Industrial Liquid Filter Bags: Types, Sizes, and Selection
An industrial filter bag is a porous sleeve in a sealed housing: liquid enters, passes through the bag wall, and exits clean, leaving solids behind in or on the media. The bag that fails early is almost never the wrong media choice; it is usually the wrong size for the housing or the wrong collar for the temperature.
Why plants run bags instead of a wall of cartridges
The large filtration area of even one bag lets it handle flow that would otherwise need several cartridge housings in parallel: a single Size #2 bag, 7 inches by 32 inches, handles flow up to 34 cubic metres per hour through one housing and one change-out. For high-volume, continuous processes, that lower footprint and lower operating cost per litre filtered is why bags are the practical first stage, with a finer cartridge often staged behind them for polishing duty.
Felt or mesh: the decision that comes before everything else
Felt bags compared with mesh bags
| Criteria | Felt (depth filtration) | Mesh (surface filtration) |
|---|---|---|
| Construction | Needle-punched polypropylene or polyester fibre | Woven monofilament nylon |
| Rating basis | Absolute: 99 percent or more at the stated size | Nominal: typically 50 to 70 percent at the stated size |
| Micron range | About 1 to 200 micron | About 50 to 300 micron |
| Reusable | No, single-use | Yes, back-flushed, rinsed, or ultrasonically cleaned |
| Temperature limit | 80C (PP), 130C with a stainless 316 ring (PE) | 130C with a stainless 316 ring |
| Best fit | High particulate load, single-pass duty | Coarser contaminants, high volume, reuse desired |
Felt bags are needle-punched, randomly oriented fibre, usually polypropylene or polyester, and they capture particles throughout the three-dimensional depth of the media rather than only at the surface. That gives high dirt-holding capacity for heavy particulate loads, at the cost of being single-use: once loaded, a felt bag cannot be effectively cleaned. Mesh bags are woven from single-strand nylon in a precise, uniform pattern, capturing particles on the surface rather than through a depth structure. The trade-off runs the other way: mesh bags can be back-flushed, rinsed, or ultrasonically cleaned and reinstalled, which delivers a lower cost per cycle on coarser, high-volume duty such as paint agglomerates or food-processing debris. A specialty third category, oil-adsorbing bags, works on neither mechanism: it adsorbs free and dispersed hydrocarbons out of an aqueous stream rather than straining solids, for oil-water separation and produced-water duty.
Reading the micron rating correctly
Felt bags are rated on an absolute basis: 99 percent or more of particles at the stated size are captured, across a range of roughly 1 to 200 microns. Mesh bags carry a nominal rating instead, typically 50 to 70 percent of particles at the stated size, across roughly 50 to 300 microns. A finer rating than the stream needs raises pressure drop and shortens bag life for no purity gain; a coarser rating than the duty needs lets contamination straight through to the downstream process. The correct rating comes from the particle size distribution in your feed liquid and what your downstream process can tolerate, not from picking the smallest number available.
The four standard sizes, and what happens when none fit
The four standard filter bag sizes
| Size | Nominal dimensions (D x L) | Max recommended flow rate |
|---|---|---|
| Size #1 | 7 in x 16 in | 18 m3/hr |
| Size #2 | 7 in x 32 in | 34 m3/hr |
| Size #3 | 4 in x 8 in | 6 m3/hr |
| Size #4 | 4 in x 14 in | 12 m3/hr |
Non-standard sizes, including #5, #6, #2X (7 in x 32 in double) and #7 (7 in x 42 in), are available as custom orders.
These four dimensions are compatible with bag housings from most major global manufacturers, so a housing bought from one supplier can usually run a bag from another without a rebuild. Multi-bag housings, holding two, four, eight or more bags in parallel, multiply the throughput while keeping change frequency manageable per bag.
A five-step way to specify the right bag
Work through the fluid and temperature first: water-based, solvent-based, hot oil, acidic or alkaline duty decides which media survives in service, and polypropylene to 80C or polyester and nylon mesh to 130C with a stainless ring cover most industrial cases. Next, set the target micron rating from what the downstream process needs protected; without test data, a 25 micron felt bag is a reasonable trial starting point. Third, choose felt for high particulate load and single-use budget, mesh for coarser contaminants and reuse, or an oil-adsorbing bag for hydrocarbon in water. Fourth, match the bag size to the housing, keeping flow velocity below the size's maximum recommended rate. Fifth, specify the collar: polypropylene for standard temperature, stainless 316 for high-temperature or sanitary-contact duty, confirming food-contact compliance where that applies.
Direct answers
Can I reuse a filter bag?
Felt bags cannot; the fibres that capture the particles are also what degrades, so they are single-use. Nylon mesh bags are designed for reuse and can be back-flushed, rinsed, or ultrasonically cleaned across multiple service cycles, provided you inspect for holes or deformation before putting one back into service.
What if none of the four standard sizes fit my housing?
The four standard sizes (#1 through #4) fit housings from most major global manufacturers, but non-standard sizes, including #5, #6, a double-length #2X, and #7, are available as custom orders when your housing does not match the standard dimensions.
How do I know when to change a bag, rather than wait for a fixed schedule?
Watch differential pressure across the housing: a rapid rise toward the housing's rated maximum, typically 0.5 to 1.0 bar, means the bag is nearing full loading and should come out. A drop in outlet flow below your required rate is the same signal from the other direction. A fixed schedule is a reasonable starting point once you have operating data, but the pressure gauge is the actual trigger.