|
Post by daniel on Aug 22, 2015 11:47:26 GMT -8
Hi,
I have built a batch box for a 8" diameter riser according to the base number specifications and want to add a double bell with the first bell being above the riser. According to the dimensions list my riser would be 1440 high ( is this from the firebox floor?) What I don't know is what are the rules height width to use to ensure avoiding the gases to be sucked out before they have the time to rise . I have read about the widening of the bell after the hole inlet but I don't understand it too well and don't want to make a mistake . If someone please explain the basic rules (height , width and other things that I must ensure are correct). I think I will build the bell of firebrick in the hot part and brick where is not as hot so that I am not tied to a certain size.
|
|
|
Post by erikweaver on Aug 23, 2015 10:35:43 GMT -8
Hi Daniel, There are a number of threads here discussing bells. Here's some links to get you started: donkey32.proboards.com/thread/1359/bell-explanation donkey32.proboards.com/thread/1418/question-bell-sizing?page=2donkey32.proboards.com/thread/40?page=6 donkey32.proboards.com/thread/40?page=7 donkey32.proboards.com/thread/1026/ donkey32.proboards.com/thread/1505/half-barrels-bell-bypass donkey32.proboards.com/thread/1487/why-maximum-isa donkey32.proboards.com/thread/1394/bells-materials donkey32.proboards.com/thread/1065/non-metal-barrel-downdraft-chambers donkey32.proboards.com/thread/1134/heat-store-temperature-readings donkey32.proboards.com/thread/1194/regarding-bells As to your questions, I can only offer a few pointers. I'm designing a 6" batch box, not an 8" so I don't recall the numbers you'll need. But even so, there are a few basics than apply to all double bell designs (so far as I know/as best I recall): A good chimney is required. In the USA a standard code requirement is that the top of the chimney must be a minimum of 2-feet higher than anything within 10-feet of the chimney. I *think* another pretty standard rule of thumb is that the chimney is also a minimum of 12-feet in vertical height. When calculating the Internal Surface Area (ISA) do *not* include the batch box, nor the fire riser. Do *not* count the floor of the bells when calculating the ISA. Do *not* insulate the batch box/fire box; DO insulate the Fire Riser. For each bell the ISA is reduced by 15%. NOTE: There is an additional subtraction for *metal* bells, but I don't recall how much. ALSO, what about when the metal bell (a second barrel, as I may use) is enclosed in cob/brick? My recollection is that the reason a metal bell reduces the ISA more than masonry, is because the metal bell radiates heat to the room more efficiently; however, if the metal bell is enclosed in masonry this is no longer true, and I am *ASSUMING* this means only the standard 15% reduction in ISA applies. Fire brick is only required above the height of the top of the fire riser (verify this recollection of mine). The opening (manifold) between the two bells should be generous, and not constrict the flow of air. I rule of thumb is 4 times the CSA of the system size; also, the bell into which the opening enters, would ideally be 4 times the CSA of the opening into it. Likewise, the manifold leading to the chimney pipe also must allow a smooth flow of air (and a similarly sized transition is recommended). Please read the links above to get the details on this, I may be misremembering to some degree. Change of materials (between metal and masonry; between fire brick/common masonry) should include an expansion joint. This can be as simple as a sheet of cardboard, or a piece of insulation wrap. ASTM standards call for an 8" minimum masonry thickness, which also allows for less clearance to combustible walls and ceilings. Plus, one of the benefits of masonry heaters is the long-heating effect, so thicker masonry walls/tops helps in this regard too. As I recall, Ernie Wisner suggests estimating the penetration of heat through cob at about 1" per hour. I don't recall a similar estimate for common masonry materials, but I would think it to be roughly the same, perhaps a little faster through common masonry due to higher density. So that should get you started!
|
|
|
Post by satamax on Aug 23, 2015 10:52:06 GMT -8
Erik, your *do not* insulate the batch box is a bit stringent. I would say, you can skip the insulation of the firebox, if it's realy needed. But they seem to work a smidge better with an insulated firebox The do not account for the floor of the bells can be rectified too Do not account for the floors of bells if the entry and exit points are above the floor by half of their height
|
|
|
Post by martinpolley on Aug 23, 2015 10:53:59 GMT -8
erikweaver Sorry to take this thread off on a tangent, but I notice that you say not to insulate the fire box. I've seen others say the same. But others say *do* insulate. What's the reasoning here? What are the pros and cons of insulating/not insulating? (My earthen oven is powered by a 7" j-tube, and I did insulate all around the feed and burn tunnel.)
|
|
|
Post by erikweaver on Aug 23, 2015 11:17:54 GMT -8
Erik, your *do not* insulate the batch box is a bit stringent. I would say, you can skip the insulation of the firebox, if it's realy needed. But they seem to work a smidge better with an insulated firebox The do not account for the floor of the bells can be rectified too Do not account for the floors of bells if the entry and exit points are above the floor by half of their height Thanks Satamax! I was flying by memory, and my recollections are *not* to be taken as gospel! lol Please, correct me any time I am mistaken, I do appreciate it.
|
|
|
Post by erikweaver on Aug 23, 2015 11:25:54 GMT -8
erikweaver Sorry to take this thread off on a tangent, but I notice that you say not to insulate the fire box. I've seen others say the same. But others say *do* insulate. What's the reasoning here? What are the pros and cons of insulating/not insulating? (My earthen oven is powered by a 7" j-tube, and I did insulate all around the feed and burn tunnel.) In a standard J-style RMH the idea is to discourage the feed tube from acting like a chimney, and to discourage smoking up the house. But the horizontal Burn Chamber and vertical Fire Riser do work best when insulated. (EDIT: This insulation is to help increase the temperature of the fire, which leads to highly efficient burns, due to the very high temperatures. In my prototype build last winter, of a 6" J-style, I measured temperatures as high as 1950-1980 F at the opening from the Feed Tube into the Burn Chamber (and routinely in excess of 1400 F).) I don't recall why Peterburg said not to insulate the batch box. Perhaps the same reason? Fire Risers are said to always perform best when insulated. Although, Ernie Wisner once suggested a Fire Riser made from insulating kiln brick may not require insulation, and Peterburg has used designs of his batch box system which employ refractory materials which are also highly insulating by the nature of their material, and they do not require additional insulation (I believe these are sometimes called "refractory sleeves"). So one may say that not all Fire Risers require insulation, but in the two examples given above, that is because the materials themselves are already highly insulative. So to my mind, that means insulation is simply the nature of the materials, and therefore they are automatically insulated Fire Risers
|
|
|
Post by satamax on Aug 23, 2015 11:57:03 GMT -8
Erik, Peter van den Berg said that it's not absolutely necessary to insulate. Following that and his inovator gathering batch, i drew the cooking batch box in sketchup.
Tho, myself, if i don't need glass sides, i would insulate the box, especialy the bottom, it gets rid of the embers better.
|
|
|
Post by daniel on Aug 23, 2015 14:27:39 GMT -8
thank you all for the explanation, so if I understand correctly if I have a 8"ID round chimney about 24' high, a 8"id round riser I got a 50.26 sq inches area, the bell should have a CSA of 201 sq inches, the space above the riser will be higher than 1 ft. I could make the bell square or rectangular or round. The rectangular section would allow me to have the inlet and outlet opposite to each other with the inlet slightly higher. The outlet will be lenghtwise on the opposite side. First bell could be 10x20" = 200sq inches. If I understand correctly the total internal area of both bells could be, in the upper limit, for a 8 incher, 11 square meters (walls and top of bell)? the inlet and outlet of all the bells should be system size (8")?
|
|
|
Post by erikweaver on Aug 23, 2015 16:12:01 GMT -8
thank you all for the explanation, so if I understand correctly if I have a 8"ID round chimney about 24' high, a 8"id round riser I got a 50.26 sq inches area, the bell should have a CSA of 201 sq inches, the space above the riser will be higher than 1 ft. I could make the bell square or rectangular or round. The rectangular section would allow me to have the inlet and outlet opposite to each other with the inlet slightly higher. The outlet will be lenghtwise on the opposite side. First bell could be 10x20" = 200sq inches. If I understand correctly the total internal area of both bells could be, in the upper limit, for a 8 incher, 11 square meters (walls and top of bell)? the inlet and outlet of all the bells should be system size (8")? No, it looks like you are confusing the CSA of the *opening* into the bell, with the ISA of the bell. These are very different things. You appear to have correctly identified the CSA of the opening of the first bell (the barrel in which the Fire Riser is located) into the second bell. But the calculation for the ISA is a summation of all the sides and tops (interior dimensions only) of the two bells. This is, in part at least, to calculate the surface areas upon which condensation may occur; then we take a given maximum ISA and stay below that when sizing our bells. As a quick example, for my 6-inch batch box system, I am trying to stay below 50.3 square feet of ISA (not counting the floor). My first bell (in which the Fire Riser is located) is a 12" radius barrel, 34" tall, and that is going to sit on a brick ring 16" tall; this 16" tall brick ring is also where the Batch Box will enter on one side, and the exit to the second bell will be on the other side. I have calculated the approximate ISA of the first bell to be 26 sq.ft. So I subtract that from 50.3, and get a maximum ISA for my second bell of 24 sq.ft. (50.3 - 26 = 24.3 sq.ft., which I have rounded down to 24.0 sq.ft. for a small margin of error). My preliminary calculations indicate I can have a second bell, also 12" radius, and 36" tall, which will have an ISA of 22 sq.ft, which will allow an additional 2 sq.ft of ISA to make the connection manifold between the two bells. (Included in my estimated ISA's are all the square edges of brick ledges, subtractions for the opening of the Batch Box, and the CSA of the opening/manifold between the two bells.) The place to start is with some basic geometry (previously Peterburg has offered the following web site for such calculations: www.cleavebooks.co.uk/scol/index.htm ). You will see that a barrel, not counting the bottom, has a surface area of (pi*r^2) + (2*pi*r*h). The circumference of a circle is 2*pi*r, and h is the height of the barrel/bell/cylinder. Then you add the surfaces of protrusions (ledges, piers, etc) and subtract openings (where the Batch Box enters the side of the barrel, and the manifold opening, for example). I sketch the layout on graph paper and start adding up surface area. It is a little tedious but pretty straight forward. The only tricky part is remembering what you've forgotten lol (or much worse, do not yet know!) So, you still need to find out the recommended maximum ISA for an 8" batch box system. And I don't recall what that might be. If I do remember correctly, it is a matter of some debate because not many have been built which offer good stats and performance evaluations to fine-tune the estimates. EDIT: Sorry, I just re-read your post, and you have already found a figure of 11 sq.m. for your bells. Is that before of after the 15% reduction for the second bell? Apparently, I'm getting too tired and too bleary-eyed to make complete sense. Maybe you are calculating correctly?
|
|
|
Post by satamax on Aug 23, 2015 22:34:55 GMT -8
Erik, i gave that number.
Well, that's for a single bell. Two bells should total maximum 9.35m² if massonry.
|
|
|
Post by martinpolley on Aug 24, 2015 6:34:59 GMT -8
erikweaver satamax Thanks for the explanation guys. I guess for an indoor RMH, an uninsulated feed also means you get a bit more heat radiating out into the room, which is probably a good thing.
|
|
|
Post by peterberg on Aug 24, 2015 7:57:08 GMT -8
Ermm... The figure of 11 sq m for the internal surface area of a bell system being 8" isn't correct. The latest large brick batch box heater with two separate dead-end benches showed 9.5 sq m as the largest workable size. One could argue the benches, although those were part of the main bell, acted as a second bell. But beware, this is on the basis of just a single experience. So with other configurations it might well be different.
The longer I am meddling with those heaters the more complicated the rules seem to become. Depite how much I would love to work by simple rules, though.
|
|
|
Post by satamax on Aug 24, 2015 11:03:02 GMT -8
Peter, i think the simplest way for bell dimensioning is to make teh burner, and make barrel bell towers behind that, as a temporary test. One tower, one and a half, two, two and a half, three. Etc. Untill ones reaches the limit. Then stick to a square meter less in surface or so. Or at least, that would work for a dumbass like me.
|
|
|
Post by daniel on Aug 24, 2015 12:49:20 GMT -8
Thank you all, I am planning to use all these principles to build an efficient batch box heater with bells since that seem to be the best for me. Related to the flow of gases in the bells I was trying to find a video that show (perhaps in infrared)how the gases travel in a masonry heater of flues but so far I did not find any real videos just simulations.
|
|
|
Post by daniel on Aug 24, 2015 13:00:13 GMT -8
I almost forgot, now I plan to build the bells of stones (fire resistant) layered with refractory mortar, the internal surface will thus be much higher for the same dimensions (height and width,lenght) so I will have to make an aproximation. On the inside of the bells there will be some protruding stones consistent in shape. I am thinking to take some kind of foil wrap it around a few rows of stones and then stretch it and measure it to aproximate the surface area. The Top might be a refractor slab or a large stone or both.
|
|