Power Play
Decrease temps and cooling costs
Using the equipment cabinet as an
architectural feature can cool heat loads in
excess of 20 kW per cabinet.
by Ian Seaton
Case studies have shown that by employing
passive cooling technology to overcome
thermal challenges, network managers can
decrease data center cooling costs up to 40
percent. The basic principle of passive
cooling technology involves using the
equipment cabinet as an architectural
feature in the data center that secures the
isolation between chilled supply air and
heated return air. When using a ducted
exhaust cabinet, this isolation can
effectively cool heat loads in excess of 20
kW per cabinet, while maximizing cooling
unit efficiency and allowing the air
temperature in the data center to be
increased.
A case study of a casino's 600 kW data
center demonstrates the savings that can be
achieved by using passive cooling
technology. This data center contains two
rows of low-density deployments with 2
kW-2.5 kW per cabinet, two rows of 8 kW
cabinets, two rows of 16 kW cabinets, four
short rows of connectivity and three rows
with 1 kW-1.5 kW per cabinet.
The original design was based on standard
hot aisle and cold aisle separation with
free-space return air. There were severe hot
spots in the 8 kW and 16 kW cabinets.
The ducted exhaust cabinets and
improved tile locations in the
containment scenario eliminated seven
CRAC units from the plan.
The containment strategy included adding
vertical exhaust ducts to the higher-density
cabinets to remove the return air into the
suspended ceiling space, and plenum
extensions were added to the cooling units
to capture the return air. Suspended ceiling
tiles over the hot aisles of the low-density
cabinets were replaced with ceiling grates
to bring the return air into the suspended
ceiling space and minimize its impact on the
overall room.
A computational fluid-dynamics model
confirmed the site audit that the hot air
containment changes had eliminated the hot
spots. More importantly, the initial
scenario required 14 computer room air
conditioning (CRAC) units with a cooling
capacity of 385 tons and still experienced
hot spots, compared to the containment
scenario, which only required seven CRAC
units for a cooling capacity of 220 tons and
resulted in no hot spots.
The ducted exhaust cabinets and improved
tile locations in the containment scenario
eliminated seven CRAC units from the plan,
totaling 165 tons of excess cooling capacity
(40 percent of the original capacity). The
reduced capacity resulted from the improved
efficiency of the water-cooled CRAC units at
higher return air temperatures and the
elimination of the need to over-provision
airflow to compensate for the wide variation
in static pressure typically found under the
access floors of most data centers.
Following this plan, this data center
could see more than $80,000 in annual
savings, plus the reduced capital outlay for
the unneeded CRAC units.
This example demonstrates the challenge
of effective cooling in a free-space return
air environment and the benefits of
contained return air using ducted exhaust
cabinets. The free-space return air
environment requires additional cooling
capacity to overcome the mixing of hot and
cold air in the room.
Immediate savings were realized using
partial isolation with ducted exhaust
cabinets for the highest heat load and by
carefully locating supply and return vents
throughout the room. Complete isolation with
ducted exhaust cabinets and the use of
economizers can further reduce cooling cost.
Ian Seaton, technology marketing
manager for Chatsworth Products, New Bern,
N.C., serves as editor of the rack and
cabinet chapter and functions as a thermal
consultant on the mechanical working group
for the new BICSI data center design
standard.
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