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4.1.3 Eucalyptus pilularis – blackbutt 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. pilularis. For this reason, we judged existing vacuum drying schedules for Australian species unsuitable. The schedules used for each trial are shown (Table 32) 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 BBT1 was the preset schedule provided for Quercus rubra (red 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. rubra is 890 kg/m3 (Lincoln 1991), which is similar to 930 kg/m3 reported for mature E. pilularis (http://www.fpc.wa.gov.au, 2009). Also, Q. rubra and E. pilularis are similarly constrained by surface checking as the primary drying defect. 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 11% EMC. We developed the BBT2 vacuum drying schedule to reduce the unacceptable amount of surface checking resulting from the BBT1 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. Additionally, we used a higher final equilibrium MC (12% compared with 11%) to improve residual drying stress resulting from the first vacuum trial. Similarly, for the BBT3 and BBT4 vacuum trials, we increased the EMC further during the early stages of drying up to 30% MC due to undesirable checking resulting from the respective BBT2 and BBT3 vacuum trials. Hurford Hardwood sawmill (Casino, New South Wales) supplied material for conventional trial BBT1 and Boral Timber (Murwillumbah) for conventional trials BBT2 and BBT3. Due to time constraints, conventional trial BBT4 did not occur. The conventionally dried end–matched boards of trial BBT1 were air dried from green to approximately 15% MC before final kiln drying. Conventional trials BBT2 and BBT3 were kiln dried from the green condition. Commercial confidentiality prevents details of the conventional drying schedule used by Hurford Hardwood and Boral Timber from being discussed. 68 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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