Header-style heat exchanger

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Eric Johnson, Oct 20, 2007.

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  1. Eric Johnson

    Eric Johnson
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    Now I'm thinking about making a header-style heat exchanger out of 1" rigid copper with 1/2-inch copper branches connecting them. Is there an optimum spacing between the 1/2-inch sections? Or, to put it another way, how many should I put per section if the 1-inch headers are 5 feet long?

    And the way I understand it, each header has a cap on one end and an inlet/outlet on the other, so that water flows into one end of the hx, then across the 1/2-inch sections to the other header, and then on to the next section or back to the boiler, depending.
     
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  2. leaddog

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    I don't think it makes a diference how close you space them. You can space them with 1in tee's next to each other. that will give you alot of cross pieces. I'm sure that some where there is a limit on heat transfer so that you would have excess 1/2 in lines but my attitude is the more the better or untill the credit card says stop. I built a couple of these for a boiler and put them in the top to use in a pole barn. The idea was to put antifreese in them for intermitent use. I ended up useing them for the house heat and they worked well. I kept the closed system in the house that way. I think they were made with a bout 70ft of 1/2in branches with 3/4 in mains. worked very well for me
     
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  3. leaddog

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    I just hit the wrong butten and wasn't finished.
    If you look at nofossels tank you will see his house heat ex is made that way and it doesn't look like there is many feet of copper doing the job for him.
    Yes you want the water to enter one end of the manifold,travel thru the branches and exit the oposite end of the manifold. That makes the lines all about the same length so each will flow about the same water and transfer heat about the same.
    My top ex in my tank is made from 1/2 in finned stainless and is in 4 loops manifolded together into 3/4 copper.
    But you said you had alot of 1in copper and I would use that instead of 1/2in if I had to buy it. The tee's will cost the same and you would use less of them.




    but then again, You could sell the 1in stuff for scrap at $3 a lb and buy alot of 1/2in. I helped my bil last year tear out alot of copper pipe from a store last year. We cut all the 3/4in to 2in stuff we could and sold it for over 3 grand and that was at only $2 a lb. Its now $3. I wish now that I had saved alot of it as it would have worked got on some of my projects. Must of it was 1-1/2 and I didn't want to buy fittings
    leaddog
     
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  4. Eric Johnson

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    I'm glad to hear that, 'dog. I'm thinking 8 units with 6-8 crossmembers--four for stashing heat and four for recovering it. That's two separate heat exchangers in the tank, with pumps on both sides.

    I have a lot of 1/2-inch copper water pipe that I can replace with pex, particularly in the greenhouse, where periodic freezups play hell with the copper anyway. Pex is supposed to be more tolerant of the occasional lapse, and it's a lot easier and cheaper to repair/replace.

    I'm a fair hand with a torch, so I think I can drill holes, ream and solder directly pipe-to-pipe with silver solder. Save a fortune on fittings. We'll see--I'm going to try to make a prototype this weekend to see how it works out.

    I have A LOT of copper in this old house and it seems to me that a heat exchanger is the "highest and best" use for some of it--at least as much as I need right now.
     
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  5. Nofossil

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    My decision on the number of parallel 1/2" pipes was the net flow resistance. If you have them, more is better, especially at the top of the tank.

    Based on my experience with copper solar panels, make sure that you have a good fillet on you braze joints and be very sure that you won't have stress concentration points as things heat up and cool down. I got three glazed copper solar hot water panels for free because of a design defect that resulted in just such a stress concentration - broke the headers in all of them.
     
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  6. Eric Johnson

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    This is what I have in mind. Just silver solder. How would I go about avoiding excessive stress on the joints? After I build it, should I hook the thing up and run hot water through it for a few days to see if it holds up?

    My 1" is all Type M. I'm sure most of the 1/2-inch I have is Type L. Do you know if they make Type M 1/2-inch? If so, it ought to be cheaper and maybe I should just buy it.
     

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  7. Nofossil

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    Don't know about the type M 1/2". Stress example: If you had one end of each header rigidly attached to some mount point, it would have to bend there as the length of the 1/2" pipes changed with temperature. Just grab a friend and a beer and spend a little time thinking about what happens as the exchanger gets larger / smaller. A curved piece of soft copper goes a long way towards absorbing stress. We're not talking a lot of movement, after all.
     
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  8. Eric Johnson

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    OK. Type M 1/2-inch copper pipe is $1.15 per foot at my local supply house. I think with my 80 feet of 1" copper and 200 feet of the 1/2-inch, I can make a couple of kickass heat exchangers.

    My tank measures 4' x 7'. Anyone have any thoughts on the best size for each section? I'm thinking four on the bottom and four on the top, with more 1/2-inch pieces on the top. Or would it be better to have fewer sections with more soldered 1/2-inch cross sections?

    EDIT: According to an HVAC fabrication guru on another website, silver solder is stronger than standard, soft solder, so it holds up better. He said that using silver solder in this application meets code. He also said that the hx I described has enough copper in the water to get the job done. I can't wait to get started, but I have to wait for the pex-al-pex to arrive so that I can get at the copper and get to work.
     
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  9. Nofossil

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    My only caution would be that if you want to minimize the difference between the tank top and the water coming out, you can never have too much copper in the top. You've seen mine - I will be adding copper fins to my domestic hot water preheat coil (50' of 3/4 soft copper).

    My free solar panels were brazed. The joints didn't fail - the pipe next to the joints failed. Your joints look great, by the way.
     
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