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Flow rate and water temperature

Stokehold Member Posts: 43
When figuring flow rate for a given BTU load, why isn't water temperature considered in the equation?
For instance, if my heat load is 39,000 BTU, the temperature drop is 20°, and the "fluid factor" or specific gravity is 500, the rate of flow is 3.9 GPM, but isn't the temperature a consideration here?


  • hot_rod
    hot_rod Member Posts: 22,100
    You have your temperature in the formula, 20∆T.

    Doesn't matter if it is 180- 160 or 120- 100, the ∆T is the temperature factor.

    Change the ∆ and change the energy transfer. Plug a bunch of different examples into the formula to see.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SWEI
    SWEI Member Posts: 7,356
    Technically, that "fluid factor" does change a bit with temperature, but it's not significant:

    40°F = 500.8
    60°F = 500.1
    100°F = 497.3
    140°F = 492.4
    180°F = 485.9
    200°F = 482.2
  • Canucker
    Canucker Member Posts: 722
    @SWEI I'm definitely in a different line of work. That is a huge difference in density that would throw off volumes, if we didn't account for temperature. I think back to a process we used to distill where the initial volume measurements were taken under 30C and the final one was at 78C. Made for some head scratching moments with new operators when they couldn't figure out why they had 1000L of distillate but the volume in the hot vessel had barely changed. Haha
    You can have it good, fast or cheap. Pick two
  • Gordy
    Gordy Member Posts: 9,546
    The actual water temperature is covered by the selected type of emitter, and it's output rating in conjunction with a selected AWT..

    Rads, ci baseboard,fintube. Outputs are based on 170.

    Radiant depending on the type of assembly can be a few degrees above setpoint to 150ish.
  • SWEI
    SWEI Member Posts: 7,356
    edited September 2016
    Well, yes -- if you don't have an expansion tank then volume remains constant and so pressure changes. 490 is probably a bit more accurate than 500 for most radiator systems (especially if ODR is used) but 2% will rarely drive a change in emitter, pump, or pipe sizing.