[Beowulf] Reasonable upper limit in kW per rack for air cooling?

Robert G. Brown rgb at phy.duke.edu
Mon Feb 14 10:12:09 PST 2005

On Mon, 14 Feb 2005, David Mathog wrote:

> Robert G. Brown wrote:
> > In order to for liquid cooling to ever
> > make sense for COTS clusters, it would have to BECOME COTS -- basically,
> > to become cheap in both hardware and human terms.
> Shuttle's itty bitty computers have a heat pipe that goes out to
> a radiator on the back of the case.  It isn't much of a step from
> there to replacing the back radiator with a copper block.  That block
> could in turn mate with another copper block which itself was
> on a cold water line.  Ie, move the radiator even further from
> the CPU and other heat generating parts of the computer.  So
> a company like shuttle could relatively easily start selling
> liquid cooled nodes using only minor modifications to its existing
> hardware.
> In this sort of a system you might have to pay to have the pros 
> install (plumb) the rack itself, but you could still work on the
> nodes of the rack, as is true now.  It would seem to be relatively
> straightforward to have the nodes mate up copper block to copper
> block when fully inserted, so that each node is not itself part
> of the rack circulation system. The tricky part is
> that something else would have to be attached to the copper block
> on the back when the node was serviced on the bench.

I think there are lots of tricky parts, but I agree that it can be done.
In face, Eugen found this from Rittal:


where it IS being done, in the sense that one can get liquid cooling
adjuncts for racks that accept standard ported lq heat sinks for CPUs
and maybe a couple of other parts (disks, power supplies?).  Their
"mini-chiller" per rack is only around 1.3 tons (4500 "cooling watts")
which seems small, and running all the supply hoses around in and out of
the systems (especially MP motherboard or blade systems) inside
enclosures not really designed for them seems like it would be

I just don't think of this is being mainstream.  I didn't get a price
from anybody on this, but I'll bet it is an option on your newborn child
per rack.

The external heat exchanger idea is also "interesting".  I agree that
better thermal management in motherboards themselves would be desirable,
but it takes a biggish chunk of copper to make a heat pipe capable of
moving 100 W 20-30 cm at \kappa_Cu = 385 W/(m-K) and keep the end
temperature differentials in the 20-30 K range.  Maybe what, 0.5 cm in

> On the plus side your racks could replace the building's current
> hot water supply!

Not unless you permit the max T on the sink in contact with the water to
get dangerously high... (taking this as a serious, rather than a wry,
remark).  Ditto for numerous discussions of using server room waste heat
to help heat buildings -- good idea on paper, pretty difficult in
practice, and then there is summer.


> Regards,
> David Mathog
> mathog at caltech.edu
> Manager, Sequence Analysis Facility, Biology Division, Caltech
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Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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