Drying Schedules
Standard Schedules for Conventional Kilns
Experience has shown that satisfactory kiln drying can usually be best
accomplished by gradually raising the temperature and lowering the relative
humidity of the circulating air as drying proceeds. After warming the load, the
normal procedure is to monitor the fall in average moisture content of the load
and to make changes to the temperature and humidity of the inlet air when the
wettest sample reaches a prescribed moisture content.
A progressive sequence of kiln temperatures and relative humidities, together
with corresponding timber moisture contents, is referred to as a drying
schedule. Because the drying properties of different timber species vary
considerably (Appendix A), it is necessary to have a
number of different schedules. On the basis of known drying characteristics,
twelve schedules have been formulated to provide a suitable range of drying
conditions. From the results of test drying, each timber species examined has
been assigned in Appendix A to that schedule which will normally achieve the
maximum rate of drying with an acceptably low level of degrade.
The standard schedules have been formulated for use with timber of random
width and thicknesses of up to 38mm. However, for convenience, wet bulb
temperature modifications are provided with each schedule which will provide
suitable higher humidity drying conditions for thicker timbers. The schedules
are based on the assumption that drying is carried out in a forced-draught type
of kiln, with an average air speed through the load of 1 to 1.5m/s.
In general, these schedules are comparatively mild and can usually be relied
upon to give satisfactory results. They should be used as they stand unless
there are special considerations which make it clear that some modification is
necessary, or until experience indicates that modifications can be made with
advantage. Normally the schedule is followed until monitoring reveals that the
moisture content is suitably low to apply a conditioning treatment. After
conditioning the kiln is allowed to cool and the dried timber unloaded.
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