Calculate Flow Rate: Inlet & Outlet Diameters & Pressures

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Discussion Overview

The discussion revolves around calculating the flow rate of water through a device described as "restrictionless," specifically focusing on a tank and a heat exchanger system. Participants explore the relationship between inlet and outlet diameters, pressures, and the implications of flow restrictions in piping systems.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether it is possible to calculate flow rate through a restrictionless device given inlet and outlet diameters and pressures.
  • Another participant asserts that a restrictionless device does not affect flow rate, implying that flow rate is determined by the piping system.
  • A participant clarifies that the original question may need refinement, suggesting that the focus should be on pressure drop through a pipe rather than a restrictionless device.
  • Further details are provided about a specific system involving a 3" pipe and a heat exchanger, with pressures of 45 psi at the inlet and 35 psi at the outlet, raising questions about actual flow rate calculations.
  • Participants discuss the need for flow coefficients (Cv) to determine flow rates at given pressure drops, indicating that restrictions exist despite the initial description of the device.
  • Concerns are raised about the relevance of height in a closed system, with suggestions to measure pressures at various points in the system for better flow rate estimation.
  • Questions are posed about whether the heat exchanger is custom-built or manufactured, with the latter potentially providing performance curves for flow calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the ability to calculate flow rate through the described device, with multiple competing views on the nature of restrictions and the necessary parameters for such calculations remaining unresolved.

Contextual Notes

The discussion highlights limitations in the information provided, particularly regarding the specifics of the piping system and the need for additional measurements to accurately determine flow rates. The dependence on flow coefficients and the implications of pressure drops are also noted as critical factors.

DHJenkins
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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...

Thanks!
 
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No, it is not. Such a device has no effect on flow rate, if it provides no restriction.
 
DHJenkins said:
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...

Thanks!
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?
 
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 preferably 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.

Thanks
 
DHJenkins said:
Russ - I'm not looking for the effect on flow rate, I'm looking the actual flow rate - a number, specifically in GPM and preferably within 5% +/-
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:
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...
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
...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.
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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