An introduction to wood drying

1. Introduction

Wood drying refers to reducing the moisture content of wood prior to its use. It is the process of removing its moisture contents to an average equilibrium with the atmospheric conditions of the locality of use. Ideally, wood is dried to that equilibrium moisture content as will later (in service) be attained by the wood so that further dimensional change will be kept to a minimum. Wood drying has lots of benefits but this is not common practices in developing countries like Nepal, India, Bangladesh and others. Only around 6-7 percent of the total production of industrial wood produced in India is seasoned in a strict sense (Pandey and Jain, 1992) and in Nepal, it is estimated far less than India. This example shows the meager condition of wood drying.

2. Water in the wood

2.1 Wood-water relationships

The timber of living trees and freshly felled logs contains a large amount of water, which often constitutes more weight than the actual wood. Wood continually exchanges moisture (water) with its surroundings, although the rate of exchange is strongly affected by the degree wood is sealed. Water presents in wood in two forms:

• Free water: The bulk of water contained in the cell lumina is only held by capillary forces: it is not bound chemically and is termed free water. Free water may contain chemicals, altering the drying characteristics.
• Bound or hygroscopic water: Bound water is bound to the wood via hydrogen bond and it is found on the cell walls of wood.

2.2 Fibre saturation point

The point at which free water has been removed from the fibre cavities and they just begin to release their bound water is known as the 'fibre saturation point'. In most woods, the fibre saturation point is at 25 to 30% moisture content. Many important properties of wood show a considerable change as the wood is dried below the fibre saturation point. These include: shrinkage resistance, increase in strength properties and electrical resistivity.

2.3 Equilibrium Moisture Content (EMC)

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. 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 minimize 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. The primary reason for drying wood to a moisture content equivalent to its mean EMC under use conditions is to minimize the dimensional changes (or movement) in the final product.

3. Purpose and scope of wood drying

As discussed above, wood in growing trees contains considerable quantities of water. Freshly felled timber contains a large quantity of moisture in many cases roughly around 100% based on oven dry weigh of wood, though in some light timbers the quantity of water in green condition is twice as the weight of wood substance. Most of this water has to be removed in order to obtain satisfactory performance from the wood in use. Seasoning of wood is a drying process as nearly as possible to the moisture content it will attain in use or in other words, to suitable moisture content warranting equilibrium with the prevailing atmospheric conditions in service.

3.1 Advantages of wood drying:

• Reduces the weight of timber and lessens the transportation and handling costs.
• Enhances the strength properties.
• Increases easy impregnation with preservatives.
• Provides appropriate conditions for the chemical modification of wood and wood products.
• Makes easy and ensures better results in wood working, machining, finishing and gluing. Paints and finishes last longer on dry timber.
• Upgrades the electrical and thermal insulation properties of wood.
• Minimizes the decay, fungal attack, larval attack and such other damage.
• Minimizes other seasonal defects such as warping, bowing, cracks etc. after use.
• Ensures more dimensional stability. Enables substantial long-term economy in utilization by minimizing replacement
• Nails, screws, and glue hold better in seasoned wood.

4. General principles involved in wood drying

When wood is dried, surface zones dry in advance of the interior because of direct evaporation of the moisture at the surface. As a result, a moisture gradient is set-up which causes the moisture to move from the interior to the surface. The rate of drying of wood is dependent on following aspects:

• Temperature: If relative humidity is kept constant, the higher the temperature the higher is the drying rate. Temperature influences the drying rate by increasing the moisture holding capacity of air, as well as by accelerating the rate of diffusion of moisture through the wood.
• Relative Humidity: If temperature is kept constant, lower relative humidity result in higher and faster drying rates.
• Air circulation: With constant temperature and relative humidity, the rapid air circulation results the higher possible drying rates.

Besides these external factors, the species, initial moisture content of woods, grain direction and thickness of wood affects the rate and quality of drying. Some wood species are highly refractory (e.g. Shorea robusta, Teminalia spp etc), some are moderately refractory (e.g. Dalbergia sissoo, Tectona grandis etc) and some are not refractory (Bombax ceiba, Populas deltoides, Mangifera indica etc). While organizing the wood drying, these wood properties should be taken into account.

5. Methods of wood drying

Wood is generally dried by either air drying or kiln drying or by a combination of both methods. They are as follows:

5.1 Air drying

Air drying is the process of removing moisture from wood by exposing it to atmospheric conditions. It is the simplest method of seasoning mainly based on the natural wind circulation and takes longer time up to 6-12 months. The temperature, and relative humidity can not be controlled in air drying process. The technique of air seasoning consists mainly in making a good stack of sawn timber with the help of crossers on raised foundation. As the atmospheric air is the drying agent, the rate and quality of drying largely depend on the climatic conditions. The losses through cracking, splitting, warping, decay and insect attacks can be considerably minimized by paying proper attention to the method of stacking, protection of stacks against the sun and heat and dry winds, and sanitary conditions in yard. The yard layout should consider prevailing winds so that the air movement through the stacks (piles) is at a maximum.

5.1.1 Stacking: Stacking technique is the most important factor in satisfactory air seasoning timber. Proper stacking of timber ensures uniform drying and reduces seasoning degrade, particularly warping and biological deterioration through mould and stain. The drying rate in air seasoning is influenced by rate of air circulation, stack size, width and height of the stacks and crossers and their spacing. There are two types of stacking for air drying:

• Horizontal stacking: This is the most common method of stacking timber for air seasoning and is suitable for all forms of sawn timber. One in nine methods for railway sleepers, close crib method (for slower drying and thick lumbers) and open crib method (allowing more air circulation for faster drying) are the some techniques of stacking for air seasoning.
• Vertical stacking: This is done for the rapid surface drying of non refractory woods which are liable to develop mould and discoloration soon after sawing. The planks stacked should be turned frequently to prevent warping. After the partly drying by vertical stacking, the timber can be air dried by horizontal stacking too.

5.2 Kiln drying

Kiln seasoning is drying of timber by keeping it in closed chamber in controlled temperature, RH and air circulation environment. In Kiln, the timber is stacked in closed chambers where the equipments are fitted to manipulate and control temperature and relative humidity of drying air, its circulation through the timber stack. The drying is faster, precise and better than air seasoning. Kiln seasoning enables wood to be dried to any desired moisture content regardless of weather conditions. It normally takes 2-5 weeks for wood drying in kiln. Almost all commercial timbers of the world are dried in industrial kilns. Seasoning kilns differ widely in their construction, equipments and design. They vary in construction materials, modes of heating, humidification, air circulation and chamber profiles.

There are two types of kilns in broad classification. They are progressive kiln in which the kilns are long tunnels type and stacked timbers moves while drying, and other is compartment kilns in which the timber remains stationary. Compartment-type kilns are most commonly used in timber companies.

5.3 Advantages and disadvantages of air drying and kiln drying

Air drying does not require a large initial investment for buildings and equipments, but requires timber to be held in yard storage for a considerable time before it is ready for market. In contrast, kiln drying requires a comparatively large capital investment, but dries the wood in a short time and can provide dry timber for all seasons of the year where it is not possible in the case of air drying.

5.4 Challenges in drying wood

There are some negative aspects to drying wood, including:

• As wood dries, it shrinks in several dimensions. If wood is not correctly dried, the dimensional changes will cause drying defects, including checks, splits, warp, casehardening, and honeycomb
• The great amount of energy that must be expended to drive the water out of wood. This energy costs lots of investment and it requires lots of skills and technology.

5.5 Drying defects

Drying defects are the common forms of degrade in timber. They degenerate the quality, use value and price of the timbers. There are two broad categories of drying defects (some defects involve both causes):

• defects that arise due to the shrinkage assimilarity. For example warping, cupping, bowing, twisting, and spring.
• defects that arise due to uneven drying. This leads to the rupture of the wood tissue: checks, splits, honey-combing and case-hardening.

These drying defects cannot be eliminated but it could be controlled by providing appropriate amounts of heat, air circulation and relative humidity. These can be controlled more in kiln drying than in air drying hence the defects would occur less in kiln drying.

6. Conclusions

The appropriate drying of would could benefit both the producers and users of the wood. For the producers, it generates higher price and low transportation and handling cost, and for the users, however it seems little expensive in the beginning, the durability, strength, dimensional stability etc provides high quality assurance hence making long term maintenance costs minimal. Therefore the use of dry wood should be promoted.

But the techniques and resources availability might be different in country to country and places to places. Also, the situations and requirements determine mostly to go for which type of seasoning. For example, the developing countries like Nepal, might not have such a big facilities for the kiln seasoning, however they could utilize the air drying method which will cost no extra money but provides lots of benefits.

7. References

• Brown W.H. 1965. An Introduction to the seasoning of timber.
• Millet R.S. 1961. Department of Forestry, Canada. Wood Seasoning.
• Northeastern Wood Utilization Council, Inc (the USA) 1948. Economies in Seasoning.
• Pandey C.N. and V.K. Jain 1992. Wood Seasoning Technology. Indian Council of Forestry Research and Education, Dehradun, India.
• http://en.wikipedia.org/wiki/Wood_drying
• http://www.ces.purdue.edu/extmedia/FNR/FNR-155.html
• http://www.mtc.com.my/publication/library/drying/ch16.htm