Abstract:
Variability in wood properties causes boards within a stack of lumber to
dry at different rates and reach different moisture content levels after drying is
completed. The consequence is that the boards will have properties that differ from
ones that were intended to be achieved by the drying process.
The impact of basic density, initial moisture content, percentage of
heartwood, position of heartwood within a board, ring count, distance from the
pith, growth ring angle, chemical composition of wood volatile compounds,
earlywood and latewood coverage of board faces on the drying behavior was
investigated for western hemlock (Tsuga heterophylla) lumber. Board weights
were monitored during drying at 80°C and 115°C to obtain the drying rate.
Mathematical relations for the effect of wood properties and board geometric
features related to the position within its parental log were developed to predict the
drying rate at four moisture content levels. These relations were used for
predicting the drying times of individual boards which were used as a criterion for
sorting green boards prior to drying.
Basic density, initial moisture content, heartwood percentage, and growth
ring angle were found to affect drying rate at 80°C and initial moisture content
only at 115°C. The calculated final moisture content distributions for nonpresorted
and presorted boards using the results from low temperature drying
indicated that presorting reduced the standard deviation of the final moisture
content distribution by 65%. The composition of head space volatiles appeared to
be predictive of the final moisture reached by boards and with further development
might be useful for presorting.
