Cooling CPU with passive heatsink?

[spodosaurus]

Best to clarify I suppose, but I think the answer is fairly obvious.

He could've merged two thoughts: air over the CPU (beneath the heatsink)
and air through the heatsink. While *I* know (99% sure) what he meant, a
newbie might get confused.

 

[David Maynard]

Thanks for the advice. I'll certainly look at your site.

As I see it I am not proposing fanless cooling for the CPU, but rather a
setup where the fan is not mounted directly on top of the heat sink. That
way, as you say, it can be bigger and run slower. The duct is to connect the
fan to the heat sink, in effect. Also, the remote fan will dump the warmed
air outside the cas, rahter than just circulate it inside, and also double
up as general ventialtion device for the whole PC.

I think this whole issue is not that simple, and needs a bit of thinking
about, including doing a few calculations, as w_tom suggested.
Plan A is to get the PC built and running using the supplied heatsink fan
from AMD. Plan B will then be to look at the ducting etc. as discussed here.

Interestingly, my Dell started to acclerate it's fan a couple of times the
other day, making me think about that.
I'm tempted to see what I can do there to improve cooling. Initial thoughts
are to...
1) lap the heatsink and put it back on with good thermal compund (e.g.
'Arctic Silver')

Assuming the fan's speed is adjusted by a thermal sensor at the fan's
intake, improving the CPU/heatsink thermal interface may improve the CPU
temperature a bit but it won't alter when the ducted fan accelerates
because the overall CPU watts is not altered. It would simply improve the
heatsink's transfer of heat to the air a bit, which would get hot, which
causes the fan to speed up.

2) make an extra vents in the front of the case (this is a Dell Dimension
8300, and the new 8400 has the same case design except for an additional
vent in the front for "improved cooling" - admittedly for faster 3.4GHz
CPUs)

Does the 8400 have exactly the same duct and fan?

The thing to keep in mind is that 'extra' vents will alter the internal
airflow. As an extreme, but illustrative, example imagine you remove the CD
drives and front panel covers on the top 5 1/4 inch drive bays so they
become a 'big vent'. Virtually all air pulled in by the rear fan will come
through that large upper 'vent' with very little induced through the bottom
intake, leaving little airflow across the lower half of the motherboard,
PCI slots, and hard drive area.

 

[spodosaurus]

Sentence should have read "CPU heatsink". My mistake
properly noted and therefore corrected here.

^^^^^ ahaha I meant air...Ari is my first name hehe

 

[John Fryatt]

Interestingly, my Dell started to acclerate it's fan a couple of times
the

Assuming the fan's speed is adjusted by a thermal sensor at the fan's
intake, improving the CPU/heatsink thermal interface may improve the CPU
temperature a bit but it won't alter when the ducted fan accelerates
because the overall CPU watts is not altered. It would simply improve the
heatsink's transfer of heat to the air a bit, which would get hot, which
causes the fan to speed up.

Yes, I see your point. I'll have to find out where the sensor is. I was
assuming that the fan responded to the actual CPU's temperature, in which
case better heat trtansfer to the heatsink would delay acceleration of the
fan, yes? Assumptions are dangerous though - better to find out the facts.

Does the 8400 have exactly the same duct and fan?

I believe so, but I'd check before getting out the drill and saw. ;-)

The thing to keep in mind is that 'extra' vents will alter the internal
airflow. As an extreme, but illustrative, example imagine you remove the CD
drives and front panel covers on the top 5 1/4 inch drive bays so they
become a 'big vent'. Virtually all air pulled in by the rear fan will come
through that large upper 'vent' with very little induced through the bottom
intake, leaving little airflow across the lower half of the motherboard,
PCI slots, and hard drive area.

Absolutely. I remeber having a discussion some time ago about cooling and
someone thought cooling would be better with the case lid completely off.
That's pretty much always *not* the case though as the airflow is totally
messed up with the lid.

If I did make extra vents I'd probably be thinking in terms of making the
extra intake air flow over the hard disk(s). By default I'll do nothing
though, and only wield the tools once I feel happy I a good idea of what's
happening.

John

 

[David Maynard]

Yes, I see your point. I'll have to find out where the sensor is. I was
assuming that the fan responded to the actual CPU's temperature, in which
case better heat trtansfer to the heatsink would delay acceleration of the
fan, yes?

One would think so.

Assumptions are dangerous though - better to find out the facts.

You bet, which is why I made a point of what my assumption was. I guessing
as I've not dissected that model but self contained thermal control fans
are just so 'dime a dozen' cheap and easy. Slap it in: no 'special' BIOS,
wires to the heatsink, or anything else needed.

I believe so, but I'd check before getting out the drill and saw. ;-)




Absolutely. I remeber having a discussion some time ago about cooling and
someone thought cooling would be better with the case lid completely off.
That's pretty much always *not* the case though as the airflow is totally
messed up with the lid.

Yeah. That's true if the case depends on specific airflow paths but not
necessarily so with the swiss cheese, vent holes all over the place, models.

If I did make extra vents I'd probably be thinking in terms of making the
extra intake air flow over the hard disk(s).

That makes good sense and you could always 'fine tune' it with tape if needed.