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manipulation between vacuum drying trials, where the WBD was increased during the early stages of drying, thus creating a milder schedule. Except for the BBT2 trial, vacuum drying trials produced a higher percentage of select boards than conventional trials for surface checking. Surface checking generally occurs during the early stages of drying when MC gradients and surface tension are at their highest (McMillen, 1963). For future vacuum drying E. pilularis trials, we need to reconsider early vacuum drying schedule conditions to reduce the severity of surface checking within acceptable grade quality. Internal checking was present in boards for all vacuum and conventional trials. Unacceptable internal check grade quality resulted for the BBT1 vacuum, BBT2 vacuum and conventional and the BBT3 conventional trials. Again, we can attribute the improvement of internal checking to within acceptable limits for the BBT3 and BBT4 vacuum trials to using a milder schedule. Table 39. Surface checking results – E. pilularis BBT1 Vacuum Conventional Surface checking (mm) BBT4 Vacuum 85 1061 0 204 NA BBT4 Vacuum BBT2 BBT3 Vacuum Conventional Vacuum Conventional 119 627 1260 1900 00 224 0.250 662 148 1181 1370 1900 00 320 770 < 0.0001 Average Maximum 1260 Minimum 0 Standard deviation Significance (p) Table 40. Surface checking grade results – E. pilularis 119 627 1900 0 662 224 < 0.0001 BBT2 BBT3 Vacuum Conventional Vacuum Conventional 54 62 00 46 38 39 51 84 79 82 35 00 18 65 70 17 93 88 Grade Select (%) Medium feature (%) High feature (%) Surface check free boards (%) Internal check free boards (%) End splits BBT1 Vacuum Conventional 79 33 200 1967 22 5117 65 8793 98 78 Significant differences exist between the end split population means of all conventional and vacuum end-matched trials (Table 41). End splitting was more prevalent in the conventionally dried boards, which achieved the unacceptable grade quality D. All vacuum trials achieved acceptable grade quality A. It may be that due to extra internal overpressure caused by the vacuum itself, more end drying is present during vacuum drying compared to conventional methods. This theory is reinforced by the test board thermocouple results presented in section 4.3.4 Board temperature of this report. The thermocouple results show noticeably lower temperatures at the board ends than the centres indicating a higher degree of free water at the ends of the boards. This would certainly reduce the level of shrinkage variation at the board ends and hence reduce the incidence and severity of end-splitting. Table 41. End split results – E. pilularis 73 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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