Chilled Water Buffer Tanks?

[tlhoo]

General question regarding buffer tanks for chilled water systems.

I have a primary-secondary system with two 70 ton air-cooled screw chillers. My total system volume is approximately 950 gals. The system is mainly serving fan coils units with 2-way modulating valves. Do I need a buffer tank?

Cemline recommends a minimum sys volume of 3-6 gallons/ton. I have 6.7 gallons per ton, so I think I'm okay. However, another engineer in our office said that I should only consider the volume in the primary loop, not the total system volume? Is this correct?

Also, since a screw chiller can modulate continously throughout its operating range, does this lessen the need for a buffer tank.

Any thoughts?

Thanks in advance.

 

[KiwiMace]

I take it you are trying to address the concern that if your primary loop volume is too low, you may end up tripping your chillers on startup when they get a fast recirc on the cold primary.

Soooo, you can either increase your primary chw header size (to a point), add a buffer tank as you say, or solve the problem with the controls - ensure that your controls start the screw chillers on low load and/or start the secondary circulation before starting the chillers.

 

[KenRad]

How many fan coils are in this system? I agree with CinciMace that you should be able to keep out of trouble by ensuring that you have secondary flow before the chillers start. Also, do you have a means of maintaining a minimum flow on your secondary pumps, like a bypass line or a 3-way valve on a few of your fan coils?

---KenRad

 

[tlhoo]

Yes, I have 3-way coils on a portion of the loads to maintain a minimum circulation in the secondary loop.

The chillers will not be operating unless the secondary pumps are on, so the "system" volume should be the entire secondary system as well as the primary.

Thanks for your help.