Effect of pre-drying schedule ramping on collapse recovery and internal checking with Victorian Ash eucalypts |
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Authors: | Philip Blakemore Timothy A G Langrish |
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Institution: | (1) CSIRO Forest Biosciences, Private Bag 10, Clayton South, Melbourne, VIC, 3169, Australia;(2) School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSW, 2006, Australia |
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Abstract: | Matched sample boards from 20 quarter-sawn boards of Victorian Ash (Eucalyptus regnans F. Muell and E. delegatensis R.T. Baker) were dried using three different levels of ramped pre-drying schedules to investigate the effects of moisture
gradients on collapse recovery and internal checking. Prior to reconditioning, most wet cores were found in highly collapsed
boards with low density. Reducing the gradients in these boards is crucial for recovering collapse and closing internal checks.
If time allows the boards to be equilibrated prior to steam reconditioning, a target mean moisture content of ≤20% with a
moisture gradient of close to 5% (core to surface moisture content) is likely to recover slightly more collapse than targeting
a mean moisture content close to ≥15%. However, if time or kiln restraints limit equilibration it is likely to be better to
target a percentage moisture content of closer to 15% in order to ensure that the core to surface moisture gradients are below
8–10%. The slight reduction in collapse recovery with this second approach is less important than the possibility that collapse
and internal checks in the centre of boards with wet cores will not be closed. Care needs to be used with this latter approach
not to over-dry some boards, since moisture contents below 15% will progressively reduce collapse recovery. For boards within
these moisture content guidelines, the application of heat, rather than moisture pick-up, appears to be the most important
component of the steaming reconditioning process. Hence, steaming only needs to be undertaken for long enough to heat the
core of the board close to the target temperature of 100°C. A simple method for estimating this heat-up time for different
thicknesses and species was demonstrated based on a key dimensionless group for heat transfer, the heat-transfer Fourier number. |
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