sammorris
kardson

Well, Sam, you'll get this in the morning then. If Travis wakes up from the anaesthetic, I wonder also what his thoughts would be.

Here's a thought: The 120 is 3/2 the size of the 80. Assuming same rpms, I would expect the 120 to move cfm per ratio of the surface area of the fan, which I think would correspond roughly to the square of the blade size. (I might be on the low side - are the 120s also deeper fans, in addition to having a larger diameter? Just going by diameter, this gives us 3/2 x 3/2 = 9/4 larger, for 9/4 more cfm.

Is that how they work in the real world Sam?

But in this case, 1/3 of that area is blocked. So 2/3 x 9/4 gives me 6/4 as much surface area exhausting air out the vent (into the screen as you say regarding the fan and square vent pictures.)

...

Now, if I am not far off so far, I realize that I haven't taken into consideration the fact that the part that is blocked is the part of the fan farthest from the center, which is doing the most pushing. So the blockage cuts into the superiority of the 120, giving me a ratio of cfm, from the two proposed solutions, more like 5.5/4, or even 5/4. This would go up if it is true that 120s are deeper, as well as wider = more blade surface creating more airflow.

In any event, I believe I will get more cfm from a partially blocked 120mm, than from the completely unblocked 80mm. Would you agree?

Rich

This message has been edited since posting. Last time this message was edited on 14. September 2007 @ 19:46

One last thought to unblock the 120 a bit, (other than drilling out my side cover.) What if I were to get several case fan gaskets (thanks Travis for the proper name) and mount the 120 maybe 1/2" off the surface of the side cover.

Would that help, or hurt?

By backing the 120 off the surface of the side cover in this way, would that allow the blocked air more room to collect and get sucked into the air stream of the rest of the air, reducing the blockage?

Or, conversely, would this actually allow the the turbulence from the blockage to propagate further, disrupting the laminar airflow of the unblocked portion even more, for an overall decrease in cfm?

I was even wondering, if I were to back off the 120 a bit, would that give me room to tape a piece of stiff paper to the blocked area, angling from the top of the opening across the top of the fan gaskets, before I adhere the fan, creating a ramp or duct funneling smoothly into the rectangle?

Here's a sideways view of my proposed "funnel":

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How many fan gaskets do I need for 1/2" of pushoff? Any opinions?

This message has been edited since posting. Last time this message was edited on 14. September 2007 @ 19:47

Right.

Of course, mate, I was following you. That's why I said with a 1600 rpm scythe, even with the blockage, then I'm twice the cfm of the 80mm. But that's $30 extra - oh I'll probably get it eventually - I'll have two 120mm holes in my case by the time I'm done, and only one fan so far.

In terms of getting rid of the blockage and squaring out the vent hole, it shouldn't be a problem, I just have to raise the height of the vent by 40mm. In fact, I don't have to cut it out, I can just drill out a bunch of small holes. Look at this nifty grill cover I just found from Travis' old link:
...

That grill will hide any unsightly holes - and then I'll just add one case fan gasket for vibration suppression. Isn't that a beauty!
rich

This message has been edited since posting. Last time this message was edited on 15. September 2007 @ 10:11

Afterdawn Addict // Silent PC enthusiast // PC Build advisor // LANGamer Alias:Ratmanscoop
PC Specs page -- http://my.afterdawn.com/sammorris/blog_entry.cfm/11247
updated 10-Dec-13

This message has been edited since posting. Last time this message was edited on 15. September 2007 @ 10:19

Thanks for the link,

http://www.frozencpu.com/products/2636/fgc-02/120mm_Black_Fan_Grills.html

Yeah, I looked at those a time back. The price is right at $3, versus $13. However, If I try to go circular, I'm going to have unsightly little square vent holes coming out from the side of that grill, right? For $10 I'd be blowing my bling!

Travis would not be pleased. So I'll probably get the square, eh what?

Rich
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This message has been edited since posting. Last time this message was edited on 15. September 2007 @ 10:30