Having used a collapsible fan duct for a few months, I finally decided that it was too much of a pain in the neck. First I had to modify it out of the box to fit on the fan for my CPU heatsink, then I had to duct tape it in place, and it would sometimes grind against the fan when it ran - it was a pain. I took it out and finally built something I'd been knocking around my head for a while.
This project is designed to fit an 80mm side vent, which seems to be standard on most mid-range cases nowadays. It will work with larger (or smaller) sided vents, but obviously you'll have to modify your materials, calculations, and assembly for that.
Step 1: Materials and Tools
I used the following tools and materials in this build (there are other ways to do this, but this is what I had available to me):
- tape measure
- calculator with trig functions
- Miter saw (can be replaced by a regular hand saw)
- 5/32" drill bit
- 3" Hole saw (can be replaced with a scroll saw or a square saw)
- drill press (makes using the hole saw easier)
- calipers (not necessary, but helps)
- computer case with side intake vent close to CPU fan (if you're handy enough you can make yourself a vent)
- 80mm by 80mm (3.15" x 3.15") piece of 1/4" plywood
- 5" piece of 3" PVC piping
- 4 regular case fan screws (or just two if that's all you can scrounge)
- cyanoacrylate (superglue)
Step 2: Measurements
Begin by measuring the distance from the top of the case to the top part of the vent hole (measuring to the screw hole is good). Do the same for the side and write these measurements down. My original prototype was for example 4-5/8" from the top and 3-1/2" from the side.
Now remove the case door and measure the position of the CPU fan. Use the same method as above, except you'll have to eyeball it a little. It doesn't need to be incredibly accurate, just within an eighth of an inch or so. Mine were 5-7/8" (top) and 3-7/8" (side).
After this, determine how deep into your case the CPU fan rests. If you have a calipers, you can span a straightedge across the case opening and use the depth gauge to find it. If not, using the measuring tape and the straightedge works fine too. Mine was 3-1/2" deep.
Step 3: Calculations
First we need to figure out the displacement of the CPU fan when looking at it from above. In my example, the fan was 1-1/4" lower and 3/8" to the right of the vent hole. Plug this into the Pythagorean formula to get a displacement of 1.305" (about 1-5/16").
The number we just found is crucial for the next step. We need to figure out how long to cut the PVC and at what angle. We know the CPU fan sits 3-1/2" into the case, but we can subtract 1/4" because of the plywood piece that mounts the PVC. That leaves us with 3-1/4" deep and skewed 1-5/16" laterally. The Pythagorean formula gives us a total length of 3-1/2" for the PVC.
The angle can be figured out using tangent. With the opposite and adjacent sides known, one can easily figure out the angle theta in the third diagram. However, the saw must be set to cut at an angle of 22 degrees, not the 68 we found (90 - 68 = 22).
Step 4: Cut the Mounting Plate
After this it is time to cut the hole for the air to flow through. It just so happens that the hole for the fan in a standard 80mm fan is exactly 3" diameter, so a 3" hole sawing bit is just the thing. It's easiest if you mount the bit on a drill press and clamp the mounting plate to the bench. Or, if you can't find your hole saw like me, use a scroll saw to cut the hole.
Step 5: Cut the PVC
Alas, here's the tricky part. You not only have to keep the PVC from rolling (and thus messing up your angles), but you also have to get it right length. I measured straight down the length of the PVC 3-1/2" and lined up the blade before I cut. It never hurts to double check the measurements before you cut. But if you do screw up, at least the PVC was (hopefully) not even $2 for the 5" length.
Step 6: Dry Fit
Once you're sure the PVC is right, carefully remove the panel (as not to move the PVC) and take the mounting plate off. Now place the plate on top of the PVC and eyeball it square. Carefully replace the case panel and get the mounting plate as close as you can. Remove the panel carefully again once you're satisfied.
With the plywood still on the PVC, take a permanent marker and make some registration marks on the piece. You'll need these to line it up again after gluing.
Step 7: Assembly & Mounting
Give the glue some time to cure (30 minutes should be plenty, just to be safe). You're now ready to put it on the case. When screwing the fan screws into the plywood, you just want to get them snug; don't overtighten the screws or you'll strip the plywood and lose all the grip from the screws.
Put the panel back on and fire the computer up. Put your hand to the vent hole and you should feel a good air flow into the case. Enjoy a better cooled CPU. Next step (when we all get the money for it) is water cooling :-)