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<div class="moz-cite-prefix">On 08/22/2016 11:22 AM, Stu Midgley
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>
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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
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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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Any failsafe mechanism would be designed to shut the system down
with a wide safety margin. <br>
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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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In reality, I don't think any of this stuff would need to vent, or
even have vents, I was just explaining that there are a multitude of
ways to mitigate the alleged explosion risks based on my Chem Eng.
education and several years of experience as a practicing Chem E.
I'm afraid I may have gotten a little carried away and distracted
from the 'in actual practice' argument I was trying to make. <br>
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If you look at the IceTope literature, their Novec loops are
designated 'low pressure' so, I'm assume there is plenty of excess
capacity in them for pressure to build up safely. <br>
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<div>With a non-phase change solution, this isn't an issue.</div>
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Novec is a common fluid used in fire suppression
systems in computer rooms... so you shouldn't
have too much of an issue with the flammability
of the fluid, but it is an entirely different
issue to get them to warrant the solution.</div>
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</span> If it's already used in fire suppressions
systems, where it's going to deliberately be sprayed
into the atmosphere, I don't see how a system where it's
designed to be completely contained would more of a
safety issue, but this could be a local practice issue
(USA vs. Australia laws, etc.) In the event of an
emergency venting, as stated above, the vents are hooked
up to exhaust piping so that that gases released are
piped away to a location to where it can be safely
released to the environment, so a venting event would
pose no risk to the occupants of the data center. This
is done all the time at chemical plants. In fact,
venting like this happens quite regularly in those
environments, but I honestly don't even see this type of
venting being needed in a system like this. <br>
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<div>If you look at the US laws around this (I've only been
through the Houston documentation - but I assume all
states are roughly the same) their is a massive different
between holding a few hundred litres of fluids (which is
what is in a fire supression system) and say 30000L, which
is what you'll have in 30 tanks.</div>
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I don't think you'd anywhere near that volume of Novec in a system.
The cooling systems volume are only partially filled with Novec,
since you want to have a low pressure and room for the gas to
expand. This system only works if the Novec can easily change state.
Since the change of state absorbs a lot of heat, you need a lot less
of the fluid to have the same thermal capacity. <br>
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Going back to my home's steam boiler example, My boiler only holds a
couple of gallons of water. If I had hot-water radiators or
hot-water baseboards, I'd have you use a lot more water. <br>
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Also, the Novec is only in the primary loop. It condenses on another
heat exchanger which transfers the heat to a secondary loop which
carries the heat out of the data center, so not as much Novec would
be need as you might be assuming. <br>
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From what I recall, the Green Revolution system is a single loop
filled with mineral oil all the way to the external heat exchangers.
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<div class="gmail_extra">Perhaps the final nail in
the coffin of the Novec solutions... it has been
on many different booths at SC for many years...
now go and try to find a vendor that will
actually sell you a solution...</div>
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</span> I guess you never stopped by the Icetope booth
at SC, then. They've had solutions on the market for
several years now, and have had booths at SC for several
years now, too. <br>
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<a moz-do-not-send="true"
href="http://www.iceotope.com/" target="_blank">http://www.iceotope.com/</a><span
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I've seen them, I hadn't realised they were using Novec. Even
a 5min look through their website doesn't make that clear.</div>
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Yeah, I noticed that when I checked the link, and I was very
disappointed in that, since if I couldn't prove it, it wouldn't help
my argument. They used to be much more 'vocal' about that. Icetope
worked with University of Leeds to develop and test their system. If
you remember, there was member of this list involved in that work
who would occasionally post updates on their work here. <br>
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<div class="gmail_extra">Which brings us to another topic... and
that's price. The icetope stuff looks very very custom... and
thus very very expensive. I can purchase gigabyte or
supermicro equipment already for the fluid we are using (they
modify the power supplies, leave the thermal paste off
components etc)... no modifications at our site necessary -
and relatively cheap (is any HPC gear cheap?)<br>
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Damn it! I can't argue this point at all. You are absolutely correct
that Icetope's design requires purpose-built cases. I think with
enough volume (no pun intended), the cases could be manufactured
almost as cheap as existing air-cooled cases, but until they get
that kind of volume, I'm sure they're more expensive another con is
that I don't think any of the big OEMs (Dell, HP, etc) are working
with them at the moment, limiting the options. <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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