American Power Conversion White Paper Sample

The airflow in and around the rack cabinet is critical to cooling performance. The key to understanding rack airflow is to recognize the fundamental principle that IT equipment cares about two things:

1. That appropriate conditioned air is presented at the equipment air intake
2. That the airflow in and out of the equipment is not restricted.

The two key problems that routinely occur and prevent the ideal situation are

1. The CRAC air becomes mixed with hot exhaust air before it gets to the equipment air intake
2. The equipment airflow is blocked by obstructions.

The common theme throughout the next sections is that well-intentioned implementation decisions that appear to be inconsequential actually create the two problems above, and that the common solutions routinely used to address the symptoms of these problems significantly compromise availability and increase costs.

Airflow in the rack cabinet
Although the rack is frequently thought of as a mechanical support, it provides a very critical function in preventing hot exhaust air from equipment from circulating back into the equipment air intake. The exhaust air is slightly pressurized, and this combined with the suction at the equipment intake leads to a situation where the exhaust air is induced to flow back into the equipment intake. The magnitude of this effect is much greater than the magnitude of the effect of buoyancy of hot exhaust air, which many people believe should naturally cause the hot exhaust air to rise away from the equipment. The rack and its blanking panels provide a natural barrier, which greatly increases the length of the air recirculation path and consequently reduces the equipment intake of hot exhaust air.