Air Filtration: An Integrated Approach to the Theory and Applications of Fibrous Filters PDF
Preview Air Filtration: An Integrated Approach to the Theory and Applications of Fibrous Filters
A R FILTRATION An Integrated Approach to the Theory and Applications of Fibrous Filters R C BROWN . . Pergamon Press Pergamon Titles of Related Interest BANHIDI Radiant HeatingSystems DODGSON Inhaled ParticlesVII MASUDA & TAKAHASHI Aerosols:Science,Industry,Healthandthe Environment VINCENT Ventilation1988 VISUALIZATION SOCIETY OFJAPAN Atlasof Visualization WILLIAMS & LOYALKA Aerosol Science:Theoryand Practice Pergamon Related Journals Annalsof Occupational Hygiene Atmospheric Environment: Part A: General Topics, Part B:UrbanAtmosphere BuildingandEnvironment Current Advancesin Ecologicaland Environmental Science Environment International InternationalJournalof Heatand MassTransfer InternationalJournal of Multiphase Flow Journal of Aerosol Science Free sample copies of journals are gladly sent on request - A I R F I L T R A T I O N 7 An Integrated Approach to the Theory and Applications of Fibrous Filters Yr / / 4 R. C. BROWN Health andSafety Executive Research and Laboratory Services Division, Sheffield,UK PERGAMON PRESS X)X , <’) I * (\ / OXFORD NEW YORK SEOUL TOKYO 'if i UK PergamonPress,Ltd,HeadingtonHillHall, Oxford0X3OBW,England USA PergamonPressInc.,660WhitePlainsRoad, Tarrytown,NewYork10591-5153,USA KOREA PergamonPress Korea,KPO Box315,Seoul110-603, Korea JAPAN PergamonPressJapan,Tsunashima BuildingAnnex, 3-20-12Yushima,Bunkyo-ku,Tokyo113,Japan Copyright©1993R.C. Brown AllRightsReserved.Nopartofthispublicationmaybe reproduced,storedinaretrievalsystemortransmittedin anyformor byanymeans:electronic,electrostatic, magnetictape,mechanical,photocopying,recordingor otherwise,withoutpermissioninwritingfromthe publishers. Firstedition1993 BritishLibraryCataloguinginPublicationData Acataloguerecordforthisbookisavailablefromthe BritishLibrary. Libraryof CongressCataloginginPublicationData Brown,R. C. (RichardColin),1944- Airfiltration:anintegratedapproachtothetheoryand applicationsoffibrousfilters/R.C.Brown.—1sted. P- cm. Includesbibliographicreferences. 1.Airfilters. 2.Fibers. 3.Air-Purification. I.Title. TH7683.A3B76 1993 628.5'.3—dc20 ISBN0080412742 STATKF TECHNICKA KKIHOVNA . lYlarians!5 C,i,'307F a::aI , lOv 1 oO) JO, ^ L 1883 27 XI. ItviTbS X \ X ( T,ivn ’ Li l, , ^ PrintedinGreatBritainbyB.P.P.C.WheatonsLtd.Exeter t Contents INTRODUCTION xiii SYMBOLIC NOTATION xv 1 Macroscopic Behaviour of Filters 1 Surface filtration and depth filtration 1 Types of filter 2 Method of filtration and assessment of efficiency 4 Layer efficiency 5 Layer efficiency for monodisperse aerosols 5 Layer efficiency for polydisperse aerosols Quality factor 9 Single fibre efficiency 9 References 11 2 Filter Structure 12 -i Introduction 12 Paper filters 13 Manufacture h 13 Description of structure 14 Fractal dimensidn 18 Carded filters ; " ~ 18 Manufacture i ; « 18 Description of structure 19 * Filters with short fibres 20 Felting '!21 Porous foam Model filters 23 (cid:127) Standard sieves ip 25 * Wound wire f25 Photo-etched filters 126 Lithographically produced filters 26 References 27 V Vi Contents 3 Flow Patterns and Pressure Drop 29 Nature of airflow through a filter 29 Steady state flow and Stokes flow 30 Simple model of airflow around a filter fibre 32 Simple description of pressure drop 33 General fluid dynamics theory 34 ' Microscopic airflow patterns in filters 35 Filters approximated by single layers of fibres 37 Single fibre theory 39 Cell models 40 Hydrodynamic factor 43 Flow parallel to the fibre axis, and at arbitrary 44 inclination Refinement of single fibre theory to describe arrays of 45 fibres Continuum theory 46 Flow through two-dimensional arrays of fibres 47 Many-fibre theories 48 Variational method 48 Boundary element method 51 Filters of irregular and imperfect structure 52 Empirical models of pressure drop 52 The effect of fibres of non-circular cross-section 53 Pressure drop of filters made from polydisperse fibres 54 Flow in conditions other than simple Stokes flow 56 Flow in fibre arrays at finite Reynolds number 56 Molecular nature of airflow through filters, and the effects 58 ! of aerodynamicslip on flow pattern and pressure drop General analysis 59 Aerodynamic slip applied tofibres 60 Macroscopic flow patterns 62 Flow through pleats 63 Flow through filters of non-uniform structure 64 Non-ideal structure on a microscopic level 64 Non-ideal structure on a macroscopiclevel 66 Filters heterogeneous in directions perpendicular to the 66 flow Filters heterogeneous in directions parallel and 67 perpendicular to the flow Flow through pin holes and leaks 68 References 70 Contents vii 4 Particle Capture by Mechanical Means 73 Introduction 73 Generalised theory of singlefibre efficiency 73 Limitations to thesingle fibre approach 75 Particle capture by direct interception 75 Simplified approach to direct interception 75 Derivation of single fibre efficiency by direct interception 77 Relationship between single fibre efficiency by 78 interception and pressure drop Effect of aerodynamicslip on particle capture by 79 interception Experimental observation of particle capture by 79 interception Particle capture by inertial impaction 80 Stopping time, stopping distance and Stokes number 81 Rigorous theory of inertial effects 82 Inertial impaction at low Stokes number 83 Inertial impaction at high Stokes number 84 Calculation of inertial impaction efficiency over a range of 84 Stokes numbers Formulae for single fibre efficiency fitted to the results of 85 calculation and experiment Experimental measurements on model and real filters 86 Capture of particles by gravity 90 Simplified approach to gravitational capture 91 Experimental observation of gravitational capture 92 Theory of gravitational capture 92 Capture of particles by diffusional deposition 93 Diffusive motion of particles 94 Simplified calculation of single fibre efficiency 96 Rigorous theory of diffusional capture 98 Experimental observation of diffusional capture 99 Effect of aerodynamic slip on diffusional capture 100 Capture of non-spherical particles by fibrous filters 101 Dynamics of fibrous aerosols 101 Interception and fibre aspect ratio 102 Inertial impaction and aerodynamic diameter 103 Diffusive motion in translation and rotation 104 Alignment of fibres in shear flow 104 Measurements of filtration efficiency against fibrous 105 aerosols Combined effects of two or more capture mechanisms 106 Diffusion and interception 107 Most penetrating particle 108 viii Contents Gravity and interception 109 Interception, diffusion and gravity 111 Inertial impaction and gravity 111 Size-selection by interception, gravity and inertial 111 impaction Specification of mechanism-dependent regimes 112 Variation of deposition site with capture process 113 Effect of leakage on filter performance 114 Potential problems with model filters 115 References 116 5 Electrically Charged Filter Material 120 Introduction 120 Basic mechanisms of action 120 Classification of material 121 Triboelectrically charged material 122 Resin wool material 122 Mixed-fibre material 125 Corona charged material 126 Split-fibre material 127 Material charged as a whole 127 Material charged by induction 127 Air currents in electric charging 129 Non-fibrous electrostaticfilter 129 The importance of electric charge configuration 130 Measurement of filter charge 131 Measurement by ionising radiation 131 Fibre-scanning 133 Dipolar charge measurement 133 Charge stability and the effect of storage on filter 134 performance Theory of charge loss 134 Observed behaviour of filters at elevated temperature 135 References 137 6 Particle Capture by Electric Forces 139 Introduction 139 Capture by permanently charged fibres 139 Electric charge on aerosol particles 140 Breakdown charge distribution 141 Equilibrium charge distribution 141 Contents ix Miscellaneous charge distributions 142 Capture of charged particles 142 Capture of neutral particles 143 Effect of particle size and filtration velocity 145 Mathematical theory of capture 145 Capture of particles by solenoidal forces 145 Capture by central forces 148 Iterative numerical calculations 148 Calculation of single fibre efficiency for polarisation forces 149 Particle capture by fibres with non-uniform charge 150 Electric field caused by fibres with complicated charge 150 configurations Particle capture by multipole charge configurations 152 Combined effect of electrostatic forces with interception 156 and other mechanisms Capture by image forces 158 Combined effects of image forces and interception 160 Experimental observation of electrostatic capture 161 Capture efficiency compared with dimensionless capture 166 parameters Functional form of single-fibre efficiency by electrostatic 167 effects and diffusion Augmentation of filtration efficiency by external electric 168 fields Capture of charged particles 168 Capture of neutral particles 169 Direct observation of particle trajectories 170 Experimental measurements on model filters 171 Effect of single fibre position and composition 172 Measurements on real filters in an external electric field 173 Augmentation of filtration efficiency by charging of particles 174 References 174 7 Particle Adhesion and Particle Bounce 178 Introduction 178 Elementary description of capture mechanism and bounce 178 probability Relative importance of bounce and re-entrainment 180 Re-suspension of particles by the airstream or by 180 particle-particle impact Adhesion forces between particles and fibres 181 Van der Waals forces 181 Surface tension (capillary)forces 183 x Contents Electricforces 184 Relative magnitude of adhesion forces 185 Range of values of adhesion forces 186 Effect of particle and fibre properties 188 Influence of filtration conditions 189 Dynamics of impact 190 Face velocity and impact velocity 192 Factors influencing bounce probability 193 Effect of oil on the fibre surface 193 Effect of particle size and shape 194 Effect of collector surface 194 Critical impact velocity and direct observation of impact 195 Adhesion and bounce of very small particles 196 Transfer of electric charge on contact 197 Capture of liquid droplets 198 Fibre shedding 198 References 199 8 Effects of Loading 201 Introduction 201 Qualitative description of filter clogging 201 Dendrite formation 202 Dendrite formation by interception 203 Dendrite formation by inertial impaction 203 Dendrite formation by diffusional deposition 204 Dendrite formation by electric forces 204 Illustrations of deposit pattern 205 The influence of aerosol properties on clogging rate 205 Macroscopic theory of filter clogging 206 Airflow pattern and dragforce acting on particle complexes 208 Theory of airflow around an isolated sphere 208 Aerodynamic slip 210 Drag force acting on agglomerates 210 Aerodynamic slip 211 Dragforce on a single particle attached to a collector 212 Dragforce on a dendrite attached to a collector 213 Theoretical description of clogging 214 Calculation of dendrite formation 216 Numerical simulation of dendrite growth 218 Experimental observation of dendritestructure on a single 221 fibre Experimental observations on model and real fibrousfilters 221 Behaviour related to capture mechanisms 222
The list of books you might like
