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5.2 Applied drying - Economic model Using fundamental drying economics, we made and economic application with the ability to compare vacuum and conventional kiln drying from the green condition. We used this model to determine the economic viability of vacuum drying for each species over three different sized operations. We labelled the different sizes small, medium and large relating to wood capacities of 10, 35 and 50m3. Kiln input data was provided by Brunner-Hildebrand, a project collaborator, and drying trial data were provided from the ‘best outcome’ applied drying trial for each species, based on a combination of the best drying quality and fastest drying time encountered. From the analysis the following conclusions can be made: For all sized operations, we found vacuum drying E. marginata and E. pilularis to be more economically viable than using current conventional methods. For large drying operations, we found vacuum drying C. citriodora and E.obliqua to be more economically viable than current conventional methods. This was not the case for the medium and small drying operations. The larger the drying operation the more economically viable vacuum drying becomes compared to conventional drying. The model is based on a number of assumptions and the case studies provided may not apply to all operations. The provision of the model as an application allows industry to tailor their own case studies to more closely simulate their own operation. Drying modelling - Measurement of kiln conditions and wood drying properties A number of specialised instruments were used on test and flying wood boards to measure wood shrinkage, moisture content and temperature, and kiln airflow during vacuum drying trials. Through continuous ‘real–time’ measurements, a greater understanding of the kiln conditions and wood properties during vacuum drying were obtained. The data produced is essential to input into and validate the modelling component of this project. 5.4 Drying modelling - Measurement of essential wood properties We measured a number of wood properties that are essential provide the necessary structure for developing an accurate heat and mass transfer drying model. They were wood density, initial MC, fibre porosity, gas permeability, bound water diffusion and shrinkage. For many of these measurements sophisticated equipment was required. As a result of these measurements the following conclusions were made: • Shrinkage data varied between species and was consistent with published data. • Shrinkage values varied greatly between the tangential and radial directions, except for C. citriodora, and shrinkage values between species differ due to variations in physical and chemical properties. In terms of shrinkage, C. citriodora is more isotropic compared with the other species tested. • Collapse shrinkage was clearly evident for Eucalyptus obliqua, but not with other species, consistent with industrial seasoning experience. The wood-water relations of this species, free of collapse, was characterised by using thinner sample sections (in the radial-tangential plane) as theorised. • A clear difference in fibre porosity, size, fibre cell wall thickness and orientation is evident between these species. • A strong correlation was found between measured and calculated wood density, based on fibre porosity. • C. citriodora diffusion coefficients were much lower in all directions compared with the other species, where E. obliqua was the highest. The radial/tangential ratio of 116 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report 5.3 • • • •PDF Image | Evaluation of super-heated steam vacuum drying
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