ASHRAE Journal - February 2014 - 35

TECHNICAL FEATURE
FEA

is above the low limit for almost all chillers). In practice, this lower limit varies and is dependent on the
type of chiller. The closer to full load the system runs,
the greater the energy savings from such strategies.
However, most chiller systems operate at less than full
load for the majority of time.
While it may seem counterintuitive, many designers
and operators have found that using less cooling tower
energy reduces overall system energy at many off-design
conditions. At such conditions, ancillary equipment
(condenser pumps and cooling tower fans) operating at
full design speed becomes a larger portion of the system
energy use, especially when variable speed chillers are
used. Reducing cooling tower fan speed can reduce associated fan power significantly while increasing chiller
power only marginally. For example, slowing the tower
fan speed to 80% of design reduces tower fan power by
about half, while only raising the cooling tower leaving
water temperature about 3°F (1.7°C). Depending on the
specific load point, the increase in chiller energy consumption from the higher condenser water temperature
may or may not be less than the reduction in cooling
tower energy. The key is to balance the performance
of the system components so overall performance is
optimized. Articles such as Taylor's2,3,4 excellent series
on chilled water system design provide more details on
such strategies.

Closer Approach Selections
Another method to reduce system energy is to select a
cooling tower using a closer approach than might be typical for a particular area. The tower approach is defined
as the difference between the water temperature leaving the cooling tower minus the entering wet-bulb
temperature. When a closer design approach is chosen,
the resulting cooling tower provides colder water to the
chiller condenser, even on a design day, which in turn
reduces compressor energy. This, of course, assumes the
system designer has not taken advantage of the colder
design water temperature to reduce the condenser surface area of the chiller.
The added cooling tower cost and potentially greater
tower fan horsepower and pumping head must be evaluated versus the expected chiller energy savings. Facilities
with constant year-round loads, such as those experienced in data centers or certain manufacturing facilities,
typically derive the greatest benefit from this technique.

PHOTO 1 Four cell crossflow open circuit cooling tower.

Fan Speed Control
There are several specific methods to optimize cooling tower energy use. First, as required by Standard
90.1-2013, cooling tower fan speed must have the capability to be controlled proportional to the leaving fluid
temperature or condensing temperature/pressure.5
This can be accomplished in several ways, including
the use of two-speed motors or variable speed drive
technology. For multi-cell cooling towers, all of the
fans should be operated simultaneously at the same
fan speed, maximizing the heat transfer surface area
used in the evaporative cooling process, for the lowest
energy use. This is opposed to the traditional manner
of fan cycling (on/off) that provides step control (for
example, Tower 1 fan on, then Tower 2 fan on, etc., as
the load increases). This operating sequence becomes
even easier to apply today with the widespread use of
cost-effective variable speed drives (VSD), though the
sequence can also be used with either multi-speed or
pony motors.
To illustrate the potential energy savings, let's
look at the case of a four-cell cooling tower with
and without variable speed fan drives as shown in
Photo 1. With full water flow over all cells, operating the fans in two cells at full speed with the fans
in the other two cells idle produces essentially the
same leaving water temperature off the tower as
when running the fans in all four cells at approximately 56% fan speed. However, by running all fans
simultaneously at the lower speed, the fan energy is
reduced by more than 60% compared to step control
ABOUT THE AUTHOR Frank Morrison is manager, global strategy at Baltimore Aircoil Company
in Jessup, Md. He is chair of ASHRAE TC 3.6, Water Treatment, and a voting member of
ASHRAE SSPC 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings.

FEBRUARY 2014

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