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Old Jan 5th 2015, 06:00 PM   #1
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Water Flow Through 1/2 Cu Tube

Hello. I am an electronics manufacturer with a problem outside my expertise. I realize this may be simplistic in how I am posing but was just hoping someone could help me.

I want to get a general sense on expect water flow through a 1/2 copper tube. I have 60 feet of level 1/2 copper tube. I want to get 8 gpm flow out at 80psi pressure in. I understand that there are other variables that may affect flow. However, can someone give me a sense of what the expected flow rate is or whether I am in the ball park for what I am looking for.
Thanks, Brian
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Old Jan 6th 2015, 10:57 AM   #2
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An electrical analogy might help you...

Consider the pipe as a resistor,
The pressure is analogous to the voltage across the resistance,
The Flow Rate is analogous to the Current through the resistance.

Thus it would seem high pressure gives high flow rate, however there is a catch.
Consider the power supply, where the pump is analogous to a battery.

A small battery might be able to deliver 12 Volts, but not many amps.
Similarly a small pump might be able to deliver 80psi, but not at a high flow rate.
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Old Jan 6th 2015, 02:14 PM   #3
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Quick calculation:

Required volume flow: 8 GPM = 1.07 ft^3/min = 18532 in^3/min = 30.9 in^3/s
For 1/2" pipe, with internal area of 0.2 in^2, required flow velocity = 30.9/0.2 = 157 in/s = 13.1 ft/s.

If the pump can deliver 80 psi for 8 GPM, the outlet velocity of the pump would be v = sqrt(2gh), where h = pressure in feet of head. 80 psi is equivalent to 185 feet of head (i.e., the pressure at the bottom of a tank that is 185 feet tall.) That's a lot of pressure! The expected outlet velocity is therefore sqrt(2 x 32.2 x 185) = 109 ft/s. In reality it would be a bit less due to losses, but this is a reasonable ball park calculation.

There will also be some loss of flow due to resistance of water along the inner wall of the 60 feet of pipe, but you have about a factor of 8 safety, so I see no problem in being able to deliver 8 GPM. In fact you would likely get close to 50 GPM, if the pump can maintain 80 psi at that flow rate.
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