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Published byDana Hall Modified over 9 years ago
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Particulate Control-2 Fabric Filters Particulate Scrubbers
Lecture notes adapted from Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu
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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
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Fabric Filters
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Fabric Filters A shaker baghouse
Filter compartements
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Fabric Filters
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Fabric Filters
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Filtration Theory
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Filtration Theory
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Filtration Theory Figure 6.2 pp 186
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Filtration Theory
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Filtration Theory
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Design Considerations
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Cleaning Cycles tf: time interval between two cleanings of the same compartment tr: time interval between cleanings of any two compartment
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Variation of pressure drop with time
DPm DP tr tc Time
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Cleaning Cycles
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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
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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
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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
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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
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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)
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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
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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
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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)
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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)
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Example
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Example
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Example
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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
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Educational Videos Animation showing the pulse jet filtration A presentation from Baghouse and Wet Scrubber Production Firm, MikroPul
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Wet Scrubbers
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Particulate Scrubbers
Reading: Chap. 7 Types of scrubbers: spray chamber and venturi scrubber Theory and design consideration Pressure drop Contacting power 2017/4/26 Aerosol & Particulate Research Lab 32 32
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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) 2017/4/26 Aerosol & Particulate Research Lab 33 33
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Cyclone Spray Chamber & Impingement Scrubber
Flagan & Seinfeld, Fundamental of Air Pollution Engineering, 1988 2017/4/26 Aerosol & Particulate Research Lab 35 35
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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. 2017/4/26 Aerosol & Particulate Research Lab 36 36
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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) 2017/4/26 Aerosol & Particulate Research Lab 37 37
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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 2017/4/26 Aerosol & Particulate Research Lab 38 38
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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 2017/4/26 Aerosol & Particulate Research Lab 39 39
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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)? 2017/4/26 Aerosol & Particulate Research Lab 40 40
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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) 2017/4/26 Aerosol & Particulate Research Lab 42 42
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Impaction only (Impaction parameter Kp is used in textbook; Kp = 2 St)
rp = 2 g/cm3 43
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Venturi Scrubber Use intertial impaction of suspended particles on water droplet formed by gas atomization
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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 2017/4/26 45 45
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Aerosol & Particulate Research Lab
Pressure Drop Venturi Scrubber lt: venturi throat length X: dimensionless throat length Ex: 10” water, 2 mm, h = ? 2017/4/26 Aerosol & Particulate Research Lab 46 46
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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
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Contacting Power Approach
Venturi scrubber collecting a metallurgical fume Contacting power, hp/cfm 48
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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
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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
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Example Substractin Eq A from Eq B: A B 51
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Problem 7.1
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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
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Solution
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Solution
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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. ADD
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