Page 209 - WaterSense at Work

October 2012
Conservation Mechanical Systems, Inc.
Water Use in Cooling Towers
Replacement Options
Replacing a chilled water system involves significant capital cost and involves many
design considerations. Before replacing an existing chilled water system, first consid-
er implementing efficient operation and maintenance and procedures and perform-
ing any retrofits available to optimize the current chilled water system. After con-
sidering the costs and benefits of installing a new chilled water system, if the facility
plans to do so, the design process should take into account all system components.
Facility managers and design professionals should consult design guides for efficient
chilled water systems.
Because chillers are central to chilled water system design, replacing an existing chill-
er might allow for efficiency improvements. If the existing chiller is inefficient, and
the potential energy and cost savings merit a replacement, both water and energy
efficiency should be considered as part of the planned design. Water-cooled systems
are typically the most efficient option for larger facilities with cooling towers. Alter-
nate technologies, such as ground source heat pumps, can also be more energy- and
water-efficient than traditional chiller and cooling tower technology. Choose a sys-
tem that will operate most efficiently under typical load conditions. For most cooling
loads less than 100 tons, air cooling is just as cost-effective as a water-cooled system.
An analysis of the total cost of cooling with air versus cooling towers should include
the cost of the water, wastewater, water treatment to prevent scale and corrosion,
and labor needed to operate a cooling tower versus the 0.2 to 0.3 kilowatt-hour/ton-
hour savings realized with chilled water/cooling tower/chiller systems.
Savings Potential
Chilled water systems are completely closed loops and thus consume no water when
operating properly with no leaking components. However, if cooling towers are
used to operate the refrigeration loop, the tower requires approximately 2.0 gallons
per hour of evaporation for each ton of cooling.
By improving the efficiency of the
chilled water system, the heat load on the cooling tower can be reduced, thereby
reducing the evaporative cooling load and the water use of the system as a whole.
Additional Resources
DOE, Energy Efficiency & Renewable Energy. October 2005.
Improving Chilled Water
System Performance: Chilled Water Systems Analysis Tools (CWSAT) Improves Efficiency
Energy Design Resources. December 2009.
Chilled Water Plant Design Guide
Building Upgrade Manual, Chapter 9: Heating and Cooling
Chilled Water Systems