[Beowulf] Q: Cooling units? Raised floors? General machine room stuff..

David Mathog mathog at mendel.bio.caltech.edu
Fri Jul 2 11:16:17 PDT 2004

---- Original Message -----
From:  Jim Lux james.p.lux at jpl.nasa.gov 
-To: <beowulf at beowulf.org>
Sent: Friday, July 02, 2004 6:42 AM
Subject: [Beowulf] Q: Cooling units? Raised floors? General machine room

|> Secondly, given this is a rather small machine room (aiming for a max
|> capacity of 5 42U racks), what advantage would be found in
|> going with a raised floor?  And does anyone have any cost
|> estimates on raised floors for rooms on the order of, say,
|> 15' by 15'?

> Kind of depends on how you're arranging the racks.
> If you've got them all side by side in the middle
> of the room (so the cold air comes in the front
> and blows out the back of a "wall o'racks") the
> raised floor probably isn't worth it. 

The facilities types here always want to put the air outlets/inlets
in the ceiling.  No matter how well you place these in a small room
the hot air out of the back of the racks is going to have some
time to mix with the room's cold air before it gets returned to the
A/C.  And strange things have been known to happen when a third
rack is added, such as, turn on the third rack and it messes up
the air flow in the room enough that the first rack overheats.

All the computers I've seen lately exhaust their hot
air in defined spots, usually through one or two holes in the back
of the case.  Our 20 unit Beowulf rack (and likely most others
in the world) have a nice neat linear array of these
going from the floor to the top of the rack.  It seems to me
that rather than putting the return opening in the ceiling
what one would really like are a series of openings in the ceiling
(normally capped) into the main (large) return duct.  To these
one could attach (smaller) vertical ducts containing a series
of holes aligned more or less with the hot air outlets on the
back of the machinery.  These smaller ducts would hang down
behind the racks and suck the hot air out of the room very close
to the back of each rack.  Assuming
the openings in the ceiling are some standard size
the vertical duct could then be purchased inexpensively
at Home Depot or the equivalent, and the holes
cut with a pair of tin snips.  The bottom of the duct would likely
have to be fastened down somehow to avoid vibration.  Ugly yes,
but when new equipment arrives one wouldn't have to pay two
arms and one leg to have the A/C guys come out and move
the ducts around above the ceiling.  Probably if there
were multiple vertical ducts in use each would need an
inline impeller (available from  Fantech, EBM, and
probably others) to avoid problems with the closer duct
short circuiting the vacuum available to the second.  These
could be built into the ceiling inlets, and the controls wired
to one panel at the time the room was constructed. Most of these
types of fans can be speed controlled, so one
can imagine being able to balance the flow as needed by just
turning these dials, rather than having to redo the ductwork.

Has anybody seen an installation like this?  The closest thing
I've seen in practice is in some woodshops, where they have a
central sawdust repository and multiple ducts going to various
machines.  (A more difficult job surely, since they also have to
transport the sawdust.)  Aside from the aesthetics the biggest
flaw I see is that if an impeller dies that rack will overheat.
The impellers tend to be pretty reliable though, since they
are designed to sit (nearly) unmaintained in relatively inaccessible


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

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