Case Fan Duct


My PC case is designed to be able to run without a fan on the CPU heatsink (a Thermalright Le Grand Macho RT, one of few heatsinks specifically marketed to work like this), relying on a case fan to keep it cool. After originally completing the case, I used a sheet of plastic to direct air from the fan into the heatsink. Without doing that, much of the air simply flows around the heatsink without contributing to cooling it. I never ran any proper tests or compared different arrangements though... until now.

The inside of the case. There are three 140mm case fans at bottom of the case. The centre fan blows air towards the CPU heatsink, then out through the top of the case.
With nothing to direct the air from the case fan, it goes wherever it feels like, which is mostly not through the heatsink.

The first attempt was a tube. The idea was to match the shape of the fan, which should minimize turbulence. The height of the tube is lower nearer the motherboard, partly so it doesn't completely block air over the RAM, and partly because it would be physically impossible to squeeze the tube into place if it was full height all around its circumference.

A piece of card crudely formed into a tube and placed over the fan, towards the heatsink.
It's made from a box of Christmas chocolates.

The tube didn't work as well as I wanted, and I surmised that this was due to air flowing out the sides of the heatsink, not all the way through. So the second attempt was a partial box made of sheets of plastic long long enough to go from the fan all the way to the top of the heatsink.

Sheets of plastic aligned vertically to cover the sides of the fan except the side closest to the motherboard.
It turns out that polypropylene sheets don't appreciate being bent at right angles, and snap. Thus the brackets at the sides holding three separate sheets together.

There were some other variations and permutations that I tried, which I shall not document here, as it would be too much data. These three are the most interesting.

To test the performance of the different arrangements, I ran IntelBurnTest 100x (which takes about 12 minutes). This fully loads all cores of the CPU (a Ryzen 5950X), maximizing its power consumption at the default 140W limit long enough for the CPU to reach maximum temperature. I used HWiNFO to record both CPU temperature (Tctl) and fan speed at the default 2s interval, saving it to a CSV file which I could then chart with Veusz.

The lines are colour-coded to the previous photos. The data are all averaged over 5 samples, otherwise the lines are too spiky to be read clearly. Temperatures are normalized to a room temperature of 20°C.

It's obvious that the "square" is best, reducing CPU temp by about 4°C and fan speed by about 175rpm. It actually completes IBT a few seconds faster too, since the CPU will boost to higher frequencies when it's kept cooler. The tube is only half as good.