Felt filter bags – technical overview
Felt filter bags are selected for depth filtration where higher dirt holding capacity and stable operation are required. Contaminants are retained throughout the nonwoven felt structure rather than only on the surface, which supports longer service life in applications with variable or elevated solids loading.
Filtration principle
Felt filter bags provide depth filtration by capturing particles within a three-dimensional media matrix. Compared to surface filtration (e.g. monofilament mesh), felt typically offers higher dirt loading and a more gradual differential pressure increase over the service cycle. This makes felt bags suitable for general duty process filtration and water treatment duties where robust operation is required.
Felt materials and options
- Polypropylene felt – common choice for many aqueous and alkaline streams, offering broad chemical resistance for general process and water applications
- Polyester felt – widely used for industrial filtration, with good dimensional stability and suitability across many process fluids
- Nomex felt – selected for elevated temperature applications where polymer alternatives are not sufficient
- PTFE felt – selected for demanding chemical environments where high resistance is required
Retention behaviour
- Depth retention – particles are retained throughout the felt thickness, supporting higher dirt holding capacity compared to surface filtration
- Nominal performance – felt bags are typically selected for nominal micron duties where service life and robustness are priorities
- Process stability – stable operation depends on matching micron rating, bag size and housing seal to the application to minimise bypass
Bag formats and dimensions
Felt filter bags are supplied in common industrial bag sizes (e.g. Size 1 and Size 2) and can be configured with different sealing options (ring types and constructions). Correct fit between bag and housing is a key factor to prevent bypass and maintain consistent filtration performance.
Typical applications
- Process filtration (chemicals, coatings, resins and general industrial liquids)
- Water treatment and utility water filtration
- Pre-filtration to protect downstream cartridges or membrane stages
- Protection of pumps, valves, spray nozzles and heat exchangers
Key selection criteria
- Required micron rating and expected contaminant profile
- Solids loading and target service interval (dirt holding capacity)
- Media compatibility with the process fluid (chemistry and temperature)
- Bag size, sealing type and housing configuration (bypass control)
- Allowable differential pressure and change-out procedure
Felt filter bags are sometimes also referred to as depth filter bags, nonwoven filter bags or high dirt holding capacity filter bags, depending on the industry and application context.
Felt filter bags are frequently used upstream of absolute filter bags or monofilament filter bags as part of a filtration train, depending on whether the next step requires defined retention or surface filtration.