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Post by rectifier on Nov 14, 2011 21:48:35 GMT -8
I have not run the stack to below the firebox - however, I have run it level with the firebox a few times, 0' of theoretical stack. It will operate for some time, and can even run quite clean during the initial stage where riser is hot and barrel is cold, giving a few feet of theoretical stack. However, as barrel temperature increases, the theoretical stack approaches 0, the draft falters and the fire begins to weaken and smoke badly. At that point, as there is no chimney to take smoke away, the tests tend to be rapidly terminated  Here's a question for Donkey and other RMH owners, what is the 'standard' run-time you operate your stoves for? I have found that for the first hour or two, a significant external stack is not required - the 3 footer is quite adequate - and with a larger riser and barrel, you may even get a bit more time than that. However, as time passes and temperatures equilibrate within the riser/barrel system, that stack becomes more and necessary. I'm shooting for +8 hour runtimes - I think that's a bit outside of typical rocket firing patterns, but you tell me. Rural, how are the Alberta winters, how many cords of wood do you go through with your regular stove? I'm in BC myself, plan on making the jump over the mountains next year, hope it's not too cold for the wife over there. |
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Post by Donkey on Nov 15, 2011 2:16:17 GMT -8
8+ hours is WAY outside what I need here..
All we need is an hour or two per day (sometimes 3).
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Post by rectifier on Nov 16, 2011 17:51:29 GMT -8
I believe that is probably well within the time in which the initial temperature differential will remain adequate to drive the stove with a minimal rise on the stack.
From photos of your stove/house design, my guess is that you live somewhere in the southern USA? Where it can get cold for awhile (usually overnight) but not the sustained cold of Canada that seeps through everything and freezes it solid?
I'd like to hear results from an RMH builder in a colder part of Canada, like the prairies... in Calgary this weekend it is supposed to be below -10C the entire weekend - that's a lot of heat required from any thermal mass, and it's not even really 'winter' yet... But I wouldn't depend on one to heat a house without proven results. A backup heat source is essential to avoid frozen pipes or literally freezing to death!
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Post by Donkey on Nov 16, 2011 19:08:50 GMT -8
I believe that is probably well within the time in which the initial temperature differential will remain adequate to drive the stove with a minimal rise on the stack. From photos of your stove/house design, my guess is that you live somewhere in the southern USA? Where it can get cold for awhile (usually overnight) but not the sustained cold of Canada that seeps through everything and freezes it solid? Northern California, right on the coast.. VERY forgiving climate both for heating AND cooling. I would as well. I'm sure that there's room for improvement, what better place to do that work than in a cold climate? OTOH, these stoves ARE experimental, backups ARE prudent. Don't risk everything on an experimental stove. I CAN say that masonry stoves in general were invented and used primarily in places that experience EXTREME cold conditions. Masonry stoves major trick is thermal mass. In my mind, RMH's are essentially a variant of Masonry stoves. Same idea, different firebox. Granted, I advocate (and rocket stoves (and stovers) tend to be) a dirt simple, DIY, trash-and-mud building style that may not be entirely appropriate for some extreme conditions. OTOH, the concepts here can be EASILY adapted to just about any level of sophistication in terms of material or fabrication. Conversely, the ability to make functioning rocket stoves out of (literally) trash and mud or even JUST mud makes them a (potentially) fantastic survival tool for just about anywhere. |
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rural
New Member
Posts: 38
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Post by rural on Nov 17, 2011 6:31:36 GMT -8
Rectifier, your results are consistent with mine. Not exactly the same, given the fact that my setup is pretty wonky, but consistent. I'm now looking at an angled fuel feed tube, maybe at 45-degrees, to drop the air intake. Still experimenting with a clay-ash mix before going full-bore with this. Sad to say, but Alberta's winters seem to be less cold and snowy than they were 20 years ago. Now, from mid-November to late February we get lows of around -20C and highs of about -15C. That's my guess as to the averages. We will get bursts of really cold weather as low as -40C one or two times a winter. Daytime highs start to drop below zero in mid-October with the lows around -10C. We don't heat with wood currently (natural gas), but people who do tend to use 6 or 7 cords. I know of one person in Calgary (slightly warmer than us) with an efficient masonry woodstove that would burn much less than a cord. But they are efficiency gurus. Rural, how are the Alberta winters, how many cords of wood do you go through with your regular stove? I'm in BC myself, plan on making the jump over the mountains next year, hope it's not too cold for the wife over there. |
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Post by rectifier on Nov 17, 2011 21:54:05 GMT -8
I agree that RMH are simply a breed of masonry heater. In fact, the 'bell rockets' on this site are very similar to a masonry heater. The firebox isn't even that different and principles are similar, the insulated riser is the only significant difference and all that does is create a hotter burn area.
I would think that in Canada the thermal mass may have to be a bit more advanced than dirt and rock, as it simply must hold more heat. Water of course comes to mind as the most easily handled material that can absorb a lot of heat without taking up too much space. In fact, water is odd due to its hydrogen bonding, and "At 4181.3 J/(kg·K), water has the second highest specific heat capacity of any known substance (after ammonia)." Make sure the tank is open to the atmosphere so you don't blow it up and water is pretty safe.
Less than a cord in Calgary! Is this a modern, powered stove with blowers/condensers etc?
I noticed that we have drifted off the RMH drive topic a little, maybe Donkey could fork off some of these last posts into another thread, maybe called "RMH in extreme cold" or something like that?
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Post by Donkey on Nov 18, 2011 9:29:17 GMT -8
I agree that RMH are simply a breed of masonry heater. In fact, the 'bell rockets' on this site are very similar to a masonry heater. The firebox isn't even that different and principles are similar, the insulated riser is the only significant difference and all that does is create a hotter burn area. Simply a hotter burn area is probably good enough. It will, assuming enough air, burn more of the fuel, especially the stuff that's only burnable at higher temperatures. Beyond that (and ON topic) it seems to me that the sudden 90deg. turn sets up a little cyclone, improving mixing and introducing the possibility of the creation of an INTENSELY hot zone inside the cyclone. I think that (just maybe) the sudden expansion created by this hot zone could be adding force to the system. If the above effect IS happening, it's clearly NOT at work in every rocket stove. I've noticed that 6 inch stoves (and smaller) don't usually exhibit the same dynamism as 8 inch stoves, though sometimes they do. I've not been able to confirm (or shoot down) my hypothesis as of yet. Peterberg's little beauty on the small scale development thread has been exhibiting some interesting behaviors.. The better (way expensive) ones that I have seen are soapstone. True of course.. Water as heat storage has disadvantages too. From day one it works to get out of whatever you stick it in. It's great stuff, probably the absolute best for some applications, but it adds complications. My criteria is pretty simple. I ask, "does THIS solution add more problems than it solves and does the weight of these new complications overbalance the problem I was trying to solve in the first place?" Might be a well insulated house w/good air exchangers and a quality masonry stove w/ lotsa mass. Ya, noticed that. Considered what you've said.. I like to let things lay as much as possible here. Things can be learned from the string of consciousness of the posts, new relationships between things, etc. This is a good subject that should be over in the heating area.. Go for it, I'll sticky it. |
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Post by mintcake on Nov 21, 2011 22:28:21 GMT -8
Make sure the tank is open to the atmosphere so you don't blow it up and water is pretty safe. But if you've got a large volume of water, open to the dust etc, which never gets much above 50 degrees C then that's a breeding ground for all sorts of nasties, e.g. Legionnaire's disease. I'd stick with more earth/rocks myself. |
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Post by pyrocasto on Nov 25, 2011 18:39:12 GMT -8
Water is a great storage device and with a little chemical(bleach or such) you can keep it germ free. Use a water trap or expansion exhaust(hole in a ballon) and you can keep fresh air out anyway for the most part.
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Post by kwillets on Dec 11, 2011 15:33:45 GMT -8
This doesn't seem like a very hard calculation. If you take the formula for stack effect at en.wikipedia.org/wiki/Stack_effect, you can plug in a stack temp and height, eg 1000F and 3 feet: 0.0188*15 *3 ft*((1/(460+1000F))-1/(460+70F))=-0.00102 psi pressure difference. The hard part is calculating the friction losses, which determine the maximum air velocity. There are some formulas here: www.engineeringtoolbox.com/major-loss-ducts-tubes-d_459.html. I haven't tried that calculation, but from the example they give it looks like 0.001 psi would give you a few feet per second in a 6" system. |
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Post by Donkey on Dec 11, 2011 15:51:30 GMT -8
The formula for stack effect is fine as long as the heat riser exhausts into the environment directly.
The numbers stop making sense in a real application, with down channels and so on. The product of the stack formula for the riser will need to be (at least) balanced against the opposite in the down channel, etc. Also as you pointed out, friction needs to be included and whatnot.
A few years ago, I did some calcs along these lines and discovered that the actual performance of these stoves didn't match the theoreticals based on numbers. Seems to me that there's more going on than pure stack effect. Also when you consider everything that SHOULD be working against the system, it's sort of amazing that they work at all.
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Post by kwillets on Dec 11, 2011 18:01:47 GMT -8
You can use that same formula for the downward leg -- I get about .0004 for 250F, or .0003 for 200. There's also a buoyancy vs. resistance draft calculator at www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html . It seems to give high figures: Outside density (kg/m3): 1.201 Inside density (kg/m3): 0.435 Natural draught pressure (Pa, N/m2) : 6.8 Duct Velocity (m/s) : 3.3 Air Flow (m3/s) : 0.06 Air Flow (m3/h) : 208 Air Flow (cfm) : 122 for these values 21 outside temperature (oC) 538 inside temperature (oC) (1000F) .9 height (m) .15 duct hydraulic diameter (m) 2.5 duct length (m) (added some extra) 2.6 Σξ minor loss coefficient (summarized) (best guess based on one 90 deg bend and entrance/exit, from tables) Do people really see 3.3 m/s (11.8 kph) air speeds? I suspect the real speed would be a fraction of that if all obstructions are accounted for. |
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Post by kwillets on Dec 25, 2011 22:28:02 GMT -8
I found a fairly extensive paper that does more detailed calculations for rocket stoves here:
A Simplified Model for Understanding
Natural Convection Driven Biomass Cooking Stoves
A simplified model of the fundamental stove flow physics is developed for predicting bulk flow rate, temperature, and excess air ratio based on stove geometry (chimney height, chimney area, viscous and heat release losses) and the fi repower (as established by the stove operator). These parameters are intended to be fundamental inputs for future work understanding and improving biomass cook stove emissions and heat transfer.
Experimental validation is performed and the simpli ed model is shown to be both
accurate and applicable to typical stove operation.
digitool.library.colostate.edu/exlibris/dtl/d3_1/apache_media/L2V4bGlicmlzL2R0bC9kM18xL2FwYWNoZV9tZWRpYS84ODM2MQ==.pdfThere's a lot in there, but he basically starts out with some buoyancy and friction loss calculations and adds a lot of corrections. I'm not an expert in this area, but I did learn enough from my earlier searches that this is a low-Reynolds-number regime, and a bit outside of normal flow calculations -- for instance the "minor losses" are major, and viscosity varies quite a bit with temperature. This paper seems to cover these concerns. One thing I did notice in all the data is that mass flow is almost flat vs. power output; lower levels simply pull the same amount of air. Possibly that's due to gas viscosity increasing with temperature and making it harder to push gas up the stack, even as buoyancy increases. I can't say for sure yet unless I digest all the equations. |
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Post by bmeagle on Jan 20, 2012 11:39:26 GMT -8
kwillets thanks for the link to that thesis, there is a lot of useful information in there should one want to simulate a RMH in more detail.
One day, when I have more time!
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Post by woodburner on Feb 9, 2012 2:23:32 GMT -8
GOOD! We need more open minded skeptics. I should be ok then. Ouch! Me and my big mouth Now I will have to do some work. Useful comment. I find this in my work looking after peoples oil heaters, so I should realise, but nevertheless I tried a stack of loose bricks as a first attempt. No surprise, It didn't work too well. Another priceless tip. Many thanks to you and peterberg. I'm so glad I found this forum, I could have been wasting years getting to the same place you are at. As it is I still have a long way to go, but it's much less than it would have been. I have a big pack of ceramic fibre board which I intend to use for for the burn tunnel which will thus have good insulation. It will be easy to form a trip wire with a bit of sanding, and will have minimum joints. I have a Kane 455 analyser so I can get some measurements. Unfortunately I use Macs, and so the software for the analyser is no use for data logging as it's Windoze only. When the weather is a little less cool, I will get a stove running and let you know. |
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