Raised Floors; Air Supply Plenum - Compaq BL10e - HP ProLiant - 512 MB RAM Introduction Manual

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.
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