<br><br><div class="gmail_quote">On Sat, Apr 4, 2009 at 8:23 AM, Bill Broadley <span dir="ltr"><<a href="mailto:bill@cse.ucdavis.edu">bill@cse.ucdavis.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div class="im">Robert G. Brown wrote:<br>
> On Fri, 3 Apr 2009, Greg Lindahl wrote:<br>
><br>
>> On Fri, Apr 03, 2009 at 09:14:37AM -0400, Robert G. Brown wrote:<br>
>><br>
>>> b) The idea is to get the heat production OFF the motherboard. One<br>
>>> really interesting thing about the google design is that they hang the<br>
>>> stock OTC power supply off the back, altogether outside of the case<br>
>><br>
>> It looks to me like 100% of the airflow through the case goes through<br>
>> the PS, and in fact the PS fan is the only one driving front-to-back<br>
>> flow.<br>
><br>
> As it is in a lot of PC cases, but I can't help but think that having<br>
> the PS in the case itself does raise the temperature in there because it<br>
> itself gets warm and is in contact with the air in the dead air pocket<br>
> underneath the PS bracket.<br>
<br>
</div>Most normal cases I've seen have a power supply in the top rear and suck air<br>
from the bottom of the power supply (usually very near the cpu) and exhaust it<br>
out the back. So while it burns about 10% of the energy for the PC (typical<br>
efficiencies in the nicer power supplies are around 90%) very little of that<br>
heat should flow upstream.... unless you lock your PC in a cabinet. For this<br>
reason I've found it quite strange that high end cases (apple, antex, lian-li,<br>
and others) have put the power supply in a separate airflow chamber in the<br>
bottom of the case. Seems of no benefit to the CPU/GPU/motherboard where most<br>
of the heat is generated. </blockquote><div><br>It's of benefit for the PSU and it's done on purpose. It's done to extend the lifespan of the PSU's components and to make it run cooler and more silently. In high-end systems with kW PSUs you have a lot of heat generated by the PSU itself, which when pulling hot air from 2 or 3 graphics cards and an overclocked CPU starts having an intake not of 20-25ºC but of 35-40ºC. No PSU can handle this kind of temperature silently. It's manageable in winter but a problem in summer, when PSUs should not intake air at more than 50ºC or it will exhibit sub-optimal power delivery.<br>
Also, the separate air-channel usually serves HDDs and PSU, which go very well together.<br><br>Best regards,<br> Tiago Marques<br><br> </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
I guess it helps the power supply since it's<br>
sucking air at ambient, or past a few drives which shouldn't change the air<br>
temp much.<br>
<div class="im"><br>
> I was trying to figure out if I "liked" having the battery up front so<br>
> that air flowed over it before reaching the motherboard, as batteries<br>
> heat too when they're being charged<br>
<br>
</div>Indeed, although if it's fed 12V I'd expect once charged (i.e. 99.9% of it's<br>
life) that the heat would be minimal. Sure you have a heat spike after a 3<br>
minute outage.<br>
<div class="im"><br>
> but then I remembered that battery<br>
> lifetime is pretty strongly tied to temperature, so perhaps it makes<br>
> sense to keep it as cool as anything in the system.<br>
<br>
</div>Indeed, it wasn't clear to me if the power supply was the only fan of if the<br>
racks themselves had additional fans in them. If the powersupply is the only<br>
fan then at least the fan would be running during an outage. Not too sure<br>
about the chemistry, but I suspect a discharge in 5 minutes would be quite a<br>
bit hotter than a 30-60 minute charge that seems commonly required to get back<br>
to 90% capacity or so.<br>
<div class="im"><br>
> The disk stack did look like it might get a bit hot; I'm guessing that<br>
> Google disks are pretty hammered.<br>
<br>
</div>Disks are only what 5-10 watts each these days? Doesn't seem like you need<br>
much air relative to the rest of the system. Also I believe Google published<br>
some disk data on some n*10k drives over a few years of use and basically<br>
found no correlation between temperature (within normal usage limits) and rate<br>
of failures. While not great the google design certainly seems better than<br>
many hotswap bays I've seen that often mostly enclose the drive (sometimes in<br>
plastic even *ugh*), then put it in a tiny enclosure with a tiny air holes<br>
drilled in the backplane. Fortunately the migration from SCA and IDE to SAS<br>
and SATA seem to have helped this a good bit with smaller cables/pinouts.<br>
<div><div></div><div class="h5"><br>
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