Archived from groups: alt.comp.hardware.overclocking (
More info?)
The short answer:
There is not really an "optimum" in the abstract. More is better. A better
question is how MUCH air flow is needed to keep the temperature of the CPU
below your target temperature. A compromise is made between how much noise
is permissible, how high a CPU temperature is permissible, and cost.
The long answer for air cooling:
The CPU temperature can not be lower than the air used to cool the heatsink.
The temperature of the CPU will depend on-
CPU power consumption (which is all dissapated as heat, well 99.999999...%)
Thermal resistance of the thermal compound and thermal resistance of the
heatsink base and fins (not the overall thermal resistance of the whole
system, which includes ALL these factors except the CPU power consumption
and entering air temperature)
Temperature of the air entering the heatsink
Volume of air flowing through the heatsink
Turbulence of air flowing through the heatsink (a moderate amount is best)
Efficiency of heat transfer from the heatsink to the air (includes the
turbulence factor)
Air at room temperature and pressure has a heat capacity of about 24 Watt
seconds per second per cubic foot per degree K (one degree Kelvin is the
same as one degree C, but the zero point is absolute zero rather than the
freezing point of water; when measuring temperature differences the results
are the same in Kelvin and Celsius.) If a 20 degree C rise of the air
temperature is acceptable, then a CPU dissappating 100 Watts of heat would
require 24/30 CF per second = 24/30 X 60 = 48 CFM (at least.)
If the air temperature entering the heatsink is 25 degrees C, the thermal
resistance of the heatsink body and thermal compound is 0.05 degree C per
Watt, then the BEST that air cooling can do given
100 Watts power dissapation
25 degrees C air temperature
0.05 degree C thermal resistance between the CPU and the heatsink fins
50 CFM air flow
is 50 degrees C for the CPU temperature [(entering air temperature) +
(thermal resistance X power dissapation) + (cooling air temperature rise)] =
[25 degrees C + (0.05 degrees C per Watt X 100 Watts) + (20 degree C)] = [25
+ 5 + 20] = 50 degrees C.
If the power dissapation is less the CPU temperature would be lower, or a
lower air flow could be used for the same temperature.
Generally heatsink manufacturers give a thermal resistance specification
that combines the effect of thermal compound, heatsink material, transfer
efficiency, and air flow volume, giving a thermal resistance of something
like 0.15 to 0.35 degrees K per Watt. For 100 Watts CPU dissipation and an
entering air temperature of 25 degrees C, the corresponding CPU
temperature would be from 40 degrees C(0.15 degrees C per Watt) to 60
degrees C (0.35 degrees C per Watt).
There Ain't No Such Thing As A Free Lunch; the heat has gotta go some where,
and the less air in this case, the higher the temperatures. Watercooling
has a BIG advantage, by volume water has about 5000 times the heat capacity
of air at ordinary room temperature and pressure. On the other hand, the
same amount of heat that went into the water has to be taken out before
recirculation, usually by air cooling, requiring at least as much cooling
air as if watercooling had not been used.
--
Phil Weldon, pweldonatmindjumpdotcom
For communication,
replace "at" with the 'at sign'
replace "mindjump" with "mindspring."
replace "dot" with "."
<edek> wrote in message news:m01eb0ds445id7ttfmfk1lgqgb89t1k201@4ax.com...
> Thanks for the advice.
>
> When choosing a cooling rig what sort of air flow parameters are
> optimal for modern cpu's?
>
> regards
>
> On Thu, 27 May 2004 23:21:49 GMT, "Phil Weldon"
> <notdisclosed@example.com> wrote:
>
> >A watercooling system is great for getting a lower CPU temperature; to
> >within a few degrees of room temperature. However, it is a very
expensive
> >choice for getting a QUIETER CPU cooling system. The particular water
cooler
> >you mention has some odd specifications, especially the air flow, 17.18
CFM,
> >which is so low that you end up with less cooling than with more or less
> >ordinary aircooled heatsinks which use more than twice the air flow.
>
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