<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Aug 22, 2016 at 11:22 PM, Stu Midgley <span dir="ltr"><<a href="mailto:sdm900@gmail.com" target="_blank">sdm900@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><span class=""><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><span><br></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 appropriate</div></blockquote><div><br></div><div><br></div><div><br></div></span><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><div><br></div><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><div><br></div><div>With a non-phase change solution, this isn't an issue.</div><span class=""><div><br></div></span></div></div></div></blockquote><div><br></div><div><br></div><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><div><br></div><div> * complexity (all that plumbing and getting it to 8 phi's + 2 cpu's all crammed in 2RU)</div><div> * nodes have to be modified after the come out of the factory</div><div> * not all the components are cooled (ie. ram, disks etc) You still have to run some form of air cooling.</div><div><br></div><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><div> </div></div>-- <br><div class="gmail_signature" data-smartmail="gmail_signature">Dr Stuart Midgley<br><a href="mailto:sdm900@sdm900.com" target="_blank">sdm900@sdm900.com</a></div>
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