An Air Filter Box

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Building a filter box for blower cooling.–Dale DeHoff

This should lead to a dust free case.

It took all day to build; working at it off-and-on, it isn’t the prettiest thing I ever built out of wood – looked OK till I put the filters on it with some wood screws.

I built this “filter box” to filter the air going into the PC case. The blower used for ‘cohesive air cooling’ creates a high positive pressure inside the case and there is air coming out everywhere. When I was using a smaller blower, I put an 80mm fan filter on the intake. It cut down on airflow so much that I needed to increase the speed of the blower. As you can see, in about 2 weeks time the filter was getting clogged with dirt and dust.


The concept of filtering the air was good, but what I needed was more filter surface area.

After getting a larger blower, I considered using the top half of a small cabinet or printer stand, mounting the blower underneath and porting the air up and over with ductwork. A large filter could be positioned horizontally, like a shelf and the area above enclosed.

The first problem was finding a source for 3″ square ducting or making it from metal or plastic. Second, there would be more things to fit together and disassemble when I wanted to get into the case. So, I took a different approach. Since the blower was already working very well mounted on the side of the case and held on by only 2 bolts, the best solution was to leave it there and build on an addition.

What I built is a wooden box 10″ x 11″ x 8.5″ high. This size was determined to be the max I could fit into the area on the desk and fit underneath the blower. The ½” plywood sides were routed and fitted together at the corners and before assembly, I cut the centers out of three of the sides leaving about 1.5″ on all edges.

A solid piece of plywood is used for the bottom and a ¼” piece of masonite for the top. I cut a 5″ diameter hole in the masonite where the blower intake would be and lined the edge of the hole with some ¾” x 1″ foam used to seal around air conditioners. The height allows the foam to compress when the blower is set on top. I just tilt the case and slide the box up next to it and ease the blower down on top.

So, the box is not connected to the case or the blower and the foam acts as a cushion between. The image below shows the airflow through the filter box and into the blower. Please excuse the use of a poor 3-D image as I refuse to shell out $200 for a decent digital camera.


There are 3 filters measuring 7.5″ x 6″ or approx. 135 sq. inches. These were cut from a large furnace filter and new cardboard glued onto the edges. The blower intake opening measures about 20 square inches, 5″ diameter, so deleting for the metal backing on one side of the filters there still should be over 5 times as much intake area than what is required by the blower.

It works great. While the PC was off and the blower was running at about 75%, I tilted the case away from the filter box and couldn’t tell the difference in air volume at all. In fact, I can’t tell the difference from the way it ran before. The whole thing is about a loud as an 18″ pedestal fan on low speed. Compared to a Delta fan, it is silent! The best test will be to see how much dirt and dust the filters pick up in a month or so.

This really is a great hobby, this overclocking and all that it encompasses. Always something else to consider and something to build or rebuild.

Hey, how about a larger filter box that could employ the means to cool and dry the air!! That one would work for the small cabinet idea and you could vent it into the PC from the bottom.

Not now, I’m tired. Maybe another day.

Dale DeHoff


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