A Practical Guide to Working with Filter Bags

Filter bags are one of the simplest and most widely used first stages of liquid filtration in industrial processes. They are valued for their cost-effectiveness, ease of use, and ability to handle high flow rates. But how do you get the best performance from a bag filter system—and why are they applied across so many industries?

In this guide, we’ll break down the principles of bag filtration, explain the key components, and highlight where and why they are best applied.

What is a Filter Bag?

A bag filter is a pressure-driven filtration device designed to remove bulk solids from a liquid stream. The “bag” is the filter medium: a fabric pouch (typically made of felt or mesh) that traps particles as liquid flows through it.

The filter bag is held inside a pressure vessel—commonly called a bag filter housing. Liquid enters the housing, passes through the filter bag, and exits as clarified liquid.

How Bag Filters Work

The operating principle is simple but effective:

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

The choice of bag material determines performance:

• 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).

Key Design Considerations

When specifying or operating a bag filter system, engineers should account for:

• Micron Rating

  • Defines the particle size retention capability (typically 1–1000 µm).
    • Needlefelts (polypropylene, polyester, nylon, PTFE & Nomex) range from 0.5 – 200 µm.
    • Meshes (Nylon, polypropylene, polyester, PVDF and ETCFE) range from 1 -  1,200 µm.

       

       

• Bag Size & Surface Area

  • Larger bags (e.g., Size 2 vs. Size 1) support higher flow rates and longer service life.
  • As a guideline:
    • Size 1: up to ~12 m³/hr
    • Size 2: up to ~26 m³/hr

       

       

• Bag Filter Housing Design

  • Single-bag housings (sizes 1, 2, 3 or 4) are common for small to medium flows.
  • Multi-bag housings (with up to 24 bags or more) are used for large-scale industrial systems.

     

     

• Operating Pressure & Temperature

  • Both bags and housings must be compatible with the process.
  • Housings are typically designed to meet UK, European, and US pressure vessel regulations.

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.

Typical Applications

Bag filters are widely used wherever bulk liquid filtration is needed:

• 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.

Summary

Filter bags work on a straightforward principle: force liquid through a porous bag to separate solids from the stream. Despite their simplicity, they offer versatility, cost-effectiveness, and reliability.

For engineers and technicians, understanding how bag filters work—and how to specify the right size, micron rating, and media type—ensures systems run efficiently, downtime is minimised, and processes are protected

Looking for Bag Filter Products? Here’s our full range of Filter Bags and Bag Filter Housings. And If you have any questions on bag filtration, then you can give us a call or send us an email - we’d be more than happy to help. 

You can also read more in our blogs:  

• Bag Filtration: Often the First Step in the Filtration Process
• Bag Filtration: Comparing Needlefelts, Monfilment Meshes & Other Materials
• Choosing the Right Filter Bag for Your Application

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