Abridged from “The Advantages and Disadvantages of Pre-Coating Pleated Filter Cartridges” 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. 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.
PRE-COATING FOR HIGHER INITIAL EFFICIENCY
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. 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.
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.
Pre-coating filter media for enhanced filtration efficiency is only a temporary fix.
PRE-COATING FOR LONGER LIFE
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. However, 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.
Figure 1. The effect of pre-coating material on a filter.
A BETTER SOLUTION
Of the premium grades, nanofiber filters offer the best option in most cases. Efficiency of a clean and new nanofiber cartridge will be much higher than a generic cartridge; typical nanofiber 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 nanofiber filter will increase very quickly.
Users also seeking longer filter cartridge life will benefit from the use of premium grade nanofiber filters.
Finally, the enhanced pulse cleaning performance of premium nanofiber 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 nanofiber filters operate at a lower average operating pressure drop, the premium nanofiber 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 nanofiber cartridges mean they are generally a more cost effective solution than pre-coating.
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