Two fans - same CFM, different static pressure - how is it possible?

[thefreeaccount]

It seems to me that in order to make air move, you must create a pressure void. Air will then move from the area of higher pressure to the area of lower pressure. If a difference in pressure makes air move, then it should follow that two fans with the same cfm rating should have exactly the same static pressure. Yet, they don't.

So, can anyone tell me why I am stupid? Thanks!

 

[KillerCharlie]

What do you mean by static pressure? Actual static pressure? Measured where? I think I'm confused.

 

[Red Squirrel]

If I understand correctly, static pressure is not something a fan "puts out", it's what it can handle. Static pressure increases as the air path is more obstructed, and air is overall harder to push. Ex: a fan just on in middle of nowhere has very little static pressure to worry about. A fan such as the blower in a furnace has tons of static pressure as it has to push air through all the vents. You can replace the blower in a furnace with a basic fan that has same CFM rating but if it's not strong it wont be able to deal with the static pressure so it wont actually be putting out the rated CFM. So things like blade design, motor etc affect how much static pressure the fan can handle.

In the case of computer fans if you have lot of intakes and not enough exhaust holes then there is more static pressure, thus less air flow. More powerful fans might be able to overcome this and force air more through all the little gaps and such.

At least that's my understanding of it.

 

[Rubycon]

Fans are supposed to have at least two ratings - free air and under a specified static pressure. The computer cooling business is about as sneaky as speaker manufacturers when it comes to publishing of specification! Sure the CFM looks really good but that's under FREE AIR.

The STATIC airflow would vary considerably and the clear winners would stand out like a sore thumb! Not so fast! The final specification comes into play - operating noise! (since PC users are so picky about noise!)

 

[thefreeaccount]

Ok - I think I see what you guys are saying. When reviews talk about a certain fan having exactly twice the static pressure of another fan, what they are really saying is that if you put both fans in a chamber with a fixed sized hole for air exit, one fan will move exactly twice the amount of air as the other.

So, static pressure is unlike CFM in that you can't compare the static pressure measurement from different reviews, correct? Because it will depend on the size of the chamber and the size of the exit hole?

 

[canis]

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[canis]

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[canis]

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