Rescuing Your Old Dust Collection System

February 28, 2006
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Caught in the old bind of owning an overtaxed dust collection system but lacking the funds to replace the system? Instead of replacing it, try improving the performance of system components. Tweaking one thing or nudging another can often improve the entire system.
Most dust collectors, and particularly pulse-jet systems (the most commonly used type), don't just "wear out." They might corrode and require replacement, but pulse-jet collectors have few moving parts to wear. A well-maintained dust collector and fan that is not subject to corrosion or erosion can perform just as well when it is 10 or 20 years old as it did when it was first installed. The problem might be that maintenance has not been adequately carried out, resulting in an underperforming system, or that the requirements of the system have changed.



Caught in the old bind of owning an overtaxed dust collection system but lacking the funds to replace the system? Instead of replacing it, try improving the performance of system components. Tweaking one thing or nudging another can often improve the entire system.

Most dust collectors, and particularly pulse-jet systems (the most commonly used type), don't just "wear out." They might corrode and require replacement, but pulse-jet collectors have few moving parts to wear. A well-maintained dust collector and fan that is not subject to corrosion or erosion can perform just as well when it is 10 or 20 years old as it did when it was first installed. The problem might be that maintenance has not been adequately carried out, resulting in an underperforming system, or that the requirements of the system have changed.



How to Spot Problems

If you see any of these signs, you'll need to upgrade one or more components:
  • A high pressure drop (Δ P) across the filter media, resulting in low airflow
  • Worker overexposure to dust caused by poor dust capture at the hoods, typically resulting from low airflow at the hoods
  • Frequent filter changes due to uncontrollable filter Δ P or leakage through the filters
  • Excess emissions from the collector or poor collection efficiency


An integrated dust collector, such as the Dalamatic(r) DLMV supplied by Donaldson Co., Inc., Minneapolis, Minn., can eliminate ducting, reduce overall energy consumption and reduce maintenance associated with dirty air duct abrasion.

Causes and Solutions

Over the years, your plant's production demands have probably increased. Additional production machine exhausts might have been added to the system. Speeds of mixers, belt conveyors and other processes might have increased, generating more dust and requiring additional airflow to control the contaminant.

Over this same period, safe worker exposure levels have significantly decreased for many contaminants. For instance, the threshold limit value for silica in sand has been reduced from 10 mg/cubic meter to 0.05 mg/cubic meter. Lower exposure levels require more airflow at the hoods to increase the capture efficiency. In systems that recirculate filtered air to the workspace, higher media capture efficiencies are also required.

One approach often used to update older baghouses is replacing the filter bags with pleated media bags. This might help fit more media into the collector housing in an effort to increase airflow, but it also morphs the collector into a rather odd cross of an old baghouse and newer cartridge-style collector, without the advantages found in either.

Of the many variables in a dust collector system, performance is mainly affected by the fan, filter media and collector positioning. To update an older system, you'll want to:

  • Optimize your fan performance
  • Improve dust collector performance by upgrading to newer, high-performance filter media
  • Install dedicated integral collectors on selected dust sources to provide additional site-specific coverage

All of these steps can increase filtration performance, helping to lower worker exposure, extend filter life and reduce emissions. In the process, you might be able to retain the existing filter, fan and most, if not all, of the existing ducting.



Optimizing Fan Performance

Faced with an underperforming system, your first thought might be to change fan performance by increasing the fan speed. While this is a legitimate way to increase flow, it will not fully optimize fan performance. In a fixed system, a change in airflow is proportional to the change in speed. For a 15% increase in flow, you can simply increase the fan speed 15%. However, this flow change is limited because power requirement is a cube function of speed; thus, a 15% increase in speed requires a 52% increase in horsepower.

Before increasing the fan speed, check the fan rotation. Centrifugal fans move air even when rotating in the wrong direction-they just do it poorly. It is not unusual to find fans in small and large systems rotating backward. If there is any doubt about the condition of the fan, have it checked by a qualified fan technician.

You should also ensure the fan inlet/outlet arrangement is not affecting fan performance. A common fan outlet problem is the absence of an outlet duct. While it may seem counterproductive, a straight section of duct on the outlet is required for a fan to perform properly. To correct the matter, attach a duct at least four times longer than the diameter of the outlet area to the outlet. (Example: If the outlet area equals 2 ft in diameter, the attached duct should be 8 ft long.)

A common inlet arrangement affecting performance is the use of an elbow directly in front of the fan. If at all possible, remove the elbow or install an elbow with turning vanes. These two changes can add up to 8% of additional suction to a typical dust collector fan.



Click on image for larger version.

Upgrading to Better Filter Media

The next option is to change the filter fabric. Many new filters are designed to decrease the Δ P across the filter, thus allowing the fan to deliver more airflow through the hoods. This increase in airflow is also limited, but combined with an increase from fan speed changes, it can allow the dust collector to use the maximum available motor power for a significant and noticeable increase in airflow at the hoods. New filter media technologies can also maximize the retention of dust on the surface of the media so that less energy (i.e., less pulse cleaning) is required to remove the collected dust from the media. When pulse cleaning is minimized, emissions are also reduced, along with wear/abrasion on the filter bag structure.

Filter Media Types
While pulse-jet collectors-those that perform a self-cleaning process via a reverse blast of clean air through the filters-are available in both felted (bag) media and cartridge filter design, this discussion is limited to felted media designs. (Cartridge collectors are generally installed in newer systems and are not normally subject to upgrade demands.) Table 1 lists the most common fabric types and their performance characteristics. Over the last decade, a variety of new composite media (base plus a coating or embedded chemicals) have been developed to meet specific application or performance requirements, but these specialty media are beyond the scope of this article.



Figure 1. Needling vs. hydro-entangling.

Filter Media Surface Finishes
Surface treatments can enhance the performance of filter media by increasing efficiency, decreasing the filter Δ P, increasing resistance to moisture and chemistry, providing better dust cake release and reducing bridging. Surface treatments can be mechanical, chemical or a combination. The primary goal of most finishes is to create a surface that both retains the majority of the particulate on the filter and releases particulate easily during pulse cleaning. A secondary goal can be to increase resistance to moisture and chemistry, and/or to reduce bridging.

Some of the most commonly used treatments include:

  • Plain Finish - Standard filter bags have a plain felt finish with a natural softness attributed to the open fibers. These fibers aid in the capture of fine particulate and hold the dust cake, which presents a problem for dusts like soda ash that form hard cakes in the presence of humidity.
  • Singed Finish - Singed finishes are produced by melting the surface fibers with a gas flame to reduce the tendency of dust particles to stick to the surface. A singed finish often provides better dust cake release than a plain finish.
  • Glazed Finish - Also known as "eggshell," a glazed finish involves the melting and smearing of a microscopic layer of the media fibers to form a slick surface for better dust cake release. It provides a short-term improvement for dust cake release but can cause additional Δ P because the surface of the pore structure closes during the smearing process.
  • Silicone Treatments - Silicone aids the initial buildup of the dust cake and reduces moisture absorption into the fiber, allowing better release of moisture-sensitive dust.
  • Oleophobic, Hydrophobic and Universal Chemical Finishes - These are all terms describing media that has been immersed in a fluorocarbon bath, squeezed and then heated to set the fluorocarbon into the felt. The fluorocarbon reduces the absorption of moisture and acids into the fibers and provides a slick surface for dust cake release. It can increase fiber resistance to hydrolysis from moisture and heat, and increase resistance to acid attack.
  • Expanded PTFE - An expanded PTFE membrane thermobonded to the surface of conventional needled felts or woven fabrics can provide high efficiencies, superior dust cake release and improved airflow.

Various other surface treatments, such as acrylic foam, are available and have been developed primarily for specific industries and applications.



Figure 2a. A scanning electron microscope image (300X magnification) of a standard 16 oz. polyester media. Mechanical needling allows dust to migrate through the pore structure. Dust particles become lodged within the depth of the felt, blocking airflow and causing excess Δ P across the filters.

New Media Construction

The newest felted fabric,* produced through a proprietary process called hydro-entanglement, is a departure from conventional felted construction (see Figure 1). Traditional manufacturing of felted fabric uses steel needles that pull, weave and mix the fibers together to form a thick felt. The drawback to this mechanical needling process is the inconsistency of fiber pore spacing and size. This inconsistency affects filter efficiency and Δ P by allowing dust to migrate through the pore structure and allowing dust particles to become lodged within the depth of the felt, blocking airflow and causing excess Δ P across the filters.

Hydro-entanglement creates a bag material with a higher proportion of very fine fibers, and a smaller and more consistent pore size. This consistency and uniformity result in a bag material that more effectively loads dust on the surface, allowing for more efficient pulse cleaning and a lower Δ P. The smaller pore size also retards depth loading, thus promoting more efficient filter cleaning, lower pressure losses, higher airflows, longer filter life and lower emissions.

In both laboratory tests and field installations, the hydro-entangled fabric has proven to last two to three times longer than traditional 16 oz polyester needled felts. Twice the life equals half the bag change-outs and half the bag purchases thoughout the life of the collector. Figure 2, from a scanning electron microscope at 300 times magnification, shows the "clean air side" of filter bags that have been used for 2700 hours on pulverized fly ash at an air-to-media ratio of 4.5 to 1. The hydro-entangled fabric has clearly outperformed the polyester media.



Figure 2b. A scanning electron microscope image (300X magnification) of a 10.5 oz hydro-entangled felt. Hydro-entanglement creates a bag material with smaller, more uniform pores that retard depth loading, thus promoting more efficient filter cleaning, lower pressure losses, higher airflows, longer filter life and lower emissions.

Installing Integrated Collectors

System changes that will have the greatest effect on airflow are changes that optimize fan performance or lower the Δ P across the media. These changes typically involve removing certain hoods from the system to allow increased airflow at the remaining hoods. Filtering the air from the disconnected hoods can be accomplished using more modern, self-contained collectors dedicated to the hood or a group of removed hoods-thus, salvaging the original collector, fan and the majority of the ducting.

These additional collectors can be stand-alone collectors with ducting and an integral fan, or directly integrated into the hoods. Integrated collectors** have the advantages of eliminating ducting, reducing overall energy consumption, and reducing maintenance associated with dirty air duct abrasion, which is a real problem in many ceramic manufacturing plants.

The use of stand-alone or integrated collectors provides the maximum improvement to existing systems when the hoods selected are exhausting processes with very heavy dust burdens. Treating these hoods separately provides the maximum reduction in dust burden to the existing collector. Hoods located the farthest from the collector are also good candidates for independent collectors, since removing them from the system will often maximize the reduction in static requirement to the original fan, thereby providing the best opportunity for the existing fan to be reused. Figure 3 illustrates how an integrated collector might be installed in a bucket elevator casing and a conveyor transfer enclosure.



Figure 3. An integrated collector in a bucket elevator casing.

An Economical Alternative

Most older baghouse dust collection systems requiring upgrades have multiple problems, and no single change will provide a complete solution. The best approach is to make several changes that can provide an economical improvement in airflow and system performance, reduce dust levels, provide lower worker exposure levels, and decrease maintenance costs. Optimizing fan performance, switching to high-performance filter media, alleviating the heavy dust burden and airflow requirements on existing collectors using integrated collectors in hoods, and following a conscious maintenance program can provide an economical alternative to complete system replacement.

For more information about optimizing dust collection systems, contact Donaldson Torit at P.O. Box 1299, Minneapolis, MN 55440-1299; (800) 365-1331; fax (952) 703-4865; e-mail tgodbey@mail.donaldson.com or filters@mail.donaldson.com; or visit www.donaldsontorit.com.

FOOTNOTES:

*Hydro-entangled felt is available in different fiber materials such as polyester, Nomex and others, and is marketed as Dura-Life(tm), a trademark of Donaldson Co., Inc., Minneapolis, Minn.

**The Donaldson Dalamatic, supplied by Donaldson Co., Inc., is an example of an integrated collector.



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