[Beowulf] bring back 2012?

Stu Midgley sdm900 at gmail.com
Mon Aug 22 08:46:46 PDT 2016

On Mon, Aug 22, 2016 at 11:22 PM, Stu Midgley <sdm900 at gmail.com> wrote:

>> 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.
>> 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.
>> 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.
>> 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).
>> 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.
>> 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
> 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.
> 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.
> With a non-phase change solution, this isn't an issue.

FWIW the direct contact solutions (wether they use water or some other
dielectric fluid) as far as I can see have several main problems

 * complexity (all that plumbing and getting it to 8 phi's + 2 cpu's all
crammed in 2RU)
 * nodes have to be modified after the come out of the factory
 * not all the components are cooled (ie. ram, disks etc)  You still have
to run some form of air cooling.

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).

Dr Stuart Midgley
sdm900 at sdm900.com
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