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4.1.4 Eucalyptus obliqua - messmate 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 schedules used for each trial are shown (Table 46) 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 MES1 was the preset schedule provided for E. globulus and was chosen because E. globulus and E. obliqua share the same recommended quartersawn conventional drying schedule (Rozsa and Mills, 1997). They also have similar drying defect limitations, particularly surface and internal checking, and collapse. 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 a 12% EMC. We developed the MES2 vacuum drying schedule to reduce the unacceptable amount of internal checking resulting from the MES1 vacuum trial. Changes to the schedule include higher initial relative humidities down to 30% MC. Due to favourable dried quality resulting from the MES2 vacuum trial, we increased the DBT during the latter stages of the MES3 schedule, from 30% MC to final dry, to increase the drying rate and improve overall drying time. Unfortunately, although instructed to completely wrap these boards block packed with impermeable plastic after sawing, the MES3 pack was presented unwrapped and exposed to the weather before tagging. The pack had been exposed for approximately 3-4 weeks during December/January; Tasmanian summer. As these boards were partially dried before testing, the results of this trial have been compromised and do not represent a complete drying trial and thus cannot directly be compared with other trials. For this reason, we repeated the MES3 vacuum drying schedule for the MES4 vacuum trial. Integrated Tree Cropping (ITC) Timber (Huonville, Tasmania) (now Gunns Timber) supplied material for conventional trials MES1 and MES2, and Gunns Timber (Launceston, Tasmania – sourced from Huonville) for conventional trial MES3. The conventionally dried end–matched boards of trial MES1 were air dried from green to approximately 15% MC before final kiln drying. Material for the conventional MES2 and MES3 were kiln dried from the green condition. Commercial confidentiality prevents us discussing the details of the conventional drying schedules used by these mills. 80 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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