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T able 18. V acuum Trial MC change points (%) Heat-up Green - 70 70-60 60-50 50-40 40-30 30-25 25-20 20-15 15 - final Equalise (48 hrs) drying schedules - Eucalyptus marginata JAR4 WBD (°C) 2.0 3.0 3.0 3.5 4.2 5.4 7.7 9.4 14.0 13.8 4.0 JAR1 JAR2 JAR3 DBT (°C) WBD (°C) EMC (%) DBT (°C) WBD (°C) EMC (%) DBT (°C) WBD (°C) EMC (%) 44 44 44 45 45 (54) 45 (59) 48 (64) 53 (69) 60 (73) 67 (76) 2.0 18.0 3.0 16.0 3.5 15.0 4.0 14.0 4.8 13.0 5.4 12.0 7.7 (6.5) 9.4 (9.0) 9.0 14 (13.0) 13.8 (14.0) 6.9 10 (11.0) 6.7 (7.0) 44 2.1 18.0 44 2.8 16.0 44 2.8 16.0 45 3.6 14.5 48 4.8 13.0 53 5.6 12.0 58 7.1 10.5 63 9.3 9.0 67 13.0 6.9 70 13.0 6.9 44 2.0 18.0 44 3.0 16.0 44 3.0 16.0 45 3.5 14.5 45 4.2 13.5 45 5.4 12.0 48 7.7 10.0 53 9.4 9.0 60 14.0 6.7 67 13.8 6.9 70 4.8 11.0 70 4.8 12.0 70 4.0 13.0 DBT (°C) 44 44 44 45 45 45 48 53 60 67 70 EMC (%) 18.0 16.0 16.0 14.5 13.5 12.0 10.0 9.0 6.7 6.9 (72 hrs) 13 Moisture content The initial MC results are shown (Table 19). For each trial, initial MC populations were not normally distributed, likely because of high MC outliers (Figure 22). Multiple pairwise comparisons produced no significant differences between sample populations of any trial. 120 JAR1 110 100 90 80 70 60 50 40 JAR2 JAR3 JAR4 Table 19. Initial MC results – E. marginata Trial Average Maximum Minimum Standard deviation Initial MC (%) JAR1 JAR2 72.0 73.5 105.3 95.9 51.5 55.4 10.7 8.2 JAR3 JAR4 74.2 72.5 100.4 110.3 56.7 59.6 8.7 9.3 Figure 22. Scattergram of initial MC – E. marginata For each trial, we recorded significant differences between the final cross-sectional MC population means of end-matched conventional and vacuum trials (Figure 23 and Table 20). In accordance with both AS/NZS 4787:2001 and AS 2796:1999 trials JAR1 vacuum, JAR2 conventional and both JAR4 trials did not achieve acceptable grade quality in terms of average final cross-sectional MC. All other trials achieved acceptable grade quality. By observing the scattergrams and output results it can be surmised that the JAR2 conventional trial did not meet acceptable standards for final cross- sectional MC due to under-drying (average 14.4), and the JAR1 vacuum trial did not meet grade due to a large proportion of ‘wet’ boards occurring after drying. The ‘wet boards are evident in Figure 23 as the top thin section of the vacuum scattergram (i.e. boards vastly wetter than the average). The most likely cause of this phenomenon is due to ‘tension set’ caused by too harsh drying during early drying periods. This well known phenomena is commonly known as ‘case hardening’ (McMillen, 1963). It can be observed that by reducing the DBT and increasing the WBD between subsequent trials the number of ‘wet’ boards is reduced (and standard deviation) so that for the JAR3 vacuum trial the final cross-sectional MC spread is more uniformly distributed practically eliminating the under-dried boards. 58 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report Initial MC(%)PDF Image | Evaluation of super-heated steam vacuum drying
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