# [Beowulf] More about those underwater data centers

Joe Landman joe.landman at gmail.com
Thu Nov 8 08:06:23 PST 2018

```On 11/8/18 10:46 AM, Prentice Bisbal via Beowulf wrote:
>
> One comment - my dissertation below is specifically about
> non-ebullient immersion cooling. As Jim Lux pointed out in a later
> e-mail, in ebullient cooling, some kind of surface feature to promote
> nucleation could be beneficial. Ebbulient cooling is a whole different
> beast from normal (non-ebullient) immersive cooling, since in that
> case you have changes of state and gas bubbles flowing through a liquid.
>
> However, in all of the live and video demonstrations I've seen of
> Novec, the processors were completely bare, bubbles were forming at a
> pretty rapid rate, so again I think creating some sort of heat sink
> for this would add cost with no significant benefit.
>
I get to use physics ... whee!

Short version ... most (all?) heats of vaporization (the energy you have
to pour into a liquid to turn it from a liquid to a gas at its boiling
point temperature/pressure) are (much) higher than the energy you
deposit into the same mass of liquid to bring it from just above
freezing to boiling.

Cv for water is about 4.186 J/(gram * C), so 1 gram of water, going from
just above 0 C (freezing point) to 100 C (boiling point at sea level)
means Q = m Cv delta_T = 418.6 J.

Take that 1g of water at 100 C, and turn it into vapor at 100 C, and you
get Q = m Hv = 2256 J.

Put another way, evaporation cooling allows you to absorb more heat
(about 5x in this case) for the same mass.

That said, convective cooling is a somewhat different beast. Immersion
cooling is, I believe, primarily convective in nature.

I think DUG is primarily convective based, and it is sufficient for
their use case (please correct me if I am wrong).

There is a fundamental danger with evaporative cooling, in that one has
to make sure one does not ignore the vapor, or potential heat induced
reaction products of the vapor.  Fluorinert has some issues:
https://en.wikipedia.org/wiki/Fluorinert#Toxicity if you overcook it ...

--

Joe Landman
e: joe.landman at gmail.com
t: @hpcjoe
w: https://scalability.org
g: https://github.com/joelandman