Improving dust collection
When it comes to dust collection, the first consideration is what type of collector best fits the application. The choice comes down to the baghouse collector—which some refer to as the grand daddy of filtration technologies—or its offspring, the cartridge dust collector. For some filtration projects, there really is a choice.
When it comes to dust collection, the first consideration is what type of collector best fits the application. The choice comes down to the baghouse collector—which some refer to as the grand daddy of filtration technologies—or its offspring, the cartridge dust collector. For some filtration projects, there really is a choice. For others, the application dictates either a baghouse or cartridge collector.
With improvements in both cartridge and baghouse dust collection technologies in the past 10 yr, filtration customers now have access to more reliable, higher performance filtration technologies no matter what type of collector they choose.
Modern baghouse designs use more efficient media, improved cleaning, and structural enhancements to provide a more predictable, cost-effective, and energy-efficient filtration solution. Advanced cartridge collectors provide more airflow and longer filter life in a compact footprint.
Introduced in the 1970s, cartridge collection technology developed over the years into an extremely efficient, high-performance, and easy-to-maintain dust collection solution for a broad spectrum of applications. It's an ideal solution for many processing applications from polymer and composites, cosmetics, pharmaceutical, and metal grinding to thermal cutting, woodworking, and sandblasting.
The first cartridge collectors had hopper inlets. Later designs utilized top-to-bottom downward airflow that used gravity to automatically carry dust particles into the hopper. They were subsequently redesigned with horizontally mounted cartridge filters and easy access porthole covers for simplified filter changing and maintenance (Fig. 1).
Access porthole filter service eliminated the need to deal with OSHA's confined space rules during filter changeouts. Development of fine fiber filter media in combination with new pulse cleaning technology eliminated baghouse-like blow pipes, replacing them with individual pulse valves that allow dust to release easily from the cartridge filter.
Advances in fine fiber filter media technology increased the operating efficiency of cartridge dust collectors while providing users with durable and efficient filters that last longer.
When particulates load a horizontal cylindrical cartridge filter, the effective media area of the filter is reduced, because particulates tend to blind off the top half of the media section quickly, resulting in increased velocity and turbulence. The effective media area is increased by re-shaping the cartridge filter from circular to oval — without adding more media or increasing the size of the collector.
Along with new pleat configurations, increased space between pleats, shorter pleat height, and improved nanofiber filter media technology, oval-shaped filters increase the effective media area and virtually eliminate clogging problems (Fig. 2).
Narrow oval filters open up cabinet space and allow increased cross-sectional flow area, which lowers cabinet velocities. Convex side panels accommodate the oval shape and eliminate turbulence inside the collector, minimizing filter erosion problems.
High performance, small package
Cellulose and polyester filters depth-load, trapping particulates within the filter media and prohibiting increased airflow. Nanofiber media keeps dust on the surface of the filter and allows it to be easily released through pulse cleaning, which helps maintain a greater effective media area over a longer period of time.
Pulse cleaning technology, which works in tandem with the filter media to manage airflow through the collector, accommodates the oval-shaped cartridge filter design. The result is a uniquely shaped, computer-modeled venturi that manages pulse air volume and air delivery to the cartridge. This ensures the right pressure is uniformly applied when the system pulses air through the filter.
Today's cartridge collectors with oval filters offer up to 25% more airflow capacity, an improved airflow path, and up to 30% more pulse cleaning energy — all in a smaller collector.
While improvements in cartridge dust collection technology over the last three decades have allowed some users to trade in their baghouse collectors for cartridge collectors, the baghouse still has a strong and enduring presence in the industrial processing world.
Considered a reliable workhorse, the baghouse is often the best or only solution for industrial processes with high air volumes, heavy dust loading, and coarse, agglomerative dust particles.
The baghouse is used in many industrial processing markets including grain, metal, woodworking, food, foundry, cement, pharmaceutical, chemical, plastic, coal, and fertilizer. There have been several key improvements in bag filter media, filter cleaning technology, and the baghouse collector itself.
Improving the bag
Baghouses clean dirty air by collecting dust on the surface of the bags, removing dust from the bag surface, and then depositing dust into a collection container. If dust does not come off the bags, the pressure drop increases, resulting in reduced suction at the collection point.
In many applications, simple polyester bags allow the dust to be dropped off into a container. When processing sticky or hygroscopic dusts, special bags may be needed. Previously, the only options were singed or glazed polyester bags and fabrics such as polypropylene. Today, many more options are available. Bags can be chemically treated with materials such as silicone to improve the bag's release characteristics.
Polytetrafluoroethylene (PTFE) membranes can also be laminated to the outer surface of the bags, which allows the dust to stay on the surface of the bags rather than imbedding deep in the fibers of the fabric. When bags are pulsed for cleaning, the dust easily slides off the slippery PTFE membrane and drops into a collection bin.
Further improvements have occurred with oval-shaped bags. Bags are now available in oval and envelope shapes, which provide two distinct advantages. Oval or envelope-shaped bags increase the amount of bag material that can fit in a given area, increasing the collector capacity possible in tight spaces.
Oval and envelope-shaped bags flex better during cleaning and allow the dust to be easily knocked off the bags, which results in lower pressure drop and longer bag life. New technologies are now in development to provide lower cost, higher performance bag filters with longer life.
Baghouses can require 500 or more bags with lengths to12-ft or more, so upfront costs can be sizeable. With bag life running from 6-mo. for more difficult applications, and up to 7-yr for less taxing applications, bag life and cost are chief among the concerns of baghouse users today.
Collector enhancements have occurred to ensure proper cleaning and more efficient, cost-effective operation. In the past, dust could only be removed from the exterior of bags with high-cost, labor-intensive compressed air, typically 90 psi, or with a fan, which momentarily reverses the airflow and blows dust off the bag.
Now, a medium-pressure cleaning system, that uses a pump dedicated to the collector, pressurizes a tank to 8 psi and releases air at precise intervals to clean dust off the bags.
Medium air pressure systems also use a smaller motor than reverse airflow-type units, reducing annual energy costs. Moving the air tank from outside the collector to inside, closer to the bags, enhances the cleaning performance of medium-pressure collectors.
The inside air tank allows cleaning air to travel a shorter, more direct path to the bags, reducing energy loss and noise levels, while improving bag cleaning.
Envelope-style baghouse collectors are also available that require fewer bag changes and increase flexibility with compact, modular designs (Fig. 3). The flat fabric envelope-style bags are manufactured to high tolerances and mounted on a wire frame to ensure optimum airflow and thorough bag cleaning.
With envelope-style collectors, dust accumulates on the outer surface of the bag as air penetrates the fabric. A blow-pipe then injects a burst of compressed air into the filter bag, briefly reversing the airflow, inflating the bag, and dislodging the dust.
If you have any questions about dust collection, call the authors at 888-338-3878 or visit donaldsontorit.com . Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org .
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Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.
There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.
But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.
Read more: 2015 Salary Survey