ASHRAE Journal - February 2014 - 34

PHOTO COURTESY OF NSCA (NATIONAL CENTER FOR SUPERCOMPUTING APPLICATIONS,
LOCATED ON THE CAMPUS OF THE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN)

TECHNICAL FEATURE
FEA

Saving Energy With
Cooling Towers
BY FRANK MORRISON, MEMBER ASHRAE

Water-cooled systems offer lower energy use than air-cooled alternatives. Many years
ago, the first water-cooled systems used potable water directly in the condenser to
provide heat rejection with the cooling water wasted to a drain. Cooling towers were
developed to recycle more than 98% of this water, resulting in tremendous reductions
in water and energy use as these systems grew in both size and popularity.
Since then, water-cooled systems have steadily
improved their performance. For instance, the efficiency of a 500 ton (1757 kW) water-cooled centrifugal
chiller has improved by over 50% since 19751 as indicated by the requirements of ASHRAE/IES Standard 90.1
(hereafter referred to as Standard 90.1). Cooling towers
have also evolved from centrifugal fan units to much
more energy-efficient axial fan designs with improved
heat transfer surfaces, known as fill. In addition, independent certification of thermal performance for open
circuit cooling towers per the Cooling Technology
Institute's Standard 201* has become widely accepted in
the marketplace and became required by Standard 90.1
in the 2007 edition.
While the efficiency improvements of individual system components have certainly lowered overall energy
use, even greater improvements are possible by optimizing the way cooling systems are designed and operated.
For instance, the full load energy use in a 500 ton (1757

kW) water-cooled chiller system, based on Standard
90.1-2013 minimum efficiencies, is roughly broken
down as follows: chiller - 77%, cooling tower - 8%, condenser pump - 7%, and chilled water pump - 8%. With
the chiller accounting for the majority of the energy use,
many contend that it makes sense to operate the cooling
tower fan and condenser pump such that compressor
energy use is reduced-since it is by far the largest motor
in the system.
For instance, to lower chiller energy, the cooling tower
is often operated at full fan speed and flow until ambient conditions allow the minimum condenser water
temperature limit to be reached. Below this level, the
fan speed of the cooling tower is modulated, typically
by a variable speed drive (VSD), to maintain the setpoint. This is essentially the operating sequence for the
water-cooled baseline buildings found in Appendix
G of Standard 90.1-2013, which uses 70°F (21.1°C) as
the lower condenser water setpoint (though this value

*CTI Standard 201 has been replaced by two 2013 standards: STD 201-OM, Operations Manual for Thermal Performance Certification of Evaporative Heat Rejection Equipment and STD 201RS, Performance
Rating of Evaporative Heat Rejection Equipment.
34

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