Powder and Bulk Engineering, July 2002 15

Making molehills out of mountains: Simple ways to improve pulse-jet dust collector performance

Matt Gorecki TDC Filter Manufacturing

Sometimes serious dust collection problems have simple solutions. Here are tips for improving your pulse-jet dust collector’s efficiency if you’re using cellulose or spun-bond polyester fdter media.

aybe you’ve heard the old expression, “You bought a cannon to shoot a mouse.” Well, this ar- M ticle describes some fast, easy, and inexpensive

ways you can improve your pulse-jet dust collector’s per- formance without buying the proverbial cannon.

When your dust collector isn’t performing up to snuff, it’s safe to say that one of three factors is at fault: dust loading, filter cleaning, or the filters themselves.

Dust loading Improvements in your dust loading can lead to longer filter life, better collection efficiencies, lower emissions, lower compressed-air use, lower electricity use due to a lower operating pressure drop, and fewer labor hours spent on changeouts.

Contrary to popular belief, you can’t throw unlimited amounts of dust at your collector’s filters and expect them to perform well. Instead, your workers will have their heads in a dust cloud, or your filters will fail under the strain. Recommended air-to-cloth ratios will tell you how much air (in cubic feet per minute at 0.5 inches water gauge) you can put through I square foot of filter media, but if the air is too loaded with dust, you’ll need to modify the air-to-cloth ratio downward by adding more media sur- face area or reducing the collector’s airflow volume (nearly always impossible) or both. If you have a bag-

house, an easy way to increase media surface area is by in- stalling pleated bags, which can increase the area by as much as 33 percent, usually without other modifications.

If you have a cartridge dust collector, increasing the filter media surface area reaches a point of diminishing returns. After a certain point, the pleats will be so close together that the effective surface area will be reduced and the fil- ters won’t be able to pulse clean. However, if you can’t ef- fectively increase your media surface area, you can try a different media.

For example, you can switch from cellulose or cellulose- synthetic blended media to a spun-bond polyester media. Cellulose and blended media have depth-loading proper- ties that allow them to soak up dust like a sponge. A new cellulose or cellulose-synthetic blended cartridge filter with a 12.75-inch outside diameter and 26-inch length will weigh about 18 pounds when it’s installed. However, at changeout it will weigh two to three times as much. In con- trast, spun-bond media has surface filtration properties that allow it to shed the dust more easily. A spun-bond car- tridge filter with the same dimensions as the cellulose filter will weigh 15 pounds when new but gain only 5 to 7 pounds during average service life. (This dust-shedding quality also makes spun-bond filters a good choice if you’re reclaiming the dust to recycle it to your process.) Your filter supplier can give you a guide for calculating how much spun-bond media to use when converting from a cellulose-based media, based on factors such as airflow volume, inlet dust loading, temperature and humidity, and other factors.

In addition, if you have dust-loading problems be sure to keep your dust collector hopper empty. It may sound obvi- ous, but don ’t use the hopper for storage. A lot of users for- get to empty the hopper regularly, and dust there is easily picked up and re-entrained on the filter surfaces.

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Filter cleaning There are just no two ways about it -properly cleanedfil- ters will ensure longer filter life. You’ll also get better throughput: More air and dust will flow through the sys- tem, creating more suction and evacuating dust particles more rapidly through the dust collection system so your workers won’t have their heads in a dust cloud.

The dominant filter-cleaning method is back-pulsing the filters’ clean side with compressed air. In a typical pulse cleaning system, a solenoid valve controls aifflow from the compressed-air tank to one or more diaphragm valves. Each diaphragm valve can supply air to a pipe that directs the cleaning pulse into one or more filters. You can dis- cover whether your cleaning system is working effectively by answering these questions:

Is the compressed-air pressure at the header correct? (The correct pressure normally is 80 to 100 psi.)

After a pulse, does the compressed-air header in the col- lector have enough time to recover to that full pressure before the next pulse?

If the compressor or any piping is new, is it thoroughly clean and dry? (Blowing compressed air containing traces of oil or water onto the filter’s clean side is a quick way to reduce collector performance as well as ruin a filter.)

Do all the diaphragm valves sound the same and operate correctly? (If you hear one valve pulsing with a normal high-pitched short tone, then another valve pulsing with slightly lower tone and longer pulse, check that second

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18 Powder and Bulk Engineering, July 2002

valve; a new diaphragm may be in order. E a valve doesn’t sound at all, look into repairing or replacing the solenoid valve, and check your timer or on-demand gauge. Keep extra solenoid valves in stock-they’re inexpensive.)

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Are the rows of diaphragm valves silent between pulses? (If not, look inside each diaphragm valve and solenoid valve for dirt, a failed diaphragm or solenoid plunger, or a leak in the tubing between a diaphragm valve and the so- lenoid valve.)

How many filters are you asking one diaphragm valve to clean? (A single %-inch valve should not be cleaning eight filters of 200 or more square feet of media each. A good filter supplier should give you recommendations for valve size based on media area.)

Filters The filters are, of course, the most important factor in dust collection. Select your filters to suit your application. If you’re handling or processing lower-loading dusts, such as lightweight metal or wood dusts, consider cellulose filters. For a heavy dust-loading application, such as metal grind- ing, paint powder, or any application with voluminous amounts of dust, consider spun-bond polyester media. If the dust is particularly sticky, agglomerative, or just plain nasty, a spun-bond media with an expanded polytetrafluo- roethylene (ePTFE) membrane may be the answer. This type of media is expensive and won’t work well in applica- tions with silicones or hydrocarbons in the airflow, but it sheds dust better than any other media on the market.

As with cellulose media, if you’re using spun-bond poly- ester, you must consider the balance between pleat spacing and the total media surface area. Spun-bond polyester fil- ters don’t require as much media. In fact, packing too much spun-bond polyester media into a filter will make it more difficult to pulse clean. A good rule of thumb is to keep at least % inch of space between neighboring pleat tips.

Filter finishes can also affect your filter’s efficiency. Cellu- lose filters can be impregnated with a flame retardant, but they don’t have other optional finishes. Spun-bond poly- ester media can have many finishes, including ePTFE membrane and these:

Hydro-oleophobicfinish, which is applied to the media to add water- and oil-resistance, excellent efficiency, and strength.

One way to improve dust collecior performance is to use filters with special finishes. The standing filter here has an aluminized finish and ground wire (not shown) fbr dissipaiing stcriic, fhere6y improving dust Elease.

Flame-retardantfinish, which is applied by spraying the media or dipping it into a bath. Be aware: This coating only slows down combustion; it isn’t fire-proof.

To determine the best filter for your application, ask your filter supplier for recommendations based on your appli- cation (including such factors as your particle characteris- tics, air-to-cloth ratios, and dust loading) and the improvements you’d like to see. PBE

For further reading Find more information on pulse-jet dust collectors and fil- ters in articles listed under “Dust collection and dust con- trol” in Powder and Bulk Engineering’s comprehensive “Index to articles” (in the December 2001 issue and at www.powderbulk.com).

*Metalizedjnish, which is used with a ground wire to dis- sipate static that can prevent full dust release during cleaning.

Matt Gorecki is sales mnager andajltration consultant at TDC Filter Manufacturing, 1331 South 55th Court, Cicero, IL 60804; 800-424-1 91 0, fcwc 708-863-4472 (goreckim@td cfiltezcom, www. tdcfiltezcom). He has a BA in businessfiom Southern Illinois University, Carbondale, Ill.

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