ASHRAE Journal - July 2019 - 58

COLUMN DATA CENTERS

in Figure 3. These arrangements are often utilized on
higher-powered racks in a data center.
Liquid-cooled electronics defines the cases where liquid
is supplied directly to the electronics at the board level
for cooling with no other form of heat transfer, as shown
in Figure 4. The use of such liquid-cooling systems can
range from cooling all the electronics in a data center to
cooling only the higher-powered equipment in a facility
to ensure its performance as well as reduce overall cooling load. This latter method can often be found in mixed
use, colocation-type data centers.
The fluid used in liquid cooling can be either water
or a refrigerant, depending on the specific system.
Aqueous glycol solutions are typically not used within
the data center due to the logistical challenge of cleaning
up a leak inside the facility. When glycol is required for
freeze protection of outside heat rejection equipment,
an isolation heat exchanger is often used with water on
the opposite side. Alternatively, a freeze-protected dry
cooler employing a refrigerant loop can be utilized.
In another embodiment, the electronics can be fully
immersed in a non-conductive cooling liquid, such as
mineral oil or a refrigerant. For these applications, an
isolation heat exchanger is also used to cool the immersion fluid, which is typically viscous, saving pumping
energy while avoiding level issues in the immersion
baths for the servers as shown in the system in Photo 1.
Immersion cooling has several additional advantages:
* No cooling fans are required in the servers, offset by
any additional pumping energy, resulting in the potential for lower PUE values;
* Oxidation is reduced because the electronics are not
exposed to large volumes of air, which can carry particulates, gases, etc.;
* Failures due to thermal expansion/contraction are
eliminated; and
* The immersion fluid provides greater resiliency in
the event of a cooling system failure-up to 30 minutes
versus approximately two minutes for cooling systems
using air.
Successful application of liquid cooling in a datacom
facility requires following key design considerations
centered on the piping of the liquid throughout the electronic space. These include, but are not limited to the
following:
* Route liquid piping below the electronics-avoid
overhead piping that can drip on the equipment wher58

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FIGURE 3 Liquid-cooled datacom equipment.

Rack or Cabinet
(Side View)

Facility Coolant
Return
Supply

Raised Floor

Foundation

FIGURE 4 Flexible hose connects CPU cold plates and other cooling structures in

a server.

ever possible;
* Confine piping to pipe galleries, chases, utility pits,
etc., wherever possible;
* Use higher liquid-cooling loop temperatures where
possible along with the appropriate insulation on piping
to avoid the possibility of condensation;
* Use shut-off valves to enable safe servicing and
modification of sections of the liquid-cooling loop;
* Properly locate balancing and/or control valves in
the liquid-cooling loop to balance flows to each area;
* Properly size the circulating pumps and where possible, use variable speed pumping to enhance temperature control and save additional energy;
* Oversize piping, use large radius elbows, etc. and
specify premium pump motors, all of which offer short
paybacks due to 24/7 operation;
* Allow for expansion and contraction in the pipework as well as design for seismic forces;
* Where required, use good quality, drip-less quick
connects and flexible hoses that run to and from the
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ASHRAE Journal - July 2019

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