Saturday, May 12, 2012

Dust control: Shop air filter

Here's a simple shop dust filter* -- a copy of the one that's been working great in my wood shop for several years. It's basically a 20 x 20 inch box with a blower inside, pulling air through a couple of 20" furnace filters. I cut "L" shaped strips to create slots for the filters (aluminum angle would work great too). The fit on the filters is fairly loose, but the suction from the fan pulls them in for a tight seal.

The whole thing cost about $70-$75 and took an afternoon to build.

The blower is from a Jen-aire range hood that I picked up at a local rebuilding center for $15.  It's a little big for the 20 x 20 box, so I mounted it diagonally. The previous one I built used the motor and blower from a window-mount air conditioner salvaged from an appliance recycling depot -- for free.

Ugly but effective. Here's the first filter I built, that's mounted in the rafters of my garage. The slots on this one are big enough for 2 filters stacked, and I put a cheap fiberglass filter on the outside to catch the big chunks. I'm not sure how much good that does, so I left it off the new design. The exhaust port is the angled aluminum flashing on the left. I typically leave this running the whole time I'm working in the garage, and unless I'm doing a bunch of sanding, very little dust settles out on surfaces.

1 --- 1/2" AC fir plywood  -- $29
1 ---2x4 kiln dried fir -- $3
2 ---20x20 furnace filters -- $20 ea. (you can get much cheaper ones, but I wanted as close to HEPA as I could find.)
Misc --  Switch, electrical box, wire, wire nuts, glue, screws, duct tape, aluminum flashing (to extend exhaust port)

Please note: this design is only as good as the filters you choose. Most furnace filters only filter out large particles. I used 3M's Filtrite filters with their highest proprietary MPR rating of 1900. They claim to filter out 90% of particles down to .3 micron. From what I can tell, they achieve this by relying on the physical filter to trap particles in the 10+ micron range, and an electrostatic charge to get the smaller ones. I'm skeptical of the longevity of the electrostatic charge, so I'm only going to assume this filter will trap the larger dust particles and also use dust collection at the router and ventilation to control dust.

*years ago, I taped a furnace filter to a box fan for a dust filter. It worked, but not well. A radial fan doesn't do well with restricted air flow. The the airflow over the blades stalls and pressure drops off significantly. A centrifugal (squirrel-cage) blower operates much better under conditions with resistance to air flow.

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