ASHRAE Journal - February 2009 - (Page 52) Redundancy for Data Centers By Zuhair Hasan, P Associate Member ASHRAE .E., C ooling or power interruptions can dramatically affect a data center’s environment. Because system failure can be prohibitively expensive, reliability and redundancy are vital. Redundancy in cooling systems minimizes risk of system failure and increases system performance by decreasing downtime for maintenance and repairs. The main advantage of redundancy is that it increases system reliability. The range of environmental conditions for operating a data center is defined by ASHRAE.1 Class 1 is usually used in mission-critical operations and requires more stringently controlled environmental parameters than other classes. Classes 2, 3, and 4 require some or little control of the environmental parameters. Environmental requirements for computer equipment also can be obtained from equipment manufacturers.2 ASHRAE’s Thermal Guidelines for Data Processing Environments1 recommends a relative humidity (RH) range of 40% to 55%. Exceeding recommended environmental conditions can negatively affect the performance of data equipment and its reliability, and possibly result in corrupted 52 ASHRAE Journal system performance with resource restrictions. Regardless of the evaluation’s findings, system performance and the required environmental conditions must be maintained. For a cooling system to successfully meet required cooling demands in a high reliability mission-critical data center, the following practices are recommended. Chilled Water System data or system failure and shutdown. For instance, exceeding the manufacturer’s recommended temperature range can cause expansion or contraction of sensitive computer components resulting in disconnects among the circuits on the printed circuit boards. Also, whenever the RH is too low, there exists a threat of spontaneous electrostatic discharge, which can damage data and hardware. High humidity levels can cause condensation, corrosion, and “tape and surface” wear and tear. Providing accurate and strict environmental conditions at all times can be challenging because of budget restrictions, schedules, and space limitations. Often, a system evaluation needs to balance ashrae.org • Provide the chilled water system with a minimum of N+2 chiller equipment redundancy, where N is the number of online units to meet the cooling demand. This protects the center from losses in available cooling capacity if one unit fails, and an additional redundant unit is either being serviced or fails to operate simultaneously. Implementing this recommendation ensures the system cooling demand is met. Other redundancy configurations About the Author Zuhair Hasan, P is an engineering and energy .E., consultant in Atlanta. February 2009 http://www.ashrae.org
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