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4.1.2 Eucalyptus marginata - jarrah Drying schedules The vacuum kiln used for the study was supplied with a standard software package that includes a number of pre-set schedules for various species (predominantly American and European species) of varying thickness. Only two pre-set schedules pertaining to native Australian eucalypts exist (Eucalyptus regnans and Eucalyptus globulus). The drying properties of these ‘ash’ type species are very different from those for E. marginata. For this reason, we judged existing vacuum drying schedules for Australian species unsuitable. The schedules used for each trial are shown (Table 18) including for each MC change point, the dry bulb temperature (DBT), wet bulb depression (WBD) and equilibrium moisture content (EMC). The initial schedule chosen for vacuum drying trial JAR1 was the preset schedule provided for Quercus alba (white oak) and was chosen on the understanding (supported by Simpson and Verill, 1997) that in general, timbers of similar densities often have similar drying characteristics. The average dried density (12% MC) of mature Q. alba is 760 kg/m3 (Lincoln 1991), which is similar to 820 kg/m3 reported for mature E. marginata (http://www.dpi.qld.gov.au, 2008). Although presented as a conventional stepwise schedule, the dry and wet bulb temperatures were actually ramped between MC change points. A 48 hour equalisation phase was performed at the end of drying at an EMC of 11%. After approximately 11.5 days of drying, the average MC of the test boards had only fallen to 43% and we decided that the drying rate had become too slow. We changed the schedule to the values shown in brackets (Table 18) in association with Brunner-Hildebrand, the kiln manufacturer. We developed the JAR2 vacuum drying schedule to reduce the unacceptable amount of surface checking resulting from the JAR1 vacuum trial. Changes to the schedule include higher EMCs from green to 50% MC, to slow down drying during the early surface check prone drying stage, and lower dry-bulb temperature thereafter and a higher final equilibrium MC (12% compared with 11%). Similarly for the JAR3 vacuum trial the EMC was further increased during the early stages of drying up to 40% MC due to undesirable checking results from the JAR2 trial. Lower DBTs during the latter stages of drying and we employed a further increased equalisation EMC (13%) to reduce MC gradients and associated drying stresses after drying. For the JAR4 vacuum trial we chose the same schedule used for the JAR3 trial except the equalisation time was increased from 48 to 72 hours. We made this change in an attempt to alleviate drying stress, which was not within acceptable limits for the JAR3 vacuum trial. As is generally the case when drying Australian hardwoods, the material was dried 1-2% below the target average MC and then re-wet during the equalisation phase to reach the target MC to minimise MC gradients and associated residual drying stresses. The fact that re-wetting timber (sorption) leads to lower values of MC at the same EMC than drying timber (desorption) is known as sorption hysteresis (Siau, 1984). For this reason, we increased the equalisation EMC for the JAR2 and JAR3 trials by 1% MC between subsequent trials to bring the final average MC closer to the target 11% and reduce unacceptable stress levels encountered in the first two trials. Gunns Timber (Manjimup, Western Australia) supplied material for all four trials. Commercial confidentiality prevents us from discussing details of their conventional drying schedules. 57 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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