SA-4 Spark Arrester for 4" Vacuum System

• Introduction

As a knifemaker I work with a variety of materials from exotic woods, synthetic laminates, brass, copper and a variety of steels. Dust is a fact of life in many processes in my shop. I have installed a 600 CFM dust collector and was planning drops to my different pieces of machinery. When I got to my belt grinder I had a big concern. I want to be able to collect the wood dust and the synthetics, but what about the sparks coming from the steel? If there was only a way to separate the wood dust from the steel dust. As we know, dust, sparks and fast moving air are dangerous together. I know other knifemakers who have their shop vacuums catch on fire from sparks. This is a very serious matter for us. Some have installed steel baffles in metal cans to catch the sparks and filings, others use water to quench the hot sparks.

Luckily I have a little experience making conical dust separators for my shop vacuums and Bernoulli's Principle, so I set out to make a prototype of an in-line separator that would catch the sparks and steel, but let the lighter dust pass through. Commercially available units are on the market, but the costs are largely prohibitive, especially for hobbyists.

Before starting the build process, I must say:

Builders be warned. This information is presented for educational purposes only. I cannot be held responsible or liable for any issue that arise from the use of this information. Use this information at your own risk. Always abide by the laws and codes in your jurisdiction.

The basic concept

The air and dust particles are moving into the system from a boot (scoop) along a hose to the vacuum. If we can slow the particles down enough they can be captured. One way to do this is to spin them around. Spinning and increasing the area of the duct should slow the heavier particles enough to be captured from the stream. (see graphic).

The resulting product must meet the following criteria:

• Must be effective at catching sparks and steel.
• Must be inexpensive to build
• Must be made from readily available materials. (cannot be made with antimatter or Californium)
• Must be cleanable to remove the particle that are captured.

A measuring 6" in height and with a base diameter of 4" is required.

The cone was constructed from a steel oven pan. The basic geometry was planned on card paper and then transferred to the steel pan and cut out. Once cut out, I put the gloves on and gently shaped the steel into a conical shape. as it's difficult to get into the tip of the cone for forming, I made it into a frustum by clipping the top 1/2" off the cone. This let me get my finger in there.

Once a basic cone shape is formed, overlap the steel by about 3/8" and clamp the base with vise-grips. Now drill and rivet with 1/8" rivets (qty 3). Make the top of the cone rivet about 1" down from the top.

Four vanes mounted to the cone will cause the incoming air to go into a spin. This is probably the most trickiest part of the build. I made some ad-hoc vanes from the same oven pan. Their shape was not perfect, but some foil tape filled in the gaps.

The vanes are placed at 90° around the cone. I started each vane at the bottom and gave it a 90° twist on its way to the top. Have a few pieces of foil tape handy as you shape the vane to the cone.

Once the four vanes are attached with foil tape, make a small tapered wooden plug to fit in the top of the cone. I some 5 minute epoxy to hold it in place. Once the epoxy has set, drill a 3/16" hole through it parallel to the base of the cone.

Next I applied a generous amount of fibreglass autobody filler. The fibreglass has two purposes. To strengthen the vanes and to make the surface rough and introduce turbulence to the airflow, slowing the air a bit. I know that seems counter-intuitive, but we want to drop the air velocity as we start the particles spinning outwards.To avoid getting too much body filler in the 3/16" hole near the top of the cone, I pushed a piece of wire through there.

The outlet (vacuum side) is made from a 6" duct cap, a short section of 4" duct (I used a peanut can) and a 4" plastic dust port.

Mark out a 4" hole in the centre of the 6" duct cap. I used the 4" dust port and traced the circle. Drill a couple of holes inside the big 4" hole and using the aviation snips, cut out the 4" circle. If you are having a hard time with the tight curve, you can snip little segments and bend them to the line with pliers. Caution! This can be very sharp. Gloves are recommended.

Cut a short section of 4" duct to 2.5" and slip this into the hole in the cap. Press it in until the duct is flush to the cap flange.

Apply a bead of silicone to the dust port flange and attach with 4 sheet metal screws

Place the impeller in a flat surface and slip the 6" to 4" reducer over top of it. Drill two 3/16" holes in the 4" sleeve of the reduce that line up with the 3/16" hole. These holes should be opposite each other so that the impeller sits nicely in the centre of the reducer.

Measure and cut two spacers from the 3/16" ID tubing to hold the impeller in place. The idea is secure the impeller and not allow the sleeve to crush when tightening things up.

The inlet and outlet slide together to form the body. At this stage we want to make sure that there is an even 1" gap between the outlet duct and the bottom of the impeller cone. This 1" gap has the same area as the area of the inlet, that is 12.57 inches. If the gap is set too small it will restrict air flow. This is a good staring point for testing. If you wish to adjust the gap and experiment, please do so and let me know the results.

The last step is to make a band that seals the joint between the inlet and outlet sections. This must be removable for cleaning out the particles trapped in the arrester. I used a length of rubbery toolbox liner, but some weather stripping or closed cell foam would work better.

Now that it's assembled you can add labels if you wish.

Now one with the testing...

For testing I attached the SA-4 spark arrester to my vacuum line and attached about 3 feet of hose to a boot. I prepared a test bed shown. With the vacuum running, I sent streams of sparks from an angle grinder into the boot and into the arrester. The results were impressive.

I have a video on YouTube showing just how effective this is at capturing the heavier particles.

Conclusion

After multiple tests, I concluded that a simple arrester like this could prevent fires in a vacuum system in which sparks may be present. It should be noted that not all dust collection systems will produce the same results. Higher air velocities may require a larger unit or a modified impeller design, more vanes etc.

I am looking forward to your feedback. Bear in mind, I am not expert in fluid dynamics, and I realize there could be many improvements to this device. I hope that it will be beneficial in reducing fire risk for my fellow knifemakers.

More of this kind of stuff on my blog at http://dcknives.com

Best wishes,

Dan

Materials:

1 - Steel oven pan (Dollar Store)
1 - 6" to 3" duct reducer (Home Depot)
1 - 6" duct connector (Home Depot)
1 - 6" duct cap (Home Depot)
1 - 4" flanged dust port (Amazon)
4 - #10 x 3/4" sheet metal screw
1 - 10-24 threaded rod, approximately 5" long. (Home Depot)
3 - 10-24 nuts. (Home Depot)
3 - 1/8" rivets.
1 - Short length of 3/16" ID tubing (I used a stainless steel drinking straw. Dollar store)
21" of 1" wide strip light gauge steel (for sealing body)
1 - #10-24 x 1" machine screw.
21" of 1" x 1/8" closed cell foam, weather stripping etc. for seal around body.

Miscellaneous: Foil tape, fibreglass automotive body filler, high-heat paint, silicone.

Tools:
Drill, with bit set
Permanent marker
Aviation snips
Rivet tool
Vise-grip pliers
Leather gloves
Hacksaw & vise
A few different files for rough edges

Now on to construction...