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8: What About Chillers, Cooling Tower Fans, and All That
Cooling Equipment Usually Ignored by IT? 141

Digital Scroll compressor technology offers a newer way to precisely
match capacity and power consumption to the desired load and can deliver
significantly lower energy consumption compared to standard fixed-capacity
compressors.
Traditional modulation technologies (cycling units on and off to match
load conditions) often consume close to full-load energy regardless of the
required capacity. In a system designed for high reliability, the compressors
do not just turn on and off. There is a turn-on delay period and a turn-off
pump-down period where the compressor is actually running, ensuring
proper oil lubricant to the compressor bearings before power is removed.
Improving Sensible Heat/Latent Heat Removal Capacity: IT equip-
ment generates sensible (dry) heat. Latent heat comes from people and out-
door humidity infiltration (which can be minimized through the vapor seal
discussed previously). As server density or capacity increases, it creates a cor-
responding increase in the sensible heat load. The latent heat load is unaf-
fected. Thus, using cooling solutions that can operate at a 100 percent
sensible capacity, except when dehumidification is required, can result in
reduced energy consumption. Operating a variable capacity compressor at a
ptg
lower capacity raises the temperature of the evaporator coil. This means less
latent cooling takes place. Under the vast majority of load conditions, the
evaporator coil temperature will be high enough to achieve 100 percent sen-
sible cooling. Thus, no energy will be required to add humidity that was
inadvertently removed.
Improving Coordination Across Multiple Units: The data center envi-
ronment has become more diverse as newer high-density servers are deployed
alongside older systems. As a result, without proper coordination between
room cooling units, air conditioners might be operating in different modes of
temperature and humidity control. For example, a unit on the north side of
the room might be sensing low relative humidity conditions and adding
humidity, while a unit on the south side of the room is sensing high relative
humidity and removing moisture from the air. The actual moisture in the air
is equal, but because the measurement is a relative measurement, the higher
the temperature, the lower the relative humidity. Advanced control systems
can be deployed across all the CRAC units in a room to enable the units to
communicate and coordinate their operation, preventing the “fighting
mode.”