Continuous measurement and recording of the core temperature in wood samples during convective drying in an airstream nveals that the temperature difference between Ihc airstream and the core of the wocd reflects almost quantitatively the influence of che external conditions and the characteristic f a m s of the drying wood material on the course. of the drying velocity. Conclusions concerning tbe drying practice of timber kiIn+ng which arise from the experimental findings are discussed. Key words: c o m e of wood temperature; drying velocity; kiln control The course of the tempemure within the drying body in conjunction with dah of the exremal , conditions is of considerable imponance in understanding the drying behaviour of wood. Such dam bas often been published (Voigt el al. 1940; Keylwerth 1952; Kollmann 1954; Malmquist and Noack 1960; Schneider 1972) but a close analysis and interpretation of the experimental findings has not been attcmted so far.I h e objective of the experimental sludy qorled hen was lo obrain detailed and reliable data on the course of the temperature of wood samples during convective drying under constant external conditions and to come to a convincing explanation of the experimental fmdings. A second goal was to evaluate the feasibility of a wood-tempenture-dependent mnml system for the kiln drying of timber. '
2087The material for the test drying runs consisted of single planks 440 x 100 x 30 mm which were cut from beech logs (Fagus s,ilvatica L.) and oak logs (Quercus robw L.) with initial M.C. 70 ... 80 %. Tbe planks were fined with calibrated temperature probes PT 100, placed into rhe centre of the plan&. ,The drying runs were conducted in an electically heated forced-air circulation experimental kiln of the Swiss Federal Institute of Technology, Chair of WoodTechnology in Zurich. The variation in the relative humidity in the kiln during the drying runs was about 2 46 and the temperature variation was about k0.4OC. Air velocity used during the runs was 2 mls. The temperature recording within the.kiln (dry-bulb and wet-bulb temperature of the circulating air) was conducted with the same type of calibrated PT 100 temperature probes as for the measurement of the core temperature of the samples.The kiln was equipped with a weight-measuring system using a load cell of high sensitivity. The system permitted a continuous recording of the M.C. decrease of the wood samples during the test runs with an accuracy of 4. I g.
RESULTS AND DISCUSSIONThe drying course of beech wood within the fmt 20 hours of 3 characteristic test rum under different sets of tempencure and relative humidity conditions is graphicaly presented in Fig. L.3. Fig. 4 shows the drying course of a drying run of oak wood. 'The difference-in-tempenture curves shown in the graphs represent the temperature depression in the core of the wood samples over the dry-bulb temperature of the circulating air. The moisture content curves in the graphs show the weight decrease of the wood samples due to the removal of moisture. The slope...
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