Bag Filter

What is a Bag Filter

A bag filter is a mechanical separation device in which fluid, liquid or gas, is forced through a porous fabric bag so that suspended solids are retained while the clarified fluid passes through. In a liquid bag filter the dirty fluid enters the top of a sealed housing, flows from the inside of the bag outward through the fabric wall, and exits clean; the captured solids accumulate inside the bag, which is lifted out and replaced when the pressure drop rises. In a dust-collection bag filter the dirty gas flows from outside the bag inward (or inside-out, depending on geometry), building a dust cake on the fabric that is periodically cleaned off and dropped into a hopper.

The defining feature of any bag filter is that the separation medium is a sewn or welded fabric bag, not a rigid pleated cartridge, a metal screen, or a wound element. That fabric form gives the bag filter two structural advantages: a very large dirt-holding volume relative to its footprint, and a fast, low-cost change-out. A single liquid filter bag can capture a pound or more of contaminant before it must be swapped, and the swap takes a minute with no tools. These two properties are the reason bag filters dominate high-solids, coarse-to-medium duties where finer cartridge or membrane elements would clog quickly and cost too much to replace.

Structurally, a liquid bag filter system has three parts: the housing (a pressure vessel of stainless steel or reinforced plastic with an inlet, an outlet, a swing-bolt or band-clamp closure, and a vent and drain), the restrainer basket (a perforated metal cage that supports the fabric bag against the differential pressure so it does not balloon or rupture), and the filter bag itself (the consumable fabric element carrying a sealing ring at its mouth). The basket-and-ring interface is where most bypass leakage occurs, so manufacturers invest heavily in positive-seal ring designs.

The filter bag concept is old: woven cloth has been used to strain liquids for centuries, and fabric dust filters appeared in foundries and flour mills in the nineteenth century. The modern industrial bag filter took shape after the 1950s, when needle-punched felt media, standardized bag-and-housing dimensions (the familiar Size 1 through Size 4 family), and welded stainless pressure vessels turned a craft item into a catalog product. The pulse-jet cleaning system, patented in the late 1950s, did the same for dust collection by allowing fabric bags to be cleaned online without shutting the gas flow.

In application scale, bag filters span an enormous range. A single Size 2 liquid housing the size of a fire extinguisher polishes coolant on a machine tool at a few cubic meters per hour, while a multi-bag vessel clarifies hundreds of cubic meters per hour of cooling water or process chemical. On the gas side, a baghouse on a cement kiln or power-plant boiler can hold thousands of bags and treat hundreds of thousands of cubic meters of flue gas per hour, dropping particulate emissions below a few milligrams per normal cubic meter. No single bag filter covers that whole range; selection is the act of matching the fluid, the solids load, and the cleanliness target to a specific bag, basket, and housing.

Liquid and Dust Bag Filter Types

Bag filters divide first by the fluid they treat, liquid or gas, and then by construction within each family. The split matters because the vocabulary, the standards, and the failure modes are different. The table below maps the main types against their defining traits before each is discussed in turn.

Single-bag liquid housing is the workhorse of liquid filtration. One fabric bag sits in a restrainer basket inside a vertical pressure vessel; flow is inside-to-out. It suits flows up to roughly 100 gpm (23 cubic meters per hour) on a Size 2 bag. Eaton's FLOWLINE II and the wider 304 and 316L single-bag families from many suppliers are typical examples. When the differential pressure across the bag reaches the set change-out point, an operator opens the swing-bolt cover, lifts out the dirty bag, and drops in a clean one.

Multi-bag liquid housing parallels several bags (4, 8, 12, 16 or more) in one large vessel to handle higher flow without raising velocity through any single bag. A 16-bag 304 stainless vessel can pass over 2,000 gpm with flanged connections. Multi-bag units lower the cost per unit of flow and stretch the change-out interval, but the manifold and the larger flanged closure raise the capital cost, so they pay off only above the flow where a bank of single housings becomes unwieldy.

Self-cleaning bag filters replace the disposable bag with a permanent fine screen or a long-life bag and add a motorized scraper or backwash that removes the cake without opening the vessel. They suit continuous, high-solids, or viscous service where manual bag changes would be too frequent. They cost more upfront and add moving parts, so they are justified only where bag consumption or downtime would otherwise be excessive; SpecForge treats them as a distinct category.

On the gas side, pulse-jet baghouses dominate new dust-collection installations. They use felt bags, clean online with a short blast of compressed air fired down each bag, and tolerate high air-to-cloth ratios, which keeps the unit compact. Reverse-air baghouses use woven bags and clean by gently back-flowing clean gas through an isolated compartment; the soft action suits fragile glass-fiber media and high-temperature flue gas in utility and process boilers. Shaker baghouses agitate the bag tops mechanically and must clean offline, so they survive mainly in smaller, intermittent, low-temperature streams.

Filter Bag Media and Materials

The fabric bag, not the housing, sets the chemical and temperature envelope of a bag filter, and choosing it wrong is the most common field error. Media split into two construction families: felt or needle-punched fabrics, which capture particles by depth and carry nominal ratings, and woven monofilament mesh, which captures by surface and carries a defined opening size. The table below compares the mainstream polymers used for liquid filter bags by temperature limit and typical chemistry.

Polypropylene felt is the default liquid bag media: it resists most acids and many solvents, is inexpensive, and gives good depth capture, but it softens near 82 degrees Celsius (180 F) so it is limited to ambient and warm streams. Polyester felt handles higher temperature, near 135 degrees Celsius (275 F), and suits oils and neutral water, but it is attacked by strong alkali and hot concentrated acid. These two felts cover the large majority of liquid duties.

Nylon monofilament mesh is a woven, reusable bag with a precise opening, typically from 25 to 175 microns, used where the goal is to recover a product or screen coarse debris rather than achieve fine clarity; it can often be washed and reused. Nomex (meta-aramid) felt extends the temperature ceiling to about 200 degrees Celsius (400 F) for hot oils and high-temperature process liquids, at a higher cost than polyester. For aggressive chemistry or sustained service above 200 degrees Celsius, PTFE media is the durable answer.

On the dust-collection side the same logic applies but the polymers differ. Polyester is the workhorse fabric below about 135 degrees Celsius; polyphenylene sulfide (PPS) handles acidic flue gas to roughly 190 degrees Celsius; PTFE membrane laminate gives the highest temperature and the lowest emission; and woven fiberglass with PTFE finish serves the hottest utility-boiler streams. A PTFE membrane laminated onto a felt backing shifts capture from depth to the membrane surface, which both lowers emissions and improves cleaning because the cake releases cleanly.

Two construction details matter regardless of polymer. First, the sealing ring or collar at the bag mouth: a plastic snap ring, a steel ring, or a flange determines how positively the bag seats against the basket and therefore how much fluid bypasses unfiltered. Second, the seam: sewn seams leave needle holes that can pass fine particles, whereas welded (heat-sealed) seams give a true absolute path, which is why high-efficiency bags are welded rather than sewn.

Housing Construction and Standard Sizes

The housing is the pressure vessel that holds the bag, contains the differential pressure, and provides the inlet, outlet, vent, drain, and closure. Its two defining choices are the body material and the standard bag size it accepts. Liquid bag filter sizes are standardized across the industry so that bags from any supplier fit any housing of the same size designation; this interchangeability is one of the bag filter's biggest commercial advantages. The table below lists the common standard sizes with verified dimensions and typical capacity.

Size 2 is by far the most common because it doubles the media area and dirt-holding of Size 1 in a housing only modestly larger, giving the best balance of flow, change-out interval, and cost. Sizes 3 and 4, with their smaller 4.12 inch diameter, serve compact or low-flow installations and OEM skids. A multi-bag housing simply parallels several Size 2 bags, so a 4-bag vessel handles roughly four times the single-bag flow at the same per-bag velocity.

Body material follows the fluid. 304 stainless steel suits water, coolant, and mild chemistry and is the economical default. 316L stainless steel adds molybdenum for chloride and acid resistance and is standard for chemical, food, pharmaceutical, and brine duty; sanitary versions are electropolished and use Tri-Clamp connections for clean-in-place. Polypropylene and PVC housings serve aggressive acids and deionized water where even 316L would corrode or contaminate, at the cost of lower pressure and temperature limits. Carbon steel with a coating or lining appears in low-cost water and oil service.

The table below summarizes housing material against pressure and temperature envelope so the body choice can be made against the duty before bag selection.

Where a housing must be a code pressure vessel, two regimes apply. In North America the ASME Boiler and Pressure Vessel Code Section VIII governs design and stamping; Eaton's SIDELINE vessel, for example, carries an ASME Code Stamp. In the European Union the Pressure Equipment Directive 2014/68/EU (PED) sets the conformity route and CE marking. Economy and small housings often fall below the code threshold and ship unstamped, which is acceptable for low-pressure or low-volume duty but must be checked against site requirements. The closure type, swing-bolt, band-clamp, or eye-nut, trades opening speed against pressure rating and is chosen for the change-out frequency.

Key Specification Parameters

Reading a bag filter datasheet means separating the bag specification from the housing specification. The bag carries the filtration rating; the housing carries the mechanical envelope. Seven parameters drive most selection decisions: micron rating and rating basis, efficiency or Beta ratio, flow rate and area, differential pressure, dirt-holding capacity, media and ring construction, and the housing pressure and temperature rating. Each is decoded below.

Micron rating is the headline number, available across an enormous range from 1 to 1,000 microns. The number is the particle size at and above which the bag is designed to capture; a 25 micron bag passes finer particles and stops coarser ones. The practical working bands are roughly 1 to 5 microns for fine and polishing duty, 10 to 25 microns for general clarity, 50 to 100 microns for sediment removal and pre-filtration, and 150 to 200 microns for coarse equipment protection. The micron number alone is meaningless without the rating basis.

Rating basis, nominal or absolute, is the single most misread item on a bag spec. A nominal rating implies single-pass removal of only 50 to 80 percent of particles at the rated size, typical of felt and needle-punched bags. An absolute rating certifies 90 to 98 percent or higher single-pass capture and is usually tied to a Beta ratio under the ISO 16889 multipass method: Beta 10 equals 90 percent, Beta 100 equals 99 percent, Beta 1000 equals 99.9 percent. Specifying a nominal bag where the process needs absolute clarity is a frequent and expensive mistake.

Flow rate and media area set the velocity through the fabric, which in turn governs both capture efficiency and pressure drop. A Size 2 bag at 4.4 square feet handles up to about 100 gpm (23 cubic meters per hour) on clean, low-viscosity liquid, and far less as viscosity rises; a fluid at 100 cP may halve the rated flow. Designing for a lower velocity, by adding bags or going multi-bag, improves capture and stretches the change-out interval, at the cost of a larger and more expensive housing.

Differential pressure is the operating signal of a bag filter. A clean bag typically starts at 0.1 to 0.3 bar; as solids accumulate, the differential rises, and a set change-out point (commonly around 1 to 1.5 bar above clean, never exceeding the bag and basket rating) tells the operator to replace the bag. Dirt-holding capacity is the mass of solids a bag retains before that limit, and the high value, often a pound or more per Size 2 bag, is the bag filter's economic advantage over cartridges.

Housing pressure and temperature rating are mechanical limits independent of the bag. Stainless single and multi-bag housings are commonly rated to 10 bar (150 psi) working pressure, plastic vessels to about 5.5 bar (80 psi), and high-pressure designs to 16 bar design. The temperature ceiling, however, is almost always set by the bag media, not the steel, so the assembly is limited by whichever of bag or housing is lower. Construction details, the sealing ring (plastic snap, steel, or flange) and the seam (sewn or welded), decide how much fluid bypasses the bag and whether the rating is truly absolute.

Selection Decision Factors

To turn the preceding chapters into a specific bag and housing, work through the decision sequence below in order. Most selection failures come not from a single wrong number but from deciding a downstream item, such as housing size, before an upstream one, such as the required rating basis. These steps double as an RFQ template.

1. Fluid and chemistry: First fix the fluid, its temperature, and its chemistry, since these set both the bag polymer (Chapter 3) and the housing material (Chapter 4). Get the lower of the two temperature limits and design under it.
2. Particle size and rating basis: Define the smallest particle to remove and the required removal efficiency, then choose nominal or absolute. Pre-filtration and sediment knock-down can be nominal; polishing and pre-membrane duty must be absolute with a stated Beta ratio.
3. Flow rate and bag count: Divide design flow by the recommended per-bag flow (about 100 gpm for a clean Size 2), derate for viscosity and solids, then round up and add margin. Decide single-bag versus multi-bag from the resulting count and the available footprint.
4. Bag size and media area: Pick Size 1 through Size 4 from the flow and the desired change-out interval. A larger area lowers velocity, improves capture, and extends bag life; specify the size before the housing so the vessel matches.
5. Housing material and connections: Choose 304, 316L, or plastic from the chemistry, then the connection type (threaded, flanged DN50 PN16, or sanitary Tri-Clamp) and the closure (swing-bolt, band-clamp) from pressure and change-out frequency.
6. Pressure-vessel code: Confirm whether the duty requires an ASME Section VIII stamp or PED 2014/68/EU conformity, and verify the maximum working pressure and the differential at which the bag and basket are rated, leaving margin for pump shut-off head.
7. Cleaning strategy (dust duty): For dust collection, set the air-to-cloth ratio (0.9 to 2.4 m/min for pulse-jet felt, 0.3 to 1.2 m/min for reverse-air or shaker woven), then size the bag count and choose the cleaning method from temperature, dust combustibility, and online-cleaning needs.
8. Total cost of ownership: Add bag consumption (rate of change-out times bag price), labor per change, disposal of the spent bag and its captured solids, and any downtime. A cheaper nominal bag that change out twice as often, or an undersized housing that runs at high differential, often costs more over a year than the correct selection.

One last dimension is serviceability and supply. Because the bag is a frequently replaced consumable, confirm that the standard size you choose (ideally Size 2) is stocked by more than one supplier, that the ring and media you need are held locally, and that the housing's basket and gaskets are spare-able. Eaton, Pentair, Rosedale, Parker, and 3M, alongside cost-competitive makers such as Hongtek, Filson, and Brother Filtration, all build to the standard size family, which keeps the consumable market open and protects against single-source price risk over the housing's long service life.