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