[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
>   B.  the reduction in temperature of that air across the A/C unit
> The latter part is specified in tons.  The A/C guys I've spoken
> with recently utilize some more or less standard relationship
> between cubic feet per minute (cfm) and A/C tons for the units they
> maintain.  These run off the campus cold water supply, so
> it makes sense that heat out is proportional to flow across, assuming
> that the cold water has a very large heat capacity.
> However, in terms of cooling the units themselves, the amount of
> air flow through the racks is also important.  That flow is
> also in cfm.  Ideally cfm through the racks would be equal to cfm
> through the A/C, ie, all air goes once through the racks and then
> directly through the A/C.  Even more ideally cfm through _each_ rack
> could be modulated somehow, since some racks move much more 
> air than others and putting a low flow rack next to a high flow rack
> might drive the air the wrong way through the low flow unit.
> How does one calculate an optimal cfm through a rack? 
> For a specific example with round numbers, let's say it's a
> 25U rack, dissipates 10kW, and has a single 50 cfm per minute output
> fan per 1U node.  (Ie, all air out must go through that path.)
> There seem to be a bunch of variables that are hard to deal with. 
> For instance, adding the exhaust fans would be 50*25 = 1250 cfm.
> Is that all there is to it? But that type of fan only runs at
> the stated flow rate if the pressures are exactly as specified. 
> Without incredibly careful balancing of the pressure across the
> rack it won't generally run at 50 cfm.
> Is cfm the key unit here or should one think in terms of pressure
> 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


Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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