Please explain the relationship b/t "static pressure" and "cfm" (blower vs. fan)

[MichaelD]

I know I've just been filling up Cases & Cooling lately...but hey, inquiring minds want to know!

I've been thinking about going with a blower for an exhaust fan...that would be something different.

Specifically, looking at this blower. Thinking of cutting out the rheostat/resistor and wiring it straight to a fanbus.

The specs say:

APPLICATION: PC system
DIMENSIONS: 80 x 80 x 70 mm
WEIGHT: 102 g
RATED VOLTAGE: 12 V
BEARING TYPE: Ball Bearing
INPUT CURRENT: 0.30 - 0.90 A
INPUT POWER: 3.6 - 10.8 W
RATED SPEED: 1,900 - 3,500 RPM
AIR FLOW: 10.9 - 20 cfm* (*Air Pressure: 2.9 - 10.7 mm H2O)
NOISE OUTPUT: 23 - 36 dBA
LIFE: 50,000 hrs


It's CFM is not that impressive at all, but it's max pressure of 10.7mm is much higher than say

this fan and it's rated at 48.5 CFM!! :Q

Can someone explain this to me? Is the blower a bad idea?

 

[WarCon]

Normal bladed fans will move a certain volume of air when there is no or low pressure gradient (alot of the moving air is doing so on a radial tangent because the air is moved cyclonically - from the spinning blades - but there isn't enough vacuum created to draw the air back to the center as in a tornado so pressure is lost to the air moving at an angle away from the center). Blowers create a much more aligned flow of air which can push a gradient (gives them a higher max pressure). The blades of the blower push air instead of using an airfoil to draw it from the high pressure side to the low pressure side. The pushed pockets of air are able to push on each other and can develop back pressure. The air from a fan starts to create a back pressure and that back pressure ruins the high / low pressure zones that airfoils (fan blades) use to move and they sit in the backwash and waffle and move significantly less air. You can increase the pitch on the blade to fight this problem somewhat, but as the pitch increases so does the noise. That is why the delta fans are so effective and so noisy, they have a very steep pitch to their blades.

A blower or high pitched fan can really help out more in a circumstance where the air from a fan would catch alot of backwash (like with pinned heatsinks like the Alpha's ).

It is perfectly ok to use a case fan for just moving air through your case as there isn't much chance for building a pressure gradient (cut out back fan grill/holes and use a wire one). A blower will not give any better results cfm versus cfm and are typically much noisier for the same cfm ratings.

Hope this helped.

 

[MichaelD]

Whoa.~:Q That was good! Lots of physics classes, you have taken. :beer:<--you with a glass full of algorithms and such.

I get what you're saying . Thanks a lot, it DOES help. I would've been using the blowers for case fans, but obviously that's not a really good idea. I guess no blowers, then. Now I just have to find medium-high speed case fans WITH rpm sensing. *searching high and low*