Re: DATACENTER: AC and colo sizing question.

[Sean Donelan <sean@donelan.com>]

On Wed, 09 February 2000, foo wrote:
> The first question I have is what your experiences have been regarding how
> to size ac unit(s) for an unknown amount of total dissipated heat. We can
> determine what stuff is likely to have a permanent home in our space, but
> other things will come and go or make short pit stops while on the way to
> the larger colo space. I'm familiar with the ACCA Manual J, but its focus
> seems to be purely residential. Is there a standard for industrial or
> computing facilities?

Since the heat dissipation requirements of computers can vary so greatly,
most design standards usually calculate it based on actual demand.  There is
a rule of thumb of 50-100 sqft/ton, but I have never been able to trace
the original source for that estimate.  I don't like the number because
it is based on gross space, and people seem to pack their computer rooms
as tight as they can.  I tend to assume people will keep putting equipment
in until circuit breakers start tripping, so I use the electrical method.

Most computer room cooling equipment designs include multiple, multi-speed
compressors.  So the unit can bring up additional capacity as needed.  For
example, some Liebert systems come with two compressors and space for two
additional compressors; and will run them at hi and low speeds.  This allows
it to handle a pretty wide heat load.

I wouldn't use standard residential or commercial air conditioners for a
computer room.

> Since it is not possible to over-engineer AC and experience the desired
> result (i.e. if your ac is too big it'll short cycle and wont remove
> moisture from the air), this is an important design decision.

Computer rooms should use process cooling, not sensible load.  But you are
correct, a mis-sized unit will use a lot more energy.  Equipment designed
specifically for computer rooms will run both the reheat coil and the coiling
cool at the same time trying to maintain the temperature and humidity. Over-
engineering will hurt your pocket book, but generally not your computers.
Under-engineering is another matter.