Oscillating drying of red oak wood blanks

Abstract

The reasons for the reduction in the price of the lamella made of oak wood have been determined. It is established that during the drying of oak wood at low temperatures there is an accumulation of moisture in the near-surface zone, the so-called “conditional source of moisture”. This leads to the formation of light spots in the middle of the lumber, which have long been in conditions of high air humidity and had a low rate of moisture removal. To avoid this defect, it is proposed to intensify the drying process by using oscillating schedules, which consist of periods of heating and cooling of wood. During the heating period, the temperature of the material gradually rises, the wood receives heat stroke and accumulates heat, drying occurs only due to the moisture gradient. During cooling, the air temperature decreases, the degree of saturation increases and, accordingly, the equilibrium moisture content increases. As a result, the surface layers of the material are moistened, the moisture is leveled along the thickness of the wood, there is a relaxation of drying stresses. In the laboratory, experimental studies of oscillating drying of red oak wood blanks with a thickness of 30 and 50 mm were performed. The amplitude of temperature fluctuations is determined - 30 ° С and the expediency of cyclic heating only until the average moisture content of the material reaches 20%, after which the wood should be dried to the required final moisture content at a constant heating temperature. A method for calculating the duration of oscillating drying is proposed, which takes into account the peculiarities of the kinetics of the periods of heating and cooling of sawtimbers made of red oak wood with a thickness of 30 mm and 50 mm. It is recommended to consider the process of temperature change kinetics as the sum of the processes of cyclic heating, which occurs in polynomial dependence, and cyclic cooling, which occurs in exponential dependence. It is determined that the ratio of the duration of cyclic heating to the duration of cyclic cooling is 1/3. Using heat and mass transfer criteria and the experimentally determined coefficient of moisture conductivity of red oak wood, an adequate model for calculating wood temperature and relative humidity during wood heating and cooling cycles has been developed. Based on the obtained results of theoretical and experimental studies, oscillating drying schedules of red oak blanks of different thickness are proposed, which exclude the formation of white spots in the middle of the material. The process was reduced by 1.5–2.4 times and energy consumption was reduced by 1.53 times. Experimentally determined indicators of residual stresses, the magnitude of the stress difference in the workpieces after drying and the values of the tensile strength showed that the quality of drying corresponds to the I quality category according to DSTU 4921: 2008. Comparison of the obtained values with the maximum allowable values of tensile strength across the fibers in the tangential direction σm = 6.5 MPa showed a margin of safety - 25%, which indicates the possibility of using oscillating drying schedules for high-quality drying of red oak wood blanks. Keywords: Red oak (Quercus rubra), moisture conductivity coefficient, oscillating schedules, drying quality, drying duration.