American Fabric Filter Co

Improve performance by installing a larger filter bag

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Courtesy of American Fabric Filter Co

YOU MIGHT NOT NEED a more powerful dust collector after all. That’s the word from woodworkers who’ve replaced the original equipment filterbags on their collectors with bags that are more efficient. You might ask, “What can be more efficient than the 1 micron bag that came with my collector?” The answer is a 1 micron bag that’s properly sized for your machine and surface-treated for optimal performance, according to the folks at American Fabric Filter Company(AFF). They specialize in custom-made filter bags for dust collection systems.

The right size bag

A properly sized bag provides the correct air-to-cloth ratio, which allows most of the dust that enters to slow down and fall out of the air stream before it reaches the filter. the ratio is determined by the collector’s rated air output (calculated in cubic feet per minute, or CFM) and the filter’s surface area and CFM rating. ideally, according to AFF, the bag’s total CFM rating should be 50% to 100% greater than the collector’s rated air output.

Many single-stage dust collector manufacturers include the bottom bag when calculating the air-to-cloth ratio. this allows using a smaller bag on the top, which minimizes the collector’s overall size. however, as the bottom bag fills with debris, the machine’s air-to-cloth ratio and its dust collection capability both decrease. a better solution is to use a non-breathing collection bag on the bottom and a properly sized filter bag on top. the only drawback is that this setup usually requires more space.

Low can velocity

Here’s a simple test to see if a replacement bag will improve your dust collector’s ability: turn it on. if the top bag inflates with a pop and is firm to the touch, a new bag could help.

A rigidly inflated bag is a sign that its filter media is clogged, or that the collector’s can velocity is too high (which is probably why the filter is clogged). Can velocity is the speed at which air enters the filter. For optimal filtration, can velocity should be minimized; otherwise, the dust in the air stream gets forced into the filter, where it inhibits the airflow. Once the filter is impregnated with dust, it’s difficult to clean.

A larger bag lowers the can velocity, because it has more volume, which allows the air to slow down before it contacts the filter. A filter will operate most efficiently if there’s a large difference between collector’s can velocity and it’s inlet velocity (the air speed at the impellor). When that’s the case, most of the dust will fall out of the air stream, and the dust that’s left won’t be densely packed against the inside surface of the filter. In fact, because of the low can velocity, the dust will typically fall off on its own.

Like a larger filter bag, a cartridge style pleated filter is more effective than a typical OEM filter bag, because the pleated filter’s surface area is much larger. However, cartridge filters typically have higher can velocity than a large bag, according to AFF, because of their compact size (usually considered an advantage), which keeps the filter close to the air stream. The high can velocity won’t allow the dust to fall off the pleated filter on its own, so the filter is likely to require more frequent cleaning.

Replacement filter bags that are properly sized to match a dust collector’s rated airflow are an economical upgrade. prices start at $75 for straight tube-type bags (1). efficient replacement bags can be shaped to fit the available space (3), and should be supported so they don’t collapse when the collector is turned off (2). according to american fabric filter Company, which specializes in custom-made dust collection bags, most duxt collectors come equipped with bags that are too small.

A thin dust cake

The layer of dust that forms on the inside surface of the filter is called the dust cake. A thin dust cake is important for peak performance. In addition to providing most of the fine filtration, this thin cake creates a barrier that keeps the incoming dust-laden air from impregnating the filter. Of course, if the dust cake becomes too thick, filtration suffers. That’s why low can velocity is so important—it allows most of the dust cake to slough off when it gets to heavy, leaving only the thin layer, which permanently remains on the filter.

A singed surface

Polyester felt is preferred for filtering wood dust, but not all polyester felt bags are the same, because the surface of the felt can be treated to address specific types of dust. Wood dust doesn’t easily stick to felt that has what’s called a “singed” finish. Untreated felt, on the other hand, acts like a wood dust magnet. AFF recommends polyester felt with a singed finish for capturing wood dust, because of its high breathability, fine-particle filtering capability and excellent dust cake build/release properties.

In a nutshell

By pairing the correct filter fabric with the proper surface treatment and the correct air-to-cloth ratio for the collector’s air flow, it’s possible to capture dust particles as small as 1 micron. Initially, the felt fabric does the filtering, while a very thin layer of fine dust builds on its singed surface. this thin dust cake then does the actual filtering of all subsequent dust that’s blown into the bag. as the cake builds in thickness, the felt’s singed surface prevents it from adhering permanently. When the cake becomes too heavy, it simply sloughs off, leaving behind a thin layer to continue the process.

Here’s an easy way to improve dust cake release on any new filter: pre-coat it with flour. Just use the collector to suck up 1 lb. of flour for every 10 sq. ft. of filter surface. This does the same thing as sprinkling a handful of flour before rolling out a pie crust—the flour releases the dust cake much as it releases the dough. For more dust collection tips, go to www.americanWoodworker.com/Webextras.

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