Air Flow and Pressure

Calculating air flow and the amount of air pressure needed to cool the data center involves a number of factors:

• The initial temperature of the air.

• The initial pressure (velocity) of the air.

• How much cooling is needed per rack. (Is it the same for all racks, or do some racks need more cooling than others?)

• The arrangement of solid and perforated tiles (in different perforation percentages) to deliver air at the correct pressure to each rack.

The following figure shows an example of how pressure (in this case, water) diminishes as it is systematically leaked. The example shows a hose with three identically sized holes. The greatest amount of water pressure is leaked from the first hole. The pressure from the second hole is less, and the pressure from the third hole is less still.

Figure 6-6. Reduction of Pressure With Distance

In the case of air travelling through a plenum and escaping through the holes of the floor tiles, the same principle applies even if you use only perforated tiles with the same pass-through percentage. The air escaping through the holes of the tile closest to the source (HVAC unit) will move at a greater pressure than the air escaping through the holes in subsequently placed tiles. Therefore, racks directly above the first perforated tile will receive more cooling than racks above perforated tiles farther down the plenum. The last rack in the line might not receive enough air for proper cooling.

To regulate the air more efficiently , perforated tiles of different air flow percentages can be used. The first tiles would have fewer holes relying on the greater pressure to move the required volume into the racks. Subsequent tiles would have more holes to allow volume to move through them despite the drop in pressure.

Solid tiles can also be used to control air flow. Where no air is needed (areas with no racks above them and in the hot aisles in a back-to-back cooling model), solid tiles should be used to maintain the optimum pressure. Or perforated tiles can be placed in locations with no racks if air pressure needs to be reduced, or the room requires more general cooling.

The following figure shows a suggested perforated tile placement to cool racks with a front-to-back cooling model.

Figure 6-7. Suggested Perforated Tile Placement

Pressure Leak Detection

It is important to maintain pressure under the floor and allow air flow only through perforated tiles in the specific areas where it is needed. This will help to maximize the efficiency of the HVAC systems. However, rooms are not always perfectly square nor level, so voids in the raised floor, especially near walls and around pipes and conduits , occur. These voids allow air to escape from the floor void and decrease pressure.

The raised floor should be inspected routinely and any voids should be filled. Also, the perforated tiles that were used to direct air to machines that have been moved to a different location should be replaced with solid tiles. Replacing perforated tiles with solid tiles should be part of the standard procedure when a machine is removed or relocated .