In message , Jerry Stuckle
writes
On 6/27/2015 5:35 AM, Ian Jackson wrote:
In message , Irv Finkleman VE6BP
writes
Thanks Ian,

There were existing fans and holes in the rear panel, and I was
just curious before replacing the fans. Both had become noisy and
I removed them without checking air flow direction.

Ah....
I was thinking more of a 19" rack horizontal fan tray (holding maybe a
couple of 6" diameter fans - which can sound like a hovercraft on full
power!).



Ian, we do a couple of dozen racks a year. The best cooling (even
recommended by the rack manufacturers) is fans with filters at the
bottom, blowing air directly into the rack. If you need additional
cooling, you can put fans at the top pulling the air out.

You don't want fans just pulling air through, as you would have if the
fans were at the middle or top; you have no control of how or where the
air enters the rack; air can be pulled in through spaces between units,
or even through holes in equipment, for instance. But you can place
fans at the top if you have fans at the bottom; the bottom fans will
still supply the air and the top fans will increase airflow.

The biggest problem with cooling a cabinet is equipment placement. You
generally want the heaviest items near the bottom for stability, but
these also generally generate the most heat. Additionally, deeper items
will block more of the airflow. You don't, for instance, want a
full-depth item immediately above one that generates significant heat.
But most items that run hot generate the heat near the rear of the
chassis, so you can place a low depth item above it, if convenient. If
not, you may have to place a 2 or 3 RU blank panel above it to allow for
sufficient airflow.

You also need to consider things like too many full-depth items you have
in the cabinet. Each one of them adds air resistance, lowering air
flow. You may, for instance, want to go to a 28" deep rack, even if
everything will fit into a 23" deep one, to allow for more airflow. Or
you may go for two 28RU racks instead of one 44RU rack to split the heat up.

There is a certain art to good rack design; you don't just slap
everything together and hope it works. When you're done, you should
power up everything in the cabinet and monitor temperature for a few
hours. And if you generate significant heat (or even moderate heat in
unconditioned space), it's a good idea to have a thermometer with remote
alert capabilities in case of a problem like a fan failure.

Your experimental experience is noted. I have to admit that I haven't
personally experimented with trying to optimise cooling in racks.

My suggestion of 'halfway up' is essentially a gut feeling - based on my
experiences where the equipment doesn't extend all the way to the back
to the rear door/wall of the rack - ie there is a gap, say, of least 6",
where the air can get from bottom to top without having to pass
completely through the intervening hot equipment.

By the very nature of things, and with the best will in the world, racks
can rarely be made into air-tight tubes, and as a result, there is
usually quite a fair bit of leakage. If the fan tray is located at the
very top of the rack, a lot of the lower equipment doesn't really feel
their benefit. This may not matter much to the very lowest equipment, as
it probably doesn't really need the fans. It will be the equipment in
the centre of the rack that will suffer, because it will get the hot air
from the equipment beneath, but the fans won't have enough 'reach' to
extract it. That's why I feel that locating the fan tray about halfway
up might be the best compromise.
--
Ian