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Originally Posted by shakran
But it can still leave slower than it can come in. Look at a soaker hose to get an idea of how this works. You can open up the faucet, and the water comes in quickly until the hose is full. Then there's lots of pressure on the intake side but, even though the soaker hose is not water tight, it still comes out slowly. Now, add a heat source to the soaker hose (leave it out in the sun) and wait about half an hour. Check the water coming out of the soaker holes. It'll be warm, even though the water going in is cool.
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I'm not sure what you're trying to say with the "soaker hose" analogy nor am I sure if you understand what I'm saying (or not saying).
So, lets clarify this much. Do you believe that the same amount of water that you push into a (filled) soaker hose will be pushed out?
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It works the same way for a computer case. If you put, say 300CFM worth of fans in the front, but the case can only evacuate 75CFM out the back, then you'll get higher air pressure inside the case than you normally would. It'll eventually equalize, but at a higher pressure than when it's off. And that extra air will just heat up instead of removing heat from the case. That's why you want a fan or two in the back - to speed up the egress of air from the case.
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I'm getting the sense that there might be some inkling of understanding when you admit that the pressure will "equalize," so lets run with that.
I think your characterization of "eventully" when you refer to the equalization of pressure is dishonest in that the pressure will equalize in a fraction of a second. Even when you have intake fans pushing air into the case faster than the outtake fans are pushing it out, while there is a positive pressure differential, the pressure difference will always remain minimal because the computer case is so porous... It will even be pushed out the outtake fan valve faster than the outtake fan can push it!
Now, it's probably important to note that I have, yet, to make any statement on the thermal propterties of such a system, if that's what you're trying to debate. The only thing I've discussed, so far, is the volume of air in the case and how it must remain (relatively) constant.
Outtake fans are useful (in addition to intake fans) if you want to
shape the air flow. So, if you want air to be pushed out a specific valve (rather than just any and all pores), then an outtake fan will help do that but, make no mistake, (and this was my only point) that air will be pushed out whether you have an outtake fan or not!