CFM vs Static Pressure

[Guest]

CFM vs Static Pressure

I'm just not sure. I thought that cfm and static pressure are directly proportional so that higher CFM always means better static pressure. But when looking at reviews for water cooling, there seems to be a more significant difference between cfm and static pressure. Can anybody clarify for me?

 

[iam2thecrowe]

its not necessarily true that they are proportional. For a basic example, a high RPM smaller radius fan pushing the same CFM as a fan twice the size of the same design will likely have completely different static pressure. The smaller fan would likely perform better in a higher pressure environment as it is producing more pressure initially, forcing more air through a smaller opening. But there are other factors such as the power of the motor, fan type, blade angles/curves etc, etc.

 

[RJR]

Check FPI of radiator,
denser fins = use higher pressure fans
lower fin density = concentrate on cfm

 

[Guest]

Ok. But between 120mm fans, what will determine cfm and static pressure?
My scythe fan had 1800rpm and 100+ cfm but reviews said it had low static pressure.
The gentle typhoon fans have 3000rpm and lower cfm but reviews praise it for its high static pressure.
The difference in design i see is that the typhoon has more but shallower blades while scythe has fewer but steeper blades.

So does higher rpm, shallower blades, or more blades usually give higher static pressure?

 

[Guest]

Check FPI of radiator,
denser fins = use higher pressure fans
lower fin density = concentrate on cfm

I just saw that.
So by denser fins do you mean more fins?

 

[spiralsun1]

Ok. But between 120mm fans, what will determine cfm and static pressure?
My scythe fan had 1800rpm and 100+ cfm but reviews said it had low static pressure.
The gentle typhoon fans have 3000rpm and lower cfm but reviews praise it for its high static pressure.
The difference in design i see is that the typhoon has more but shallower blades while scythe has fewer but steeper blades.

So does higher rpm, shallower blades, or more blades usually give higher static pressure?

Had to comment on this old thread. This is the best answer I have seen. To get clarity on this issue, think of your beginning physics class and the nature of a screw -- which is a spiral plane or wedge. In aviation, they call propellers "airscrews" and this is good to remember -- a screw with many threads at low angle needs to turn a lot to move the same distance as a screw with fewer threads at a higher angle. the lower angle threads can have a much higher RPM to move the same distance -- and have MUCH more power to keep moving under resistance -- because everyone knows that a slope that is gradual is much easier to go up than a very steep one. Simple machines my friends. Therefore the slope and width of the fan blades counts as does RPM these are the two interacting variables. You would need a much more powerful motor to move steep fan blades through air at the same given RPM as more gradually-sloped fan blades. ALSO - and here is the KEY where pressure comes in -- lower angle, wide, fan blades moving faster would be able to produce MUCH more pressure with the same motor because of the relentless gradual slope screwing more slowly but with much greater force because each turn of the engine (engine torque and power) is concentrated in a smaller amount of air movement but a larger amount of potential pressure against restriction.