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What is the true for fans situation?

Do all fans take away air from case?

Or can there be one with take in function, e.g. in front?

My installation looks like:

           Two Top coolers (OUT)
                           |   |
              -------------|---|----
              |\           v   v    \
              | \          O   O     \
              | |\____________________\
      cdrom-> |\  |                    | 
              |\\ |        CPU Fan->   O <---- System Cooler (OUT)
              | \\|                    |
              |  \|          O    O    |
              |\  |           ^---^----------- Two side coolers (IN/OUT)
              | \ |                    |
      front ->| O\|                    |
      cooler   \  |                    |
       (IN)     \ |                    O <--- Power supply 
                 \|____________________|      with embedded cooler (OUT)

Am I right that have put two fans, front fan and one side fan to (IN) mode?

Or must all fans take air away?

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  • all your fans should be input except the top and back. you want the output fans to blow the hot air out, not the room temperature air, so by evacuating heat from the top (as it rises) and the back where the CPU exhaust goes, and feeding air in from the opposite ends is a winning strategy. Nice graphic btw. Jul 17 '14 at 15:48
  • I'm just thinking that all fans must be output. Like power supply's fan by default everywhere is output. In this case, all fans move heats out, by default inside the tower heat. This heat must be taken away anyhow. From the physics sight, less air - less corpuscles, less heat. Jul 17 '14 at 15:56
  • but you are neglecting the mechanism by which the heat is moved: air. air at room temperature is draw into the box, where it is heated, and then blown out, leaving the system with less heat. if I'm understanding your use of the term Corpuscle, you are thinking on way way too small a scale with your physics. think simple thermodynamics and Bernoulli's principal. inside the box, heat migrates to the matter with the lowest energy state (the room temp air), and is distributed away from the sensitive components. moving air also has some effect from the same mechanism that makes sublimimation work. Jul 17 '14 at 16:23
  • @testaccount Fans don't move heat. Fans move air. Heat is transferred away by moving air (this is "convection") as long as the cooler air passes over the hot component and then out of the machine. If all fans are blowing out then you better also have some large open vents to pull sufficient air in and across the hot components, otherwise you're just generally lowering air pressure in the case with minimal heat transfer, and making the fans work hard to keep the air out. If you're not pulling air across the hot components, you're not doing much.
    – Jason C
    Jul 17 '14 at 16:24
  • Air however moves like any fluid, so Bernoulli describes how best to move air in, and out of the system. the output fans cannot move more air than is coming in, and the input fans can't push more air in that is blowing out. there is some slight variance from pressure differentials, but its the same principal as putting water through a pipe. the faster the air enters and leaves the system, the more cooling you can achieve by leveraging the temperature difference between the room temp and the component temp. remember you can never cool to below ambient (room) temp, without active refrigeration. Jul 17 '14 at 16:28
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Generally, front and side fans are intake (move air in), and top and rear fans are outtake (move air out).

Edit: a picture (not mine) for visual learners airflow

3
  • Are you can put kind of source for this subject? Jul 17 '14 at 15:53
  • @testaccount Source is common sense. Create as much airflow as possible over the hot components, because your goal is to carry away heat by convection. Personally I go in the front and out the back because the back tends to collect more dust and be harder to clean, and the back is usually in a tighter area that may not provide as much fresh, cool air as the front (and if your power supply is venting out the back you dont want to suck that back in), but it doesn't matter and depends on your situation. The real thing is to just move cooler air across the hot components however you can.
    – Jason C
    Jul 17 '14 at 16:18
  • 1
    @testaccount Everyone has a different opinion on this, but the fact of the matter is, the case manufacturers themselves mount the fans this way.
    – Jason
    Jul 17 '14 at 16:18
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It's called 'air flow' for a reason... air comes in cool at one end & goes out warm at the other. It needs to pass effectively across all hot surfaces & take as much of that heat as it can along with it.

Hot air rises, so it's sensible to put inlets at the bottom & outlets at the top. Convention dictates that the front is 'cold' & the back is 'hot' - perhaps initially even for such simple reason as to not blow hot air at the user.

In a perfectly-sealed box, having all fans extracting would provide no flow, only a pressure-drop, as good as the fans were capable of maintaining - putting undue stress on them. Hot air would have no-where to go & no cold air could be brought inside the unit to replace it.

In the diagram in the question, it would make most sense to have front & side as input, back & top as output; which would give 3 fans doing each task - nicely balanced, plus the PSU fan, which isn't switchable.

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