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Static Pressure - Simple question.

post #1 of 8
Thread Starter 
Ok... so I am in the market for new fans. I have an accumulation of all sorts of fans I have had throughout the years. I mainly bought on CFM's moved and dba's heard. I never gave static pressure a thought. I am now though. I call this the "the holy trinity of fans".

Question: Static pressure tends to be HIGHER at slower fan speeds in general, right?
post #2 of 8
Not in general...blade shape has a lot to do with it.
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post #3 of 8
Static pressure as in CFM? Fans don't build pressure as they are not in closed systems.

As far as rotational speed goes, the size of the fan comes into account too

3000 rpm 80mm > 2000 rpm 80mm
3000 rpm 80mm < 2000 rpm 120mm

Pushing air has more to do with fan shape and size than anything else really
post #4 of 8
Static pressure is the force the fan applies onto the surface the air is directed to.

Generally speaking, the static pressure only matters when moving air thru a heatsink or a radiator, since the more force it applies, the deeper the air can reach, and the faster it will reach.

If the fan is just for ventilation, then you only need CFMs, since you don't care how strong the air column is, but you do care on how much air is moved.

And no, more pressure doesn't mean less speed. What gives the pressure is the pitch of the fan blades, in other words, the thicker the fan, the higher the pressure. Some blade designs, like Scythe's Gentle Typhoon outperform by far other fans of its size in pressure, because the GT's blade design picks air from the center, compresses it pretty hard, and then sends it to the blade tips, which increase its speed. That makes a pretty high static pressure for a common 25mm fan.

The Ultra Kazes, also from Scythe, have a very high static pressure value because they pick air and compress it all along the blade, which has got a very high pitch grade. It is also thicker than normal fans (38mm).

And generally, when fans are placed very close to their target equipment (such as fans in a radiator), what determines their performance is the tip stall spressure value. Yep, more or less like a propeller in an airplane.
If your fan moves shizloads of air, but it stalls on high pressures, it won't be any good for a radiator, since once the blade stalls, the fan moves zero air. And if the tip has stalled, the whole blade will stall unless the center of the fan still picks air, in that case more air will be sent to the tips and therefore the fan will not stall.

Returning to the Ultra Kazes, those ones are very good for radiators (also VERY loud) because they need massive pressures for their tips to stall, basically because the motor has got craploads of torque, and because the blades are thick and short (less friction ergo less power wasted in moving air from center to blade tips).


In conclusion:
-Heatsinks: High pressure average air volume.
-Case fans: Low pressure high air volume.

Remember that higher the pressure higher the noise, so you have to find the balance between those.
   
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post #5 of 8
Thread Starter 
Thanks guys... Reason I ask is because I came across an "explanation" of "The Fan Curve" and this is what is making me think here. The explanatory graph is as follows:



This is a "general" application graph of course and the reason for my question. My source is here.
post #6 of 8
Yeah, mostly the limit relies in the friction of the fan. There's a point where the motor can't spin faster due to blade friction with the air.

In that case, moving to a more powerful motor is the solution. That's why Ultra Kazes and Deltas move so farst, because their motors are immense
Edited by Artikbot - 1/24/11 at 3:06am
   
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post #7 of 8
Thread Starter 
My comments in bold/blue within quote. Thanks Artikbot... + rep to ya!

Quote:
Originally Posted by Artikbot View Post
Static pressure is the force the fan applies onto the surface the air is directed to.

Generally speaking, the static pressure only matters when moving air thru a heatsink or a radiator, since the more force it applies, the deeper the air can reach, and the faster it will reach.

If the fan is just for ventilation, then you only need CFMs, since you don't care how strong the air column is, but you do care on how much air is moved.

Ok... On the above, this was part of my thought process but needed confirmation.

And no, more pressure doesn't mean less speed. What gives the pressure is the pitch of the fan blades, in other words, the thicker the fan, the higher the pressure. Some blade designs, like Scythe's Gentle Typhoon outperform by far other fans of its size in pressure, because the GT's blade design picks air from the center, compresses it pretty hard, and then sends it to the blade tips, which increase its speed. That makes a pretty high static pressure for a common 25mm fan.

Ok... this helps me understand the why of higher pricing on better SP'ed fans. Better engineering basically.

The Ultra Kazes, also from Scythe, have a very high static pressure value because they pick air and compress it all along the blade, which has got a very high pitch grade. It is also thicker than normal fans (38mm).

And generally, when fans are placed very close to their target equipment (such as fans in a radiator), what determines their performance is the tip stall spressure value. Yep, more or less like a propeller in an airplane.
If your fan moves shizloads of air, but it stalls on high pressures, it won't be any good for a radiator, since once the blade stalls, the fan moves zero air. And if the tip has stalled, the whole blade will stall unless the center of the fan still picks air, in that case more air will be sent to the tips and therefore the fan will not stall.

Let me see if I understand this correctly then... a low static pressured fan would have it's "airflow" bounced back at it by a radiator negating the initial flow of air causing the fan to stall? A high static pressured fan would just push through (not at 100% obviously) the radiator fins.

Returning to the Ultra Kazes, those ones are very good for radiators (also VERY loud) because they need massive pressures for their tips to stall, basically because the motor has got craploads of torque, and because the blades are thick and short (less friction ergo less power wasted in moving air from center to blade tips).


In conclusion:
-Heatsinks: High pressure average air volume.
-Case fans: Low pressure high air volume.

This seems to sum it up for me basically... Thanks.

Remember that higher the pressure higher the noise, so you have to find the balance between those.
post #8 of 8
Basically yes.
If the fan doesn't have enough pressure the air makes a chamber between the radiator and the blades, and since the fan can't compress the air enough for it to cross the radiator fins, it stays in surface of the radiator (making some weird unwanted turbulences) and stalls the fan blades.

And stalled fan blades means no airflow, so you get the idea
   
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