Thermal Problems

Jim Lux James.P.Lux at jpl.nasa.gov
Wed Jul 23 18:19:00 EDT 2003


At 09:04 AM 7/23/2003 -1000, Mitchel Kagawa wrote:
>I run a small 64 node cluster each with dual AMD MP2200's in a 1U chassis.
>I am having problems with some of the nodes overheating and shutting down.
>We are using Dynatron 1U CPU fans which are supposed to spin at 5400 rpm but
>I notice that a lot (25%) of the fans tend to freeze up or blow the bearings
>and spin at only 1000 RPM, which causes the cpu to overheat.  After careful
>inspection I noticed that the heatsink and fan sit very close to the lid of
>the case.  I was wondering how much clearance is needed between the lid and
>the fan that blown down onto the short copper heatsink?

To a first order, the area of the inlet should be comparable to the area of 
the outlet.  A 60 mm diameter fan has an area of around 2800 mm^2. If you 
draw from around the entire periphery (which would be around 180 mm), you'd 
need a gap of around 15 mm (probably 20 mm would be a better idea)  That's 
a fairly significant fraction of the 45 mm or so for 1 rack U.



>  When I put the lid
>on the case it is almost as if the fan is working in a vaccum because it
>actually speeds up an aditional 600-700 rpm to over 6000 rpm... like there
>is no air resistance.  Could this be why the fans are crapping out?  I was
>thinking that a 60x60x10mm cpu fan that has air intakes on the side of the
>fan might work better but I have not seen any... have you?
>
>Also the vendor suggested that we sepetate the 1U cases because he belives
>that there is heat transfer between the nodeswhen they are stacked right on
>top of eachother.  I thought that if one node is running at 50c and another
>node is running at 50c it wont generate a combined heatload of more than 50c
>right.

So, your vendor essentially claims that his 1U case will work just fine as 
long as there is a 1U air gap above and below?

Let's look at the problem with some simple calculations:

Assume no heat transfer up or down (tightly packed), and that no heat 
transfers through the sides by conduction, as well, so all the heat has to 
go into airflow.
Assume that you've got to move about 200W out of the box, and you can 
tolerate a 10C rise in temperature of the air moving through the box. The 
question is how much air do you need to move. Air has a density of about 
1.13 kg/m^3 and a specific heat of about 1 kJ/kgK.
200W is 0.2 kJ/sec, so you need to move 0.02 kg of air every second (you 
get a 10 deg rise) is about 0.018 cubic meters/second. To relate this to 
more common fan specs: about 40 CFM or  65 cubic meters/hr. (I did a quick 
check on some smallish 60mm fans, and they only flow around 10-20 CFM into 
NO backpressure... http://www.papst.de/pdf_dat_d/Seite_13.pdf
for instance)

How fast is the air going to be moving through the vents?  What's the vent 
area... say it's 10 square inches (1 inch high and 10 inches wide...).. 40 
CFM through .07 square feet is 576 ft/min for the air flow (which is a 
reasonable speed.. 1000 ft/min is getting fast and noisy...)

But here's the thing.. you've got 32 of these things in the rack... are you 
moving 1300 CFM through the rack, or are you blowing hot air from one 
chassis into the next.






>Mitchel Kagawa
>Systems Admin.
>
>
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James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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