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By Mark Belcher, Donaldson Torit Filtration Engineering Manager
Filter pre-coating is the practice of intentionally loading a porous particulate layer onto a filter media in a dust collector to enhance some aspect of performance. For reasons discussed below, pre-coating is sometimes considered advantageous for pleated cartridge filters, but you may not find it to be the best choice for many applications.
There are two basic reasons pre-coating would be considered for new clean filter media. Before considering the possible use of pre-coating, you should understand the differences and impacts of it, including how a single application of pre-coating may only be a temporary fix.
Pre-coating is often suggested as a means of increasing the initial efficiency of lower performance filter media. New, unused cellulose or cellulose-synthetic blend filter medias often offer initial efficiencies of only 30% on sub-micron particles (based on ASHRAE 52.2 testing). The filtration efficiency performance does gradually improve as dust is deposited on the media (as it’s loaded), but this enhanced efficiency performance can be accelerated with the use of pre-coating. As the pre-coating builds a layer of particulate on the surface of the filter media, it acts as a pre-filter, capturing submicron and other particles from the airstream before they can penetrate into the filter media, so overall filter efficiency improves.
Pre-coating suppliers often suggest amounts of pre-coating material application, from as little as 0.5 lb (or 227 grams) of pre-coating per cartridge filter to up to 2.5 lbs (or 1,134 grams) of pre-coating per filter. It is worth noting that while the addition of pre-coating to the filter can increase initial efficiency, the addition of pre-coating also means there will be an increase in filter restriction to airflow. Depending on the type and amount of pre-coating, this increase in filter restriction can become very significant and the increased pressure drop will require more energy to move air. Additionally, there is a cost associated with the application of the pre-coating, as well as the disposal of the secondary waste stream.
Pre-coating filter media for enhanced filtration efficiency is only a temporary fix. Cartridge collectors typically extend filter life by using compressed air cleaning pulses to periodically remove accumulated “dust cake” from the surface of the filter media. When cartridges are pulse cleaned, substantial amounts of collected particulate and pre-coating materials are both dislodged and removed from the media surface. The media once covered by pre-coating now behaves more like untreated filter media. The efficiency adjusts accordingly because the filter media is exposed to fine particles that can penetrate into and bleed through the media.
The end of life for filter media occurs when pressure drop across the filter media becomes either high enough to substantially restrict airflow or the increase in energy cost to operate the fan against the increased resistance reaches such a level it becomes impractical to continue to operate the fan with plugged filters. The general logic behind claims of extended filter life through pre-coating is dependent on the assumption that pre-coating minimizes the amount of small particles penetrating into the depth of the filter media (minimizing the rate of plugging for the media). That may be a reasonable assumption immediately following the application of pre-coating material because the pre-coating material acts as a barrier to smaller particles entering the actual media. However, as discussed earlier, any pulse cleaning of the filter media will disrupt that barrier, and since the pre-coating is removed with each pulse cleaning, its benefits are also removed. After pre-coating has been disturbed and lost during pulse cleaning, the filters begin to load just like untreated filters, and the operating pressure drop again begins to increase accordingly as the media begins to plug, see Figure 1.
Premium grade filter media can offer the enhanced filtration characteristics promised by pre-coating without the potential loss of filtration performance that occurs with the loss of the pre-coating materials during pulse cleaning. This opportunity for enhanced performance is explained below.
To avoid the disadvantages of pre-coating, users could instead utilize premium grade filters. Premium grade filters generally offer some form of permanent surface treatment that collects particulate on the surface of the media and protects the substrate from dust loading. The most common types of surface treatments include directly-applied fine fibers, meltblown laminations, or PTFE layers. Meltblown-coated premium filters offer efficiency but don’t pulse clean effectively because sub-micron particles are often captured in the depth of the meltblown layer. This results in shorter overall filter life. PTFE-coated filters are also usually efficient and in most situations pulse clean effectively, but they typically operate at a higher air resistance or pressure drop (and use more energy). Typically, they are also much more expensive than other premium grade filters. Filters with a directly-applied fine fiber surface layer offer excellent efficiency and pulse clean effectively - plus they operate at relatively low pressure drops and are generally are very cost effective, see Figure 2.
Of the premium grades, fine fiber filters offer the best option in most cases. Medias for premium filters with fine fiber are composed of a substrate (often similar to generic cellulose cartridge media) with a permanent surface layer of fine fibers. This layer is often less than one micron thick, so plugging or depth loading in this layer isn’t an issue. The premium fine fibers on high quality fine fiber cartridges will be fine fibers, usually measuring 0.3 microns or smaller in diameter, and they will form a permanent matrix of small pores on the surface of the filter media. Because the fibers in the surface layer are so small in size, they don’t noticeably increase the resistance to air so initial pressure drop stays lower.
If a user wants higher initial efficiency, fine fiber filters are an excellent option. Efficiency of a clean and new fine fiber cartridge will be much higher than a generic cartridge; typical fine fiber cartridges are at least 65% efficient on sub-micron particles while a generic filter may struggle to achieve 30 % initial efficiency, and operating efficiency for the fine fiber filter will increase very quickly. ASHRAE 52.2 testing demonstrates that after one ounce (or 28.35 grams) of particulate is fed to a fine fiber cartridge, its sub-micron efficiency can easily exceed 90%.
Users also seeking longer filter cartridge life will benefit from the use of premium grade fine fiber filters. In operation, dust loads on the surface of the fine fiber media. When the media is pulse cleaned, most of the dust cake is dislodged from the media and is removed from the filter. The fine fiber layer though remains intact, and continues to protect the substrate from depth loading and blinding. Because depth loading is no longer significant, the fine fiber cartridges continuously clean to a lower operating pressure drop than any of the other media options, and produce significantly longer life, see Figure 3.
Finally, the enhanced pulse cleaning performance of premium fine fiber media results in less frequent demand for cleaning and a lower average resistance to airflow (pressure drop) across the filters because the substrate does not suffer the depth loading associated with generic media filter. The lower pulse cleaning demand conserves compressed air, and because premium grade fine fiber filters operate at a lower average operating pressure drop, the premium fine fiber media cartridges save horsepower during fan operation. The combination of saved horsepower and compressed air savings can total significant energy savings over their useful life.
The extended life, improved efficiency, and energy savings offered by premium grade fine fiber cartridges mean they are generally a more cost effective solution than pre-coating.