- Modifications Before Drying Begins
Air Speed
Air speed, like temperature and relative humidity, has a significant effect
on the behaviour of wood during kiln drying. The standard schedules are
formulated for use with a kiln air flow rate of between 1 and 1.5m/s. If flow
rates are significantly smaller or greater than this assumed value there may
be a need to consider modifying the schedule. Under slower air speeds the
timber is exposed to milder drying conditions and the relative humidity
(recommended in the schedule) can be lowered, which will reduce the overall
drying time without any increase in the risk of drying degrade. For example if
the air speed is 0.6m/s or less the relative humidity can be lowered by
between 5 and 10%. In modern kilns employing very fast air circulation, say 2
to 3m/s, there may be an increased risk of damage with timber species which
are susceptible to degrade, and with these it is recommended that kiln
humidities are increased slightly above those provided in the schedules.
Thickness
As might be expected, the thicker the timber the more slowly moisture is
removed. To minimise degrade, drying rates at the surfaces must not greatly
exceed the movement of moisture from the centre and, generally, slower drying
is necessary for thicker material if degrade such as checking, honeycombing
and collapse is to be avoided.
The drying schedules in Appendix B are intended
for use with timber up to 38mm thick, but they include wet bulb modifications
which will give suitable increases in relative humidity for thicknesses of
38mm or more. Unless experience with a species indicates otherwise, relative
humidities throughout the schedule must be increased by 5% for material
between 38mm and 75mm thick and an increase of 10% relative humidity is
normally necessary when drying timber over 75mm thick. The recommended
increases in relative humidity are simple additions. For example, a schedule
condition of 60°C dry bulb/30% relative humidity for material up to 38mm
thick becomes 60°C/35% relative humidity for material over 75mm thick.
Width
For any given thickness, a reduction in width will normally result in more
rapid drying characteristics and a reduced tendency to split and check.
Therefore narrow pieces or squares can normally be dried at lower relative
humidities and faster drying times can be achieved.
Strength
As indicated, high temperature drying can reduce the strength properties of
timber and where strength is of particular importance, the kiln temperature
should not exceed about 60°C.
It has also been found that timber which has been impregnated with some
types of water-borne flame retardant salts suffers a loss in strength when
kiln-dried at high temperatures. Therefore it is important that such treated
timber, if intended for structural use, should be re-dried at low or moderate
temperatures in order to keep strength losses to a minimum (see Appendix
F).
Resin Exudation
The resins and gums contained in some species are liable to exude on to the
surface of the timber if drying is conducted at temperatures much above 50°C.
In some instances this effect is considered beneficial because the resin
removed during drying cannot exude later when the timber is in use and
therefore drying is carried out at the highest temperature which can be
tolerated by the timber. When the appearance of the timber is important and
resin exudation would be unacceptable, the kiln temperature should be limited
to 50°C and a slower rate of drying accepted.
Colour
Many timbers tend to darken in colour when dried at high temperatures and
this effect is greatest when high humidities are also employed during the
early stages of drying. Where it is important to retain the natural light
colour of the wood, it is probably best to maintain a temperature of about
35°C throughout and to commence the drying at a relative humidity 10 to 15%
lower than would normally be used, even though this may cause some checking to
occur on plain-sawn surfaces. It should be noted that the lower operating
temperature will normally extend drying times.
Air Dried Timber
For a particular average moisture content, timber which has been air-dried
will have a comparatively higher surface moisture content and a shallower
moisture gradient than similar material which has been dried from green in a
kiln. Because drying schedule conditions are based on the behaviour of timber
which has been wholly dried in a kiln, the conditions recommended for a
particular average moisture content may not be suitable for air dried timber.
Therefore in the kilning of timber which has already been partially air-dried
it is recommended that the initial schedule conditions are at least two stages
less severe than is indicated by the average moisture content of the wetter
pieces in the load. These conditions need only be maintained long enough to
dry off the surface layers, the period varying roughly from half a day to four
days depending on the species and thickness being dried. The appropriate
schedule conditions should then be established by gradual stages.
Mixed Loads
As mentioned, it is generally not advisable to mix different species and
thicknesses in the same load, but when such mixing is unavoidable, the
schedule used should be suitable for the species or thickness which requires
the mildest treatment.
Initial and final quality of timber
Lower quality timber with irregular grain, shakes or other growth defects
is more liable to split and distort, and to minimise degrade it may require
more careful and slower drying than timber of higher quality.
For some uses the final quality of the dried timber is not important and if
checks and distortion can be tolerated in the material, drying can be carried
out at a faster rate at lower relative humidities and higher temperatures.
- Modifications During Drying
It may not always be appropriate to adhere strictly to a drying schedule.
If observations reveal that an unacceptable level of degrade is developing
under certain schedule conditions, the relative humidity should be raised
immediately or, at least, a delay introduced before implementing the next
stage of the schedule. Either of these measures should help to reduce the
further development of degrade and provides an opportunity to check that the
kiln is operating correctly and to select less severe drying conditions if
necessary. If the timber is distorting unduly or there are signs of collapse
developing, high temperature conditions in the later stages of a schedule may
need to be replaced by milder conditions.
The influences that kiln conditions have over the development of the
various categories of degrade are listed in Appendix C.
- Modifications Based on Experience
The suggested use of only twelve standard schedules is a compromise and
clearly a schedule may not be equally well suited for all the timber species
which have been assigned to it. For example a few timbers will dry acceptably
at relative humidities below 70% (the lowest initial relative humidity
recommended in the standard schedules). On the basis of experience it may
therefore be possible to implement lower initial relative humidities and
achieve faster overall drying times with these species. Similarly when drying
large quantities of the same species, it may often be possible to modify the
recommended kiln schedule to reduce drying times if, after kilning the first
few loads, degrade is found to be well within acceptable limits: initially the
temperature could be raised by 5% or the relative humidity reduced by 5%
throughout the schedule. If results are still satisfactory, further
modifications of a similar order can be made until the optimum treatment has
been evolved.
After long experience with the same type of load, it is sometimes possible
to apply a schedule on a time basis; the changes in temperature and humidity
being made and the treatment terminated at prescribed times after the
commencement of drying. The advantage with this approach is that the periodic
weighing of samples is no longer necessary. Nevertheless, because timber is a
variable material, it is advisable to monitor for degrade during drying, and
to carry out normal estimates of final moisture content before unstacking
begins.
MTC’s
