Papermaking Science and Technology a book series covering the latest technology and future trends

Book 9 Papermaking Part 2, Drying

Second Edition Totally updated version

Book editor Markku Karlsson, D.Sc. (Tech.), Senior Vice President, Technology, UPM

Publisher Paper Engineers’ Association/Paperi ja Puu Oy

Paperi ja Puu Oy Table of Contents

1 Introduction to paper drying and its principles ...... 13

2 Fundamentals of paper drying ...... 39

3 Multicylinder dryer and drying concepts ...... 78

4 Air impingement drying ...... 126

5 Drying of tissue ...... 162

6 Drying of paperboard and packaging grades ...... 211

7 Drying and paper quality ...... 236

8 Automation in paper drying ...... 296

9 Energy management ...... 336

10 Steam and condensate systems ...... 374

11 Drying section ventilation and heat recovery ...... 436

12 Web handling ...... 484

13 Dryer fabrics ...... 542

14 Drives ...... 578

15 Future outlook ...... 617

Conversion factors ...... 626

Index ...... 628

12 CHAPTER 1

Introduction to paper drying and its principles

1 Introduction...... 14 2 Basic requirements for a modern drying section ...... 16 2.1 Tasks and requirements for the drying section ...... 16 2.2 Production speed ...... 17 2.3 Production line efficiency ...... 18 2.4 Space efficiency ...... 19 2.5 Paper quality ...... 19 3 Drying methods...... 19 4 Energy consumption in paper drying ...... 21 5 Developments in paper machine drying technology ...... 23 5.1 From two-tier to single-tier drying section ...... 23 5.1.1 Two-tier configuration ...... 23 5.1.2 Single-tier configuration ...... 23 5.1.3 Web handling and runnability ...... 24 5.1.4 Dryer fabrics ...... 24 5.1.5 Inside the drying cylinder ...... 25 5.1.6 Inside the hood ...... 25 5.2 Alternative drying methods ...... 26 5.2.1 Air dryers ...... 26 5.2.2 Infrared dryers ...... 27 5.2.3 High-intensity processes ...... 28 5.2.4 Impingement drying ...... 30 5.2.5 Steam drying ...... 31 6 Modern drying section design and future trends ...... 31 7 Conclusions and future outlook ...... 33 References ...... 35

13 CHAPTER 2

Fundamentals of paper drying

1 Introduction...... 40 2 Properties of moist air ...... 40 2.1 Moisture in air ...... 40 2.1.1 Relative humidity of air ...... 41 2.2 Specific enthalpy and specific heat of moist air ...... 41 2.3 The i/x chart ...... 42 2.4 Basic definitions...... 46 3 Drying process – the characteristic drying curve ...... 47 4 Evaporation from the web ...... 50 4.1 Vapour partial pressure ...... 51 5 Sorption isotherm ...... 52 5.1 Hysteresis ...... 53 5.2 Heat of sorption ...... 54 6 Boundary conditions for heat transfer ...... 56 7 Influence of evaporation on convective heat transfer ...... 57 8 Changes in web structure during drying ...... 59 9 Capillary flow of free water ...... 63 10 Movement of bound water ...... 65 11 Vapour flow ...... 66 12 Thermal conductivity of paper ...... 69 13 Summary ...... 73 References ...... 74

39 CHAPTER 3

Multicylinder dryer and drying concepts

1 Introduction...... 80 1.1 Cylinder dryer configurations ...... 80 1.1.1 Double-felted configuration ...... 80 1.1.2 Single-felted configuration ...... 81 2 Basic design concepts for most common paper grades ...... 82 2.1 Design concepts for different paper and board grades ...... 82 2.1.1 Wood-containing printing papers (newsprint, LWC and SC) ...... 82 2.1.2 Woodfree papers ...... 83 2.1.3 Linerboard machines...... 84 2.2 Basic requirements for cylinder drying with different paper grades ...... 85 2.2.1 Newsprint ...... 85 2.2.2 Woodfree papers ...... 85 2.2.3 SC (supercalendered) and LWC (light weight coated) papers ...... 86 2.2.4 Liner and fluting ...... 86 2.2.5 Sack paper...... 86 2.3 Use of energy in multicylinder drying concepts...... 86 2.4 Drying section curl control ...... 88 2.4.1 Double-felted drying section ...... 89 2.4.2 Single-felted drying section ...... 89 2.4.3 Curl control with impingement drying ...... 90 3 Drying section components ...... 91 3.1 Cylinders and vacuum rolls ...... 91 3.2 Fabric lead rolls ...... 91 3.3 Drying section runnability ...... 91 3.4 Tail threading in cylinder drying section...... 93 3.4.1 Tail threading conditions in a cylinder drying section ...... 94 3.4.2 Open draw between press and drying sections ...... 95

78 Multicylinder dryer and drying concepts

3.4.3 Closed draw between press and drying sections ...... 95 3.4.4 Ropeless tail threading through drying section ...... 96 3.4.5 Threading through surface sizing unit with ropes ...... 96 4 Heat transfer in cylinder drying ...... 98 4.1 Total heat transfer coefficient ...... 98 4.2 Periodic variation of dryer surface temperature ...... 101 4.3 Condensate behaviour in the cylinder ...... 101 4.3.1 Dryer drive power ...... 102 4.3.2 Condensate oscillation ...... 103 4.3.3 Condensate film thickness ...... 104 4.4 Spoiler bars ...... 105 4.5 Condensate coefficient ...... 106 4.5.1 Dryer with smooth inner surface ...... 106 4.5.2 Dryer with spoiler bars ...... 108 4.6 Contact coefficient ...... 108 4.6.1 Moisture content ...... 108 4.6.2 Fabric or web tension ...... 109 4.6.3 Cylinder and web smoothness ...... 110 4.6.4 Number of fabric contact points ...... 110 4.6.5 Air film between cylinder and web ...... 110 4.6.6 Influence of cylinder temperature ...... 111 4.6.7 Use of press roll to produce contact ...... 112 5 Drying rate of multicylinder drying section ...... 112 5.1 Dryer steam pressures ...... 115 5.2 Other factors influencing drying rate ...... 116 5.3 Drying section simulation ...... 117 5.3.1 Single-layer model ...... 117 5.3.2 Multilayer model ...... 119 5.3.3 Example of simulation application ...... 120 6 Future outlook and summary ...... 121 References ...... 123

79 CHAPTER 4

Air impingement drying

1 Introduction...... 128 1.1 Direct impingement ...... 129 1.2 Indirect impingement ...... 129 2 Impingement heat transfer ...... 130 2.1 Hydrodynamics of the impingement flow ...... 131 2.2 Impingement flow – process description ...... 131 2.3 Conventional empirical correlation for impingement heat transfer ...... 132 3 Influences of process parameters on impingement heat transfer ...... 134 3.1 Effect of evaporation on heat transfer ...... 135 3.2 Effect of surface movement on heat transfer ...... 136 3.3 Effect of jet angle on heat transfer ...... 136 3.4 Effect of through-flow on heat transfer ...... 137 3.5 Effect of the cross-flow ...... 137 3.6 Effect of large temperature differences ...... 138 4 Experimental determination of convective heat transfer coefficient (CHTC) ...... 138 4.1 Influence of thermal radiation on total heat transfer ...... 139 4.2 Empirical correlation for impingement heat transfer ...... 140 4.3 Modelling of the impingement heat transfer process ...... 141 5 Impingement dryer design considerations ...... 141 5.1 Heat transfer coefficient ...... 142 5.2 Geometry optimization ...... 142 5.3 Drying parameters ...... 143 5.4 Optimal air-system ...... 144 5.5 Heat recovery system ...... 144 5.6 Control system ...... 144 5.7 Thermal expansions ...... 145 5.8 Energy consideration ...... 145

126 Air impingement drying

6 Impingement drying applications ...... 145 6.1 Impingement dryer with big vacuum roll ...... 145 6.2 Impingement dryer with big vacuum rolls immediately after press section ...... 148 6.3 Impingement dryer on small vacuum rolls ...... 148 6.4 Impingement dryer on grooved rolls ...... 149 6.4.1 Design concept for increasing production capacity ...... 149 6.5 Design concept for beginning of drying section with grooved supporting rolls ...... 152 6.5.1 Draw management with cylinder drying section ...... 152 6.5.2 Technology ...... 153 6.5.3 Drying capacity ...... 154 7 Draw management ...... 154 8 Bulk saving potential with copy paper ...... 154 9 Sticking prevention with impingement drying ...... 155 10 Summary ...... 156 References ...... 157 Nomenclature ...... 160

127 CHAPTER 5

Drying of tissue

1 Introduction...... 164 2 Yankee drying ...... 164 2.1 Tissue drying process ...... 164 2.1.1 Fundamentals of heat and mass transfer in tissue drying ...... 166 2.1.2 Sheet adhesion ...... 169 2.1.3 Dryer coating ...... 170 2.1.4 Chemicals for controlling coating and sheet adhesion ...... 171 2.1.5 Yankee doctors ...... 171 2.1.6 Web structure and properties ...... 172 2.2 Yankee dryer design ...... 173 2.2.1 General description ...... 173 2.2.2 Manufacturing and materials ...... 174 2.2.3 Cylinder design ...... 174 2.2.4 Steel Yankee drying cylinders in tissue machines ...... 178 2.3 Steam and condensate system for a tissue Yankee dryer ...... 179 2.3.1 Design and functional description ...... 179 2.4 Safety considerations ...... 181 2.4.1 Stress analysis ...... 181 2.4.2 Deformation analysis ...... 182 2.5 Reconditioning of Yankee cylinders by thermal spraying ...... 184 3 Yankee hood drying, ventilation and heat recovery ...... 185 3.1 Yankee hood drying process ...... 185 3.2 Yankee hood ...... 185 3.2.1 Yankee hood types ...... 186 3.2.2 Design criteria ...... 186

162 Drying of tissue

3.3 Yankee hood ventilation ...... 187 3.3.1 Yankee hood air systems ...... 187 3.3.2 Yankee hood air profiling ...... 189 3.3.3 Yankee hood air balancing systems ...... 191 3.3.4 Tissue machine energy balance ...... 191 3.3.5 Yankee hood retraction mechanism ...... 192 3.3.6 Auxiliary equipment of Yankee hood ...... 192 3.3.7 Process air fans ...... 193 3.3.8 Air heating ...... 194 3.4 Yankee section heat recovery ...... 194 3.4.1 Heat recovery from exhaust air ...... 194 3.5 Control systems ...... 195 3.6 High-temperature hood ...... 196 4 Through-air drying of tissue (TAD) ...... 197 4.1 Description of the through-air drying process ...... 198 4.2 Through-air drying vs. other drying methods ...... 199 4.2.1 Drying rates ...... 199 4.2.2 Product properties ...... 200 4.2.3 Production costs ...... 201 4.3 Different machine configurations ...... 201 5 Advantages of hybrid technologies in making high-quality tissue paper ...... 202 5.1 ATMOS tissue concept ...... 202 5.2 Advantage NTT tissue making concept ...... 205 6 Summary ...... 207 References ...... 209

163 CHAPTER 6

Drying of paperboard and packaging grades

1 Introduction...... 212 2 Containerboards ...... 213 2.1 Layout ...... 213 2.2 Design criteria and drying rate ...... 215 2.3 Effect on board quality ...... 216 2.4 Sack paper ...... 217 2.5 Other drying methods ...... 218 2.5.1 Condebelt drying ...... 218 2.5.2 High Efficiency Dryer ...... 221 3 Cartonboards ...... 223 3.1 Layout ...... 223 3.2 Design criteria and drying rate ...... 225 3.3 Effect on board quality ...... 227 3.4 Metal belt calender ...... 227 4 Speciality papers and boards ...... 229 4.1 Machine glazed drying for wrapping paper ...... 229 5 Heat and mass transfer considerations in paper and paperboard finishing and converting ...... 230 5.1 General ...... 230 5.2 Corrugated board manufacture ...... 230 5.2.1 Preheating, preconditioning and corrugating ...... 231 5.2.2 Gluing ...... 232 5.2.3 Preheater tower ...... 232 5.2.4 Double-facer ...... 232 6 Summary ...... 233 References ...... 235

211 CHAPTER 7

Drying and paper quality

1 Introduction ...... 238 2 Drying of a single fibre ...... 238 2.1 Fibre shrinkage and removal of water fractions ...... 238 2.2 Inter-fibre bonding ...... 240 2.3 Mechanical properties of fibres ...... 241 3 Impacts of fibre morphology during drying ...... 243 3.1 Plasticity and viscoelasticity of material ...... 243 3.2 Fibre softening ...... 243 3.3 Hornification of cellulose fibres ...... 245 3.4 Fibre sticking and adhesion to cylinder surface ...... 246 4 Drying of a paper web ...... 249 4.1 Different phases of paper drying ...... 250 4.2 Drying and web rheology ...... 251 4.3 Creep and tension relaxation ...... 251 5 Paper shrinkage during drying ...... 252 5.1 Definitions of drying stresses and deformations ...... 252 5.2 Paper deformation during drying ...... 253 5.3 In-plane shrinkage ...... 253 6 Development of mechanical properties of a web ...... 255 6.1 Effects of machine-direction draw during drying ...... 257 6.2 Effects of cross-direction shrinkage during drying ...... 258 6.3 Development of hydrogen bonding of the web during drying ...... 259 7 Web peeling from press roll ...... 261 7.1 Tension relaxation of the wet web ...... 263 8 Effects of drying temperature on paper quality ...... 265 9 Web drying in thickness direction ...... 269

236 Drying and paper quality

10 Flatness deviations ...... 270 10.1 Challenges caused by efficiency improvement ...... 270 10.2 Curl ...... 271 10.2.1 One-sided drying ...... 272 10.2.2 Asymmetric moisture treatment ...... 273 10.2.3 Curl and its control ...... 274 10.3 Cockling ...... 275 10.3.1 Streakiness and waviness in machine direction ...... 276 10.3.2 Lumpiness ...... 280 11 Drying-induced profiles of a web ...... 280 11.1 Shrinkage profile of a web ...... 281 11.2 Tension profile of a web ...... 283 12 Drying of paper coatings and surface size ...... 285 12.1 Drying of precoat and surface size ...... 285 12.2 Drying strategy and quality in pigment drying ...... 285 13 Moisture changes in printing and end use ...... 287 13.1 Heatset waviness ...... 287 13.2 Stack waviness ...... 288 14 Development trends ...... 289 References ...... 290

237 CHAPTER 8

Automation in paper drying

1 Introduction...... 298 1.1 Control theory – opportunities and limitations ...... 299 1.2 Water to be removed in papermaking ...... 302 1.3 Cylinder drying and steam system ...... 302 1.4 Disturbances to be eliminated by automation ...... 302 1.5 Paper quality measurements and problems in paper scanning ...... 303 1.6 MD and CD controls ...... 303 1.7 Grade changes ...... 304 2 Cylinder drying and steam system ...... 304 2.1 Cylinder drying ...... 304 2.2 Steam and condensate system ...... 305 2.3 Capability to control moisture level in paper ...... 307 3 Paper quality measurements ...... 308 3.1 Scanning measurement ...... 308 3.2 Control challenges related to measurement principle ...... 309 4 Steam box ...... 310 4.1 Product and its applications ...... 310 5 Moisturising system ...... 312 5.1 Product and its applications ...... 312 6 MD controls ...... 314 6.1 Products and applications ...... 314 6.2 Dryer control concept ...... 316 6.3 Stock preparation and wet end controls ...... 318 6.4 MPC based control of paper moisture ...... 319 7 Impingement drying ...... 319 8 Hood and heat recovery management ...... 321

296 Automation in paper drying

9 CD controls ...... 323 9.1 Products, technologies and applications ...... 323 9.2 CD moisture control ...... 326 9.3 MPC technology for CD moisture control ...... 328 10 Grade change ...... 328 11 Development opportunities ...... 331 References ...... 334

297 CHAPTER 9

Energy management

1 Energy generation and use in integrated pulp and paper mills ...... 338 1.1 Introduction ...... 338 1.2 Energy use at integrated mills ...... 340 1.2.1 Energy use in unit processes ...... 340 1.2.2 Energy quality ...... 341 1.2.3 Influence of paper drying method on energy use ...... 341 1.3 Energy production at integrated mills ...... 342 1.3.1 Energy balances ...... 342 1.3.2 Energy system design ...... 344 1.3.3 Benefits of integration ...... 344 1.3.4 Trends in energy use and their consequences ...... 347 1.3.5 Fuel quality improvement by drying ...... 348 1.4 Energy efficiency ...... 349 1.4.1 Energy efficiency indicators ...... 349 1.4.2 Conceptual challenges related to energy efficiency ...... 350 1.4.3 Energy efficiency management ...... 351 1.4.4 Process systems analysis methods ...... 353 1.4.5 Total efficiency aspects ...... 355 2 Energy management at paper machine level ...... 356 2.1 Drying section as part of an integrated mill ...... 356 2.2 Benchmarking papermaking energy consumption ...... 357 2.3 PM line energy consumption allocation ...... 359 2.4 Drying section energy consumption and related factors ...... 361 2.4.1 Electricity consumption ...... 362 2.4.2 Heat consumption ...... 363 2.4.3 Energy efficiency of different drying methods ...... 366

336 Energy management

2.4.4 Energy efficiency analysis based on automation system data ...... 367 2.4.5 Energy efficiency monitoring ...... 367 2.4.6 Example of energy efficiency program ...... 368 2.5 Future development of energy efficiency in papermaking ...... 369 References ...... 371

337 CHAPTER 10

Steam and condensate systems

1 Introduction ...... 376 2 Steam requirements ...... 378 2.1 Steam pressure ...... 379 2.2 Steam temperature ...... 379 3 Equipment ...... 380 3.1 Condensate tanks ...... 381 3.2 Condenser ...... 382 3.3 Condensate pumps ...... 382 3.4 Vacuum pumps ...... 383 4 Design criteria of steam drying systems ...... 383 4.1 General ...... 383 4.2 Design principles by cylinder dryer configuration ...... 383 4.3 Design principles by grade ...... 385 5 Dryers condensing and condensate removal ...... 386 5.1 Steam and condensate connections ...... 388 5.2 Condensate removal equipment ...... 389 5.2.1 General ...... 389 5.2.2 Stationary siphon ...... 391 5.2.3 Rotary siphon ...... 397 5.3 Controlling pressure and differential pressure ...... 402 5.3.1 General ...... 402 5.3.2 Differential pressure control ...... 403 5.3.3 Flow control ...... 404 6 Steam and condensate systems ...... 404 6.1 Cascade system ...... 405 6.1.1 Operating and design principles ...... 405 6.1.2 Division of steam sections ...... 407

374 Steam and condensate systems

6.1.3 Applications ...... 409 6.1.4 Advantages and disadvantages of cascade systems ...... 412 6.1.5 Simulation ...... 412 6.2 Thermocompressor systems ...... 413 6.2.1 Construction and operation of a thermocompressor...... 413 6.2.2 Performance ...... 415 6.2.3 Thermocompressor pressure control system ...... 418 6.2.4 Thermocompressor pressure control system with differential control valve...... 419 6.2.5 Dryer differential control using the thermocompressor spindle ...... 419 6.2.6 Advantages and disadvantages of thermocompressor systems ...... 421 6.3 Cascade thermocompressor system...... 421 6.4 Thermocompressor system for Yankee dryer ...... 423 6.5 Vacuum control and air venting ...... 424 7 Heating air with steam ...... 426 7.1 General ...... 426 7.2 Applications ...... 427 8 Heating water with steam ...... 429 8.1 General ...... 429 8.2 Applications ...... 430 9 Condensate recovery and return ...... 430 10 Development trends ...... 432 References ...... 433 Appendix ...... 434

375 CHAPTER 11

Drying section ventilation and heat recovery

1 Introduction...... 438 2 Drying section air systems ...... 439 2.1 Design criteria ...... 439 2.2 Drying section hood ...... 440 2.2.1 Hood types ...... 440 2.2.2 Exhaust air humidity ...... 441 2.2.3 Dew point ...... 442 2.2.4 Pressure levels and air balance ...... 442 2.2.5 Air handling in the closed hood ...... 444 2.2.6 General aspects of hood design ...... 445 2.3 Drying section ventilation ...... 446 2.3.1 Pocket ventilation concepts ...... 447 2.3.2 Pocket ventilation for different paper and board grades ...... 448 2.3.3 Ventilators for single-felting ...... 449 2.3.4 Pocket ventilation parameters ...... 450 2.3.5 Profiling by means of ventilation ...... 451 2.3.6 Supply air distribution ...... 451 2.4 Integration of drying section air systems and sheet runnability systems ...... 452 2.4.1 Design criteria related to sheet runnability systems ...... 453 3 Drying section heat recovery ...... 454 3.1 Drying section energy balance ...... 454 3.2 General aspects of drying section energy use and management ...... 459 3.2.1 Process parameters ...... 459 3.2.2 Control systems ...... 461 3.2.3 Heat recovery systems ...... 461 3.2.4 Process improvements ...... 462 3.2.5 Application of new technologies ...... 462

436 Drying section ventilation and heat recovery

3.3 General basis for heat recovery system design ...... 463 3.4 Energy prices ...... 464 3.5 Continuity ...... 465 3.6 Heating requirements...... 466 3.7 Basic types of heat exchanger ...... 467 3.7.1 Air/air heat exchangers ...... 467 3.7.2 Air/water heat exchangers ...... 468 3.7.3 Scrubbers ...... 469 3.7.4 Air coils ...... 469 3.8 Different types of heat recovery system ...... 470 3.8.1 Earlier types of heat recovery system ...... 470 3.8.2 Contemporary heat recovery system designs ...... 471 3.8.3 High-temperature heat recovery ...... 472 3.8.4 Heat recovery in air dryers for coating ...... 472 3.8.5 Heat recovery in pulp dryers ...... 473 3.8.6 Other applications ...... 473 3.8.7 Sizing and optimisation of heat recovery systems ...... 474 3.8.8 Design criteria for heat recovery equipment ...... 475 4 Environmental aspects related to drying section ventilation ...... 475 4.1 Sound attenuation ...... 475 4.1.1 Absorptive (resistive) silencers ...... 477 4.1.2 Reactive silencers ...... 477 4.2 Plume remediation ...... 478 5 Process ventilation control ...... 479 5.1 Basic control system ...... 479 5.2 Energy management systems ...... 480 6 Development trends and future concepts in paper machine ventilation ...... 481 References ...... 483

437 CHAPTER 12

Web handling

1 Introduction...... 486 2 Dynamics of a moving web ...... 487 2.1 Forces influencing a web ...... 488 2.1.1 External forces ...... 488 2.1.2 Acceleration forces in out-of-plane direction of the web ...... 490 2.2 Thread-line model ...... 491 2.2.1 Longitudinal wave equation ...... 491 2.2.2 Out-of-plane wave equation ...... 492 3 Material properties of a web ...... 493 3.1 Rheology of a web ...... 493 3.2 Models of web rheology ...... 500 3.2.1 Uni-axial models ...... 500 3.2.2 Multi-axial models...... 503 4 Aerodynamics of moving webs ...... 504 4.1 Boundary layers on a flat plate ...... 504 4.2 Pressure development in pockets ...... 505 4.3 Web flutter and aerodynamic added mass ...... 507 4.3.1 Effect of geometry on web flutter ...... 507 5 Air flows and runnability systems in drying section ...... 508 5.1 Paper machine speed development ...... 508 5.2 Air flow-induced runnability problems ...... 509 5.3 Double-felted drying sections...... 510 5.4 Single-felted drying sections ...... 510 5.4.1 Single felting with small-diameter vacuum roll ...... 511 5.4.2 Single felting with grooved larger-diameter vacuum roll ...... 512 5.4.3 Drying section geometry with perforated bottom rolls ...... 515 5.5 Principle for app lication of runnability systems ...... 516

484 Web handling

6 Fluid flow analysis and computational fluid dynamics ...... 516 7 Web tension ...... 519 7.1 Strain behaviour of a moving web ...... 519 7.2 Unsupported web ...... 520 7.2.1 Tension behaviour of steady-state web ...... 520 7.2.2 Tension behaviour of unsupported continuous web...... 521 7.3 Tension behaviour of a supported web ...... 525 7.3.1 Effects of fabrics on strain behaviour...... 525 8 Two-dimensional planar strain and tension fields ...... 526 8.1 Planar stationary elastic and visco-elastic web ...... 526 8.2 Effects of the moving web ...... 527 8.3 Internal strains and shrinkages ...... 528 8.4 Tension profile ...... 529 9 Future trends in web handling ...... 532 References ...... 533 Nomenclature ...... 539

485 CHAPTER 13

Dryer fabrics

1 Introduction...... 544 2 Role of dryer fabrics in paper drying ...... 544 2.1 Fabric as part of the drying process ...... 546 2.1.1 Heat transfer ...... 546 2.1.2 Evaporation ...... 548 2.1.3 Ventilation ...... 549 2.1.4 Air permeability ...... 551 2.2 Web support and runnability...... 551 2.2.1 Effects of fabrics in air transport ...... 553 2.2.2 Fabric-to-web contact and CD web shrinkage control ...... 554 2.2.3 Effects of dryer fabrics on web strain behaviour and speed differences ...... 557 2.2.4 Dryer fabric marking ...... 559 3 Dryer fabric structures ...... 560 3.1 Raw materials ...... 560 3.2 Traditional and modern structures ...... 563 3.3 Seams ...... 564 3.4 Dryer fabric dimensions ...... 565 4 Selection of dryer fabric type ...... 566 4.1 Dryer fabrics for impingement drying units ...... 567 5 Operative use ...... 568 5.1 Installation ...... 568 5.2 Guiding ...... 568 5.3 Cleaning ...... 569 5.3.1 Consequences of contamination ...... 569 5.3.2 Types of contaminant ...... 569 5.3.3 Fabric application and cleaning recommendations ...... 570

542 Dryer fabrics

6 Clothing follow-up service ...... 572 6.1 Fabric inspections ...... 573 6.2 Troubleshooting ...... 574 6.3 Optimising tools ...... 574 7 Future dryer fabrics ...... 575 References ...... 576

543 CHAPTER 14

Drives

1 Introduction...... 580 2 Dimensioning and selection of components ...... 580 2.1 General principles ...... 580 2.2 Dimensioning of mechanical drives ...... 581 2.2.1 Criteria for selecting separate transmissions ...... 582 2.2.2 Criteria for selecting couplings and universal shafts ...... 583 2.3 Dimensioning of electrical drives ...... 586 3 Mechanical drives ...... 587 3.1 Main shaft drives ...... 587 3.1.1 Belt drives ...... 588 3.1.2 Differential drives ...... 588 3.2 Sectional drives ...... 589 3.2.1 Separate transmissions ...... 591 3.2.2 Roll gearbox ...... 593 3.2.3 Drying section drives ...... 595 4 Electrical drives ...... 597 4.1 History ...... 597 4.2 Variable-speed drive ...... 597 4.2.1 DC drives ...... 597 4.2.2 AC drive s with asynchronous induction motors ...... 598 4.2.3 AC drives with synchronous permanent magnet motors ...... 600 4.3 Sectional drive system ...... 600 4.3.1 AC or DC drive with a speed reduction gear ...... 601 4.3.2 Direct drive without speed reduction gear ...... 602 4.3.3 Machine integrated motor ...... 603 4.4 Energy efficiency of drive system ...... 603

578 Drives

4.5 Controls ...... 604 4.5.1 Different running modes ...... 605 4.5.2 Different control modes ...... 605 4.5.3 Control modes in a paper machine ...... 606 4.5.4 Speed reference chain ...... 607 4.6 Control performance ...... 609 4.6.1 Configuration ...... 609 4.6.2 Control accuracy and resolution ...... 609 4.6.3 Static accuracy ...... 609 4.6.4 Dynamic accuracy ...... 610 4.6.5 Paper transport considerations ...... 610 4.6.6 Controllers ...... 611 4.7 Operator interfaces ...... 613 4.7.1 Local control devices...... 613 4.7.2 Supervisory systems ...... 614 4.8 Maintenance tools ...... 614 5 Future of drives ...... 615 References ...... 616

579 CHAPTER 15

Future outlook

1 Drying concepts ...... 618 2 Energy management ...... 620 3 Ventilation, heat recovery and runnability systems ...... 620 4 Paper and board grades ...... 621 5 Other drying section systems ...... 622 6 Modelling and simulation of complicated interactions in paper drying ...... 623 7 Conclusion ...... 624 References ...... 625

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