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Fan clearance - how much needed ?

post #1 of 10
Thread Starter 
Hi guys,

advice sought.....please

I am considering putting my bottom intake fans outside the case - under the floor & blowing up into the case.

How much higher than the desk should the bottom of the fan be ??

I know I will have to extend the case feet some but what is thought to be the minimum gap needed for fan to do it's job properly - not staving it of fresh air to blow in


If you see my build log here - you should understand why I am thinking of doing this

Fans in question are these;
Skythe slip-stream fans - Kaze-JYuni Slim 120mm @ 1600rmp

post #2 of 10
At least 20mm, IMO.
Don't bother with slim fans. You will most likely need filters and even a "airflow" type normal width is defeated by a filter
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Ayase Eli
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post #3 of 10
Thread Starter 
Quote:
Originally Posted by DaveLT View Post

At least 20mm, IMO.
Don't bother with slim fans. You will most likely need filters and even a "airflow" type normal width is defeated by a filter

I won't be using filters - will deal with dust, but don't want restriction.
Don't get me wrong the advice appreciated thumb.gif
BUT most cases have less than 10mm or so clearance undernieth, so how do you figure 20mm ??
Is that including the case feet OR are you advising 20 betewen bottom of fan and desk - making the case feet 30mm high ?
30mm is too much for my liking - that will look stupid I reckon.
20mm total height I can deal with (maybe) - which would be an 8mm gap, less would be better if I can.
enough ??
If not I will have to elongate the fan holes after all redface.gif
post #4 of 10
When i say 20mm of course i mean there's a gap of 20mm with the case feet
And now, slim fans also have a hard time getting past honeycomb fan holes.
Ayase Eli
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Ayase Eli
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post #5 of 10
The rule of thumb is at least a fan diameter clearance, less than that and we'll be choking the fan.



HAL-9011
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HAL-9011
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post #6 of 10
What TELVM says is true in a perfect world. But 1/2 the diameter of fan for clearance works very well.. even 1/3rd works way better than stock clearance does. I don't know of any case manufacturer that allows 1/4th fan diameter clearance.. It's usualy more like 1/6th or 1/7th the diameter.. rolleyes.gif

I try for the circumference of fan times clearance being the same a area as fan. IE 120mm fan has 113sq cm of area.. and a circumference of 37.7cm.. times 3cm clearance is 113sq cm of area to draw air.

Problem is if you have 2 120mm fans side by side you you loose 1/2 of each circumference and add 2 times 12cm to get the total circumference available to draw air.. meaning you have 226sq cm fan area and 61.7cm circumference 4cm clearance is 246.8sq cm of area to draw air.

And now you see half of the reason my builds almost always have castors under them. thumb.gif
Edited by doyll - 10/29/13 at 11:34am
post #7 of 10
Quote:
Originally Posted by doyll View Post

What TELVM says is true in a perfect world ...

I'm seriously afraid it's true in the real world biggrin.gif .

cute-winking-witch-in-black.jpg
HAL-9011
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HAL-9011
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post #8 of 10
In some of the real world but by no means all of it. biggrin.gif
post #9 of 10
Thread Starter 
Well thanks for the replys and info.
In myt real world I ended up with just under 10mm clearance - last pic makes it look less than it actually is.
I used an assorment of stacked case feet to raise it up.
The end result with maybe not ideal is best compromised situation for my circumstance.







post #10 of 10
226sq cm of fan area (201sq cm w/ motor hub area removed) with only 61.7sq cm of intake area. rolleyes.gif

That's like putting a 120mm fans on 60mm vents. applaud.gif

Hope it works to your satisfaction.
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