HP ML150 Optimizing facility operation in high density data center environment - Page 15

Figure 8. Mixing of supply air and exhaust air Raised floors Raised floors typically measure 18 inches (46 cm) to 36 inches (91 cm) from the building floor to the top of the floor tiles, which are supported by a grounded grid structure. The static pressure in the supply plenum pushes the air up through perforated floor tiles to cool the racks. Most equipment draws in cold supply air from the front and exhausts warm air out the rear of the racks. Ideally, the warm exhaust air rises to the ceiling and returns along the ceiling back to the top of the CRAC units to repeat the cycle. Administrators should take into consideration that a higher supply air plenum (36 inches) will provide additional air flow for high density configurations. Air supply plenum The air supply plenum must be a totally enclosed space to achieve pressurization for efficient air distribution. The integrity of the subfloor perimeter (walls) is critical to prevent moisture retention and to maintain supply plenum pressure. This means that openings in the plenum perimeter and raised floor must be filled or sealed. Subfloor plenum dividers should be constructed in areas with large openings or with no subfloor perimeter walls. The plenum is also used to route piping, conduit, and cables that bring power and network connections to the racks. In some data centers, cables are simply laid on the floor in the plenum where they can become badly tangled (Figure 9). This can result in cable dams that block airflow or cause turbulence that minimizes airflow and creates hot spots above the floor. U-shaped "basket" cable trays or cable hangers can be used to manage cable paths, prevent blockage of airflow, and provide a path for future cable additions. Another option is to use overhead cable trays to route network and data cables so that only power cables remain in the floor plenum. 15