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Distortion We measured distortion on undressed boards before and after drying. For all trials, cupping was not included in the total percentage of boards within acceptable limits as all cupping was removed after dressing. Significant differences do not exist between paired conventional and vacuum population means of bow, twist and cup for trial SPG1, bow for trial SPG2, cup for trial SPG3, and bow and cup for trial SPG4 (Table 9). All other comparisons exhibited significant differences. Distortion results were acceptable (total > 90%) after drying for all trials except for the SPG3 and SPG4 conventional trials. Twist was the predominant type of distortion for boards failing to meet the standard (Table 10). Table 9. Distortion results after drying – C. citriodora Trial SPG1 SPG2 SPG3 SPG4 Table 10. Distortion grade results – C. citriodora Trial SPG1 SPG2 SPG3 SPG4 Surface and internal checking Distortion (mm) Spring 2.4 12.0 0.0 2.2 2.5 10.0 0.0 2.3 4.3 21.0 0.0 3.5 3.3 12.0 0.0 2.7 Distortion - % In grade Spring Bow NA NA 100 100 NA NA 100 97 NA NA 99 98 NA NA 99 93 Conventional Bow Twist Cup 3.5 1.7 0.0 10.0 8.0 0.0 0.0 0.0 0.0 2.7 1.8 0.0 5.4 3.3 0.2 16.0 11.0 2.0 0.0 0.0 0.0 3.3 2.5 0.5 5.0 5.1 0.8 21.0 35.0 6.0 0.0 0.0 0.0 3.9 5.4 0.9 4.8 2.8 0.3 17.0 15.0 7.0 0.0 0.0 0.0 3.9 3.2 1.0 Conventional Twist Cup Total NA NA NA 98 100 98 NA NA NA 97 97 93 NA NA NA 87 81 86 NA NA NA 84 97 83 Vacuum Spring Average 3.0 Maximum 10.0 Minimum 0.0 Standard deviation 1.9 Significance (p) 0.013 Bow Twist Cup 3.6 1.8 0.0 13.0 9.0 0.0 1.0 0.0 0.0 2.2 1.4 0.0 0.640 0.747 1.000 Average 1.9 Maximum 8.0 Minimum 0.0 Standard deviation 1.9 Significance (p) 0.024 5.4 2.0 15.0 8.0 0.0 0.0 2.6 2.0 0.961 0.001 0.0 0.0 0.0 0.0 < 0.0001 Average 2.2 Maximum 12.0 Minimum 0.0 Standard deviation 1.8 Significance (p) < 0.0001 2.2 10.0 0.0 1.9 < 0.0001 1.5 0.8 8.0 2.0 0.0 0.0 1.4 0.7 < 0.0001 0.878 Average 2.7 Maximum 11.0 Minimum 0.0 Standard deviation 2.6 Significance (p) 0.049 5.2 1.3 0.4 16.0 11.0 3.0 0.0 0.0 0.0 2.8 2.0 0.7 0.146 < 0.0001 0.171 Vacuum Spring Bow Twist Cup Total Before drying 100 100 100 100 100 After drying 100 99 99 100 99 Before drying 100 99 95 100 94 After drying 100 98 96 100 94 Before drying 100 99 100 100 99 After drying 100 100 98 90 98 Before drying 100 95 100 100 95 After drying 100 98 97 91 96 Significant differences do not exist between the surface checking populations of the SPG1 and SPG3 conventional and vacuum trials (Table 11). For every vacuum drying trial and the SPG3 conventional trial, an acceptable (> 90%) number of boards achieved select grade for surface checking in accordance with AS 2796:1999 (Table 12). For the SPG2 conventional trial only 8% of boards achieved this grade. This result may be due to this trial being pre-air dried in an uncontrolled environment prior to final kiln drying. The other conventional trials were kiln dried from the green condition. In most cases, we achieved better surface checking grade recovery through vacuum drying than conventional drying. 50 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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