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January 19, 2011 -
Today’s tip: Pay attention to chiller system design to improve data center energy efficiency.
The early data center design phase is the time to address chiller system configurations, pumping system topologies, chilled water loop supply temperatures and selection of energy efficient equipment.
Today's chillers can be selected for far lower kW/ton electrical use than in the past. Equipping the chiller with a VFD and condenser water reset is usually the most efficient option, because the VFD will adjust the chiller performance as the compressor load and condenser water supply temperature vary. Newer, highly efficient cooling towers with drift eliminators can be selected to reduce water use by as much as 40 to 50 percent.
Chiller systems for data centers have historically been oversized, resulting in less efficient operation until the data center is completely populated and full design loads are realized. Chillers are designed to operate at a specific difference between supply and return temperature or "delta-T." An efficient and properly-designed system will allow the chillers to operate closer to the design delta-T over the entire range of expected load conditions. A number of design strategies can contribute to achieving this goal, including pumping system configuration and reducing system bypasses.
Variable, primary-only pumping eliminates unnecessary chilled water bypass and allows the chiller to operate nearer to its optimized delta-T both during full- and part-load conditions. A traditional chiller design, in which a constant flow through the chiller is maintained, results in the delta-T staying proportional to the load. In a variable primary-only flow pumping system, the flow tends to stay proportional to the load while the delta-T stays closer to the optimized condition for the selected chiller. Also, a variable primary system requires fewer chilled water pumps (which lowers first cost), has fewer single points of failure (which makes it more reliable) and simplifies the chiller plant installation and controls.