extend burn times, something I might actually need in my application) -- does this mean stretching it to base*5.5 or does it mean stretching it beyond these stated design limits? Also, is it the depth of the firebox itself that makes it more inefficient or is it the increased length of wood in proportion to the system size? It means stretching it to base*5 wouldn't affect burn quality. Anything above that isn't tested so that's unchartered territory. Efficiency of a larger firebox is somewhat lower in the sense that the chimney temp will be higher. So it's the greater fuel load in proportion to system size. Don't try to pursue longer burn times, it's different with these combustion cores. It is also stated several times the burn time will be marginally affected by more fuel. Also, I've read Peter state in other threads that fuel loaded into the firebox should allow for 2" of airspace both in front and behind (and that not leaving space will just result in a less efficient burn). Is this recommendation general or is it perhaps assuming an average 6-8" system's firebox? Does it apply to all system sizes or is it proportional to overall system size or depth? The recommendation is assuming a 6" to 8" system since these are tested. Larger systems tend to be more stable, smaller systems more finicky. Probably due to the more favourable surface/volume ratio. Surface area (inside the firebox) goes up linear while scaling up, volume exponentially. Air space isn't a fixed number since it is greatly influenced by the back end of the system, heat extraction by bells for example and quality of the chimney.
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hanee
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Post by hanee on Nov 17, 2018 9:05:13 GMT -8
Thanks for the clarification. Apologies for the additional questions; I am trying to wrap my head around scaling these systems down. Efficiency of a larger firebox is somewhat lower in the sense that the chimney temp will be higher. So it's the greater fuel load in proportion to system size. The chimney temp being higher just means that the bell ISA isn't correctly sized to recover that amount of additional BTU's being burned, correct? Don't try to pursue longer burn times, it's different with these combustion cores. It is also stated several times the burn time will be marginally affected by more fuel. At 30-60 minutes, I think the burn times are fine, in fact perfect for most cooking (which usually begins with high BTU and ends with low BTU simmers). What I am really pursuing is more frequent burns (lower BTU per load, thus the small system size) or occasionally in shoulder seasons "partial loads" (or softwoods) if needed to achieve that frequency. Since cooking is a prime goal I need at least 2 burns, preferably 3. But, I need those to add up to only between 0.6-1.2kw mean daily output, depending on weather. My only other option I can find to achieve that low output in multiple sessions would be a small conventional wood stove, the kind they use in boats and tiny houses, none of which are efficient or low emissions relative to rocket combustion cores. Not only that, but if people think the firebox in a 4" system begins to be unusable they should compare it to these tiny house stoves: the one most sized to my anticipated BTU needs requires a 5.75" wood length and a 3" flue. I'm sure that is not any more inconvenient or likely to be inefficient than even a 3" BBR/DSR2. The point is, that I am pretty sure that a sometimes less efficiently fired DSR2 (due to a partial load) will still be far more efficient and safe (due to lower chimney temp) than any conventional wood-burning/cooking option. Correct me if I'm wrong. (My only other conceivable option being to have a separate cook stove, which ideally achieve the same efficiencies and generate only the BTU needed for cooking with no excess heat and yet reasonable temperature stability: a challenging design. Perhaps it could be done with an extremely small DSR2/BBR fueled with charcoal harvested from the primary heater during the heating season to allow more stable burns. I may start a post about this and would love to experiment but I am concerned it is beyond my level of experience/understanding. On permies there is a recent post with someone building a supposedly successful 2.5" BBR (https://permies.com/t/95083/Smallest-batch-box-feasible), but obviously they've not run a testo on it) The recommendation is assuming a 6" to 8" system since these are tested. Larger systems tend to be more stable, smaller systems more finicky. Probably due to the more favourable surface/volume ratio. Surface area (inside the firebox) goes up linear while scaling up, volume exponentially. Is the favorability of low-surface high-volume a matter of air-flow issues or is it just about maintaining proper temperature in the firebox (i.e. greater surface area in proportion to volume translates greater heat-loss in proportion to loaded fuel -- a problem that could perhaps be mitigated through more insulation)? Air space isn't a fixed number since it is greatly influenced by the back end of the system, heat extraction by bells for example and quality of the chimney. Based on this I'm wondering what would define a "partial load" in terms of consequences to efficiency. Clearly there is some wiggle room in terms of length of wood depending on system design (in my case, I'd be going for the smallest load size, so shortest wood that still could burn efficiently). I'm assuming based on batchrocket.eu site saying "Leave at least 50 mm (2") free between the fuel and the ceiling of the firebox" that (a) this is again based on 6-8" systems and (b) there is also an "at most", which means another opportunity for scaling down the fuel load without significant adverse consequence? All of this leads me to question what the real design-limit is as to how little BTU of wood can be burned effectively in a given system size when the system has been designed for that purpose (down-sized bell, shortest possible firebox, highest possible insulation, plus whatever tweaks to air supply/chimney are needed). I'm considering all these things only because I've been told that the 4" size is most likely at the lower limit of a BBR (or J-tube), but if you feel that going below 4" is more likely to be successful than "under loading" a 4" system, then I perhaps should be attempting to build one of those systems tightly to the standard rather than exploring "under loading".
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Post by peterberg on Nov 17, 2018 9:20:32 GMT -8
There's a 4" batchrocket heater that would fit your needs, in my opinion. What it doesn't do is cooking at the same time although it sports a steel top so you could heat water on it. You seem to be over thinking the whole matter, adding more stringent specifications as you go. And at the same time asking more questions than one would care to answer. Smallest practical size is 4", due to the afore mentioned unfavorable ratios. Have a look at the concept of Rocket Mass Heaters Portugal. And their Facebook page www.facebook.com/BatchRocketPT/
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Post by martyn on Nov 17, 2018 10:21:22 GMT -8
How do you intend to do the cooking, I mean on a hot plate or in a closed over?
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Post by satamax on Nov 17, 2018 11:34:29 GMT -8
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hanee
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Post by hanee on Nov 17, 2018 12:27:15 GMT -8
How do you intend to do the cooking, I mean on a hot plate or in a closed over?
Hi Martyn, I'm aiming at a hot plate/cooktop. Enough room for two small pans preferably. It would be replacing a typical 2-burner coleman propane range.
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Post by martyn on Nov 17, 2018 12:48:36 GMT -8
The main issue I came across while revising rocket stoves for cooking was the required hight of a cook plate. I got around the issue by building a 6” J tube and partly sinking the unit down in the floor but that might not be a optsion for you. Your proposed 4” system might still need quite a high heat riser?
I think Peter has a new design very nearly operation to his standards that uses a low heat riser, that might be interesting to you.
Personally I wanted a top feeding fire that can be regulated with the amount of wood so to help with cooking temps. I love cooking on my stove, I stand at my satsion and feed the fire as nessary to get the temperatures I require. I can get a steady 200c in about 20 mins from light up, nice for a lot of cooking but I like more for some things, luckily the plate will go much higher if I push it with lots of small dry wood.
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hanee
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Post by hanee on Nov 17, 2018 13:27:12 GMT -8
There's a 4" batchrocket heater that would fit your needs, in my opinion. What it doesn't do is cooking at the same time although it sports a steel top so you could heat water on it. You seem to be over thinking the whole matter, adding more stringent specifications as you go. And at the same time asking more questions than one would care to answer. Smallest practical size is 4", due to the afore mentioned unfavorable ratios. Have a look at the concept of Rocket Mass Heaters Portugal. And their Facebook page www.facebook.com/BatchRocketPT/Hi Peter, You're right, the heating problem is very straightforward to solve. Just build a 4" system of any design and fire less frequently.
It is the cooking problem *plus* small system size that adds complexity. I'm actually looking to solve that. Either through two separate units or through a single unit. It appears to me quite possible to design a 4" system with a DSR2 of reasonable cookstove height and a very functional cookstove. I had thought the idea of the forum was to help define/refine the exact parameters and limits of this technology and that very specific questions were preferable to general advice or ready-made designs. Also, of course, you're certainly not required to respond to my post if you find it troublesome. Perhaps I have the tone wrong and will try again.
1. How "bad" (inefficient/unsafe/polluting) would it be to habitually fire a 4" system with small amounts of fuel (as compared to a TLUD or normal wood stove)? And, if it's not a good idea, then I assume I should separate cooking/heating, : 2. Can anyone put forward their opinion as to the most efficient option for year-round, indoor wood-fueled cooking needs? (These questions are to anyone interested in answering them, and it's also fine if no one does. I appreciate these forums have learned a lot and look forward to continuing to do so.)
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Post by peterberg on Nov 17, 2018 13:45:52 GMT -8
1. How "bad" (inefficient/unsafe/polluting) would it be to habitually fire a 4" system with small amounts of fuel ( as compared to a TLUD or normal wood stove)? No problem, you could fire it using half loads (or less) without compromising on safety or clean burning. Keep in mind every burn is running the same amount of time, irrespective of the fuel load. As long as one can manage to keep the rocket sound audible to be sure the afterburner flame is going there won't be a problem at all. A full load burns much faster and delivers more power than half a load in the same time frame. Whether or not efficiency is lower depends on how the heat extracting end is organized. Bell ISA could be larger than strictly needed, in combination with a bypass one could regulate the chimney temperature. Before you ask more questions about why every run is over and done in the same time frame: given the same fuel and other circumstances like wind or temperature between indoors and outdoors, half a load takes the same time as a full load. Why it does that is still not fully understood.
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hanee
New Member
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Post by hanee on Nov 17, 2018 13:52:37 GMT -8
The main issue I came across while revising rocket stoves for cooking was the required hight of a cook plate. I got around the issue by building a 6” J tube and partly sinking the unit down in the floor but that might not be a optsion for you. Your proposed 4” system might still need quite a high heat riser? I think Peter has a new design very nearly operation to his standards that uses a low heat riser, that might be interesting to you. Personally I wanted a top feeding fire that can be regulated with the amount of wood so to help with cooking temps. I love cooking on my stove, I stand at my satsion and feed the fire as nessary to get the temperatures I require. I can get a steady 200c in about 20 mins from light up, nice for a lot of cooking but I like more for some things, luckily the plate will go much higher if I push it with lots of small dry wood. Hi Martyn,
Yes, agreed, it's the riser height. The DSR2 (double shoebox rocket 2) that peter is currently finishing up the testing/design phase of fixes the riser height problem, which is why I am following that thread closely and have spec'd out my core based on his current measurements (and why my questions in this post are based on using a DSR2 core).
I've thought about the J-Tube as my solution several times -- as it seems to be more of a steady-feed like you described. Sounds good in many ways. 6" would be too big though I think for my needs in terms of output. 4" could do, but everyone talks them down as being less reliable than 4" batchbox or DSR.
Your system looked highly functional when I read your thread a few days ago (beautiful too!), but I believe you said you're using it outdoors or in a large uninsulated space? I've also seen Donkey's incredible outdoor cookstoves. They all seem great but not sure how to scale them down to cooking oatmeal for 2 in a 192sqft well insulated house, indoors, 3x a day .
Meanwhile there are tons of backpackers and campers using typical TLUD or "rocket" stoves to boil a single quart of water, but my understanding has been they are going to create a lot more smoke and not burn very efficiently and only a few of these designs would even work with a chimney and all the other safety features you'd want indoors. That's why my separated-cookstove ideas tend to lean towards using homemade charcoal as a fuel source (it would remove a lot of fuel variables).
I get the impression that those looking to cook full-time (or even half-the year full time) on their stoves are in the minority, though I think a lot of newcomers (like myself) are excited about the possibility. In my case I'm trying to not treat it like a possibility but as a requirement. Definitely sick of burning propane. I barely need electricity and have no running water: I should be able to figure out how to cook with wood. Most homesteaders just go with a wood burning stove but I was hoping to not wantonly cut down any more trees than I have to .
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hanee
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Post by hanee on Nov 17, 2018 14:06:28 GMT -8
1. How "bad" (inefficient/unsafe/polluting) would it be to habitually fire a 4" system with small amounts of fuel ( as compared to a TLUD or normal wood stove)? No problem, you could fire it using half loads (or less) without compromising on safety or clean burning. Keep in mind every burn is running the same amount of time, irrespective of the fuel load. As long as one can manage to keep the rocket sound audible to be sure the afterburner flame is going there won't be a problem at all. A full load burns much faster and delivers more power than half a load in the same time frame. Whether or not efficiency is lower depends on how the heat extracting end is organized. Bell ISA could be larger than strictly needed, in combination with a bypass one could regulate the chimney temperature. Before you ask more questions about why every run is over and done in the same time frame: given the same fuel and other circumstances like wind or temperature between indoors and outdoors, half a load takes the same time as a full load. Why it does that is still not fully understood.
Sounds like the 4" DSR2 cookstove is worth a trial run. I picked up firebrick splits today and a glass cooktop, a cast iron stove door and a metal barrel at the dump. Not the best weather for doing test builds outside but I will start digging in anyway after thanksgiving. Aiming for something similar to the Allerton Alley DSR, but without the risk of overrun.
A nice plus for the 4" system is that the standard 9" long bricks work perfectly to span the top of the 5.78" firebox! One less problem to solve. I will make any additional updates in an appropriately titled thread.
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graham
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Posts: 74
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Post by graham on Nov 17, 2018 14:29:49 GMT -8
Before you ask more questions about why every run is over and done in the same time frame: given the same fuel and other circumstances like wind or temperature between indoors and outdoors, half a load takes the same time as a full load. Why it does that is still not fully understood. I roasted some pork belly in my rocket stove oven yesterday. It was 600 g instead of 1200 g and it still took the same time, which was 1.5 hours. I think the analogy also applies to your firebox where you are cooking wood. Once you reach the maximum temperature of your firebox, the wood is going to take the same time to cook regardless of whether it's half or completely full.
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Post by coastalrocketeer on Nov 17, 2018 14:46:58 GMT -8
half a load takes the same time as a full load. Why it does that is still not fully understood. I suspect this has something to do with the larger amount of fuel surface combusting, building to more intense heat in the batchbox's "gassification area", creating faster outgassing of the remaining fuel, and accelerating the latter 2/3 of fhe burn of the larger load significantly. Thus, the loads of different size wind up consuming themselves in the same amount of time. I could be totally off base in this, but that is my notion of it.
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