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By Fran Escott, Leo Soucy, and Bill Flaherty, Facilities Engineering Associates, Farmington, CT; and Kishor Khankari, Fluent Inc.
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As the computational power of servers
increases, so does the heat load associated
with them. Air conditioning systems that
were the norm for data centers of the recent past
can now no longer handle the increased load. As
new data centers come on-line or existing data
centers are upgraded, improved air conditioning
methods are now mandatory. The problem isn’t
that sufficient air conditioning cannot be provided,
but that the air conditioning must be provided
where it is needed. This means that the flow
characteristics of the air stream that provides the
cooling must be carefully studied as the air progresses
from the computer room air conditioner
(CRAC), through the racks of equipment, and
back to the CRAC.
Pathlines, colored by temperature, for the
parallel rack orientation showing the
recirculation of hot air into the cold aisle;
the high density rack is shown in red
Temperatures on the rack surfaces for the
parallel orientation, showing the excessive
heating of the upper portion of the low
density rack, left, resulting from the
entrainment of heated air from the high
density rack, second from left
Pathlines, colored by temperature, for the
perpendicular rack orientation showing the
entrainment of hot air from two hot aisles
into the cold aisle between them; the high
density rack is shown in red
Temperature on the rack surfaces for the
perpendicular orientation, showing the
impact of the entrained hot air on the
surface temperature of the upper servers
facing the cold aisle
Facilities Engineering Associates (FEA), in conjunction
with Fluent, has recently completed an
analysis of a representative data center. The room
contains four racks of computers and four
CRACs, located in pairs at opposite ends of the
room. In addition to the main floor, an elevated
floor, 14 inches above the main floor, is used to
support the racks. The data center is laid out with
a hot-aisle / cold-aisle arrangement by careful
positioning of the racks and perforated sections
in the elevated floor. The CRACs dispense cooling
air under the elevated floor. This air rises through
the perforated sections, forming cold aisles. The
cold air flows through the racks where it picks up
heat before exiting from the rear of the racks.
The warm exit air forms hot aisles behind the
racks, and the hot air returns to the CRAC
intakes, which are positioned above the floor.
Two orientations for the data center were studied
– one with the racks (and perforated flooring)
parallel to the CRACs, and the other with a perpendicular
orientation. For both cases, one server
rack (shown in red, far left) was assumed to
have a higher (about 2.5 times) power density
rating than the others.
Pathlines colored by temperature were used
to show the progression of the air streams from
the perforated flooring, through the racks, and
back to the CRAC units for the two configurations
studied. Both designs were found to be
problematic as a result of the high-density rack in
the room. For the parallel orientation, hot air
exiting from the high-density rack circles over the
top of the rack and reenters the cold aisle, where
it is drawn into the servers located in the upper
section of the neighboring low-density rack. The
lower portion of the low-density rack is properly
cooled by air entering through the floor tiles. For
the perpedicular orientation case, a similar problem
occurs: hot air exiting not only from the
high-density rack but also from the adjacent low
density rack circles above and around the sides of
the rack and reenters the cold aisle, compromising
the cooling air that is delivered through the
floor tiles. A display of the temperatures on the
surfaces of the racks indicates that while the high
density rack is adequately cooled for both cases,
the neighboring racks are not.
By using CFD in its design process, FEA identified
the potential cooling problems before they
occurred, thereby saving the time and expense
required for repairs or retrofitting once the construction
of the room is complete and the room is
operational.
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