Ashrae Journal - October 2008 - (Page 34) Energy-Efficient Data Centers A Close-Coupled Row Solution By John Bean, Member ASHRAE; and Kevin Dunlap T he predominant architecture for cooling data centers since the inception of the mainframe has been raised floor air delivery from perimeter computer room air handlers (CRAH). In this approach, CRAHs are placed around the perimeter of the room, and they distribute cold air through a raised floor with perforated floor tiles or vents to direct the air into the room (Figure 1). At lower densities (1 to 5 kW/rack) adequate cooling is provided to sensitive IT equipment, despite the mixing of air throughout the room. A similar air delivery system that has been used to cool data centers is central air-handling units (CAHU) (Figure 2). These systems use much larger, more centralized cooling units with similar air delivery to perimeter CRAH cooling of either raised floor or custom overhead ductwork. 34 ASHRAE Journal As rack power grew beyond 5 kW, air delivery and heat removal challenges with use of CRAH and CAHU systems became evident. The major obstacle in these architectures is the length scale of air delivery. Distance between the cooling units and the heat load make it difficult to properly remove the heat generated ashrae.org from IT equipment without mixing with supply air. This separation results in hot spots and a complicated design approach to air distribution. To add to this problem, the airflow demands of the IT equipment also increases with power density. Since CRAH and CAHU systems use a plenum for supply air delivery (and warm air return in certain designs), a significant amount of fan horsepower is required to pressurize the plenum and overcome resistances in the airdistribution system. Additionally, to overcome the effects of mixing the net volume of air circulated is significantly greater than actual air volume required About the Authors John Bean is director of innovation for racks and cooling and Kevin Dunlap is general manager for cooling solutions at American Power Conversion in O’Fallon, Mo. October 2008 http://www.ashrae.org
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