# When is cooling air cool enough?

David Mathog mathog at mendel.bio.caltech.edu
Fri Oct 17 16:47:08 EDT 2003

```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?

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.

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.

Anybody have a good estimate for what this upper limit is.
For instance, from a computer room with an A/C that failed
slowly?

There's clearly a lower temperature limit too.  However on cold
days opening a feedback duct from the outlet back into the intake
should do the trick.  In really cold climates the intake
duct might be closed entirely - when it's 20 below outside.

Thanks,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech
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