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WOOD DRYING PRINCIPLES

 

Wood is a hygroscopic material that contains significant quantities of water when freshly cut or 'green' (up to 200% of oven dry weight). After sawing, the moisture content of wood will decrease until equilibrium is attained with ambient conditions of temperature and atmospheric humidity. This is between 6% and 18% moisture content depending on location (arid, tropical, and temperate). In the majority of cases, wood should be dried before it is used because it shrinks as it is dried, and wood moisture content should be in equilibrium with the atmospheric conditions so that the material is dimensionally stable while in service. Other properties such as strength, stiffness, electrical conductivity, treatability, resistance to decay and gluing properties also change as wood dries.

Wood comprises three principle chemical components, which are cellulose, hemicellulose and lignin. Water in wood exists partly as either free or very loosely bound water, or water that is chemically bound very strongly to cellulose and hemicellulose structures that make up wood. Shrinkage occurs only when the strongly bound water is removed.

The moisture content at which bound water begins to leave wood is called the fiber saturation point (FSP), which is between 20% and 30% moisture content for most timber species. Free water is readily removed from wood under ambient atmospheric conditions. Removing bound water however requires comparatively large amounts of energy or long periods of time in the air. The process of drying therefore is initially quite fast as free water is removed early in drying, and then slows considerably during the phase of bound water removal.

To accelerate drying, wood drying kilns are used to supply energy to remove chemically bound water. In some instances, air-drying is undertaken initially, followed by kiln drying. In other cases, where the cost can be justified, wood may be kiln dried from 'green'.

Due to shrinkage in wood that occurs during drying, wood may be subject to substantial internal forces or stresses during drying. The magnitude of these forces and the distribution within a wood section is dependent in part upon the rate at which drying occurs. This in turn is dependent upon the conditions that are maintained inside the kiln.

For example, a convection kiln delivers heat through the surface of a board, and therefore the surface wood dries before the inner core of a section of a board. This results in shrinkage at the surface and the development of tension stress, while the core is compressed.

If the tension stress exceeds the strength of the wood then splits or 'checks' may occur. In general, if a moisture content gradient develops across the cross section of a board, then shrinkage will vary with this gradient, and differentials in shrinkage lead to tension or compression forces distributed across the section.

Wood behaves like “plastic” to a certain degree, and can be permanently stretched by tension forces that develop during the early stages of drying. Tension at the surface stretches the surface wood, and the stretched form becomes permanent after drying is complete. This state is sometimes called 'tension set' or 'case hardened'. When drying progresses and the inner part of a section dries, it too shrinks, but is restricted by the stretched or “tension set” case. This leads to tension in the core and compression at the surface late in the drying cycle.

Stresses can remain after drying is complete and can result in dramatic changes in form when the timber is reworked by planing or molding. Similarly, gradient in moisture content can equalize leading to shrinkage or swelling after drying, also causing movement.

Long term exposure to high temperature (>100°C) causes densification and embrittlement. As a consequence high temperature drying is in appropriate for large section material when drying times are very long.

Drying timber in the air, or in a convection kiln, requires control of drying conditions to prevent damage or degrade. Different species require different conditions to prevent degrade because some timbers, such as very heavy or high density timbers, are prone to development of much steeper moisture gradients and larger stresses. Some lighter more porous timber species can be dried more quickly at higher temperatures, however this becomes more difficult in larger sections. In all cases however, the drying conditions must be controlled to attain dry timber of the highest quality.

Controlling the amount of heat in the kiln and the amount of vapor in the kiln atmosphere controls the rate of drying. Several types of conditions can be attained. High humidity and high temperatures can be used to heat a charge without significant drying. High temperature and low humidity is applied during quicker drying phases.