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By Ted Henderson, Donaldson Torit Applications Engineer
Proper maintenance of your dust collector is important in extending filter life and ensuring the collector consumes as little energy as possible. Dust collector owners and operators sometimes assume they’re being frugal by replacing a single damaged filter rather than all the filters. While this simpler and quicker fix may seem like an economical choice, it may actually result in more frequent filter replacements, more downtime, and higher electric bills – meaning increased cost.
Dust collectors are sized based on a number of variables including the specific particulate the collector is expected to remove from an airstream, as well as total air volume. The filtration velocity, or air-to-media ratio (AMR), is the ratio of the volume of air being filtered compared to the total filtration area available in the collector. In general, the smaller the particulate being filtered, the lower the recommended filtration velocity. This is true in part because smaller particulate presents specific challenges to filtration.
First, smaller particulate packs together much closer as it builds up on the filter surface, leaving less open space for air to pass through the collected dust. This increases pressure resistance and increases the frequency of filter cleaning required. A lower filtration velocity allows the particulate to build up with more open space between the deposited particles, reducing resistance and lowering the cleaning frequency. Secondly, smaller particulate has a much greater potential to penetrate the outer fibers of a filter where it can become embedded in the filter media. This depth-loading phenomenon is very difficult - if not impossible - to reverse, and will eventually create a permanent pressure rise in the media, leading to replacement. A lower filtration velocity reduces the energy available to drive small particulate into the media, resulting in less depth-loading and longer filter life.
Notice the particulate (or dust) gathering on the surface of the filters restricts the openings for airflow through the filter media. Some of this restriction is observed as an increase in resistance that can be managed with effective periodic cleaning, but eventually even excellent surface-loading filters experience long-term increases in resistance, or pressure drop.
This inevitable increase in pressure drop is typically a design consideration when sizing a fan. Fan selection must anticipate the final pressure drop across filters just as they are replaced to ensure the fan can deliver design air volume throughout the filters’ life. This typically means fans include four to six additional inches of static capacity to account for the eventual rise in pressure drop over the total life of the filters.
To ensure the fan does not pull excessive air volume when the filters are clean and do not have their final static resistance, the designer should include a damper or other means to control air flow. One technique involves the use of an Airflow Controller in combination with a variable frequency drive (VFD) to adjust the fan speed to control air volume to design levels while resistance across the filters is low.
Partial filter replacement is not ideal because the resistance across all the filters in a dust collector will be uniform during operation, but when a single new filter is installed, it has much less resistance than the other dirty filters. In fact, new filters often display less than one inch of resistance at design flow while seasoned filters often have 3 to 4 times that much resistance. When a single new filter is installed in the dust collector with other filters that have an existing resistance because of dust, the new filter experiences a significant airflow increase until the resistance across the new filter equals the resistance across the dirty filters.
Instead of the new filter handling a proportionate percentage of design flow, the airflow increases well above design flow and, as a consequence, the new filter experiences immediate depth-loading, radically reduced filter life, and the collector runs at a higher pressure drop.
This reduction in filter life for the replaced filter will impact all the filters and may result in more filter purchases and certainly creates more downtime. In addition, running a collector at a higher pressure drop increases compressed air consumption and requires more energy to maintain airflow through the dust collector.
A complete filter change out allows a uniform pressure drop across all the filters inside the dust collector. This allows all the filters to carry a proportionate amount of the air volume, minimizing pressure drop. Uniform pressure drop from a complete filter change also allows the operator to control the filtration velocity through all filters. This saves compressed air and extends the life of the filters by minimizing filter cleaning. The design airflow can also be more easily maintained so the proper AMR is maintained, and depth loading of the filter media is minimized. Less depth-loading results in longer filter life and saves money in the long run.
And don't overlook the likelihood the damaged filter being replaced is not the only damaged filter in the collector. Replacing that single filter leaves other partially-damaged filters in service only to have them fail in the coming days or weeks. Delayed replacement of older filters means operators will have to shut down and investigate multiple filter failures at an increasing rate, which increases downtime, increases labor costs, and forces the collector to operate at the elevated pressure drop of plugged and damaged filters for an increased period of time, increasing operating costs.
Bottom line: when you discover a damaged filter in your collector, consider the relative age of the filters and the potential advantages of reducing the operating pressure drop of the collector with a new set of filters. Consider the possibility to avoid additional unscheduled downtime and maintenance cost, and remember that sometimes replacing all the filters that are about to wear out is a better investment than only replacing the ones that have already failed.