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Question on tank immersed heat exchangers.

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Donl, Dec 29, 2007.

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  1. Donl

    Donl Feeling the Heat

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    I am planning on using copper coils placed vertically in the storage tank for heat exchangers. I want to use the same coils for both heat input and output. Does the direction of flow matter with regards to whether you are storing or retrieving heat? I have been looking at some of the designs and it appears that some change direction between storage and retrieving. Nofossil's design for example. Is there any benefit to changing flow direction in the heat exchangers? If so, why?

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    To store heat, you want the flow to go from the top of the coil to the bottom. That way, the water exiting the coil is meeting the coolest water in the tank before heading back to the boiler. Conversely, for heat recovery, you want the flow to start at the bottom and exit at the top, so that the hottest water in the tank meets the heating water as it exits the coil and heads into the zones to heat the house.

    If your flow only went one direction, the coil would be good at one thing, but not very good at the other. Imagine, for example, that you were trying to heat your house with water flowing out from the bottom of the tank: You'd never get it very hot, no matter how hot the tank got on top.

    That's my layman's understanding of how it works, anyway. Remember that the most efficient heat transfer is the one with the greatest range between the water being heated and the water doing the heating. When heating up a cool tank, your heat transfer is very efficient, because 180 degree water is meeting say, 120 degree water. Your Delta T is 60 degrees. As the tank heats up, less heat is transferred per cycle, because your Delta T is getting smaller. To me that seems almost counterintuitive, but it's a fact of life.
  3. ericjeeper

    ericjeeper New Member

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    Us po folks can not afford fine copper.. So I chose a six way manifold.of half inch non barrier pex.Works like a champ.I cross flowed the fluid. basically water comes in the thumbs and the first loop on one goes out the index. But it returns to the other manifold at the pinky end, So in theory each loop has the same amount of friction and each loop should flow equally.
  4. Grover59

    Grover59 Member

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    How much pex would you need for a decent HX?
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think about 3 times as much as comparable size copper.
  6. Grover59

    Grover59 Member

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    Tomorrow I will be putting my liner in the tank, sounds like it is going to be fun. As you may know I plan on using the dip tubes and just pull the water out of the tank to a flat plate heat exchanger to charge the tank. But I was thinking I could use pex to take the heat back out in a pressurized seperate system.

    Steve
  7. ericjeeper

    ericjeeper New Member

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    Maybe 3 times the amount.. But pex is dirt cheap.. especially since you do not want or need barrier pex. I have 500 feet in each of my manifolds in my tank. I bought my manifolds off of fleabay. I bid on the six or eight of them once lost the bid by a nickel.. bidder was a non payer.. seller put them back up. I bought em for 20 bucks less the next time he listed em. I still have a few of them laying behind me on the floor saving them for another project later down the line you know..
  8. Grover59

    Grover59 Member

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    Just how did you make your heat exchanger?

    Steve
  9. ericjeeper

    ericjeeper New Member

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    I took a roll of 500 feet 1/2 inch non barrier pex. cut it into six pieces 83 feet long.. Then took end of one line to the fitting closest to the 1 inch inlet, the other end went to the fitting furthest from the 1inch on the other manifold..did this on each line.. By all rights each loop should have the exact same flow as the others.
    This should be done with plenty of help on a warm day.. Cause the rolling out and holding it to the tape, sort of resembles a monkey playing with a football.I have no neighbors so we were saved the embarrassment..
    My dump for the solar is wedged to the bottom of the tank.. and the pickup for the house is wedged to just beneath the water surface.
  10. Grover59

    Grover59 Member

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    So you have 2 500 foot rolls one for the solar one to take heat out?

    steve
  11. ericjeeper

    ericjeeper New Member

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    Yes 1,000 feet of pex. 12 lines of 83 feet each. One manifold puts heat in from the solar(glycol system) and the other picks heat up for the house also glycol. The storage tank is just plain old nasty stinky water.
    http://www.pbase.com/ericjeeper/image/68615789
  12. Grover59

    Grover59 Member

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    Do you have the coils of pex in your tank as pictured or did you spread them out at all, I would have to suspend the pex somehow at the top of my tank.
  13. ericjeeper

    ericjeeper New Member

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    I spread them out the best I could.. But it aint easy.. trying to reach into the top of the tank Granted the water was luke warm then.. I used some wire ties to hold the loops elongated.. But the stuff likes to go back to the shape it was originally rolled.
    Here are all of my solar /boiler
    http://www.pbase.com/ericjeeper/solar
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