Bag Filter: Complete Technical Guide

The bag filter is the last line of defence between the cement plant and the atmosphere: it takes the dust-laden gas from the kiln, the mills, the cooler, the silos and the conveying points, and it returns a stream clean enough to meet the emission limits of the permit, while the collected dust becomes raw meal, cement or saleable product: the presentation “294798440 Bag Filter” from the cementequipment.org library documents the fabric filter from the gas flows to the bag cleaning: the air-to-cloth ratio, the can velocity, the pressure drop, the pulse-jet cleaning, the filter bag materials and the design arithmetic: this article expands the full logic of the file into a complete technical reference for the process and the maintenance engineers who own the dust collection of their plants.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes this bag filter presentation together with the bag filter calculations workbooks, the ESP courses and the air pollution control handbooks: this article walks the file for the designers, the operators and the auditors, and it reproduces the formulas and the typical values so that a filter can be sized, checked or troubleshot with the numbers on the table.

The bag filter is at once the simplest and the most demanding machine of the plant: its principle is a permeable cloth and a pressure difference, but its performance depends on the gas volume, the dust load, the temperature, the moisture and the cleaning discipline, and a miscalculated air-to-cloth ratio shows up months later as the visible plume that the neighbours photograph: this article is the complete operation of that machine.

1. The Place of the Bag Filter in the Cement Plant: the dust applications

The bag filter appears at every dust source of the cement works, and each application places its own temperature, moisture and dust-load demands on the same machine: the complete list of the duty points is the census that the presentation opens with, because the design of each filter follows the dust and the gas it must serve:

• The kiln and the calciner main bag filter: the largest filter of the plant, receiving the cooler and the preheater gas after the conditioning, near 200 to 250 degrees Celsius at the inlet, with the typical dust loads of 50 to 150 g/Nm3 entering the filter;
• The raw mill filter: the filter that follows the raw mill circuit, seeing the moist meal-laden gas and operating as part of the drying loop, with the dew point protection as its design constraint;
• The cement mill and the separator filters: the finish grinding filters with the fine cement dust, the sticky fine particles and the thermal load of the mill ventilation;
• The clinker cooler filter: the cooler dedusting with the hot, abrasive and coarse clinker dust, where the abrasion protection and the temperature control dominate the design;
• The silo top, the packing plant and the conveying point filters: the many small filters of the dispatch and the materials handling, with the light dust loads and the short operating cycles;

The file’s census matters because the same filter model repeats across all the applications with the different internals: the kiln filter gets the high-temperature bags and the conditioning; the raw mill filter gets the insulated casing and the dew-point protection; the cooler filter gets the abrasion-resistant inlet and the high air volumes; and the dispatch filters get the compact modules: the engineer who understands the family understands every member, and this article covers the family.

2. How the Bag Filter Works: the cloth, the dust cake and the clean gas

The bag filter is a pressure-driven separator: the dust-laden gas enters the filter casing, flows to the inside or the outside of the filter bags, and passes through the fabric while the dust is arrested on the cloth surface: the cake of the collected dust that builds on the cloth is itself the effective filter medium, and the cleaning cycle removes that cake periodically so that the pressure drop stays in its window: the machine is a cake-building and cake-removing cycle repeated every few minutes for the lifetime of the bags.

• The filtration mechanism: the interception, the inertial impaction and the diffusion trap the particles on the fibres and in the cake: the fine particles that diffuse by the Brownian motion are captured even at the sub-micron sizes, which is why the bag filter reaches the emission levels that the filters of the past could not;
• The cake as the filter: once the first layer of the dust is laid, the subsequent filtration happens through the cake, and the collection efficiency rises with the cake: the freshly cleaned bag is momentarily less efficient until the cake re-forms;
• The gas paths: the tube-sheet separates the dirty-gas chamber from the clean-gas chamber, the bags hang or stand on the tube-sheet, and the cleaned gas leaves through the outlet plenum to the fan and the stack;
• The control purpose: the operator’s instruments are the differential pressure across the bags, the inlet and the outlet temperatures, the gas flow and the opacity of the stack: the bag filter is run by keeping those four in their windows;

The mechanism defines the two design numbers that everything else follows from: the air-to-cloth ratio, which sets how fast the gas passes through the cloth, and the pressure drop, which sets how much force drives that passage: both appear in every section below, and both are the numbers every bag filter conversation begins with.

3. The Air-to-Cloth Ratio (A/C): the master design number

The air-to-cloth ratio, abbreviated A/C and called the filter velocity or the filtration velocity in the other schools, is the gas volume per unit of filter cloth area: it is the master number of the bag filter design because it trades the filtration efficiency against the filter size and the energy: the lower the A/C ratio, the gentler the filtration, the lower the pressure drop and the longer the bag life, but the larger and the more expensive the filter must be: the design chooses the ratio from the dust, the temperature, the moisture and the cleaning system.

A / C = Q / A-cloth

Where Q is the gas volume flow in cubic meters per hour (or per minute) at the operating conditions and A-cloth the total fabric area in square meters: the common units are m3/h per m2, or the equivalent m/min in the American practice: the typical values of the cement plant group by the cleaning system and the duty:

Application	Cleaning system	Typical A/C m3/h per m2	Notes
Pulse-jet, general process dust	Pulse-jet	90 – 150 (1.5 – 2.5 m/min)	Compact, high on-line cleaning
Reverse-air, large kiln filters	Reverse-air	24 – 48 (0.4 – 0.8 m/min)	Gentle, low pressure, larger units
Kiln / raw mill filters (fine, moist)	Pulse-jet or reverse-air	60 – 120	Dew point and temperature dominated
Fine cement and separator dust	Pulse-jet	60 – 100	Sticky fines need the lower ratio
Silo top and light dusty air	Pulse-jet	120 – 180	Light load allows the higher ratio

The A/C ratio is chosen at the standard conditions but must be corrected to the actual operating temperature and pressure, because the gas volume expands with the temperature: the filter handles the actual cubic meters at the inlet condition, and the designer who sizes at the normal conditions and forgets the temperature correction under-sizes his filter by the ratio of the absolute temperatures: the file’s design sheets carry the correction as the first step: the A/C ratio is the number that must be honestly computed before a single bag is quoted.

4. The Pressure Drop: the driving force and its limits

The pressure difference that pushes the gas through the cloth and its cake is the filter’s working force, and it is measured as the differential pressure (dP or delta-P) between the dirty and the clean side: the pressure drop is the sum of two parts: the drop across the clean cloth, which is small and constant, and the drop across the dust cake, which grows as the cake thickens between the cleaning cycles: the operator controls the filter by watching this delta-P and by triggering the cleaning when it reaches the set limit.

dP-total = dP-cloth + dP-cake = (K1 × mu × u) + (K2 × mu × u × W-cake)

Where K1 and K2 are the resistance coefficients of the cloth and the cake, mu the gas viscosity, u the filtration velocity (the A/C ratio in m/s or m/min) and W-cake the dust mass per unit of the cloth area: the formula embodies the operating logic: the pressure drop rises with the velocity and with the accumulated dust, and the cleaning resets the cake term while the cloth term always remains: the typical operating delta-P of the pulse-jet filters runs 800 to 1,500 Pa online and 1,500 to 2,500 Pa with the heavy loads, and the trip limits usually sit between 2,500 and 3,000 Pa, where the cleaning can no longer keep up.

• The delta-P and the fan: the pressure drop is the resistance the fan must overcome, and a rising delta-P is a rising energy bill: the difference between 1,000 and 1,800 Pa on a large kiln fan is measurable electricity every hour;
• The cleaning trigger: the differential pressure controller initiates the cleaning cycle at the high setpoint and stops it at the low setpoint, so the cake is kept thin enough to be economical and thick enough to filter well;
• The cake control: the over-cleaning leaves no cake and the emissions spike until the cake re-forms; the under-cleaning lets the cake harden and the delta-P climbs to the trip: the cleaning discipline is the operator’s daily art;
• The temperature effect: the gas viscosity grows with the temperature, so the same filter at a hotter gas draws the higher pressure drop: the temperature correction belongs to the pressure budget as much as to the A/C sizing;

The pressure drop is the measurable voice of the filter: a sudden rise says the cake is wet, the bags are blinded or the rotation is short; a sudden fall says the bags are torn or the hopper is plugged and turning to the short-circuit; the delta-P trend, plotted by the control system, is the earliest and the most reliable diagnostic of the whole machine, and the file builds its troubleshooting section on exactly that trend.

5. The Can Velocity and the Gas Distribution: the internal aerodynamics

Two more velocity numbers govern the filter’s internal behaviour: the can velocity, the upward velocity of the gas within the filter casing below the tube-sheet, and the inter-bag velocity, the velocity in the space between the bags: the can velocity must stay low enough that the falling dust, dislodged by the cleaning, falls through the gas and into the hopper instead of being re-entrained onto the neighbouring bags: the typical can velocities run 1.5 to 3.0 m/s, lower for the fine and the sticky dusts.

• The can velocity limit: the dusty and the fine-material filters hold the can velocity below 2 m/s, so the re-entrainment stays weak and the cleaning is productive;
• The inlet distribution: the inlet gas must spread evenly across the tube-sheet, or the bags near the inlet choke while the far bags filter nothing: the guide vanes and the baffles of the inlet plenum make the flow uniform;
• The inter-bag velocity: the spacing of the bags sets their local gas velocity, and the manufacturer’s bag-spacing tables keep that velocity inside the limit that prevents the local over-filtration and the abrasion;
• The hopper aerodynamics: the dust must descend into the hopper and out through the rotary valve or the screw, and the hopper is designed so that the gas does not re-circulate and the dust does not pile above the outlet;

The internal aerodynamics are where the well-made filters earn their reputation: two filters with the identical bags and the identical A/C ratio can perform differently because one distributes the gas evenly and the other sends the whole flow through the near bags: the presentation teaches the plenum design and the computational-flow check that the serious suppliers run, and the auditor’s on-site inspection of the baffles and the wear patterns reads which design philosophy the filter was built with.

6. The Cleaning Systems: the reverse-air, the shaker and the pulse-jet

The cleaning of the bags is the rhythm of the filter’s life, and the cement industry uses three families: the reverse-air cleaning, where the gas flow is reversed through the off-line bags to collapse the cake; the mechanical shaker, where the bag is shaken to dislodge the cake; and the pulse-jet, where a short, high-pressure air pulse from inside the bag flexes the fabric and snaps the cake off: the pulse-jet dominates the modern plants because it cleans on-line with the small compressed-air pulse and allows the high A/C ratios and the compact filters:

• The reverse-air system: the gentlest cleaning, with the low-velocity reversed flow, suited to the large kiln filters with the woven bags and the delicate cakes: the filter compartments are isolated in rotation, so the effective cloth area during the cleaning is reduced and the A/C ratio must allow for the off-line compartments;
• The mechanical shaker: the simple, rugged cleaning of the small process filters, with the top-mounted shaker motors and the rigid bag frames: the shakers clean the bags gently but cannot reach the deep cake;
• The pulse-jet system: the modern standard: the compressed-air pulse at 3 to 6 bar is injected through the venturi at the top of each bag row, the shock wave travels down the bag flexing the fabric, and the cake falls into the hopper: the pulse-jet runs on-line, so the full cloth area is always in service;
• The cleaning control: the pulse frequency is set by the differential pressure, and the intelligent controllers adapt the pulse interval to the dust load, pulsing often when the load is heavy and sleeping when it is light;

The choice of the cleaning system sets the bag material, the A/C ratio, the casing geometry and the compressed-air consumption, and the file compares the three on the cement duty: the reverse-air on the big kiln filters for the long bag life at the low pressure, the pulse-jet everywhere else for the compactness and the on-line cleaning: the compressed air of the pulse-jet is an operating cost, and the file quantifies it so that the plant chooses the cleaning economics with the hardware.

7. The Filter Bag Materials: the fabric, the finish and the temperature rating

The filter bag is the consumable heart of the machine, and the material choice is a design decision as important as the A/C ratio: the bag must survive the gas temperature, the chemical attack of the sulfur and the alkalis, the abrasion of the dust and the flexing of the cleaning, and the choice trades the bag price against the bag life and the operating window: the presentation’s material tables carry the rating that the engineer must match to the gas at his inlet:

Bag fibre	Continuous temperature C	Key property	Typical cement duty
Polyester (PES)	130 – 150	Good strength, low cost, hydrolysis risk on the moisture	Cool process gas, silos, medium temperatures
Acrylic (homopolymer)	110 – 140	Excellent hydrolysis and moisture resistance	Moist raw mill and cooler applications
Polyimide (P84)	230 – 250	High temperature, fine filtration, higher cost	Kiln and preheater gas after cooling
PTFE (Teflon)	230 – 260	Chemical resistance, low friction, highest cost	Difficult chemical and high-temperature duty
Aramid	200 – 220	High temperature and mechanical strength	Kiln baghouses with the cooling towers
Fiberglass	230 – 260	High temperature, needs the surface finish	Large kiln reverse-air filters

Beyond the fibre, the bag construction carries its own choices: the woven versus the felted cloth, with the felts giving the finer filtration and the higher initial efficiency; the surface finishes such as the PTFE membrane and the singed-and-calendered felts that shed the cake easily; and the bag sealing, the cap and the cages that hold the bag geometry: the modern high-efficiency filters use the surface-finished felts that release the cake cleanly, keeping the pressure drop low and the bag life long: the file’s selection is the reference for matching the fabric to the gas, and every plant should have its own material matrix written from it.

8. The Bag Design Arithmetic: the cloth area, the number of bags and the housing

The design of a bag filter from the process data is the arithmetic that ties the sections together, and the presentation works the procedure in full: the engineer collects the gas volume at the operating conditions, the dust load, the temperature, the moisture, the emission limit and the cleaning system, and the design follows:

1. The gas volume: the actual cubic meters per hour at the filter inlet condition, corrected for the temperature and the pressure, plus the leakage margin of 5 to 10 percent;
2. The cloth area: the required fabric area equals the gas volume divided by the chosen A/C ratio, with the safety factor and, for the reverse-air, the off-line cleaning allowance:

A-cloth = Q / (A/C)

3. The number of bags: the number equals the cloth area divided by the area of one bag (the perimeter times the length): a standard 150 mm diameter bag of 6 meters gives about 2.8 square meters per bag;
4. The housing: the bag arrangement on the tube-sheet, the spacing and the can velocity set the plan area of the casing, and the hopper angles, the inlet plenum and the clean-gas plenum close the geometry;

The worked example of the file takes a cement mill filter at 200,000 actual cubic meters per hour, the fine cement dust, the pulse-jet cleaning and the A/C of 100 m3/h per m2, giving the required cloth area of 2,000 square meters and, at 6 meter bags, about 700 bags: the same duty with the reverse-air at half the A/C doubles the cloth and nearly doubles the casing: the comparison is the file’s demonstration of why the A/C choice is the economic core of the design, and why the honest computation of the actual gas volume is the fundament of the whole method.

9. The Emissions, the Dust Load and the Compliance: the filter as the environmental guard

The bag filter exists to meet the emission limit, and the modern limits frame its design: the European and the tightening national limits run near 10 to 30 mg/Nm3 for the cement dust emissions, and the well-designed and well-run bag filters reach 5 to 15 mg/Nm3 comfortably: the stack opacity, measured continuously, is the real-time witness, and the file treats the limit as the fixed design target that the A/C, the bags and the cleaning must serve:

• The dust load entering: the kiln filter sees 50 to 150 g/Nm3, the mill filters 30 to 100, and the design must arrest the whole load onto the cloth between the cleaning cycles: the load per cycle sets the cake mass per unit of cloth and the required cleaning intensity;
• The collection efficiency: the modern felts and the surface finishes reach efficiencies of 99.9 percent and above on the total dust, and the sub-micron fraction is captured by the diffusion and the cake action: the visible plume is a failure of the cleaning or the sealing, not of the fabric;
• The opacity monitoring: the stack monitor reads the plume in real time, and the alarm thresholds alert the operator to the tearing bags before the limit is breached: the bag-leak detection systems with the triboelectric or the photometric sensors ring at the moment of the damage;
• The mass balance and the product: the collected dust returns to the process (the raw meal to the feed, the cement to the silo, the kiln dust to the kiln or the bypass), and the filter is also a material-recovery machine: the dust that the filter returns is worth real money, and the capture is a product-saving as much as an environmental act;

The compliance discipline is the last control of the machine: the continuous emission measurement, the annual stack test, the pressure and the temperature logs, and the bag-house maintenance records form the evidence chain that the auditor and the regulator read: the file’s message is that the bag filter is not a cost center but the plant’s environmental reputation and its product recovery machine, and the same care that sizes it must run it.

10. The Operation and the Maintenance of the Bag Filter

The best-designed filter fails without the operating and the maintenance discipline, and the presentation devotes its closing technical sections to the daily and the campaign routines: the operator’s rounds, the readings, the cleaning adjustments and the condition monitoring are the habits that keep the emissions low and the bag life long:

• The daily readings: the delta-P, the inlet and the outlet temperatures, the gas flow, the stack opacity and the compressed-air pressure, logged on the shift sheet and compared with the trend;
• The cleaning adjustment: the pulse interval is trimmed to the dust load and the season: heavy load pulses more, light load sleeps, and the delta-P band is held without the over-cleaning;
• The bag leak detection: the opacity and the triboelectric sensors flag the torn bags, and the compartment isolation and the bag replacement follow during the short stops;
• The campaign maintenance: at the major stops the bags are inspected for the wear, the blinding and the tears, a percentage sample is replaced, the cages and the seals are checked, and the hopper can be cleared and the internals washed;
• The compressed-air system: the pulse-jet’s oil-free dry air is its lifeblood: the moisture and the oil in the pulse line blind the bags and freeze the solenoid valves, so the air treatment is a maintenance priority;

The bag life is the filter’s economics: the good pulse-jet filters run the bags 2 to 4 years on the normal process duty, and the premature failures trace to the temperature excursions, the dew point, the abrasion, the poor air quality or the over-cleaning: the file’s life-cycle tables let the plant price the bag cost per year and per tonne, and the condition monitoring extends that life by the timely intervention: the operation and the maintenance are the two hands of the same filter, and the file treats both.

11. The Troubleshooting of the Bag Filter: the case table

The presentation closes with the operating cases, and the table below condenses the classic ones, because the value of the file is highest when the plume appears:

Symptom	Likely cause	Cure
Visible plume, emissions rising	Torn bags, poor bag seals, hopper short-circuit	Leak detection, compartment isolation, bag and seal replacement
Delta-P climbing to the trip	Wet or blinded cake, blocked cleaning, plugged hopper	Dew point check, cleaning repair, hopper clearing, air quality
Delta-P unusually low	Broken bags, torn tube-sheet gaskets, gas bypass	Inspection and repair of the bags and the tube-sheet
Rapid bag wear and tearing	Abrasive dust and high velocities, cage corrosion	Lower the can velocity, protect the inlet, choose the tougher fabric
Bag blinding by the cake buildup	Over-cleaning robbing the cake, or sticky fines	Balanced cleaning, lower A/C, surface finish, larger bags
Air pressure low, pulsing weak	Compressor or dryer failure, solenoid blockages, leaks	Air system maintenance, oil-free dry air, pressure checks

The table is the condensed memory of the file, but the method behind it is the one the file teaches through all its sections: read the delta-P curve, read the opacity, read the temperatures, and connect each symptom to its physical cause before the tools are touched: the bag filter misbehaves for reasons, and every reason has its name: the operator who learns the names from the case pages will cure the plant’s visible plume in the first hour instead of the first week.

12. The Frequently Asked Questions

What is the air-to-cloth ratio and why does it matter?

The A/C ratio is the gas volume per unit of filter cloth area, the key sizing number of the bag filter: the lower the ratio, the gentler the filtration, the lower the pressure drop and the longer the bag life, but the larger and the costlier the filter: the cement plant’s pulse-jet filters run about 60 to 150 cubic meters per hour per square meter depending on the dust, and the ratio is chosen from the dust load, the temperature, the moisture and the cleaning system.

What pressure drop should a bag filter run at?

The operating delta-P of the pulse-jet filters runs about 800 to 1,500 Pa in the normal service, rising to 1,500 to 2,500 Pa with the heavy loads, with the trip limits near 2,500 to 3,000 Pa: the differential pressure is the sum of the cloth drop (always present) and the cake drop (grows between cleanings), and the cleaning is triggered by the controller to hold the delta-P in its window: the rising delta-P is both a process signal and a fan-energy cost.

How often are the bags cleaned and replaced?

The bags are cleaned every few minutes in the pulse-jet filter, with the pulse interval set by the differential pressure and the dust load: the bags themselves are replaced on the 2 to 4 year cycle of the normal process duty, with the partial replacements at the campaign stops and the torn bags replaced on detection: the bag life is governed by the temperature excursions, the dew point, the abrasion and the cleaning discipline.

Which bag material should the kiln filter use?

The kiln and the preheater gas, cooled and conditioned to 200 to 250 degrees Celsius, is served by the high-temperature fabrics: the aramid and the fiberglass for the large reverse-air filters, the polyimide (P84) and the PTFE for the pulse-jet duties: the choice is scored against the exact gas temperature, the sulfur and the alkali content, the bag cost and the required bag life, and the plant’s own material matrix is written from the file’s tables.

Why does the bag filter sometimes show a visible plume?

A visible plume means the bag filter is failing its primary duty: the common causes are the torn bags, the leaking bag seals, the hopper short-circuiting that sweeps the settled dust into the gas, or the cleaned bag with the lost cake that momentarily lets the fine dust through: the leak-detection systems and the opacity monitor ring at the moment of the damage, and the cure is the isolation and the replacement of the offending bags: a well-run filter shows no plume at all.

13. Conclusion

The bag filter is the plant’s environmental guard and its dust-recovery machine: the air-to-cloth ratio that sizes it, the pressure drop that runs it, the can velocity that protects its cleaning, the pulse-jet that keeps it alive, the fabrics that survive its gas, the design arithmetic that builds it and the cases that keep it clean: this article walked the file from the gas volume to the stack, and the engineer who applies the method will hold his emission inside the limit, his delta-P in its window and his bag cost per tonne at its minimum: the last line of defence, made into the first line of the plant’s good name.

The Complete Cement Technical Package includes the bag filter presentation together with the bag filter calculations workbooks, the ESP and the air pollution control handbooks: the one-time $249.99 purchase, the instant download and the lifetime access: the cloth, sized right: the delta-P, in its window: the cleaning, disciplined: the stack, invisible: the emissions, under the limit.

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.