Bag Filters: Complete Technical Guide

Bag filters are the fabric filtration systems that have served the cement industry for generations: a dust-bearing gas is intercepted by a permeable fabric in such a manner that all the gas passes through the fabric while the dust impinges on the fibres and is thereby retained. As the dust accumulates on the fabric a cake is formed, which aids filtration by improving the particle capture and raising the collection efficiency; at the same time the resistance to the gas flow increases, and when that resistance reaches an unacceptable level the fabric has to be cleaned to dislodge the cake. This complete technical guide, based on the classic engineering document “Bag Filters” (the Rushworth treatise held in the Complete Cement Technical Package), walks the full design and operating discipline of fabric filters: the mechanisms of particle capture, the cleaning methods, the temperature limitations, the sizing calculations with the filtration velocity and the estimation of the dedusting air flow rates, the choice of the correct fabric for each application, the troubleshooting of the operating failures, the comments on practical application, and the recent developments of the technology.

Fabric filtration is a deceptively demanding technology because its physics are subtle: the particles are captured not by sieving but by a combination of inertial impaction, direct interception, diffusion and electrostatic forces, and the performance of the filter is governed as much by the dust cake as by the fabric. The design numbers matter with precision: the filtration velocity decides the cloth area and the pressure drop, the gas temperature decides the fabric family, the dust characteristics decide the cleaning method, and the emission guarantee decides the margin. This guide gives the engineer the complete picture, from the fibre physics to the field troubleshooting, with the numbers, the tables and the application experience that the original document collects.

1. The Principle of Fabric Filtration and the Role of the Dust Cake

In essence, a dust-bearing gas is intercepted by a permeable fabric in such a manner that all the gas passes through the fabric whilst the dust impinges on the fibre of the fabric and is thereby retained. The retained dust accumulates as a cake on the surface of the fabric, and the cake is the true filter medium of the system:

• The initial capture: the clean fabric captures the coarsest particles by impingement and interception, while the finest particles penetrate into or through the weave until the first cake layers establish the filtering surface.
• The cake as the filter: as the dust accumulates, the cake of collected particles, with its fine pore structure, captures the sub-micron dust that the fabric alone would pass; the collection efficiency of the cake-covered filter is therefore far higher than that of the new fabric.
• The pressure penalty: at the same time the resistance to the gas flow increases, and to maintain the same gas flow rate as at start-up the system fan has to work harder; the pressure drop across the filter is the mirror of the cake thickness.
• The cleaning event: when the resistance reaches an unacceptable level, the fabric has to be cleaned to dislodge the cake; the cleaning is a compromise, because it must remove enough cake to restore the flow without removing so much that the filtering surface is lost.
• The residual resistance: the pressure drop across the fabric will always be greater than the initial value, that is, greater than with new fabric, because some of the dust particles become permanently embedded in the fabric structure; the operating pressure drop stabilises at the equilibrium between the cake growth and the cleaning effectiveness.

This cake-based understanding explains every operating phenomenon of the baghouse: why a new collector filters poorly at first and improves as the cake builds, why over-cleaning causes the emission spikes (the cake is stripped and the fines pass through the bare fabric), and why the pressure drop never returns to its new-fabric value. The designer and the operator of the fabric filter are, in reality, managing the dust cake.

2. The Mechanisms of Particle Capture: How the Dust Is Actually Stopped

The capture of the dust particles by the fibres of the fabric is not a sieving process but the sum of several physical mechanisms, each dominant in a different particle size range. Understanding the mechanisms explains the efficiency curves of the filters and the behaviour of the fines:

• Inertial impaction: the larger particles, following the gas streamlines around the fibre, are too massive to follow the curvature of the flow, leave the streamline, and impact the fibre surface where they adhere; the mechanism is strong for the coarse particles and negligible for the sub-micron dust.
• Direct interception: the particles that travel within one particle radius of the fibre surface touch the fibre even if the streamline passes around it, and are retained by contact; the mechanism operates across the size range and is the geometric minimum of the filter’s capture.
• Diffusion: the very fine particles (below about 0.3 microns) are in constant random (Brownian) motion, which carries them across the streamlines onto the fibre surface; the diffusion capture is strongest for the smallest particles, so the collection efficiency curve of the bare fabric has a minimum in the intermediate sub-micron range where both the inertia and the diffusion are weak.
• Electrostatic forces: the triboelectric charge of the dust and the electrostatic fields around the charged or charged-fabric surfaces attract the fine particles to the fibres; the effect is exploited in the electret and charged-media filters and is the reason the antistatic and conductive fabrics exist for the combustible dusts.
• Gravitational settling: the very coarse dust settles in the lower velocity zones of the collector and the ducts, contributing a minor fraction of the total capture inside the fabric structure itself.

The practical consequence of the mechanism mix is the familiar efficiency curve: the total penetration of a well-operated fabric filter is lowest for the coarse and the finest particles and highest in the intermediate sub-micron window, and the cake, with its fine pore structure, closes that window. This is why the emission of a properly caked filter is measured in milligrams per cubic metre while the emission of a badly managed filter rises in the sub-micron range that the visible opacity monitors detect first.

3. The Filter Media: Woven Fabrics and Needle Felts

The fibres used in the manufacture of the fabrics for filtration are almost exclusively synthetic, and they are either woven or needle-felted. The two families have distinct personalities that decide their applications:

• Woven fabrics: are smoother and more easily cleaned than the felts and, sometimes at low loads, no cleaning devices are needed because the fabric is self-cleaning; on the other hand they often cannot be cleaned too vigorously, because the breakdown of the entire dust cake would force the dust between the fibres and raise the emission. The woven fabrics are the traditional media of the reverse-air and the shaker collectors.
• Needle felts: are less permeable than the woven fabrics, but they can be operated at considerably higher filtration velocities; the pores in the needle felts are very small compared with the woven fabrics, so the dust penetration is low, which is why the felts dominate the pulse-jet collectors with their high air-to-cloth ratios and their low-emission guarantees.
• The element geometry: generally the filter elements, whether of woven or felted fabric, are cylindrical bags, but some manufacturers have adopted flat panel, or envelope, elements whose higher cloth-to-volume ratio compacts the collector; the element shape is a design choice traded against the cleaning effectiveness and the maintenance access.
• The fabric construction variables: the yarn type (staple or continuous filament), the weave or the felt density, the weight of the fabric, the surface finish (singed, calendered, coated) and the seam construction all participate in the performance: the surface treatments improve the cake release and the coated fabrics reduce the pressure drop and the dust penetration.

The choice between the woven fabric and the felt is therefore a design decision about the filtration velocity, the emission guarantee and the cleaning energy, and the operating economics of the collector follow that decision for its whole life. The modern cement plant runs its pulse-jet collectors on felts at the high velocities, and its large hot-gas units on the woven glass fabrics at the low velocities, exactly the split the original document describes.

4. The Cleaning Methods: Shaking, Reverse Air and Pulse-Jet

The cleaning of the filter fabric is the operational core of the system, and the three classical methods define the three collector families. The cleaning methods in service are:

• The mechanical shaker: the bags are shaken mechanically at their tops, flexing the fabric and dislodging the cake; the shaking is effective only on the woven fabrics whose cake is brittle, and the collector must be taken off line (compartment by compartment) during the cleaning, so the method is confined to the older and the small units.
• The reverse-air cleaning: a low-velocity flow of clean gas is passed through the fabric in the reverse direction, gently flexing the woven bags against their internal rings and dropping the cake; the method operates off line by compartment, suits the large units and the fragile high-temperature fabrics, and works at the low filtration velocities of the woven glass media.
• The pulse-jet cleaning: a short, high-energy pulse of compressed air (at the typical pressures of 5 to 7 bar) is fired through the blow pipe into the bag, momentarily reversing and collapsing the felt against its cage and cracking the cake off the surface; the cleaning operates on line (the collector keeps filtering during the pulse), which allows the high filtration velocities and the continuous operation that dominate the modern plant.
• The combined and sonic aids: some large units add the sonic horns or the rapping devices that supplement the primary cleaning where the cake is sticky, and the newer controllers sequence the pulses row by row on the differential pressure demand, the mode the source document’s photohelic-and-module system implements in the field.

The cleaning method decides the collector architecture, the filtration velocity, the bag design (with or without the cage, with or without the rings), the fabric family and the emission performance. The industry’s movement has been one-way: from the shaking and the reverse air to the pulse-jet, driven by the higher velocities and the continuous operation, with the reverse air retained where the very large volumes or the very hot gases make the low-velocity woven construction the safest engineering answer.

5. Temperature Limitations: The Gas Temperature Defines the Fabric Family

The gas temperature of the application is the master constraint of the filter design, because every fabric family has its safe continuous operating ceiling, and the temperature profile of the cement plant spans the whole range of the fabric families. The temperature limitations in practice are:

Fabric family	Typical continuous operating limit	Typical cement plant services
Polypropylene (PP)	~90 degrees C	Low-temperature conveying and storage vents
Polyester (PET)	~130-150 degrees C	Crusher vents, raw mill, cement mill, conveying
Acrylic	~130-140 degrees C	Moderate temperature services with moisture
Nomex / meta-aramid	~200 degrees C	Kiln system and process vents at elevated temperature
P84 / polyimide	~250 degrees C	High temperature filter bag applications
PTFE / fluoropolymer	~250-260 degrees C	Aggressive chemical and low-emission duties
Glass fibre	~260 degrees C	Kiln, clinker cooler and hot gas filters

The temperature discipline goes beyond the fabric ceiling, because the decisive limit is the acid dew point: when the gas contains the sulfur oxides of the fuel and the temperature falls below the dew point, the sulfuric acid condenses on the fabric and the dust, blinding the bags and corroding the casing. The design therefore sets the operating temperature with the margin above the acid dew point, and the plant protects the filter during the startups, the shutdowns and the fuel changes when the temperature excursions are the most dangerous. The temperature measurement points, the interlocks and the cold-start bypasses are as much a part of the filter system as the fabric itself.

6. Bag Filter Sizing: The Filtration Velocity and the Air Flow Estimation

The sizing of a bag filter is the calculation of the cloth area from the gas volume and the chosen filtration velocity, and the source document treats the two halves of the calculation in detail: the filtration velocity and the estimation of the dedusting air flow rates. The sizing discipline is:

• The filtration velocity (air-to-cloth ratio): the gas volume passing through one square metre of fabric per minute; the selection depends on the dust fineness, the loading and the cleaning method: the reverse-air woven collectors run at approximately 0.6 to 1.2 m/min, while the pulse-jet felt collectors run at approximately 2 to 6 m/min, with the fine and cohesive dusts (such as the cement and the kiln dust) demanding the lower end of the range.
• The cloth area: the required gas volume divided by the filtration velocity gives the minimum cloth area; the practical area adds the margin for the peak loads, the uneven distribution and the cleaning cycles, because the collector that runs at its mechanical limit at the design point has no reserve when the process breathes.
• The estimation of the dedusting air flow rates: the volume that the collector must handle is the sum of the process vent gas (the mill vent, the cooler air, the extraction at the transfer points) plus the infiltration air of the system; the estimation corrects each flow to the filter conditions (the temperature and the pressure), because the volume at the filter is the volume that sizes the cloth.
• The number and the geometry of the elements: the cloth area is divided into the bags of the practical diameter and length (or the panel elements), giving the collector dimensions, the compartment count and the hopper geometry; the aspect ratio of the casing and the gas distribution decide whether every bag does its share of the work.
• The fan and the system curve: the fan is selected for the collector’s design pressure drop plus the duct losses at the design flow, and the system curve is checked against the fan curve at the operating points, because the fan and the filter are a matched pair.

The sizing calculation is the moment where the economics are set: every square metre of cloth is capital, and every unit of filtration velocity is operating risk. The experienced designer sizes the collector so that the dust cake, the cleaning cycle and the pressure drop all stabilise inside the operating window at the design gas volume, leaving the reserve that the operator will need on the day the process runs at its peak.

7. The Choice of the Correct Fabric for the Application

The fabric selection is the design decision that fixes the collector’s life and its emission, and the correct choice is a systematic comparison of the application demands against the fabric properties. The selection route of the industry is:

1. Temperature first: the maximum continuous and the peak gas temperatures select the fabric family, with the acid dew point margin imposed on the operating point.
2. Chemistry second: the gas composition (the sulfur oxides, the chlorides, the humidity, the reducing conditions) is checked against the fabric’s chemical resistance, because the polyester that survives in a clean air stream fails in months in a chloride-laden kiln gas.
3. Dust characteristics: the fineness (the median size and the sub-micron fraction), the abrasiveness, the stickiness, the hygroscopicity and the explosivity of the dust decide the fabric weight, the surface finish, the coating and the antistatic provisions.
4. The cleaning energy: the fabric must release its cake under the cleaning energy of the chosen method; the felts with the calendered surfaces and the release coatings are chosen for the pulse-jet, and the woven fabrics for the gentle cleaning of the reverse air.
5. The emission guarantee: the required outlet concentration (the local limits, often of the order of 10 to 30 milligrams per cubic metre, and lower for the sensitive locations) decides whether the standard fabric suffices or whether the PTFE membrane or the coated media are required.
6. The commercial confirmation: the fabric is confirmed by the reference installations and, for the critical duties, by the pilot testing on the actual gas and dust, because the fabric data sheet and the field behaviour are two different documents.

The result of the systematic route is a fabric specification, not a fabric name: the fibre family, the construction (felt or woven), the weight, the finish, the treatment and the required accessories (the grounding wire, the anti-static provisions, the membrane). The document’s treatment of the choice is the reference for that specification, and the practical rule that underlies it all is simple: the correct fabric is the cheapest fabric that survives the temperature, resists the chemistry, releases the cake, and meets the emission guarantee at the chosen filtration velocity.

8. Troubleshooting: The Symptom-Cause-Cure Discipline of the Bag Filter

The troubleshooting of the operating failures is the field discipline that keeps the collector alive between the redesigns, and the source document’s treatment organises the experience into the symptom-cause-cure structure. The principal symptoms and their readings are:

• Rising differential pressure at constant flow: the cake is not being cleaned (weak or missing pulses, low compressed air pressure, failed diaphragm valves, blocked bleed-off tubes), or the fabric is blinding (moisture condensation below the dew point, oil or tar aerosols, over-fine dust packing); the cure follows the cause: repair the cleaning system, restore the temperature margin, or review the fabric.
• Falling differential pressure with rising emission: a bag has failed (a hole, a split seam, a worn cage abrading the fabric, a broken snap band seal) or the tube sheet seals leak; the emission monitor rises because the clean gas path bypasses the fabric; the cure is the bag inspection and replacement campaign, finding the failed row by the compartment isolation.
• Emission spikes after cleaning: the over-cleaning strips the cake and the fines pass through the bare fabric; the cure is the reduction of the cleaning intensity (the pulse pressure or the frequency), the adjustment of the cleaning trigger, or the pre-coating of the bags after the extreme events.
• Uneven bag wear: the bags nearest the gas inlet wear first, abraded by the impinging coarse dust; the cure is the baffle improvement, the wear protection, or the better gas distribution in the hopper.
• Hopper discharge failures: the bridging, the ratholing and the plugging of the hoppers (from the sticky or the moist dust) starve the discharge and re-entrain the dust; the cure is the hopper heating, the air injection or the continuous discharge operation.
• Corrosion of the casing: the condensation below the acid dew point attacks the steel; the cure is the insulation, the gas heating and the operation above the dew point, with the corrosion-resistant materials where the exposure is chronic.

The troubleshooting discipline has one master rule: read the differential pressure and the emission together, because the pair of trends identifies the failure class before the inspection. The combination of the falling pressure and the rising emission says a bag failure; the combination of the rising pressure and the stable emission says a cleaning or a blinding problem; and the routine that reads the pair daily catches every failure in its early, cheap stage.

9. Comments on Application: The Cement Plant Experience

The practical application of the bag filters in the cement industry has accumulated a body of experience that the original document summarises, and the principal comments of the application experience are:

• The mill vents: the raw mill and the cement mill vents run on the pulse-jet felts at the high filtration velocities, with the dust returned to the product; the moisture of the mill atmosphere and the mill temperature govern the fabric choice and the dew point margin.
• The kiln system: the kiln gases carry the volatile-rich dust and the fuel sulfur; the fabric is chosen for the temperature and the acid dew point, and the kiln dust return is managed for the alkali and sulfate cycles of the process; the bypass systems of the volatile-rich plants vent through their own filters.
• The clinker cooler: the cooler vent handles the hot, abrasive clinker dust with the temperature excursions; the glass or the high-temperature fabrics with the abrasion protection serve the duty, and the recovered dust returns to the product.
• The coal and the fuel systems: the combustible dust demands the antistatic and the grounded fabrics, the temperature interlocks, the explosion protection and the housekeeping discipline of the safety codes; this is the application where the fabric’s grounding wire is not an option but a requirement.
• The loading and the dispatch: the silo vents, the packer dust and the loading points are the low-technology applications where the simple collectors on the polyester felts deliver the environmental compliance with the minimum sophistication.
• The operator’s reality: the field experience across the applications confirms the same truths: the collector lives on its differential pressure discipline, its temperature margin and its maintenance routine, and the emission guarantee is a design promise redeemed only by the operating practice.

These comments are the accumulated wisdom that no data sheet contains: which applications tolerate the high velocities and which punish them, which fabrics survive the sulfur and which fail, which maintenance short-cuts are fatal and which are harmless. The plant that inherits this experience inherits the collector’s reliability.

10. Recent Developments: The Direction of the Technology

The technology of the fabric filtration has moved steadily in the directions that the source document’s final chapter records, and the modern developments complete the picture:

• Higher performance media: the PTFE membranes laminated onto the support fabrics, the coated felts and the high-temperature fibres (P84, PTFE, the fiberglass with the advanced finishes) have pushed the emission guarantees down and the operating temperatures up, allowing the bag filters to serve the duties that previously demanded the electrostatic precipitators.
• Long-bag and compact designs: the bag lengths of 6 metres and beyond, the panel elements and the optimised gas distribution have compacted the collectors, reducing the footprint and the capital cost of the new installations.
• Smart control: the differential pressure demand cleaning, the emission feedback, the valve diagnostics and the plant integration have made the collector an instrumented, automated unit whose controller talks to the plant control system.
• Online monitoring: the continuous emission monitors, the bag leak detectors per compartment and the pressure mapping of the casing give the operator the real-time picture that the troubleshooting section above describes as the master rule.
• The low-carbon interface: the new process streams of the low-carbon era (the capture units, the calcined clay systems, the alternative fuel handling) bring new gas and dust chemistries that the fabric selection must serve, extending the technology into the decarbonised plant.

The direction of the technology is the direction of the whole industry: lower emissions, higher temperatures, tighter control and smaller footprints, with the physics of the cake, the mechanisms of the capture and the discipline of the differential pressure unchanged at the core. The engineer who masters the classic fundamentals is the engineer who rides the modern developments.

11. The Sizing Worked Logic and the Design Files

The design documentation of the source document includes the worked logic of the filter sizing and the design data (the appendix material of the original), and the working procedure the engineer follows is the following:

1. Establish the duty: the gas volume at the filter conditions, the temperature, the moisture, the dust concentration and the particle size distribution of the application, each measured or estimated with the process data.
2. Select the cleaning method and the velocity: the method follows the duty (the temperature, the dust, the volume, the emission), and the filtration velocity follows the method and the dust fineness.
3. Calculate the cloth area: the volume divided by the velocity, plus the margin; distribute the area into the elements and the compartments of the practical geometry.
4. Select the fabric: by the temperature, the chemistry, the dust and the cleaning energy, per the selection route of the document.
5. Check the pressure drop: the fabric resistance plus the cake resistance at the design loading and the cleaning interval, against the fan’s available pressure; iterate the velocity or the margin until the window closes.
6. Specify the auxiliaries: the fan, the ducting, the hoppers, the discharge airlocks, the instrumentation and the safety provisions, and the collector is defined.

This worked logic is the practical application of everything the document teaches, and the design files of the package carry the numbers and the formats that the engineer needs for the audits, the extensions and the new projects. The design discipline is the same whether the collector is a small silo vent or a kiln system unit of half a million cubic metres per hour: define the duty, choose the velocity and the fabric, check the pressure drop, and leave the margin.

12. Frequently Asked Questions

What are the mechanisms by which a bag filter captures dust?

The capture is the sum of inertial impaction (the coarse particles leave the streamlines and hit the fibres), direct interception (the particles within one radius of the fibre touch it), diffusion (the Brownian motion carries the sub-micron particles onto the fibres) and the electrostatic forces; the dust cake adds the fine-pore filtering surface that closes the sub-micron window of the bare fabric. Sieving is the least of the mechanisms.

What is the difference between a woven fabric and a needle felt?

The woven fabrics are smoother and more easily cleaned (often self-cleaning at low loads) but cannot be cleaned too vigorously and pass more dust; the needle felts are less permeable but operate at considerably higher filtration velocities, with small pores and low dust penetration, which is why the felts dominate the high-velocity pulse-jet collectors with the low-emission guarantees.

What filtration velocities are typical for cement plant bag filters?

The pulse-jet felt collectors run at approximately 2 to 6 metres of gas per minute per square metre of cloth, while the reverse-air woven collectors run at approximately 0.6 to 1.2 metres per minute, with the fine and cohesive cement and kiln dusts demanding the lower ends of the ranges. The velocity times the cloth area must deliver the rated volume at the rated pressure drop.

Why does the pressure drop never return to the new-fabric value after cleaning?

Because some of the dust particles become permanently embedded in the fabric structure and cannot be removed by the cleaning; the operating pressure drop stabilises at the equilibrium between the cake growth and the cleaning effectiveness, always above the initial clean-fabric resistance. A rising equilibrium over the months is the sign of the progressive blinding.

How is the dedusting air flow rate of a collector estimated?

As the sum of the process vent gas (the mill vent, the cooler air, the transfer extractions) plus the system infiltration, each corrected to the filter conditions of temperature and pressure, because the volume at the filter is the volume that sizes the cloth. The estimation is checked against the fan curve and the system curve at the design operating point.

Which fabric should be chosen for a kiln system collector?

The choice follows the temperature and the chemistry: the glass fibre fabrics (up to about 260 degrees Celsius) for the hot kiln and cooler gases, or the high-temperature synthetics (P84, Nomex, PTFE) where the gas chemistry and the dew point demand them, with the operating temperature held safely above the acid dew point of the fuel’s sulfur.

13. Conclusion and Summary

The bag filter is the fabric filtration workhorse of the cement industry, and its complete engineering discipline is the subject of the classic document this guide summarises. The physics of the method are the particle capture mechanisms of impaction, interception, diffusion and the electrostatic forces, organised by the dust cake that is the true filter medium; the cleaning methods of the shaker, the reverse air and the pulse-jet define the three collector families; the temperature limitations tie the fabric families (polyester, acrylic, Nomex, P84, PTFE and glass) to the services; and the sizing calculations of the filtration velocity and the dedusting air flow rates fix the cloth area, the collector geometry and the fan.

The fabric selection is the systematic comparison of the temperature, the chemistry, the dust, the cleaning energy and the emission guarantee against the fabric properties; the troubleshooting is the reading of the differential pressure and the emission pair; and the application experience and the recent developments (the membrane media, the long-bag designs, the smart control and the online monitoring) complete the picture. The cement plant that masters the cake, the velocity, the fabric and the pressure drop runs its collectors at the emission limits the law demands and the mass balance returns, and the full engineering documentation of the design, the sizing, the fabrics and the troubleshooting is available in the Complete Cement Technical Package.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.