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Minimum CFM's for Heatsink & Case Fans?

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March 17, 2003 7:46:41 AM

Hello folks.

In order to keep an Athlon XP CPU based PC system within safe and cool enough temperature levels...

What would be the minimum CFM's pushed by the heatsink fan?

What would be the minimum CFM's pushed by the case fan?
(please specify exhaust and/or intake, and number of fans)


(If it helps to know, I am considering using an evidently very effective heatsink such as the Themalright SLK800 or Thermalright SLK900 with an Asus motherboard in a PC case such as an Evercase ECE4252 Mid Tower or an Antec SLK3700.)


Thanks much!
DuckTape
March 17, 2003 1:21:15 PM

simple rule...if you're not overclocking, use the stock heatsink

the cfm rating of fans are often not very accurate, and to get a better judgement, you want to look whether your using 60, 70, or 80 mm fans, and the rpms they run at.

Pretty much any decent case will have the area for one 80mm exhaust and intake fan, and most midi or full towers have up to 4-6 intake and exhaust fan locations. Any generic 80mm fan will do, doesn't have to push much or or be loud, just as long as they work. I'd go Coolermasters or Panflo's for 80mm fans, cheap and quiet, and generally just get the job done.

ONLY if you're overclocking would I consider buying such a large and expensive heatsink (which I do overclock ^^). But with the larger heatsinks, i'd look into one of these depending on budget, in ascending order

Alpha Pal 8045t
Thermalright Sk7
Swiftech MCX-462+
Thermalright SLK 900

you'll get good cooling even if you use a low rpm fan. The gains of using a higher rpm fan are much more apparent on the full copper ones (being the Sk7 and slk 900), but not anything near double performance (like going from 2500 to 5000 rpms).

Instead of Rdram, why not just merge 4 Sdram channels...
March 17, 2003 6:26:35 PM

well with an slk800 or slk900 it depends on your cpu, but you can usually get off using low rpm panaflo (24cfm) or the 30cfm version... nice and quiet ~20db and perfect for the application

<b>people are only idiots when they don't realize - when they do it just gets funnier, like a dog chasing its own tail, or like george bush's public address(es)</b>
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March 17, 2003 9:13:06 PM

About cooling...

The safety limit for AMD processors is 85c... you want a heatsink that can keep you as far below that limit as possible.

For a heatsink the CFM of the fan is not the imporatant spec. The best specification to look for is "Thermal Resistence" which is a measure of how much heat the heatsink/fan combination can handle. This is generally stated in "Degrees per watt", indicating how much the temperature of the heatsink (and thus the CPU) will rise for every watt of thermal energy. Obviously this needs to be a very small number, in fractions of a degree per watt to keep a furnace like the AMD CPU cool.

The real math is complex, but this will give you a working approximation:

<b>T == (P x TR) + A</b>

T = approx CPU temperature.
P = cpu power dissipation in watts.
TR = the thermal resistence of your heat sink.
A = ambient temperature (inside the case)

For example... 50 watt cPU, Heatsink TR = .5, Ambient = 25

T == (50 x .5) + 25 == 25 + 25 == 50c. <i>approximately</i>

In practice the final temperature is usually somewhat lower than this approximation indicates... which is a good thing.

The construction of the heatsink also has a bearing on it's eficiency. Believe it or not there is a good reason most look like little aluminum boxes with the fans sitting on top... that's what works.

On a CPU with a built in heat spreader, you really only need an aluminum heatsink to draw heat off the spreader and radiate it out into the air. Copper heatsinks are getting popular but, while they do absorbe heat better, it is far more difficult to get that heat back out and into the air. Plus copper is heavier than aluminum, more than twice as heavy, which puts extra strain on the mouting points. But, this suggests an excellent work around... Hybrids... AMD cpus do not include heat spreaders, like <i>everyone else's</i> cpus do... therefore you've got to find some way of replacing it. This is done by a hybrid construction, a copper slug in the bottom of an aluminum heatsink. The copper draws heat off the cpu and the aluminum cools the copper... It's a very successful combination.

The fan does play a part. Thermal Resistence is usually specified with the fan running at full speed. Reduce the speed of the fan more than a little bit and the TR starts climbing... so rather than worrying about the amount of air moved, you need to look at the heatsink and fan as a system, giving you a specific thermal resistence.

So, what am I suggesting?

For Athlon XP processors you need a hybrid heatsink with thermal resistence below .6

Here's an example:
<A HREF="http://www.spirecooler.com/asp/fcc.asp?ProdID=95" target="_new">http://www.spirecooler.com/asp/fcc.asp?ProdID=95&lt;/A>

This is a good cooler, I've used them, but I provide the picture mostly as an example... there are lots of others like it and better.

As for the case cooling question...
This isn't really about CFMs as much as it is about airflow management...
You want to draw cool air into the front of the case, circulate it through as much of the case as you can and then send the warm stuff out the back.

To do this, the intake area of the case must be sufficient that your exhaust fans (in the back) can draw air feely into the case; you need at least as much intake area as you have for exhaust. Too many exhaust fans and all you do is create a vaccuum in the case. Too many intake fans and you create pressure... neither of which is the result of good airflow. In extreme cases, all you end up doing is blowing the hot air around inside the case. The goal is always to move cool air in the front, circulate it through the case, let it pick up heat and the draw it out the back.

Taking a common sense approach, pays bigger benefits than high CFM fans or multiple fans. With good air intakes, on the average computer one standard 80mm, 2700 rpm exhaust fan will provide adequate cooling if you handle it right. (FWIW... that's about 25cfm of airflow)


Hope this helps....


--->It ain't better if it don't work<---
March 17, 2003 10:44:47 PM

Thanks for the informative replies, folks!

I have another couple related questions, if anyone would be so kind as to offer their thoughts and experiences...


First -- standard case fans and heatsink fans are typically 80mm, correct?


Now, for Athlon XP CPU based PC systems...

What are your different experiences with what you have found to be typical cfm ratings for standard, commonly used stock heatsink fans? (25, 30, 35, 40 cfm's?)

How about for case fans? (25, 30, 35, 40 cfm's? for only one exhaust fan, or for both an intake and an exhaust fan?)

Thanks again!
DuckTape
March 18, 2003 3:49:37 AM

Cooler fans are mostly 60 or 70mm fans in the 10 to 20 cfm range spinning anywhere from 3300 to 6000 rpm.

Case fans are almost always 80mm, 25 cfm units spinning at 2800rpm. The higher CFM fans also spin faster, some hit 4800rpm, and make a LOT more noise.





--->It ain't better if it don't work<---
!