Equipment Rack Cooling 101

Thermal management in equipment racks is of grave importance. Here's how to ensure it's done properly.

All the magic of the Great and Powerful Oz are typically hidden behind the curtain, in the AV closet in a rack.

While there have been a few exposés on racks in the past, they were mostly beauty pageants to show off the LEDs and shots of the back, where the real work takes place.

I want tackle thermal management in rack-based systems. It’s not the most exciting topic, but it’s of grave importance.

First, here are a few things to consider when planning thermal management.

1) How dense will the rack be? Afford blank 1RU sections between gear that is more susceptible to heat (like PCs) or produces more of it (analog amplifiers).

2) How efficient is the equipment? How much of the energy it consumes goes to product performance and how much is dissipated as waste heat?

3) Where will the equipment reside in the rack? If possible, items that produce more heat should be placed toward the top to give a quicker exit to the heat it creates and to minimize the transfer of that heat to other gear on the way out.

4) What environment will the rack be placed in? You want to let every client know the rack room needs to be at ASHRAE 2008 standards (64.4-80.6 degrees F and no more than 60 percent relative humidity)

Passive vs. Active Cooling
Passive cooling uses physics to cool the rack, letting heat rise. Racks are wider than the equipment to create a chimney effect on the left and right sides. The heat rises, hits the shelf or piece of equipment above and then moves left and right to find the chimneys and proceed out the top of the rack. In many cases, this is the best solution if the heat needing to be dissipated is not extreme, and the rack is low density.

Active cooling assists the cooling of the rack with fans. The amount of air that needs to be evacuated from the rack is obviously contingent on the total BTUs of waste heat it produces, which determines the fan size and throughput. There are different approaches to active cooling.

Top Fan(s), Top Exhaust: The most common method assisting the natural chimney effect already present, placing the fan(s) at the top of the rack to pull air out.

Bottom Fan(s): I have seen fans installed low at the front of racks to pull cool air in at the bottom and assist the top fan(s) by pulling cool air in so it can be pulled up through the rack. Most of the time this is ill-advised. Cool air accumulates low, so does dust and debris. Creating a vacuum to bring that dust into the rack usually hurts more than it helps. There are exceptions in data center or clean room environments, but they are atypical of the rooms we typically are asked to place our equipment in.

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.