Adding a pre-separator to your single-stage dust collector will save you time and money. It’ll save you time because it’s a lot easier to empty one of these than the lower, or “chip,” bag of a bag-over- bag collector. It’ll save you money in the long run: Your dust collector will last longer since the impeller won’t be under a constant barrage of heavy chips, or damaging bits of metal such as the occasional screw or nail that finds its way to the floor. And it will allow your filter bags to do their real job—filter out dust, not collect chips.
As I discussed in Chapter 1, a filter bag loses its ability to filter efficiently as the lower bag fills up, because as it fills, the filter surface area shrinks. Less filter, same pressure—more blow-through. This occurs at a much slower rate when you’ve got a pre-separator attached between the ductwork and the collector. Note: Here again, this pre-separator will add resistance to your system. Typically, it’ll add anywhere from 2” to 3” of static pressure loss.
A mini-cyclone Yet another “trash can” pre-separator, this pre-separator is quite different from the drop box shown on page 91. Instead of using an internal baffle to “knock down” heavy chips, this pre-separator uses a combination of PVC pipe and fittings and a round trash can to create sort of a mini-cyclone.
Here again, I used a Rubberrnaid heavy-duty trash can since it will hold up under the suction of a 1 ‘li-hp or smaller collector. Don’t be tempted to use a thinner-gauge can: It will surely collapse as soon as you turn on your collector.
How It works Dust and chips flow into the inlet pipe and travel down into the container, where they exit into the container through a 90-degree elbow angled toward the container wall.
Just as in a cyclone, the heavy chips spin around the walls of the container, losing their momentum until they fall to the bottom. Lighter chips and dust are pulled up through the outlet port into the dust collector, where they are captured by the filters. Just as with the drop box, pre-separator efficiency drops as the container fills. Check it often, and always before a big job.
Assemble the unit The pre-separator is quite easy to build. The only challenge is cutting the large-diameter holes in the lid. The best way I’ve found to do 45-degree angle—the idea here is to swirl the chips around the inside of the can. Seal around both inside and outside edges of the pipe with silicone caulk.
Safety Note: Since dust and chips are moving rapidly through a large plastic container, take the time to ground the container, to reduce the risk of sparks caused by static electricity. Running bare copper wire through the interior of the container and connecting this to your metal ductwork will do the job.
Plastic blast gates Anytime you insert a plastic part in your ductwork, you break the ground path. This can lead to static problems, including the possibility of a dust explosion.
When installing plastic blast gates, like the one shown in the photo, it’s important to reconnect the ground path. A simple way to do this is to run a ground strap or wire from pipe to pipe, attaching the wire with sheet-metal screws. (Position the wire so that it doesn’t interfere with opening or closing the gate.)
PVC fittings If you must use PVC pipe or fittings, you’ll need to run a ground wire both inside the pipe and around the exterior to prevent static electricity from building up. Most dust collection companies that sell plastic parts also sell a grounding kit.
Make sure to follow their installation directions to the letter, and use a continuity tester or a multimeter to verify that the ground circuit is in fact continuous. To prevent leaks, use silicone to seal any holes you drilled into the pipe for the ground wire.
Flexible hose To prevent static problems with flexible hose, make sure to use wirewrapped helix hose and prepare the ends to guarantee that your ground path is complete. To do this, use a utility knife to remove a 1” to 2” section of the plastic covering the wire inside the hose, as shown.
Then as you install the hose and tighten down the hose clamp, the wire inside the hose will make solid contact with the metal C flange of the blast gate or pipe you’re hooking it up to. It’s best to bare the wire in a couple of places around the inside perimeter to ensure that it makes contact with the pipe or blast gate.
This article is excerpted from Controlling Dust in the Workshop by Rick Peters".