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The Greening of IT
138 How Companies Can Make a Difference for the Environment

This approach is most effective when cold and hot air do not mix.
Therefore, perforated floor tiles should be removed from hot aisles and
used only in cold aisles. Blanking panels should be used to fill open
spaces in racks to prevent hot air from being drawn back through the
rack.
Some type of cabling grommet should also be used to prevent the cold air
from entering the space through cable openings, which are typically at
the rear of the rack. Additional steps such as using a return ceiling
plenum to draw the air back to the CRAC and physical curtains at the
ends of the cold aisles have also proved to be very effective in minimizing
mixing of the hot and cold air.
■ CRAC Placement: When using the hot-aisle/cold-aisle approach, CRAC
units should always be placed perpendicular to the hot aisle to reduce air
travel and prevent hot air from being pulled down into the cold aisles as
it returns to the air conditioner. A return ceiling plenum can be effective
in minimizing the mixing of hot and cold air.
■ Cable Management: The explosion in the number of servers that data
centers must support has created cable management challenges in many ptg
facilities. If not properly managed, cables can obstruct air flow through
perforated floor tiles and prevent air from being exhausted out the rear of
the rack. Check the under-floor plenum to determine if cabling or piping
is obstructing air flow. Overhead cabling is becoming increasingly popu-
lar, which eliminates the potential for obstruction. Deeper racks are now
available to allow for increased airflow. Sometimes existing racks can be
equipped with expansion channels to add depth for cables and airflow. Be
cautious when using cable management “swing arms” because they are
not compatible with all IT equipment air-flow patterns.

Finally, but perhaps most significantly, investigate bringing high-voltage
three-phase power as close to the IT equipment as possible and increasing the
voltage of the IT equipment. These steps will minimize the number and size
of the power cable feeds under the floor. This can sometimes be accomplished
by using high-voltage three-phase managed power strips within the rack, but
it might also require the use of multiple-pole distribution panels or PDUs
located within the row of IT equipment racks.
You can add fans to the rear of racks to draw hot air out of the rack, but be
aware that these fans consume energy and generate additional heat that must
be removed from the room.