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Head loss

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rhl
rhl Member Posts: 100
edited January 2019 in THE MAIN WALL
The Taco Pump Sizing guide has a head loss formula, the text says that if you have a parallel runs then the head loss is computed for each parallel run. It separately discusses taking the maximum over all the parallel runs. Is the total head loss for the system the sum of the losses over these parallel runs? For example in a manifold.
Reference: https://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

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  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    No. If the various runs are parallel, then the run with the greatest head loss is the maximum head loss the pump would see.

    The flows in the various runs add.

    At the risk of confusing you, if you have multiple parallel runs open at the same time, the flow will divide between the runs such that the head loss in each run is the same.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    ZmanRich_49
  • Voyager
    Voyager Member Posts: 395
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    rhl said:

    The Taco Pump Sizing guide has a head loss formula, the text says that if you have a parallel runs then the head loss is computed for each parallel run. It separately discusses taking the maximum over all the parallel runs. Is the total head loss for the system the sum of the losses over these parallel runs? For example in a manifold.

    Reference: https://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

    No, it is just what the Taco guide says: “In a system with multiple zones, the head loss needs to be calculated for each parallel loop. The loop with the highest head loss will be used in figuring the total equivalent length in Example 4.”

    The loop with the highest head establishes the design head for the distribution system. The total flow, however, is the sum of the flows of each loop.
  • Brewbeer
    Brewbeer Member Posts: 616
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    Also, the head loss values will change in a system with zone valves, depending on which and how many zones are actively calling at the same time.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,605
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    The reason they tell you to calculate all parallel runs is so you can find the head loss of the highest loop.
    and see which zone has more resistance.

    only the loop with the highest resistance is considered for head loss
  • SuperJ
    SuperJ Member Posts: 609
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    You take the highest head requirement, but have to sum the flows to select the appropriate pump. Since the differential pressure a pump can create varies depending on flow rate. You may need balancing devices to keep the lower head loss parallel loops from "stealing" too much flow from the higher head loop.
  • nibs
    nibs Member Posts: 511
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    I understand that for practical purposes, two parallel loops can be considered the same as one loop, but something tells my gut that there must be a small difference. Would like to know if the flow rate co-efficiency of friction in pipes is a straight line.
    Spent some time last year working on head loss so am curious.
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    Yes, two or more parallel pipes can be treated as a single, larger pipe for some types of calculations.

    The relationship of head loss to flow in a pipe is not a straight line. It's a power curve; the exponent is commonly taken as 1.83
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,245
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    This journal has a good explanation on pump sizing and circuit calculations.
    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • rhl
    rhl Member Posts: 100
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    Thanks guys. FWIW I’m a mathematician not a plumber, and Jamie it’s a polynomial function not a power function. Typically power functions mean exponential, i.e the formula is something like x^2 rather than 2^x, the latter is considered a power law.
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    uh huh. And when I said power function, that's what I meant, But whatever... head loss is proportional to Q raised to the 1.82 power...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Voyager
    Voyager Member Posts: 395
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    rhl said:

    Thanks guys. FWIW I’m a mathematician not a plumber, and Jamie it’s a polynomial function not a power function. Typically power functions mean exponential, i.e the formula is something like x^2 rather than 2^x, the latter is considered a power law.

    As a mathematician, you should have known that Jamie was correct. I prefer exponential function rather than power function, but same thing. I prefer to use power in the physics domain rather than mathematics to avoid ambiguity.
  • nibs
    nibs Member Posts: 511
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    I remember looking it up in my Marks Handbook and the formulae were longer than three months in jail. Now I have time to ponder this, my Marks is at home.
    Thanks @Jamie Hall
  • rhl
    rhl Member Posts: 100
    edited January 2019
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    Yes and f(q) = q^1.82 is a polynomial not an exponential in q
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    I stand corrected as to mathematical terminology. A mathematician I am not, and don't pretend to be. I just know how to apply the blessed things...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    DZororhl
  • Gordy
    Gordy Member Posts: 9,546
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    Parallel designed piping systems is the longest loop plus piping back to the boiler headloss for the pump sizing.

    Head balances out through out the system. While flow rates will differ.

    A little more discription of your particular example will shed more finite light.
  • rhl
    rhl Member Posts: 100
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    That’s all I wanted to know. Therefore a manifold at the boiler room with parallel runs is likely no worse than running big mains in a direct/reverse return with respect to head loss