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.

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About the author

Mark Coxon

Mark started his technology career at IBM in 2000 before migrating into AV integration in early 2002. He currently works at Orange ProAV, a division of Mad Systems Inc., where he helped the company transition from the niche Museum marketplace to the corporate, education, government, and retail markets. Mark lives in Orange County with Lesley, his wife of 11 years and his 3 children.

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