[rescue] Deciding how to place machines in a rack.

Robert Darlington rdarlington at gmail.com
Fri Mar 7 11:13:49 CST 2008

Racks with the fronts and backs off are ugly to most sane people.
Racks with the doors closed are ovens.  The fans at the top assist in
cooling by drawing air up through the bottom of the rack and out the
top, but also up through the front and out the back of the machines
(and again, out the top).

Remember, heating air makes it expand and that expanding air has to go
somewhere.  The back of the rack will provide more resistance to the
escaping air than you can imagine, and if you have spaces between
servers, a lot of it goes through those spaces, right back to the
front of the servers.  The fans on top help prevent this.  It also
helps vent the air that is heated by the sides of the computers in
that dead airspace under the side skins.  The only way out is via
convection in most cases.


On Fri, Mar 7, 2008 at 9:51 AM, Earl Baugh <earl at baugh.org> wrote:
> Ok, I've parsed all this advise, and am going to place the machines as close
>  as possible.
>  I do want to understand one thing a bit better regarding the cooling.....
>  all of the machines I'll have
>  in the rack do front-to-back air flow (fans in the front and back which pull
>  air thru the case front to back)
>  It seems that the recommendation is to keep the back door off, and either
>  keep the front door off, or get one
>  that has a lot of ventilation.    This all makes sense with the front to
>  back air flow.
>  But I'm curious about two things.  First, I'm trying to understand the
>  purpose of the fans at the top of the rack.
>  If this approach is to move the hot air out the back, I'm thinking I'll just
>  have a hot spot behind the rack and that the top fans will
>  just be pulling air from the front and back near the top.   Is this really
>  assisting the cooling?  I can believe with the doors off
>  that it's going to move that much air from the bottom of the rack.  Or is it
>  to pull heat from the "stack" of machines?
>  I'm just trying to understand this....  In my head I was thinking having the
>  doors would help the top fans pull air from the
>  bottom.  But I hadn't thought about the way they'd also hold in heat from
>  all the machines, and that you'd get that hot air
>  exhaust coming around and re-circulating thru the machines.  Very very
>  helpful to know that that happens....
>  Also, if I have any open space in the rack, I should bunch it up... I'm
>  thinking it should be near the top.... or does
>  it really matter where it is?
>  In all cases, I really appreciate the advise, real world data, and modeling
>  results!  Way cool... and really helps to
>  bust the myths that we all build up in our heads...
>  Earl
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>  rescue list - http://www.sunhelp.org/mailman/listinfo/rescue

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