Bag Filter Calculations Mr Bokaian S: Complete Guide & Downl

Bag filter calculations are the core of every cement plant dedusting design: before any bag filter can be selected, the engineer must determine, for every dedusting point in the plant (conveyor transfer points, elevators, silos, crushers, mills, kiln feed, packers and loadout), the air volume that must be extracted to keep the dust where it belongs. This article is based on the classic Krupp dedusting design procedure, the spreadsheet-based calculation workbook (“Bag Filter Calculations”, the Krupp Design Manual – Dedusting, with its 24 calculation sheets covering 38 standard dedusting points) that has been used by plant engineers and engineering offices for decades. It explains the complete calculation route: the classification of dusts into wearing (abrasive) and non-wearing types, the recommended air velocities in dedusting pipes, the air volume formulas for every equipment type (troughed belt conveyors, apron feeders, bucket elevators, vibrating troughs and screens, Aeropol and Poldos pressure vessels, Fluidor fluidization conveyors, silo feed and discharge points, bins, crushers and coolers), the temperature corrections that every volume must pass through, and finally the filter area, bag count and fan sizing that turn the volumes into a working dedusting system. The complete workbook is part of the Complete Cement Technical Package, the 931-file engineering library from cementequipment.org.

Getting the air volume wrong in either direction is expensive: too little air leaves dust escaping into the plant and the environment, while too much air oversizes the filter, the fan and the ductwork, wasting capital and energy for the life of the plant. The Krupp procedure’s value is its systematic, per-equipment treatment of the problem: every dedusting point type has a characteristic formula and characteristic coefficients, so that a complete plant volume estimate can be built sheet by sheet, and the total volume then drives the filter selection. This guide reproduces the logic, the formulas and the typical values so that any engineer can repeat the calculation for a real plant.

1. The Purpose of Dedusting Design in a Cement Plant

A cement plant moves dusty material at every step, and each material transfer generates dust: the falling stream of material displaces air, the impact creates fine particles, and the air currents of the process carry them away. The dedusting system has three simultaneous tasks. First, it protects the health of the workforce and meets emission limits: the plant extracts dusty air at the point of generation and cleans it in a bag filter before release to the atmosphere. Second, it protects the equipment: dust in the atmosphere enters bearings, gearboxes, electrical rooms and control panels, and abrasive dust shortens the life of every machine it reaches. Third, it protects the process: dust recovery returns valuable material (kiln dust, coal dust, cement) back into the process instead of losing it, and dust-free surroundings allow the instruments (weigh feeders, level probes, optical sensors) to work reliably.

The design task is defined by the number and type of dedusting points. In a typical integrated plant, the dedusting network covers dozens of points: the crusher house, the raw mill feed and reject circuits, the raw meal silo and kiln feed system, the preheater tower (through the process gas filter), the clinker transport and cooler dedusting, the cement mill circuits, the cement silos and the packing and loading halls. The Krupp procedure classifies these points into 38 standard types, each with its own air volume calculation, and this systematic classification is what makes a complete plant calculation possible without missing a single transfer point.

2. The Dust Classification: Wearing and Non-Wearing Dusts

The first decision in the calculation procedure is the classification of the dust, because it determines the design air velocities and the equipment wear protection. The procedure distinguishes two classes:

• Non-wearing (non-abrasive) dusts: limestone dust, raw meal, cement kiln dust, gypsum and raw materials with low quartz content, and most preheater dusts. These dusts can be conveyed at higher velocities without excessive pipe wear, because the particles are soft and rounded.
• Wearing (abrasive) dusts: dust from slag, dust from iron ore and from materials with high free silica or hard angular grains. These dusts abrade the ductwork, the bends and the filter inlet at high velocity, so the design sets lower velocities, thicker pipe walls, and wear protection (hard liners or ceramic lining at the bends and at the filter inlet).

The classification also influences the cleaning and filtering parameters: abrasive dusts justify a more robust filter construction, while fine, sticky dusts (such as kiln dust with alkalis) influence the cloth selection and the cleaning frequency. The design table of the procedure gives the recommended air velocities in the dedusting pipes for each class, typically in the range of 13 to 18 m/s for the horizontal conveying lines (with a minimum of about 13 m/s to keep dust from settling in horizontal ducts) and somewhat higher values where the dust is non-wearing and the transport distance short. The rule of the calculation is that the computed velocity must lie between a defined minimum and maximum for the pipe section: below the minimum, dust settles in the horizontal runs; above the maximum, the pipe erodes and the energy consumption becomes unreasonable.

Dust class	Examples	Recommended duct velocity	Wear protection
Non-wearing	Limestone dust, raw meal, kiln dust	13–18 m/s	Standard steel duct
Wearing (abrasive)	Slag dust, iron ore dust, high-silica dust	12–15 m/s	Thicker walls, liners at bends
Very fine / cohesive	Cement, coal dust	15–18 m/s	Grounding for coal, anti-stick design

3. The Standard Dedusting Points and Their Air Volume Formulas

The heart of the Krupp calculation procedure is the standard list of dedusting points, each with its own air volume formula derived from the equipment geometry and the rules of the art. The full list of 38 types includes the following principal families, and the formulas below show the structure of the calculation:

• Troughed belt conveyor transfer points: the air volume Vf is calculated from the belt width and the material drop height, using formulas of the form Vf = ka × (width in mm / 1000) × (length in m) × 60 × ((273 + temp) / 293) × 1.1 for the feed point, and correspondingly smaller volumes for the discharge point, where ka is a coefficient that depends on the material and the drop height. The width term reflects the opening size of the skirted transfer hood, and the 1.1 factor is the standard leakage allowance.
• Steel apron feeder transfer points: similar formulas with the feeder width, with slightly larger coefficients because the apron surface and the material stream expose more area.
• Deep bucket conveyor and bucket elevator points: the feed (bottom boot) and discharge (top hood) points are calculated separately; the boot volume formula Vd = km × (width in mm) × 3.6 reflects the bucket chain width and the air displaced by the buckets, while the head hood volume is based on the discharge opening and the material stream, with the discharge hood formula using the width and the drop. The bottom boot for a deep bucket conveyor has its own coefficient (0.75 × width × 3.6) that reflects the larger opening of the deep bucket boot compared to a chain elevator.
• Box feeder and weigh belt: volumes based on the opening dimensions (width times height of the feed opening), with formulas of the form Vf = (width of opening in mm × height of opening in mm / 1000) × factor × 3.6, where the factor increases with the drop height of the material (1.5 for moderate drops, 2.0 for higher drops with more air displacement).
• Vibrating troughs and vibrating screens: with and without flex seals, with formulas that use the trough or screen width and the conveying length; the flex seal (a flexible skirt closing the gap around the vibrating machine) reduces the required volume substantially, because it limits the air displacement. For vibrating screens the discharge end uses the width and the number of decks.
• Pneumatic systems and fluidized conveyors: the Aeropol (fluidized air conveyor), the Poldos pressure vessel and the Fluidor fluidization conveyor receive their own formulas based on the fluidization air flow or on the vessel size: for the pressure vessel discharge, the volume is derived from the aeration air and the discharge rate, and for the Aeropol the formula uses the conveying capacity in tph with a factor of 1.4 m³/h per tph of material flow.
• Silo feed and discharge points: open and covered silo feed, silo top vents and silo bottom extraction points, with formulas using the silo diameter: the cone vent volume is based on the aeration air and the cone geometry, and the top vent volume uses the silo diameter and the filling rate, with formulas such as Vsc = (Dsc/1000)²/2 − 0.5 × (Dsc/1000)² × π/4 × 2 × 3600 for the cone volume correction, and the stack-volume formulas Vs = π×(D/1000)²/4 × (D/2000) × tan(30°)/3 × factor for the material pile geometry in the silo. Covered (enclosed) silo tops need only the venting air, while open silo tops need the full displacement volume.
• Bin on top with feed openings: one or two feed openings on the bin top, with volumes based on the opening dimensions and the drop height, and pneumatic feed points for the bins, where the volume includes the conveying air of the pneumatic system itself.
• Polycom (high-pressure grinding rolls), crushers and hammer crushers: volumes based on the crusher opening, the material drop and the moisture level; the moisture level is decisive because dry material produces far more dust, and the calculation applies an additional factor (for example 0.4 or 1.0–1.1) according to the moisture class of the feed.
• Bulk loading points, cement coolers and other machines: the bulk loading of tankers and the ship and truck loading points use the loading spout cross-section and the material rate, and the cement cooler receives a volume based on its cooling air flow, since the cooler exhaust must pass through the filter.

The power of the list is that it forces completeness: the plant engineer walks the process and assigns every dust source to one of the 38 types, so that the final volume table covers the entire plant. The typical result of such a plant-wide count is a list of 20–80 dedusting points, from which the volumes are then grouped into filters according to the plant layout.

4. The Temperature Correction of the Air Volumes

Almost every air volume in the dedusting calculation is computed at the equipment condition and must be corrected to the actual gas temperature before the filter and the fan are sized, because the gas volume grows directly with the absolute temperature. The standard correction used throughout the procedure is the ratio of absolute temperatures:

Vactual = Vcalculated × (273 + t) / (273 + 20)   (for t in °C, referred to 20 °C)

which appears in the belt conveyor transfer formulas as the factor (273 + temp)/293. For cold dust (ambient temperature), the correction is near unity; for hot points (clinker transfer, cooler dedusting, kiln feed), the correction is decisive: a gas at 200 °C occupies about 60 percent more volume than the same mass at 20 °C, so the filter, the fan and the duct must all be sized for the hot volume. The calculation also applies the leakage factor: every hood, enclosure and duct joint admits false air, typically accounted at 10–15 percent of the calculated volume (the 1.1 and 1.15 factors seen in the formulas), and the fan is selected for the total volume at the fan inlet conditions, which includes both the temperature correction and the leakage allowance.

The temperature also sets the filter technology: at up to about 130 °C, standard polyester needle-felt bags are used; between 130 and 200 °C, the design changes to homopolymer acrylic or PTFE coated fabrics or to woven glass bags with appropriate treatment; above 200 °C, either the gas is cooled (air dilution, water injection or heat recovery) before the filter, or high-temperature filter media are required. The procedure therefore ends the volume calculation with a clear statement of each point’s gas temperature, because that single value drives the filter media choice, the casing insulation and the fan material.

5. Filter Area, Bag Count and the Air-to-Cloth Ratio

With the total volume per filter group established, the bag filter itself is sized through the air-to-cloth ratio (also called the filtration velocity or the specific gas load), the fundamental design parameter of fabric filters. The filter area is simply:

A = V / vf

where A is the effective filter cloth area in m², V is the gas volume at filter conditions in m³/h, and vf is the air-to-cloth ratio in m³/(m²h), also expressed as the filtration velocity in m/min. The air-to-cloth ratio is chosen from the dust class and the cleaning system: for pulse-jet filters with polyester bags handling cement and raw material dusts, the design value is typically in the range of 1.0 to 1.8 m/min (60–110 m³/(m²h)) for continuous operation; for reverse-air cleaned woven bags the values are lower (0.5–1.0 m/min), and for heavy dust loads or fine cohesive dusts the design is set at the conservative end. The number of bags follows from the filter area and the bag geometry:

n = A / (π × d × L)

where d and L are the bag diameter and length (typical modern pulse-jet bags: 130–160 mm diameter and 2.5–6 m length). The bag count then determines the filter housing layout: bags are arranged in rows in compartments, with the pulse cleaning system (compressed air manifolds, blow pipes and venturis) above each row, and the total filter is divided into compartments so that one compartment can be cleaned or isolated while the others operate.

The complete sizing loop closes with the dust load check: the filter inlet dust concentration (for a transfer point typically 5–50 g/m³, for a mill or cooler exhaust 50–200 g/m³) times the volume gives the hourly dust mass that the filter must collect and discharge, which sizes the hopper, the rotary airlock and the dust conveying line; and the outlet emission (typically below 10–30 mg/m³ at the stack) verifies the separation efficiency of the chosen cloth and cleaning system.

6. Duct Design and System Layout Rules

With the volumes and velocities defined, the duct system is laid out to satisfy the two velocity constraints at every section: the minimum velocity that keeps the dust entrained (about 13 m/s in horizontal runs for cement dusts) and the maximum velocity that limits erosion and noise. The duct diameter follows from the continuity equation d = 1.128 × (V/(3600 × v))^0.5 (d in m, V in m³/h, v in m/s), and the system is then balanced so that every hood receives its designed share of the volume. Balancing is achieved by sizing the branch ducts and, where necessary, by dampers at the branches, and the procedure’s recommendation is to prefer self-balancing design (correct duct sizing) over dampers, because dampers drift with time and the point with the shortest duct always wins the volume.

The layout rules of the procedure follow the physical realities: ducts run as short and as straight as possible; long horizontal runs are avoided (or equipped with clean-out doors) because dust settles when the system runs at reduced duty; bends are radiused (radius at least 1.5–2 times the diameter) and are the first places to be lined for abrasive dusts; the fan is placed after the filter (clean-gas side) so that the impeller sees clean air and the whole system runs under negative pressure, which prevents dust leaking outward through every joint; and each filter group is laid out so that the filter can be isolated from the process for maintenance without stopping the process line. The fan sizing closes the design: the fan total pressure is the sum of the duct friction losses (from the duct dimensions, the velocities and the bend losses), the filter casing and bag losses (typically 100–250 mm WG), and the stack losses, with the standard safety margin; and the fan volume includes the temperature correction and the leakage allowance calculated above.

7. The Worked Example: A Complete Dedusting Point Calculation

To make the procedure concrete, this section works a typical example from the workbook’s logic: a troughed belt conveyor transfer point handling limestone, with a belt width of 1000 mm, a drop height of 2 m at the feed point, and a discharge end with a hood at the head pulley. The feed point volume is calculated with the transfer formula: for a moderate drop and a 1000 mm belt, the base volume is of the order of 0.075 times the width times the length-plus-drop terms, giving a feed volume of about 1000 to 1300 m³/h for the enclosure around the chute; the discharge hood contributes about 0.7 to 1.0 times the width term, around 700 to 1000 m³/h, and the leakage allowance of 10 percent is then added to each. At ambient temperature (20 °C) the temperature correction is unity, so the two points sum to roughly 2000–2500 m³/h; with the belt running at 2.5 m/s and the material at 25 °C (minor correction), the design total for the transfer station is about 2300 m³/h.

For comparison, the workbook shows the same structure for the other transfer types: a deep bucket conveyor boot takes about 0.75 × 1000 × 3.6 = 2700 m³/h for a 1000 mm bucket chain, a vibrating screen discharge takes about 27.41 × (1000)^0.741 ≈ 6300 m³/h for a 1000 mm screen without flex seal, and a silo top with a 12 m diameter and two feed openings needs the pile displacement and venting volumes computed from the silo diameter formulas, on the order of 10,000 m³/h for an open top. The aggregate of these point volumes for a whole plant is what the 24-sheet workbook accumulates: the final summary sheet lists each point, its volume at equipment condition, the corrected volume at the filter, and the assigned filter group, producing the total gas volume that the plant’s dedusting fan capacity must match.

With a total of, say, 120,000 m³/h collected from a cement mill circuit (mill vent, elevator, separator, airslides and silo vents), the filter sizing proceeds as described: at a filtration velocity of 1.2 m/min (72 m³/(m²h)), the required cloth area is 120,000 / 72 ≈ 1667 m², which with 150 mm diameter by 5 m long bags (2.36 m² per bag) gives about 700 bags, arranged in 6–8 compartments; the pulse system is sized for the compressed air consumption of roughly 1–2 m³/min per compartment at 5–7 bar, and the fan is selected for the corrected volume plus a 10 percent margin at a pressure of 180–250 mm WG. The example shows how the procedure converts a plant walk-down into a concrete equipment specification, sheet by sheet.

8. Filter Types, Cleaning Systems and Media Selection

The bag filter hardware follows from the design data. Three filter families dominate cement plant practice. Pulse-jet filters are the modern standard: bags are cleaned by a short compressed air pulse (0.3–0.5 MPa) through the venturi at the bag top, which snaps the bag and releases the dust cake; they run at the highest air-to-cloth ratios, operate continuously with on-line cleaning, and are built for volumes from a few hundred to over a million m³/h (the kiln and raw mill baghouses). Reverse-air filters use a gentle reverse air flow to collapse the bags and crack the cake off; they run at lower ratios, are gentler on the cloth, and were the standard for large process filters before pulse-jet designs improved. Shaker filters mechanically shake the bags; they are limited to moderate volumes and low temperatures and are today found mostly in small applications.

The media selection follows the gas temperature and chemistry: polyester needle felt is the default for temperatures up to 130 °C (abrasion resistant, good cake release, moderate cost); homopolymer acrylic serves up to about 160–180 °C; PTFE and PPS products serve the high-temperature and chemical-duty ranges; and for hot process gas (150–250 °C) woven glass or ePTFE membrane fabrics are used with appropriate treatment and more conservative filtration velocities. For dedusting points with moisture (grinding plants, wet materials), the filter must be insulated and the hoppers heated or the cloth will blind with sticky dust; for coal dust, the entire filter is grounded, spark-proofed and provided with explosion venting and fire suppression, and the collected dust is returned to the coal circuit. The cleaning control (pressure-differential triggered, timed or a combination) completes the package: the filter is controlled by the pressure drop across the cloth, with the cleaning cycle set to hold the drop in the range of 80–150 mm WG for pulse-jet filters, and the cake thickness on the bags is the wear parameter that sets bag life, typically 2–5 years in cement service.

9. Hood and Enclosure Design: Capturing the Dust at the Source

The air volumes of the procedure are only meaningful if the hoods and enclosures actually capture the dust, and hood design is therefore part of the same calculation sheet. The three hood principles that govern the point volumes are enclosure, partial enclosure and capture-at-the-opening. Fully enclosed points (elevator boots, mill vents, enclosed silo tops, fully skirted transfer points) need only the ventilation volume that keeps the enclosure under slight negative pressure: the volume replaces the displaced air and the leakage, and the formulas for these points are the small ones with the 0.3–0.75 coefficients. Partially enclosed points (open transfer hoods, crusher openings) need the air displaced by the falling material stream and by the induced air of the machine, which is why the opening-area formulas with the 1.5–2.0 factors apply. Open capture points (loading spouts, open chutes, hoppers) need capture velocities at the opening edge, typically 0.5–1.0 m/s for dust-laden openings, which is why the loading point formulas use the spout area and the drop energy.

The enclosure design rules that make the volumes effective are simple and strict: the enclosure must extend below the falling material stream (the skirt should dip into the material on the belt so that the stream seals the opening), the hood must be large enough that the dust cloud forms inside it rather than at its edge, the extraction point must be placed at the top of the hood or on the side opposite the material flow (where the displaced air exits), and the gaps must be minimized and sealed, because every square centimeter of gap admits leakage air that reduces the effective capture velocity at the opening. The calculated volume already contains the standard 10–15 percent leakage allowance, but a poorly sealed hood can double that allowance silently, starving the actual capture point. The procedure closes the hood section by checking the capture velocity at each opening against the minimum value for the dust class, and it records the hood dimensions and the extraction position on the sheet so that the layout drawing and the calculation agree.

10. Frequently Asked Questions

Q1. What is the first step of a bag filter calculation?

Enumerating the dedusting points: walking the plant and assigning every dust source to one of the standard types (belt transfer, elevator boot, silo top, crusher, and so on), because each type has its own air volume formula and no volume can be calculated for a point that was not identified.

Q2. Why is the air velocity in dedusting pipes kept above about 13 m/s?

To keep the dust entrained: below the minimum velocity, particles settle in horizontal duct runs, the duct fills with dust, and the system loses its extraction capacity. The velocity is capped on the high side to limit erosion and energy consumption.

Q3. How is the filter area of a bag filter determined?

By dividing the gas volume (corrected for temperature and leakage) by the air-to-cloth ratio: A = V / vf. The air-to-cloth ratio is chosen from the dust class and the cleaning system, typically 1.0–1.8 m/min for pulse-jet filters in cement service.

Q4. Why must the gas temperature correction be applied before sizing the filter?

Because gas volume is proportional to absolute temperature: the same mass flow at 200 °C occupies about 60 percent more volume than at 20 °C. Sizing the filter and fan on the cold volume would under-size them, and the filter media choice also depends on the operating temperature.

Q5. What distinguishes a wearing dust in the calculation?

Slag, iron ore and high-silica dusts are classified as wearing: they force lower duct velocities, thicker duct walls and liner protection at the bends and at the filter inlet, because the abrasive particles erode unprotected steel rapidly.

Q6. How many dedusting points does a typical cement plant have?

Depending on the plant configuration, between 20 and 80 dedusting points are counted for the crusher, raw mill, kiln feed, clinker transport, cement mill, silo and packing areas, and each appears in the volume table and in the filter grouping.

11. Final Summary

The bag filter calculation procedure developed in the Krupp design manual is the complete, systematic method for designing cement plant dedusting systems: it classifies the dusts, assigns every dust source to a standard point type with its own air volume formula, applies the temperature correction and leakage allowance, balances the duct network, sizes the filter cloth area and bag count through the air-to-cloth ratio, selects the cleaning system and media for the temperature and chemistry, and sizes the fan for the corrected volume and pressure. The spreadsheet workbook (“Bag Filter Calculations”) turns this procedure into a practical tool: 24 sheets, 38 standard point types, and the complete example calculations that let the plant engineer produce a defendable volume table and filter specification in days instead of weeks.

The workbook is part of the Complete Cement Technical Package, the 931-file engineering library that covers the entire cement process from quarry to loadout, including the dedusting calculations, the conveyor and elevator design tools, the mill and kiln calculators, and the process books and courses. Get the full package with one PayPal payment — instant download, lifetime access, and the complete engineering desk of the cement industry on your screen.

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.