[Beowulf] Re: Rear-door heat exchangers and condensation

Mark Hahn hahn at mcmaster.ca
Thu Dec 11 17:57:30 EST 2008


> Netshelter VX racks and the powered ventilation rear doors are not
> sufficient any more.

why do you have doors on your rack?  normally, a rack is filled with
servers with fans that generate the standard front-to-back airflow.
that means that you want no doors, or at least only high-perf mesh ones.

I've seen some wacky things in machinerooms - closed racks with 
just a small fan in the top, for instance.  or racks with 1U 
servers each carefully separated by 2-3U of open space.

here's the way I think of it: try to make your airflow cycle as 
close to a simple cycle as possible.  get all the air coming out
of the chiller to impinge (only and as naturally as possible) 
on the front of the rack(s).  get all the hot air from the back 
to to the chiller intake as naturally as possible.  no mixing,
no bypass, no counter-rotation, minimizing total air path as well
as changes in the airflow vectors.  ideally, machine room should be 
divided into hot and cold halves, with no free flow between them.
there are, of course some sweet spots (as well as "sour" ones):
very close to the chiller, cold air velocity may be high enough
to under-supply hot racks.  far away from the chillers, the problem 
is both supply and the hot-air/return path.  ducting and plenums
are invaluable, but IMO the main goal is partitioning hot from cold.

once airflow is relatively sane and controllable, it's more doable
to measure dissipation in a rack, as well as its airflow and delta-t
to see whether an auxiliary chiller is necessary.  I would be very 
reluctant to add "spot fixes" to a particular rack without having 
a very good handle on the full flow/circulation/temp/power picture...

>> I'm especially interested in feedback about condensation, and operational
>> water temperature.

incidentally, before committing to chilled water, make sure that it runs
year-round with reasonable temperature and flow.  our first machineroom
was stable only during summer, because the campus chilled water loop
was warmer and poorly chilled during the winter when (office) load was low.
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