Cramer
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Post by Cramer on Dec 17, 2013 18:42:54 GMT -8
...When you stated the number of cubic feet the bell should contain, were you referring to inside or outside dimentions? The volume I am referring to is always inside. In theory, that's correct. So, when you are calculating the total volume, first and second bell together, try to end up with a figure that is around 40 cubic feet max. My 6" double bell system had a total volume of 25 cu feet and produced a very low output temperature. Approximately 104 F for the first 60 minutes, starting from cold. Yours would be an 8" system, about 1.77 times as large as mine. The volume multiplied with 1.77 makes 44.25 cu feet. Regard that as a maximum without safety margin. |
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Cramer
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Posts: 129
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Post by Cramer on Dec 17, 2013 18:44:06 GMT -8
The volume I am referring to is always inside. In theory, that's correct. So, when you are calculating the total volume, first and second bell together, try to end up with a figure that is around 40 cubic feet max. My 6" double bell system had a total volume of 25 cu feet and produced a very low output temperature. Approximately 104 F for the first 60 minutes, starting from cold. Yours would be an 8" system, about 1.77 times as large as mine. The volume multiplied with 1.77 makes 44.25 cu feet. Regard that as a maximum without safety margin. |
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Cramer
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Posts: 129
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Post by Cramer on Dec 17, 2013 19:00:23 GMT -8
I do hope I have used the quote thing correctly, I was really only after this small snippet so I guess I did not. Peter, you said "My 6 [inch] double bell system had a total volume of 25 cu feet and produced a very low output temperature. Approximately 104 F for the first 60 minutes, starting from cold." A 3X3X3 foot cube has 27 cubic feet. Is this correct? Not my calculation of cubic feet but your statement of the bell volume for a 6 inch burner. If so it is really very much the information I have been seeking. I have been armchairing the subject of rocket heaters and stoves for a couple of years now. (Armchairing is a colloquialism Peter, meaning: I have not really been building or testing just opining to myself and reading, rather like an "armchair quarterback" in American football). I know English is not your primary language so I did not want to confuse. (That being said you have a very good mastery of it most of the time). I asked the question in another thread about an abbreviation regarding surface area but after getting the answer I just could not wrap my head around the idea. Cubic feet I can deal with though so a 6 inch setup would use a bit less than 25 cubic foot of volume as the bell? Would one make the bell slightly larger using a single bell since the interior surface area (ISA) would not be including the extra surface of the division between the two bells in the double bell system? Or have I completely misunderstood?
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Post by Dan (Upstate NY, USA) on Dec 18, 2013 2:27:46 GMT -8
Well you could have a bell of 25 cubic feet that is a cube and this would have the minimum ISA. Or it could be a narrow, long bell which would have a much larger ISA for the same amount of 25 cubic feet. The narrow long bell would give off much more heat than the cube even though both have the same 25 cubic feet inside.
See?
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Post by peterberg on Dec 18, 2013 11:47:31 GMT -8
Yes, Wolf is right. At first I used the volume to define the bell size. But that is utterly wrong, the internal surface area is the base of the measuring system. And because the exhaust opening is in the side of the bell close to the bottom, the floor of the bell doesn't count much for heat extraction. There's an exception, though. When the exhaust opening is in the floor of the bell, the hot gases will touch the floor as well.
Armchairing the subject of rocket stoves, funny expression!
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Cramer
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Posts: 129
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Post by Cramer on Dec 18, 2013 15:49:37 GMT -8
Okay, that completely clears up the question, except I do not recall after reading the thread a recommended ISA of a 6 inch system. I was thinking before the question was cleared up that the cubic footage was the determining factor but now I see that if I increase the ISA I can use a larger (maybe 8 inch) system with more output and that if the cube shape is not used but rather a longer more narrow shape with a greater internal surface area (ISA) is used then the 6 inch system would likely stall?
So the question then becomes what is a rule of thumb on the ISA of a 6 inch system and is the ISA linear in regards to proportion when up-sizing? Another question comes to mind here that is inextricably linked to this question. What would be the effect of insulating the exterior of the bell as in providing a double walled bell with an insulating air space between the interior bell that gasses pass through and an exterior "shell" that the gasses would not pass through? I ask this only as a hypothetical, not that I would plan to do such a thing. But for fine tuning a belled system it seems to me that if the bell is made with too large an ISA thus causing the system to stall because the chimney will not draw then insulating one wall would have the same affect as decreasing the ISA because thermal energy would not be expended as rapidly as it would through a non insulated wall. Is this assumption on the mark or off base?
Which, alas brings me to another question, does the material of the bell affect the calculation of ISA? If dense fire brick is used with more heat adsorption capability would the ISA calculation then be smaller than, for example using red brick for the bell? And if a steel "lid" is used for the bell which gives up heat readily then should the ISA be reduced by some margin?
Peter, "armchairing"... just an attempt to impart a little information to you (very little) in return for your tireless efforts to all of us. I am glad you enjoyed it.
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Post by satamax on Dec 19, 2013 2:10:11 GMT -8
Yes, Wolf is right. At first I used the volume to define the bell size. But that is utterly wrong, the internal surface area is the base of the measuring system. And because the exhaust opening is in the side of the bell close to the bottom, the floor of the bell doesn't count much for heat extraction. There's an exception, though. When the exhaust opening is in the floor of the bell, the hot gases will touch the floor as well. Armchairing the subject of rocket stoves, funny expression! Peter, here's a question for you, The bell with the least ISA would be round, square would have a slightly larger ISA, rectangular even more; for a given volume. Star shaped would have an even greater ISA for the volume. I have thought about this, since i've put the green machine in the workshop, and noticed that despite being all metal, it keeps heat. I'm under the impression that a normal bell, without the burner inside should keep even more heat, since the hot gases tend to remain up the top of the bell, when they aren't disturbed. Normal cooling of the gases on the walls of the bell occurs, then they get dragged down slowly. Temperature between the center of the gas volume and the walls of the bell evenize. And it all cools down because of the heat release of the outside walls of the bell. But gases not being very good at transfering heat, they kind of insulate themselves. When the walls of the inside of the bell are at near equal temps as the gases. The cooling occurs slowly, keeping heat also in the bell. So we have another way for heat storage. I wonder which is the most intresting, a large volume small isa, where the heat is slowly released to the dense material walls, because there's less exchange surface storing the heat inside the bell, which can be cooled by the stream of colder gases comming from the burer because of the draft. Or smaller volume, bigger isa bell, where the radiating surface is greater, transfering more heat to the walls, and letting the inside cool faster. I know that, anyways this happend, there's a tendency to evenisation between inside bell temps, bell wall temps, and outside of the bell temps. But i wonder if there's any interest in making a square versus round, versus rectangular bell. I know it's all mater of compromise. But have you given a thought about this? Cramer, latest "consensus" For a 6 incher J tube, 4m² or slightly more than 40sqf For a 6 incher batch, 6m², or slightly more than 60sqf |
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Dec 19, 2013 4:03:38 GMT -8
Thanks satamax, useful information. I will check back this evening for other answers, should any be forthcoming. I feel rather honored to have the conversation moved with the colloquialism in the title  |
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Post by peterberg on Dec 19, 2013 8:57:38 GMT -8
Okay, that completely clears up the question, except I do not recall after reading the thread a recommended ISA of a 6 inch system. I was thinking before the question was cleared up that the cubic footage was the determining factor but now I see that if I increase the ISA I can use a larger (maybe 8 inch) system with more output and that if the cube shape is not used but rather a longer more narrow shape with a greater internal surface area (ISA) is used then the 6 inch system would likely stall? Very likely yes. So the question then becomes what is a rule of thumb on the ISA of a 6 inch system and is the ISA linear in regards to proportion when up-sizing? This is a tricky question. Up to this day I don't know of an 8" batch burner used inside a large bell system. The accepted maximum seems to be 6 m² for a 6" system. A poor chimney would reduce this number significantly. How the bell ISA should be scaled, linear or exponentially, I don't know. What would be the effect of insulating the exterior of the bell as in providing a double walled bell with an insulating air space between the interior bell that gasses pass through and an exterior "shell" that the gasses would not pass through? Depends on the material of the bell, steel bells would tend to get hotter inside and the stack temperature would rise accordingly. Brick bells are slower to take up heat and pass through. That would result in slower dissipation of heat by the outer shell. But for fine tuning a belled system it seems to me that if the bell is made with too large an ISA thus causing the system to stall because the chimney will not draw then insulating one wall would have the same affect as decreasing the ISA because thermal energy would not be expended as rapidly as it would through a non insulated wall. Is this assumption on the mark or off base? I think you are right, shielding part of the bell's inside area will result in less absorpsion of the produced heat. Which, alas brings me to another question, does the material of the bell affect the calculation of ISA? If dense fire brick is used with more heat adsorption capability would the ISA calculation then be smaller than, for example using red brick for the bell? Strange enough, the material isn't playing such a large role here, or so it seems. I have three stacked 55 gallon drums as the heat exchanger, together with a brick column to support the fire box it's got an ISA of 5.5 m². The barrels do shed lots of heat, totally uncomparable with a brick bell. But the supported ISA isn't that much smaller, surprisingly. And if a steel "lid" is used for the bell which gives up heat readily then should the ISA be reduced by some margin? That margin wouldn't be large as you might think. All this has something to do with the way the material is tranporting the heat. The metal bell recieves its heat from the gases inside, no difference to a brick bell there. Inside the metal heat is transported by conduction, outside the material by radiation helped by convection of the heated air. That latter effect isn't the case with brick, the heat is transported by conduction. Probably the only transport for one or two hours, depending on the thickness of the material. After that, the heat leaves the brick by radiation again, but at a much lower rate as compared to the steel example. Not much convection there, and a lower infrared wavelength. The effect is that using whatrever brick for the bell wall, it's possible to burn two or more batches before the heat comes through. The changeover from conduction to radiation is shifted to a later point in time. Perhaps, in these circumstances the conduction through the brick is a bit slower, but not very much, as compared to the combined conduction and immediatly radiation of the thin steel. Interesting stuff isn't it? |
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Post by peterberg on Dec 19, 2013 9:14:01 GMT -8
I wonder which is the most intresting, a large volume small isa, where the heat is slowly released to the dense material walls, because there's less exchange surface storing the heat inside the bell, which can be cooled by the stream of colder gases comming from the burer because of the draft. Or smaller volume, bigger isa bell, where the radiating surface is greater, transfering more heat to the walls, and letting the inside cool faster. I know that, anyways this happend, there's a tendency to evenisation between inside bell temps, bell wall temps, and outside of the bell temps. But i wonder if there's any interest in making a square versus round, versus rectangular bell. I know it's all mater of compromise. But have you given a thought about this? Yes, but things are getting more and more complicated (which I don't like much). Let's say this: the bell csa should be at least 4 times the intake and exhaust channels. But in a single bell there's a riser spewing heat up to the ceiling which turns around and go down again along all the walls of the bell. This is principally different to an empty second bell where stratification do occur. So, I would say keep the first (single) bell round or square, i.e. uncomplicated. The second bell do enclose a less turbulent environment in order to get stratification, there are no violent up and down streams. This second bell could have any shape, as long as the rule of 1 to 4 is maintained. |
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Post by satamax on Dec 19, 2013 9:29:30 GMT -8
Thanks a lot Peter. Well, it's a complicated game. And as some say, Rome hasn't been built in a day. So may be with some of our efforts (look at my signature  ) We might know the rockets and bells better one day! I agree on the KISS side of things. |
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Cramer
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Post by Cramer on Dec 19, 2013 18:13:44 GMT -8
Peter said: Interesting stuff isn't it? My Reply: It most assuredly is!
This one quick question is a bit off topic but how do you quote the small snippets and reply to those individually? Edit: Never mind, I figured it out.
So, if the metal were as "thick as a brick" then the conduction rate would be marginally similar to that of the conduction rate of the brick bell wall? One may store heat a bit better than the other but they would store a similar amount? I think that is what I gleaned from the answer above but please correct me if I am in error.
Then the ISA would be pretty similar whichever substance is used for the walls of the bell. In a two bell system then the dividing wall between the two bells would allow one to use a smaller footprint than a single bell because that extra surface area, though it conducts heat but does not for the most part transmit it's heat through radiation (except from the warmer side to the cooler side before equilibrium is reached) does figure in to the ISA calculation. Correct?
I think I see why a batch box can use a larger ISA. More heat is dumped at once (uninterrupted by refueling) into the bell?
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Dec 19, 2013 18:26:27 GMT -8
Hey, I just thought of this! If I put two tea kettles on top of a double bell stove, when the water boiled, I would have "bells and whistles"...
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Dec 20, 2013 4:00:10 GMT -8
Well, I answered one of my own questions. The question about how to quote snippets of a post has been discovered!
That being behind me now I have a proposition for thought. Since the above is true, maybe the relation of surface area inside a bell has naught to do with absorption but everything to do with adsorption of the gas molecules upon the solid. Since adsorption is a surface phenomenon whereas absorption involves the action of the entire mass then the gas molecules must be adsorbing onto the surface of the material, being brick, glass, steel et. al. then being absorbed into the material and conducting through it like water being sucked into a paper towel.
To test the hypothesis one could create a smaller than optimum bell and "corduroy" the inside of it producing a greater surface area inside the bell effectively increasing the surface area for adsorption of the hot gasses and measure the output temperature of the flue gasses?
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Post by peterberg on Dec 20, 2013 11:54:20 GMT -8
So, if the metal were as "thick as a brick" then the conduction rate would be marginally similar to that of the conduction rate of the brick bell wall? One may store heat a bit better than the other but they would store a similar amount? I think that is what I gleaned from the answer above but please correct me if I am in error. The steel would shed the heat quicker, but that should be all. So the all steel stove would generate more power, everything else being equal. Then the ISA would be pretty similar whichever substance is used for the walls of the bell. In a two bell system then the dividing wall between the two bells would allow one to use a smaller footprint than a single bell because that extra surface area, though it conducts heat but does not for the most part transmit it's heat through radiation (except from the warmer side to the cooler side before equilibrium is reached) does figure in to the ISA calculation. Correct? Not entirely. A double bell system is about 15% more efficient as compared to a single bell setup. So the total ISA of the bells should be smaller. I think I see why a batch box can use a larger ISA. More heat is dumped at once (uninterrupted by refueling) into the bell? Yes, the batchbox is about 50% more powerful, hence the difference between a bell for a batch box and a J-tube. |
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) We might know the rockets and bells better one day!