Gandalf said:
Usually moving the air will do the most good. You shouldnt need to cool it
unless it is a really hot room. Then you might as well cool the room.
there are some limits, but here's the basic calculations...
there are 3 basic thermal conductive layers you have to deal with. The
first two are fixed and more or less cannot be changed (the CPU material
and substrate, followed by the heat sink material). The differential
temp across each of these components is directly proportional to the
heat transfer rate, and inversely proportional to its thermal
conductivity. So double heat transfer, double differential temp.
Ideally in a balanced system the heat transfer rate will be more or less
a constant, determined by the power load of the CPU in watts. This
means that the differential temp of the CPU case and substrate and heat
sink are more or less constants. You can probably read them from the
appropriate tech manuals.
Next is the variable part, the air temp. The average air temp
determines the average conductivity so you can look at the differential
temp of the outlet - inlet air, which is proportional to the heat
transfer rate, and inversely proportional to the mass flow rate of the air.
To reduce the temp of the CPU you must increase the mass flow rate such
that the differential temp of the CPU + heat sink plus the average temp
of the flowing air is equal to the temp that you want to maintain in the
CPU.
So if the CPU is at 140 degrees F, and the temp drop is 40 degrees F
(heat sink + CPU case and substrate), then the average air temp needs to
be below 100 degrees F in order to keep the CPU temp at or below 140
degrees F. If the inlet temp is 80 degrees F and the exhaust temp is
120 degrees F, this would work (in a properly designed system).
However, if the inlet temperature were raised to 90 degrees F, you would
have to exhaust the air at 110 degrees F to maintain the desired CPU
temperature. To accomplish this, the flow rate of air would have to
double because the differential temperature is now 20 degrees instead of
40 degrees. In addition, the fan would consume 8 times the power in
order to double the flow rate (an important factor for laptops) and
would generate additional heat, requiring (possibly) even MORE cooling
air to compensate.
Placing a peltier (or other cooling) device on the inlet would, in fact,
offset this by lowering the inlet temperature, which would then allow
for higher differential temperature of the air, and thereby prevent the
need for a larger fan. It is quite possible that the peltier device
would consume less power than the larger/faster fan (though I'm only
guessing at this point), but only in some configurations (in other
configurations, larger fans might be more efficient).
Basic heat transfer/fluid flow stuff, at any rate.
