September 26, 2011 | by Mark Coxon Top Fan(s), Rear Exhaust: There are fans that are very low profile that can reside above a heat offending component. They pull the air out of the top of that equipment and turn the collected heat out of the rear of the rack. These are great solutions where one or two pieces of high BTU output equipment needs to be placed near more sensitive pieces.

Rear Fan(s): These are typically seen in data center environments, where fans are placed at the rear of the rack in a vertical array. They pull cool air in from the front of the rack, bathing the equipment in room temperature air and collecting the heat as it travels around the components. This heat exits out of the rear of the rack. Sometimes, this is coupled with a chiller, a radiator if you will, between the fans and the equipment, that actually returns the air to room temp as it is expelled.

Most commonly, however, rear fan environments lay racks out in “hot” and “cold” aisles. The “hot” aisles are between the rear of two rows of racks, where the heat accumulates, and the HVAC intakes are on these rows. The “cold” aisles are between the fronts of two rows of racks, where the air is being pulled from by the rear fans, and the HVAC registers are on these rows, blowing the newly cooled air from the “hot aisles” back into the system. There are a couple ways to do hot and cold layouts, so I encourage you to read more on that if applicable.

I have seen racks that are sealed after they are built to prevent air intake from non-planned entry points. As in the “bottom fan” scenario, this can be ill-advised. Sealing the racks means the vacuum pressure is increased at the openings that have been unsealed and have the propensity to pull in more dust and debris. It increases the needed maintenance on the system in many cases, as opposed to decreasing them as sealing the racks suggests. Again, there are controlled conditions where this can be an advantage, but we rarely see those ideals.

In all cases, maintenance is an integral part of equipment life, as all intakes on the racks and equipment should be cleaned on a regular basis to give the system owner the maximum performance and not decrease the overall life of the equipment in the process.

For those wanting more, Middle Atlantic has a great White Paper on this subject and a course for BICSI/CEDIA/CTS CU credit as well.