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Calculating flow rate

  1. Dec 9, 2008 #1
    Is it possible to calculate the flow rate of water through a restrictionless device (a tank, for example) if you know the inlet & outlet diameters and pressures?

    I've been looking all over, but all I can seem to find is formulas for pressure drop in a length of pipe...

  2. jcsd
  3. Dec 9, 2008 #2


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    Staff: Mentor

    No, it is not. Such a device has no effect on flow rate, if it provides no restriction.
  4. Dec 9, 2008 #3


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    A pipe is not a "restrictionless device", there is a very real restriction to flow through a pipe, so the question needs a bit of help. Are you looking to find out how to determine pressure drop (ie: restriction to flow) through a pipe? Or are you asking about something else?
  5. Dec 10, 2008 #4
    Thanks for the replies.

    Russ - I'm not looking for the effect on flow rate, I'm looking the actual flow rate - a number, specifically in GPM and preferrably within 5% +/-

    Q_goest - Basically, I have a 3" pipe that enters the heat exchanging portion of a packaged A/C unit. The water flows in, flows through 4 different tube-style heat exchangers (in parallel), then out of a 3" pipe; I'd say total trip of about 22 linear feet from pressure gauge to pressure gauge.

    This small loop is tapped off of a larger supply/return loop. There are 2 valves - an inlet & and outlet. The inlet remains wide open so as to not starve the coils for water, and flow rate is adjusted by opening/closing the outlet valve. The inlet pressure is 45psi, and the outlet pressure is 35 psi - what is the flow rate of water through this machine? Also, maximum pressure drop (with both handles wide open) is 20 psi - I imagine this has to do with the height; the machine is located approximately 50' below the pumps.

  6. Dec 10, 2008 #5


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    You missed my point. My point was that if the device has no effect on the flow rate, then the flow rate is determined entirely by what is going on with the input and ouput piping: and you provided no information about them besides their size.

    That said, I should have realized by your wording that your "restrictionless" device is not restrictionless:
    If there is a pressure drop, then that means there is a restriction. The restriction causes the pressure drop. Unfortunately, you need to know the flow coefficient (Cv) to find the flow at a given pressure drop. That's based on this formula (in step 3): http://www.cheresources.com/valvezz.shtml
    If it is a closed system, the height doesn't matter. Do you have pressure gages anywhere else? At the pumps? At the mains before the valves? It sounds like the pressure gages you are referring to are on either side of your heat exchanger, between the valves. Is that correct?

    Is this heat exchanger something you built yourself or is it manufactured? If it is manufactured, it should have a performance curve. If nothing else, the valves should have published Cv's, so if you can measure the pressure drop across them, you can calculate the flow that way.
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