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Post by peterberg on Apr 3, 2015 3:11:35 GMT -8
Douglas sent me this as a personal massage. In order to keep the conversation global, not private, I opened a new thread just for his project. --------------------------------------------
Hello Peter,
Thank you for your technical comments as Global Moderator on Donkey32.proboards regarding rocket stoves.
I have referenced your batch rocket table dimensions and find your additional comments regard table dimension basis to be easy to work with but now the barrel parameters are a bit fuzzy. Allow me explain where I am at...
The riser is being made from 9"x4.5"x2.5" Homedepot firebricks
I plan to cut each riser firebrick to 7.75" length then apply a 45 degree bevel across the brick's width to achieve a more uniform outside riser distance when butted end to end into a square shaped riser... the space/distance from riser to the propane tank is more uniform around the inside of the tank as 1.8" to 2.1".
The firebox will be 7" width x 13.5" height x 18" deep. On top of the firebox will be a 100# cut propane tank at 36" tall x 14.5" OD placed over the brick riser. So the riser inside will be 5.25" square x 49" tall from the firebox base. I was planning on a total riser height of 45" which permits a 4.5" space between the riser and tank-barrel top.
Question1 what is the formula to determine the space or distance between the riser outside and barrel-tank inside be? Question2 what is the formula for the space or distance between the top of the riser to the inside of the barrel-tank top? Question3 Is there a known riser to barrel space formula vs the effect on temperature? Question4 what is the formula to determine the diameter size of the barrel-tank exhaust exit if placed 2" above the tank base?
Thanks again for your continued support. If there are formulas or metrics to my questions, please add them to your spreadsheet as comments.
Cheers,
Douglas
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Post by peterberg on Apr 3, 2015 6:57:39 GMT -8
Douglas, welcome to the boards. The first thing that could be the wrong dimension is your riser. That one will be 5.25" inside this way. This is just too cramped for a 6" system. The circle is the best aerodynamically speaking, octagon is a close approximation and square is not good because of the corners. To compensate for that, the sides of the square should be 6". There should be insulation around the riser when it isn't built out of insulative material. You could use split firebricks, the half thickness ones, to build the riser. Otherwise your tank is too small. You could decide to build a smaller batch box. The firebox should be 8.64 wide, 12.96 high and 17.28 deep. As you can see, your dimensions are a bit off. The depth is alright but height and width aren't, sorry. As for the barrel: side gap at least 2" but preferably more, top gap 1 foot. There's some conformity about the internal surface area inside a completely steel bell around a 6" batch box rocket. The maximum would be 5 m2 or 53 sq ft. There's a formula to determine the diameter of the exhaust, see link
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Post by drpohl on Apr 3, 2015 13:13:48 GMT -8
The following are comments from forum postings that I've attempted to apply (http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions)
I'd appreciate new comments.
The dimensions of the batch box rocket stove follow: The assumption is, there should be a common base number to which 'all' of the other dimensions are related. When there is no correlation another formula or basis is given. That BASE number is calculated from the diameter of the exhaust riser diameter. (treat round, hexagon and square riser diameter dimensions all the same as round in that preferred build order) BASE reference number is 72.34% of riser diameter. i.e. 5.25" inside-diameter round or 5.25" inside-square exhaust riser x .7234 = 3.8 BASE REFERENCE NUMBER Width of firebox is 2 times BASE. 3.8" x 2 = 7.6" Height of firebox is 3 times BASE. 3.8" x 3 = 11.4" Depth of firebox is 4 to 5.5 times BASE. 3.8" x 4 = 15.2" to 3.8" x 5.5 = 20.9" Height of 'throat' port from firebox into riser is 2.2 times BASE. 3.8" x 2.2 = 8.4" Width of 'throat' port is 0.5 times BASE. 3.8" x 0.5 = 1.9" Tuning the "throat" dimensions controls the gas mixing and burn turbulence which will greatly affect the stove draw... watch the flames and listen for the air rushing 'rocket sound'. Height of exhaust riser is 8 to 10 times BASE, measured from the firebox floor. NEVER SMALLER ELSE IT WILL NOT DRAW WELL. 3.8" x 8 = 30.4" to 38" The firebox floor consists of a narrow flat surface the width of the firebox throat port. Left and right there are 45 degree slopes to form a "V" shaped firebox in order to concentrate the glowing charcoals into the middle as the wood burns down. Those 45 degree chamfers are inclusive part of the dimensions of the firebox. In addition, there’s also a similar shaped piece(s) at the rear bottom of the riser opposite the 'throat' acting as a flame 'up' deflector. The total (primary 20% & secondary 5%) air inlet is 25% of the riser cross section area (csa). Riser can be round, octagon or a square fire-brick shape... square works ok but not as optimum as round. For a first stove build it ok to use/build a square using fire bricks.
P-channel is 5% riser csa. 5.25" diameter, area of circle = PiRsquared = 3.15 x 2.625 squared = 21.7 sqin x 0.05 = 1.1 sqin
Main inlet plus window wash is 20% riser csa. 4X P-channel = 1.1 x 4 = 4.4 sqin or a 2.5" hole = 4.9 sqin - adjust as required for best fire Main inlet could be larger when starting cold and is situated level with the floor of the firebox.
P-channel should be as wide as the port or slightly more, for the calculation of the 5% you should take the width of the port, not the actual width of the duct. This duct is hanging over the top of the port the same distance as the depth of the duct. Build the box first then add the p-channel port and optimize with a few test fires.
The back of the p-channel which is resting against the firebox rear wall has been cut away over the height of the overhang. Barrel exhaust is never smaller than the diameter of the riser. When in doubt use the next standard size larger, i.e. a 6" riser uses a 8" diameter flue.
I appreciate being told answers but would like a math basis to calc the following?
1. What is the math basis to calculate the space and volume between the top of the riser and the top of the barrel? 2. What is the math basis to calculate the space between the insulated riser outside and the inside of the barrel? I've seen Pi R2 : Pi D= height of "ring" (Is the "ring" the distance for the vertical riser to barrel or the horizontal insulated riser to barrel distance? Then... 5.25" x Pi x r2 = 18.375 x 2.625 = 48.2" Pi x D = 3.15 x 5.25 = 16.5" 48.2 / 16.5 = 2.92" must be the horizontal space between insulated riser to barrel since Peter says the vertical space from the top of the riser to the top of the barrel is one foot (12 inches?)? I seem to remember other builds less than 4 inches... confused? 3. What is the math basis to calculate the optimum diameter of the barrel exhaust flue?
I'm undoubtedly over thinking this matter but if I cannot put it down on paper in black and white its not ready to be constructed.
Thank you!
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Post by peterberg on Apr 4, 2015 0:30:32 GMT -8
I've spotted one mistake: the firebox floor is a narrow flat surface the width of the port, not the firebox.
We are talking about a batch box which does need a spaciously top gap. The recommended minimum top gap of the j-tube is much smaller, hence the confusion. As far as I am aware, there's no math basis as yet to calculate the above for the batch box. The same goes for the side gap. The math basis for calculating the exhaust diameter, very much depending on the side gap and the distance from the floor, I provided in my first answer, see that link.
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Post by drpohl on Apr 4, 2015 13:15:56 GMT -8
I took a look at increasing the riser size to 6" and its possible using cut 9"x4.5"x1.25" firebricks to 7.5"x4.5"x1.25" for the riser which will allow a 2-3" space between the riser OD and the tank ID. (I'd recommend butting with one end width cut at a 45 degree bevel to maximize the 2" space at the corners and hence a 3" space along the brick length to the tank ID.) Attached is a drawing with my notes. If anyone has developed more formulas with a brief explanation I'd appreciate learning your thoughts. Specifically: 1. Space between top of riser and top of tank - now at 12" per Peter - no formula. 2. Space between riser OD and Tank ID - calc as 2" to maintain CSA of firebox gases but feel a 150%, i.e. 3" space better to allow for friction loss etc. Thanks Doug Attached: 20150404_dpohl_batchbox_rocketheater.jpg How much heat is generated in the firebox? American Beech 22,700,000 btu per cord 128cuft = 1 cord 22,700,000btu / 128cuft = 177,000 btu per cubic foot If a 6" riser batch rocket firebox was used: 8.6" wide 12.9" height 20" deep 8.6 x 12.9 x 20 = 2219 cubic inches Cubic foot is 12" wide x 12" height x 12" deep = 1728 cubic inches per cubic foot 2219 / 1728 = 1.28 cubic feet Hence 1.28 cuft x 177,000btu/cuft = 226,000 btu/firebox burn (if you could pack the firebox solid, therefore say 80% volume = 180,000btu per firebox load. If a 3 hr burn, about 60,000btu/hour. How much of that heat goes up the flue? With more than 100 views not one additional comment or reply... guess school must be out... forum experts how about some your 2-cents worth of wisdom... Attachments:
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Post by drpohl on Apr 5, 2015 17:59:10 GMT -8
Can Peter or Donkey explain why it was recommended that the riser to tank top distance be 12" when it was stated here as 2" - See: donkey32.proboards.com/thread/124/good-infoI would think the "science" or basis for the riser to tank "space" to be a common application of combustion gases moving through a chamber for heat extraction and to maintain the draft. The Ideal Gas Law is just that... a law or science. All I can think of is a: 12" space - promotes the transfer of heat because of a longer dwell time. while a 2" space - promotes the drafting energy to continue further down-stream in a longer flue Comments please.
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Post by peterberg on Apr 5, 2015 23:59:29 GMT -8
A batch box is quite another beast as compared to the J-tube with the same riser diameter. The gas stream of a J-tube is steady, not a terribly large volume. The batch box is running full tilt at some point with all the fuel burning at the same time. The output at any given time is much higher, so both systems aren't comparable directly.
In short: not all rocket heaters are equal. Different heaters, different numbers, OK? Don't mix up the numbers for both systems.
The side gap I've used for a batch box has been 6" at times, I don't know what the smallest possible gap is, you have to try for yourself. Also, dependent on the rest of the system and the amount of draw your chimney generates. So every given situation need its own adjustments.
As a side note: you are asking for more formulas two posts back. When nobody answers, could there be a possibility that there aren't any for what you ask but just rules of thumb?
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Post by toshvan on Apr 6, 2015 3:40:25 GMT -8
A batch box is quite another beast as compared to the J-tube with the same riser diameter. The gas stream of a J-tube is steady, not a terribly large volume. The batch box is running full tilt at some point with all the fuel burning at the same time. The output at any given time is much higher, so both systems aren't comparable directly.In short: not all rocket heaters are equal. Different heaters, different numbers, OK? Don't mix up the numbers for both systems. Peter, if you don't mind me asking... How would you explain usage goals for J-tube and batch box, appropriately? The way I get it ATM, J-tube does slower steadier burn while your batch box aims for stronger faster burn. I see the batch box very useful for very fast heating of sporadically used areas. But if used with thermal mass, it should also do the job better then the older concept J-tube - meaning that it can be used for heating living quarters. After all, basic rocket stove concept is fast and clean burn (and user can decide what to do with the energy, store it in thermal mass, or not). other beast as compared to the J-tube with the same riser diameter. The gas stream of a J-tube is steady, not a terribly large volume. The batch box does all this, only faster and cleaner. Am I missing something?
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Post by drpohl on Apr 6, 2015 7:56:28 GMT -8
Be sure to mention that a "J" tube heater requires the user to feed every 10-15 minutes (extremely annoying to spend your valuable time splitting small sticks and feeding wood) while a batch box allows you to load common 'cord firewood' and burn in a 'batch' process (bell curve?) allowing you to attend to other matters.
(Sure I realize you could have long 'stickwood' standing upright in the feed to a "J" heater but not all firewood is straight for EZ feeding...)
Peter, what would your estimate be for a 6" or 8" riser "J" heater be for the weight of 15-20% moisture wood be per hour as compared to the same riser diameter 'batchbox'?
How long has your 6" or 8" riser batch boxes taken to burn a batch or charge of wood from start to end?
Trying to get a handle on how much heat is produced per hour for a given riser size by using the weight of wood burnt in an hour.
Thanks
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Post by peterberg on Apr 6, 2015 8:43:30 GMT -8
In general terms, you are both got the right picture. Feeding every 10 minutes is exaggerated I would think it could be stretched out to 30 minutes using hardwood.
Fuel consumption of a flat out running 6" batch box is about 6 kg softwood per hour. A J-tube would do about half of that.
A given batch box combustion chamber/heat extraction/chimney combination will always take the same time for a complete burn with the same size of fuel. Irrespective of the amount of fuel more or less, very peculiar to see that for the first time! Mine took 45 minutes for a burn on average. Refilling the firebox with only two very large pieces took about twice as long. The burn wasn't as clean though but there wasn't any smoke from the chimney. So this heater happened to be very predictable.
A 6" version is a very potent heater, an 8" is a monster. I don't want to imagine what a 10" would be like.
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Post by Deleted on Apr 6, 2015 9:38:14 GMT -8
Carbon content of wood ~ 50 W%. 6kg wood 20% moisture about 2.4kg carbon ~ 2.4*32.808 ~ 78,74 Megajoules or 21,87 kilowatt-hour. At 90% efficiency ~ 19.68 kilowatt-hour ~ 67150.95 BTU At 85% efficiency ~ 18.59 kilowatt-hour ~ 63431.71 BTU Which is pretty close to the values given in Wood Fuels Handbook www.aebiom.org/IMG/pdf/WOOD_FUELS_HANDBOOK_BTC_EN.pdf
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Post by toshvan on Apr 6, 2015 12:42:11 GMT -8
...Refilling the firebox with only two very large pieces took about twice as long. The burn wasn't as clean though but there wasn't any smoke from the chimney. So this heater happened to be very predictable. A 6" version is a very potent heater, an 8" is a monster. I don't want to imagine what a 10" would be like. Peter, if down-scaling the whole thing to 4" - does the batch box combustion chamber need scaling too? One big advantage of the batch box rocket, I see in being able to use common 'cord firewood' (and thus avoiding the tedious wood splitting to sticks). But if one characteristic of the batch box rocket is a double increase in wood burned (per time) - and double increase in energy output (per time, and compared to J-tube rocket), than it is an imperative to properly size the dimension of the power output, of the batch box rocket stove. Or suffer the excess heat. I understand that there are many factors in the stove design that affect the power output, but generally - is there any loose consensus about correlation of size vs kilowatt/BTU? For instance, Karl rated Peters's 6" batch box rocket ~ 19.68 kilowatt-hour ~ 67150.95 BTU (@ 90% efficiency). Can we speculate on downsized 4" batch box rocket BTU/kilowatt-hour values? I'm trying to understand what rocket stove size I need...
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Post by Deleted on Apr 6, 2015 13:20:54 GMT -8
6*6=36 4*4=16 If the box follows the same ratio: For 4" wood and air can be delivered and flue gas moved away by 16/36, thus the power output will follow this ratio too.
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Post by toshvan on Apr 6, 2015 13:41:35 GMT -8
Tnx, I've got dimensions from Peter's spreadsheet. My question was about the power output estimate of 4"/3" variants. But I shouldn't hijack the thread, apologies. I will ask elsewhere.
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Post by peterberg on Apr 6, 2015 13:44:26 GMT -8
Peter, if down-scaling the whole thing to 4" - does the batch box combustion chamber need scaling too? Yes, the whole thing, definetely. See the spreadsheet in the Reference Library. I think you need to chance your way of thinking about mass heaters, those are completely different from steel box stoves. Provided there's adequate mass downstream from the combustion unit, the batch box is able to charge that heat sink in a shorter span of time as compared to the J-tube. So there won't be such thing as "suffering from excess heat".
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