Moisture Distribution and Flow During Drying of Wood and Fiber

Zink-Sharp, Audrey ; Hanna, Robert B.

Virginia Polytechnic Institute and State University, Blacksburg, VA (United States)

关键词: Optical Properties;    Heat Transfer;    Energy Consumption;    Thermal Conductivity;    Fibers;   

DOI  :  10.2172/789798
RP-ID  :  DOE/ID/13537
RP-ID  :  FC07-97ID13537
RP-ID  :  789798

美国|英语

来源: UNT Digital Library

【 摘 要 】

New understanding, theories, and techniques for moisture flow and distribution were developed in this research on wood and wood fiber. Improved understanding of the mechanisms of flake drying has been provided. Observations of flake drying and drying rate curves revealed that rate of moisture loss consisted of two falling rate periods and no constant rate drying period was observed. Convective heat transfer controls the first period, and bound water diffusion controls the second period. Influence of lower drying temperatures on bending properties of wood flakes was investigated. Drying temperature was found to have a significant influence on bending stiffness and strength. A worksheet for calculation of the energy required to dry a single strandboard flake was developed but has not been tested in an industrial setting yet. A more complete understanding of anisotropic transverse shrinkage of wood is proposed based on test results and statistical analysis. A simplified mod el of a wood cell's cross-section was drawn for calculating differential transverse shrinkage. The model utilizes cell wall thickness and microfibrillar packing density and orientation. In spite of some phenomena of cell wall structure not yet understood completely, the results might explain anisotropic transverse shrinkage to a major extent. Boundary layer theory was found useful for evaluating external moisture resistance during drying. Simulated moisture gradients were quire comparable to the actual gradients in dried wood. A mathematical procedure for determining diffusion and surface emission coefficients was also developed. Thermal conductivity models of wood derived from its anatomical structure were created and tested against experimental values. Model estimations provide insights into changes in heat transfer parameters during drying. Two new techniques for measuring moisture gradients created in wood during drying were developed. A new technique that utilizes optical properties of cobalt chloride was developed for nondestructive determination of surface moisture content. Fundamental new understanding of drying characteristics in wood and fiber has been provided that can be used by researchers to improve drying of wood and fiber. The three techniques for measuring moisture content and gradients provided in this study are efficient, practical, and economical - easy to apply by industry and researchers. An energy consumption worksheet is provided as a first step toward reducing energy consumed during drying of lumber and strandboard flakes. However, it will need additional verification and testing.