Heatsink Fan Distance - Please Help

[Pasta]

Right now I have a WBK38 with a Sunon 60x60x25 fan (23.5 CFM @ 34 dBA). I know this is waaay better than the stock Delta, but it's still too loud for me. An option I've read about is to use a fan adapter to attach an 80mm fan. This sounds great, except that the fan clips on my WBK38 don't allow fan adapters, they're damn expensive, and they don't work too well. I read in a review of the Plycon adapters that the flow coming out of the small end simply doesn't feel as great as the 80mm fan without the adapter. So my question is, will I be able to achieve the same / better results by suspending a large, quiet (92 or 120 mm Panaflo L1A) fan over the heatsink? Can this fan be mounted on the side of my case, or does it need to be touching the heatsink?

 

[Hoot]

When you squeeze an airflow down to fit through a smaller opening, you encounter resistance to airflow. This is often referred to as Static Pressure. Axial fans, in general, do not perform well in the presence of static pressure. A few do, but they are the louder Delta versions. The thickness of the fan is the biggest indicator of how well they perform with static pressure. 80x10, 80x15 and 80x25mm fans do the poorest. The same applies to 95 and 120mm fans. The common 25mm thick ones do not perform well, whereas the 32 and 38mm ones do better. The shape of the reducer (adapter) can be a big factor. Those plastic ones that look like a small bathroom sink basin are the poorest thought out design I've ever seen. If you are going to squeeze down the airflow, you want to do it over a longer distance than those bowl adapters give you. A four sided funnel type, like the more expensive fanadapters are a much better approach. Ideally, the distance from the fan blades to the point of maximum constriction "running room" should be a minimum of the diameter of the fan. This places physical challenges from the height of the total assembly being too great. The worst you want for "running room" is 1/2 the diameter of the source fan. If you choose to try a 92mm solution, I recommend the Sanyo Denki 92x32mm 55cfm fan. It holds up well, in the presence of modest static pressure and for the amount of air it delivers, it is relatively quiet.

Hoot

 

[Pasta]

Thanks for the prompt reply, Hoot. What I'm hoping to do is to either drill a hole on the side of my case directly over the heatsink and mount a fan, or rig up a spare expansion slot cover with a fan and position it over my heatsink.

I'm not planning on forcing any air through a funnel, but what I need to know is the effectiveness of a fan mounted a couple of inches above the heatsink instead of directly on the heatsink. Do you think that will work?

 

[Pasta]

Quick bump...

 

[Hoot]

Okay, now i understand what you are trying to accomplish. With a centrifugal blower, you can get away with that as the air does not disperse much out to about 4 or 5 inches. Quite the contrary with an axial fan. The air coming out of them expands in a cone of spiraling air and very little will penetrate the HS down to where the fins join the baseplate (hottest area). You may be able to get away with this by placing a piece of square or round duct on the outflow side of the fan that extends down to and stops just short of the top of the HS. The duct will help keep the axial fan air from dispersing, but it causes resistance to the flow of the air. That Sanyo Denki fan I mentioned will tolerate a fair amount of resistance without letting too much air slip out the back. If you combine that duct solution with an equal amount (cfm) of exhause air, that will help with the resistance also. Remember to count the cfm exhausting from the PSU fan(s). A 13 oz. (used to be 1 lb) Coffee Can makes an excellent duct for a 92mm fan. Snip here, snip there, bend here, bend there, punch mounting holes and away you go.

Hoot