ASHRAE Journal - July 2019 - 59

COLUMN DATA CENTERS

liquid-cooling loop and the liquid-cooled equipment;
* Select the appropriate piping and heat exchanger
materials and an appropriate water treatment system
for the closed liquid-cooling loop (TCS);
* Monitor and maintain the proper ﬂuid quality in
both the FWS and the technology cooling system (TCS);
* Evaluate the various piping layouts that are possible, considering reliability, redundancy, serviceability,
ﬂow balancing, system complexity, and lastly installation costs (piping is not an area to cut costs in a liquidcooled datacom facility!); and
* Consider any future expansion plans for the facility,
including the use of higher power racks.
Another important consideration is the temperature control system, which is important to protect the
electronic equipment, prevent condensation, and save
energy. Many liquid-cooled data centers utilize coolant
distribution units (CDU) to help properly distribute
and temper the ﬂuid entering the electronics from the
TCS. CDUs, which may be internal or external to the
racks, typically house a heat exchanger, one or more
circulating pump(s), and service valves. These units
also isolate the TCS from the FWS, which allows the
use of a different ﬂuid while reducing the potential
leak volume and the risk of condensation. Because of
their importance, CDUs will often be placed on a UPS
to ensure continued operation in the event of a power
interruption.
For further detail on liquid-cooled data centers, be
sure to read the feature article in this month's Journal,
"Moderating the Impact of Integrating Water-Cooled
Servers into Data Centers" (Page 20).

Heat Rejection Options
Liquid-cooled equipment and electronics offer abundant opportunities for handling higher power in a facility in an energy-efﬁcient manner. The use of water-side
economization complements this trend by being able
to provide a less energy intensive and lower cost source
of cooling. For instance, a processing chip may operate
at 185°F (85°C) which can be readily cooled by a ﬂuid
stream with an inlet temperature of 104°F (40°C).* As
discussed earlier, specialized racks are also available that
completely submerse the servers in a non-conductive
liquid. Heat is then rejected from this ﬂuid either

PHOTO 1 Liquid-cooled high performance server array with cooling distribution

unit (CDU) in tank. Courtesy DownUnder GeoSolutions

directly or indirectly to the cooling system. In all these
cases, temperature control is essential to keep from
undercooling or overcooling the electronics or causing
thermal shock due to very cold ﬂuid cooling very hot
chips. Hence, the frequent use of CDUs to control the
ﬂuid temperatures to what is required in the various
areas of the facility.
Liquid-cooling systems offer the possibility of designing 100% water-side economization cooling systems for
the electronics (Classes W2 through W5, depending on
design temperatures and climate). Cooling of the economizer ﬂuid can be handled through a variety of heat
rejection methods, like those described in our March
2019 column on "Designing Waterside Economizers-
Part 3." Due to the need to protect the liquid-cooling
loop inside the data center, all heat rejection options
involve closed circuit cooling to ensure a clean, closed
system, reducing fouling and system maintenance.
This system also accommodates variable speed pumping, which offers the potential for further energy savings. Keep in mind that even in a fully liquid-cooled
data center, a more conventional, though smaller HVAC
system would still be required to maintain the overall
room conditions for both the building environment and
the datacom technicians who work in the facility. Since
liquid cooling is available, these HVAC systems are often
water cooled.
Heat rejection options for a liquid-cooled data center
depend on the speciﬁc needs of the facility driven in

*Liquid Cooling in Data Centers. ASHRAE Transaction Paper CH-09-025, T. Cader, L. Westra, V. Sorel, and A. Marquez, 2009, page 235.
J U LY 2 0 19

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ASHRAE Journal - July 2019

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