[Beowulf] Rear-door heat exchangers and condensation

[Bill Broadley]

Since you mentioned the rear door exchangers, I figured I'd mention a related
solution for machine rooms that can't handle the heat density of today's
1U/blade racks.  Liebert makes a rack top cooler that blows air in front of
the rack (the cold isle) and sucks in hot air from the rear, and dumps the
heat into a water source.  Seems like a pretty reasonable design and seems to
work well.

It doesn't make it any harder to work on the rack/nodes, although I do
recommend a wide brimmed hat if you don't like high volumes of cold air
blowing on your forehead when you are working on the console.

One complications I saw of a design with the retrofitted rear rack cooler was
the maximum flow rate they were designed for and how changes in that rate
would effect the resulting cooling.  Vendors I talked to didn't immediately
have CFM numbers for nodes, nor did the rear door vendor have any graphs for
cooling delivered vs air temperature and pressure.  Nor the the 1U vendors
have graphs of airflow delivered relative to backpressure (potentially caused
by the rear door).  It wasn't at all clear to me if a rear door would work
similarly with a 10kw rack with zero room cooling as it would with a 20kw rack
with 10kw of room cooling.  Not to mention blades/1Us designed to dissipate 20
kw per rack would likely have significantly higher airflow.

With all that said I've seen installations that were pretty happy with the
rear door solutions as well.