- Preliminary Procedures
Any pre-start maintenance procedures recommended by the kiln manufacturer
should be followed. The normal procedures associated with examining and piling
the timber should be carried out and appropriate drying conditions selected.
When loaded in the kiln the correct alignment of any baffles against the
timber load should be checked and any gaps which might allow short circuiting
of air should be sealed. Kiln monitoring equipment should be checked and
calibrated if necessary and the kiln started up.
- Warming Up
Generally no harm will result from fairly rapid warming of the load to
the first stage temperature of the particular drying schedule which is
recommended for the timber species being dried. During warming, the
temperature of the wood will lag appreciably behind that of the air and
there will be some risk of condensation on the wood. To minimise this it is
recommended that a suitably low air relative humidity is maintained by
keeping the wet bulb temperature about 5°C below the dry bulb reading.
- Maintaining Humidity
Air conditions in conventional kilns are controlled by applying heat to
raise the temperature and by adjusting ventilation and moisture spray
equipment to control relative humidity. Traditionally these conditions were
controlled manually and considerable expertise was required to keep kiln
conditions in line with a particular drying schedule. In most modern kilns,
automatic air control systems are now standard equipment, often providing a
range of control functions. Semi-automatic operation (i.e. control of a set
temperature/relative humidity condition) may be the most appropriate approach
when drying a timber species for the first time or when drying a timber which
is known to have unpredictable drying properties. Some kilns are equipped for
fully automatic control although at present this approach is generally
considered to have its most successful application in the drying of timbers
which have predictable drying properties. Modern fully-automatic systems,
incorporating microprocessors and remote sensing equipment measuring weight or
moisture content of the load can control the complete drying operation from
warming up right through to the cooling of the dried load.
With dehumidifier kilns the air conditions are primarily controlled by
adjustment of the temperature setting, although some kilns may be fitted with
humidification and ventilation facilities.
In the setting of kiln air conditions it is important to appreciate that
the quality of drying is very dependent on the relative humidity of the air,
as indicated by the wet bulb depression. This is demonstrated in Appendix
G, Table G2, which shows how even small deviations from a required wet bulb
depression can have a significant effect on relative humidity. In practical
terms this means that small deviations from the schedule dry bulb temperature
have little effect on drying, provided the correct wet bulb depression is
maintained. For example if the schedule conditions were dry bulb 50°C/wet
bulb 47°C (giving a relative humidity of approximately 84%), drying could be
carried out equally satisfactorily if the actual kiln conditions settled at
dry bulb 52°C/wet bulb 49°C where the wet bulb depression of 3°C would
again result in a relative humidity close to 84%. If however the kiln
conditions settled at dry bulb 52°C/wet bulb 47°C, giving a wet bulb
depression of 5°C, the relative humidity would be only 75% and could be too
severe for satisfactory drying.
With automatic control systems and under frequent air flow reversal the
correct positioning of the sensing equipment is an important consideration.
Kiln drying schedules are based on the condition of the air as it enters the
load after re-heating and air sensing equipment for slow air speed kilns
should be installed to enable control of the air conditions at whichever side
the reheated air enters the load. This is perhaps not so critical with fast
air flow side fan kilns where the sensing equipment can be positioned on the
side opposite the fan: in this position the air is sensed half way along its
passage through the timber load, whichever way the air is circulating, and
differences between the air conditions at the inlet and sensing positions will
normally be small.
- Kiln Records
Many modern kiln control systems will automatically provide a record of the
air conditions throughout the drying operation and have largely eliminated the
need to make repetitive written records. However, monitoring of air conditions
should proceed hand in hand with monitoring of the timber to assess its
moisture content and the risk of degrade. Such information on the condition of
the timber is difficult to obtain automatically and normally this is monitored
by examining a series of withdrawable timber samples which are incorporated in
the load.
Inevitably this kind of monitoring requires a certain amount of manual
recording and it is recommended that record sheets are kept of the behaviour
of the timber during a kiln operation. These not only help to keep track of
events during the drying operation, but can also be valuable later,
particularly in terms of modifying schedules to advantage, assessing the
effectiveness of treatments which have been applied to correct drying degrade
or evaluating the performance of a kiln.
- Completion of Drying and Cooling
Towards the end of the drying schedule a reassessment of the condition of
the load is normally carried out. The purpose of this is to recalculate and
check the moisture content of the load and to ascertain whether a
casehardening relief treatment is necessary before either applying a
conditioning treatment or allowing the kiln to cool. A few timber species can
be withdrawn from the kiln immediately after completion of a schedule or
conditioning treatment. However, generally this practice is not recommended,
particularly when high kiln temperatures have been used; the cool outside air
heats up and becomes drier as it passes over the hot timber and this may renew
casehardening stresses.
During cooling, the relative humidity should be kept as high as possible
and a wet bulb depression of not more than 5°C is recommended until the kiln
temperature is within 20°C of the outside temperature, at which point the
load can be removed from the kiln. Kilns may not always have facilities to
allow such accurate control of conditions during cooling and an alternative
method is to raise the humidity for a short time before cooling and then
switch off all kiln systems, allowing the kiln to cool without further
attention until, as before, it is within 20°C of the outside temperature.
This approach may result in a very slight increase in the moisture content of
the timber.
- Kiln Maintenance and Testing
A regular programme of day to day and long term preventive maintenance
should be worked into the operating timetable for each kiln. it is recommended
that the kiln and kiln equipment manufacturers should be consulted about the
maintenance programmes which are suitable for particular categories or models
of kiln. The high operating temperature and humidity conditions inside kilns
require that a number of fairly basic maintenance procedures are carried out
on a regular basis; many of these may not require the attention of a
specialist technician.
Normally it should not be necessary for a newly installed kiln to be
tested. However, older kilns may develop faults and often these can be
diagnosed by simple tests. Usually the first sign of a fault will be the
occurrence of poor or irregular drying which cannot be attributed to poor kiln
practice or low quality timber. Tests using a number of hygrometers positioned
strategically within the kiln can be carried out to evaluate whether
temperature/humidity conditions are suitably uniform within the kiln; lack of
uniformity may often be attributed to fairly straight-forward causes such as
poorly fitting doors/door seals or inadequate insulation.
Inadequate or non-uniform air flow rate within the kiln is also a common
cause of poor drying and velometers or hot wire anemometers can be used to
check flow rates over the timber at appropriate locations.
- Selection of Kilns
Capital and running costs are of prime importance when selecting kilns and
kiln equipment, but choice may be controlled by a number of other factors, in
particular the types of kiln load which will normally be dried. Also choice
may often be predetermined by existing site facilities; for example if spare
heating capacity is available, it may be best when expanding kilning capacity
to select new plant which can be incorporated into the existing heating
system. In general, kiln plant should be selected to be versatile and to
anticipate possible future changes in volume, species and thickness of the
timber to be dried.
Intending purchasers of kilning equipment are advised to seek the
recommendations of more than one kiln manufacturer, together with independent
advice if necessary. Obviously, satisfactory kiln installations depend upon
individual requirements and limitations. Nevertheless, if possible, the
following information should be available to assist selection.
MTC’s
