ASHRAE Journal - July 2019 - 60

COLUMN DATA CENTERS

part by the operating temperature of the liquid-cooling
loop and the local climate. For centers requiring relatively cold design fluid temperatures or are in a "hot"
climate, a conventional chiller, either water-cooled or
air-cooled, can be used (typically Class W1 or W2). The
higher the fluid temperature that can be tolerated by
the equipment, the more efficient the chiller will operate and the greater opportunity for economization. Both
air- and water-cooled chillers can be equipped with
economizer circuits to allow a reduction in the mechanical cooling energy as outside temperatures and loads
decrease throughout the year.
For most facilities, evaporative heat rejection, which is
based on the wet bulb of the air (which is typically lower
than the dry bulb), offers the highest number of potential hours of economization. By designing for higher TCS
liquid temperatures (i.e., 85°F [29.4°C]), chiller-less
data centers can be considered (Class W4). Open circuit
cooling towers offer the most efficient operation thanks
to the direct cooling of the recirculating water by the
entering air to the unit (Photo 2). However, because of the
need for clean, closed loops, open circuit cooling towers must be isolated from the FWS by plate & frame heat
exchangers. The approach across the heat exchanger,
typically about 3°F (1.7°C), must be considered in the
design. As an example, for a cooling tower producing
85°F (29.4°C), the design fluid temperature to the liquid-cooled equipment would be 88°F (31.1°C) from the
other side of the heat exchanger.
A closed-circuit cooling tower, or fluid cooler, can also
be used, which combines the function of an open circuit
cooling tower and a heat exchanger together into one
compact unit (Photo 3). The coil in the unit is typically
hot dip galvanized steel though stainless steel coils are
also available if required due to corrosion considerations. The fluid from this unit can be used directly in
the FWS, eliminating the need for a separate plate &
frame heat exchanger. Fluids other than water can also
be cooled in closed circuit cooling towers, such as aqueous glycol solutions, which may be required in colder
climates.
Open circuit cooling towers can operate with entering
water temperatures up to a maximum of approximately
120°F to 140°F (51.7°C to 60.0°C), depending on the fill
type and material. Closed circuit cooling towers generally have an entering fluid temperature limit of approximately 180°F (82.2°C). While open circuit cooling towers
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ASHRAE JOURNAL

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J U LY 2 0 19

PHOTO 2 Open circuit cooling towers on a liquid-cooled data center. Courtesy of

National Center for Supercomputing Applications (NSCA)

PHOTO 3 Closed circuit cooling towers being installed on a liquid-cooled data cen-

ter in Houston. Courtesy of DownUnder GeoSolutions

will typically offer the lowest fan horsepower for the
cooling tower, both tower types offer lower fan and system energy use than air-cooled alternatives while being
able to achieve lower design fluid temperatures. Since a
closed loop is required, the cost of a closed-circuit cooling tower will be comparable to an open circuit cooling
tower coupled with a plate & frame heat exchanger for
the same cooling duty. Closed-circuit units can also be
selected to operate dry in the event of a water disruption
to the facility. Their dry capability can be enhanced by
finning the coil surface in the unit.
For areas where makeup water is expensive or in
short supply, hybrid closed circuit cooling towers are
available which contain both evaporative and finned
heat exchange sections. These units are capable of
operating in either "wet" (evaporative) mode or dry
mode, saving water and reducing visible plume during colder weather. Hybrid units can be selected for

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ASHRAE Journal - July 2019

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