Springer Series in Wood Science

Transcription

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2 Springer Series in Wood Science Editor: T. E. Timell Martin H. Zimmermann Xylem Structure and the Ascent of Sap (1983) John F. Siau Transport Processes in Wood (1984)

3 John F. Siau 1Iansport Processes inwood With 123 Figures Springer- erlag Berlin Heidelberg New York Tokyo 1984

4 JOHN FINN SIAU Professor of Wood Products Engineering Department of Wood Products Engineering SUNY College of Environmental Science and Forestry Syracuse, New York 13210, U.S.A. Series Editor: T. E. TlMELL State University of New York College of Environmental Science and Forestry Syracuse, NY U.S.A. Cover: Transverse section of Pinus lambertiana wood. Courtesy of Dr. Carl de Zeeuw, SUNY College of Environmental Science and Forestry, Syracuse, New York ISBN-13 : DOl: / e-isbn-13 : Library of Congress Cataloging in Publication Data Siau, John F. Transport processes in wood. (Springer series in wood science; v. 2) Based on: Flow in wood, Wood-Permeability. 2. Wood-Moisture. 3. Wood-Anatomy. I. Title. II. Series. TA419.s ' ISBN-13 : (U.S.) This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under 54 of the German Copyright Law, where copies are made for other than private use, a fee is payable to "Verwertungsgesellschaft Wort", Munich by Springer-Verlag Berlin Heidelberg 1984 Softcover reprint of the hardcover 1st edition 1984 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. 2131/

5 Preface This book has a similar subject content to the author's previous Flow in Wood but with substantial updating due to the abundance of research in the wood science field since Several different concepts have been introduced, particularly in regard to wood-moisture relationships. The role of water potential in the equilibria between wood and its humid and moist environments is considered. Two theories are introduced to explain the nonisothermal transport of bound water in the steady and unsteady states. As in the former text, the wood-. structure relationship is emphasized.. The author is especially grateful to Dr. C. Skaar for his careful and critical review of much of the manuscript and for the productive discussions of many of the concepts. Dr. T. E. Timell, the series editor, rendered major assistance in the preparation of Chap. 2 and in his editing of the manuscript. The author wishes to thank Dr. W. A. Cote, Mr. A. C. Day, and Mr. J. J. McKeon for providing electron micrographs, Mr. G. A. Snyder for his photography of much of the art work, Dr. C. H. de Zeeuw for his advice in the field of wood anatomy, and Ms. Mary M. Siau for her careful rendition of the art work. Appreciation is extended to Miss Judy A. Barton and Mrs. Stephanie V. Micale for their work in typing and checking the manuscript. Mr. J. A. Meloling of Carrier Corporation provided the psychrometric charts, and the U.S.D.A. Forest Service furnished several items of art work. Finally, the author wishes to thank Dr. G. H. Kyanka, Dean W. P. Tully, and President E. E. Palmer of SUNY College of Environmental Science and Forestry for permitting the expenditure of time and the use of college facilities for writing this book. Syracuse, N. Y. November 1983 JOHN F. SIAU

6 Contents 1 Basic Wood-Moisture Relationships 1.1 Introduction Saturated Vapor Pressure Relative Humidity Use of the Psychrometric Chart Measurement of Relative Humidity Control of Relative Humidity Equilibrium Moisture Content and the Sorption Isotherm The Effect of Changes in Pressure and Temperature on Relative Humidity Specific Gravity and Density Specific Gravity of the Cell Wall and Porosity of Wood Swelling and Shrinkage of the Cell Wall Swelling and Shrinkage of Wood Wood Structure and Chemical Composition 2.1 Introduction The Cell Wall Structure of Softwoods 2.4 Types of Pit Pairs Softwood Pitting Microscopic Studies of Flow in Softwoods. 2.7 Structure of Hardwoods Hardwood Pitting Microscopic Studies of Flow in Hardwoods 2.10 Chemical Composition of Normal Wood Cellulose Hemicelluloses Introduction Softwood Hemicelluloses Hardwood Hemicelluloses Lignins Chemical Composition of Reaction Wood Introduction Compression Wood Tension Wood Topochemistry of Wood

7 VIn Contents 3 Permeability 3.1 Introduction 3.2 Darcy's Law. 3.3 Kinds of Flow 3.4 Specific Permeability 3.5 Poiseuille's Law of Viscous Flow 3.6 Turbulent Flow Nonlinear Flow Due to Kinetic-Energy Losses at the Entrance of a Short Capillary. 3.8 Knudsen Diffusion or Slip Flow Corrections for Short Capillaries Permeability Models Applicable to Wood Simple Parallel Capillary Model Petty Model for Conductances in Series Comstock Model for Softwoods Characterization of Wood Structure from Permeability Measurements 3.11 Measurement of Liquid Permeability Measurement of Gas Permeability The Effect of Drying on Wood Permeability Treatments to Increase Permeability The Effect of Moisture Content on Permeability The Influence of Specimen Length on Permeability Permeability of the Cell Wall Zones of Widely Differing Permeabilities in Wood General Permeability Variation with Species Capillary and Water Potential 4.1 Surface Tension Capillary Tension and Pressure Mercury Porosimetry Influence of Capillary Forces on the Pressure Impregnation of Woods with Liquids III 4.5 Collapse in Wood III 4.6 Pit Aspiration The Relationship Between Water Potential and Moisture Movement Notes on Water Potential. Equilibrium Moisture Content, and Fiber Saturation Point of Wood Thermal Conductivity 5.1 Fourier's Law Empirical Equations for Thermal Conductivity Conductivity Model Resistance and Resistivity; Conductance and Conductivity Derivation of Theoretical Transverse Conductivity Equation

8 Contents IX 5.6 Derivation of Theoretical Longitudinal Conductivity Equation Rand U Values; Convection and Radiation Application to Electrical Resistivity Calculations Application to Dielectric Constant Calculations Steady-State Moisture Movement 6.1 Fick's First Law Under Isothermal Conditions Bound-Water Diffusion Coefficient of Cell-Wall Substance The Combined Effect of Moisture Content and Temperature on the Diffusion Coefficient of Cell-Wall Substance Water-Vapor Diffusion Coefficient of Air in the Lumens The Transverse Moisture Diffusion Model The Importance of Pit Pairs in Water-Vapor Diffusion Longitudinal Moisture Diffusion Model Nonisothermal Moisture Movement Measurement of Diffusion Coefficients by Steady-State Method Unsteady-State Transport 7.1 Derivation of Unsteady-State Equations for Heat and Moisture Flow Derivation of Unsteady-State Equations for Gaseous Flow in Parallel-Sided Bodies Graphical and Analytical Solutions of Diffusion-Differential Equations with Constant Coefficients Solutions of Equations for Parallel-Sided Bodies Solutions of Equations for Cylinders Simultaneous Diffusion in Different Flow Directions Significance of Flow in Different Directions Special Considerations Relating to the Heating of Wood Relative Values of Diffusion Coefficients Retention Unsteady-State Transport of Liquids Parallel-Sided Bodies, Permeability Assumed Constant with Length Parallel-Sided Bodies with Permeability Decreasing with Length (Bramhall Model) Cylindrical Specimens Square and Rectangular Specimens Unsteady-State Transport of Moisture Under Nonisothermal Conditions Heat Transfer Through Massive Walls. 222 References Symbols and Abbreviations Subject Index