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mixture of pipe sizes for non-pressurized storage heat exchanger?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by scottinkn, Nov 1, 2012.

  1. scottinkn

    scottinkn Member

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    Hello,

    I am FINALLY ready! I am building a 5' round x 5' high tank (pool metal) with epdm liner; around 900 gallons of storage.

    I need your help and guidance!

    My boiler is 130,000 btu & my system (as a temp setup) is hooked up in series: wood - oil w/ dedicated circ running constantly,(through tankless DHW on oil) I have 2 heat zones, circ based, baseboard. And right now I light 1 small fire a day because I do not have storage and do not want to overheat.
    (but you guys knew that, lol)

    I, so far, have 60' of copper pipe; 10' pieces of: 1-1/4", 1" & 3/4" (2 each)

    I THINK I want 2 separate heat exchangers in the tank. 1 for DHW preheat and the other for boiler heat exchange, therefore the tank of water does not circulate.

    Is that best?

    (I would really-really like to minimize the amount of electricity required to run this system.)

    Can I use the different sizes of pipe for heat exchange in each coil or should I go with larger pipe for boiler and smaller for preheat How much (length) do I need (minimum) for each run?

    Thanks for your time and consideration in advance!

    Scott

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  2. If you want use the pipe you have I'd use the larger diameters for the dhw preheat. The pipe will hold more water and it will be used in 'batches'. So it will have more time in the tank to transfer btu's while waiting for the next time someone turns on a faucet.
  3. scottinkn

    scottinkn Member

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    Mike,

    Thanks! that is the opposite of what i thought...
    I now still need to know:
    How many feet of pipe exposed in the tank for either the boiler charge or DHW preheat side!

    Thanks,
    Scott
  4. Short answer, you need more pipe.

    Long answer, assuming all 3/4" pipe
    130k btu requires 13 gpm (BTUper hr/500*delta T=GPM)
    To keep the flow less than 4' second you'll want to use 3 separate loops in the tank.
    So 130/3 =43,333 btu per coil
    The transfer rate for copper in a tank is 60-80BTU/hr*ft^2*F
    Assume a 20 degree delta, and 70 btu, you need 31 ft^2 of copper
    To convert to 3/4" pipe, pi*d = circumference = 2.35/12= .196 ft^2 per foot
    31/.196 = 158' of 3/4 pipe per coil
    Or 474 feet to transfer full boiler output.

    Real world, you could probably get by with less since the house might be calling for heat at the same time. So you wouldn't need to transfer all of the output to storage.
  5. scottinkn

    scottinkn Member

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    Mike! Thanks!

    That is a LOT of pipe! I think it would be a tight fit in the tank, that's for sure!

    So are there advantages/disadvantages from using a larger pipe for heat exchange?
    I am thinking larger might be better for DHW preheat as it will "hold" more hot water readily BUT when in use will efficiency of heat transfer go down?
    Secondly, can I circulate the unpressurized tank water in my heating system and therefore heat the storage directly?
    I would imagine there is probably some loss through all of that exchange...
    Thanks !
    Scott
  6. Bad Wolf

    Bad Wolf Minister of Fire

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    I use 300' of 3/4" copper refrigeration tubing in 2 150' coils for the boiler HX This is fed from a 1" line from the boiler. I always get full transfer i.e. my return water is the same temp as the bottom of the tank.
    DHW is a 150' coil.
  7. Nofossil

    Nofossil Moderator Emeritus

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    Assuming that you have a normal pressurized heating system, you can't circulate unpressurized storage tank water through it. I have a smaller boiler and therefore need less HX surface area, but the other problem is the delta T term in the equation above. As your tank gets closer to your boiler outlet temp, your heat transfer capability drops. Bottom line: you can never have enough surface area. I've toyed with the idea of forming clip-on fins out of copper flashing scraps. Probably a good idea, but my tank is really hard to get to.
  8. scottinkn

    scottinkn Member

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    I have thought of using finned baseboard for heat exchange, I can get it locally for about 5$ a foot.
    Maybe that is the way to go, for at least some of it.
  9. scottinkn

    scottinkn Member

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    - I don't mean to seem ignorant but concerning circulating unpressurized tank (with the least amount possible of circulators in system)
    because I want to design it so that my elec consumption NEED is low

    COULD I run it so that only the old oil system is 1 of the loops for HX in the tank ( for circulator - > heat zones heat exchange)
    (Omit using tankless coil on oil for DHW)
    Use coils for preheat DHW HX in the tank.
    Then I would only run wood circ when charging the tank (circulating its water)
    Am I completely wrong? ( don't spank me too hard)
  10. ozzie88

    ozzie88 Member

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    I used finned baseboard for my heat exchanger for DHW and it is working fine, got it from the dump [price was right,lol] I have them in an propane tank 90gal. and run 170 to 210 for my heat.
  11. ozzie88

    ozzie88 Member

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    Here a photo of when i started putting them together in tank. The tubes are thin so the two pipes sticking out of tank I soilder type k3/4 copper to them then compresion fitting to brass fitting.

    Attached Files:

  12. Nofossil

    Nofossil Moderator Emeritus

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    The whole issue is that in order to prevent corrosion of your system components, you need to keep the water that you circulate away from air. Most hydronic systems are pressurized to around 15-20 psi, and circulate the same water forever. If the system were not closed and pressurized, it would be difficult to pump water to any points in the system above the point where it's open to the air (probably the top of storage in your case). The benefit of pressurized storage is that you don't have a heat exchanger between storage and the rest of your system. If you're set up like me with unpressurized storage, then you have to have a heat exchanger. I actually have three: one bidirectional coil to transfer heat from the boiler into storage and from storage back to the heating zones. The second is DHW preheat, and the third is for charging storage from solar panels in the summer. I have a maze of copper in my storage tank, as you might expect. I wish I had more.

    [​IMG]
  13. scottinkn

    scottinkn Member

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    Thanks Nofossil! Looks like I am going to do what you did! Wish I had your tank! That's beautiful!
    - Wish me luck

    Scott
  14. Nofossil

    Nofossil Moderator Emeritus

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    You're welcome. Here's how I plumbed mine if that helps. Only one circulator on at a time, except for a few minutes when the fire starts, Then I run the tank and boiler circs to get warm water in the boiler jacket.
    [​IMG]
  15. scottinkn

    scottinkn Member

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    I am going to have to get a lot more copper! I guess I will go with coils... Any place to get it relatively inexpensive?

    Thanks,

    Scott
  16. ozzie88

    ozzie88 Member

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    Check on line E-bay I have found some good deals there for copper, come right to your door too.
  17. ozzie88

    ozzie88 Member

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    E-bay also have some already made up heat exchangers out of stainless around $300 big for a steam cleaner, large coile with ends on also, I would check into this if I was building a new one.
  18. scottinkn

    scottinkn Member

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    Because copper is so expensive & I need so much of it to have adequate heat transfer...

    Could I "make" a heat exchanger from copper flashing? Similar to this:

    HX.gif

    Attached Files:

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      HX.gif
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  19. scottinkn

    scottinkn Member

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    This idea should probably be its own thread.
  20. scottinkn

    scottinkn Member

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    Almost ready to put in tank. They are both 45" wide and tube risers are 40" long. Larger one is made with 2" & 1-1/4 (9 - DHW preheat) and the other is 1-1/2" & 1" (18 - for boiler HX. Should I put them low in the tank? ...How low? (so that I can add solar and hot tub HX later)

    Attached Files:

  21. Fred61

    Fred61 Minister of Fire

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    How did you leak est them?
  22. scottinkn

    scottinkn Member

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    I am going to with air & soap bubbles.

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