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Post by PNW Dave on Nov 10, 2014 12:02:02 GMT -8
I want to improve my conceptual understanding of bell sizing. What is the determining factor for the size of bell that a given J-tubes system size can serve? Is it that if the bell is too large the gasses will be cooled so much that they aren't going to rise up and out of the exit flue? (I hope it is this) Is it that a bell that is too large just won't ever be sufficiently heated? (I hope it is not this) I figure that even a larger than recommended bell would eventually have an even distribution of the hottest gasses along the entire highest point. My design will significantly reheat the gasses before the final exit, so I'm not concerned with temps being too low when exiting the bell. Here are a couple pics to put my question into the context of my build Half-barrel bench layout- Exit temps increased by passing through the second barrel (shown in green) While I don't expect a strong flow of "hot" air rushing up through the seat, I would rather the barrels get too hot then not hot enough. If the barrels were to get too hot that the radiant heat made it unsafe to be enclosed in the wood bench, I plan to set a second layer of half-barrels with a couple inch air gap to act as a heat shield. The heat shield should also improve radiant-to-convective heat transfer, so I'll probably just do it anyway.
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Post by matthewwalker on Nov 10, 2014 12:08:17 GMT -8
In my experiments the former has been the limiting factor, and yep, powering up the chimney can overcome a lot of that. I have successfully used fans, bypass designs, and chimneys close to the barrel to help power through cold flow problems. All of them work in their way.
I don't think you are going to want to double those half barrels, they will be warm but with the airspace you show between barrel and bench top, I think you will want them as warm as they can get, which tends to not be all that hot. The bare barrels radiate a ton of heat, so you will be limiting your bench length by going "bare" like that, but it will be quick to heat and warm, so there is that.
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Post by PNW Dave on Nov 10, 2014 12:36:03 GMT -8
In my experiments the former has been the limiting factor, and yep, powering up the chimney can overcome a lot of that. I have successfully used fans, bypass designs, and chimneys close to the barrel to help power through cold flow problems. All of them work in their way. I don't think you are going to want to double those half barrels, they will be warm but with the airspace you show between barrel and bench top, I think you will want them as warm as they can get, which tends to not be all that hot. The bare barrels radiate a ton of heat, so you will be limiting your bench length by going "bare" like that, but it will be quick to heat and warm, so there is that. Thanks for the quick response! The first half of your reply is good news. The airspace between barrel and seat in the drawing is not accurate, just a quick concept drawing. I think I need to clarify the second layer of half-barrels. They will be a space for air to pass through, pulling all that radiant heat-
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Post by DCish on Nov 10, 2014 12:43:01 GMT -8
Another thought... with that much heat harvest you may cool the gasses to below the condensation point (about 150° if I recall correctly). That's great in the sense that you recover that latent heat, buy it requires that you have a plan for dealing with the condensate.
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Post by PNW Dave on Nov 10, 2014 12:49:13 GMT -8
...it will be quick to heat and warm, so there is that. Great, since this is outdoors and will generally be fired up on a whim for just a few hours of use. Also I want to thank you for all of the work you've done, as most of my build has been inspired by your builds.
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Post by PNW Dave on Nov 10, 2014 13:00:56 GMT -8
Another thought... with that much heat harvest you may cool the gasses to below the condensation point (about 150° if I recall correctly). That's great in the sense that you recover that latent heat, buy it requires that you have a plan for dealing with the condensate. Thanks DCish, I don't have a plan in place to deal with that. I assume you are saying this could occur within the half-barrels? My plan for the ground underneath the barrels is to rototill a trench to increase the area inside the barrels. Then cover the ground in aluminum foil and hand pack an inch or two of "fireclay coated perlite" over the foil for insulation to keep the heat being absorbed into the ground. I do expect the exit temps to be well above 150°F
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Post by DCish on Nov 10, 2014 21:04:12 GMT -8
I assume you are saying this could occur within the half-barrels? ... I do expect the exit temps to be well above 150°F Yes, the half-barrels (i.e., the bell) is where I am guessing this has potential to occur. Peterberg says that 6 sq meters of internal surface area is about what a 6" batch box can support without stalling. You will be re-heating your exhaust, so you have a way around that problem. However, if your ISA is significantly above the 6mSq, you have the potential to drop the temps low enough to hit the condensation point inside the bell. A couple of recent threads explore intentionally doing something like this to recover latent heat (the significant energy required to evaporate water, which heat is captured at condensation). Donkey has even proposed a re-heater such as yours to act as a "fan" to keep draft going. Do you know the expected ISA of your bell? And as always, I defer to the experienced builders here to correct any mistaken assumptions I may have made...
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Post by PNW Dave on Nov 12, 2014 1:09:24 GMT -8
...I think you will want them as warm as they can get, which tends to not be all that hot. The bare barrels radiate a ton of heat... Do you think the concept shown in the pic would help transfer that radiant heat to the air, and in a way that would actually be worth while? I figure the radiant heat would be somewhat contained between the layers, allowing the fresh air to be more effectively heated as it enters the bench, then it is directed towards the backrest bell. I wonder if it would also help keep the main barrels at a higher temperature across the entire bell, considering I don't expect much air to actually exit through the seat boards (therefore not much fresh air cooling the barrels) as there will be butts or seat cushions blocking the only exit. With a very low flow of fresh air the heat shields might actually function as a third bell in the bench since the inlet is lower than the exit. Do you know the expected ISA of your bell? I don't, I currently intend to use 8 half barrels for the bell. A quick search didn't find the ISA of a full barrel, which I'd divide by 2 then multiply that by 8. But I'd have to compensate for the lid and bottom since they will be removed. Any help with that would be appreciated.
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Post by Donkey on Nov 12, 2014 8:29:26 GMT -8
A standard 55 gallon barrel is 22 1/2 inches in diameter and (oops, forgot.. Running out to the metal pile with a ruler) 35 inches tall. The circumference then would be: Pi times diameter = 70.65 inches The area of the cylindrical walls would be: circumference times height = 2472.75 square inches The area of the barrel ends (individually) are (Pi radius squared) 397.41 square inches
Adding one barrel end to the cylinder is: 2870.16 square inches, or 19.93 square feet or 6.07 square meters Cutting it in half, I would halve the cylinder first, then add in the ends (or half-ends) that count. SO! half of 2472.75 is: 1236.38 square inches, 8.59 square feet, 2.61 square meters half of one end is: 198.71 square inches, 1.38 sq. ft., .42 sq. M.
From here, you can add them or take them away at will. A half barrel bench that is 2 barrels long will have the SAME area as the whole barrel minus an end (our original number), just about the perfect bell size for a six inch Rocket Stove (6.07M Sq).
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Post by wiscojames on Nov 12, 2014 9:15:18 GMT -8
The conversion between square feet to square meters is about 11:1, so if Donkey's calculations on the half barrel are correct, a half of a barrel plus half of an end should be about a little less than a square meter.
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Post by matthewwalker on Nov 12, 2014 9:23:53 GMT -8
...I think you will want them as warm as they can get, which tends to not be all that hot. The bare barrels radiate a ton of heat... Do you think the concept shown in the pic would help transfer that radiant heat to the air, and in a way that would actually be worth while? Uh, well, with regards to what I think about the concept in general... I'm having a great time following your build and thought process and am learning right along with you. I think the process is more important than the end result, since nothing is permanent and we will all learn a lot along the way. Bare barrels shed a ton of heat, I think you will be asking too much of a fairly small firebox to try to warm that much air. Your shroud idea has some merit, I suppose. Not sure it's going to do what you want, but your concepts make sense to me. I personally am a fan of a thin mass of cob over the halves for outdoor seating. It works well. But, I'm very interested in all of your experimenting, it's a lot of fun to follow along, so...good luck man! As you are doing your calcs, remember that bare metal is a different ISA calc. than masonry.
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Post by satamax on Nov 12, 2014 9:26:42 GMT -8
One end and the walls of a barrel if it's 58cm diameter, makes an ISA of 1.86m²
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Post by DCish on Nov 12, 2014 13:40:19 GMT -8
As you are doing your calcs, remember that bare metal is a different ISA calc. than masonry. Oops, I'd forgotten about that... Wasn't it 5.5 mSq for metal?
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Post by peterberg on Nov 12, 2014 14:46:07 GMT -8
A standard 55 gallon barrel is 22 1/2 inches in diameter and (oops, forgot.. Running out to the metal pile with a ruler) 35 inches tall. The circumference then would be: Pi times diameter = 70.65 inches The area of the cylindrical walls would be: circumference times height = 2472.75 square inches Donkey, your calculation isn't entirely correct. My calculator says one barrel cylinder following the numbers you quote is 2474 sq in, which equals 1.6 sq m. according to Cleave Books converters. The area of the barrel ends (individually) are (Pi radius squared) 397.41 square inches One barrel end should be 397.6 sq in, equals 0.257 sq m. My three barrel tower happened to be 5.057 sq m, minus the steel part where the firebox is plus half of the firebox inside the barrel, plus a column of bricks. That'll sum up to a little more than 5 sq m, and at the same time just the maximum the 6" batch box could serve. Calculated like this, a half barrel bench of six halves and two half ends is about the same as my three barrel tower. Quite a bit larger than your example, I would say. For some years I've used the conversion calculators of Cleave Books, those are quite good and very comprehensive.
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Post by ericvw on Nov 12, 2014 15:29:07 GMT -8
Hello fellas, I humbly request a brief explanation or directed to some links that describe the difference between the ISA of metal VS. masonry. Not sure where I missed that? ?! Thanks a bunch, Eric VW
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