When is cooling air cool enough?

Robert G. Brown rgb at phy.duke.edu
Fri Oct 17 21:21:42 EDT 2003

On Fri, 17 Oct 2003, David Mathog wrote:

> Most computer rooms shuttle the air back and forth
> between the computers and the A/C.  I'm
> wondering if one could not construct a less expensive
> facility (less power running the A/C which is rarely
> on, smaller A/C units) if the computer room was a
> lot more like a wind tunnel: ambient air in (after
> filtering out any dust or rain),
> pass it through the computers, and then blow it out
> the other side of the building.   Note the room
> wouldn't be wide open like a normal computer room.
> Instead essentially each rack and other largish
> computer unit would sit in its own separate air flow,
> so that hot air from one wouldn't heat the next.
> The question is, how hot can the cooling air be and
> still keep the computers happy?

I personally have strong feelings about this, although there probably
are sites out there with hard data and statistics and engineering

70F or cooler would be my recommendation.  In fact, cooler would be my
recommendation -- 60F would be better still.

I think the number is every 10F costs roughly a year of component life
in the 60-80F ranges and even brief periods where the temperature at the
intake gets significantly above 80F makes it uncomfortably likely that
some component is damaged enough to fail within a year.

> The answer will determine how big an A/C unit is
> needed to handle cooling the intake air for those
> times when it exceeds this upper limit.

It costs roughly $1/watt/year to feed AND cool a computer, order of
$100-150/cpu/year, with about 1/4 of that for cooling per se.  The
computer itself costs anywhere from $500 lowball to a couple of thousand
per CPU (more if you have an expensive network).  The HUMAN cost of
screwing around with broken hardware can be crushing, and high
temperatures are an open invitation for hardware to break a lot more
often (and it breaks all too often at LOW temperatures).  It just isn't
worth it.

> I'm guessing that so long as a lot of air is moving through
> the computers most would be ok in a sustained 30C (86F) flow.  
> Remember, this isn't 30C in dead air, it's 30C with high
> pressure on the intake side of the computer and low
> pressure on the outlet side, so that the generated heat
> is rapidly moved out of the computer and away.  (But not
> so much flow as to blow cards out of their sockets!)
> Somewhere between 30C and 40C one might expect poorly
> ventilated CPUs and disks to begin to have problems.  Above
> 40C seems a tad too warm.  At that temperature it's going
> to be pretty uncomfortable for the operators too.

So an 86F wind keeps YOU cool in the summer time?  Only because you're
damp on the outside and evaporating sweat cools you.  Think 86F humid,
and you're only at 98F at core.  The CPU is considerably hotter, and is
cooled by the temperature DIFFERENCE.


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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