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). For another article about a different type of logging, see data logging. ...

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 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 wood from Eucalyptus delegatensis is in the order of 4.6x10e-18 m2, whereas the permeability of green wood of Pinus radiata is 263 to 410x10e-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 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 micro meter in diameter (Desch and Dinwoodie, 1996). These vessels are sometimes blocked by tyloses. Tyloses are the bubble or balloon like outgrowth 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.

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) 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 microns 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).

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 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.

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 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 microns 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): (mg-mod)/mod (1.1) 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 +/- 2 deg C 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). For example, the average green moisture content for ten samples (collected from ten different blackbutt logs) was found to be 0.59 kg/kg (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 micron) 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 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) is dependent on the temperature T (deg C) 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. 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 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 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 (Panshin and de Zeeuw, 1970). The ultrastructure of the wood cell wall helps to explain why longitudinal shrinkage is negligible, but transverse shrinkage is appreciable, as discussed in the next section.

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.

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 micron in diameter, 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.

Wood drying

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 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. 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 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. 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 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 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).

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). 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 cell wall openings in the heartwood. This is why sapwood generally dries faster than heartwood under the same drying conditions. 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. Most of the moisture is removed from wood by lateral movement during drying.

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 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 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 100 deg C and wet-bulb temperature 60 deg C, 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 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):

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).

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 85 deg C 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 100 deg C) 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

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 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. 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).

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, 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).

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 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 60 deg C 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 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. 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. 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. 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. 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. 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.

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. 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).

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.

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External links

• This is edited extract from the Chapter 1 of Ph.D. Thesis by Dr. Nawshadul Haque