ASHRAE Journal - July 2019 - 62

COLUMN DATA CENTERS

technology than that of either open or closed-circuit
cooling towers for the same heat load.
No matter what heat rejection alternative is chosen,
the ability of the system to reject the load at the specified
temperature conditions is critical. To achieve the full
energy-saving potential of liquid cooling, variable speed
fan systems should be utilized on heat rejection equipment to enhance system control, minimize energy use,
reduce fan system maintenance, extend fan system life,
and comply with code requirements.
Furthermore, equipment with independently verified thermal performance should be specified or, in
lieu of certification, a field test should be required.
For instance, the thermal performance of open circuit
and closed-circuit cooling towers should be certified
per CTI STD-201RS, Performance Rating of Evaporative
Heat Rejection Equipment, or a thermal test by a licensed,
third-party testing agency should be performed, using
CTI ATC-105 for open circuit cooling towers and CTI
ATC-105S for closed circuit cooling towers. Note that
the Cooling Technology Institute has recently published their Acceptance Test Code for Dry Fluid Coolers (ATC105DS) which can be used to verify performance claims
in the field for these devices. In Europe, Eurovent
Certita offers a certification program for dry coolers.
Additionally, an acceptance test code for adiabatic fluid
coolers is currently under development by the CTI.
The higher fluid temperatures possible with liquidcooling systems also increase the potential for the
reclaim of the waste heat for many symbiotic uses such
as building heating, greenhouses and fish farming
(typically Liquid Cooling Class W5, but all classes have
the potential for some heat recovery, depending on the
leaving fluid temperature). This, in turn, can reduce
the amount of heat that needs to be rejected to the
atmosphere.
Lastly, natural sources can be utilized for cooling the liquid-cooling loop. These include seawater,
lake, or ground source cooling if available. Each of
these options present unique challenges that must be
dealt with, such as corrosion and long-term reliability. A hybrid approach is often called for due to high
expected uptime requirements for data centers, such
as a ground source cooling system integrated with a
closed-circuit cooling tower for redundancy in the
likely event of problems with a well field in a coolingonly application.
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Summary
With the relentless increase of heat loads within datacom facilities, the trend to liquid cooling is growing
significantly. Higher temperature liquid cooling offers
the prospect of greatly reducing or even eliminating the
need for mechanical cooling by replacing it with 100%
fluid economizer systems. The potential for heat reclaim
is also enhanced thanks to the higher allowed fluid
temperatures. Liquid cooling can be selectively utilized
for handling the higher-powered racks in a data center, helping to unload the main cooling system. New, or
greenfield datacom facilities should, at a minimum, be
designed to accommodate the use of liquid-cooled electronics in the future.
Heat rejection options involve the use of closed-circuit
cooling to maintain efficiency and reduce maintenance.
Open circuit cooling towers with heat exchangers or
closed-circuit cooling towers can be used for high efficiency heat rejection, while hybrid closed circuit cooling
towers or adiabatic fluid coolers offer the advantage of
lower water use. With higher operating liquid temperatures and the proper climate, dry coolers can be utilized
to eliminate water usage but at a higher parasitic energy
cost.
Like those data centers that rely on air for cooling
described in our last column, the key to proper design of
liquid-cooled data centers is to define the owner's project
requirements along with those required by code. The
design should not detract from the mission of the facility while preferably increasing both the redundancy and
reliability of the operation. A total cost of ownership (TCO)
analysis can be performed to help select the optimum
cooling system for the data center, whether air or liquid
cooled, from the many alternative designs available.

Bibliography/References

ASHRAE TC 9.9, Liquid Cooling Guidelines for Datacom Equipment
Centers, Second Edition, 2014.
ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments,
Fourth Edition, 2015.
ASHRAE TC 9.9, Water-Cooled Servers: Common Designs, Components,
and Processes, 2019.
D. Beaty, D. Quirk, F. Morrison. 2019. "Designing data center
waterside economizers - part 3: heat rejection alternatives."
ASHRAE Journal 61(3):78-86.
D. Beaty, R. Schmidt. 2004. "Back to the future: liquid cooling-
data center considerations." ASHRAE Journal , (12):42-46.
F. Morrison, A. Rushworth. 2018, "Closing the Loop - Which
Method is Best for Your System?" CTI Technical Paper 18-15
R. Schmidt. 2019. "Moderating the impact of integrating watercooled servers into data centers." ASHRAE Journal (7).

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ASHRAE Journal - July 2019

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