Trends in Pleated Cartridge Filters
Choosing the right pleated cartridge filter can improve your cartridge collector's dust capture efficiency, help it operate more reliably, and reduce its energy and maintenance requirements. To help you make an informed choice, this article discusses some recent advances in pleated cartridge filter technology.
A cartridge collector is the most popular dust collector for bulk solids handling operations today. This high-efficiency unit is more compact than a baghouse collector and operates at a lower pressure drop. The cartridge collector can be designed to reduce emissions well below OSHA limits, often allowing it to recirculate the air back to the plant for significant energy savings. However, many of us mistakenly think of the collector's replaceable pleated cartridge filters as generic, interchangeable items. In fact, design and performance can vary markedly from filter to filter.
Making an informed decision when selecting a cartridge filter can improve your collector's dust capture efficiency and reliability while reducing its energy and maintenance requirements. This means you need to consider several inter-related factors - including the filter's mounting position, media type, configuration, and pleat spacing - and recent technology advances for each. During your selection process, it can be helpful to work with an independent dust collection consultant. This expert can help you sift through the many new filter technologies available and find the right combination of filter features for your application.
Figure 1 - Cartridge Collector Designs
How a cartridge filter is mounted or installed in the collector has a major impact on the pleated media's performance. In early cartridge collectors with vertically mounted filters and a clean-air plenum located at the collector top, dirty air would enter the collector's hopper and flow upward into the filter housing, as shown in Figure 1a. During the cleaning cycle, dust could be cleaned off the filters only if the upward-flowing dirty air was flowing slowly enough to allow the dislodged dust to fight its way back down through the dirty airstream. This arrangement worked well for heavy dusts, but not for lighter ones.
The upward airflow through these cartridge collectors, where the media was packed much more tightly than in a baghouse collector, resulted in an upward can velocity (the average velocity of air approaching the filters using the collector's entire cross-sectional area) that created a limit on the amount of air that could enter a collector of a given size. For this reason, the next cartridge collector design incorporated filters mounted on their sides - that is, horizontally - so that the clean air plenum was now at one side of the collector, as shown in Figure 1b. The incoming airflow entered through the collector top, creating a downflow effect to help dust dislodged during the cleaning cycle overcome the upward can velocity found in the older vertically mounted cartridge collectors. This design change improved the cartridge collector's performance over that of the older hopper-entry collector.
Figure 2 - Dust accumulation on horizontally mounted cartridge filters.
Yet mounting a filter on its side has some limitations. The biggest problem is that the dust doesn't get cleaned off the filter's upper side, causing the dust to blind at least one third of the entire filter, as shown in Figure 2. This dust accumulation increases the air-to-media ratio (determined by dividing the process air volume by the filter's media area; also called air-to-cloth ratio) and can lead to premature filter failure. One way to reduce this problem is to rotate each filter 180 degrees once a month. Another problem with horizontal mounting is that, because all the incoming dust enters at the collector's top and lands on the filter tops, there's no chance to preseparate heavy or abrasive particles from the dirty airstream. This can shorten the filter life.
Today's newest cartridge collectors have vertically mounted filters and a high, side-entry airflow inlet that directs the air into a cross-flow pattern across the collector at the same height as the filters, as shown in Figure 1c. The air first flows through a series of staggered baffles. These baffles distribute the air throughout the housing and act as a classifier, separating the heavier particles so they drop straight into the hopper without contacting the filters. This cross-flow effect eliminates upward can velocity, thus providing a downward airflow pattern without having to turn the filters on their sides.