ASHRAE Journal - July 2019 - 57

COLUMN DATA CENTERS

* Reducing the overall volume of
FIGURE 1 Sealed hot aisle/cold aisle rack configuration. (NREL report number BR-7A40-47201, February 2010,
original source Rumsey Engineers)
airﬂow needed by the equipment
racks, lowering fan energy;
Return Air Plenum
* Increasing computing performance, extending server life, and
Physical
Separation
Hot Aisle
Hot Aisle
reducing the risk of failure of the
electronics, all thanks to cooler operating temperatures;
Cold Aisle
* Increasing usable data center
space by reducing the size or even
Raised Floor
eliminating traditional cooling
equipment, such as chillers and air
handlers; and
FIGURE 2 Typical infrastructure design schematic for each liquid cooling class.
* Reducing overall energy use of the datacom
CDU + IT
facility by increasing cooling efﬁciencies and
Heat Exchanger
Cooling Tower
Equipment
thus power usage effectiveness (PUE).
Supply Temperature to CDU:
W1/W2/W3
Keep in mind that water and other heat trans2°C to 17°C (W1)
Water Chiller
2°C to 27°C (W2)
fer ﬂuids can move, on a pound-for-pound
2°C to 32°C (W3)
basis, nearly 4.2 times the amount of heat as
compared to air.  As water is also signiﬁcantly
CDU + IT
Dry Cooler or
Heat Exchanger
Equipment
denser than air, on an equivalent volumetric
W4
Cooling Tower
Supply Temperature
ﬂow rate basis, water can transport over 3,700
to CDU:
times more heat than air.  This signiﬁcantly
2°C to 45°C
reduces the "pumping" (fan) energy along with
CDU + IT
Dry Cooler or
the thermal transport conduit size (duct versus
Heat Exchanger
Equipment
Cooling Tower
W5
pipe) within the facility.
Supply Temperature
So just what is "liquid cooling"? In our conto CDU:
Building Heating
>46°C
System
text, liquid cooling refers to bringing a cooling
ﬂuid close to the heat source, which ultimately
are the processor chips and other electronics in the data units are placed in the aisles of the data center, includcenter. This is not a new concept as many high-powered ing in-row and overhead locations to either deal with
mainframes of the past had liquid-cooling circuits
hot spots or provide dedicated cooling on speciﬁc highinside the cabinetry to carry away the large quantities
power racks. These cooling modules can also be placed
of heat generated. The speciﬁc problem was that the
in the racks or cabinets themselves. While the cooling
density of the heat load did not allow enough air to ﬂow
ﬂuid is closer to the electronics in these embodiments,
through the electronics to keep them from overheating
note that air continues to perform the cooling function
and subsequently failing. The industry moved to cooling at the actual heat sources.
with air as mainframes were replaced with distributed
The real opportunity for liquid cooling lies in bringing
computing and air proved to be a lower cost alternative. the cooling ﬂuid more directly in contact with the heat
Additionally, many data center managers were consources in the data center. Because the electronics opercerned about bringing cooling ﬂuids in close proximity
ate at a much higher temperature (up to 212°F [100°C] or
to the electronics where a leak could affect reliability
more), the ability of warmer liquid ﬂows to carry away
and uptime.
this heat is signiﬁcantly increased thanks to the greater
Liquid cooling can take many forms. One of the simtemperature difference. Liquid-cooled datacom equipment
plest involves ﬁnned coil modules, either with or withhas liquid circulated within the datacom equipment for
out fans, that are supplied with a cooling ﬂuid and proheat removal. The heat from this ﬂuid can be rejected
duce a cooling airstream directed toward the racks. Such directly to the facility water system (FWS) as shown
J U LY 2 0 19

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