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Why not use Static Pressure fans for ALL case fans? - Page 2

post #11 of 15
Air flow fans do work good as exhaust. If you have good positive pressure they can work very nicely. I do use higher static pressure for intakes because all mine are filtered and most regular case fans just can't move much through a filter.
Edited by gdubc - 9/22/13 at 9:02am
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post #12 of 15
Really when the smoke clears it's a combination of static pressure and amount of air moved.
Problem is:
* 2.2mm H2O rating is the highest point water is pushed up the tube when the fan stops flowing air.
* 80cfm is the amount of air the fan will flow setting on an open bench... or possibly suspended so the surface of bench doesn't lower it's cfm rating.

Neither rating has any real world use by itself. We need to know how much air the fan will move at say... 0.4-1.1mm H20... and let's guess that is 40-60cfm. Problem is that is nothing but a guess.

Here are 2 fans:
* Cougar Vortex PWM = 2.2mm H20 & 70.5cfm
* Gentle Typhoon AP-15 = 2.03mm H2O & 58cfm
Now we could assume that if at 1.02mm H2O the Cougar would flows about 37cfm the AP-15 = 28cfm . Cougar being 9cfm better than AP-15
When really
The plotted data shows Couger flows about 34cfm while AP-15 = 42cfm . AP-15 is 8cfm better than Cougar.

http://martinsliquidlab.org/2013/02/18/why-static-pressure-max-flow-specs-are-poor-measures-of-fan-performance/

So don't rely on fan specs. biggrin.gif

Find a good review that tests them on a cooler or radiator.

Because in reality most case intake vents are about as restrictive as coolers.. with grills, doors, HDD cages, cables, etc. After all a simple wire grille will restrict 2-28% depending on fan and who did the testing.
http://www.pugetsystems.com/labs/articles/Effects-of-Grill-Patterns-on-Fan-Performance-Noise-107/
http://www.silverstonetek.com/techtalk_cont.php?tid=wh_chessis&area=en
post #13 of 15
SCIENCE
Dynamic Pressure = Total Pressure - Static Pressure

(via Bernoulli equation)

Dynamic pressure "Q" in martinsliquidlab graph is proportional to air speed "v" (sometimes "u" in aerodynamics) (i.e. q = 1/2 * rho * v^2 , where rho =density of air)

Total pressure aka stagnation pressure is pressure when the fluid stops moving.

This phenomenon is known as the Bernoulli effect. (Velocity inversely proportional to static pressure)


SHORT ANSWER

In short: There's no point in giving up airflow for static pressure if you don't need to , but keep in mind grilles, HDD cages, and filters are flow restrictions.

doyll gave you a good answer, static pressure and airflow numbers are meaningless by themselves

martinsliquidlab piece doyll is referring to: http://martinsliquidlab.org/2013/02/18/why-static-pressure-max-flow-specs-are-poor-measures-of-fan-performance/

Hope that helps smile.gif

P.S. Air is compressible too but is approximated as incompressible below Mach number of 0.3. Air density also depends on temperature (so mass flow changes but the volume flow "CFM" stays the same).

P.S. #2 I think ASHRAE changed their fan specification method to total pressure.
Edited by AlphaC - 9/22/13 at 11:04am
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post #14 of 15
the need for static pressure is directly related to the amount of resistance the air is expected to overcome. with a free flowing environment there is little need for static pressure. however if you need to overcome airflow resistance such as heatsinks, grills, filters, radiators, etc... that's when you need increased static pressure. the higher the resistance to airflow, the higher the need for static pressure.

from an acoustic point of view, a "push" fan produces a quieter acoustic signature then a pull fan. however sometimes a "pull" fan is required to get the job done (such as the fan behind an air filter )
post #15 of 15
Indeed. Good input. thumb.gif

OP:
I do use static pressure fans for ALL applications. thumb.gif

Bottom line: It's better to have it and not need it than need it and not have it.
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