What are filter bags?
Filter bag filters are used to remove large amounts of solid particles from liquids. A “filter bag” refers to the filter medium: a fabric bag (usually made of felt or mesh) that traps particulate matter as liquid flows through it.
The filter bag is placed inside a pressure vessel—often called a filter bag housing. Liquid enters the housing, passes through the filter bag, and then flows out as a clear liquid.
How Bag Filters Work
1.Liquid Entry
The unfiltered liquid (often called the feed or dirty stream, and in the case of water, raw water) enters the housing—usually through a top or side inlet.
Flow is directed into the inside of the filter bag.
2.Filtration Process
As liquid passes through the bag wall, particles larger than the pore structure of the media are trapped inside.
Smaller particles flow through with the filtrate.
Needlefelt bags filter through the full thickness of the media (depth filtration).
Mesh bags capture particles only on the surface of the fabric (surface filtration).
3.Filtered Outlet
The clean liquid exits through the outlet port, ready for use or further processing.
4.Bag Removal & Replacement
As solids build up, the differential pressure across the bag increases. Once it reaches around 1.5 bar, the bag should be changed.
Needlefelt bags are typically disposable, while mesh bags can sometimes be cleaned and reused.
Types of Bag Filter Media:
Needlefelt Filter Bags (Depth Filtration)
Made from non-woven fabrics such as polypropylene or polyester.
Trap particles throughout the thickness of the media.
Offer high dirt-holding capacity and broad particle removal.
Mesh Filter Bags (Surface Filtration)
Made from woven nylon, polyester, or metal mesh.
Trap particles only on the surface of the bag.
Best for applications requiring precise, consistent particle size cut-off (e.g., 50 µm, 100 µm).
Advantages of Bag Filtration:
High dirt-holding capacity.
Capable of handling high flow rates in a compact footprint.
Quick and simple bag changes minimize downtime.
Lower initial capital cost compared to cartridge filtration systems.
Flexible and effective across many liquid types and industries.
Applications:
Water Treatment – pre-filtration ahead of cartridge or membrane systems.
Paints, Coatings & Inks – removal of gels, agglomerates, and debris.
Chemicals & Petrochemicals – filtration of process streams, acids, solvents, and resins.
Food & Beverage – clarification of syrups, oils, and beverages (non-critical applications).
Metals & Mining – removal of fines, scale, and process solids.
