Data centre info

JC Dill jcdill.lists at
Thu May 7 11:53:49 CDT 2009

>> JC Dill <jcdill.lists at> writes:
>>> Why do you believe that it is proper to deliver the cooling "top
>>> down"?  Heat rises!  The cool air needs to be delivered to the bottom
>>> (raised floor) or front (alternate row cooling) so that it flows
>>> over/thru the computers on its way to the hot air intakes which should
>>> be located above the back of the cabinet (above the "hot row") or top
>>> of the room (for raised floors).
>> This would be true if the computers were all mounted within several
>> inches of the floor, but they're not.
>> Just as heat rises, cold sinks.  Discharging cool air up high in a
>> data center will cause it to end up where it needs to be for all
>> computers in a rack, not just the ones in the bottom.
In theory, this should work.  In theory, there is no difference between 
theory and practice.  In practice, there is.
>> Visit any Equinix for an example of this principle in action

I disagree, and I've been in several Equinix locations.

Computers produce heat / hot air, and this hot air rises.  As the hot 
air rises and moves away from the computers, new air has to circulate in 
from *somewhere*.  There isn't a vacuum of "no air" between the cold air 
sinking/sitting at the bottom of the room and the hot air rising up to 
the ceiling.  :-)

When you have cold air entering the bottom and/or front of a rack, and 
hot air exiting out the back (and then up) then you don't have cold air 
mixed with hot air circulating in the room before it gets to the 
computers.  The fans located in the building's hot air intakes, the 
action of the rising hot air, and the fans blowing the cold air into the 
data center all create the air flow that pulls the "heavier" cold air up 
into and across the computers.

When you have cold air coming down in the same area with hot air going 
up, it just mixes up into warm air, and truly cold air never gets to the 
bottom - you get warm (or hot) air flowing across the bottom computers 
in the racks.  This is not an efficient air flow, and not a good way to 
keep the hot and cold air separate so you can get cold air (not warm 
air) flowing over the equipment.  It works great for offices full of 
people (where you want an even temp everywhere in the room) but isn't 
great for computers (where you want cold air entering the equipment for 
maximum cooling effect and hot air being removed as quickly as possible.)

I've never been happy with the room temps or equipment temps in any data 
center that used this "blow cold air from the ceiling" approach.  But 
hey, if it works for your equipment, more power to you.  I encourage all 
my competitors to do this.


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