Filter bag

A filter bag is the replaceable fabric element inside a fabric filter or baghouse. It provides the surface on which dust is captured, held as a filter cake and periodically released by pulse-jet, reverse-air or shaker cleaning.

Construction

Most industrial bags are cylindrical or oval tubes sewn from needle felt, woven fabric or laminated membrane media. Pulse-jet bags are supported by metal bag cages and cleaned from the clean side by compressed-air pulses. Reverse-air and shaker bags are often larger, unsupported or lightly supported, and cleaned more gently. Seals can be snap bands, cuffs, clamps, rings or sewn-in hardware depending on tubesheet design.

Media selection

Bag media is selected from gas temperature, moisture, acid gases, oxygen, alkalis, hydrocarbons, dust abrasiveness, spark risk and cleaning method. Polyester suits cooler dry duties. Aramid, PPS, polyimide and PTFE cover more severe temperature and chemical ranges. Fibreglass is used in high-temperature reverse-air service. PTFE membrane improves dust release and fine-particle capture but costs more and requires careful handling.

Failure modes

Filter bags fail by abrasion, flex fatigue, thermal shrinkage, chemical attack, hydrolysis, acid condensation, spark holes, cage cuts, seam failure, poor sealing and blinding. Symptoms include higher outlet dust, broken-bag alarms, visible stack emissions, rising differential pressure, short cleaning cycles and dust in the clean-air plenum.

Maintenance implications

Good bag life depends on correct installation, cage condition, pulse pressure, compressed-air dryness, temperature control, hopper evacuation and avoiding direct inlet impingement. Operators should treat repeated failures in the same row or compartment as a gas-distribution or mechanical problem, not simply a bag-quality problem.

Acoustic cleaning relevance

Sonic horns help filter bags indirectly by reducing residual cake, loosening dust between rows and preventing hopper build-up. They are most valuable when the primary cleaning system works but cannot fully release cohesive fine dust without excessive pulse energy or frequent cleaning.

Inspection notes

A failed bag investigation should keep the removed bag, cage and position record together. The failure location often identifies the root cause: top cuff dust marks suggest sealing problems, vertical wear lines suggest cage damage, bottom wear suggests bag movement or hopper abrasion, and brittle media suggests temperature or chemistry. Bag replacement campaigns should include tubesheet cleaning, cage inspection and pulse-pipe alignment. Installing new bags into a dirty or mechanically damaged compartment usually repeats the same failure pattern.

Selection variables

A filter bag is specified from the gas and dust duty, not only from the collector model. Engineers consider operating temperature, excursions, acid and alkali chemistry, moisture, oxygen, sparks, dust abrasiveness, particle size, required emissions level, cleaning method, bag length, cage fit, tube-sheet attachment and whether a membrane is needed. The same baghouse can need different media after a fuel, raw material or reagent change.

Fit and support are as important as fabric. A loose bag can rub on its cage or neighbouring bags. A poor snap band can leak at the tube sheet. A cage with broken wires can cut the fabric. Long bags need vertical alignment and adequate spacing so they do not touch during pulsing or high can velocity.

Failure and maintenance checks

Common failures include pinholes, seam failure, blinding, hydrolysis, thermal shrinkage, acid attack, spark holes, abrasion at the inlet side, cage wear and failed tube-sheet seals. Operators track differential pressure, outlet particulate, triboelectric leak detection, pulse frequency, compressed-air quality and compartment performance. A sudden emissions rise after a pulse often indicates a damaged bag or tube-sheet leak.

Acoustic cleaning can support bags by preventing heavy bridging between rows or helping dry cake release, especially in fine-dust service. It does not replace correct pulse-jet settings, dry compressed air, suitable media or systematic leak testing.

Replacement practice

Bag replacement should include cage inspection, tube-sheet cleaning and confirmation that old failure patterns are understood. Installing new bags onto bent cages or into a wet, leaking compartment usually repeats the same failure. A short baseline run after replacement helps confirm seating, pulse response and emissions stability.