Next, we are going to make a fan shroud..a fancy way of saying we are going to block off the square corners, so just the round fan is exposed. This was discussed here by Jim Rosenthal.
Short version – the round fan does not reach the square corners, so there’s a gap there, and is not efficient. David Elfstrom, “At the tips of the fan blades there is negative pressure which sucks air in through the front (outlet) of the fan. By covering up this area, more air will flow through the filter on the inlet side and noise is reduced.”
As you can see below, the green arrows are where the air is flowing out of the fan, and the red arrows are where the negative pressure is sucking air back in the opposite way. Picture from here.
When you make the shroud, you will be putting it on the outside of the fan. On the side that the air is blowing from. And you only need one shroud, not one on the inside AND the outside. Just the outside.
You could do this with tape:
So – there have been two fans that have had their fans optimized by David Elfstrom. They are here. After this, will show you how to find the shroud size for your specific fan.
All others, you could can use tissue paper to determine how big the hole in the middle should be. As the fan is on, pull a piece of tissue across the front of the fan, slowly. You will very quickly see the part of the tissue that is being sucked back INTO the fan. You’ll want to cover that part with the shroud. In the picture below, you’ll see the green arrows up, and red arrows down – the red arrow down is the part you want to cover.
After determining the size, draw a circle on your cardboard. If you draw an “X”, corner to corner, that will give you the middle. Then, you can use this technique with a pin and string to draw that circle.
Some people put contact paper over their cardboard parts – like the shroud.