Filter Media For Dust Control
Filter bags made with spunlaced microfiber nonwovens are used to recover dust from production of foodstuffs and other materials and prevent contamination of the recovered products.
Jürg Rupp, Executive Editor
Years ago, the biggest nonwovens producer in the world, Germany-based Freudenberg Nonwovens, started the successful launch of its Evolon® product. This fabric made of spunlaced microfibers became a global success in many applications, even for apparel. One of the latest innovations in adapting Evolon for new end-uses is Viledon® NEXX filter media for foodstuff production and other end-uses.
This scanning electron micrograph (SEM) shows the size of a human hair, shown in front, compared to Evolon® microfibers, shown in the background.
These filter bags are the newest generation of surface filters and offer advantages when compared to traditional filters manufactured using needlepunch technology. According to Freudenberg, these filters have a considerably lower weight than traditional filters but are stronger and longer-lasting, and are more cost-effective and have lower emission values. Compared with needled felt filter bags, the Viledon NEXX bags use about half of the resources during manufacture, and filtration performance is higher than that of the needled filters. The NEXX filter bags are available with and without antistatic finishes. Freudenberg developed the Evolon microfiber base material specifically with dust removal technology in mind.
In many production areas, high dust load occurs, which disrupts the manufacturing of the products. This high dust load can occur in processes for production of sugar, plastics and chemical products; and even in steel mills or foundries, or for surface treatment processes. For these areas, Freudenberg developed Viledon NEXX filter bags. The bags are especially suitable for the recovery of costly products used in food and pharmaceutical industries, or for silo storage applications. In the flour milling process, they can be used to recover flour dust from extracted air. The extra-fine continuous-filament microfiber surface protects the recovered products from contamination.
The basic Evolon material is a 70-percent polyester/30-percent polyamide bicomponent nonwoven. The cake-like cross-section of the filament is eventually fibrillated using hydroentangling technology. The fibrillation results in a very fine microfilament. The basic Evolon fabric as well as the new high- quality filter media are especially suitable for surface filtration applications.
This SEM shows the dense structure of Evolon microfiber.
Initial conditioning of the Viledon NEXX filter material virtually prevents dust penetration into its structure. ”The integrated microfiber layer creates a stable dust cake which prevents irreversible adhesion of dust particles inside the filter medium,” Freudenberg explains. “The result is to create a stable operating condition and reduced resistance to flow. Continuous operation over long periods at minimum pressure loss becomes possible.”
Long Filtration Cycles
One of the key issues for an economical product is its lifecycle. The NEXX filter bags are said to be “sturdy and resilient.” As Freudenberg reports, compared to traditional filters, “they are subject to lower mechanical stresses owing to lower operating pressure differential. This advantage extends the filtration cycles and the cleaning intervals. Costs for maintenance and replacement are minimized and the consumption of expensive compressed air for pulse jet cleaning is reduced.” These advantages can potentially reduce electrical energy consumption.
Thanks to the microfiber layer of such a filter bag, removal of dust from the media surface can be achieved easily. Continuous clean gas values of less than 1 milligram per cubic meter are possible. At 240 grams per square meter, the filter media is approximately half the weight of traditional needle felts, and its mechanical stability is high, Freudenberg notes. The light weight facilitates handling during filter bag installation and removal, even when the filter is contaminated with dust. Traditional installation methods can be used, and no special tools are needed.
Enhancing The Performance
If the workplace requires very low dust concentrations or separation of harmful or carcinogenic dust, a second filter stage with static filters can be installed after the NEXX filter bags. Such an arrangement also allows air to be recirculated into the workplace, which can result in reduced heating costs, particularly during cold weather. The filter bags can be pre-coated to comply with user specifications, for example, to handle sticky, oily or other dust types.
Inorganic Pigment Separation
According to Freudenberg, the Viledon NEXX filter media has already realized successful applications for the separation of inorganic powder coating pigment dusts, whose particle sizes are mainly in the nanometer range. “To create the best surface finish, the (powder coating) pigment industry aims to manufacture pigments with the smallest possible particle diameters,” the company reports. “This type of dust does not create a suitable dust cake for surface filtration. For a filter medium to work effectively on this application it is necessary to have extremely fine fibers and pores like that exhibited by Viledon® NEXX filter media.”
The company further notes that because silicones are not used in the manufacturing process, the filter media also is suitable for the recovery of powder coating pigments used in the automotive industry.