[Beowulf] Innovative liquid cooling

Prentice Bisbal prentice.bisbal at rutgers.edu
Thu Feb 28 07:59:11 PST 2013


On 02/28/2013 05:00 AM, Hearns, John wrote:
>
> I think this has been discussed here before, but it is a pretty 
> innovative product:
>
> http://www.theregister.co.uk/2013/02/28/wet_servers_cut_cooling_costs_research_leeds_university/
>
>

I really should just save my rants about immersive liquid cooling on a 
web page somewhere so I can just provide a link every time this topic 
comes up. I'd just provide links to my previous rants here in the 
archives, but I'm feeling lazy this morning.

Seriously, this article is both interesting and confusing, and there are 
some stupid/ridiculous statements in the article.

1. The interesting:

How easy is it to clean this liquid up? Is it oily like mineral oil? If 
it's not slippery and it's easy to clean up, that addresses my biggest 
problem with immersive cooling. The Novec didn't look as thick or 
slippery as mineral oil in the video, but it's hard to tell something 
like that from a web-quality video.

2. The confusing:

Is the Icetope system an immersive cooling system? From the video, it 
looked more like a "direct contact" cooling system where the liquid is 
run through pipes into "direct" contact with the processors, but the 
demo made me think they are talking about immersive cooling. Even for 
direct-contact cooling (if that is the right term) having a 
non-conductive liquid is a better option than water if a leak occurs.

3. The stupid/ridiculous:

> Dunking servers in new magic liquid 3M Novec reduces the cooling 
> system's energy use by 80 - 97 per cent, compared to cooling systems 
> that use air. Air cooling is inefficient because it is a poor 
> conductor, produces diffuse general heat and requires energy-chomping 
> high powered fans, said the boffins.
>
> 3M Novec is also a thousand times better at conducting heat than 
> water, and one low-powered fan in a chamber of 3M Novec is adequate to 
> chill a server array.
>

At some point or system size, you're still going to need pumps to 
circulate the liquid. While natural convection is fine for gases, it's 
usually inadequate for liquids due to their higher viscosity. And if you 
need to transport that liquid horizontally away from the heat source to 
the heat sinks, you're definitely going to need pumps. Liquids might 
have thermal capacities and thermal conductivities that are about 1000x 
that of air, but I think the viscosity of a liquid has got to be at 
least 1000x that of a gas. In this case, the pumps are still probably 
using less electricity than all those fans, but I think these quotes 
distort some of the facts.

> The fact that this system is completely enclosed raises a host of 
> possibilities. It does not interact with its environment in the way an 
> air-cooled server does, so you could put it in an extreme environment 
> like the desert

Ummm, no. that heat still has to go somewhere. And that "somewhere" has 
to be at a temperature low enough for there to be a temperature 
difference large enough create the "driving force" necessary for useful 
heat transfer. That's probably not going to happen in the desert.

> It is also completely silent. You could have it on a submarine or in a 
> classroom.

See my earlier comment about the need for a pump. I guarantee that even 
the smallest production systems will need some kind of circulation pump. 
you can probably locate that pump further away from the system being 
cooled, but it will be producing some level of noise, somewhere.

Overall, if this 3M Novec overcomes the drawbacks of mineral oil, this 
is great, but I feel that this article and this research is more like 
press release for 3M Novec.

Rant over. You may now return to your regularly scheduled work day.

--
Prentice


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