Question about airflow in a duct

[revanrules]

I'm going to have a duct go from a 92mm fan mounted to my heatsink to an 80mm blow hole fan on the top of my case. There is also an 80mm intake fan in the rear of the case blowing towards the heatsink.

Here is an approximation of what it will be like (ignore periods)
..............................[80]
...............................|..|
...........................__/..|
[80]->{HS}[92]->____/
duct curves at a 90 degree angle up to top of case

Anyways, my question is will the 80mm fan at the top of the case hinder the airflow caused by the 92mm fan? The 80mm will have a lower CFM than then 92mm fyi. Also, will the fact that the 92mm fan is sucking air across the heatsink rather than blowing it make a difference in cooling ability?

 

[BonzaiDuck]

First, I don't want to rain on your parade. And, second, there are many approaches which I haven't tested, but the basic principles work like this.

For ducting to be effective, the intake CFMs must be greater than the exhaust CFMs. Even a narrow margin will be effective.

In addition, serial use of fans generally doesn't enhance airflow, unless you are taking higher CFM airflow from a larger intake fan or fans and pushing it through a smaller fan.

Without asking about the type of case or other factors in your setup, my first objective in preparing a case for ducted cooling is to find opportunities for high-CFM intake with low-rpm fans -- larger fans. Then the trick is to seal the case except for the intake and exhaust fans, and put the ducting over the warm components so air is forced past those components at high velocity and forced out of the case quickly.

Ducting to a case-top fan is something I've considered in the past, but because of the distances involved, it becomes more complicated. If you want to do it, I'm sure you can, but I don't think going from 80mm to 92mm and back to 80mm is going to be optimal. It may "work" but it won't be optimal.

Anyway, if you are going to experiment with that idea, make sure the ducting is well-sealed, since this will force the air from the 92mm fan to push through the lower-CFM(?) 80mm.

 

[revanrules]

Thanks for your replies to all my threads BonzaiDuck, unfortunately I wont be building my new computer until October so I cant experiment. But I'll make sure to post my experiences once I do.

 

[Bluefront]

When you think about it, you really don't need the duct from the 92mm to the top of the case. The heated air will always tend to go upward, and fans at the top of the case will exhaust that air.....duct or no duct.

I seriously doubt you could measure any significant temperature rise by removing that duct.....actually you might get lower temperatures. But trying it is the only real way.