Kiln-Drying of Lumber

At present, no single book adequately covers a basic understanding of wood book satisfies the need for such a work. It describes drying in practice. This the fundamental basis of kiln-drying technology, to enable forest companies to imFrove their drying operations as high-quality timbers become scarcer and of yesteryear can no longer be tolerated. Adaptive the wasteful practices is no longer good enough. Innovations change based on past experience of the material being dried and the processes require a sound understanding of drying. Newer techniques, such as the use of ultrahigh temperature sea­ soning and superheated steam under vacuum, require an even greater depth of physical understanding for these methods to be used effectively and economically. book provides a description of modern ideas about wood structure, This moisture movement and stress development, from which models of the drying process are developed to give the kiln operator important information about the course of drying under specified conditions, and thus a means is compared with practice wherever for rational process improvement. Theory possible.

Contenu

1 The Structure of Wood.- 1.1 The Structure of Softwoods.- 1.2 The Structure of Hardwoods.- 1.3 Cell-Wall Structure and Composition.- 1.3.1 Cellulose.- 1.3.2 Hemicellulose.- 1.3.3 Lignin.- 1.3.4 The Cell-Wall Ultrastructure of a Softwood Tracheid.- 1.3.5 Water in the Cell Wall.- 2 Wood-Water Relationships.- 2.1 Water in Wood.- 2.2 Moisture Sorption.- 2.3 Fibre Saturation and Maximum Hygroscopic MoistureContent.- 2.4 Theories of Sorption.- 2.5 Heat of Sorption.- 2.6 Response to Environmental Changes.- 3 Evaporation and Humidification.- 3.1 Moisture in Air.- 3.2 Enthalpy of Moist Air.- 3.3 Adiabatic Saturation and Wet-Bulb Temperatures.- 3.4 Humidity Charts.- 3.5 Ideal Heat Demand.- 3.6 Evaporation from a Wood Surface.- 3.7. Subsurface Evaporation.- 3.8 Mass-Transfer Coefficient Measurements.- 4 Wood-Drying Kinetics.- 4.1 Empirical Observations.- 4.1.1 Permeable Wood.- 4.1.2 Impermeable Wood.- 4.1.3 Empirical Models.- 4.1.4 Graphical-Analytical Methods.- 4.2 Normalisation of Drying-Rate Curves.- 4.3 Pathways for Moisture Movement in Wood.- 4.3.1 Longitudinal Movement.- 4.3.2 Transverse Movement.- 4.4 Selection of Drying Models.- 5 Moisture Diffusion.- 5.1 Driving Forces for Diffusion.- 5.1.1 The Kirschhoff Transformation.- 5.1.2 Moisture-Content Driving Force.- 5.1.3 Vapour-Pressure Driving Force.- 5.1.4 Chemical-Potential Driving Force.- 5.2 Penetration Periods and Regular-Regime Drying.- 5.3 Theoretical Modelling of Diffusion Coefficients.- 5.4 Experimental Measurements of Diffusion Coefficients.- 5.4.1 The Effect of Temperature on Diffusion Coefficients.- 5.4.2 The Effect of Moisture Content on Diffusion Coefficients.- 5.5 Conclusions.- 6 Multiple-Mechanism Models.- 6.1 Fundamental Equations.- 6.2 Experimental Observations.- 6.2.1 Temperature Profiles.- 6.2.2 Moisture-Content Profiles.- 6.2.3 Pressure Profiles.- 6.3 The Physical Process of Drying for a Softwood, Pinus radiata.- 6.3.1 Heartwood.- 6.3.2 Sapwood.- 6.3.3 The Moisture-Transport Equations.- 6.3.3.1 The Movement of Free Moisture.- 6.3.3.2 The Movement of Water Vapour.- 6.3.3.3 The Movement of Bound Water.- 6.3.4 The Evaporative Zone.- 6.3.5 Mass and Energy Conservation Equations.- 6.3.6 Initial Conditions.- 6.3.7 Boundary Conditions.- 6.4 Mixed-Wood Boards.- 6.5 Conclusions.- 7 Lumber Quality.- 7.1 Gross Features of Wood.- 7.1.1 Greenwood Moisture Content.- 7.1.2 Wetwood.- 7.1.3 Heartwood.- 7.1.4 Knots.- 7.1.5 Spiral Grain.- 7.1.6 Juvenile and Mature Wood.- 7.2 Intrinsic Features of Wood.- 7.2.1 Density.- 7.2.2 Collapse.- 7.2.3 Warp.- 7.2.4 Reaction Wood.- 7.2.4.1 Compression Wood.- 7.2.4.2 Tension Wood.- 7.3 Processing Implications.- 7.3.1 Sawing Strategies.- 7.3.2 Warp on Drying.- 8 Stress and Strain Behaviour.- 8.1 Mechanical Analogues.- 8.1.1 Elastic Element.- 8.1.2 Viscous Element.- 8.1.3 Maxwell Model.- 8.1.4 Kelvin Model.- 8.1.5 Burgers Model.- 8.2 Shrinkage.- 8.3 Instantaneous Strain.- 8.3.1 Linear Loading.- 8.3.2 Non-Linqar Loading.- 8.3.3 Unloading.- 8.3.4 Slow-Loading Tests.- 8.4 Viscoelastic Strain.- 8.4.1 Mechanical Analogues.- 8.4.2 The Bailey-Norton Equation.- 8.5 Mechanosorptive Strain.- 8.5.1 Qualitative Observations.- 8.5.2 Quantitative Analysis.- 8.6 Relative Magnitude of Strain Components.- 8.6.1 Elastic and Other Strains.- 8.6.2 Viscoelastic and Mechanosorptive Strains.- 8.7 Solution Procedures.- 8.7.1 One-Dimensional Analysis.- 8.7.2 Two-Dimensional Analysis.- 8.8 Experimental Apparatus.- 8.9 Applications.- 9 Airflow and Convection.- 9.1 Airflow in a Batch Kiln.- 9.1.1 Velocity Distributions over a Kiln.- 9.1.2 Geometrical Considerations.- 9.1.3 Pressure Drops over Kiln Sections.- 9.1.4 Stack-Velocity Distribution.- 9.2 Flow between Boards.- 9.3 Convection in Kilns.- 9.3.1 Airflow Maldistribution.- 9.3.2 Board Irregularities.- 9.3.2.1 Uneven Thickness of Boards.- 9.3.2.2 Stack Ends.- 9.4 Bypassing.- 9.5 Kiln Economics.- 10 Kiln Operation.- 10.1 Drying under Constant External Conditions.- 10.2 Drying under Variable External Conditions.- 10.3 Practical Kiln Schedules.- 10.4 General Practical Considerations.- 10.4.1 Species-Grouped Schedules.- 10.4.2 Species-Specific Schedules.- 10.4.3 Schedule Development.- 10.4.4 Stacking.- 10.4.5 Fan Speeds and Reversals.- 10.4.6 Kiln Monitoring.- 10.4.7 Volatile Emissions and Kiln Corrosion.- 10.4.8 Equalisation.- 10.4.9 Stress Relief.- 10.4.10 Destickering.- 10.5 End-Moisture Specification.- 10.6 Handling Kiln-Dried Lumber.- 11 Pretreatments of Green Lumber.- 11.1 Protecting Wood Prior to Drying.- 11.1.1 Wet Storage.- 11.1.1.1 Control of Microorganisms.- 11.1.1.2 Relaxation of Growth Stresses.- 11.1.2 Antisapstain Treatments.- 11.1.3 Brownstain Control.- 11.2 Physical Methods to Improve Permeability.- 11.2.1 Incising.- 11.2.2 Compression Rolling.- 11.3 Low-Temperature Predrying.- 11.4 Heat Treatment.- 11.4.1 Steaming and Soaking in Hot Water.- 11.4.2 Dry Heat.- 11.5 Prefreezing.- 11.6 Antishrink Chemicals.- 11.7 Presurfacing.- 11.7.1 Problems with Surface Checking.- 11.7.2 Problems with Moulds.- 11.8 Green Finger-Jointing and Cutting Blanks.- 11.9 Precoating.- 11.10 Presorting.- 12 Less-Common Drying Methods.- 12.1 Solar Kilns.- 12.1.1 Insolation Rates and Kiln Locations.- 12.1.2 Absorbers.- 12.1.3 Glazing.- 12.1.4 Temperature Control.- 12.1.5 Humidity Control.- 12.1.6 Air Circulation.- 12.1.7 Energy Losses.- 12.1.8 Economics.- 12.1.9 Mathematical Modelling of Performance.- 12.2 Dielectric Drying.- 12.2.1 Mechanisms of Heating.- 12.2.1.1 Dipolar Rotation.- 12.2.1.2 Ionic Conduction.- 12.2.2 Interactions.- 12.2.2.1 Moisture Content.- 12.2.2.2 Density.- 12.2.2.3 Temperature.- 12.2.3 Internal Pressures.- 12.2.4 Drying Times Relative to Conventional Kilns.- 12.2.5 Economics.- 12.3 Superheated Steam Drying.- 12.4 Vacuum Drying.- 12.5 Dehumidifier Kilns.- References.- Species Index.