Baghouse versus cartridge dust collectors
Q: What factors should we consider when deciding between a baghouse or a cartridge dust collector?
A quick search in any venue related to the dust collection industry will uncover a number of divisive opinions and experiences on this question. Perhaps the question is better phrased as, "When should someone choose to use a cartridge dust collector instead of a 'traditional' baghouse dust collector using bags and cages?" The answer is straightforward:
A cartridge dust collector should be considered instead of a traditional baghouse when the application has space limitations and the operating conditions match those in which cartridge dust collectors exhibit the best performance. I emphasize the word and because space considerations or equipment costs are often the driving factors in purchasing decisions, and sufficient consideration isn't given to the potential problems that can occur with this dust collector.
Cartridge collector development
Cartridge collectors were developed for applications with a low grain loading of dry, fine particulates, such as fumes and dusts associated with paint spraying, welding, and foundry venting. Cartridge collector manufacturers promoted the ability to handle the same volumetric gas (air) rate as a traditional baghouse in a smaller space or footprint
because of the cartridge filter element design, and a new market segment took off, with many companies offering some variation on the general design and seeking to apply the technology to different applications.
Operational history shows the cartridge filter element attributes that facilitate success in some applications become hindrances in applications with higher grain loadings, problematic process conditions, different shapes, and material properties other than those of dry, fine particulates.
General issues with cartridge elements
A typical cartridge filter element consists of pleated filtration media placed around a typically metal core, outer straps attached to the pleat pack to keep the pleats from blowing out, end plates required for the installation (top-open-bottom-closed or open-open for stacking purposes), and sometimes an outer cage around the pleat pack. Typical cartridges have pleat depths between 11⁄2 inches to 2 inches and 300 or higher pleat counts approximately 12 or 13 inches in diameter.
The cartridge configuration (see Figure 1) that enables it to handle a high volumetric gas rate also inherently creates narrow pockets where material will collect. One fairly common configuration has an approximately 1
-inch pleat gap at the pleat tip. The narrow pockets render this configuration unsuitable for materials that are fibrous or agglomerative (either in their typical state or because of a process upset) or grain loadings where the pockets may substantially fill between cleaning pulses.
In many installations with higher grain loadings or material concerns, a cyclone or other separation device may be required before the collector to reduce the likelihood of the aforementioned issues. These additional devices not only increase project costs, they increase the energy costs for running the system (higher fan static-pressure requirements).
Horizontally mounted cartridges versus vertical
To minimize the footprint of a cartridge collector, several manufacturers mount the cartridges horizontally in columns several elements tall. Two or more columns, often referred to as banks, are typically installed in a single plenum.
This orientation can be problematic because the pockets' top sections often fill with dust that returns to the pockets after a cleaning pulse or with dust that's discharged from an element higher in the column.
Retained material can eventually fill the pockets in this area, which reduces the effective cloth area and leads to higher filtration velocities. The resulting higher filtration velocities can then affect the elements' cleaning performance, leading to a higher differential pressure across the media, higher cleaning energy, and reduced media life.
To account for this potential issue with this style of collector and to keep filtration velocities within recommended limits at the system's design volumetric rate, you would need to increase the number of elements accordingly. Another option is to install a separation device before the dust collector to reduce the grain loading to the media. In either case, the project cost would increase.
Several cartridge collector designs have vertically oriented elements to facilitate dust discharge, which is certainly an improvement over a collector with horizontal elements, but results in a larger footprint and installation height because of the required hopper and plenum designs. Moreover, an often-overlooked operating parameter can become especially problematic with media of this configuration and orientation.
Interstitial velocity concerns
The interstitial velocity through the cross section of the dirty air plenum is an important design parameter regardless of the collector design or media. High interstitial velocities impede dust settling into the hopper, increase the dust loading to the media because of material recirculation, and often lead to material buildup underneath the tube sheet or between the elements, thereby reducing effective cloth area.
For vertically mounted filtration media (cartridges, bags and cages, or pleated filter elements), cloth area is often increased by increasing the length of the media element (including cartridge stacking). Increasing cloth area (for an increase in volumetric gas rate) without adjusting the housing cross-sectional dimensions results in higher interstitial velocities. This issue will also occur with horizontally mounted elements if the cloth area is increased by adding elements to the column height rather than adding additional columns.
The interstitial velocity in a collector becomes an issue much quicker with filter elements comprised of pleated filtration media than with traditional bags, or socks, because pleated media provides a higher increase in flow-handling capability per unit length than a traditional bag. This aspect can impose a law of dimensioning returns because the size of the required cleaning components can also be readily affected by the increase in flow-handling capability.
A better option for tight spaces
Pleated filter elements are similar in general construction to cartridge filter elements. Although they were originally developed as a solution for problematic operations in existing collectors with bags and cages, smaller footprints for new equipment installations can be achieved when using them.
Figure 2 shows a typical pleated filter element; the design has a much wider pleat gap and shallower pleat depth than a cartridge filter element. This enables the technology to be applied to a wider range of dusts and materials as well as gas streams with higher grain loadings.
A dust collector using pleated filter elements in many cases will have a larger footprint than a cartridge collector handling the same volumetric gas rate. However, the difference in a pleated filter element's configuration significantly reduces the likelihood of issues when process upsets occur or the application has a higher grain loading; in which case, the more reliable operation of a properly designed dust collector with pleated filter elements will offset the additional costs, if any, above those for a cartridge-style collector.
There's a drawback to purchasing either style of collector that should be taken into consideration:
When installing a collector that uses pleated filter media (cartridges or pleated filter elements), unless appropriate future operating levels are considered in the design of the collector, you will impart a maximum flow limitation for the system as there are no media options that will increase the cloth area other than increasing the filter elements' number of pleats or the pleat depth. Depending on the situation, such changes may work to the detriment of the media's performance.
My intention in this discussion isn't to assert that cartridge collectors will only work satisfactorily in specific applications or with specific dusts — there are certainly examples to the contrary. Rather, I strongly believe that cartridge collectors can readily exhibit problematic or inconsistent performance in applications that fall out of their niche or when the process conditions change, and that modifications or additions made to the basic equipment configuration of a cartridge collector to combat potential issues can quickly diminish the economic benefit.
For space-limited installations, a properly designed dust collector with pleated filter elements will generally operate with much less risk and provide better performance than a cartridge-style collector and therefore should be the first consideration as an option to a traditional baghouse with bags and cages.
David Renfert, PE, is a consulting mechanical engineer at Axis Engineering LLC, a consulting practice headquartered in Kansas City, Mo.
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