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Cooling a pond with river water?

  1. Sep 24, 2012 #1
    Cooling a pond with river water??

    Hi all, this is kind of a basic question and I am looking for a simple formula to use to illustrate how an idea is not practical. I worked almost this exact same problem years ago in a limnology class but can’t remember how I did any of it. I help manage a small pond, for this exercise let’s say it has a volume of ~7.2 million liters (roughly 6 acre feet). The maximum summer temperature of the pond is, for the sake of argument 30⁰C. Some stakeholders would like to see the temperature lowered to 25⁰C. Their idea is to construct a closed loop of piping carrying river water through the pond, at night when the river is at it's "coolest", although there's not much of of a difference, actually. The idea is that the river water is cooler and would therefore cool the pond. But the river water is at, let’s say 22⁰C. Given the relatively large volume of water in the pond, the high thermal inertia of water, and the relatively small temperature differences, I can’t see how this idea would be practical (I’ll completely agree that it is theoretically possible). If I know the difference in temperature, and the heat conductivity of likely piping materials, then it’s sort of a rate of time question, right? Is there an equation that would let the stakeholders know that if we constructed a loop of pipe holding a volume of water equivalent to the entire pond it would take X amount of time for the temperature to equilibrate? Conversely, could I then show that with a loop of pipe holding ¼ the volume of the pond water it would take 4X time, and so on? And I realize that the relationships are not linear and the heat transfer rate would change as the differential narrows etc. I’m really just looking for an simplified way to show these folks that it’s just not a practical idea for our purposes or budget. Or, conversely, if someone can show me such an equation where it might be practical I guess I’d have to go talk to my engineers. Either way, thanks, and cheers!
     
    Last edited: Sep 24, 2012
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  3. Sep 24, 2012 #2

    Vanadium 50

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    Re: Cooling a pond with river water??

    If you're talking a pipe 1 foot wide and 200 feet long, that's 4500 liters. So we're talking 0.06% of the volume of the pond. That seems awfully small.
     
  4. Sep 24, 2012 #3
    Re: Cooling a pond with river water??

    The way I understand it, you are going to have a certain length of pipe laid out in the pond, and you are going to be pumping river water through the piping. So for a specified layout of the piping, you are going to want to be able to calculate, for different flow rates, the pressure drop in the piping (to guarantee structural integrity of the pipe), the power requirements of the pump, and the heat load that can be removed. If you had an infinite flow rate, and the water in the pond were well-mixed (which it would probably not be), you could get the pond temperature down to the river water inlettemperature. If you actually want to see the best you can do, then you assume that the pond is well mixed, and all the resistance to heat transfer resides inside the pipe. See the Chemical Engineer's Handbook by Perry, or Transport Phenomena by Bird, Stewart, and Lightfoot for equations to calculate the pressure drop in the pipe, and the heat transfer coefficient within the pipe, the temperature rise of the river water, the heat removal rate, and the transient temperature variation of the pond water. The heat transfer coefficient is the most important parameter you are going to need. In the end, you're going to find that it is impractical.

    chet
     
  5. Sep 25, 2012 #4

    AlephZero

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    Re: Cooling a pond with river water??

    I agree this doesn't sound very pratical, but the previous posts seem to have missed one thing. In the long term the issue is not how to cool the pond "quickly" from 30 to 25. It is how to keep it at 25 and take away the excess solar heat input. In other words, if the pond heats up from 25 to 30 in say 4 weeks in summer, your system needs to be able to cool it down to 25 again over 4 weeks to maintain the temperature at 25.

    Even if your cooling system was 100% efficient, with cooling water at 22 you would need about 1/4 of the volume of the pond to reduce the pond temperature from 26 to 25 and increase the coolant temperature fom 22 to 25. (I've no idea how efficient the cooling system would actually be, but for just pumping water through a large diameter pipe I would probably guess nearer 1% than 100%)

    That sort of calculation might be enough to show the idea won't work.
     
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