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Q: How do filter media, air-to-media ratio, and airflow affect dust collection in our tableting operation?
A: David Steil, Camfil Air Pollution Control, says:
To keep your dust collector operating efficiently over its lifetime, you must select the right filters and filter media, which requires that you understand how the system's air-to-media ratio affects its operation. A dust collector's air-to-media ratio, sometimes referred to as the air-to-cloth ratio, is the volume of air flowing through each square foot of filter media per minute.
An incorrect ratio can lower filtration efficiency, exposing workers to more particulates and risking cross contamination. In addition, improper dust filtration can create conditions that foster dust explosions.
To calculate the optimal air-to-media ratio for your dust collector, divide the system's airflow in cubic feet per minute (cfm) by the total square footage of the filter media in all of the cartridges.
The right ratio
Generally, the higher the airflow and dust concentration in your dust collection system, the more filter media you need. High volumes of airflow and dust require a low air-to-media ratio. For tablet-press applications, the typical air-to-media ratio should be 2.5 to 3.0 cfm of air per square foot of media.
Some equipment suppliers recommend a small collector because it requires less space and costs less, but the air-to-media ratio may be as high as 5 to 1. Overly high ratios can cause inconsistent airflow that can create problems, including static pressure that is out of specification, causing the tablet press to malfunction. A too-high ratio can lead to frequent and excessive pulse filter cleaning, shortening filter life and impeding the operation of the dust collector and tablet press.
When a filter becomes overloaded, with dust penetrating deeply into the filter media faster than the pulse cleaning system can remove it, the negative pressure increases, and cleaning becomes less effective. As a result, the filter clogs faster and needs to be changed more frequently, increasing the cost of consumables, requiring maintenance downtime, and delaying tablet press production.
On the other hand, if the air-to-media ratio is too low or if the collector uses too much filter media for the volume of particulates collected, it can severely reduce your dust collector's efficiency.
Determining the right ratio can enable your dust collector to perform reliably for 1 to 2 years without filter changes and will reduce maintenance costs (photo).
Amount of airflow
On average, dust collectors used in coating and fluid-bed dryer operations need higher airflows and pressures than those serving tablet manufacturing. This is especially true for continuous coating processes, because dust loads are heavier. Also, the additional moisture involved with coating and drying operations compared to tableting may require more frequent filter changes if you don't use the proper filter design and media.
A dust collector for solid dose operations can come equipped with one or two primary filter cartridges. A single primary filter can serve processes that don't require more than 600 cfm of airflow, depending on the square footage of the media in the cartridge.
Collectors with two filters are typically designed for airflow volumes between 590 and 1,765 cfm to increase filtration capacity and allow for a properly sized collector based on a specified air-to-media ratio.
Some suppliers pleat the filter media to maximize the square footage they can fit into each cartridge. This can result in the filter being packed so tightly with media that it becomes inefficient.
In that case, much of the media isn't usable because it's blocked from the airstream and unavailable for filtering. This means that the filter can't load as much dust, and pulse cleaning is much less efficient.
Filters with separators to hold the pleats open and keep them evenly spaced expose much more of the media to the airstream for filtering. This design enables a much higher airflow per square foot of media and allows more dust to be loaded onto the filter and then released during pulse cleaning. The open, breathable design maintains a lower pressure drop across the media, cuts down on compressed-air usage, and reduces the energy demand on the dust collector's motor.
Type of filter media
Tablet presses generating hazardous, fine, dry dust often require high-efficiency filter media for the dust collector to adequately capture the particulates. For example, filter media made of a polyester-cellulose blend with a nanofiber surface layer can provide 99.999 percent efficiency for 0.5-micron-and-larger particles.
When a tablet press uses a wash-in-place system, the filter media is exposed to moisture during the cleaning process. In that case, consider a high-efficiency, spun-bond polyester media that can include an oleophobic treatment.
David Steil is pharmaceutical market manager at Camfil Air Pollution Control, Jonesboro, AR. The company manufactures dust collection equipment and replacement filters for the pharmaceutical and other industries. For information call 800 479 6801 or email the company.
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