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indirect tank boiler piping heat transfer question

ron
ron Member Posts: 316


For a typical indirect domestic hot water tank, hot water from boiler goes in (5) and flows in a coiled loop then exits tank (6) and goes back to boiler.

question is wouldn't u want hot boiler water entering near the bottom of the indirect tank or specifically nearest where the potable cold water inlet enters the tank? In doing so wouldn't there be a greater temperature differential and better heat transfer?

Comments

  • Noel
    Noel Member Posts: 177
    Hottest boiler water in goes to the hottest water in storage so that the heat transfer there is still high. The cooler boiler water leaving the coil heats the coldest water in storage, which is still a wide enough temperature difference to have effective heat transfer. More total heat is transferred this way. Noel
    mattmia2
  • HVACNUT
    HVACNUT Member Posts: 4,873
    Now you tell us.
  • ron
    ron Member Posts: 316
    edited August 2019
    i would think more total heat xfer would happen having "greatest" delta temperature so hottest boiler water contacting coldest water within the tank?

    I sort of see having hottest boiler water near the potable outlet would tend to give hotter potable water during usage when cold water is flowing into the tank, mixing, then exiting?
  • John Ruhnke
    John Ruhnke Member Posts: 882
    edited August 2019
    Ron,

    The amount of energy used by the tanks is measured by the delta tee between the hot pipe entering the tank and coming from the boiler and the cold water exiting the tank to the boiler. This and the flow rate and diameter of the pipe at the place of flow rate measurement.

    I think this makes Noel right.
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    Heat transfer is highest with the greatest total delta t across the entire heat exchanger. If you reverse flow, heat transfer would pretty much stop 2/3rd up the coil.

    Also, cool water falls so the boiler water naturally wants to flow downward as it cools.

    Plate heat exchangers are always piped this way as well. Cool water in or out the bottom by water in and out the top.

    Because if this you can use water only 150f, make 140f water in a tank and still have 110f return water and run a condensing boiler at 92% efficiency instead of 87% if you reversed it and return water is 140f.
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    Do the math. If you have 150f coming from the boiler at 10gpm if you have a 140f tank setpoint the best you can do is a 10f delta T. So just 50000btu with parallel flow.

    With counter flow you will likely get 125f return with let’s say 60f incoming water. 125000 btu
  • hot_rod
    hot_rod Member Posts: 18,279
    With a plate HX you have two moving flows to leverage those delta Ts and close approach sizing.

    With a tank the coil is pretty much in stagnant water so heat transfer is a bit different, slower and also dependent on how much cold enters as the tank is running into the tank.

    I know a coil up top a tank to pull heat out doesn't perform near what a plate HX does due to stagnant conditions.

    It is all about surface area with heat exchange. Some of the tanks that had coils from top to bottom of the tank performed really well. IF you have the BTU and flow to drive them.

    Heat-Flo offers tanks with 3 options standard, high, and extra high output, by increasing coil or doubling them up.

    Niles and others have packages with external plate HX, like the System 2000 setup.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dopey27177
    dopey27177 Member Posts: 885
    All this discussion about whats the best way to get hotter water more economically is quite funny.

    When buying a tank for hot water storage the manufacturer can provide the proper piping configuration for your needs. ADDITIONALLY , a circulating pump is needed to assure the tank will have hot water from top to the bottom of the tank. To produce hot water from a coil mounted inside the boiler the boiler water needs to be at 180 degrees,

    To store water in the tank at 140, the water is at scalding temperatures, therefore a hot water mixing valve needs to be installed at discharge side if the tank, the setting should not be above 115 degrees to the fixtures.

    Jacob Myron