Variation property of steam pressure and temperature in wood during microwave drying

In this research, the measured point in the dried wood was connected with temperature sensor and pressure sensor by taking advantage of a polyfluortetraethylene T-bridge piece so that the measurement of both temperature and pressure could be available simultaneously during microwave drying. The experiment was mainly conducted to investigate the variations of the inner temperature, steam pressure and both relations, and the correlation between the variations and some drying defects such as inner cracking and charring were discussed as well.Resultsare as the followings: during microwave drying of wood, there are three stages of temperature changing: rapid temperature-increasing stage, constant temperature stage and later temperature-increasing stage; the higher the power is, the quicker the temperature increases. In the period of constant temperature stage, the higher the temperature is, the shorter the duration is. If the power of microwave radiation increases, the steam pressure increases quickly in wood, its peak value becomes higher and the holding time of maximal pressure becomes shorter. If temperature increases quickly, the speed of pressure will increase quickly. When the temperature reaches the constant value, the pressure will reach the maximum meanwhile. Inner cracking usually appears in the initial stage of the drying on account of the excessive steam pressure in high moisture wood; charring usually appears in the final stage of the drying on account of the dielectric characteristics of high temperature point in low moisture wood.