1. Particulate Control-2 Fabric Filters Particulate Scrubbers
Lecture notes adapted from
Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu

2. Fabric Filters
Well known and accepted method for separating dry particles from a gas stream
Many different types of fabrics, different ways of configuring bags in a baghouse and different ways of flowing the air through the bags.
There are 3 common types of baghouse based on cleaning method
Reverse-air
Shaker
Pulse-jet

3. Fabric Filters

4. Fabric Filters A shaker baghouse
Filter compartements

5. Fabric Filters

6. Fabric Filters

7. Filtration Theory

8. Filtration Theory

9. Filtration Theory
Figure 6.2 pp 186

10. Filtration Theory

11. Filtration Theory

12. Design Considerations

13. Cleaning Cycles
tf: time interval between two cleanings of the same compartment
tr: time interval between cleanings of any two compartment

14. Variation of pressure drop with time
DPm DP tr tc
Time

15. Cleaning Cycles

17. Table 6.4 Total Number of Compartments N uj/uN-1 fN 3 0.87 4 0.80 5
0.76 7
0.71 10
0.67 12
0.65 15
0.64 20
0.62
Ratio of actual filtering velocity uj to average filtering velocity uN-1 in a multicompartament baghouse

18. Maximum Filtering Velocities in Shaker or Reverse Air Baghouses
Dusts
Max. Filtering V (ft/min)
Activated charcoal, carbon black, detergents, metal fumes
1.5
Aluminum oxide, carbon, fertilizer, graphite, iron ore, lime, paint pigments, fly ash, dyes 2
Aluminum, clay, coke, charcoal, cocoa, lead oide, mica soap, sugar, talc
2.25
Bauxite,ceramics,chorme ore, feldsapr, blour, flint, glass, gypsum, plastics, cement
2.5
Asbestos, limestone, quartz, silica
2.75
Cork, feeds and grain, marble, oyster shell, salt
3-3.25
Leather, paper, tobacco, wood
3.5
Table 6.1

19. Fabric Selection Fabric Max Temp, C Acid resistance Base resistance
Dynel 71
Good
Cotton 82
Poor
Wool 93
Nylon
Polypropylene
Excellent
Orlon
127
Fair
Dacron
135
Teflon
204
Glass
288
Table 6.2

20. Pulse Jet Filters
Introduced 45 years ago captured one-half of the industrial air filtration market
Air is filtered through the bags from outside to the inside, a cage inside each bag prevents the bag from collapsing
The bags are cleaned by short blast of high pressure air ( psi)
Each bag is pulsed every few minutes
On stream use

21. Pulse Jet Filters
There are no compartments and thus no extra bags which reduces size and cost (for a large coal-fired power plant, the baghouse is so large that it is designed with separate compartments)
Since bags are placed from the top, no need to provide walkways between rows of bags (reducing the size)
Felted fabrics can be used at much higher air to cloth ratio (higher filtering velocities)

22. Pulse Jet Filters
Table Maximum Filtering Velocities for Various Dust or Fumes
Dusts or Fumes
Maximum Filtering Velocity (ft/min)
Carbon, Graphite, Metallurgical Fumes, Soap, Detergents;Zinc oxide
5-6
Cement (Raw), Clay (Green), Plastics, paitn Pigments, Starch, Sugar, Wood, Gypsum, Zinc
7-8
Aluminum oxide, cement (finished), Clay (vitrifies), Lime, Limestone, Mica,Quartz, soybean, Talc
9-11
Cocoa, Cholocate,Flour,Grains, Leather Dust, Sawdust,tobacco
12-14

23. Advantages -High collection efficiency on both coarse and fine PM
-Insensitive to fluctuations in gas stream
-Filtered outlet air is very clean and may be recirculated within the plant
-Collected particles are dry for subsuquent processing or disposal
-Operation is relatively simple
-Unlike ESP, do not require the use of high voltages, maintenance is simplified
-The use of selected fibrous or granular filter aids (precoating) permits the high efficiency collection of PM1 smokes and gaseous contaminants
-Can be modular

24. Disadvantages
Over 290 C expensive special refractory mineral or metallic fabrics are required
Concentrations of some dusts in the collector, may represent a fire of explosion if a spark or flame is accidentally admitted.
Fabrics can burn if readily oxidizable dust is being collected
Fabric filters have relatively high maintenance requirements (periodic bag replacement)

25. Disadvantages
Fabric life may be shortened at elevated temperatures and in the presence of acid or alkaline PM or gas constituents
Cant be operated in the presence of hygroscopic materials, condensation of moisture or tarry adhesive components may cause plugging of the farbri or reqire sepcial additives
Pesprotory protection for maintenance personel may be required when replacing fabric
Medium pressure drop is required (4 to 10 inches of water column)

26. Example

27. Example

28. Example

29. Other Considerations
Temperature and Humidity : Fabrics have different maximum allowable teperatures. Low T can cause condensation of acid and/or blinding of the fabric with wet dust
Chemical nature of gas: Different fabrics have different resistance to acids or alkalies
Fire/explosion: Some fabric are flammable; Some dust are explosive
Dust Handling: dust removal rate, conveyor system, and hopper slope should all be considered

30. Educational Videos
Animation showing the pulse jet filtration
A presentation from Baghouse and Wet Scrubber Production
Firm, MikroPul

31. Wet Scrubbers

32. Particulate Scrubbers
Reading: Chap. 7
Types of scrubbers: spray chamber and venturi scrubber
Theory and design consideration
Pressure drop
Contacting power
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33. Aerosol & Particulate Research Lab
Spray Chamber
Collecting medium:
Liquid drops
Wetted surface
Recirculated water
Water to settling basin and recycle pump
Vertical spray chamber (countercurrent flow)
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34. Aerosol & Particulate Research Lab
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35. Cyclone Spray Chamber & Impingement Scrubber
Flagan & Seinfeld, Fundamental of Air
Pollution Engineering, 1988
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36. Aerosol & Particulate Research Lab
Venturi Scrubber
High efficiency even for small particles
QL/QG:
VG: m/s
Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995.
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37. Aerosol & Particulate Research Lab
Theory: Spray Chamber
Volume of each droplet
Total number of droplets that pass the chamber per second VG
QL: volumetric liquid flow rate
Droplet concentration in the chamber Vd
Vtd
Vd: droplet falling velocity relative to a fixed coordinate
Vtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow)
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38. Aerosol & Particulate Research Lab
At a given time dt, the distance a droplet falls is
Volume of air that flows through the cross-section area of a single
droplet during the time dt
Total effective volume of gas swept clean per second by all droplets in dz
Total number of particles swept clean per second by all droplets in dz
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39. Aerosol & Particulate Research Lab
Total number of particles removed per second over dz QL
Penetration can be defined as the fraction of particles of a specified diameter (Pd) that are not capture. hd=1-Pd
Particle penetration in a countercurrent vertical spray chamber:
Cross-sectional area of all the droplets QG
Overall penetration then will be sum of Pd
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40. Aerosol & Particulate Research Lab
If QL in gal/min and QG in cfm, z in ft and dd in mm
Particle penetration in a cross-flow spray chamber
Q: How do we have higher collection efficiency?
Q: What are the collection mechanisms (we need it for hd)?
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41. Aerosol & Particulate Research Lab
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42. Deposition of Particles on a Spherical Collector
Particle Reynolds #
Particle Schmidt #
Particle Stokes #
Diameter ratio
Viscosity ratio
Single droplet collection efficiency d
(diffusion)
(interception)
(impaction)
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43. Impaction only (Impaction parameter Kp is used in textbook; Kp = 2 St)
rp = 2 g/cm3 43

44. Venturi Scrubber
Use intertial impaction of suspended particles on water droplet formed by gas atomization

45. Venturi Scrubbers: Calvert Design
Particle penetration through a venturi scrubber
Kpo=2St (aerodynamic diameter) using throat velocity
f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials
Atomization produces a wide distribution of droplet size. However using the Sauter mean droplet diameter (dd) equation can be solved with satisfactory results.
s in dyne/cm, rL in g/cm3 and m should be in poise
QL and QG should be of the same unit
k1 = if VG is in cm/s
= 1920 if VG is in ft/s
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46. Aerosol & Particulate Research Lab
Pressure Drop
Venturi Scrubber
lt: venturi throat length
X: dimensionless throat length
Ex: 10” water, 2 mm, h = ?
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47. Contacting Power Approach
When compared at the same power consumption, all scrubbers give the same degree of collection of a given dispersed dust, regardless of the mechanisms involved and regardless of whether the pressure drop is obtained by high gas flow rate or high water flow rate
Nt: Number of transfer unit (unitless)
(PT :contacting power in hp / 1000 cfm)
a and b: coefficient and exponent of PT
PT should be determined from the friciton loss across the wetted portion of the scrubber. 47 47

48. Contacting Power Approach
Venturi scrubber collecting a metallurgical fume
Contacting power, hp/cfm 48

49. Example (PT contacting power in hp / 1000 acfm)
Q: Tests of a venturi scrubber show the results listed on the right. Estimate the contacting power required to attain 97% efficiency.
Friction loss (in H2O)
h (%)
12.7 56
38.1 89
(PT contacting power in hp / 1000 acfm)
Nt: Number of transfer unit
(unitless)
(1 inch of water = hp/1000 cfm) 49

50. Example
Convert friction loss to contacting power (hp/1000 cfm): 1 in H20 = hp/1000cfm
Friction loss (in H2O)
PT hp/1000cfm
12.7 2
38.1 6
h (%) Nt 56
0.821 89
2.207 97
3.506 50

51. Example
Substractin Eq A from Eq B: A B 51

52. Aerosol & Particulate Research Lab
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53. Problem 7.1

54. Solution Impaction parameter Kp is used in textbook
Determine the density of water and the viscosity of the air at 80 °F from Appendix B

55. Solution

56. Solution

57. Elimination of Liquid Entrainment
To avoid recontamination of the gas by the entrainment of liquid in the exiting gas stream, it is neccessary to remove at least 95% of the liquid carryover
In spray scrubbers, louver-type mist elimanators are frequently used.
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