HP ML150 Optimizing facility operation in high density data center environment - Page 19

At the ends of rows, exhaust from the high-density rack will wrap around the end of the row infiltrating the cold aisle and device inlets. Deep into room corners, heat will be trapped in corner and eventually increase inlet temperatures (see Figure 14). Figure 14. Exhaust from the high-density rack wrapping around the end of the row Cold Aisle Cold Aisle Cold Aisle Cold Aisle Cold Aisle R9C1 •Max. inlet temperature occurs at R9C1: 84.1oF R10C1 Closely coupled cooling Cooling coupling is defined as how intimate and sensitive the cooling solution is to the individual IT equipment heat load. Close-coupling focuses on areas where the need for cooling is more pronounced, such as a rack populated with blade servers, rather than the heterogeneously-populated open space of the data center room. Close-coupling can result in shorter air paths that are more effective at heat extraction and require less fan power. Close-coupled heat removal minimizes the mixing of cool and hot air, since the airflow is completely contained in the row or rack. Examples of closely coupled solutions are ceiling mounted heat exchangers, which target individual racks, and liquid cooled racks. Liquid cooled racks are particularly effective since they isolate the rack from the ambient conditions in the data center. There is no air mixing, and the cooling solution is immediately coupled with the rack which makes it highly energy efficient, as described in the section titled "Liquid cooled racks". Computer room air conditioners A common question with respect to cooling resources is how much capacity in kilowatts a particular CRAC unit can cool. Assuming a fixed heat load from the equipment in its airflow pattern, the answer depends largely on the capacity of the CRAC unit, its placement in the facility, and its discharge velocity. Capacity of CRAC units The heat load of equipment is normally specified in kilowatt per hour (kWh) or British Thermal Units per hour (BTU/hr). However, in the U.S., CRAC unit capacity is often expressed in "tons" of refrigeration, where one ton corresponds to a heat absorption rate of 3.5 kWh (12,000 BTU/hr). 19