Why don't we see more top fans used as intakes?

[fyleow]

On the majority of cases with either user installed or factory installed top fans they are typically used as exhausts. The most cited reason for this is because hot air rises so it makes sense to use it as an exhaust. While that may be true in natural convection once there is airflow in the case the effect of convection is minimal.

Silverstone is one of the few exceptions to this and they have repeatedly introduced cases which use an AP181 as a top intake. This arrangement has garnered good reviews and results even in less space constrained cases like the FT-01. In fact in their tech talk post a few years ago they specifically call out the arrangement of having both an exhaust at the rear and the top as being "poor".

http://www.silverstonetek.com/techtalk_cont.php?area=en&tid=wh_airflow

So why the discrepancy between Silverstone and other manufacturers and even enthusiasts? I don't see anything inherently different between most of Silverstone cases and other manufacturers that would explain why increased effectiveness with a top intake. It seems advantageous to use the top fan as an intake which can draw in cool air directly near the CPU area, and it also helps maintain positive pressure.

 

[dealmaster]

Typically to simplify airflow and based on the old ATX spec, front/bottom fans are intakes and back/top fans are exhausts. Nothing will prevent you from reversing that, but that's just the way most fans are set up in cases.

 

[timta2]

I say put them where you want, but it isn't going to change science. The facts about how airflow works aren't going to change based on how you want to market a product, etc.

I personally like my intakes at the front/bottom of the case where it's cooler and the exhausts at the rear/top where it's warmer.

 

[Cbshahji]

I say put them where you want, but it isn't going to change science.

Hot air rises from bottom to top, and that sinks cold air to bottom which is faster than hot air turning cold air hot so it's best to have top fans as exhaust. you can switch them as you like but it'll only help if you put your case sideways or upside down.

 

[fyleow]

The effects of natural convection is negligible when there is an outside force such as case fans to disrupt the flow. Puget did thermal analysis on the FT-02 and the P183 in various orientations.

http://www.pugetsystems.com/labs/articles/Vertical-vs-Horizontal-Case-Cooling-89/page3

No science is being changed here and there are some calculations done on the Puget conclusions to help explain why natural convection is not a factor in a computer case with airflow.

 

[D-Wiz]

CPU coolers, simple fact of the matter is that there are many different cooler configurations and an exhaust setup for the fan ensures maximum compatibility.

 

[teh_chem]

I thought that it had more to do with the fact that the top fans typically move a greater volume of air--and coupled with the generalization that a more-negative pressure gradient in the case helps defeat dead pockets, whereas a more-positive pressure gradient in the case tends to create dead pockets. If you reverse the top fan, it would move much more air into the case on a volume vs. fan speed basis, and lend to dead pockets (that is, assuming you're not using a wire-mesh as your computer "case," lol).

It's all about case design though. Assuming you had enough vents in your case, you could theoretically get away with ALL of your fans either blowing in or out and still perform just fine.

The fact that hot air rises, etc., isn't much of an issue since the forced convection from the fans is orders of magnitude greater than passive convection from temperature.