chapter 1: introduction 1.1 background information on …

Introduction Chapter 1

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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.


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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


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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


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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.


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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).


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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


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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).


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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).


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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).


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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


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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)


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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


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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


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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


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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


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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).


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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,


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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


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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


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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.


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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


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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


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


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)


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


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


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)


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


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).


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


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.


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


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


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.


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.


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.


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).


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


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.


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


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


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;


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


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


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.


46

REFERENCES

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Baker, R.T. (1919). The Hardwoods of Australia and Their Economics. TechnicalEducation Series, No. 23. The Government of New South Wales, William ApplegateGullick, Government Printer, Sydney. 522p.

Bootle, K.R. (1994). Wood in Australia: Types, Properties and Uses. McGraw-Hill Book Company, Sydney. 443p.

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Rowell, R.M. (1983). Chemical modification of wood. Forest Product Abstract,6(12):363-382.

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Thompson, F.B., Bailey, P.B. and Plumptre, R.A. (1999). Modelling thePerformance of Solar Kiln to Improve Design and Operation in Range of ClimaticConditions. In: Vermaas, H. and Steinmann, D. (eds). Wood Drying Research andTechnology for Sustainable Forestry beyond 2000. Proc. 6th International IUFROWood Drying Conference. University of Stellenbosch, South Africa. pp195-203.

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