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Post by rhyddid on Mar 6, 2008 10:48:56 GMT -8
Have been pondering the subject of water heating... Having checked out the back boiler system for a traditional woodburner at the bothy and musing over a brew from my lovely kelly kettle... a thought occoured to me. Why not build the heat riser out of a sealed cylinder containing water? Cold water intake at bottom of heat riser and hot water exit at top. Copper Pipes come to and from a header tank somewhere above stove (and through barrel) by thermosyphoning. Tank has a vent to highest point of building to avoid too much pressure. Hot water could then be used for house or excess sent to bench to charge using a valve for draining down system at either end of bench. Geddit? Can draw a picture but difficult to post. Very keen to know if this has been thought of before. Surely must have? My only question is whether there needs to be a temperature difference between inside/outside of heat riser to make the thing draw. If it works, then adding a vented solar hot water system to the tank should make for very cheap and easy to build water heating. Whaddya reckon Donkey? Rhyddian
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Post by Donkey on Mar 6, 2008 19:47:50 GMT -8
The heat riser MUST be insulated. For that complete, clean burn you need to keep the heat in for an appropriate run. Afterwards, you could do just as you've said, place the water cylinder over the heat riser.. It would look like a double height heat riser. The thing I've found though is that this kind of arrangement provides a relatively small surface area for collecting heat into the water. I have stuck a central flue water tank over a rocket stove.. It does work, though a whole lot of heat just leaves the system out the top. This is another reason why I'm so attracted to the down-flow method in the image below.
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Post by canyon on Mar 10, 2008 1:15:57 GMT -8
I really like Donkey's downflow design but I think you need even more surface area of heat exchange than just a tank unless the flue continues through a mass bench or something as there will be a lot of heat left. A tube shell tank of sufficient length with turbulators in the tubes to break up the laminar flow is what I have in mind. This is common in efficient clean burning wood gasification boilers. Another interesting method is the Garn unit which after high temp combustion in a long refractory the hot gases pass back and forth through a giant water storage tank. The 2000 plus gallons of water is a heck of a lot of thermal storage that can be used to heat several buildings and domestic hot water as well. I have been intrigued with the idea of one of these rocket fired instead of induced draft as the Garn is.
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johng
New Member
Posts: 10
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Post by johng on Apr 3, 2008 3:24:22 GMT -8
Hi Guys,
I'm new here, and have yet to build a rocket stove, but am starting to plan...
Here is my thought on heating water in a rocket stove: Why not bury a copper coil in the cob bench, surrounding the exhaust tube, and have it tie into a h2o tank. It will take awhile to heat a large volume of water (since you have to bring the cob up to temperature first), and you won't get near-boiling temperatures, but it seems like it would tie in well as a secondary source of heat for hydronic heating (e.g. to supplement solar hot water during cloudy conditions.)
Any thoughts?
-john
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Post by canyon on Apr 3, 2008 19:12:34 GMT -8
Johng, check out the thread "Water Pipes In The Bench" in the Experimenters Corner section!
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johng
New Member
Posts: 10
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Post by johng on Apr 4, 2008 2:15:01 GMT -8
Thanks, that's what I was thinking... Can't believe that I missed it there. Will reply on that thread.
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Post by Donkey on Apr 6, 2008 18:04:27 GMT -8
Something that I forgot to add here and don't think anyone else mentioned yet.. Heating water creates a GREAT deal of condensation on the water tank. I learned this one from experience and have had to tear a system back to the ground to re-design in drainage. The cob below was showing wet and the thought of a 50 gallon tank coming down on someone.... There is a great deal of water vapor present in flue gasses, even from the driest wood. It WILL turn liquid again the moment it hits cooler metal. Provide an easy escape and all will be well.
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Post by canyon on Apr 9, 2008 23:44:35 GMT -8
Excellent point Donkey! What are you providing for an easy escape? Stainless steel seems like an obvious choice to me. The condensate is a bit acidic as well as goey after a few uses. Small drains can plug up in a hurry. Corosion and cleaning can be problems as well as drainage. Good access for cleaning along with larger drainage is what I'm thinking.
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Post by Donkey on Apr 10, 2008 16:40:38 GMT -8
Well, for that project I built a gravel drainage channel and connected it up to the french-drain foundation trenches. The drain ended up being everything under the tank.. A circle with a radius of around 2 feet to the depth of the wall drains, about 3 feet down.
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morpho
Junior Member
Posts: 50
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Post by morpho on Jun 12, 2008 19:04:20 GMT -8
Okay, I'll bite! I just finished Ianto and leslie's book (new edition) and my gears are turning like mad. The first thing that popped into my head was hot water experiments. My thought was to build the RS outside and run a 30 foot flue in an equally long insulated trough of water. I was thinking a channel made of straw bales with a poly pond liner inside, suspend the flue and Insulate the top with more bales and fire it up. Probably not as efficient as the things I have seen in a very quick look around the net, but not going to blow up either. Would it be a stupid waste of wood? Have I lost the plot? ....again.
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Post by canyon on Jun 13, 2008 8:39:00 GMT -8
Morpho, what is the plot you might of lost? Sounds like a cool experiment but what are you wanting to do? The bales would need staking/backing up with mass as the hydrostatic pressure of such a trough will push hard. How would you suspend the flue without melting/puncturing the pond liner? The pipe would need to be water tight with welded joints. Again, check the web for the Garn to see a solid wood fired hot water unit that is an open system. I'd love to see a rocket fired version!
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morpho
Junior Member
Posts: 50
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Post by morpho on Jun 13, 2008 8:50:58 GMT -8
Hi Canyon,
I have a tendency to let my mind wander into fanciful creations...that sometimes are a bit too far off course. I am always open to people offering advice to direct my mental wanderings. So, the I was thinking the bales could be pinned with upside down U shaped braces so the pipe could be suspended from above. That way no heat element directly touches the liner. I live in oil country so long lengths of pipe are everywhere...do you think the oil company would miss 30 feet of their pipline? hahaha...just kidding guy's! I'm not sure what I want the hot water for yet. Maybe to pump through the pex tubing in the floor....take a shower....probably more efficient ways to do both. Thanks for the heads up on the wood fired hot water heater. I'll scour the web.
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Post by Donkey on Jun 15, 2008 8:57:54 GMT -8
Morpho, Sounds like a cool experiment to me.. 30 feet of pipe inside a jacket of water. Only difficulty I see here is condensation inside your pipes, as stated before.. Otherwise I see no reason your idea wouldn't work. Dealing with a big floppy pond liner may be a pain in the butt, still.. Do-able.
Hmm.. It is possible that (especially when first lighting) this stove may back up and not work properly. 30 feet is quite a long run through water. It is possible to cool the flue gasses to the point where they are colder than outside air conditions. when this happens, they will no longer rise but fall (relative densities, etc). You could actually run the exit stovepipe downhill to increase draft. Then, as the system heats up and flue gasses become warmer than ambient, you would need to rotate the exit chimney back upwards again for proper draft. ... I would skip the chimney in your design.. Just pop the pipe (horizontally) straight out, or angle the whole thing ever so slightly down so condensation will drain and rely on the heat riser alone for draft..
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morpho
Junior Member
Posts: 50
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Post by morpho on Jun 16, 2008 20:02:27 GMT -8
Hi Donkey,
Thanks for that...more good ideas. Didn't think about water to air temp.....hmmmmm?
I re-read Ianto and leslie's book and a lot of the posts here and got more ideas/questions....that I will post in the appropriate list.
Thanks.
-Keith.
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Post by dakman on Feb 3, 2011 18:45:53 GMT -8
Hi all; I am new to the forum, but I've been spending time here trying to learn all I can about Rocket technology. I am interested in building an "on demand" system for water heating. My goal would be to have a 20 to 30 thousand BTU / Hr. system running at a duty cycle of say 20 to 50 percent. Has anyone tried running burn cycles intermittently by sealing off inlet air for a period of time, then allowing air back in so spontaneous combustion takes place? They do this type of thing with large wood fired boilers. I have a preliminary design using a 4" combustion chamber and heat riser. My inlet for the combustion air is low, actually in line with the horizontal combustion chamber. An "L" shaped system similar to the rocket cook stoves. My fuel magazine is vearly vertical. 60 degrees from horizontal. This allows for the intersection of the mag and the combustion chamber to be very close to the elbow. I intend to use an air tight cover to cap the fuel magazine to prevent smoke back. The fuel magazine is actually an integral part of my water tank, so it is completely surrounded by water. I have borrowed this idea from the Hampton Jetstream wood fired furnace. The idea is to keep the magazine cool to help assure that only the tips of the wood burn to minimize smoke. The barrel too is built into the tank assembly. I also plan to run a 4" dia. exhaust heat exchanger through which the exhaust gases will assend thru the tank as they exit the system. The whole design is far from traditional I know. This is partly due to the fact that I've spent the last 17 years working as a designer for a large metal fab company. We also work with castable refractories. I'm afraid I would have a very difficult time doing this with fire brick and cob. I have a .jpg image of my design, but since this is my first post, I have yet to figure out how to attach it. Maybe one of you can enlighten me? I look forward to any replies.
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