DATACENTER: power requirements and thermal issues

["Roeland M.J. Meyer" <rmeyer@mhsc.com>]

> -----Original Message-----
> From: owner-datacenter@itchy.shorty.com
> [mailto:owner-datacenter@itchy.shorty.com]On Behalf Of Martin Hannigan
> Sent: Saturday, June 05, 1999 6:06 AM
>
> Good point, applicable to AC only.

Not true. In AC you have Vrms that fluctuates and Irms  varies to
maintain P (wattage). This is why power companies measure Kilowatts/hr.
BTW, for those who don't know, calculating AC values for this stuff
involves trig (Vac != Vdc and V/A is only = P in a pure resitive circut
where capacitance and inductance are small). DC is straight-forward
algebra. The only common term is wattage (Pac = Pdc), which is a measure
of produced/dissipated heat. This ALSO speaks to another question, put
forth here, about thermal requirements.

For first-approximation "rule of thumb" guide-lines;

An electronic devices (router/computer/etc), the power-supply/regulator
will be rated at the Maximum instantaneous power use of the device. A
device, with a 200 watt power supply, will draw a maximum of 200 watts.
Exceeding that would fry the supply. Switching-mode supplies are
especially sensitive to this. Most hardware engineers design their loads
(actual devices) to never exceed 63% of this limit (why they pick that
number is complicated and out of scope here), in steady-state operation.
Therefore, you can be assured that if you go by the power supply rating,
you will never be caught by surprise wrt power-up current surges, using
solid-state devices (motors and other high inductive/capacitance loads
are another issue). You can also be assured that the device will never
generate more than 200 watts of heat, in normal operation. However, it
may generate less, but don't count on it.

When you are using a UPS it not only protects you from power failures,
it ALSO regulates your load feed-back. In other words, it presents a
constant load to your power feed. Start-up current spikes (drive
spin-up) are absorbed by the UPS. If your UPS doesn't do this, it isn't
large enough for your application anyway and it might as well not be
there. Conversly, a properly designed UPS will also act like a large
capacitor bank (complete with resonance issues) wrt lighting strikes,
inductive surges, and other voltage problems, thus regulating the power
to your devices. A UPS is also a voltage regulator. If you think about
it properly, that is its primary function. It will supply a controlled
voltage to within a milli-joule of its life <grin>.

Now comes the interesting part;

Watts can be converted to BTU/hr by the relation ( P * 3.413 =
BTU/hr )[ref #1 - page 72]. Most airconditioning units are rated in BTUs
(still).

>From the previous discussion, power usage in a device can be safely
determined from the rating of its power supply. In the case of redundant
supplies, only one needs to be counted (times 1.15).

Given a device (above), of 200 watt rating, and a UPS of 1400 V/A
rating, it becomes quickly apparent that one would never attach more
than 7 devices to this UPS. A conservative upper limit would be six
devices (1200 watts). However, these devices would also generate 4095.6
BTUs of heat per hour. You could circulate the air all you want to
(fans) but the room would still get noticably warmer over time. For
guide-lines on what to do about this see reference #2 section 4-76, wrt
"Heating-load determination".

The joke I was making earlier was regarding power-factor games, with
large industrial loads that are largely reactive. Such plants "correct"
their power-factor by running large synchronus motors in order balance
the power-factor and sometimes "fool" the power meter (kilowatt/hour) to
indicate less apparent power than true power. For such plants, this
translates into significant power budget reductions. I have never heard
of anyone applying this to even large data-centers because, unlike a
large industrial plant, electronic equipment is largely resistive in
nature. Whereas the large industrial plant is predominantly reactive.
The joke is that Joe Pointy-hair doesn't know this little detail and he
could spend months of budget trying to save power budget, to no avail
<grin>. It is the management equivalent of a snipe-hunt <GRIN>.

> > They only thing that counts is wattage. You ARE running
> UPS's, right?
> > Except during boot, most server power usage is constant. No
> one running
> > production servers uses "power management" features on
> their DASD farms
> > and RAID packs.
>

references
---------------------------------------
[1] "Handbook of Electronic Tables and formulas" ISBN 0-672-21532-2
[2] "Standard handbook of engineering calculations" ISBN 0-07-028734-1