- Air Drying
With air drying there is virtually no control of the temperature, relative
humidity or speed of the air passing through the timber stacks. The rate of
drying is therefore dependent on all the vagaries of the local climate and can
vary between practically zero on a calm, damp day to quite fast enough to
cause surface checking during dry, windy weather.
Because of the comparatively high humidity conditions in ASEAN countries,
air drying is a slow process. Times taken to reach 20-25% moisture content
vary from 2 or 3 months to 1 or 2 years, depending upon the species and size
of the timber. With air drying, wood cannot be dried below its equilibrium
moisture content and this will vary depending on the atmospheric conditions.
So, except in unusually hot and dry weather, the lowest moisture content
obtainable is around 16-17%; air drying alone is not sufficient for timber
intended for most interior uses in Europe, Japan or N. America where a
moisture content of between 8 and 12% is required. In air conditioned
buildings moisture contents of about 12% should be anticipated.
- Kiln Drying
In contrast to air drying a modern conventional drying kiln provides
temperature control and a steady and adequate flow of air over the timber
surface. The air flow rate and direction is controlled by fans and the
temperature and relative humidity of the air can be adjusted to suit the
species and sizes of timber being dried. It is thus possible to make full use
of the increase in drying rate which can be achieved by raising the
temperature to the maximum value which a particular timber species can
tolerate without excessive degrade. At the same time, the relative humidity
can be controlled so that the moisture gradients in the wood are not steep
enough to cause surface checking. The same principles apply to the use of heat
pump kilns except these recover and re-use a proportion of the energy which in
conventional kilns is lost during the drying process when the warm, damp air
is vented.
In addition to the advantages of more rapid drying and limitation of
degrade, the ability to control drying conditions in a kiln means that it is
possible to achieve timber moisture contents suitable for specific uses.
The direct costs of kiln drying are much higher than those of air drying
for they include the capital costs of the equipment and the cost of fuel,
electricity and supervision. These costs are partially or wholly offset by the
reduction in stock level.
- Air Drying Followed by Kiln Drying
Kiln drying tends to become uneconomical when the species and size of
timber being dried require long kilning times. Therefore, with material taking
more than about 4 or 5 weeks to kiln dry from green it will often be more
economical to air dry the timber to about 25-30% moisture content before
completing the drying in a kiln.
The economic advantage of this approach may be lost, however, if the layout
or lack of handling facilities necessitates dismantling the air dried stack
and repiling for the kiln drying phase. Also with some species, the amount of
splitting and checking which occurs during air drying in the dry season can be
excessive.
MTC’s
