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schedule used for vacuum trial MES2 compared with vacuum trial MES1, greatly improved the number of boards free of internal checks (89% cf. 78%) to a level comparable with the conventionally dried boards of the MES1 trial (88%). Further schedule manipulations reduced the incidence of surface checking to within acceptable limits for the final MES4 vacuum trial. Table 53. Surface checking results – E. obliqua Surface checking (mm) MES2 MES3 Vacuum Conventional Vacuum Conventional 26.2 1400.0 0.0 157.6 84.2 1900.0 0.0 292.8 0.009 41.2 810.0 0.0 138.9 148.9 1900.0 0.0 438.3 0.031 Average Maximum Minimum Standard deviation Significance (p) MES1 Vacuum Conventional MES4 Vacuum Conventional 7.0 5.9 600.0 452.0 0.0 0.0 56.8 45.8 0.859 MES4 Vacuum Conventional 98 98 0 0 2 2 98 94 96 97 14.4 1070.0 0.0 104.7 33.7 2719.0 0.0 247.4 0.075 Table 54. Surface checking grade results – E.obliqua MES2 MES3 Vacuum Conventional Vacuum Conventional 96 84 00 4 16 94 71 89 96 91 78 10 8 21 80 72 95 85 Grade Select (%) Medium feature (%) High feature (%) Surface check free boards (%) Internal check free boards (%) End splits MES1 Vacuum Conventional 97 0 3 96 78 95 0 5 94 88 Significant differences exist between the end split population means of all conventional and vacuum end-matched trials, except the MES4 trials (Table 55). End splitting was more prevalent in the MES1-3 conventionally dried boards, failing to meet even the lowest grade quality E. As the MES3 vacuum boards were partially dried before testing the results cannot be directly compared with other trials. 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 section4.3.4 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 at the board ends and hence reduce the incidence and severity of end-splitting. Table 55. End split results – E.obliqua End splits (mm) MES2 MES3 Vacuum Conventional Vacuum Conventional 20.5 77.9 1082.0 989.0 0.0 0.0 107.4 151.5 < 0.0001 0.0 61.9 0.0 947.0 0.0 0.0 0.0 136.8 < 0.0001 A Fail A Fail MES1 MES4 Vacuum Conventional Vacuum Conventional Maximum Minimum Standard deviation Significance (p) Grade quality (AS/NZS 4787 ) Drying stress 6.6 53.7 6.4 300.0 980.0 390.0 0.0 0.0 0.0 39.2 147.7 40.0 < 0.0001 A Fail A 0.3 50.0 0.0 3.6 0..932 A Significant differences exist between the drying stress population means of MES1 and MES4 but not for the other conventional and vacuum end-matched trials (Table 56). All trials achieved acceptable grade quality B results for drying stress, except for conventional trial MES4. As the MES3 vacuum boards were partially dried before testing the results cannot be directly compared with other trials. 85 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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