Racking fun

Saturday, August 11, 2007 




The rack system we use to house our servers….


The house is not air conditioned and the rack is effectively in the garage, so it needs both dust filtering and self-adjusting cooling to compensate for summer daytimes without being insanely noisy all the time. The fans have taken a little trial and error to get configured right. They are connected to this industrial controller that turns on fans sequentially as the temperature rises.


I just got the door fans repaired after a minor accident that sent one blade flying, and came to two observations:


  1. The fans are loud.
  2. I’m not optimally supporting the designed cooling system for the servers.

I visited a massive colo facility a while back and they went on about using CFD to calculate the flow rates through their cages to keep 20kw racks from melting down (about 15 space heaters inside a box) and one thing they mentioned was sealing all the gaps between the front and back of the rack. Clearly I have not done this. The bit I didn’t really think of is that the rack isn’t so much a chimney filled with rising hot air as a jet with the intake at the front and the exhaust at the back. The fan system needs to enhance that jet flow, not some chimney effect with hot air exiting out the top.


That said, normally racks are put in environments where dust isn’t a problem. So I need positive pressure fans to pressurize the front of the rack through filters and compensate for the pressure drop through the media. That should provide plenty of clean, ambient temp air for the server fans to blow through the rack (and provide enough air that the server fans don’t create negative pressure in the front and draw dirty air in though other openings in the front half of the rack).


If that positive pressure turns out to be inadequate, then the temperature controller will turn on negative pressure fans at the back of the rack to exhaust the heated air, which will necessarily increase the flow through the servers and presumably drop the pressure in the front of the rack below ambient and thus, worst case, draw in a mix of dirty and clean air. The compromise is that a bit of dust is better than overheating on hot days.


The temp controller is currently set to turn on supplemental cooling at 85F (inside the rack temp). Since ambient hits 85 on the worst days, that seems like the lowest reasonable temperature to set the controller to.


In the end, if that’s not enough, then a ton of chilled water should do the trick, but I’m hoping not to have to go there…

Posted at 14:15:39 GMT-0700

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