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

Fluid-side economizers are the system of choice for most data center envi-
ronments because they are not affected by outdoor humidity levels and so
are effective in a wider portion of the temperature/humidity band. Also,
they do not add any additional air filtration requirements on the data
center. For more detailed information on economizers, see the white
paper, “Utilizing Economizers Effectively in the Data Center,” available
at www.liebert.com.

4. Increasing the Efficiency of Room Air Conditioners

Three factors are critical to optimizing the efficiency of CRAC units:

■ How efficiently the units operate at partial load
■ How efficient the units are at removing sensible heat as compared to
latent heat
■ How well multiple units work together

ptg
Increasing efficiency at part load is a significant issue. Data centers are
designed with some level of cooling system redundancy. Plus, the actual
capacity of a direct expansion or air-cooled CRAC unit increases as the out-
door ambient temperature decreases below the peak design condition (typi-
cally 95° F). This means equipment is operating at less than 100 percent load
all the time, creating the opportunity to design systems to operate more effi-
ciently during normal operating conditions. Operating conditions usually
aren’t stable, so some method of varying capacity based on operating condi-
tions is required.
There are several approaches to providing variable capacity in a direct
expansion CRAC unit. The two most common are four-step compressor
unloading and Digital Scroll™ compressor technology.
Four-step compressor unloading works by shutting off the flow of refriger-
ant to some of the cylinders within the system. This minimizes the need
to cycle compressors on and off to control capacity. Because unloading
essentially changes the compressor operating point, it enables the cooling
system to operate more efficiently at lower capacities. For example, a system
operating with two compressors “unloaded” will consume approximately
50 percent of the energy of a fully loaded system but will deliver approxi-
mately 76 percent capacity because the condenser and evaporator are sized for
full load.