[Beowulf] cooling question: cfm per rack?

Jim Lux james.p.lux at jpl.nasa.gov
Sat Feb 12 05:45:02 PST 2005

> CFM and delta-t across the machine-to-be-cooled are convolved to give
> you how much heat you're extracting.  no doubt both pressure and humidity
> are involved to some degree as well, and I don't have a good equation
> for this.

Indeed.. there is no "nice simple" equation for the general case, because of
the problem with humidity.  You really need to be worrying about enthalpy,
etc., and with any sort of significant temperature change, it's neither
constant pressure, nor constant volume, not to mention mechanical
turbulence, etc..  All that icky thermodynamics stuff.  I once spend several
weeks trying to figure out if one could make theatrical fog without using
liquid nitrogen. They do it by having a big tank of water about half full at
around 160-180F, and then they inject liquid nitrogen into the headspace
above the water.  Turns out that the heat of vaporization of the LN2 is
almost exactly balanced by the heat of condensation of the saturated water
vapor, and that the volume of nitrogen gas produced, etc, works out to the
outlet stream being around 38F, with the water droplets at the same
temperature.  Very, very tough to do this with mechanical refrigeration for
a variety of reasons.

So, as you say, unless you're airconditioning a huge building (where the
cost of excess capacity is significant, and where there all those hot, water
exhaling people inside), you can just do some quasi-worst case

  the good thing is that turning down the temperature can partly
> mitigate minor airflow problems.

> to complicate matters, HVAC folk always bring up the issue of "sensible
> load".  as near as I can tell, this is just a way of saying that if you
> to impose too much delta-T on humid air, you wind up wasting a lot of
> dehumidifying it...

Yes.. this is especially true if you're not recirculating, but chilling
fresh air from "outside".  If you've got a reasonably closed system and
there's no people inside, it's less of an issue.

> tiles between 500-2000 CFM:
> that also gives:
> CFM = btu/hr / (1.08 * dT)
> so for 1 ton = 12000 BTU/hr and 70->90, 555 CFM per ton of cooling.
> HVAC folk also tend to say 1 tile/ton, which seems about right.
> > These run off the campus cold water supply, so
> > it makes sense that heat out is proportional to flow across, assuming
> > that the cold water has a very large heat capacity.

Yes, in a theoretical sense.  However, there are two factors to be aware of:
1) run the air too fast past the coils and it doesn't have time to exchange
the heat; 2) run the air too fast and you consume power (and make heat) in
compressing it to overcome the pressure drop.  There's also a practical
limit on just how much delta T you can get in one pass through the chiller

> our experience with CW has been disasterous, but we made the huge mistake
> of not using precision/machineroom chillers (fancoils, actually).
> our old/existing machineroom, for instance, is supposed to have 2x8ton
> fancoils, but combined they never moved more than about 20 KW (should be
> unless you have pretty extreme assurances about WC quality (flow, temp),
> I would only consider using dual-cool machineroom chillers (DX + CW,
> adds about 15% to price.)
> > directly through the A/C.  Even more ideally cfm through _each_ rack
> > could be modulated somehow, since some racks move much more
> > air than others and putting a low flow rack next to a high flow rack
> > might drive the air the wrong way through the low flow unit.
> > Is cfm the key unit here or should one think in terms of pressure
> > at various points in the room?
> if your pressure is reasonably even, the same tiles should flow the
> same CFM.  I'd LOVE to find some way to measure airflow, since I'd
> actually consider doing things like adding patches of duct tape to
> the underside of too-high-flow tiles.  I suppose that the empiricist
> approach is just to sample all your system temperatures, and if some
> are too high, reduce the airflow to racks which are "too cool".

Hie thee to a company called Dwyer, who make equipment specifically designed
to measure airflow.  There are several approaches..
One is using a pitot tube with a Magnehelic differential pressure gauge.
Another is to measure the pressure drop across a calibrated orifice (again,
using a sensitive pressure gauge). http://www.dwyer-inst.com/   Another is
to use a airspeed probe (looks like a wand with a little fan in a hole on
the end).  The fancy ones will average a bunch of readings over an opening
and do the calculation to turn area*average speed into CFM.

You can find Magnehelic gauges surplus all the time.. keep your eyes open
and when one turns up for $15-20, grab it.  They're handy devices that can
measure fairly small pressures (few inches of water column), and come with
all sorts of weird scales (including some already calibrated in feet per
minute or m/sec, all ready for use with a pitot tube).  Interesting to
measure the pressure in a room (or your house) and see what happens when the
heater turns on, or the kids open and close the doors, etc.

Some time spent with the Mc-Master Carr catalog (http://www.mcmaster.com/)
or the Grainger catalog (http://www.grainger.com/) (both are large suppliers
of stuff mechanical, materials, etc.. everyone should have a copy of the
several thousand page yellow  McMaster Carr catalog on their desk...).
Omega (usually associated with temperature measuring) has a fair number of
airspeed and volume measuring devices. http://www.omega.com

However, your empirical approach of reducing the flow through the coldest
racks is probably as good as anything.

Jim Lux

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