Heating characteristics of western hemlock (Tsuga heterophylla) in a high frequency field

The research aimed to establish whether dielectric heating of wood at high frequency could be accomplished fast, without excessive temperature fluctuations and no wood degrade. The intention was to develop optimized dielectric heating schedules in order to effectively pasteurize green timbers and logs for export—an eco-friendly substitute to currently used toxic chemical method. Data analysis revealed that the electric field distribution within the specimen was a strong function of its dielectric properties. Specifically, homogeneous moisture contents generated constant electric field values while heterogeneous distributions generated sudden drops and raises of the electric field strength with heating uniformly dispersed due to a fast redistribution from hot to colder areas. Convection losses through air contact may reduce the average heating rate of the timber shell by about 50%.ZusammenfassungIn dieser Studie wurde untersucht, ob Hochfrequenzerhitzung von Holz schnell und ohne übermäßige Temperaturschwankungen und ohne Beeinträchtigung der Holzqualität durchgeführt werden kann. Ziel war es, ein optimiertes Erhitzungsprogramm zur effektiven phytosanitären Behandlung von für den Export bestimmtem Schnitt- und Rundholz zu entwickeln – als umweltfreundliche Alternative zu den gegenwärtig verwendeten toxischen chemischen Verfahren. Die Versuchsergebnisse zeigten, dass die Verteilung des elektrischen Feldes im Prüfkörper stark von dessen dielektrischen Eigenschaften abhing. Eine homogene Holzfeuchte ergab konstante elektrische Feldwerte. Eine heterogene Holzfeuchteverteilung führte zu plötzlichen Ab- und Anstiegen der elektrischen Feldstärke, jedoch wegen einer schnellen Umverteilung zwischen heißen und kalten Bereichen zu einer gleichmäßigen Aufheizung. Wärmeverluste durch Luftkontakt können die mittlere Aufheizrate der Schnittholzoberfläche um bis zu 50 % verringern.

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