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AIR FILTRATION - GBV
Transcript of AIR FILTRATION - GBV
AIR FILTRATION An Integrated Approach to the
Theory and Applications of Fibrous Filters
R. С BROWN Health and Safety Executive
Research and Laboratory Services Division, Sheffield. UK
OXFORD
P E R G A M O N PRESS NEW YORK • SEOUL TOKYO
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 7 Quality factor 9
Single fibre efficiency 9 References 11
2 Filter Structure 12
Introduction 12 Paper filters 13
Manufacture 13 Description of structure 14 Fractal dimension 18
Carded filters 18 Manufacture 18 Description of structure 19 Filters with short fibres 20 Felting 21
Porous foam 21 Model filters 23
Standard sieves 25 Wound wire 25 Photo-etched filters 26 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 aerodynamic slip on flow pattern and pressure drop
General analysis 59 Aerodynamic slip applied to fibres 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 macroscopic level 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 single fibre efficiency 73
Limitations to the single 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 aerodynamic slip 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
V I I I 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 electrostatic filter 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
Electric forces 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 drag force 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
Drag force on a single particle attached to a collector 212 Drag force 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 dendrite structure on a single 221 fibre
Experimental observations on model and real fibrous filters 221 Behaviour related to capture mechanisms 222
Contents XI
Effect of filter properties on the pattern of clogging 223 Particle bounce from clogged filters 225
Fractal structure of particle deposits 226 Effect of loading with liquid aerosols 229 Time-dependent effects in filter clogging 231 The effect of dust loading on the performance of electrically 231 charged filters
Empirical theory of charge loss 233 Loading with solid aerosols 234
Effect of aerosol charge 235 Loading with liquid aerosols 235
References 236
9 Filter Testing 240
Introduction 240 Pressure drop measurement 240 Area weight measurement 241 General problems of aerosol penetration measurement 241
Sample uniformity and consistency 241 Filter scanning 242 Line losses 243 Uniformity of air velocity through the filter 243
Re-circulating filters 243 Aerosol detectors 244 Testing with monodisperse aerosols 245
Methods of production of monodisperse aerosols 245 Aerosol produced from liquid droplets 245 Aerosols produced from condensation nuclei 246 Polystyrene latex aerosols 247 Aerosols produced by classification 247 Fibrous aerosols 248
Quantification and minimisation of random errors 249 Application to layer efficiency or quality factor 250
Automated methods of obtaining penetration curves with 250 monodisperse aerosols
Size-selective testing with polydisperse aerosols 251 Technical problems with instrumentation 251 Methods of production of polydisperse test aerosols 252 Size distribution of polydisperse aerosols 253 Filter testing by sample collection and analysis 253
Non size selective testing with polydisperse aerosols 254 Aerosol number distributions and mass distributions 255
Effect of detection method on ascribed penetration 258
xii Contents
Testing of high efficiency filters 259 Tests with electrically charged aerosols 260
Neutralisation of aerosols 260 Worst possible case testing 261 Realistic prediction of filter performance 263 Standard filters 263 References 264
CONCLUSION 268
INDEX 269