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140mm intake in 120mm hole

post #1 of 5
Thread Starter 
I am still on a quest for more CFM without an increase in dBA. I know the Thermalright TY-140 has an excellent noise/flow ratio, but I'm concerned about the 120mm mounting.

As an exhaust, the characteristics of fans in general would suggest that performance would be hindered due to the chassis blocking airflow around the edges of the blades. However, as an intake, I don't know whether the air is being sucked in primarily around the inside of the fan blades (close to the hub) or not. I'm guessing the inside of the blades cut into the air and fling it out to the edges, but I haven't found any information to support this.

Essentially, would I see any drop in performance when using this fan as an intake? Does my theory for an exhaust hold up? Any info would be greatly appreciated.
Edited by pocketdrummer - 1/24/11 at 7:09pm
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post #2 of 5
Hrm, if you have the space, then something like this but of the right size to adapt a 120mm fan hole to a 140mm fan would be best. A lot of the pressure is generated near the blade tips.
I'd suggest you'd do better in the end to go hunt yourself out some gentle typhoons, the 1450rpm fans push a LOT of air and do so very quietly.
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post #3 of 5
Thread Starter 
I know there are a lot of good 120mm options, but the Thermalright TY-140 has some impressive numbers. Unfortunately, it's mounting holes are 120mm only. As an intake, it would be impossible to use any sort of shroud.

I guess I'm just interested in the physics. Would a 140mm fan placed against a 120mm hole have any issues that would negate the point of using a larger fan?

On a side note, if it's pointless, which of the quiet 120mm fans works best under high restriction (ie, going through the case grille and fan filter)? I know Noctua has a good amount of pressure. I'm not sure about the others, though.
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post #4 of 5
Quote:
Originally Posted by pocketdrummer View Post
I know there are a lot of good 120mm options, but the Thermalright TY-140 has some impressive numbers. Unfortunately, it's mounting holes are 120mm only. As an intake, it would be impossible to use any sort of shroud.

I guess I'm just interested in the physics. Would a 140mm fan placed against a 120mm hole have any issues that would negate the point of using a larger fan?

On a side note, if it's pointless, which of the quiet 120mm fans works best under high restriction (ie, going through the case grille and fan filter)? I know Noctua has a good amount of pressure. I'm not sure about the others, though.
short answer: Yes.

longer answer:

fan pulling air and pushing it through 120mm hole
the best way to explain it is that the area of the blades is what effectively moves the air. By placing a larger diameter circle against a smaller one, you lose area where the air is effectively moved. This will cause 2 effects: less area for the air to flow through = less airflow, but also, more air being pushed into a smaller hole will result in higher pressure. Think of it like a 2 year old playing with blocks, trying to push the slightly larger peg through the hole might be possible, but only with a lot of force^^. Some fans might wear faster if you do this, depending on the quality of the bearings, I wouldn't do it on a sleeve bearing fan.

fan pulling air through 120mm hole:
worse airflow, worse pressure, more wear, wouldn't do this.
    
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post #5 of 5
Thread Starter 
Quote:
Originally Posted by ascaii View Post
short answer: Yes.

longer answer:

fan pulling air and pushing it through 120mm hole
the best way to explain it is that the area of the blades is what effectively moves the air. By placing a larger diameter circle against a smaller one, you lose area where the air is effectively moved. This will cause 2 effects: less area for the air to flow through = less airflow, but also, more air being pushed into a smaller hole will result in higher pressure. Think of it like a 2 year old playing with blocks, trying to push the slightly larger peg through the hole might be possible, but only with a lot of force^^. Some fans might wear faster if you do this, depending on the quality of the bearings, I wouldn't do it on a sleeve bearing fan.

fan pulling air through 120mm hole:
worse airflow, worse pressure, more wear, wouldn't do this.
So, essentially the Thermalright TY-140's awesome design is wasted because there are only 120mm holes. Ugh, I really want to use it for my 140mm exhausts, but they fubar'd that one.
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