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Anyone using coiled PEX as a water to water heat exchanger?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by SteveJ, Nov 25, 2007.

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  1. SteveJ

    SteveJ Member

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    Is anyone using a storage tank with submerged PEX as the HX?

    I realize that there should be 3 to 4 times as much PEX as copper required for that same heat exchange, but PEX is about 1/6 th the cost of copper.

    Just wondering how to minimize cost of a heat exchanger in a storage tank without using an additional pump for an external exchanger.

    Thanks,
    Steve

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  2. Corey

    Corey Minister of Fire

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    Interesting thought. Do you have the actual R values (or of more use, the U value) for the two materials? I would be surprised if only 3-4x as much pex was needed, but it may be so. Also something to watch out for (I don't know if this is the case with pex or not) but certain hoses are rated only to carry fluid on the inside. If they are immersed, the outside can degrade because it is a different compound. One classic example if this is automotive fuel hose. It works fine to carry the fuel, but if you use the wrong type to run a fuel pump submerged in your tank, then the hose can rupture because the fuel actually eats through the outside.
  3. SteveJ

    SteveJ Member

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    The best numbers I have right now are:

    0.25 Btu/hr-ft-F for Pex
    30 Btu/hr-ft-F for Steel Pipe

    60 - 80 Btu/hr-ft-F for Copper


    For a DHE (downhole heat exchanger), the heat extraction rate is is found to be negligibly affected by the pipe with thermal conductivity greater that 6 Btu/hr-ft-F.

    In the paper, the required PEX to replace steel pipe was 4 times as much.

    I have searched the forum and found references to 3 times as much PEX to replace copper.

    I do not know the long term effects of water on the outer sheel of PEX - anyone??

    Steve
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    Make sure you're talking about heating system pex vs. the kind used for domestic water. The heating system kind is more expensive. The pipe you use should have an oxygen barrier. No way is it 16 times cheaper than copper. More like a third, I'm guessing.

    You might check into pex-al-pex, which has a layer of aluminum sandwiched between two laters of plastic. It might transfer heat better than straight pex. I have pex-al-pex on my system, and you can actually wrap your hand around it when it's carrying 180 degree water, so it's fairly self-insulating. Don't try that with copper!
  5. Corey

    Corey Minister of Fire

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    Interesting paper. It seems like they are using the pex under water for that setup, so it might be OK for submersion.
  6. jklingel

    jklingel Feeling the Heat

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    Doesn't pex have a limit of 140 degrees F? I've thought of the same thing, but that "info" (?) destroyed the idea. BTW, this is for radiant floor pex; is there a high-temp stuff, too? Thnx.
  7. leaddog

    leaddog Minister of Fire

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    here is a link that gives some good info on pex pipe.
    http://www.pexinfo.com/

    I think pex is good to 240f.

    leaddog
  8. jklingel

    jklingel Feeling the Heat

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    leaddog: thanks. maybe the 140 degree limit is for the concrete?? or maybe i mis-heard; happens all the time. i guess you mush?? ever run either of the big races up here? j
  9. leaddog

    leaddog Minister of Fire

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    I use to back in the 70's and early 80's. Never made it up there but I wanted to. I did chair a big race here in Muskegan that was the largest ever in the state and had several of the top racers here. It was fun competeing with them. I also was a director with ISDRA for a few years so I was very involved with it. Great sport but time consuming, not counting the dollars.

    I used to love the cold but now I just want this eko80 to keep me toasty warm. I've been expermenting with wood for many years. I had a wood boiler in my basement but didn't like hauling the wood down my stairs so I build a HASHA. That worked fine untill I over heated it and melted the pvc pipe. I reworked that with copper and ran that for a few years untill I build my odw boiler. That kept me warm but it did smoke. So last winter I started to research gasifiers and ended up buying my eko80. I'm very happy with it so far. If I was 15 years younger I would have gone in the business selling and installing gasifiers. They are the future of wood boilers.

    leaddog
  10. jklingel

    jklingel Feeling the Heat

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    Yes, from what I have read, gasification is where everything will be soon. I will bet that in 3-5 yrs the prices will drop a little as the competition heats up, too. I just priced out a Tarm Excel 2200 oil/wood, complete w/ their tank and 4-180' copper coils. It pushes $19K, FOB Seattle. Apparently excellent machines, though, and not ridiculously spendy. Just spendy. BTW: A friend mushes (wife did 90% of the Iditarod twice) and he says it is like heroin, only legal and a lot safer; just as costly.
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