Variable flow vs. Constant flow through centrifugal chillers

I need some opinions on variable flow vs. constant flow through centrifugal chillers. Trane says they can do it, York prefers not to. The Tranes would use R123 and the Yorks would use R134A. Our hydraulic analysis says that variable flow will work, however, we are developing some conflict as we get closer to construction. [sig][/sig]

 

[stevenw]

Constant flow has worked for years.
We have done a varable flow job and found that we cound only vary the flow 25% and keep the chillers on-line. It was hardly worth the design time for variable flow.
R123 is a better refrigerant for variable flow but the market in moving toward R134a. [sig][/sig]

 

[bumpjones]

jad747
Are you referring to variable refrigerant flow or variable flow of chill water through the evaporator or variable flow of cooling water through the condenser. I am working on a system that reduces overall electrical power requirements by reducing the water flow to the condenser and am interested in finding some other views on the matter.

 

[malz]

I use a Chilled water pump on VFD, to augment my chilled water system, and it seems to work pretty well. Over the years, systemic changes to the building have seen an increase of fan coil unit use (they are practically everywhere!), and this was putting our system out of specification (at some point, operator would have to put on another chiller, even though from historical perspective in the building, you didn't need to). The VFD pump running with the constant speed pump, would provide enough GPM to fulfill capacity needs, without putting another chiller on. I prefer to run the VFD in manual so I have tighter control of the system, but the auto mode functions fine, as well.

 

I think you have to look at your system to see if its worth while using variable flow. It seems to me the whole basis is to save energy on the pump side. If you get a 25% saving in flow rate then this equates to a pump power saving in the ratio of its cube. This is pretty substantial savings. There is a newsletter on the trane web site that goes into it, but it does say only to use variable flow if close control is not required.