HP ML150 Optimizing facility operation in high density data center environment - Page 21

perimeter. Large, square rooms may require CRAC units to be placed around the perimeter and through the center of the room. Figure 16. CRAC units should be placed perpendicular to hot aisles so that they discharge cool air beneath the floor in the same direction. Discharge velocity To force air from beneath the raised floor through the perforated tiles, the static pressure in the supply air plenum must be greater than the pressure above the raised floor. The velocity of the cooled air is highest near the CRAC unit because the entire flow is delivered through this area. The air velocity decreases as air flows through the perforated tiles away from the CRAC unit. The decrease in velocity is accompanied by an increase in static pressure with distance from the CRAC unit. Excessive discharge velocity from the CRAC unit reduces the static pressure through perforated tiles nearest the unit, causing inadequate airflow (Figure 17). The static pressure increases as the highvelocity discharge moves away from the unit, thereby increasing the airflow through the perforated tiles. To counter this situation, airfoils under the raised floor can be used to divert air through the perforated tiles.10 Another option is to use a fan-assisted perforated tile to increase the supply air circulation to a particular rack or hot spot. Fan-assisted tiles can provide 200 to 1500 cfm of supply air. 10 From Changing Cooling Requirements Leave Many Data Centers at Risk. W. Pitt Turner IV, P.E. and Edward C. Koplin, P.E. ComputerSite Engineering, Inc. 21