Timber Drying Principles: Factors Controlling the Drying of Wood

Successful timber drying depends on controlling the movement of moisture from the centre of the wood to its surface. This page explains the main principles of timber drying, including moisture movement, permeability, moisture gradients, relative humidity, temperature, air movement and wood structure. Understanding these factors helps produce high-quality timber while avoiding drying defects.

These principles apply to virtually every timber drying method, including air seasoning, conventional kiln drying, dehumidifier drying, solar drying and high-temperature drying.

Moisture Movement in Wood

Air moving across the surfaces of the timber causes moisture to be removed from the surfaces of the wood by evaporation. This water is then replaced by water from the interior of the wood.

There are several factors that influence these processes. Some are controllable and others are not. An understanding of these factors which control the drying of wood is vital if undesirable financial losses are to be avoided.

The Importance of the Moisture Gradient

Successful timber drying depends on maintaining a moisture gradient (a reduction in moisture content from the centre of the wood towards its surface) and PRODUCING AND MAINTAINING AN APPROPRIATE GRADIENT IS THE AIM OF THE DRYING PROCESS:

• Too shallow a gradient – the wood will dry (too) slowly and as well as the possibility of biodegrade, unacceptable costs might be incurred.
• Too steep a moisture gradient can cause pit aspiration, reducing permeability and slowing drying. More importantly, it greatly increases the risk of drying defects which will occur, reducing the quality of the stock.

Successful seasoning calls for the maintenance of a balance between the evaporation of water from timber surfaces and the movement of water from the interior of the timber.

The factors controlling the drying of wood are:

Relative Humidity

The relative humidity of air determines its ability to absorb water (from the wood). The rate of loss of moisture also depends on the moisture gradient. The rate of moisture loss therefore depends on:

• the relative humidity of the air in contact with surface layers.
• the moisture content of the surface layers of the wood.

The relative humidity of air depends on its:

Temperature

Increase in temperature reduces the relative humidity so its potential for drying wood increases.

Air Movement

Moist air near the wood surface must be replaced by drier air. Therefore:

• In the “open air” it is important to ensure that appropriate air movement can occur through the stacks of timber.
• Fans are often used to provide the required air movement (typically about 2 m/s).

It is important to produce even air movement over boards, since it is important to ensure uniform drying of the stock.

Wood Structure and Permeability

The rate at which surface moisture is replaced by water from within the wood depends on the wood's permeability. Consequently, wood structure is one of the most important factors controlling timber drying and the quality of the finished product.

The flowpaths used by the moving water (including water vapour) are:

• vary between species and follow tortuous pathways through the wood.
• meet with different resistances as it moves radially (R), tangentially (T) or longitudinally (L). In other words, wood is “anisotropic” with respect to permeability: (L is much greater than either T or R) (this is because the tree is “designed” to carry fluid primarily longitudinally).
• controlled by “pits” which interconnect cells (fluids can move from cell to cell via these pits). The condition of these pits also controls permeability. During drying, these pits can close (“aspirate”), which then blocks the water flow path.

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