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5 Discussion and conclusions The principal objectives of this research were to: • establish the viability of vacuum kiln drying Australian hardwood species compared with current conventional kiln drying methods, • design a predictive drying model to accurately predict heat and mass transfer, and • validate the drying model. We achieved this through a series of end-matched conventional and vacuum drying trials, where a number of relevant wood, kiln and drying properties were measured for viability and model validation. Four trials for four species were conducted. We chose the species based on their wide ranging proximity, high commercial volume and vastly different wood properties and drying capabilities. The species were, C. citriodora (spotted gum), E. marginata (jarrah) E. pilularis (blackbutt) and E. obliqua (messmate). In addition, to establish the economic viability of vacuum drying we prepared an economic model that provided a number of comparative case studies. Alongside this report, the economic model is also available as a useful tool for industry. In keeping with the structure of this report, the following project conclusions are presented under appropriate sub-headings. 5.1 Applied drying – Experimental drying trials From the series of vacuum and conventional end-matched drying trials, we can form the following conclusions: • The vacuum drying time was greatly reduced compared with end-matched conventional drying, where the best quality outcome vacuum drying times ranged between 41% and 66% of conventional kiln drying from green, depending on the species. • For the most viable vacuum drying trials, the vacuum dried quality was either the same or better than the end-matched conventional drying counterpart, depending on the species. • Unacceptable surface checking results were obtained for E. pilularis for every vacuum and conventional drying trial. However the percentage of boards making select grade for surface checking from the vacuum drying trials was considerably higher than for the conventional drying trials. We believe with further schedule adjustment acceptable surface checking resulting from vacuum drying can be obtained. • Schedule adjustments greatly improved the dried quality between vacuum drying trials of the same species, resulting in us recommending the final vacuum drying schedule used for each species as a starting point for further vacuum drying. • Twist was the major form of unacceptable distortion encountered for both vacuum and conventional trials across all species. Twist distortion was greatly reduced to within acceptable limits for vacuum drying trials by doubling the concrete stack weighting to 1000kg / m3 between trials. • A simple and efficient method was used to quantify the effect of reconditioning on collapse recovery using the image analysis software MeshPore. The reconditioning process was proved to effectively recover collapse eliminating ‘miss’ or ‘skip’ after dressing. • For all trials, end splitting was significantly more prevalent in conventionally dried boards than vacuum dried boards. Reduction of end splitting could be a principal advantage over conventional drying. It is theorised that, due to overpressure, more longitudinal water flow occurs during vacuum drying, reducing MC gradients and associated stress at the board ends. This is a topic for further investigation. 115 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final reportPDF Image | Evaluation of super-heated steam vacuum drying
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