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4 Results 4.1 Applied drying – Experimental drying trials The results of the vacuum and conventional drying trials are presented in this section. The results of each species are reported separately. 4.1.1 Corymbia citriodora – spotted gum Drying schedules The vacuum kiln used for the study incorporates 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 (E. regnans and E. globulus). The drying properties of these ash type species are very different from those for C. citriodora. For this reason, we judged existing vacuum drying schedules for Australian species unsuitable. The schedules used for each trial are shown (Table 4) including for each MC change point, the dry bulb temperature (DBT), wet bulb depression (WBD) and equilibrium moisture content (EMC). The schedule chosen for vacuum drying trial SPG1 was the same schedule reported by Redman (2006) to have successfully vacuum dried native forest Corymbia citriodora (spotted gum) during commissioning of the vacuum kiln. This schedule was a preset schedule provided for Hymenaea courbaril (courbaril) and we chose this 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 H. courbaril is 910 kg/m3 (Lincoln 1991), which is similar to 1010 kg/m3 reported for mature C. citriodora (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. We performed a 48-hour equalisation phase at the end of drying at an EMC of 12%. For the second C. citriodora vacuum drying trial (SPG2), we repeated the schedule used for the SPG1 vacuum drying trial as, although we achieved acceptable dried quality results, the drying rate for the first trial was compromised (slowed) due to a kiln vacuum leak. Therefore, it was deemed necessary to repeat the trial. Due to favourable dried quality resulting from the first two trials, we increased the DBT throughout for the SPG3 trial with the intention of reducing the drying time. 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 SPG3 trial from 12% to 13%, to bring the final average MC closer to the target 11% and reduce unacceptable stress levels encountered in the first two trials. For the SPG4 vacuum trial we chose the same schedule used for the SPG3 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 SPG3 vacuum trial. 46 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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