[Beowulf] Rear-door heat exchangers and condensation

Kilian CAVALOTTI kilian.cavalotti.work at gmail.com
Thu Dec 11 06:47:45 PST 2008

Hi Tom,

On Wednesday 10 December 2008 21:08:12 Nifty Tom Mitchell wrote:
> Watch dew point numbers in the room.
> Dew point is dominantly a function of humidity...
>   http://en.wikipedia.org/wiki/Dew_point

Oh right, that's interesting:
The dew point is associated with relative humidity. A high relative humidity 
indicates that the dew point is closer to the current air temperature. 
Relative humidity of 100% indicates that the dew point is equal to the current 
temperature (and the air is maximally saturated with water). When the dew 
point stays constant and temperature increases, relative humidity will 
I guess that controlling the relative humidity level (most CRAC units can do 
that, can't they?) and keeping it below say 60% is a pretty simple way to 
avoid condensation, then.

> Many frost free home refrigerators solve this problem by running the
> heated exhaust air over the catch pan so any frost/ condensation is
> promptly evaporated.  With clever airflow management drains may not be
> needed but water rots wood, breeds bacteria and attracts bugs and may be
> problematic. The bacteria issue is important.... see  Legionella
> pneumophila.

I was only thinking about the hassle of having to mop down your racks every 
morning, but the point about bacteria is very relevant. That should legitimate 
a bonus, working in hazardous areas. :)

> Right now the outside air dew point in Bryan, Texas is about 19F and
> historically gets as high as 69F in December.  So yes condensation from
> 42F cooling pipes is possible and should be part of the management/
> monitoring process.  I suspect that the campus AC manages the dew point
> to the high end of a comfort range  that might be about 50 - 54°F in the
> US keeping things all OK.  i.e. If the building AC manages humidity you
> may not have to if they have the capacity to control it at the building
> air inlets.

There's no such thing as "building AC" where I am, unless you call opening a 
window "managing the dew point". I guess we won't avoid local equipment in the 
server room to control relative humidity.

> Of course the weather in France is not the same as Texas... looks nice ;-)
> something like.
>  52 °F  / 11 °C Light Rain Humidity: 82% Dew Point:  46 °F  / 8 °C

That's pretty much it, even on the south coast: gray, rainy and cold. Man, I 
so miss California... :)

Thanks for the insight,

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