Paints, inks, cutting oils, pharmaceutical and food products… Bag filters are widely used for air and, primarily, liquid filtration. Cost-effective and available in a wide range of materials, filter bags can also be easily customized in size if required. Here is a brief overview of this family of filtration devices.
Standard bag filters
Filter bags perform inside-to-outside filtration: contaminants are retained within the bag, preventing downstream contamination. Sewn or heat-welded, they are designed to be inserted into a dedicated filter housing, mostly made of 316L stainless steel.
Welded bags offer the advantage of completely eliminating bypass of unfiltered liquid at the seams.
Standard filter bags are equipped with O-rings (galvanized steel or stainless steel) or molded collars at the open end. Some filter bags use a drawstring instead.
Also called “filter socks”, they can be installed directly at the outlet of a system, mounted on the pipes without a dedicated filter housing or adapter. These filter socks are intended for gravity filtration applications, where closed-environment filtration is not required.
Expert advice
The side seams of sewn filter bags can be reinforced to increase their pressure resistance.
The materials used to manufacture filter bags are carefully selected based on the application. It is particularly important to ensure that the media do not contaminate the filtered fluids through leaching.
Example of filter-bag use in the chemical industry
Filtres à poches : quel média ?
Different materials and weaves can be used to manufacture filter bags. The choice depends on the desired filtration performance and the operating conditions (fluid temperature, pressure, flow rate, chemical properties of products and solvents, etc.).
Felts are versatile and cost-effective, efficiently retaining solid and deformable particles (colloids). They enable depth filtration with low pressure drop. Felts are available in several materials: polypropylene, polyester, long-life polypropylene, long-life polyester, Nomex®, and, under certain conditions, may be suitable for food contact (CE 1935/2004). Standard filtration ratings range from 0.5 to 200 µm.
Expert advice
To prevent fiber shedding, the external surface of felt filters is calendered. Passing the felt between two heated rollers adjusts its permeability and smooths the surface.
Example of use of felt filter bags in the cosmetic industry
Monofilament filter meshes are made from woven synthetic threads such as nylon, polypropylene, polyester, or PTFE. They are available with filtration ratings from 1 to 2000 µm, are easy to clean, and provide high tensile and abrasion resistance.
Filter bags can also be made from nonwoven, multilayer, absorbent, or antistatic materials to meet specific industrial requirements.
A solution for every need
Food and pharmaceutical industries
Filter bags used on production lines for food and pharmaceutical products must comply with European standards for materials in contact with food and health products.
Their manufacturing and storage conditions must be strictly controlled to prevent any contamination. The molded collars are made of polypropylene, and the rings are made of stainless steel.
High particulate-retention filter bag
Some applications require high filtration efficiency. By using two filter-media cylinders instead of one, the filtration surface area can be increased by approximately 70% at the same volume. These high-capacity filter devices capture a larger amount of unwanted particles. Their extensive filtration surface also allows them to handle higher fluid flow rates than standard filter bags.
Long-life filter bag
Long-life filter bags use special filter media in polypropylene or polyester with a density gradient. Larger particles are trapped on the surface, while progressively smaller particles are captured within the depth of the bag, reducing clogging. These devices offer an extended service life, lasting up to five times longer than standard filters. They are particularly recommended for production lines where bag replacement is delicate or risky. The filter bags are fully welded to eliminate any risk of fluid bypass at the seams.
Learn more about filter replacement intervals.
High-efficiency filter bag
High-efficiency filter bag
Some applications (beverages, cosmetics, electronics, etc.) require low filtration ratings (approximately 0.2 to 20 μm) and involve fluids heavily loaded with particles. For these cases, filters must retain large quantities of impurities, including very fine ones. Composite media, combining polypropylene or polyester with microfibers in multiple layers, are used. Their efficiency can reach 99.98%.
Learn more about filter efficiency
These high-efficiency filter bags can effectively replace filter cartridges in many applications, often offering superior performance at a lower overall operating cost. They can also serve as pre-filters to extend the life of absolute-rated filtration cartridges, which are very expensive.
Expert advice
High-efficiency filter bags can also be manufactured from media compliant with European and FDA standards for food-contact applications.
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