Installing an air-cooled chiller with water side economizer, in the San Francisco Bay Area

 

 

September 20, 2022

I am designing a chiller plant for a client and wondering if I should have a water side economizer (free cooling module/dry cooler module) with the air cooled chiller I plan to use.  It is not an easy decision because I do not know what percentage of cooling (chilled water) will be used in air handling units.  If 100% of the chilled water will be used in air handling units then the water side economizer does not make any sense: when it is favorably cold outside, I'll just use the air side economizer of the air handling units and keep the chillers off duty.  If 100% of the chilled water will be used in the electronic racks then water side economizer makes perfect sense: I won't have any way to use cold air inside the building, but I can use turn off the chillers and just use the dry coolers to cool down the water, and then use the chilled water in the electronic racks. Installing integral free cooling modules with the air cooled chillers also makes sense if there is no provision for outside air at the chilled water coil units (Fan coil units or Computer Room units).  In these instances, there will be no way to use the cold outside air directly in the space but using the ambient cold air to cool down the water will be possible.

The problem I am facing is having no knowledge of the use of the chilled water in future.  The chilled water will probably be used mainly in the air handling units (say 60%) where air side economizer will be an option; probably 20% used in the fan coil units and computer room units where air side economizer is not present; and probably the rest (20%) will be directly used in the electronic racks to keep the processors cool.  With this distribution of chilled water capacity, it will be hard to justify the added cost of the water side economizers.

Photo: Courtesy of Trane.