ASHRAE Journal - July 2019 - 22

TECHNICAL FEATURE

FIGURE 2 CDU Liquid cooling system within a data center.

Datacom Equipment Center
Rack
ITE

Cooling
Tower

External CDU

Rack
ITE

CDU
Chiller

Condenser Water
System (CWS)

Facilities Water
System (FWS)

Technology Cooling
System (TCS)

FIGURE 3 Non-CDU liquid cooling system within a data center.

Water Cooling With and Without a
CDU

ASHRAE JOURNAL

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J U LY 2 0 19

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Datacom Equipment Center

For those unfamiliar with waterRack
Rack
Cooling
cooled IT equipment, the first quesTower
ITE
ITE
tion will be: How does one transfer
the heat from the water-cooled IT
RFU
Chiller
equipment to the facilities water sysFFU
tem (FWS)? The most common form
of water-cooling implementation
Condenser Water
Facilities Water
within the data center today uses a
System (CWS)
System (FWS)
coolant distribution unit (CDU) to
separate the facility water system
(FWS) from the technology cooling
system (TCS) by using a water-to-water heat exchanger.
a mixing valve to regulate the fraction of water from
By using a water-to-water heat exchanger, the CDU can
the FWS loop to pass through the water-to-water heat
reject heat from IT equipment (ITE) without exposing
exchanger and forcing the remainder to bypass the
the sensitive IT cooling components to the typically less
heat exchanger.
regulated FWS water.
The CDU plays an important role in data center liquid
CDUs may be located within an IT rack, providing
cooling by often providing the following functions:
TCS water distribution to equipment within a single
* Transfer heat from the TCS to the FWS;
rack, or installed as a floor-standing external unit that
* Circulation of TCS coolant;
distributes TCS water to a plurality of racks. Figure 2
* Allows for a TCS coolant other than water (e.g.,
displays both examples showing the heat transfer from refrigerant or engineered fluid);
the water-cooled server rack through the water-to* Prevents condensation within the IT equipment by
water heat exchanger in the CDU. The basic flow and
regulating TCS water above room dew point;
heat exchange components within a CDU consist of a
* Establish and maintain a coolant quality and chemheat exchanger, flow-mixing valve, pumps, expansion
istry different from that associated with the FWS and
tank and water supply/return manifolds. The tempera- more suitable to the TCS; and
ture of the water in the TCS loop is controlled by using
* Flexible coolant temperature supplied to ITE.
22

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The number at the top of each bar
in Figure 1 is the maximum facility water temperature that can be
employed to cool the server rack.
The first seven products displayed
had maximum facility water temperatures of 10°C (50°F) to 24°C
(75°F). Of the remaining 21 products, eight have maximum facility water temperatures of greater
than or equal to 40°C (104°F), a
clear focus on warm water cooling integrated with data center
economizers and the possible use of
the heated exhaust water from the
servers to heat local buildings in
the winter.
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ASHRAE Journal - July 2019

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