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Post by garybbq on Nov 10, 2014 0:11:13 GMT -8
Hello all I'm new here and I just thought I would give you the link of the efficiency of Heat transfer for different metals. I am currently investigating rocket stoves for heating a shop and possibly for a boiler for a home. Thanks for all the info, I'm sure I will have a ton more questions.  www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.htmlGary BBQ |
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Post by quorum on Nov 12, 2014 1:46:37 GMT -8
What I really dont understand about this design is why the solar piping goes to the buffer water tank, just to preheat it ?? Why it is not connected to the copper coil? Thanks
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Post by Donkey on Nov 12, 2014 8:48:50 GMT -8
The copper coil has VERY little volume. If I connected it to the coil, it would NOT thermosiphon, it would just sit there getting hotter and hotter until it finally boils, flashing to steam and blowing up my pipes.
The way it is currently, it will heat up the buffer water very nicely and safely.
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Post by quorum on Nov 13, 2014 14:58:44 GMT -8
So If I only want to build a rocket water heater I would just use the coiled copper and not the other connections right?? I will build this design this weekend I will post some pictures so you can see some adaptations that I have to do here....Thanks
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Post by ga2500ev on Nov 15, 2014 8:01:14 GMT -8
So If I only want to build a rocket water heater I would just use the coiled copper and not the other connections right?? I will build this design this weekend I will post some pictures so you can see some adaptations that I have to do here....Thanks Please re-read Donkey's last post. The buffer tank is the safety feature of the system that keeps it from blowing up. Let me walk you through it. Physics dictates that the boiling point of a substance is based on both temperature and pressure. So water boils at 212 degrees F only in a normal surface level atmosphere. When water is in a pipe, the pressure can increase above atmospheric pressure. Which means the boiling point temperature changes upwards. So the water can be superheated above 212 F and still not boil. The problem is that if that superheated water is exposed to atmospheric pressure after being superheated, it boils explosively. So for safety's sake you never want to heat water directly in a system where the pressure can get above atmospheric pressure. So let's talk about the buffer tank. It's primary feature is that it is open to the atmosphere. So it cannot raise its pressure above atmospheric pressure. So no matter how much heat you expose that water to, it's temperature cannot get above 212 F because at that point the water simply boils off. So the system works by heating the buffer water, then immersing the coil of the water you want heated into the buffer water. Since the buffer water cannot get above 212 F, the water in the coil cannot either. So the water inside the coil cannot be superheated. This eliminates the explosion risk of the water inside the coil. So I apologize for shouting, but this is really important: DO NOT DIRECTLY HEAT WATER INSIDE A PIPE! IT'S AN EXPLOSION RISK! I do hope this helps. ga2500ev |
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Post by 2tranceform on Nov 20, 2014 11:21:47 GMT -8
Very nice build Donkey! Great idea to combine the wood and solar heat. Do you have any measurements of the operating conditions (pre-heater water temperature, temperature rise for shower water, flow rate for shower water, ...)?
Thanks for sharing the design
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Post by johndepew on Nov 20, 2014 11:34:29 GMT -8
I second 2trance's query.
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Post by patamos on Dec 12, 2014 21:12:45 GMT -8
Donkey, thanks for building this thang! Yes i too am very interested in hearing about flow rate, temperature and duration. Scanning back through this threads there is mention of 20% efficiency. Which i presume to mean 'of all the btu's/Kws created by the fire, only 20% of that is captured by the hot water'. If so then the remaining 80% is captured by the body of the stove and/or lost in the exhaust (?) I am toying with the idea of combining a RMH to heat a central living area, with a water buffer tank to pipe heat via hydronics to 3 distant rooms in the house. Two of those rooms have 1" of tile (n mortar) mass over a plywood sub-floor. The other room would fare well with a firring out of the wall with lath, cob and hydronic tubing. I can do fairly accurate heat load-need assessments for those rooms, but generating stats on heat flow is the presenting mystery. ONe aspect that comes to mind is the surface area to volume ratio. The greater the surface area relative to volume, the better the heat transfer into the liquid is likely to be. In other words, would thermal transfer efficiency be better with two 30 gal tanks side by side (above the heat riser) rather than one 55gal tank? If considering a 6" batch box with Matt's new secondary air set up taming down the combustion... would i be producing too much heat over too short a time frame for the desired amount of heat transfer into the water tanks to occur? And finally, does anyone see issues with having a bench bell (well below the bottom of the water tanks) to soak up the remaining heat. I would be inclined to have a bypass flue mid way up the water tank bell no matter what, for start up and tweaking the system. Given how weirdly laid-out many existing houses are, this kind of combination (solar hydronics included) could prove to be a very sensible retrofit.... |
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Post by Deleted on Dec 13, 2014 5:05:49 GMT -8
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Post by Donkey on Dec 16, 2014 8:17:09 GMT -8
Donkey, thanks for building this thang! Yes i too am very interested in hearing about flow rate, temperature and duration. Scanning back through this threads there is mention of 20% efficiency. Which i presume to mean 'of all the btu's/Kws created by the fire, only 20% of that is captured by the hot water'. If so then the remaining 80% is captured by the body of the stove and/or lost in the exhaust (?) I usually try NOT to toss out numbers for anything. This was one of the rare exceptions, and it was all guess work, little more. There is no scientific method to those #s, just intuitive guessing.. I imagine that it would, no telling without doing the math and then attempting it for proof. Seems to me that for the things that you want to do, stepping up to 50 or 60 gallons is appropriate, but I would ALSO step up to an 8 inch batch-box. Build them into a bell and insulate the bell itself as well as is practical. Conduction through a steel tank into water is pretty good. It might be workable.. It's likely that the water could soak in so much heat that an after bench might fail. I can't say for sure as it hasn't really been done. If you want to attempt the math, you can find all the info you need (conductivity of the materials, etc) at engineers toolbox. |
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Post by Donkey on Dec 16, 2014 8:54:57 GMT -8
Very nice build Donkey! Great idea to combine the wood and solar heat. Do you have any measurements of the operating conditions (pre-heater water temperature, temperature rise for shower water, flow rate for shower water, ...)? Thanks for sharing the design As usual, the tools that I use to measure are my own senses, so I have no numbers for you. I can report how the system "feels". When I located the shower house, I placed it high on the hill to get the best view of the ocean. This turns out to have been a mistake as the water pressure to the building is rather low (gravity fed from tanks on the hill). To make up for this I plumbed the building in 3/4 inch copper to increase volume and I bought special low pressure shower heads. This did the job nicely, it made up for lower pressure with higher volume and with the old, dangerous water heater, all was well. The coil that I built is made 60 feet of 1/2 inch copper pipe. As a result, the hot side of my system has seen a marked drop in volume/pressure. It works, but just doesn't have the same punch that the older system had. When I take the shower head off to get a better jet, the hot side can flow through just a little too fast, starting quite hot (too hot) and slowly tapering in temperature. To make up for this, I back off the hot side 1/4 turn and the output is consistently hotter than is comfortable without mixing in cold. My assessment at this time is that, my pressure issues aside, 6o feet of half inch pipe has too little surface area to be a truly rockin' system. I'm considering buying a second 60 foot roll of the 1/2 inch and making a second coil around the first, tying them together at the top, back into the 3/4 inch piping. My thought is that this would solve both issues, surface area AND volume.. Alternately, I've been thinking about different kinds of radiators in place of coils. On-demand heat exchangers would probably work great but I've noticed that they tend to have 1/2 inch connectors, which would not help my flow problems any but would work great for those of you with good pressure. Edit just to say: On the UP side, the water inside the tank gets hotter faster with less wood than ever AND we've boiled the tank a couple of times figuring out how much is right with NO stress or worry whatsoever. |
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Post by 2tranceform on Dec 17, 2014 17:52:47 GMT -8
Thanks for the details Donkey. I think that a good view of the ocean from the shower is worth the extra effort. I thought about a shower like that at my place. Unfortunately, I have the same chance of seeing any of the oceans from my shower. Also, the FAA would not approve the height of my bathroom to gain such a view.  I am interested to hear about the effects of the second coil. I feel that would be enough surface area and flow to get the performance that you expect. It sounds like you have plenty of heat. Its just a matter of getting the heat to the shower. |
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Post by patamos on Dec 18, 2014 9:17:20 GMT -8
Thanks Karl, Makes sense that all that extra surface area will enhance thermal transfer. It gets me to wondering how reliably such piping could be welded into a tank. Maybe on a slight outward angle so the flue gasses exit the upper sides of the barrel near the top of the bell. This way the flue gasses remain contained in the bell and the top of the barrel remains exposed to the room for an unpressurized system. Such metal working is well beyond my ability. But if someone were to tinker and experiment with the design enough it could become a very effective way to optimize the creation of liquid heat. On to my list it goes, somewhere in the top 30 |
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Post by patamos on Dec 18, 2014 9:52:52 GMT -8
Thanks for the feedback Donkey,
I appreciate the intuitive sense that you bring to the matter. It is bound to be accurate enough (via cumulative experience) to offer a basic sense of things. A generally low percentage of overall heat transfer into the water is what gets me to thinking about how to make use of the remaining heat, eg. lower bell bench downstream...
Ya a bigger 55gal tank makes sense. The house i mentioned has an existing 6" chimney system so not sure i could convince them of the added cost of switching all that out.
If anyone out there has an existing 6" batch box with a downdraft barrel with removable lid... it could be very interesting to remove the lid and plunk another 55gal barrel full of water on top of that and see how it performs with heat concentrated only on the bottom.
If this proves effective enough then the water tank could potentially be partially (and/or adjustably) exposed to offer radiant heat into the room. I started imagining little insulated cupboard-like doors facing into the room... as a way to tweak things at will.
If bottom-only-heat delivery proves to be not quite enough heat into the water then gradually building a bell up the sides to find the sweet spot. Perhaps the bypass exhaust can have a T downstream to help tweak the delivery of remaining heat...
I'm getting going on a system in my workshop so it might be time to put these imaginings into action.
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Post by mwalimu on Dec 26, 2014 23:58:25 GMT -8
I thought also to add a water heating device to a normal RMH. Similar to your design, but the copper pipes fittet tightly from the inside to the metal sheet of the drum.
This will make it easy to fix/clamp the pipes without additional support structure.
The drum sheet itself will increase the surface for heat transfer, reducing the amount of the pipe.
And no need for much cleaning as the space for the exhaust gasses will stay free.
One could even use rubber hosepipes for connecting to a water storage tank, as the water will never go over 100°C. So they will be flexible for removing the drum.
I built a lot of Kitchen stoves w/heat exchangers in Africa. We just used an insulated 200 liter plastic drum with thermal circulation for storing the hot water, placed a bit higher than the stove or in the attic. The trick is to have enough water storage, so you cannot overheat it. But even if it starts to boil, the plastic tank will not melt. And you need a lot of heat to convert 200 liters of water into steam...
You need an open system to let the steam out, if this ever happens. With copper pipe in the stove and the rest plastic/rubber, you will never have corrosion.
For those who fear bacteria: One could put a copper or plastic pipe spiral in the water storage tank, connected at the one end to cold water supply, and on the other end to the shower/tap. So your cold water will be heated up only when you need it. No heat for bacteria breeding... You need a certain length of pipe and a thermostatic regulated tap, as the temperature is changing in the spiral during use.
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