Ventilation for Control of the Work Environment, 2nd Edition

List of Units xiii

Preface xv

1 Ventilation for Control 1

1.1 Control Options 2

1.2 Ventilation for Control of Air Contaminants 3

1.3 Ventilation Applications 5

1.4 Case Studies 7

1.5 Summary 9

References 11

2 Principles of Airflow 12

2.1 Airflow 13

2.2 Density 13

2.3 Continuity Relation 14

2.4 Pressure 16

2.4.1 Pressure Units 16

2.4.2 Types of Pressure 17

2.5 Head 18

2.6 Elevation 20

2.7 Pressure Relationships 22

2.7.1 Reynolds Number 24

2.8 Losses 26

2.8.1 Frictional Losses 26

2.8.2 Shock Losses 28

2.9 Losses in Fittings 30

2.9.1 Expansions 30

2.9.2 Contractions 32

2.9.3 Elbows 35

2.9.4 Branch Entries (Junctions) 36

2.10 Summary 38

List of Symbols 38

Problems 39

3 Airflow Measurement Techniques 43

3.1 Measurement of Velocity by Pitot–Static Tube 45

3.1.1 Pressure Measurements 47

3.1.2 Velocity Profile in a Duc 50

3.1.3 Pitot–Static Traverse 57

3.1.4 Application of the Pitot–Static Tube and Potential Errors 60

3.2 Mechanical Devices 61

3.2.1 Rotating Vane Anemometers 61

3.2.2 Deflecting Vane Anemometers (Velometer) 68

3.2.3 Bridled Vane Anemometers 71

3.3 Heated-Element Anemometers 72

3.4 Other Devices 75

3.4.1 Vortex Shedding Anemometers 75

3.4.2 Orifice Meters 76

3.4.3 Venturi Meters 76

3.5 Hood Static Pressure Method 77

3.6 Calibration of Instruments 79

3.7 Observation of Airflow Patterns with Visible Tracers 80

3.7.1 Tracer Design 81

3.7.2 Application of Visible Tracers 84

List of Symbols 85

References 86

Manufacturers of Airflow Measuring Instruments 87

Manufacturers of Smoke Tubes 87

Problems 87

4 General Exhaust Ventilation 90

4.1 Limitations of Application 91

4.2 Equations for General Exhaust Ventilation 93

4.3 Variations in Generation Rate 99

4.4 Mixing 100

4.5 Inlet  Outlet Locations 101

4.6 Other Factors 102

4.7 Comparison of General and Local Exhaust 105

List of Symbols 106

References 106

Problems 107

5 Hood Design 108

5.1 Classification of Hood Types 109

5.1.1 Enclosures 109

5.1.2 Exterior Hoods 110

5.1.3 Receiving Hoods 115

5.1.4 Summary 116

5.2 Design of Enclosing Hoods 116

5.3 Design of Exterior Hoods 120

5.3.1 Determination of Capture Velocity 120

5.3.2 Determination of Hood Airflow 125

5.3.3 Exterior Hood Shape and Location 135

5.4 Design of Receiving Hoods 135

5.4.1 Canopy Hoods for Heated Processes 135

5.4.2 Hoods for Grinding Operations 138

5.5 Evaluation of Hood Performance 141

List of Symbols 142

References 142

Appendix: Exterior Hood Centerline Velocity Models 144

Problems 148

6 Hood Designs for Specific Applications 151

6.1 Electroplating 152

6.1.1 Hood Design 152

6.1.2 Airflow 155

6.2 Spray Painting 159

6.2.1 Hood Design 159

6.2.2 Airflow 163

6.3 Processing and Transfer of Granular Material 165

6.4 Welding, Soldering, and Brazing 169

6.5 Chemical Processing 177

6.5.1 Chemical Processing Operations 178

6.6 Semiconductor Gas Cabinets 187

6.6.1 Entry Loss 190

6.6.2 Optimum Exhaust Rate 191

6.7 Low-Volume  High-Velocity Systems for Portable Tools  192

Example 6.1 Calculation of Exhaust Rate for Open-Surface Tanks 199

Example 6.2 Design of a Low-Volume  High-Velocity Exhaust System  200

List of Symbols 201

References 202

7 Chemical Laboratory Ventilation 204

7.1 Design of Chemical Laboratory Hoods 205

7.1.1 Vertical Sliding Sash Hoods 205

7.1.2 Horizontal Sliding Sash Hoods 209

7.1.3 Auxiliary Air Supply Hoods 212

7.2 Face Velocity for Laboratory Hoods 214

7.3 Special Laboratory Hoods 216

7.4 Laboratory Exhaust System Features 217

7.4.1 System Configuration 217

7.4.2 Construction 218

7.5 Factors Influencing Hood Performance 220

7.5.1 Layout of Laboratory 220

7.5.2 Work Practices 222

7.6 Energy Conservation 224

7.6.1 Reduce Operating Time 224

7.6.2 Limit Airflow 225

7.6.3 Design for Diversity 227

7.6.4 Heat Recovery 227

7.6.5 Ductless Laboratory Hoods 227

7.7 Performance of Laboratory Hoods 228

7.8 General Laboratory Ventilation 229

References 229

Problems 230

8 Design of Single-Hood Systems 232

8.1 Design Approach 233

8.2 Design of a Simple One-Hood System (Banbury Mixer Hood) 234

8.3 Design of a Slot Hood System for a Degreasing Tank 241

8.3.1 Loss Elements in a Complex Hood 241

8.3.2 Degreaser Hood Design Using Velocity Pressure Calculation Sheet (Example 8.2) 245

8.4 Pressure Plot for Single-Hood System 247

List of Symbols 247

Example 8.1 Banbury Mixer System Designed by the Velocity Pressure Method 248

Example 8.2 Degreaser System Designed by the Velocity Pressure Method 250

References 251

Appendix: Metric Version of Example 8.1 252

Problems 252

9 Design of Multiple-Hood Systems 254

9.1 Applications of Multiple-Hood Systems 254

9.2 Balanced Design Approach 256

9.3 Static Pressure Balance Method 260

9.3.1 Foundry Cleaning Room System (Example 9.1) 260

9.3.2 Electroplating Shop (Example 9.2) 262

9.4 Blast Gate Balance Method 265

9.5 Other Computational Methods 265

List of Symbols 266

Example 9.1 Foundry Cleaning Room Designed by Static Pressure Balance Method 267

Example 9.2 Electroplating Shop System Designed by Static Pressure Balance Method 272

References 278

Additional Reading 279

Appendix: Metric Version of Example 9.1 280

10 Fans and Blowers 282

10.1 Types of Air Movers 283

10.1.1 Axial Flow Fans 283

10.1.2 Centrifugal Fans 285

10.1.3 Air Ejectors 287

10.2 Fan Curves 288

10.2.1 Static Pressure Curve 289

10.2.2 Power Curve 291

10.2.3 Mechanical Efficiency Curve 293

10.2.4 Fan Laws  295

10.2.5 Relationship between Fan Curves and Fan Tables 297

10.3 Using Fans in Ventilation Systems 298

10.3.1 General Exhaust Ventilation Systems 298

10.3.2 Local Exhaust Ventilation Systems 300

10.4 Fan Selection Procedure 305

List of Symbols 308

References 309

Problems 309

11 Air-Cleaning Devices 311

11.1 Categories of Air-Cleaning Devices 312

11.1.1 Particle Removers 312

11.1.2 Gas and Vapor Removers 322

11.2 Matching the Air-Cleaning Device to the Contaminant 325

11.2.1 Introduction 325

11.2.2 Device Selection 326

11.3 Integrating the Air Cleaner and the Ventilation System 326

11.3.1 Gravity Settling Devices 330

11.3.2 Centrifugal Collectors 330

11.3.3 Filters 331

11.3.4 Electrostatic Precipitators 334

11.3.5 Scrubbers 334

11.3.6 Gas and Vapor Removers 335

List of Symbols 336

References 337

Problems 337

12 Replacement-Air Systems 338

12.1 Types of Replacement-Air Units 340

12.2 Need for Replacement Air 341

12.3 Quantity of Replacement Air 342

12.4 Delivery of Replacement Air 344

12.4.1 Replacement-Air System 1 (RAS-1), Melting Furnaces 349

12.4.2 Replacement-Air System 2 (RAS-2), Floor Casting 349

12.4.3 Replacement-Air System 3 (RAS-3), Sand Handling 350

12.4.4 Replacement-Air System 4 (RAS-4), Shakeout 351

12.5 Replacement Air for Heating 352

12.6 Energy Conservation and Replacement Air 353

12.7 Summary 356

References 356

13 Quantification of Hood Performance 358

13.1 Hood Airflow Measurements 359

13.2 Hood Capture Efficiency 360

13.2.1 Influence of Cross-Drafts on Hood Performance 361

13.2.2 Relationship between Airflow Patterns and Capture Efficiency 363

13.2.3 Shortcomings of the Centerline Velocity Approach 370

13.3 Use of Capture Efficiency in Hood Design 372

List of Symbols 372

References 373

14 Application of Computational Fluid Dynamics to Ventilation System Design 374

14.1 Introduction 374

14.2 Methods 376

14.2.1 Grid-Based Methods 377

14.2.2 Grid-Free Methods 378

14.3 Applications 379

14.3.1 Historical Perspectives 379

14.3.2 Current Progress 380

14.4 Issues on the Use of Computational Fluid Dynamics 386

14.5 Commercial Codes: Public-Domain Information 387

References 387

Appendix 389

15 Reentry 391

15.1 Airflow around Buildings 393

15.2 Measurement of Reentry 396

15.3 Calculation of Exhaust Dilution 401

15.4 Scale Model Measurement 404

15.5 Design to Prevent Reentry 406

15.5.1 Stack Height Determination 407

15.5.2 Good Engineering Practices for Stack Design 408

List of Symbols 412

References 413

Problems 415

Index 417