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Post by twohorsepower on Jan 18, 2011 11:53:27 GMT -8
Hi Kramer, You got me wondering there when I first read 212, then i realised that whilst here we talk in celsius, your figure is the boiling point in fahrenheit. Yes, over here that would be called a vented (open) hot water tank, and the rising coil you describe is a 'thermal store' arrangement, i.e., one where the water in the tank is not the water that comes out your taps (fossets). So you could have a heat exchanger in a rocket heater either below the open vented tank or in sidearm fashion, drawing cool from the tank base and feeding hot back into either the top part of the tank or the tank's vent pipe, all by thermosyphon. No pressure relief valves, no pumps, getting simpler now! For the rising coil to deliver mains pressure hot water for regular daily use, the thermal store would have to be maintained at a fairly high temperature, well insulated etc.. Well we know the max tank temp is 100C/212F, but any ideas how big a tank is needed, what sort of minimum temperature, etc..  |
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Post by zoidberg on Feb 1, 2011 0:44:05 GMT -8
For the rising coil to deliver mains pressure hot water for regular daily use, the thermal store would have to be maintained at a fairly high temperature, well insulated etc.. Well we know the max tank temp is 100C/212F, but any ideas how big a tank is needed, what sort of minimum temperature, etc.. I believe around 40 litres @ 60ºC per person per day is what codes prescribe (at least in Spain). If that is what you were asking, of which I am not very sure. |
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Post by dakman on Feb 10, 2011 20:42:54 GMT -8
Does anyone know what a typical burn rate on a 6" RMH is? How many pounds of wood per hour?
I too like an open to the atmosphere system. One consideration is keeping oxygen out of the loop to prevent bacteria, etc. I wonder if an open topped expansion tank could have a thin layer of a synthetic oil on top of the water to seal it from oxygen. This would still allow boiling without pressure buildup in the main tank. A diffuser screen could be placed several inches above the synthetic material to prevent splatter. One would have to add water from time to time whenever the water did reach the boiling point.
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Post by zoidberg on Feb 11, 2011 10:32:13 GMT -8
I too like an open to the atmosphere system. One consideration is keeping oxygen out of the loop to prevent bacteria, etc. I wonder if an open topped expansion tank could have a thin layer of a synthetic oil on top of the water to seal it from oxygen. This would still allow boiling without pressure buildup in the main tank. A diffuser screen could be placed several inches above the synthetic material to prevent splatter. One would have to add water from time to time whenever the water did reach the boiling point. In the system described by kramer (previous page, bottom) the open loop would be a "primary" circuit, i.e.: it is separated from mains water. Being just water (or water+propylene glycol) I guess few bacteria would thrive there and the few which did would get cooked regularly, wouldn't they ? |
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Post by dakman on Feb 11, 2011 17:13:47 GMT -8
Thanks, Zoidberg.
Kramer,
Does this resemble what you had in mind? I modeled this pretty quick after work today, so there's been very little effort put into sizing. I like the concept, especially the part about no blowoff valves in the primary tank.
I'm thinking that if I could keep the primary tank small, and insulate it well, I would minimize wasted heat in the shed outside. My concern is that the maximum heat transferred to the large tank in my basement may be lower than desired. For example, the thermo switch for the circulator pum turns on when primary water temp reaches 190 F. The exchanger loop has a PG and Water solution for freeze burst protection, but this impedes heat transfer at both ends. So, what do you suppose we could expect for max. temp at the big tank? Lots of variables to consider, I know.
Maybe someone out there has experience from a similar project?
Thanks,
Dakman
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Post by zoidberg on Feb 12, 2011 5:04:39 GMT -8
I'm thinking that if I could keep the primary tank small, and insulate it well, I would minimize wasted heat in the shed outside. My concern is that the maximum heat transferred to the large tank in my basement may be lower than desired. For example, the thermo switch for the circulator pum turns on when primary water temp reaches 190 F. The exchanger loop has a PG and Water solution for freeze burst protection, but this impedes heat transfer at both ends. So, what do you suppose we could expect for max. temp at the big tank? Lots of variables to consider, I know. Sorry for quoting stuff in International / Imperial units, I am more used to the former. I am very interested also in the concept because it's the one that most convinces me. I mean, I intend to set up a similar system for both DWH and space heating. Regarding temperatures, I guess it all depends on how big the storage tank is and how many Kwh you can get out of the stove. DWH systems should be designed and set up so water in storage tanks reaches 60C / 140F (in order to prevent Legionella) but no more (at higher temperatures disolved lime in water precipitates and wreacks havoc). Since water in the primary loop will be 87C / 190F or higher there should be no problem regarding heat transfer either. The other side of the story is getting enough Kwh / BTUs out of the stove for your daily hot water needs. A very rough estimate I (kind of) work with is 3 Kwh usable energy per Kilogram of firewood (4656 BTU / pound, someboy correct me if I'm wrong here), which is the amount of energy needed to raise 100 litres / 22 gallons of water from 15C / 59F to 40C / 104F, hot enough for a couple nice showers  Then again I am no expert by any stretch of imagination, so please feel free to correct my figures and general ramblings. |
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Post by zoidberg on Feb 12, 2011 10:13:24 GMT -8
Regarding the heat exchanger, I am not convinced that a coil between raiser and barrel is the best of systems. See for instance what Thermorossi wood boilers are using: www.thermorossi.com/eng/legna_det.php?gruppo=3&padre=1&figlio=4&id=41Click on the first of "Zoomable images:"; number 7 is the heat exchanger and looks like a bundle of pipes going straight through the water jacket. Opinions ? |
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Post by zoidberg on Feb 12, 2011 10:35:05 GMT -8
And this should be a view of the HX's upper end, showing the pipes and the exit to the smoke extractor.
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Post by nedreck on Feb 14, 2011 16:31:31 GMT -8
Regarding the heat exchanger, I am not convinced that a coil between raiser and barrel is the best of systems. Opinions ? Its not. I looked at the drawing and guestimated some numbers and the performance would be pretty poor. Because I am not the most friendly person available, that may lend itself to folks disregarding data, that is anyone's call. I have been playing with heat transfer for over 30 years and teaching it the last ten, use that to determine merit vs personality issues and you might save some effort. Again, I am kind of pulling these numbers from an old farts memory and they might be a bit different, but not much, coil type exchangers kind of suck, not really but sorta, they suck if you do not have lots of space is a better way to say it. Anyway here are the numbers for the coil pack in the drawing and these numbers depend upon proper tube spacing of 125% of diameter between em center. 3/4 inch tubing 0.19625 sq ft per linear foot 18 loops 10" diameter 10*3.14*18.=565.2 565.2"/12 47.1 linear feet 47.1*.19625=9.243375 square feet copper tubing Approximate 215 btu/ F ft * 9.24 = 1986.6 20 degree rise = 39732 BTU hr 39732/500*20= 3.9732 GPM I am not sure 4 gallons per minute from 60 to 80F is your goal, but that is about the performance one can expect from the drawing. Dont get me wrong, you can get more, but your gonna have to slow the water down even more and your not going to get a lot more heat by doing so, standard rule of thumb is 10 to 20 and I happen to think 20 is going to be pushing it and is an over estimate of performance. Now we could back the water flow down in half which will give us greater delta, but we are already at 4.25 feet per second velocity which is about perfect (between 2 and 6, never over 8 is the rule) so in doing that we might be able to approach 35 delta which would give you 35 over whatever your incomming is, if preheated with solar, perhaps a workable, without, not going to be real cold weather friendly. You will find that secondary fluids and a fluid to fluid heat exchanger are not going to dramaticly improve this inless you are going to mass store the primary fluid. I am not sure if this rambling was clear enough to be helpful or if anyone really wants the input, but I would suggest a bit of effort be put into the actual possible performance and the requirements to be effective before laboring over one of these bad puppys. The drawing is likely not to scale, but it sure does not look like proper tubing spacing and 9 sq ft is not a lot of contact space compared to other potential designs. FYI Ned |
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Post by zoidberg on Feb 16, 2011 9:11:39 GMT -8
Ned, I think I got lost here: What are 500 and 20 ? |
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Post by nedreck on Feb 16, 2011 11:29:01 GMT -8
Ned, I think I got lost here: What are 500 and 20 ? 500 is a constant. It converts gallons per minute to pounds per hour. Heat content is measured by mass not volume and gpm is a volumetric measurement. 20 is the temp change in the water. Q=gpm*500*delta T Depending upon size using a mean temp works well. The relationship of gpm and Dt is linear in nature and the only two controling factors since we do not have a real good way to control the input. |
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Post by zoidberg on Feb 21, 2011 11:10:04 GMT -8
Ned, Thanks for the explanation. I have another doubt, though: what is 215 in "Approximate 215 btu/ F ft * 9.24 = 1986.6" ? I have been reading a bit about heat transfer, heat exchangers and related things and I believe it might be the Heat Transfer Coefficient between gases and liquid, along with some other coefficient multiplicatively thrown in. But, since I have been reading in International Units, formulae with Imperial Units confuse me big time, more so when degrees Farenheit are involved. Would you please enlighten me ? Regarding: I am not sure 4 gallons per minute from 60 to 80F is your goal, but that is about the performance one can expect from the drawing. I imagine we could get better performance increasing the heat exchanger area, right ? |
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Post by nedreck on Feb 22, 2011 8:20:41 GMT -8
Ned, Thanks for the explanation. I have another doubt, though: what is 215 in "Approximate 215 btu/ F ft * 9.24 = 1986.6 You have it correct, that is the heat transfer coefficient for copper. In English units it is expressed as: Btu per square foot per degree. Copper alloys have differing transfer rates and I am pulling from memory but I think pure copper is 221 btu but I could be wrong, I would have to look it up. Other materials such as steel have significantly lower rates and while that would impede a system, since the RMH is not dedicated to water heating alone, really do not want to rob all the heat from the riser, it might make a better choice despite the lower coefficient. It will sure as heck be a lot less expensive and is easier to field fabricate. I really can not see a copper one being produced without employing at least 45% silver content brazing materials which are also going to be very expensive. Steel would likely be 1/5th the cost, a whole lot stronger, more reliable, and easier to fabricate. I used copper simply because that is what the industries I serve would expect, maximum performance at a good cost, not to mention that also allows the numbers to roll off the cuff easily so to speak. I do not bother looking much of this stuff up for these discussions so I am not without error, but from my point of view we are seeking "effective" ideas and minor errors will impact it all identically across the board and the information can be used for selection thoughts. Btu to kj is fairly direct but when you add in the square foot vs square meter it starts changing the numbers a lot and while it in this case remains linear, it certainly confuses the mind if you are used to using only one method. I use both, however, since I am an American I am strapped with the stubbornness of a great many people. Despite the fact we were to be changing to the metric or international standards about 30 years ago, unless you are speaking to the engineering crowd and sometimes not even then, you might as well be speaking in Latin! I referenced early on a couple of books I would be using in reference to the upcoming testing, but you have sparked an interesting thought that might well be of significant merit for all the experimenters reading along. It is going to sound a little odd since we are talking about heating so to speak, but the fundamentals of heat transfer are in reality what we are talking about most of the time. Principles of Refrigeration by Roy J Dossat Fifth edition ISBN – 0-13-027270-1 I actually think that book would prove very valuable to anyone fooling about with heat transfer. It covers basic thermodynamics, heat transfer, applied physics and the mechanics that make it all work. It includes the basic equations in both American and the System International (SI) and would likely make your life much easier, I hate doing the conversions. A lot of times you can find or request the book at your library, or such is the case here, never really thought about what library systems might exist in other parts of the world! |
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Post by zoidberg on Mar 1, 2011 10:08:36 GMT -8
I'm all for buying books if they are worth it, but I find that lately (since lots of years ago actually) I am getting much of the information I need from the Internet. The nearest library around is some 15 km away and totally out of my way and the 'net is so much more convenient. For physics and thermodinamics I usually resort to www.engineeringtoolbox.com/ Doesn't look too bad and I find I can assimilate in my brain most of what I feel like reading. About getting libraries to find books they don't actually own at the moment, I believe it is possible to do it in my neck of the woods. I will find out for sure, though. |
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Post by fredwalter on Aug 9, 2011 10:39:06 GMT -8
I'm thinking that if I could keep the primary tank small, and insulate it well, I would minimize wasted heat in the shed outside. When I replaced the ~700sq.ft. addition on my house, I put heat tubing into the concrete slab. I'm going to build a small shed (~100sq.ft.) near my house, to hold whatever it is that I end up with, to heat water for my in-floor heat. But it is $8000+ for a commercially available wood-fired boiler... How is your project coming along? |
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