Study On Mechanism And Technology Of High-temperature And Lowering Temperature Drying Of Pinus Radiata Board

Moisture movement during wood drying process is the fundamental research of wood drying, the rate of moisture movement directly affects the drying rate and the law of drying stress changing in wood.The object of this paper was Pinus radiata board. The drying law and the transverse moisture diffusion coefficient of Pinus radiata board in the condition of constant high temperature and in that of constant high-temperature in early stage and lowering temperature in late stage were studied. It was realized to dry Pinus radiata board in the condition of constant high-temperature in early stage and lowering temperature in late stage by taking temperature of the core layer during high-temperature drying as controlling parameter and choose the appropriate moisture content of starting to lower temperature. In this paper the theoretical basis was provided for solving the practical problem in high-temperature drying of Pinus radiata board and the primarily theoretical exploration was made for unsteady-state nonisothermal diffusion of moisture during wood drying.In this paper the relation between temperature and moisture content of the core layer was obtained according to the changing rules of temperature and moisture content of the core layer and the average moisture content. By this relation, temperature of the core layer was taken as controlling parameter and the appropriate moisture content of the core layer when temperature started to decline was chosen. By the proper rate of lowering temperature, moisture content in thickness was more even by the end of lowering temperature than that after constant high-temperature drying, the drying time was cut obviously. The course of lowering temperature before conditioning was not required in lowering temperature drying in late stage so time was saved, time of conditioning was shorter and the drying quality was better than those in constant high-temperature drying.In this paper, it was obtained for the transverse diffusion coefficient of Pinus radiata board in the conditions of all constant high-temperatures and in that of lowering temperature in late stage.In the condition of constant high-temperatures, the moisture diffusion coefficient of Pinus radiata board decreased with the increase of moisture content when it was above fiber saturation point and decreased with the decrease of moisture content when it was below fiber saturation point. The moisture diffusion coefficient of Pinus radiata board in radial direction was larger than that in tangential direction. The transverse diffusion coefficient increased as temperature elevated if the relative humidity and equilibrium moisture content were constant.