Trey Breckenridge trey at ERC.MsState.Edu
Wed Apr 24 09:43:23 PDT 2002

>Steven Berukoff wrote:
>> We just purchased ~150 dual AMDs, and are cooling them with 4 Fujitsu
>> ceiling-mounted air-conditioners: about 50kW of AC cost us about $25k,
>> which is about 10% of the cost of the machines.

One of the major disadvantages of overhead A/C units is that when the
overflow drain clogs (and it will), the excess water will spill into
your machines.  Another point to consider is access for maintenance of
the A/C.  In some cases, it may require you to shut down and move your
racks for the maintenance personnel to have full access the A/C units.

A second disadvantage with ceiling mounted units (with the supply-side
vent in the ceiling) is that your cooling efficiency will be lower than
with a under-floor based system.  Basically, from a ceiling mounted
unit, the cool air from the supply will have to travel the entire
height of the room in order to "reach" the machine that is the farthest
away (your bottommost machine in a rack) which may be 10 feet.  When
cooling from under the floor in a raised floor scenario, the furthest
machine from the supply is maybe 6-8 feet away (the topmost machine in
your rack).  The difference in the cold air velocity at 6 feet versus
10 feet may be significant.  My experience is that it does make a
difference.  In our data center with ceiling mounted A/C's, the
machines at the bottom of our racks run considerable hotter than the
top machines.  However, in our second data center, we have under-floor
A/C.  The machine temperatures in the racks there are much more
consistent (and cooler on average) from top to bottom.

Of course, all of this is just my opinion.

   Trey Breckenridge - Computing Systems Manager - trey at ERC.MsState.Edu
         Mississippi State University Engineering Research Center

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