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The aim of the present study was to establish the value of desorption stresses and the rate of their increase in the samples of juvenile and mature birch wood as well as in aspen samples of different physiological age. The stress value was calculated on the basis of the measurement of the force necessary to restrain shrinkage in the sample being dried. Additionally, moisture strain and mechanical-moisture strain (both real and apparent) were measured during the experiments. The samples were dried at temperatures of 30, 50 and 70°C. It was concluded that both the rate of increase and the absolute values of stress and strain are different in juvenile and mature birch wood. Also in aspen they depend on the physiological age of the wood tissue. Juvenile birch wood and the physiologically younger aspen proved more susceptible to strain caused by desorption stresses.
Currently, the basic way of wood moisture content decrease control during drying process is measurement of that moisture content with resistance meters on a random selection of timber items. The results of that measurements are particularly unreliable, especially at wood moisture content below fibre saturation point and, on the other hand, they are influenced by accuracy of sample’s choice. Research considered the problem of monitoring of water mass flow that is removed with air in order to control the course of drying process. As a results of experiments, during drying of pinewood timber, it was stated that the monitoring of outlet air state allows for the verification of measurements of moisture content with resistance meters. The monitoring of air state at inlet and outlet of a kiln creates the possibilities of a more complex control of wood drying process.
The effect offan speed on the drying process of timber in kiln driers. The study investigated the effect of fan speed on air velocity in a stack of timber in selected types of kiln driers. The trials were conducted under technical scale conditions. The values and dispersion of air velocity were measured in the stack were measured, depending on fan speed reduced in the final stage of the drying process. Changes in fan speed were evaluated on the basis of the time of drying, energy consumption to drive fans, as well as the dispersion of final moisture content in the stack, the dispersion of moisture content in the cross-section. The study included drying of hardwood and softwood timber. On the basis of obtained results it was found that reducing fan speed results in a decrease in both air velocity in the stack and electric energy consumption used to drive fans, while no significant extension of drying time or deterioration of drying quality were observed.
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