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1 Executive Summary The primary objectives of this study were to: • Establish the viability of vacuum drying for four high commercial volume Australian hardwood species in terms of drying quality, time and cost. The four species investigated were: Corymbia citriodora (spotted gum), blackbutt (Eucalyptus pilularis), messmate (Eucalyptus obliqua) and jarrah (Eucalyptus marginata) • Develop an economic model to compare the drying costs of vacuum and conventional drying. • Develop a predictive vacuum drying heat and mass transfer deterministic model using fundamental principles. Investigate the model limitations and sensitivities. • Validate the drying model against data derived from applied drying trials. We investigated the viability of vacuum drying by conducting a series of four trials per species and developing an economic model application. For each trial 25 x 100 mm nominal cross-section dimension boards were kiln dried in a 2 m3 capacity vacuum kiln and compared with end-matched boards kiln dried in commercial conventional driers. We used the economic model to investigate the comparative costs of small (10 m3), medium (35 m3) and large (50 m3) vacuum and conventional operations using the best quality results from the drying trials as input data, and data provided by industry. The results of drying trials show that vacuum drying produces material of the same or better quality than is currently being produced by conventional methods within 41 to 66 % of the drying time, depending on the species. Economic analysis indicates positive or negative results depending on the species and the size of drying operation. Definite economic benefits exist by vacuum drying over conventional drying for all operation sizes, in terms of drying quality, time and economic viability, for E. marginata and E. pilularis. The same applies for vacuum drying C. citriodora and E. obliqua in larger drying operations (kiln capacity 50 m3 or above), but not for smaller operations at this stage. Further schedule refinement has the ability to reduce drying times further and may improve the vacuum drying viability of the latter species in smaller operations. A comprehensive heat and mass transfer drying model was developed with the ability to accurately simulate vacuum and conventional drying of Australian hardwood species. Measurement of essential wood drying properties required for the model revealed that water movement within these species occurs mostly by diffusion. Subsequently diffusion measurements were critical for refining the model accuracy. Validation of the model resulted in excellent simulated accuracy where the predicted drying times agreed with the kiln trial drying times by 91 to 98 %, depending on the species. This has the ability to greatly accelerate the time required for further vacuum and conventional drying research, and to quickly design new drying schedules. Overall, the outcomes of this research provide industry with the confidence to make informed decisions regarding their potential investment in vacuum drying technology. With the aid of the economic model application and the results of the drying trials, industry has the ability to investigate the viability of vacuum drying over conventional drying dependant in their own specific requirements. iPDF Image | Evaluation of super-heated steam vacuum drying
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