Filter bags, also called filter socks are designed to remove suspended solids, contaminants, and particulates from various fluids.
What is a filter bag?
A filter bag is a porous filtration device shaped like a bag or sock that captures solid particles as liquid flows through it. They fit inside a bag filter housing or tied to the end of pipe or hose with a clamp. Liquid enters from the open end of the bag, passes through the bag's media, and exits as filtered fluid while contaminants remain trapped inside the bag.
How do filter bags work?
As the liquid passes through the bag's filtration media, particles larger than the bag's micron rating are captured on the surface or within the depth of the material. Clean, filtered liquid then exits the housing while solids accumulate inside the bag until it requires replacement.
What is a micron rating (µm) in filter bags?
A micron rating (or micron size) refers to the size of the particles a filter is designed to remove from a liquid. Here are some examples for reference:
µm is the symbol for micron rating, also called micron
Water silt / turbidity particles 2 – 50 µm
Dust or flour particle 10 – 50 µm
Fine beach sand 90 µm
Human hair 70 – 100 µm
Red blood cell 8 µm
Sugar granules: 100-500 µm
Salt crystals: 100-300 µm
Larger sediment, rust flakes 250–500 µm
Coarse silt, large debris 1000+ µm
Picture of a used felt type filter bag
What materials are filter bags made from?
Filter bags are manufactured from various materials, each suited to specific applications:
- Polypropylene felt: Usually the first choice for industrial applications. Outstanding acid and chemical resistance. Good cost vs efficiency. Operating temperature up to 80°. Micron ratings 1, 5, 10, 25, 50, 100 and 200.
- Polyester felt: Excellent chemical resistance and temperature tolerance, ideal for general industrial applications. Operating temperature up to 120°. Micron ratings 1, 5, 10, 25, 50, 100 and 200.
- Nylon Mesh(Polyamide): Suitable for straining/sieving applications and targeted removal of particles of a specific size. Operating temperature up to 120°. Micron ratings 1, 5, 10, 25, 50, 100, 150, 200, 250, 300, 400, 500, 600, 800, 1000.
- PTFE (Teflon) felt: Superior chemical resistance and high-temperature capability for extreme conditions. Special material when other materials would not be suitable.
- Nomex felt: Heat-resistant aramid fiber for high-temperature applications. Special material for specific applications.
What's the difference between felt and mesh filter bags? Are there any other types of filter socks?
- Felt filter bags: use thermally bonded fibers creating a depth filtration media. They offer higher dirt-holding capacity, capture particles throughout the material's depth, and provide excellent efficiency for a wide range of particle sizes.
- Mesh filter bags: use woven monofilament materials providing surface filtration. They offer precise micron ratings, easier cleaning for reusable applications, and are ideal when you need to capture particles of a specific size.
- Multilayer bags: combine filtration layers for longer service life and high dirt holding capacity (Extended life bags)
- High-efficiency bags: provide absolute filtration for critical applications.
Felt bags are generally preferred for high-solids applications, while mesh bags excel in low-contamination processes requiring precise filtration.
Can filter bags be cleaned and reused?
Some filter bags can be cleaned and reused, though this depends on the material, construction, and application. Mesh bags made from synthetic materials are often reusable, while disposable felt bags are designed for single use. However, consider that cleaning costs, reduced efficiency after cleaning, and potential contamination risks often make disposable bags more economical and reliable for most industrial applications.
What are the different industrial sizes of filter bags?
Filter bags come in standard sizes designated by numbers (such as Size 1, Size 2, Size 3 and Size 4) corresponding to specific dimensions:
Size 1: 7" x 16"
Size 2: 7" x 32"
Size 3: 4” x 9”
Size 4: 4” x 15”
Larger bags provide greater surface area, higher flow rates, and longer service life. Smaller bags are suitable for batch or low flow rate applications.
What's the difference between a filter bag and a filter cartridge?
Filter bags and cartridges both remove particulates but differ in construction and application:
Filter bags are sewn or welded fabric sleeves offering economical filtration with high flow rates and dirt-holding capacity, ideal for higher contamination levels.
Filter cartridges are rigid, pleated or spun elements provide precise filtration but at a higher cost, better suited for cleaner liquids requiring finer filtration.
Many operations use bags for pre-filtration and cartridges for final polishing.
-
What are the benefits of using filter bags?
-
Cost-effective: Lower initial investment compared to cartridge filters
-
Easy to install and replace: Simple change-outs minimize downtime
-
Versatile: Available in many materials, ratings, and sizes for diverse applications
-
High dirt-holding capacity: Particularly with felt bags, reducing change-out frequency
-
Space-efficient: Compact footprint compared to other filtration methods
- Customisable: Can be manufactured to custom sizes and shapes
How long do filter bags last?
Service life depends on factors such as contamination level, flow rate, and filter media and size of the system. In general, filter bags can last from weeks to months.
Regular differential pressure monitoring helps determine when to change bags.
What are the common micron ratings for filter bags?
Filter bags come in a variety of micron ratings, a measure of particle size retention capability.
- Fine filtration: 1–10 micron (removes small particles, haze, or fine sediments)
- Medium filtration: 25–100 micron (for general process liquids or oils)
- Coarse filtration: 100 – 1000 micron (pre-filtration or large particle removal)
Choosing the right micron rating depends on your liquid type and required cleanliness level.
How do I choose the right filter bag for my application?
To select the correct filter bag, consider:
- Liquid type (water, oil, chemical, food, etc.)
- Operating temperature and pressure
- Chemical compatibility with the bag material
- Desired particle retention (micron rating)
- Flow rate and dirt load
- Filtration housing type and size
We can help with filter bag selection and sizing or answer any technical questions. Contact us today.
