ASHRAE Journal - February 2014 - 38

TECHNICAL FEATURE

Hot Water In

Warm Air Out

Warm Air Out

Hot Water In
Drift Eliminators
Hot Water In

Water

Air In

Warm
Air

Warm
Air

Water

Water
Air In

Air Inlet
Louvers

Cold Water Basin

Air In
Fill
Surface

Cooled Water Out

Cooled Water Out

FIGURE 1 Induced draft, axial fan, crossflow open circuit cooling tower.

potential condenser pump energy savings to capture,
which further alters the risk/reward ratio.
Standard 90.1-2013 now requires that all open circuit cooling towers on water-cooled chiller systems
with either multiple or variable speed condenser
water pumps have the capability of a minimum 50%
flow turndown. Note, however, that there are no
requirements or guidelines for implementing condenser flow turndown in the standard at this time
as each system is unique. As such, system designers and operators should consult with their chiller
and tower manufacturer for specific flow turndown
recommendations for their system. As mentioned
previously, the local makeup water quality, the
expected water treatment program, and the sophistication of the control system required should also
be evaluated.

Water Economizers for Computer Rooms
January's article by Mick Schwedler7 covered the
proper application of water economizers, cautioning
the reader to properly account for the changes in cooling tower approach temperature at lower wet-bulb
temperatures. Reflecting this same phenomenon, a
change to the requirements for water economizers that
are primarily used for computer room applications was
implemented in Standard 90.1-2013. Unlike a comfort
cooling application where the load decreases in winter,
the load in a typical computer room remains relatively
constant throughout the year, independent of climate
variations. The high year-round load coupled with the
38

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Fill Surface

FEBRUARY 2014

Air In
Typical on All Four Sides

FIGURE 2 Induced draft, axial fan, counterflow open circuit cooling tower.

increasing approach temperature at lower wet bulbs
results in a cooling tower selection that is oversized for
the summer duty.
Since many computer facilities are lightly loaded
during their first few years of operation, system
control can suffer, which is a serious concern for
computer system operators due to their need for
high reliability and system uptime in such facilities. To reduce the oversizing, the switchover design
temperature requirement for full economization
was revised from a single fixed design temperature
for all computer rooms (wet bulb for cooling towers
and dry bulb for dry coolers) to one that varies by
climate zone. The lower design temperature reduces
the size of the heat rejection device required for the
water economizer which in turn reduces first cost
as well as improves system control under low load
conditions.

Closed Circuit Cooling Towers and Evaporative Condensers
While this article primarily discussed open circuit
cooling towers, the fan speed control requirements
mentioned earlier also apply to closed circuit cooling
towers, evaporative condensers, dry coolers, and air
cooled condensers. Minimum efficiencies and thermal
performance certification requirements for closed circuit cooling towers were added in the 2010 edition of
Standard 90.1. Closed circuit cooling towers combine
the function of a cooling tower and heat exchanger in
one compact device, keeping the process fluid clean in
a closed loop.
ASHRAE Journal - February 2014

Table of Contents for the Digital Edition of ASHRAE Journal - February 2014
