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8: What About Chillers, Cooling Tower Fans, and All That
Cooling Equipment Usually Ignored by IT? 137

failures (HDF), tape media errors, and excessive wear and corrosion can
occur. These risks increase exponentially as relative humidity increases above
55 percent.
If humidity is too low, the magnitude and propensity for electrostatic
discharge (ESD) increase, damaging equipment or adversely affecting opera-
tion. Also, tape products and media might perform poorly when exposed to
low relative humidity.
ASHRAE has defined the optimal relative humidity for a data center envi-
ronment as 40 percent to 55 percent. CRACs control humidity through
humidification or dehumidification as required, both of which consume
energy. An effective vapor seal can reduce the amount of energy expended on
humidification or dehumidification.
The vapor seal is typically created using a combination of plastic film,
vapor-retardant paint, vinyl wall coverings, and vinyl floor systems. All
openings in doors, windows, and cable entrances should also be sealed.
This sealing is the first step in any plan to increase efficiency. If the room
is not properly sealed, all other measures for improving efficiency will be less
effective. A data center assessment, available through various consulting
ptg
engineering firms or your cooling system supplier, can help identify areas
where outside air is entering the controlled environment and recommend
strategies for proper sealing.

2. Optimizing Air Flow

After the room is sealed, the next step is to ensure efficient air movement.
The goal is to move the maximum amount of heat away from the equipment
using a minimum expenditure of energy. Optimizing air flow requires evalu-
ation and optimization of rack configuration, air conditioner placement, and
cable management.

■ Rack Arrangement: Most equipment manufactured today is designed to
draw in air through the front and exhaust it out the rear. This allows
equipment racks to be arranged to create hot aisles and cold aisles. This
approach positions racks so that rows of racks face each other, with the
front of each opposing row of racks drawing cold air from the same aisle
(the “cold” aisle). Hot air from two rows is exhausted into a “hot” aisle,
raising the temperature of the air returning to the CRAC and allowing
the CRAC to operate more efficiently.