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On 08/22/2016 11:46 AM, Stu Midgley wrote:<br>
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<div class="gmail_quote">On Mon, Aug 22, 2016 at 11:22 PM, Stu
Midgley <span dir="ltr"><<a moz-do-not-send="true"
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wrote:<br>
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</span> While the risk of an explosion is a
certainly a theoretical possibility, In
practice, the risk of this is virtually
non-existent for a variety of reasons. <br>
<br>
With water, the processors and other
heat-generating components would fail from the
heat before the boiling point of water is
reached, so there would be little to no
generation of water vapor that could lead to
an explosion. Also, any heating/cooling system
with water would be designed to included an
expansion tank to account for the thermal
expansion and contraction of water. There are
millions, if not billions, of homes and
businesses in existence with hot water heating
systems, yet, I've never heard of any of them
ever exploding. <br>
<br>
With Novec and other two-phase systems, the
gas phase is compressible, meaning it can
store energy like a spring, preventing or
minimizing the case risk of an overpressure
situation rupturing the vessel. All that is
required for this to be used safely is an
adequate volume for the gas, so that is has
excess 'capacity' to be compressed. This
simple design is what allows 20-pound propane
tanks to be used all over America (and
probably other countries) to fuel gas grills
and be left out in direct sunlight all summer
long, and be stored directly under the
heat-producing burners. If those tanks were
filled to the top, they would explode in those
conditions, but but leaving about 1/3 of the
tank empty, the risk has been virtually
eliminated. This was actually a top we spent a
lot of time discussing in my Chemical
Engineering Safety class in college. <br>
<br>
This also applies to the tanks storing liquid
nitrogen, liquid oxygen, and other
gases/liquid stored well below their boiling
point. Tanks of these substances can be found
throughout the world in industrial and
laboratory environments, yet explosions caused
by them are quite rare. When they explode,
it's usually because someone who didn't know
what they were doing overfilled the tank, or
the ambient temperature exceeded the designed
safety margins through some other catastrophic
event. (structure file, etc). <br>
<br>
Finally, all systems where this is a risk
would have plenty of safety features to
prevent this. My gas water heater at home has
a simple temperature/pressure switch to safely
discharge excess pressure/temperature event.
These are cheap, readily available items that
you can buy at any local hardware store. I
also have a steam heat system in my house. In
the early days of steam heat, it was not
unheard for a steam boiler to explode with
devastating results, but just to some simple
design elements (Hartford Loop) and basic
mechanisms (low water cut-off valve, pressure
relief valves) have virtually eliminated this
risk. <br>
<br>
Before I got in to HPC as a profession, I was
a process control systems engineer. My
companies specialty was control systems for
boilers for power generation. The pressures of
these systems were much higher than what we're
talking about here. Our systems had plenty of
pressure sensors, release valves and
failsafes. Incorporating any of these safety
elements into a cooling system like this is
trivial, and I'm sure the vendors who sell
such solutions have already done that where
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<div>But if you have 40kW of gear still running,
your not storing the liquid in the sealed
container well below boiling point - its actually
the opposite you are running at or just above the
boiling point. Even if you take the approach "our
systems will shot down if we loose the external
cooling circuit)... that still takes time to
recognise and shutdown... mean while your systems
are pumping heat into the tank.</div>
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<div>Again, with the boiler example, this isn't the
sort of behaviour you want in a computer room.
You don't want this stuff venting... and also, try
and get a permit to operate such a system in an
existing or new facility.</div>
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<div>With a non-phase change solution, this isn't an
issue.</div>
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<div>FWIW the direct contact solutions (wether they use
water or some other dielectric fluid) as far as I can see
have several main problems</div>
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<div> * complexity (all that plumbing and getting it to 8
phi's + 2 cpu's all crammed in 2RU)</div>
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This is why I said I'm glad someone else is doing all that
engineering. ;) <br>
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<div> * nodes have to be modified after the come out of the
factory</div>
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I'm not sure, but I think the vendor (Dell, HP, etc.) sends the
components to CoolIT or Asetek for final assembly. This sounds
expensive, but car manufacturers have been doing it for years for
low volume special additions where a 3rd party is more optimized for
this small volume custom work than the large volume assembly lines.
Yes, it's still a little more expensive, but not as bad as building
something, and then taking it apart again to customize it. <br>
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<div> * not all the components are cooled (ie. ram, disks
etc) You still have to run some form of air cooling.</div>
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This is true. Most of the solutions I have seen have coolers for the
CPUs, GPUs, and RAM, and leave the rest to ambient cooling, this
covers the main heat producers, but some air cooling is still
needed. <br>
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But even with mineral oil, doesn't some of the heat leave the
mineral oil and heat your data center, or is it cooled to a
reasonable amount below ambient temperatures so it can absorb heat
from the servers and still be at or below room temp? <br>
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<div>I've only run direct contact cooling on a desktop style
box and that was painful enough... let along on hundreds
of servers in a rack etc. I can't see how they will be
price competitive, given all the modifications that are
needed to the systems (I've had pricing for a single rack
system but never purchased one).</div>
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Both CoolIT and Asetek claim to be only about 10%-15% more than
air-cooling per rack, and that is quickly recovered through reduced
cooling costs. I'm skeptical of the 10%-15% claim, but I have no
doubt the cooling cost reductions would be significant. <br>
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-- <br>
<div class="gmail_signature" data-smartmail="gmail_signature">Dr
Stuart Midgley<br>
<a moz-do-not-send="true" href="mailto:sdm900@sdm900.com"
target="_blank">sdm900@sdm900.com</a></div>
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