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Controlling Data Center Cooling with Supply Air Temperature

Controlling data center cooling temperature on the supply air temperature side helps achieve maximum uptime and efficiency. Today's data center layouts typically accomplish this by segregating hot air and cold air into their own aisles.

However, air handler temperature control strategies sometimes use a legacy approach to achieve constant ambient temperature within the room. In such cases, return air temperature controls vary the temperature of the air being supplied by the individual units, which consumes more energy.

Return air temperature control can result in a wide air supply temperature variation to the server equipment, which is something neither IT directors nor facilities managers want.

Controlling the temperature at the supply side of the air handler, rather than the return, gives control back to the facility manager and makes the temperatures being supplied to servers much more predictable. This predictability leads to the potential for raising water temperatures and reducing running costs, as well as savings in capital costs during the construction of new facilities.

David Cosaboon, project engineer for Facility Engineering Associates, PC, says that "supply air temperature control minimizes 'cross-contamination' between the CRAC (computer room air conditioning) units since the sensors are located relatively close to the cooling coil of a specific unit."

Return air temperature control scenarios for CRAC units also can be influenced by ambient air mixing.

"In other words," Cosaboon explains, "the return air temperature of a CRAC unit can be influenced by the activity of an adjacent CRAC unit, resulting in a change of operation and 'fighting' between units."

For hot aisle/cold aisle arrangements in data centers, hot aisle temperatures may increase slightly, according to Cosaboon, "because the supply air temperature control strategy aims to deliver a desired air temperature to the servers," rather than maintain desired ambient air temperatures.

"Also, the server rack requirements must be monitored and adjusted to ensure adequate airflow is provided for cooling," says Cosaboon. To accomplish better control of the supply air temperature, Cosaboon says, supply air sensors are frequently located in the supply air stream, close to the CRAC unit.

He also suggests using additional sensors at server intakes for more precise supply air temperature control. These sensors also can monitor the entering air temperature at server intakes.

With these tips, remarkable savings can be accomplished simply by switching to supply air temperature controls for many data centers. For instance, one federal data center retro-commissioned by Lawrence Berkeley National Laboratory during 2012 is now operating with 20 percent of its installed computer room air handlers (CRAHs) turned off.

"No considerable change in IT equipment air intake temperatures was observed by turning off the CRAH units," says Rod Mahdavi, PE, LEED AP, program manager, Environmental Energy Technologies Division of LBL, yet LBL is saving 530 MWh annually.

While supply air temperature controls can offer substantial savings, some data centers are not configured to take advantage of this energy saving technique, observes Cosaboon.

"For data centers that are not oriented with a hot aisle/cold aisle configuration, return temperature control may provide a more consistent ambient air temperature," he says.

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