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Introduction Chapter 1
1
CHAPTER 1: INTRODUCTION
1.1 BACKGROUND INFORMATION ON WOOD STRUCTURE ANDDRYING
In the timber industry, the drying of wood is the first, and possibly the most
important, process in downstream manufacturing after the sawing of logs. To
understand important issues in wood drying, it is necessary to describe some
information about the structure of the wood. Several textbooks have covered these
aspects in great detail (Kollmann and Cote, 1968; Panshin and de Zeeuw, 1970;
Walker et al., 1993; Bootle, 1994; Desch and Dinwoodie, 1996; Keey et al., 2000).
A very brief summary about wood structure is presented here first, particularly
focusing on the structure of hardwoods. Some information is presented on the
anatomical structure of blackbutt timber, because this thesis reports some research
findings on the drying of this hardwood. Since drying involves the removal of water,
wood-water relations are described in the second section. Thirdly, general aspects of
wood drying are covered, including the various drying methods currently available.
The mechanisms of moisture movement, driving forces for moisture movement, and
the directions for moisture movement are also discussed. The effects of significant
variables, i.e. temperature, relative humidity, air circulation, on wood drying are
discussed. Various types of degrade common in wood drying are reviewed. The
reasons for drying degrade are also explained in this section. The next section
discusses the experience gained, and observations made, during industrial visits.
Thereafter the issues relevant to the present study and the scope of the current
research are described. Finally the structure of this thesis is outlined, with a
description of the contributions of this thesis to the field of wood drying.
Introduction Chapter 1
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1.1.1 Wood Structure
Wood is a porous substance composed of a large number of very small elements or
cells, the cavities of which are largely occupied by air. Wood is not a solid and
homogeneous substance like a piece of metal. Commercial timbers are broadly
classified into two categories, namely softwoods and hardwoods. This classification is
not based on softness or hardness (balsa (Ochroma pyramidale) is a hardwood) but
rather reflects different botanic origins. The origins of the descriptions "softwood"
and "hardwood" possibly derive from trade descriptions in north-western Europe.
Softwoods are derived from the plant group called gymnosperms, commonly called
the conifers or cone-bearing plants, characteristically with needle shaped leaves and
naked seeds (Desch and Dinwoodie, 1996). Examples of such conifers are pines
(Pinus spp.), the spruces (Picea spp.) and the firs (Abies spp.). Hardwoods are derived
from the plant group called angiosperms (two subgroups called monocotyledons and
dicotyledons), generally known as broad-leaved trees; their seeds are enclosed in a
seed case. Examples of such trees are eucalypts (Eucalyptus spp.), oak (Quercus spp.)
and southern beeches (Nothofagus spp.). Under the International Union of Biological
Nomenclature's naming system, every tree has a name with two parts; a genus and a
species, often called the scientific name. There are two more names of timbers by
which they are known more commonly. One is the vernacular name (or local name)
and other is the trade name (accepted and established names in international timber
industries). For example, Pinus sylvestris is the scientific name of Redwood (trade
name), locally known as Scots pine.
Softwoods are relatively simple in structure, primarily (90% of volume) composed
of one kind of axially elongated pointed cells of 2 to 5 mm in length called tracheids
(Walker et al., 1993). Softwoods are generally medium to low density timbers in the
Introduction Chapter 1
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range of 350 to 700 kg/m3 (basic density at 12% moisture content), as reported by
Desch and Dinwoodie (1996). The technologies for the processing of softwoods
(including conversion and drying) may be considered to be relatively easier, well-
established and implemented by many timber companies around the world compared
with hardwoods. Some reasons are the uniformity of the softwood resources (the
majority come from plantations) and the large amount of research concentrated on
various aspects of softwood processing (pines and spruces). The geographical location
of many softwood resources in developed countries in Europe and North America is
another reason for research, because of the availability of financial support. Research
on softwood processing is also very advanced in New Zealand and Australia due to
the availability of large areas of softwood plantation resources, predominantly of
radiata pine (Pinus radiata). The plantation area for softwoods is about 1 million ha in
Australia according to the Australian Bureau of Agricultural and Resource Economics
(ABARE, 2000) and about 1.7 million ha in New Zealand (source: New Zealand
Forestry, 2002).
The processing of hardwoods is often more complex because of the diversity of
resources (mostly from native natural forests) in terms of size, shape, species groups,
and differences in timber quality, as well as the complex structure of hardwoods. For
example, the drying of most hardwoods is generally slow compared with softwoods,
and great care needs to be taken to produce defect-free good-quality timber.
Hardwoods are generally medium to high density timbers in the range of 450 to 1250
kg/m3 (basic density at 12% moisture content), as reported by Desch and Dinwoodie
(1996). The low lateral permeability and moisture transport coefficients of
hardwoods, compared with softwoods, tend to make the drying of hardwoods more
difficult than that of softwoods. For example, the transverse permeability of green
Introduction Chapter 1
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wood from Eucalyptus delegatensis is in the order of 4.6×10-18 m2, whereas the
permeability of green wood of Pinus radiata is 263 to 410×10-18 m2 (Langrish and
Walker, 1993). The focus of this thesis is the drying of blackbutt (Eucalyptus
pilularis) which is a difficult to dry hardwood species (Bootle, 1994).
Structure of Hardwoods
The structure of hardwoods (Figure 1.1) is complex because they contain more
cell types arranged in a greater variety of patterns compared with softwoods. In terms
of microstructure, there are generally three kinds of cells present in hardwoods;
namely, vessels (conduction of sap), fibres (strength and mechanical support) and ray
cells including parenchyma (storage). The majority of hardwood cells are vessels and
fibres. Vessels comprise many individual cells or vessel elements joined end to end to
form long conducting channels. The vessels are about 0.2 to 0.5 mm in length and 20
to 400 µm in diameter (Desch and Dinwoodie, 1996). These vessels are sometimes
blocked by tyloses. Tyloses are the bubble or balloon like outgrowth (Figure 1.2) of
the adjacent parenchyma cells through pits into the vessel, which may completely or
partially fill the vessel. Tyloses are formed as a part of the process of transformation
of sapwood to heartwood in some trees. In other cases, tyloses are formed to retard
the flow of sap due to physiological reasons (during draught or low water contents in
the vessel), mechanical injury, or as a result of a viral or fungal infection (Panshin and
de Zeeuw, 1970). The structural loads in hardwood are borne by the fibres, which are
the bulk of the hardwood cells. These cells differ from softwood tracheids in a
number of ways; they are comparatively shorter (0.25 to 1.5 mm long and generally
less than 1 mm), more rounded in transverse outline, and they play virtually no role in
the ascent of sap. There are radially elongated ray tissues, which may be several
millimetres in length.
Introduction Chapter 1
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Figure 1.1: Structure of a hardwood; sweetgum wood (Liquidambar styraciflua), 75×(source: Panshin and de Zeeuw, 1970).
Figure 1.2: A tylosis blocking a vessel in Nothofagus solandri, 650× (source: Walkeret al., 1993).
Introduction Chapter 1
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There are openings (called perforation plates) in the separation wall at the end of
vessel elements for longitudinal conduction. There are also minute communication
paths (known as intervascular pits, as shown in Figure 1.3) in the longitudinal walls
for lateral flow between adjacent vessels. The pits in hardwood vessels are often
bordered and are formed by an overarching of the pit membrane by the cell walls of
the two adjacent elements, leaving an elongated opening (generally 5-12 µm in
diameter). This opening is called the pit aperture, but is lacking a torous, which is
typical of softwood bordered pits. However, the pits are much less frequent in the
walls of fibres, and these pits are mainly simple pits, i.e. without any border (Desch
and Dinwoodie, 1996).
Figure 1.3: Surface view and section through bordered pits in conducting cells; right- solid view of two pits cut in half: I, pit opening; II, torous; III, margo strands formed
from the primary wall; IV, pit cavity; V, secondary wall (source: Desch andDinwoodie, 1996).
The wood at the centre of the tree stem (called heartwood) is often harder, darker
in colour and more durable than sapwood (Desch and Dinwoodie, 1996) because of
the presence of extractives, which are terpenoids and steroids, fats and waxes, and
phenolic compounds (Sjostrom, 1993). This region is composed of dead cells and is
physiologically inactive but gives significant strength and mechanical support for the
Introduction Chapter 1
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tree. The heartwood is often impermeable. The outer part of the stem is known as the
sapwood and is often paler in colour than the heartwood for most species. As the tree
grows older, the heartwood region expands and is surrounded by a thin annulus of
sapwood, which is typically 10 to 50 mm wide (Keey et al., 2000). Sapwood is
converted to heartwood with increasing age, but heartwood never becomes sapwood.
Structure of Blackbutt
Eucalyptus pilularis Sm., locally known as blackbutt, grows abundantly in the
coastal forests of New South Wales. The colour of heartwood is pale brown (Figure
1.4), and the sapwood is distinctively paler. The texture (which is the relative size and
distribution of wood cells in a unit volume) of the wood is medium and even, i.e.
wood elements are evenly distributed, and there is no great variation in cell wall
thickness throughout the growth in the growth-rings. The grain (which is the direction
of alignment of vertical wood elements, e.g. fibres and vessels, in relation to the
longitudinal axis of the tree) is usually straight. The presence of gum veins (the
formation of which is a result of natural protective response to injury common in most
eucalypts) is common on the surface of sawn boards (Bootle, 1994). The fibres are
small and have only moderately thick walls, leaving a reasonably-sized lumen, which
is normally free of any deposit. The pores are of rather large size (Figure 1.5) relative
to fibre lumens. Vessels are sometimes plugged with tyloses (Figure 1.6). Some of the
vessels contain minute particles of silica, which are often called "grit" by wood
tradesmen. These particles are believed to cause problems during finishing (Baker,
1919). Wood parenchyma (storage cells) are rare, and rays are numerous, mostly
uniseriate (only one cell wide), and some are also multiseriate (a few cells wide), with
a deposit present in some cases (Figure 1.7).
Introduction Chapter 1
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Figure 1.4: The surface of a finished blackbutt board (source: Baker, 1919).
Figure 1.5: Cross section through blackbutt, showing numerous large pores and raysin wavy lines; the fibres are small (source: Baker, 1919).
Introduction Chapter 1
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Figure 1.6: Radial section through blackbutt, showing portions of three vesselsplugged with tyloses; rays with deposit in cells; the black needle-shaped lines are
deposits from the lumens of the fibres (source: Baker, 1919).
Figure 1.7: Tangential section through blackbutt, showing portions of two vessels;ray and wood parenchyma (right of centre) (source: Baker, 1919).
Introduction Chapter 1
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The green density of blackbutt is about 1100 kg/m3, the air dry density (at 12%
moisture content) is 900 kg/m3, and the oven dry density is 710 kg/m3 (Bootle, 1994).
Special care needs to be taken to dry this timber carefully to minimise the tendency
for surface checks on the tangential surfaces of boards. Regrowth logs are subject to
much more spring and bow than the mature stems, and their central core is likely to
suffer considerable collapse during seasoning. Collapse can be avoided by sawing
radially (perpendicular to the rays) for larger dimension boards. The major uses of
this timber are for building framework, flooring, poles, sleepers etc, and this timber is
one of the most widely used general construction timbers in New South Wales and
Southern Queensland.
1.1.2 Wood-Water Relationships
A number of textbooks have covered this aspect (Siau, 1984; Skaar, 1988; Keey et
al., 2000). A brief summary is presented here. The timber of living trees and freshly
felled logs contains a large amount of water, which often constitutes a greater
proportion by weight than the solid material itself. Water has a significant influence
on the properties of wood, affecting its weight, strength, shrinkage, and liability to
attack by some insects and by fungi that cause stain or even decay (Walker et al.,
1993; Desch and Dinwoodie, 1996; Keey et al., 2000). Wood differs from most
materials used for construction in that it is continually exchanging moisture (water)
with its surroundings, more significantly than concrete or brick.
Water in wood may be present in two forms:
(i) Free water: The bulk of water contained in the cell cavities is free from the
action of the intermolecular attraction of the cell walls and is only held by capillary
forces. It is, therefore, termed free water. Free water is not in the same
Introduction Chapter 1
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thermodynamic state as liquid water in a large container, because of the additional
force due to the capillary effect of the cell lumens (Skaar, 1988), which are 20 to 300
µm in diameter (Langrish and Walker, 1993). Furthermore, water in the cell cavity
may also contain water-soluble foreign materials (Skaar, 1988), from extractives in
wood such as polyphenolic compounds which include flavonoids, stilbenes, lignans
and tannins (Sjostrom, 1993).
(ii) Bound or hygroscopic water: Bound water is contained in the voids of the cell
wall and is more intimately associated with the wood in its sub-microscopic structure
(Keey et al., 2000). The attraction of wood for water arises from the presence of free
hydroxyl (OH) groups in the chemical structure and arrangement of the cellulose,
hemicelluloses and lignin molecules within the cell wall (Wise and Jahn, 1952;
Stamm, 1964; Rowel, 1984). The hydroxyl groups are negatively charged electrically,
and since water is a polar liquid consisting of a negative hydroxyl (OH) fraction, the
free hydroxyl groups in cellulose attract and hold water by hydrogen bonding. The
water held in the cell walls by hydrogen bonds is termed bound water.
Water vapour is also present in the cell cavities. The total amount of water in
vapour form is normally only a small fraction of the total mass and is negligible at
normal temperatures and moisture contents.
Moisture Content of Wood
The moisture content of a particular sample means how much water is present in
the sample. The moisture content of wood is generally expressed as a percentage of
the oven-dry weight of the wood and is calculated according to the formula (Siau,
1984):
100m
mm(%)Xod
odg×
−= (1.1)
Introduction Chapter 1
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Here, mg is the green mass of the wood, mod is its oven-dry mass (the attainment of
constant mass generally after drying in an oven set at 103 ± 2oC for 24 hours as
mentioned by Walker et al., 1993). This moisture content can also be expressed as a
fraction of the mass of the water and the mass of the oven-dry wood rather than a
percentage, for example, in units of kilograms of water per kilogram of oven-dry
wood (or kg kg-1). For example, the average green moisture content for ten samples
(collected from ten different blackbutt logs) was found to be 0.59 kg kg-1 (oven dry
basis) from an experiment for this thesis, according to equation (1.1). Since the
moisture content is often reported on a percentage basis in the wood science literature,
the average moisture content, in this case, is 59% (oven dry basis).
Fibre Saturation Point
When green wood dries, free water leaves the cell cavities first because it is held
by weaker capillary forces than the bound water. Then the bound water is removed.
Furthermore, most physical properties, such as strength and shrinkage, are unaffected
by the removal of free water since free water is not involved in the cell walls. The
fibre saturation point (FSP) is defined as the moisture content at which free water is
completely absent from the cell cavities, but the cell walls are virtually saturated with
bound water. FSP is the limiting value between these two forms of water (free and
bound). In most woods, the value of the fibre saturation point is 25 to 30% of the
oven-dry weight for many different species of wood. Keey et al. (2000) defined the
fibre saturation point as the equilibrium moisture content of a wood sample in an
environment of 99% relative humidity, if the capillary-condensation effects in pores
(less than 0.1 µm) having equivalent cylindrical diameters are neglected. Their
definition would yield a fibre saturation point for most common commercial species
between 30 and 32% (dry basis) at room temperature, following the desorption
Introduction Chapter 1
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isotherm (equilibrium moisture contents as a function of relative humidity and
temperature) produced by Stamm (1964). Siau (1984) reported that the fibre
saturation point Xfsp (kg kg-1) is dependent on the temperature T (oC) according to the
following equation:
Xfsp = 0.30 - 0.001 (T-20) (1.2)
Many important properties of wood show a considerable change when the wood is
dried below the fibre saturation point. Some of these properties are given below:
i) Ideally no shrinkage occurs until some bound water is lost, i.e. until the wood is
dried below FSP. However, the fibre saturation point is probably something of an
idealisation because it is not possible to see the exact point where there is no free
water but the cell wall is completely saturated. In reality, a small amount of free water
may still be present when the bound water starts to escape. The shrinkage behaviour
of blackbutt wood samples is explained in Chapter 2.
ii) Most strength properties, except the decrease in impact bending strength and, in
some cases the toughness, show a consistent increase with the first loss of bound
water when the wood starts drying below the FSP (Desch and Dinwoodie, 1996).
iii) The electrical resistivity increases only slowly with the loss of free water,
whereas it increases very rapidly with the loss of bound water when the wood dries
below the FSP.
Equilibrium Moisture Content
Wood is a hygroscopic substance. It has the ability to take in or give off moisture
in the form of vapour. The water contained in wood exerts a vapour pressure of its
own, which is determined by the maximum size of the capillaries filled with water at
any time. If the water vapour pressure in the ambient space is lower than the vapour
pressure within wood, desorption takes place. The largest sized capillaries, which are
Introduction Chapter 1
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full of water at the time, empty first. The vapour pressure within the wood falls as
water is successively contained in smaller and smaller sized capillaries. A stage is
eventually reached when the vapour pressure within the wood equals the vapour
pressure in the ambient space above the wood, and further desorption ceases. The
amount of moisture that remains in the wood at this stage is in equilibrium with the
water vapour pressure in the ambient space, and is termed the equilibrium moisture
content or EMC (Siau, 1984). Because of its hygroscopicity, wood tends to reach a
moisture content that is in equilibrium with the relative humidity and temperature of
the surrounding air.
The EMC of wood varies with the ambient relative humidity (a function of
temperature) significantly, to a lesser degree with the temperature. Siau (1984)
reported that the EMC also varies very slightly with species, mechanical stress, drying
history of wood, density, extractives content and the direction of sorption in which the
moisture change takes place (i.e. adsorption or desorption).
Moisture Content of Wood in Service
Wood retains its hygroscopic characteristics after it is put into use. It is then
subjected to fluctuating humidity, the dominant factor in determining its EMC. These
fluctuations may be more or less cyclical, such as diurnal changes or annual seasonal
changes.
In order to minimise the changes in wood moisture content or the movement of
wooden objects in service, wood is usually dried to a moisture content that is close to
the average EMC conditions to which it will be exposed. These conditions vary for
interior uses compared with exterior uses in a given geographic location. For
example, according to the Australian Standard for Timber Drying Quality (AS/NZS
4787, 2001), the EMC is recommended to be 10-12% for the majority of Australian
Introduction Chapter 1
15
states, although extreme cases may be up to 15 to 18% for some places in
Queensland, Northern Territory, Western Australia and Tasmania. However, the EMC
may be as low as 6 to 7% in dry centrally heated houses and offices or in permanently
air-conditioned buildings.
The primary reason for drying wood to a moisture content equivalent to its mean
EMC under use conditions is to minimise the dimensional changes (or movement) in
the final product.
Shrinkage and Swelling
Shrinkage and swelling may occur in wood when the moisture content of wood is
below the fibre saturation point (Stamm, 1964). Shrinkage occurs as the moisture
content reduces, while swelling takes place when water is introduced into the wood.
Shrinkage and swelling are not the same in different grain directions. The greatest
dimensional change occurs in a direction tangential to the annual rings. Shrinkage
from the pith outwards, or radially, is considerably less than the tangential shrinkage,
while longitudinal (i.e., along the grain) shrinkage is so slight that it can nearly
always be neglected. The longitudinal shrinkage ranges from about 0.1 to 0.3% of the
timber length, in contrast to transverse shrinkages, which are 2-10% of the length.
Tangential shrinkage is usually about twice as great as in the radial direction,
although in some species it may be as much as five times as great. The shrinkage is
about 5 to 10% in the tangential direction and about 2 to 6% in the radial direction
(Walker et al., 1993). This variation in the properties of wood in different directions is
termed anisotropy, i.e. the properties vary in three principal directions, namely the
longitudinal, radial and tangential ones, as shown in Figure 1.8 (Panshin and de
Zeeuw, 1970). The ultrastructure of the wood cell wall helps to explain why
Introduction Chapter 1
16
longitudinal shrinkage is negligible, but transverse shrinkage is appreciable, as
discussed in the next section.
Figure 1.8: A wedge of wood cut from a five-year old tree showing structuralfeatures in three main surfaces, namely the cross section, the radial surface and the
tangential surface (source: Desch and Dinwoodie, 1996).
Wood Ultrastructure
In terms of wood ultrastructure, the cell wall is built up by several layers, namely
the middle lamella (M); the primary wall (P); and the secondary wall (S), which is
composed of three layers, designated as the outer (S1), the middle (S2) and the inner
(S3) secondary layers; and the warty layer (Figure 1.9).
Introduction Chapter 1
17
Figure 1.9: Simplified structure of a woody cell, showing the middle lamella (ML),the primary wall (P), the outer (S1), middle (S2), and inner (S3) layers of the
secondary wall, and the warty layer (W) (source: Sjostrom, 1993).
These layers differ from one another with respect to their structure and relative
size as well as their chemical composition (the amounts of cellulose, hemicelluloses
and lignin). A simplified picture is that cellulose forms a skeleton or framework,
which is surrounded by other substances functioning as a matrix, i.e. hemicelluloses
and encrusting material, lignin. The smallest structural units of cell walls are called
microfibrils, which consist of a bundle of a number of cellulose chain molecules.
Microfibrils appear to be roughly cylindrical and about 0.01 to 0.03 µm in diameter,
Introduction Chapter 1
18
depending upon the species and the location within the tree (Sjostrom, 1993).
Microfibrils combine to form sheets of wall substance, known as lamellae. Ultimately
these sheets or lamellae form discrete cell wall layers. The microfibrils wind around
the cell axis in different directions, either to the right (like the middle bar of the letter
Z or the Z helix) or to the left (like the middle bar of the letter S or the S helix)
(Walker et al., 1993; Desch and Dinwoodie, 1996).
Cell-wall moisture is held between the fibrils, and between the micelle (or
lamellae) that compose them, and removal of hygroscopic moisture results in these
units packing closer together, causing appreciable transverse contraction, but little
change in their lengths. The central or S2 layer of the secondary wall is the thickest
layer. Its microfibrils are nearly parallel to the cell axis and tend to swell transversely
as the moisture content increases. The S1 and S3 layers of the secondary wall are thin.
Their microfibrils are oriented nearly perpendicular to the cell axis, giving rise to
slight shrinkage in the longitudinal direction. The difference between radial and
tangential shrinkage has been explained by the restraining influence of the wood rays
in the radial direction (Kollmann and Cote, 1968).
In summary, differential transverse shrinkage of wood is related to:
(i) the alternation of late (produced during winter season) and early wood
(produced during summer season) increments within the annual ring;
(ii) the influence of wood rays;
(iii) the features of the cell wall structure such as microfibril angle
modifications and pits; and,
(iv) the chemical composition of the middle lamella.
1.1.3 Wood Drying
Introduction Chapter 1
19
Wood drying (also called seasoning in the wood literature) is the removal of water
from the timber as economically and with as little damage as possible. A recent
textbook by Keey et al. (2000) covers many aspects of timber drying, including the
fundamental basis of this technology.
An important objective of seasoning timber is to dry it to the equilibrium moisture
content before use. Thus the gross dimensional changes through shrinkage are carried
out during drying and before final use.
Timber is dried to conform to the average of the maximum and minimum
equilibrium moisture contents that will be attained by the wood in service under
fluctuations of different climatic conditions. The movement in the components of the
finished product, relative to the dimensions at the times of fabrication, is also kept to a
minimum if dry timber is used. Thus drying is the first step towards realising the
maximum attainable dimensional stability from any timber during use. To eliminate
movement completely in wood, chemical modification of wood is a possible
technology. This is the treatment of wood with chemicals to replace the hydroxyl
groups with other hydrophobic functional groups of modifying agents (Stamm, 1964).
Among all the existing processes, wood modification with acetic anhydride has
considerable promise due to the high anti-shrink or anti-swell efficiency (ASE)
attainable without damaging the wood properties. However, acetylation of wood has
been slow in commercialisation due to the cost, corrosion and the entrapment of the
acetic acid in wood. There is extensive literature relating to the chemical modification
of wood (Rowell, 1983, 1991; Kumar, 1994; Haque, 1997).
Drying timber is one approach for adding value to sawn products from the primary
wood processing industries. According to the Australian Forest and Wood Products
Research and Development Corporation (FWPRDC), green sawn hardwood, which is
Introduction Chapter 1
20
sold at about $350 per cubic metre or less, increases in value to $2,000 per cubic
metre or more with drying and processing. However, currently-used conventional
drying processes often result in significant quality problems from cracks, both
externally and internally, reducing the value of the product. As an example, in
Queensland alone (Anon, 1997), assuming that 10% of the dried softwood is devalued
by $200 per cubic metre because of drying defects, sawmillers are losing about $5
million per year in that State alone. Australia wide this could be $40 million per year
for softwood and an equal or higher amount for hardwood. Thus proper drying under
controlled conditions (prior to use) is of great importance in timber utilisation in any
country, where climatic conditions vary considerably at different times of the year.
Drying, if carried out promptly after the felling of trees, also protects timber
against primary decay, fungal stain and attack by certain kinds of insects. Organisms,
which cause decay and stain, generally cannot thrive in timber with a moisture
content below 20%. Several, though not all, insect pests can live only in green timber.
Dried wood is less susceptible to decay than green wood (above 20% moisture
content).
Apart from the above important advantages of drying timber, the following points
are also significant (Walker et al., 1993; Desch and Dinwoodie, 1996):
1. Dried timber is lighter, and hence the transportation and handling costs are
reduced.
2. Dried timber is stronger than green timber in terms of most strength properties.
3. Timbers for impregnation with preservatives have to be properly dried if proper
penetration is to be accomplished, particularly in the case of oil-type preservatives.
4. In the field of chemical modification of wood and wood products, the material
should be dried to a certain moisture content for the appropriate reactions to occur.
Introduction Chapter 1
21
5. Dry wood works, machines, finishes and glues better than green timber. Paints
and finishes last longer on dry timber.
6. The electrical and thermal insulation properties of wood are improved by
drying.
Prompt drying of wood immediately after felling therefore results in significant
upgrading of, and value adding to, the raw timber. Drying enables substantial long
term economy in timber utilisation by rationalising the utilisation of timber resources.
The drying of wood is thus an area for research and development, which concerns
many researchers and timber companies around the world.
How Wood Dries: the Mechanisms of Moisture Movement
Water in wood normally moves from zones of higher to zones of lower moisture
content (Walker et al., 1993). In simple terms, this means that drying starts from the
outside and moves towards the centre, and it also means that drying at the outside is
also necessary to expel moisture from the inner zones of the wood. Wood, after a
period of time, attains a moisture content in equilibrium with the surrounding air (the
EMC, as mentioned earlier).
Mechanisms for Moisture Movement
a) Moisture passageways
The basic driving force for moisture movement is chemical potential. However, it
is not always straightforward to relate chemical potential in wood to commonly
observable variables, such as temperature and moisture content (Keey et al., 2000).
Moisture in wood moves within the wood as liquid or vapour through several types of
passageways depending on the nature of the driving force, (e.g. pressure or moisture
gradient), and variations in wood structure (Langrish and Walker, 1993), as explained
in the next section on driving forces for moisture movement. These pathways consist
Introduction Chapter 1
22
of cavities of the vessels, fibres, ray cells, pit chambers and their pit membrane
openings, intercellular spaces and transitory cell wall passageways. Movement of
water takes place in these passageways in any direction, longitudinally in the cells, as
well as laterally from cell to cell until it reaches the lateral drying surfaces of the
wood. The higher longitudinal permeability of sapwood of hardwood is generally
caused by the presence of vessels. The lateral permeability and transverse flow is
often very low in hardwoods. The vessels in hardwoods are sometimes blocked by the
presence of tyloses and/or by secreting gums and resins in some other species, as
mentioned earlier in section 1.1.1. The presence of gum veins, the formation of which
is often a result of natural protective response of trees to injury, is commonly
observed on the surface of sawn boards of most eucalypts. Despite the generally
higher volume fraction of rays in hardwoods (typically 15% of wood volume), the
rays are not particularly effective in radial flow, nor are the pits on the radial surfaces
of fibres effective in tangential flow (Langrish and Walker, 1993).
b) Moisture movement space
The available space for air and moisture in wood depends on the density and
porosity of wood. Porosity is the volume fraction of void space in a solid. The
porosity is reported to be 1.2 to 4.6% of dry volume of wood cell wall (Siau, 1984).
On the other hand, permeability is a measure of the ease with which fluids are
transported through a porous solid under the influence of some driving forces, e.g.
capillary pressure gradient or moisture gradient. It is clear that solids must be porous
to be permeable, but it does not necessarily follow that all porous bodies are
permeable. Permeability can only exist if the void spaces are interconnected by
openings. For example, a hardwood may be permeable because there is intervessel
pitting with openings in the membranes (Keey et al., 2000). If these membranes are
Introduction Chapter 1
23
occluded or encrusted, or if the pits are aspirated, the wood assumes a closed-cell
structure and may be virtually impermeable. The density is also important for
impermeable hardwoods because more cell-wall material is traversed per unit
distance, which offers increased resistance to diffusion (Keey et al., 2000). Hence
lighter woods, in general, dry more rapidly than do the heavier woods. The transport
of fluids is often bulk flow (momentum transfer) for permeable softwoods at high
temperature while diffusion occurs for impermeable hardwoods (Siau, 1984). These
mechanisms are discussed below.
Driving Forces for Moisture Movement
Three main driving forces used in different version of diffusion models are
moisture content, the partial pressure of water vapour, and the chemical potential
(Skaar, 1988; Keey et al., 2000). These are discussed here, including capillary action,
which is a mechanism for free water transport in permeable softwoods.
a) Capillary action
Capillary action causes free water to flow, for the most part through cavities and
small openings in the cell wall. It is due to the simultaneous operation of adhesion and
cohesion. Adhesion is the attraction between water particles and the walls of the pit
membrane openings, and cohesion is the attraction of water particles for each other.
When green wood starts to dry, evaporation of water from the surface cells sets up
capillary forces that exert a pull on the free water in the zones of wood beneath the
surfaces, so that liquid water flows. Much of free water in sapwood moves in this
manner.
b) Vapour pressure differences
When capillary action ceases, many of the cell cavities now contain air and water
vapour. The differences in vapour pressure cause moisture that is in the vapour state
Introduction Chapter 1
24
to diffuse through the cell cavities, pit chambers, pit membrane openings, and
intercellular spaces.
c) Moisture content differences
The chemical potential is explained here since it is the true driving force for the
transport of water in both liquid and vapour phases in wood (Siau, 1984). The Gibbs
free energy per mole of substance is usually expressed as the chemical potential
(Skaar, 1988). The chemical potential of unsaturated air or wood below the fibre
saturation point influences the drying of wood. Equilibrium will occur at the
equilibrium moisture content (as defined earlier) of wood when the chemical potential
of the wood becomes equal to that of the surrounding air. The chemical potential of
sorbed water is a function of wood moisture content. Therefore, a gradient of wood
moisture content (between surface and centre), or more specifically of activity, is
accompanied by a gradient of chemical potential under isothermal conditions.
Moisture will redistribute itself throughout the wood until the chemical potential is
uniform throughout, resulting in a zero potential gradient at equilibrium (Skaar,
1988). The flux of moisture attempting to achieve the equilibrium state is assumed to
be proportional to the difference in chemical potential, and inversely proportional to
the path length over which the potential difference acts (Keey et al., 2000). Thus, for
the moisture transfer flux j is given by:
j = − Bcz∂µ∂ (1.3)
where B is a coefficient, c is a concentration, µ is the chemical potential and z is the
dimension in the direction of transfer. If the activity a is used, an alternative
expression involving the moisture-concentration gradient is given by:
j = − zc
clnalnBRT
∂∂
∂∂ (1.4)
Introduction Chapter 1
25
Here, the term in the brackets is often called the moisture diffusivity D. Equation (1.4)
may be put into an alternative form involving the moisture content X, since the
concentration c is the product of the wood density ρw (kg m-3) and the moisture
content X (kg kg-1), according to:
j = − ( ) ( )zXD w
∂ρ∂ (1.5)
The gradient in chemical potential is related to the moisture content gradient as
explained in above equations (Keey et al., 2000). The diffusion model using moisture
content gradient as a driving force was applied successfully by Wu (1989) and Doe et
al. (1994) and their observed moisture-content profiles and with model prediction are
shown in Figure 1.10. Though the agreement between the moisture-content profiles
predicted by the diffusion model based on moisture-content gradients is better at
lower moisture contents than at higher ones, there is no evidence to suggest that there
are significantly different moisture-transport mechanisms operating at higher moisture
contents for this timber. Their observations are consistent with a transport process that
is driven by the total concentration of water. The diffusion model is used for this
thesis based on this empirical evidence that the moisture-content gradient is a driving
force for drying this type of impermeable timber.
Differences in moisture content between the surface and the centre (gradient, the
chemical potential difference between interface and bulk) move the bound water
through the small passageways in the cell wall by diffusion. In comparison with
capillary movement, diffusion is a slow process. Diffusion is the generally suggested
mechanism for the drying of impermeable hardwoods (Keey et al., 2000).
Furthermore, moisture migrates slowly due to the fact that extractives plug the small
Introduction Chapter 1
26
cell wall openings in the heartwood. This is why sapwood generally dries faster than
heartwood under the same drying conditions.
Figure 1.10: Moisture content profiles through Tasmanian eucalypts during drying.(After Wu, 1989).
Moisture Movement Directions for Diffusion
It is reported that the ratio of the longitudinal to the transverse (radial and
tangential) diffusion rates for wood ranges from about 100 at a moisture content of
5% to 2 to 4 at a moisture content of 25% (Langrish and Walker, 1993). Radial
diffusion is somewhat faster than tangential diffusion. Although longitudinal diffusion
is most rapid, it is of practical importance only when short pieces are dried. Generally
the timber boards are much longer than in width or thickness. For example, a typical
size of a green board used for this research was 6 m long, 250 mm in width and 43
mm in thickness. If the boards are quartersawn (sawing around the pith), then the
width will be in the radial direction whereas the thickness will be in tangential
direction, and vice versa for back-sawn (sawing through and through) boards, as
Introduction Chapter 1
27
shown in Figure 1.11. Most of the moisture is removed from wood by lateral
movement during drying.
Figure 1.11: Sawing pattern of hardwood logs; (a) flat-sawn or backsawn (b)quarter-sawn.
Reasons for Splits and Cracks During Timber Drying and Their Control
The chief difficulty experienced in the drying of timber is the tendency of its outer
layers to dry out more rapidly than the interior ones. If these layers are allowed to dry
much below the fibre saturation point while the interior is still saturated, stresses
(called drying stresses) are set up because the shrinkage of the outer layers (below
FSP) is restricted by the wet interior (Keey et al., 2000). Rupture in the wood tissues
occurs, and consequently splits and cracks occur if these stresses across the grain
exceed the strength across the grain (fibre to fibre bonding).
The successful control of drying defects in a drying process consists in
maintaining a balance between the rate of evaporation of moisture from the surface
and the rate of outward movement of moisture from the interior of the wood. The way
in which drying can be controlled will now be explained.
Influence of Temperature, Relative Humidity and Rate of Air Circulation
The external drying conditions (temperature, relative humidity and air velocity)
control the external boundary conditions for drying, and hence the drying rate, as well
a) b)
Introduction Chapter 1
28
as affecting the rate of internal moisture movement. The drying rate is affected by
external drying conditions (Walker et al., 1993; Keey et al., 2000), as will now be
described.
Temperature: If the relative humidity is kept constant, the higher the temperature,
the higher the drying rate. Temperature influences the drying rate by increasing the
moisture holding capacity of the air, as well as by accelerating the diffusion rate of
moisture through the wood.
The actual temperature in a drying kiln is the dry-bulb temperature (usually
denoted by Tg), which is the temperature of a vapour-gas mixture determined by
inserting a thermometer with a dry bulb. On the other hand, the wet-bulb temperature
(Tw) is defined as the temperature reached by a small amount of liquid evaporating in
a large amount of an unsaturated air-vapour mixture. The temperature sensing element
of this thermometer is kept moist with a porous fabric sleeve (cloth) usually put in a
reservoir of clean water. A minimum air flow of 2 m s-1 is needed to prevent a zone of
stagnant damp air formation around the sleeve (Walker et al., 1993). Since air passes
over the wet sleeve, water is evaporated and cools the wet-bulb thermometer. The
difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression,
is used to determine the relative humidity from a standard hygrometric chart (Walker
et al., 1993). A higher difference between the dry-bulb and wet-bulb temperatures
indicates a lower relative humidity. For example, if the dry-bulb temperature is 100oC
and wet-bulb temperature 60oC, then the relative humidity is read as 17% from a
hygrometric chart.
Relative humidity: The relative humidity of air is defined as the partial pressure of
water vapour divided by the saturated vapour pressure at the same temperature and
total pressure (Siau, 1984). If the temperature is kept constant, lower relative
Introduction Chapter 1
29
humidities result in higher drying rates due to the increased moisture gradient in
wood, resulting from the reduction of the moisture content in the surface layers when
the relative humidity of air is reduced. The relative humidity is usually expressed on a
percentage basis. For drying, the other essential parameter related to relative humidity
is the absolute humidity, which is the mass of water vapour per unit mass of dry air
(kg of water per kg of dry air). The following equation can be used to calculate the
absolute humidity from the relative humidity (Strumillo and Kudra, 1986):
Y = v
v
pPp622.0−ϕ (1.6)
Here Y is the absolute humidity in kg kg-1, pv is the saturated vapour pressure, P is
total pressure of the system and ϕ is the relative humidity, expressed as a ratio.
Air circulation rate: Drying time and timber quality depend on the air velocity and
its uniform circulation. At a constant temperature and relative humidity, the highest
possible drying rate is obtained by rapid circulation of air across the surface of wood,
giving rapid removal of moisture evaporating from the wood. However, a higher
drying rate is not always desirable, particularly for impermeable hardwoods, because
higher drying rates develop greater stresses that may cause the timber to crack or
distort. At very low fan speeds, less than 1 m s-1, the air flow through the stack is
often laminar flow, and the heat transfer between the timber surface and the moving
air stream is not particularly effective (Walker et al., 1993). The low effectiveness
(externally) of heat transfer is not necessarily a problem if internal moisture
movement is the key limitation to the movement of moisture, as it is for most
hardwoods (Pordage and Langrish, 1999).
Introduction Chapter 1
30
Classification of Timbers for Drying
The timbers are classified as follows according to their ease of drying and their
proneness to drying degrade:
A. Highly refractory woods: These woods are slow and difficult to dry if the final
product is to be free from defects, particularly cracks and splits. Examples are heavy
structural timbers with high density such as ironbark (Eucalyptus paniculata),
blackbutt (E. pilularis), southern blue gum (E. globulus) and brush box (Lophostemon
cofertus). They require considerable protection and care against rapid drying
conditions for the best results (Bootle, 1994).
B. Moderately refractory woods: These timbers show a moderate tendency to
crack and split during seasoning. They can be seasoned free from defects with
moderately rapid drying conditions (i.e. a maximum dry-bulb temperature of 85oC can
be used). Examples are Sydney blue gum (E. saligna) and other timbers of medium
density (Bootle, 1994), which are potentially suitable for furniture.
C. Non-refractory woods: These woods can be rapidly seasoned to be free from
defects even by applying high temperatures (dry-bulb temperatures of more than
100oC) in industrial kilns. If not dried rapidly, they may develop discolouration (blue
stain) and mould on the surface. Examples are softwoods and low density timbers
such as Pinus radiata.
Methods of Drying Timber
Broadly, there are two distinct methods by which timber can be dried: (i) natural
drying, and (ii) artificial drying. Air drying is a natural drying method, while artificial
drying includes kiln drying (mainly), vapour drying, solvent drying, infra-red drying,
high frequency drying, microwave drying, superheated steam drying, and chemical
seasoning using salts. Solar drying utilises solar energy in such a way that it makes
Introduction Chapter 1
31
the process relatively simple and less expensive compared with kiln drying (Desch
and Dinwoodie, 1996), although the analysis of solar kiln performance is relatively
recent compared with the use of solar kilns. The work described in this thesis focuses
mainly on solar-assisted kiln drying, which may be competitive with air drying for
predrying. Air drying will now be explained.
Air Drying
Air drying is the drying of timber by exposing it to the sun (Figure 1.12). It
depends on the natural conditions of wind, sunshine and rain. The technique of air
drying consists mainly of making a stack of sawn timber (with the layers of boards
separated by stickers) on raised foundations, in a clean and dry place, under shade if
available. Atmospheric air is the drying agent, and the rate of drying largely depends
on climatic conditions. The air enters the stack of timber at the top, particularly at the
edges of the stack, picks up moisture, is cooled and then drops to the bottom. Some
air flows horizontally through the stack, driven by the wind. For successful air drying,
positive, continuous and uniform flow of air throughout the pile of the timber needs to
be considered, including the prevailing wind direction and the layout of the air drying
yard (Desch and Dinwoodie, 1996).
Figure 1.12: The air drying yard at Boral Timber's Herons Creek site.
Introduction Chapter 1
32
Kiln Drying
The process of kiln drying consists primarily of drying wood using introduced
heat sources (directly, using natural gas and/or electricity; indirectly, through steam-
heated heat exchangers, although solar energy is also possible). In the process,
deliberate control of temperature, relative humidity and air circulation is provided to
give conditions at various stages (moisture contents or times) of drying the timber to
achieve effective drying. For this purpose, the timber is stacked in chambers, called
wood drying kilns (Figure 1.13), which are fitted with equipment for manipulation
and control of the temperature and the relative humidity of the drying air and its
circulation rate through the timber stack (Walker et al., 1993; Desch and Dinwoodie,
1996).
Figure 1.13: Conventional kiln drying for hardwoods.
Kiln drying provides a means of overcoming the limitations imposed by erratic
weather conditions. In terms of the fundamental drying process, the process of kiln
drying does not differ from air seasoning. In both cases, unsaturated air is used as the
drying medium, and the principle of drying is the same, i.e. removal of moisture from
the interior to the surface of the timber. Almost all commercial timbers of the world
Introduction Chapter 1
33
are dried in industrial kilns. A comparison of air drying, conventional kiln and solar
drying is given below:
(i) Timber can be dried to any desired low moisture content by conventional or
solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to
attain for most locations.
(ii) The drying times are considerably less in conventional kiln drying than in
solar kiln drying, followed by air drying.
(iii) In air drying, a large amount of capital investment is needed for stacking a
large amount of timber stock over a longer period than in conventional or solar kilns,
although the installation for these kilns, as well as their maintenance and operation, is
expensive (in terms of capital items).
(iv) Air drying needs a large land area, so the land rental is significant.
(v) In air drying, there is little control over the drying elements, so drying degrade
cannot be controlled.
(vii) The temperatures employed in kiln drying typically kill all the fungi and
insects in the wood if a maximum dry-bulb temperature of above 60oC is used for the
drying schedule. However, all the fungi and insects may not be killed by air drying
temperatures and may subsequently attack the timber.
(viii) In air drying, the rate of drying may be very rapid in the dry summer months,
making timber boards liable to crack and split, and too slow during the cold winter
months.
The significant advantages of conventional kiln drying include higher throughput,
and precision (better control of the final moisture content). Conventional kiln and
solar drying both enable wood to be dried to any moisture content regardless of
Introduction Chapter 1
34
weather conditions. This makes both solar and conventional kiln drying more
appropriate for most large-scale drying operations than air drying.
Compartment-type kilns are most commonly used in timber companies. A
compartment kiln is filled with a static batch of timber through which air is circulated.
In these types of kiln, the timber remains stationary. The drying conditions are
successively varied from time to time in such a way that the kilns provide control over
the entire charge of timber being dried. This drying method is well suited to the needs
of timber companies, which have to dry timbers of varied species and thickness,
including refractory hardwoods that are more liable than other species to check and
split.
The main elements of kiln drying are described below:
a) Construction materials: The kiln chambers are generally built of brick masonry,
or hollow cement-concrete slabs (Figure 1.14). Sheet metal or prefabricated
aluminium in a double-walled construction with sandwiched thermal insulation, such
as glass wool or polyurethane foams, are materials that are also used in some modern
kilns. Some of the elements used in kiln construction are shown in Figure 1.15.
However, brick masonry chambers, with lime and (mortar) plaster on the inside and
painted with impermeable coatings, are used widely and have been found to be
satisfactory for many applications.
Introduction Chapter 1
35
Figure 1.14: Brick construction of a high temperature kiln.
Figure 1.15: Sample of wall material and portion of a heat exchanger.
b) Heating: Heating is usually carried out by steam heat exchangers and pipes of
various configurations (e.g. plain, or finned (transverse or longitudinal) tubes) or by
large flue pipes through which hot gases from a wood burning furnace are passed.
Only occasionally is electricity or gas employed for heating.
Introduction Chapter 1
36
c) Humidification: Humidification is commonly accomplished by introducing live
steam into the kiln through a steam spray pipe. In order to limit and control the
humidity of the air when large quantities of moisture are being rapidly evaporated
from the timber, there is normally a provision for ventilation of the chamber in all
types of kilns.
d) Air circulation: Air circulation is the means for carrying the heat to and the
moisture away from all parts of a load. Forced circulation kilns are most common,
where the air is circulated by means of fans or blowers, which may be installed
outside the kiln chamber (external fan kiln) or inside it (internal fan kiln).
Kiln Drying Schedules
Satisfactory kiln drying can usually be accomplished by regulating the
temperature and humidity of the circulating air to suit the state of the timber at any
given time. This condition is achieved by applying kiln-drying schedules. The desired
objective of an appropriate schedule is to ensure drying timber at the fastest possible
rate without causing objectionable degrade. The following factors have a considerable
bearing on the schedules.
i) The species; because of the variations in physical, mechanical and transport
properties between species.
ii) The thickness of the timber; because the drying time is approximately inversely
related to thickness and, to some extent, is also influenced by the width of the timber.
iii) Whether the timber boards are quarter-sawn, back-sawn or mixed-sawn;
because sawing pattern influences the distortion due to shrinkage anisotropy.
iv) Permissible drying degrade; because aggressive drying schedules can cause
timber to crack and distort.
Introduction Chapter 1
37
v) Intended use of timber; because the required appearance of the timber surface
and the target final moisture contents are different depending on the uses of timber.
Considering each of the factors, no one schedule is necessarily appropriate, even
for similar loads of the same species. This is why there is so much timber drying
research, including this work, focused on the development of effective drying
schedules. An optimised drying schedule has been developed and described in detail
in Chapter 3 of this thesis.
Drying Defects
Drying defects are the most common form of degrade in timber, next to natural
defects such as knots (Desch and Dinwoodie, 1996). Drying degrade can divided into
two broad categories: a) defects that arise due to the shrinkage anisotropy, related to
the warping of timber boards; and b) defects that arise due to uneven drying,
associated with the rupture of the wood tissue.
Defects related to warping include cupping, bowing, twisting, spring and
diamonding. Defects related to the rupture of tissues include checks (surface, end and
internal), end splits, honey-combing and case-hardening. Some defects due to
shrinkage anisotropy and uneven drying are shown in Figure 1.16. Collapse is another
form of defect that usually occurs above the fibre saturation point and is not related to
shrinkage anisotropy. Collapse occurs as a result of the physical flattening of water
filled fibre cells due to the action of internal tension. Collapse is often seen as a
corrugation, or "washboarding" of the board surface (Innes, 1996).
Introduction Chapter 1
38
Figure 1.16: Some defects due to uncontrolled drying.
Australian and New Zealand Standard Organisations (AS/NZS 4787, 2001) have
developed a standard for timber quality and set five criteria for measuring drying
quality. These are the moisture content gradient; the presence of residual drying stress
(i.e., related to case-hardening); surface, internal and end checks; collapse;
distortions; and discolouration caused by drying. This standard has also described the
drying quality classification, how to assess each of these drying quality criteria, and
the limits for each criterion to be acceptable within a quality class. This classification,
and its application, will be described and applied in detail in Chapter 3.
1.2 INDUSTRIAL OBSERVATIONS
The current research project was undertaken using the timber drying operations of
Boral Timber Ltd., Australia as the central case study. There are several timber mills
run by Boral Timber in NSW. A timber mill run by Boral Timber at Herons Creek,
NSW, was selected for this study because two solar kilns (Figure 1.17) were built for
trial on this site. An important aim of this research was to develop an optimised
drying schedule and to develop a mathematical model for this solar kiln. The
optimised drying schedule is needed to reduce the drying time and improve timber
Bend
Bow
Twist
Surface ChecksEnd Split
Cup
Introduction Chapter 1
39
quality, and the mathematical model will be useful to develop better operating
conditions, together with being a tool for assessing the use of the kiln at other sites.
This operation consists of two mills; namely a sawmill and a dry mill.
Figure 1.17: Solar kilns (front view) at Herons Creek site.
Green logs are collected mainly from the adjacent state forests. The logs are fed
into circular saws and are handled mechanically using hydraulic devices.
Optimisation of conversion is attempted based on the log conditions, i.e. diameter.
Defects, for example, heart rot, are excluded where possible. The main products are
boards for structural purposes, especially in building construction, which is classed as
SD1 and SD2 structural, according to the Australian Standards (AS/NZS 2878, 2000).
The minimum bending strength and modulus of elasticity along the grain for SD1
(seasoned) products are 150 MPa and 21500 MPa, whereas for SD2, these values are
130 MPa and 18500 MPa, respectively. Roof truss members, beams, floor joists,
rafters, window head lintels, and decking materials are the primary products that are
processed further in the board mill. The "conversion products", which are not suitable
for boards, are processed as pellets for packing boxes and perlins.
Introduction Chapter 1
40
The board materials are stacked mechanically at the sawmill and kept in the open
air, generally for 9 months for 38 mm thick material and 12 months for 50 mm thick
material. The moisture content of the green timber decreases from approximately 60-
70% for blackbutt (Eucalyptus pilularis) or more to 22-28% during air drying. This
material is then dried in steam-heated kilns, making timber drying on this site a two-
stage operation. The undressed dry boards are planed and sanded for final dry dressed
products. The materials are packed for final dispatch to other points of sale. This
operation processes mainly Australian eucalypts (various tree species of the genus
Eucalyptus under the plant family Myrtaceae), which comprise primarily 90%
blackbutt (Eucalyptus pilularis). The rest is a mixture of other hardwoods such as
mountain ash (E. regnans), blue gum (E. saligna), flooded gum (E. grandis), ironbark
(E. paniculata), brush box (Lophostemon confertus), messmate (E. obliqua), and
tallowwood (E. microcorys). A typical example of drying times for various drying
methods for 30 mm thick green (25 mm thick when dry) boards is shown in Figure
1.18.
Figure 1.18: Typical examples of drying times for air, conventional, and solar dryingand in combination for 30 mm thick green boards of blackbutt.
0
10
20
30
40
50
60
70
80
0 25 50 75 100 125 150 175 200Time (days)
Moi
stur
e co
nten
t (%
)
Solar
Air-drying
Conventional kiln
Conventional kiln
Introduction Chapter 1
41
1.3 ISSUES IN INDUSTRIAL PROCESSING OF TIMBER: PRESENTSTUDY AND SCOPE OF CURRENT WORK
Current challenges for the Australian timber industry in the field of timber drying
include reducing the drying time, particularly for predrying, and drying degrade. Over
the last few decades, some development of solar kilns for timber drying has occurred.
This has led to the recent commercial use and availability of solar kilns in the timber
industry (Desch and Dinwoodie, 1996). The development of solar kilns will be
described in more detail in Chapter 4.
Since predrying takes a long time by air drying, it is necessary to keep a large
stock of timber to maintain a continuous supply for the market. It is also necessary to
keep boards of various dimensions to cater for the demands of diverse customers.
Though the cost of land rental is moderately large, the quality damage of boards
during air- drying is substantial due to the lack of control over the drying process. It is
difficult to get pre-dried timber for kiln drying without a large inventory and air-
drying yard. On the other hand, it is perceived by the industry that it is not economical
to dry green-off-saw boards in conventional kilns, and it is not always possible to
maintain overall timber quality in the case of drying green hardwood boards.
In the above context, it is necessary to look for alternative drying (particularly
predrying) methods with a reasonable cost for maintaining continuous supply and
high productivity. In practice, energy is not currently a significant concern for a
timber drying mill, where there is a boiler plant in operation for heating the dry mill,
using woodwaste as fuel from a sawmill or other processing operation adjacent to the
dry mill (industry experience). There is a growing interest in the use of solar kilns to
accelerate the pre-drying stages for hardwoods, followed by conventional drying, both
Introduction Chapter 1
42
to reduce the predrying time and the improve the product quality compared with
open-air drying.
Evaluating the features of different solar kiln designs in future will involve the
analysis of a number of issues, including energy flows around kilns, and the cost and
use of materials in the kiln structures. This research has assessed the performance of a
particular kiln design that is currently used by Boral Timber (as a case study) and for
which information on the structural details is available. This study has also validated
the model for this design, where the model has been developed based on a previous
design (the Oxford solar kiln, Thompson et al., 1999). The Oxford solar kiln design is
relatively simple in structure and could be manufactured locally in Australia at a
modest cost. The validation of this kiln model is some indication that the generic
approach to solar kiln modelling used here and for the Oxford design is appropriate.
1.4 AIMS AND OUTCOMES
The major aims of this research were:
• to develop an optimised drying schedule for blackbutt timber in a solar kiln
after the determination of the physical, mechanical and transport properties of
the timber;
• to modify, adapt and improve an existing mathematical model based on that
outlined in Chapter 4 for energy flows around solar kilns (in and around a solar
heated kiln for timber drying, in and around a stack of timber within such a
kiln, and in and around boards of timber within such a stack) by including the
drying behaviour of a stack of timber;
• to predict energy flows in and around a solar kiln;
• to assess uncertainties in the developed optimised schedule and the solar kiln
model;
Introduction Chapter 1
43
• to assess operating procedures for drying Australian hardwoods in such a solar
kiln;
• to estimate stress and strain levels that develop during solar drying of timber;
and
• to assess the quantity and quality of measurements that are required to use such
a solar kiln model.
The results of this research have yielded the following outcomes:
• an optimised drying schedule for industrial application;
• a validated solar kiln model that can be used to develop better operating
procedures for solar kilns and to assess the suitability of solar kilns for other
locations. Potential future uses for the model include developing better designs
for solar kilns, and better operating procedures; and
• knowledge of what measurements are necessary to use this solar kiln model.
1.5 THE STRUCTURE OF THIS THESIS
The first chapter of this thesis introduces wood drying concepts, including the
structure of wood and the relationships between wood and water. This chapter also
gives a broad overview of the problem, including the key issues addressed and the
scope of this research. This chapter also describes the contributions of this thesis to
the field of timber drying.
The second chapter describes the procedures for and results from measurements of
the physical and mechanical properties required to develop optimised drying
schedules for blackbutt timber (Eucalyptus pilularis). These properties include the
density, modulus of elasticity, and the instantaneous, shrinkage, viscoelastic and
mechanosorptive strains. This chapter also describes the wood drying model and
Introduction Chapter 1
44
stress model used for this thesis, and the fitting of transport properties to the
experimental data from a drying test in a laboratory drying kiln.
The third chapter focuses on the development of an optimised drying schedule
using the information described in the second chapter. This optimised schedule has
been assessed in terms of the impacts of the uncertainties in the transport properties,
for example the reference diffusion coefficient, the activation energy and other
variables including the board thickness. Finally, this chapter presents the results and
comparisons of the drying trials using the original and optimised schedules in a
laboratory drying kiln, in terms of drying times and timber quality.
The fourth chapter reviews the literature on solar kiln drying research and
development. It explains the development of a mathematical model for a specific
design of solar kiln from previous research by Thompson et al. (1999) and generalises
this model to enable its development and improvement for another design, specifically
the Boral one. This chapter describes the modifications and additions made to this
model. This chapter also reports the simulation and measurements of the stack-wide
effect in a solar kiln and the relevance of this effect for the model in terms of
achieving appropriate model complexity (reducing simulation time without
unreasonably compromising accuracy).
Chapter five describes the actual performance assessment of an industrial solar
kiln. This chapter also reports the validation of the complete model by comparing the
measured outputs with the simulated outputs, after using the measured inputs in the
simulation model. This chapter assesses the impact of the uncertainties in the model,
including the estimation of the initial moisture content, kiln design variables, the
operating variables, the thermal and solar radiation properties and the estimation of
Introduction Chapter 1
45
the sky temperature for predicting radiation losses. The assessment of the required
level of measurements necessary to use the model is also carried out here.
The final chapter summarises the conclusions drawn from this study and gives
recommendations for further work arising from this research.
1.6 THE CONTRIBUTIONS OF THIS THESIS
A major contribution of this thesis to the field of timber drying practice is the
development of an optimised drying schedule for blackbutt timber in an industrial
context. This drying schedule is expected to reduce the drying time by 10% with
similar or better timber quality to that produced by current schedules.
The second major contribution to this field of knowledge is the development of a
simulation model for solar kilns. This simulation is new because it includes drying
and stress strain models that are integrated with the prediction of energy flows in the
kiln. Information on timber properties (transport properties and stress-strain ones),
kiln design and climatic conditions is required to use this model. This simulation has
also been validated, comparing the measured outputs with the predicted outputs from
this simulation model. The required measurements for running the simulation have
been established. The effects of different operating variables, kiln designs, timber
properties, geographical locations and weather conditions on the throughput and
quality of timber can all be assessed with this model. Different operating methods
include continuous or intermittent fan operation, and different circulation fan speeds,
venting amounts and frequencies, amounts of water spray for controlling humidity
and drying schedules.
Introduction Chapter 1
46
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