Expansion Tank for closed cooling system 3

[rookie81]

I am designing a closed loop cooling system. I have all of the details worked out except for an expansion/head tank. The system will circulate 3600 GPM and water will enter a centrifugal pump at 100 deg F. I think I need an expansion tank, but i don't know if i need to elevate it to increase head on the suction side of the pump. Since it is a closed system it seems like that would just increase the head on the system. Would cavitation be a problem if the tank is not elevated? I am having a hard time finding any useful information on the subject. Any help would be appreciated.

 

[Montemayor]

Rookie:

You've got to use your terms properly. When you talk about an expansion tank, you're not necessarily talking about a suction tank for the centrifugal pump. But if you want to use the expansion tank at the pump's suction, you are free to do so, with the caveat that your entire pumping system (all the pumping fluid) must have free access to that expansion chamber when the system reaches maximum temperature due to the heat pickup (in other words, you shouldn't have check valves or block valves segregating sections of the fluid system). The reason for using an expansion tank is to allow the fluid to expand without imposing a hydraulic liquid pressure on the system's mechanical components. That is why you locate the expansion tank at the highest physical elevation and usually leave approximately 25% of it's top contents to be air or an inert (like Nitrogen) which is compressible. If you fill your closed pumping system with 100% liquid fluid, this same fluid will expand upon picking up the process heat it is designed to do and, because of the thermal condition being increased, could expand and exert an astronomical hydraulic pressure that could easily rupture your system.

You should calculate the minimal amount of liquid fluid expansion that your system has to contain due to thermal conditions and allow that equivalent gaseous volume plus a contingency in your expansion tank. Remember that your fluid may also absorb some of the gas -- that's why it's always smart to allow a generous contingency of gas space.

Since your system is closed, you shouldn't have to worry about your suction head - as long as it is generously flooded. The suction pressure at the pump's inlet flange is part of the pump's discharge head less the pressure drop in the discharge piping and return back to suction. You should not have to worry about NPSH unless you reduce your piping too much. If this is an industrial application, you should do the hydraulic pressure drop calculations to ensure the pump's suction won't be starved. And in doing those calculations you will also know the total system volume of fluid and be able to calculate it's expansion potential.

I hope this explanation helps.



Art Montemayor
Spring, TX

 

[lilliput1]

Read Amtrol engineering guide on expansion tanks. The higher you can locate the expansion tank the smaller it can be. The expansion tank & fill must be at the pump suction & adequately pressurized to make sure there is adequate pump NPSH and so all points in the line is positive. If not air could be entrained in the system. Provide also pressure regulating valve & relief valve at the water makeup. Select pressure regulating valve pressure to allow fully filling the system (up to 10' above the highest elevation of the piping). Select the relief valve so at the worst case max operating pressure (pump head + static head + max operating pressure in piping) do not exceed 150 psig say max 130 psig operating pressure (allowing 20 psig safety).

 

[abcpump]

I suggest you get hold of your local B&G rep and ask for a copy of the "Large Chilled Water System Manual" - there is an excellent section on sizing expansion tanks.
Although this may seem like an endorsement, B&G has spent a considerable amount of time and resources educating the industry and probably deserves a little plug.

Hope this helps

 

[rookie81]

Thank you all for the input. I appreciate it.