Why do HSF's push air down?

[David M]

Why do HSF's push air from the motor and fan first? Wouldnt it be more thermally efficient for them to draw cool air through the vanes first and have the fan pull out hot air rather than push cool air which has already be partially heated by the fan and its motor through the vanes??

Seems what is happening now is the air is being pre-heated from the fan (friction from the blades) and motor (waste heat from the motor) and then being pushed over the vanes. Why not reverse the airflow and make the process more efficient?

Also when air is slightly compressed, (the exhaust side of the fan) the air is heated slightly. Given this, wouldn't is be more efficient to place the vanes on the intake side of the fan? There air on the intake side would be slightly cooler than the air on the exhaust side because of the pressure differential.

Also, the air at the lowest pressure point of a venturi http://en.wikipedia.org/wiki/Venturi is cooler than the surrounding atmospheric air. Has anyone considered a venturi shaped HSF with the vanes at the coolest part of the venturi?

It just seems more could be done to make HSF's more efficient than to keep increasing the flow volume of air and the surface area of the cooling vanes.

Are there some practical considerations that prevent these things from being done?

[Tin]

Some heatsink/fan combos do draw air through the vanes first and exhaust it upwards away from the heatsink....such as the Alpha PAL and some retail Socket 775 coolers. The problem with drawing air through the vanes first, is youre drawing heat from otherwise warm components, such as capacitors, mosfets, northbridge, and video card, right through the heatsink first. When the fan is blowing on the heatsink, its taking the cool air from the point furthest from the components, which is normally cooler air.

[Iniamyen]

Also remember that expanded air is less dense than compressed air, so the amount of heat you can remove by drawing air through the vanes might not be higher than if you blew it onto them. Also, blowing air is more directed -- a fan can shoot air in a more directed jet at the heatsink, whereas sucking air will suck it from all around the intake part of the fan, and some of this won't be around the heatsink.

[Trent Steel]

There are many factors to consider. Another factor not yet mentioned in the pressure drop across the fins of the heatsink, with really dense fins you can get an interesting effect. When the fan is on the inlet pushing air it raises the pressure of the air, how much it raises it depends on the fins and fan. When the fan is on the outlet sucking air out it lowers the pressure, but there is a limit to how much the pressure can drop, much like a vacuum cleaner hose stops sucking air if stick it on your arm. I am not explaining as well or in depth to truely understand, but bottom line is you get more airflow in the end using a push method rather then pull.

[Floppyman]

Someone had a good analogy on this exact topic once that I remember: On a hot day, would you rather have a fan blowing on you or drawing air away from y ou (i.e. standing behind it)? I think the principle is the same with HSF's.

[David M]

If 100% of the air flows through the fins either way, then I dont see how it would matter.