[Beowulf] cooling question: cfm per rack?

Robert G. Brown rgb at phy.duke.edu
Fri Feb 11 11:02:23 PST 2005

On Fri, 11 Feb 2005, David Mathog wrote:

> In designing a computer room two key factors are:
> 1.  Power in   (electricity)
> 2.  Power out  (A/C)
> The second term really has two parts: 
>   A.  the amount of air moved
> with recently utilize some more or less standard relationship
> that the cold water has a very large heat capacity.
> could be modulated somehow, since some racks move much more 
> How does one calculate an optimal cfm through a rack? 
> Without incredibly careful balancing of the pressure across the
> rack it won't generally run at 50 cfm.
> at various points in the room?

I can't answer all your questions here, but you've pointed out a lot of
the problems.  You have to arrange for the blower to deliver chilled air
to the right places in the room, and you ALSO have to arrange for a warm
air return that picks up the warmed air (after it has passed through the
systems and cooled them) and returns it to be cooled and cycled again.

The overall airflow is determined by those two things -- cool air being
delivered at an overpressure, warm air being returned at an
underpressure, and the intermediate pressure gradient (interacting with
intervening obstacles such as the racks full of equipment) determining
the flow pattern.  That flow pattern needs to avoid things like "hot
spots" that are isolated from the overall cooling flow, especially hot
spots that ultimately feed rack intake, and flow that feeds the warmed
exhaust from one or more units back into the cool air intake of others.
Ultimately, this is a nonlinear problem with turbulence and other
factors and hence difficult to make pronouncements on without knowing
the geometry of your rack layout and other stuff.

One reason that raised floor designs are popular is that it makes
establishing a clean circulation pattern a bit simpler -- feed cold air
from the botton right into the rack intakess, vent the warmed air from
their outflow directly into a warm air return.  The cooling air mixes
minimally with ambient room air and is relatively easy to balance.

In a simpler overhead cold air delivery, warm air return system you'll
need to be able to balance the cold air delivery at several points in
the room, perhaps blowing it down directly into the front (intake) faces
of opposing racks, while letting the warm air get pulled along the
ceiling to one or more major return vents.  That way you can get a
delivered cold-air-down, in-through-rack, out-from-rack, warm-air-up,
warm-air-along-ceiling and returned sort of pattern established that is
consistent and balancable among the delivery registers throughout the


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