The Characteristics Of Wood Particle Drying With Convection

The drying of wood particle is an important procedure in the production of particle board. The drying quality of wood particle and the consumption of energy have a direct effect on the quality and cost of the products. This research is closely connected with practical manufacturing. A mathematical model of heat and mass transfer in wood particle drying system with convection is founded by studying the characteristics of wood particle drying process and influencing factors, which provides a theoretical basis for the optimized design of wood particle drying techniques and process control, thus has important realistic and economic significance.The major results and innovation of this research are as follows:1. The characteristics of wood particle drying with convection are systematically and thoroughly studied for the first time on the basis of a large number of experiments and theoretical analysis. It is shown that the airflow temperature and the air velocity are important factors on changing average drying rate of wood particles. The average drying rate of wood particle increased with the increasing of airflow temperature, while unit consumption decreased gradually. With the raising of initial moisture content of wood particle, the average drying rate of particle and unit consumption increased. The whole consumption energy raised largely though the unit consumption energy kept unchanged with increasing loading of wood particle. The movement of rotating tube affected the drying of wood particle and can improve the average drying rate of wood particles. The appropriate drying condition should be decided according to the practical yield and cost of production.2. The water transfer process inside wood particles was observed with the help of environment scanning microscope, and the wet transfer characteristics of wood particles are analyzed. The results show that there are three reasons for water transfer inside wood particles: capillary water movement depending on the tension of capillary pipes, steam movement through diffusion, bound water movement through surface diffusion. In the process of wood particle drying, when wood particle moisture content is above fiber saturation point, the free water and water steam in the wood particles will move outwards by way of seepage flow caused by capillary tension gradient and steam pressure gradient.Sorption water in wood particles doesn't move at this stage. When moisture content of wood particle is below fiber saturation point, water exists in wood particles in the forms of sorption water and water steam, and move in the wood particles by way of diffusion.3. A heat and mass transfer model of wood particle drying with convection is established for the first time based on the characteristics of wood particle drying with convection and characteristics of heat and mass transfer. The results of calculations and experiments show that this model accurately reflects the whole process of wood particle drying with convection, and therefore can be applied in the research and analysis of drying with convection, as well as in guiding on design of drying systems of the same kind.4. A preliminary thermodynamic analysis is made for the wood particle drying with convection. It is concluded that mass transfer can reduce heat sources and heat transfer in equivalent weight. The surface area of wood particles has greater mass transfer potentials and thus stronger mass transfer abilities. In the process of wood particle drying with convection, driving potentials of drying vary according to the types of interior wet elements and transfer mechanics. Interior wet elements transfer mainly relies on liquid water movement at the preliminary stage of drying and relies on interior water gasification and steam diffusion at the medium and later stages. The interior water gasification is not on certain gasification surface, but happens on the inside of the whole wood particle. The molecular mass diffusing of water steam is not in the direction of decreasing steam pressure, but carries out in the direction of...