sasha
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Posts: 3
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Post by sasha on Mar 14, 2015 9:33:10 GMT -8
Hello All! I am seeking advice and feedback on our design. With HUGE help from Donkey, I have started the design for a rocket mass heater, that will heat both hot water for our bath house and a sauna. We are a community of about 20 people, with frequent groups of workshops students, bringing our numbers up to between 30 and 50 on a regular basis. Our hot water needs are to be able to accommodate several short showers, with relatively short time needed for the water to recharge. When the flue gasses leave the heat riser there will be the option of sending them into the pipes in the sauna, or into the bell that the water heating tank is in. The left over heat coming out of the sauna will also go to the bell for heating water. The water storage tank that the copper pipes are submersed in will also be hooked up to a solar thermosiphon system. The water heating system will be based on Donkeys design, of submerged copper coils in a barrel of water. ( described here: donkey32.proboards.com/thread/1096/hot-water ) The rocket stove itself will be a front loading, batch box style. We have this style for heating our commons, and it seems to work well for having so many different people using it, and not necessarily knowing how to use a rocket stove. -One question I have is to do a 6" or 8" system. We have a lot of used 6" pipe, so if people think this would be sufficient I would prefer to use 6". - What size should I use for the barrel of water? I like donkeys idea of using a used hot water tank, that comes already plumbed for the hot and cold solar thermosiphon. -Is a run of 50' of 3/8" copper coil pipe seem sufficient? Here is a design of the bath house itself, as well as a breakdown of the RMH. Any ideas and feedback are helpful! Thanks!!! Attachments:
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Post by Donkey on Mar 16, 2015 12:42:44 GMT -8
Sasha, your images are huge! For future reference, images that are intended for viewing on screen (web stuff, etc.) should be a MAXIMUM of 1000 pixels (dots) wide, your images are about 5 times larger. I've replaced the large images with smaller versions. OK, the project at hand. I think that if you are going with a batch-box, you can probably get away with a 6 inch model. If you were going with the J-tube design, I'd say 8 inch. The water barrel on mine is about 20 gallons, which seems like a good size. It heats up in a reasonable time and in one good heat, we can take 3 or 4 showers. My heat exchanger coil is 60 feet of 1/2 inch copper, which in retrospect seems a little small. At fully-on, it starts hot but then tapers off; if it is run at half strength, there is no noticeable tapering of heat. Tells me that the surface area of the exchanger is too little. I've been contemplating adding a second 60 feet of 1/2 copper next to (inside) the first. I calculate that my heat exchanger (the copper coil) has about 7.8 square feet of surface area. 50 feet of 3/8 inch copper is going to have less surface area yet, about 4.8 square feet. I think you will need much more than that. It seems to me that for the cost (or maybe a lot less) of enough soft copper pipe to do the job, you could get a different kind of heat exchanger that might work better. Something like this maybe.More later.
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Post by Dan (Upstate NY, USA) on Mar 18, 2015 3:35:05 GMT -8
Donkey that is a water to air heat ex-changer, most water to water are tube and shell heat ex-changers, same with steam to water, FYI. I think you were right in the first place with the longer tube run...
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Post by Donkey on Mar 18, 2015 12:39:22 GMT -8
Wolf, I noticed that and am sure it work anyway. What would the drawbacks be?
The water to water ones have SO little surface area, I doubt that they would be nearly as useful for this purpose. 60 feet of half-inch soft-copper costs around $200.. 2 of these is an excessively expensive heat exchanger.
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Post by Dan (Upstate NY, USA) on Mar 19, 2015 4:10:41 GMT -8
We had one water to water heat exchanger in our plant that was similar in a way, it was supposed to be super efficient in heat transfer with forced circulation on BOTH sides... it got clogged with particles over and over and had to be cleaned out all the time. It got "retired" in place.
I am NOT an engineer but engineers are paid to make things work the best with the least amount of capital/materials. 99 percent of the forced circulation to natural circulation water to water heat exchangers are all tube and shell, with the forced circ water in the tubes and the natural circ water in the shell. Who knows? The only way to find out is to try. I would like to but I have too little time or money.
What about 100' 1/2" of pex for less than 30$? I am sure it doesn't transfer heat as well as copper but if you got the room in the barrel who cares?
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sasha
New Member
Posts: 3
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Post by sasha on Mar 19, 2015 7:10:16 GMT -8
Thanks for the thoughts. I was also wondering about the 'water to air' vs water to water heat exchangers. This part feels like the biggest mystery to me. I like the idea of using the heat exchanger rather then the coils, as it seems a lot easier.
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Post by Bobwieser on Mar 19, 2015 7:27:36 GMT -8
linkBuilditsolar.com and linkwww.n3fjp.com/solar/index.htm have lots of DIY heat exchangers. I think it's best to have turbulence and surface area. Pex will work though it does not conduct as well as copper. Larger pipes hold more water but has less surface area to absorb the heat. As I recall they concluded that fins like on a radiator did not help much. Someone used pipes with corrugations for turbulence and surface area with good results, a lot more expensive though.
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Post by Donkey on Mar 20, 2015 15:41:38 GMT -8
Today I called the business that makes those heat exchangers, talked to the guy for some time (nice man, interested in the project, tried to help with suggestions). Upshot of the conversation: those heat exchangers probably won't work very well for this application. The surface area of the pipes inside the exchanger is a LOT less than a reasonable copper coil and the added surface area of the fins isn't enough to compensate.
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Post by Dan (Upstate NY, USA) on Mar 21, 2015 15:09:11 GMT -8
THe hardest part about PEX would be making sure it doesn touch the sides of the barrel in case it boils dry...
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Post by Donkey on Mar 21, 2015 18:50:23 GMT -8
So.. Wolf1004 suggested to me that PEX might be a good choice. I honestly hadn't thought of it, as it's a material that I know so very little about. Wolf said:
I think you would need to make some kind of scaffold or framework to wrap it around, something that can be suspended and keep the PEX from touching the sides. I'm also assuming that the PEX will not be as conductive as copper, so you will need a lot more of the stuff. This should be OK, as long as the price is right.
Any suggestions or ideas about how much would do the job?? Anyone here work with the stuff enough to have an idea?
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sasha
New Member
Posts: 3
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Post by sasha on Mar 21, 2015 19:47:05 GMT -8
Seems worth trying, since it would be so much cheaper. Maybe I will try that first, and then it wouldn't be too difficult switch it out with copper if it doesn't work well. Thanks again so much for the help!
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Post by Bobwieser on Mar 23, 2015 7:10:55 GMT -8
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Post by matthewwalker on Mar 23, 2015 7:19:43 GMT -8
I'm not following this thread very well, and it's outside of my experience for the most part, but I wanted to throw out that 1) I've used PEX quite a bit for little DHW preheat projects over the years and it is an easy/cheap way to get a system up.
And 2)...There is a cheaper material that I use for my exchangers, but I'm not sure how it will hold up at higher temps. Home Depot sells 500' of 1/2" black poly thin walled "drip line" for drip irrigation systems for $44. It's super weak, but I have 500' on the roof in boxes and 500' as the DHW preheat sitting in an old hot tub that acts as the buffer tank. The stuff works great for this application and you can't beat the price. Highest I've gotten it is in the 120°F range, so it may or may not work in this case. I imagine it's getting way hotter on the roof when the pump isn't running, and it did spring a leak up there but it was where it was chafing at a fitting. I think it might be worth a try if you are staying w/w for the exchange.
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Post by Bobwieser on Mar 24, 2015 1:17:10 GMT -8
Gary at builditsolar reported on an experiment with irrigation pipe for a DWH heat exchanger. He found that when it was heated to his max solar storage tank temp that the water in the pipe had a plastic smell. Mat does yours smell of plastic? I would love to have free heated 120 deg water for showers. The next concern is Legionella from stored water between 68 to 113 deg F and not heated to 140 deg F to kill it. Maybe I'm just to paranoid.
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Post by matthewwalker on Mar 24, 2015 7:15:09 GMT -8
I didn't notice any smell, but I guess I wasn't looking for it either. I took a lot of showers with that water though, I would have noticed if it was significant.
The Legionella thing is a non issue as far as I'm concerned. I mean, I wouldn't use a single tank system as that's just asking for problems, but I can't see the difference in an exchange system like mine and a normal water heater. Every house in this country has a heater set at below 120 and I might go on vacation for two weeks and come home and never think twice about it. So, yeah, .....I dunno, but I'm comfortable with this for myself and my guests.
By the way, the whole set up, coils, $12 12vdc pump running off of my solar, and all that was around $100. A little more to plumb it into the house with PEX and valves and stuff, but it knocked $40 off my electric bill in August and September, and I am just about ready to start it up again for the season as we get out of frost mode and into the sun here. It was able to easily keep the hot tub at >110°F 24 hours a day even with moderate use. I expect it will basically shut my water heater down for 4-5 months, and that is my largest purchased energy expenditure month to month for sure.
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