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8: What About Chillers, Cooling Tower Fans, and All That
Cooling Equipment Usually Ignored by IT? 139

3. Using Economizers to Achieve Free Cooling

In many locations, you can use outside cool air to supplement data center
cooling and provide “free cooling” during colder months. This is accom-
plished through the use of economizer systems. A study on building control
systems by Battelle Laboratories found that, on average, the normalized heat-
ing and cooling Energy Use Intensity (EUI) of buildings with economizers
was 13 percent lower than those without economizers.
The two basic types of economizer systems are air-side economizers and
fluid-side economizers. Choosing the type for a specific project is a function
of climate, codes, performance, and preference:

■ Air-side economizer: The air-side economizer uses a system of sensors,
ducts, and dampers to allow entry of the appropriate volume of outside
air to satisfy facility cooling requirements. Air-side economizers are avail-
able in two types—a “dry air” system and an “evaporatively conditioned”
air system. The former is the most common, but its use is restricted to a
few geographic locations because of the high cost of energy required to
add moisture to the room when the ambient dew-point is below 35° F. ptg
The evaporative conditioned solution is an economical method for condi-
tioning the air prior to introduction to the data center, but uncertain reli-
ability and high-maintenance requirements have generally made this
approach unattractive to most data center operators.
The key to either of these solutions is proper control. The control should
be based on enthalpy comparisons, not just dry-bulb temperature. Also
controls must be implemented to detect incidences of high pollen, dust,
or other external contaminants and effectively lock out the economizer
during these conditions.
■ Fluid-side economizer: A fluid-side economizer system is typically
incorporated into a chilled water or glycol-based cooling system, and
works in conjunction with a heat rejection loop consisting of a cooling
tower, evaporative cooler, or drycooler. CRAC units incorporate a conven-
tional glycol-cooled unit, along with a second cooling coil, control valve,
and temperature monitor. During colder months, the glycol solution
returning from the outdoor drycoolers or cooling tower is routed to the
second coil, which becomes the primary source of cooling for the room.
As long as the “free cooling” fluid is 8° F below the CRAC return tem-
perature, there is some benefit to having the “free cooling” running
because it minimizes the load on the primary cooling method.