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120mm fan question

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January 15, 2011 9:35:58 PM

Hi everyone,

I am planning to buy several case fans or cpu fans (all 120mm). But want I want to know is WHAT static pressure is and WHY it helps? Like the Noctua NF P12 doesnt have very high CFM but it cools so well (because of static pressure I assume) !!
Thank you

Olivier

More about : 120mm fan question

a b à CPUs
January 16, 2011 1:51:25 AM

The following is just an educated guess based on the few physics notions I have so I could be very wrong :p 

First of all, pressure itself can only help cooling in one way: when a gas goes from a high pressure environment to a lower pressure one, it absorbs heat. Could be what is happening, but increasing the pressure in the first place would, reversely, release heat so not much benefit when both pressure change happen next to each other (one side of the fan to the other).

On a thermodynamic point of view, the speed of the heat transfer is related to the difference in temperature. This means that cold air actually cools faster, not only more. So as soon as air get hotter, you want it out of the HSF so you can replace it with cooler air. That is why high CFM usually mean better cooling.

Why then would a lower CFM fan cool better than a higher CFM one and can pressure have an indirect impact? CFM is most probably measured in ideal circumstances, when the fan is blowing freely in standard atmospheric pressure, but then blowing against a HSF, it meets resistance. How much air a fan is actually able to push through a HSF could actually be a function of the air pressure. So one fan with higher rated CFM might not be able to push as much air through a HSF as the one with lower rated CFM but with higher static pressure.

As I said, this is just a guess.
January 16, 2011 8:10:06 AM

Zenthar said:
The following is just an educated guess based on the few physics notions I have so I could be very wrong :p 

First of all, pressure itself can only help cooling in one way: when a gas goes from a high pressure environment to a lower pressure one, it absorbs heat. Could be what is happening, but increasing the pressure in the first place would, reversely, release heat so not much benefit when both pressure change happen next to each other (one side of the fan to the other).

On a thermodynamic point of view, the speed of the heat transfer is related to the difference in temperature. This means that cold air actually cools faster, not only more. So as soon as air get hotter, you want it out of the HSF so you can replace it with cooler air. That is why high CFM usually mean better cooling.

Why then would a lower CFM fan cool better than a higher CFM one and can pressure have an indirect impact? CFM is most probably measured in ideal circumstances, when the fan is blowing freely in standard atmospheric pressure, but then blowing against a HSF, it meets resistance. How much air a fan is actually able to push through a HSF could actually be a function of the air pressure. So one fan with higher rated CFM might not be able to push as much air through a HSF as the one with lower rated CFM but with higher static pressure.

As I said, this is just a guess.


Tanks anyway :) 
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a c 229 à CPUs
January 16, 2011 3:51:54 PM

Let me start off with an analogy that may be more familiar..... when ya pump water through a pipe or a hose at the same elevation, there is friction loss to be overcome. Having 60 psi at a pump outlet might be 57 psi at the end of the hose / pipe dur to friction loss.

Same thing with a fan which is just an "air pump". In an intake fan, the air is drawn thru a grille, forced into an fan duct, past blades and into the case .... on an exhaust fan, same thing but in reverse direction. Each of those things results in friction loss. The static pressure provided by the fan, must be enough to overcome the friction loss plus any elevated pressure within the case.

A fan, like a water pump has what is called a performance curve. You might get the following points on a curve:

98 cfm @ 0.0" SP
85 cfm @ 0.1" SP
75 cfm @ 0.2" SP
35 cfm @ 0.3" SP
10 cfm @ 0.4" SP

A 2nd fan has a curve:

75 cfm @ 0.0" SP
70 cfm @ 0.1" SP
65 cfm @ 0.2" SP
60 cfm @ 0.3" SP
55 cfm @ 0.4" SP

The fans above get their labels designed by marketing types so the labels will say 1st fan is a 98cfm fan and the 2nd is a 75 cfm fan. Which one cools better ?

For case fan usage where you might need a static pressure of 0.1 , the 1st fan is the better choice @ 85 cfm (as compared to 2nd fan's 70 cfm). However the roles reverse say for a CPU heat sink fan where greater pressure is needed to push air thru the closely spaced fins requiring 0.3" static pressure to overcome. In this case the 2nd fan produces 60 cfm to the others 35.

More about fans here:

http://www.silentpcreview.com/article63-page2.html

Note the Noctua NF series review:

Quote:
A very unusual fan from an Austrian company posing as a research institute. Unusual blade design produces more airflow than usual, but possibly at the cost of reduced pressure. Reports from real users have suggested that, despite measuring higher, actual cooling power is slightly less than it should be. Noise character was very broadband and became inaudible at a relatively high voltage. However, it lacks the buttery smoothness of some other fans. An effective choice for a case fan, but the questions raised about its pressure make us hesitate to recommend it for use on a heatsink.


A more engineering based explanation here:

http://www.esmagazine.com/Articles/Feature_Article/51e3...
January 16, 2011 5:27:41 PM

JackNaylorPE said:
Let me start off with an analogy that may be more familiar..... when ya pump water through a pipe or a hose at the same elevation, there is friction loss to be overcome. Having 60 psi at a pump outlet might be 57 psi at the end of the hose / pipe dur to friction loss.

Same thing with a fan which is just an "air pump". In an intake fan, the air is drawn thru a grille, forced into an fan duct, past blades and into the case .... on an exhaust fan, same thing but in reverse direction. Each of those things results in friction loss. The static pressure provided by the fan, must be enough to overcome the friction loss plus any elevated pressure within the case.

A fan, like a water pump has what is called a performance curve. You might get the following points on a curve:

98 cfm @ 0.0" SP
85 cfm @ 0.1" SP
75 cfm @ 0.2" SP
35 cfm @ 0.3" SP
10 cfm @ 0.4" SP

A 2nd fan has a curve:

75 cfm @ 0.0" SP
70 cfm @ 0.1" SP
65 cfm @ 0.2" SP
60 cfm @ 0.3" SP
55 cfm @ 0.4" SP

The fans above get their labels designed by marketing types so the labels will say 1st fan is a 98cfm fan and the 2nd is a 75 cfm fan. Which one cools better ?

For case fan usage where you might need a static pressure of 0.1 , the 1st fan is the better choice @ 85 cfm (as compared to 2nd fan's 70 cfm). However the roles reverse say for a CPU heat sink fan where greater pressure is needed to push air thru the closely spaced fins requiring 0.3" static pressure to overcome. In this case the 2nd fan produces 60 cfm to the others 35.

More about fans here:

http://www.silentpcreview.com/article63-page2.html

Note the Noctua NF series review:

Quote:
A very unusual fan from an Austrian company posing as a research institute. Unusual blade design produces more airflow than usual, but possibly at the cost of reduced pressure. Reports from real users have suggested that, despite measuring higher, actual cooling power is slightly less than it should be. Noise character was very broadband and became inaudible at a relatively high voltage. However, it lacks the buttery smoothness of some other fans. An effective choice for a case fan, but the questions raised about its pressure make us hesitate to recommend it for use on a heatsink.


A more engineering based explanation here:

http://www.esmagazine.com/Articles/Feature_Article/51e3...


aha! :)  thanks for the helpful answer.. I am currently using 2x coolermaster sickleflow fans on my hyper 212 plus. It says: static pressure- 3.04mm? Is this good for cpu cooling? I dont want to sped too much on a Noctua fan, so if the 2 sickleflows are better than the one noctua for half the price, well yeh :p  I am currently getting 2x Nanoxia fx12-2000 fans and 2x Xigmatek XLF f1253 fans! I will use these as case fans, or are they better for cpu fan? Sorry for all the questions!
!