Vacuum in loop 1

[mjpetrag]

I've always noticed that at the top of cooling water loops at a high point (80-100' above grade), a vacuum develops at the top when a high point bleed is opened. The downleg of the pipe goes back to the cooling tower 70' below. I've associated this with the freefall of liquid accelerating in the down leg and detaching flow. Am I correct in my understanding?

I can't find any literature online about this. Is there a way to calculate the vacuum developed?

-Mike

 

[TD2K]

Partly pressure drop but mostly static head.

70' static head is about 30 psi of water. Check to see what the return pressure to your cooling tower is, I suspect it's likely 10 to 20 psi, that's enough to be well below atmospheric pressure at the highest point. Then on top of that, you need to include the frictional line losses which will further decrease the pressure at the highest point though I suspect that's a smaller effect.

 

[mjpetrag]

aha! That's the answer I was looking for. I felt like my understanding was missing something. Thank you.


-Mike

 

[katmar]

I have always used permanently open vents at the top of my cooling water return piping. If you allow a vacuum to develop you can get localised boiling and vibration. The cooling towers I have worked with required very little head to work with, and I have never had water back up and overflow the vent. A potential problem that having an open vent introduces is that air can be entrained in the cooling water and cause problems in the piping and tower nozzles. So my second rule of cooling water return piping is that the downleg must be sized to be self venting.

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