On Mon, Aug 22, 2016 at 11:22 PM, Stu Midgley <sdm...@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 sdm...@sdm900.com
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