Fabric filter

A fabric filter removes particulate by passing dusty gas through rows of woven or felted filter bags. The bag fabric supports a dust layer, or filter cake, that performs much of the fine-particle capture. Fabric filters are often called baghouses when the complete housing, bags, hoppers and cleaning system are discussed together.

Main types

Type	Cleaning method	Typical duty
Pulse-jet	Short compressed-air pulses through cages	Cement, steel, biomass, WtE, process dust
Reverse-air	Low-pressure reverse flow collapses bags gently	Large utility boilers, high-temperature fibreglass bags
Shaker	Mechanical shaking of hanging bags	Smaller or older dust collectors

Operating variables

Performance depends on air-to-cloth ratio, inlet dust loading, particle size, bag material, gas temperature, moisture, acid gases, spark risk and cleaning setpoints. Differential pressure is the main operating signal. Too little cake can reduce collection efficiency; too much cake raises fan power, reduces flow and risks bag collapse or blinding.

Failure modes

Common failures include bag blinding, broken bags, cage wear, pulse-valve failure, compressed-air moisture, hopper bridging, abrasion at the inlet, acid condensation, thermal damage, hydrolysis and spark burn-through. A single failed bag can contaminate the clean-air plenum and stack, while systemic blinding can derate the entire process.

Design and maintenance implications

Good fabric-filter design distributes inlet gas evenly, protects bags from direct abrasion, provides hopper evacuation, isolates compartments for maintenance and matches media chemistry to the gas. Maintenance teams trend differential pressure, pulse frequency, compressed-air quality, opacity or dust monitor response, broken-bag detector alarms and hopper discharge.

Acoustic cleaning relevance

Sonic horns do not replace pulse jets or reverse air. They help by loosening fine dust between bags, reducing residual cake after cleaning cycles, preventing hopper arches and lowering average differential pressure. They are most useful where dust is fine, cohesive or difficult to release from dense bag rows.

Specification notes

Fabric-filter specifications should state the design air-to-cloth ratio, inlet loading, expected particle size, temperature range, dew-point margin, cleaning system, compartment isolation philosophy and hopper discharge basis. The bag supplier's media recommendation is only valid if gas chemistry and upset temperatures are honest. Commissioning should include leak testing, pulse-valve checks, baseline pressure drop and broken-bag detector setup. If sonic horns are added later, their firing should be coordinated with pulse cleaning so released cake reaches the hopper rather than being re-deposited immediately.

Design and operating variables

Fabric-filter performance is controlled by air-to-cloth ratio, bag length, bag spacing, inlet distribution, gas temperature, moisture, dust loading, particle size, filter-media selection, cleaning pressure and the discharge system below the hoppers. The unit needs a stable dust cake, but not a cake so thick that differential pressure rises and flow collapses. Inlet design matters because a high-velocity dust jet can abrade bags, overload one compartment or carry sparks deep into the filter.

Temperature margin is also critical. Operating below the acid dew point can blind bags with sticky salts or corrosion products. Operating above the media limit can shrink, embrittle or melt fibres. Combustible dust service adds explosion-venting, isolation, bonding, housekeeping and ignition-source control requirements. In waste, biomass and metal applications, spark detection and inlet dropout arrangements are often as important as the bag material.

Failure modes and cleaning relevance

Common failures include bag blinding, pinholes, cage wear, broken snap bands, tube-sheet leakage, pulse-valve failure, compressed-air moisture, hopper bridging and re-entrainment from overloaded compartments. Maintenance teams look for outlet dust traces, differential-pressure trends, pulse header pressure, solenoid performance, leak-detection signals and visible bag wear during shutdown.

Acoustic cleaners are sometimes fitted to reduce cake bridging between bags, keep hoppers clear or improve dust release in zones that pulse cleaning does not reach evenly. They work best on dry, friable dust. They should not be treated as a cure for poor pulse-air quality, wrong media, wet operation, chemical blinding or an undersized filter.

Commissioning evidence

Commissioning should record baseline differential pressure, pulse frequency, clean-gas dust level, hopper discharge behaviour and compressed-air dryness at representative load. Without that baseline, later bag failures are harder to separate from process changes.