Drying Kilns and Their Operation
A timber drying kiln may be defined simply as 'a closed structure designed or
adapted for the purpose of reducing the moisture content of timber and wood
based panel products'.
An efficient kiln will provide controlled heating, air circulation,
humidification and ventilation or dehumidification.
Heating is required to:
- Increase the rate of movement of moisture from the centre to the surface
of the wood.
- Increase the rate of evaporation of moisture from the surface of the wood.
Air circulation is required to:
- Ensure satisfactory heat transfer from the heat source to the air and from
the air to the timber surface.
- Mix the heated and humidified air so that it is distributed uniformly
throughout the timber load.
- Remove evaporated water from the wood surfaces.
- Aid removal of excess moisture from the kiln by means of ventilation.
Ventilation of the kiln chamber is needed to keep the kiln relative
humidity down to the required levels when large quantities of moisture are being
evaporated from the timber.
Dehumidification is an alternative means of removing surplus moisture
from the kilns.
Humidification is required to maintain the kiln humidity at the
desired level when the moisture coming from the wood is insufficient. It is
usually needed during the warm-up phase and towards the end of drying,
particularly when applying final high humidity treatments.
Heating
There are two basic ways of supplying the heat required for drying. The heat
source may be direct or indirect.
Indirect Heat Sources
- Steam. Steam is produced by a boiler and fed under pressure
through heating pipes and heat exchangers or radiators within the kiln.
The boiler may be fired by oil, gas, coal, or wood waste, or a combination
of these fuels. Electricity is seldom used to produce steam for heating
except in very small kilns.
- Hot water. This is produced and circulated in the manner
described for steam.
- Thermal liquid. A suitable liquid, such as mineral oil, is heated
in special equipment and circulated at atmospheric pressure through heat
exchangers in the kiln.
- Burners. Burners, usually gas or oil fired, transfer heat to the
kiln air by means of heat exchangers. This type of equipment is usually
individually fitted to each kiln.
- Electricity. Some kilns may be heated by electric heaters, but
the method is generally far too expensive compared with the alternatives
listed.
Direct Heat Sources
With direct heating the air of the kiln is ducted through a controlled gas or
oil flame in a burner. The kiln air will therefore contain the products of
combustion. However, direct heating is claimed to be more energy efficient
than indirect heating where a proportion of heat is lost at source.
When drying timber in a conventional kiln, energy is required for the
following processes:
- To elevate the wood and the water it contains to the kiln operation
temperature ,(the sensible heat requirement).
- To increase the temperature of the kiln structure to the operation
temperature.
- To maintain elevated kiln operating temperature by reheating the
circulating air and by warming the inlet air which is drawn in to replace
vented moist air.
- To overcome the binding energy of the bound moisture within the wood.
- To provide the latent heat of evaporation (the latent heat requirement).
- To compensate for conduction, radiation and convection losses from the
kiln structure.
- To provide the kiln humidification.
- To power the air circulation fans.
Therefore the total amount of energy required for drying will be clearly
much greater than that which is needed simply to evaporate a quantity of water
from the timber. However, with conventional kilns, significant reductions in
expenditure on energy may often be possible. For example many kilns were
constructed when fuel costs were lower, and energy cost savings may often be
possible by simply upgrading the insulation of the kiln shell. Significant
heat losses may also occur through air leaks, frequently caused by poorly
fitting main doors, and periodic checks for (and elimination of) any leakage
may be worthwhile. The accurate control of kiln conditions, in particular the
operation of vents, is another important consideration and the installation
and careful maintenance of automatic control mechanisms may be a cost
effective way of reducing heating costs.
The increases in fossil fuel prices in the 1970s stimulated the evaluation
of kiln types and equipment designed to reduce running costs. One of the major
advances in this area has been the development of heat pump kilns. These
operate on the same basic principles as conventional kilns but instead of
venting the warm moist air, the incorporated heat pump recovers a proportion
of the latent heat contained in the water vapour which is present in the
circulating kiln air and from the outside air.
Solar-heated kilns represent another potentially important development.
These have been designed to operate on the glass house and on the solar
collector/heat exchanger principles, and eliminate the need for expenditure on
energy except for that required to run the fans and provide humidification.
Although their development is only at an early stage, these appear to be
effective in parts of the world which receive high levels of solar radiation.
Kilns can be divided into two broad categories - progressive
(continuous process) and compartment (batch process).
In a progressive kiln the timber is loaded on bogies and is moved at
intervals through a long chamber in which it is subjected to hotter and drier
air conditions as it progresses towards the exit. If properly designed and
used these can provide conditions appropriate for good drying.
In a compartment kiln the timber remains stationary and the temperature and
humidity of the circulated air are altered in a set sequence (a schedule)
which has been selected to suit the material being dried.
For the same total dried wood output, a large progressive kiln is cheaper
to install and operate than a battery of compartment kilns. However,
approximately uniform final moisture contents can only be attained when a
continuous supply of timber is available with closely similar drying
characteristics and not varying too widely in initial moisture content. Since
such a supply will very seldom occur in the ASEAN countries these are not
considered further in this guide.
Compartment kilns, on the other hand, are more flexible because a wide
variety of loads can be accepted and each given an appropriate drying
treatment.
- Conventional Compartment Kilns
Development of Kiln Design
In the earliest kilns the timber was open stacked over a basement
containing some form of heating, for example steam pipes or flue pipes from a
wood burning furnace. The only movement of air was that caused by the hot air
producing a natural upward draught augmented by the chimney effect of vents in
the roof. Inevitably the air circulation was very slow and irregular air
speeds seldom exceeded about 0.15m/s through the load.
These natural draught kilns were followed by various designs of forced air
circulation kilns in which the air was heated and humidified outside the
drying chamber. The air was then delivered through a system of ducting into
one side, passed through the timber pile and then pulled back into an external
fan. In these kilns an average air speed of up to about 0.5m/s was obtainable,
but difficulty was encountered in achieving satisfactory longitudinal
uniformity of air flow through timber of commercial lengths. Also, without an
additional system of ducting, reversal of the air flow through the load was
not possible.
It was found that an adequate and sufficiently uniform distribution of air
through the whole load could best be obtained by situating fans within the
kiln itself. All modern compartment kilns are of this forced draught internal
fan type and in nearly all of these the internal fan (or fans) have reversible
pitch, allowing frequent reversal of the air flow through the kiln load.
Modern internal fan kilns may conveniently be classified as either overhead or
side fan types and the commonest designs of these are described briefly in the
following sections.
Overhead Fan Kilns
- Longitudinal Shaft Design.
In two designs of overhead fan kiln the fans are mounted at regular
intervals on a longitudinal drive shaft. The air is diverted by baffle boxes
to flow across the top of the kiln above a false ceiling, down the side and
through the load as in the designs shown below ( Figure 3 ) sometimes called
cross circulation kilns.
In design A, in spite of correcting plate baffles fixed as indicated,
there is a tendency for the circulation to be stronger at the end towards
which the fans are blowing. This effect is eliminated in design B in which
left and right hand fans are fitted alternately on the shaft, although here
a slight loss in efficiency of the fans will occur because opposing pairs
will set up back pressure.
 Figure 3 Fan
arrangement in three designs of overhead fan kiln
In these longitudinal shaft kilns, air speeds through the load average only
from 0.5 to 1 m/s, unless fans larger than the usual 0.8 to 1.Om diameter
are used. In America kilns are built to design B using fans up to 1.8m in
diameter.
- Cross Shaft Design.
In the cross shaft overhead fan kiln, a commonly used design, the fans are
mounted on short shafts aligned across the kiln as shown in Figure
6. The air is delivered primarily in the required direction and, apart
from the vertical bulkhead around the fans which prevents the air from short
circuiting, no deflecting boxes and baffles are required. In this design the
difficulties of alignment and maintenance of long drive shafts are avoided
but provision of a suitable fan drive mechanism may need careful
consideration when cross shaft kilns are built side by side in large
batteries.
The fans used in cross shaft kilns range in diameter from 0.8 to 1 .8m
and average air speeds through the load vary from about 0.9 to 1.8m/s with
satisfactory uniformity along the length.
Double load units of the overhead cross shaft kiln can be built with the
left and right hand fans being fitted above each load on a common shaft.
With these the cost of drying per unit volume of timber will be somewhat
less than for two single units.
Side Fan Kilns
There are two distinctive types of side fan kiln - the vertical flow
(Figure 4, below) and the horizontal flow (Figure 4, design E, below).
In both, large propeller type fans are placed to one side of the timber load.
With side fan designs it is possible to take advantage of the greater energy
efficiency of the slower running, larger fans without the need for increased
kiln height which would be necessary if these were used in an overhead fan
design.
In design D1 the air is delivered or returned through a duct which passes
air above the timber load and no transverse baffles are required. In a
variation of this design (D2) smaller fans are mounted to one side of the
lower half of the timber load and the air return not through a duct but
through the upper half of the load.
 Figure 4 Fan arrangement
in three designs of side fan kiln
In design E, which is a type commonly installed in the UK and in some European
countries, the air flow is horizontal throughout. This is achieved by filling
the kiln to its full height and, with the wing baffles forming a fan box, air
is pulled or pushed (according to the direction of fan rotation) through the
portion of the load opposite the fan and pushed or pulled through the two end
portions.
In designs D2 and E, the air has to pass through the load twice before it
is reheated (unless booster heating is placed in a position remote from the
fans) and fast air speeds are necessary to minimise the difference in drying
rate across the width. Average speeds of the order of 1.5 to 2.4m/s are
obtained without excessive power consumption by the use of large fans 1.4-2.4m
in diameter. The air speeds through various parts of the timber load are not
usually as uniform as in the overhead cross shaft type of kiln but the lowest
speed within the load is sufficient for satisfactory drying. It is now normal
to reverse the direction of air flow frequently and this compensates for local
variations in air speed.
Side fan kilns can also be built as double load units, the one large fan
pulling air through the load on one side of it and pushing the air through
that on the other.
Comparison between overhead and side fan kilns
In the selection of either overhead or side fan kilns a number of factors
should be considered:
Advantages of side fan kilns
- Large fan is accommodated in a moderate height of kiln.
- High average air flow is attained at reasonable power cost.
- Air flow is not affected much by irregularities in pile face.
- Fans and heating equipment are supported on the ground so that the kiln
shell requires less strength.
- Fans and pipes are more easily accessible for maintenance.
Disadvantages of side fan kilns
- The length of air travel through the timber load is usually greater.
- The variation in air speed through the load from one part to another is
greater.
- The kiln takes up more width for equivalent capacity.
Construction of Kiln Shell
Previously nearly all timber drying kilns were of brick and/or concrete
construction. Prefabricated metal kilns were then introduced and, although at
first more costly, they have now become competitive in price and an increasing
number have been installed.
Most overhead fan kilns are brick built as the shell has to support the
fans and sometimes the heating pipes. Side fan kilns are more usually made of
prefabricated, insulated aluminium panels. When aluminium kilns are well
designed and manufactured from high purity metal they should withstand
standard kiln conditions with the minimum of maintenance.
Aluminium kilns, being light in weight, do not need such deep and expensive
foundations as brick kilns; a major advantage if the only site available on
in-filled land. They can be erected quickly and can be dismantled for
installation elsewhere.
When plant is used for short drying runs on timbers which tolerate high
temperatures, metal kilns are more durable and can be warmed up more rapidly
than those of brick and concrete construction.
A kiln fitted with a heat pump unit dries timber in basically the same way as
a conventional kiln: heated air is the drying medium and this is circulated by
fan through the timber, which is piled in the normal way. However, with heat
pump kilns the warm moist air which is formed during the process of drying is
not directly vented to the outside. Instead the circulating air is passed
through the heat pump unit. Here the latent heat contained in the air is
largely recovered; the water vapour it contains condenses on the cold coil and
the heat energy is transferred back into the air flow at the hot coil.
Therefore, in correctly designed, well insulated heat pump kilns very little
heat energy should be lost to the outside and potentially the energy
efficiency is higher than that of conventional kilns.
Fresh air is introduced as required and the heat it contains is extracted
by the heat pump and introduced into the kiln.
The term kiln instrumention is taken here to encompass the equipment which is
installed to provide information on the temperature and relative humidity
within a kiln as well as any equipment which is used to control these air
conditions. The ancillary equipment which is required for measuring the
moisture content of wood has been mentioned in The Theory and Practice of
Drying.
The traditional and simplest instrument for measuring air conditions is the
wet and dry bulb hygrometer (Figure 5, below). The dry bulb thermometer
measures the actual temperature in the kiln while the wet bulb reading enables
the relative humidity of the air to be estimated. The bulb of the wet bulb
thermometer is surrounded by a sleeve which is kept moist with distilled water
from a reservoir. Evaporation from this sleeve cools the wet bulb below the
temperature of the dry bulb and the magnitude of this wet bulb depression is
related to the relative humidity of the air.
This relationship which is presented in Appendix G,
Table G2, enables the relative humidity to be estimated from the wet and
dry bulb readings. This method used to be the standard way of determining kiln
air conditions and later the wet and dry bulb principle was employed in the
development of recording and semi-automatic control equipment. Because some
kiln installations still rely on methods involving the use of wet and dry
bulbs the standard kiln schedules (see Appendix B)
and discussions concerning the control of kiln conditions still make reference
to wet and dry bulb temperatures. It is recognised, however, that many kiln
air monitoring and control systems now incorporate humidistat which allow
direct control of relative humidity.
 Figure 5 Wet and
dry-bulb hygrometer
In these cases the references to wet bulb temperature are essentially
redundant although they may have a use if, for example, wet and dry bulb
hygrometers are used as a way of checking conditions within a kiln.
Microprocessor and computer technology has already assumed a major role in
the development of control systems for kiln drying. Most computer based
systems can be programmed to operate the kiln according to a predetermined
schedule, while more advanced systems are able to control a sequence of kiln
conditions on the basis of the moisture content of the timber, which is
monitored by remote sensors in the load. There is still technical difficulty
in measuring accurately moisture contents above about 30% and at present fully
automatic control systems require a greater margin of safety above this level
and consequently drying times may be slightly longer.
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