· influences moisture movement rates via the permeability.
· Wood is anisotropic and hence the permeability is anisotropic (different in different
directions -
· Pit “aspiration”. Bordered pits (in softwoods) can become closed as a water meniscus moves through the pit and the torus deflects to touch and become stuck to the pit border (see diagram below). This happens more in earlywood than latewood – hence on drying wood earlywood permeability is reduced more than latewood permeability (see below).
The internal structures of Softwoods are simpler and much less varied than those in hardwoods. Also more is known about softwood structure than hardwood structure because a great deal of research has been undertaken on softwoods because of their importance to the pulp and paper industries.
In softwoods flow of fluids (Note: for our purposes sap can be considered to be mainly water) is:
· along tracheid cells.
· cell to cell via bordered pits (visualise wood being made up rather like a bundle
of drinking straws (with their ends sealed) -
· dependant on the condition of the bordered pits – they might be open for flow or closed for flow (called “aspirated”).
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Above: diagram of a bordered pit on the tangential walls of a softwood tracheid (cross section)
Key:
1 -
2 -
3 -
4 -
5 -
Above: diagrammatic representation of the tracheid cells in softwood.
Bordered pits on the tangential walls allow flow of liquid from cell to cell -
In sapwood:
Green condition:
· Torus is central -
· Earlywood flow is greater than latewood flow
During drying:
· The air-
· Earlywood bordered pits have more flexible margos and less deep “chambers” and are thus more easily aspirated.
· Latewood bordered pits have more rigid margos and deep “chambers” and are thus less easily aspirated.
· Thus drying reduces the permeability of earlywood more than it does latewood.
In heartwood:
· Sapwood dries as it becomes heartwood.
· many pits (esp. earlywood) aspirate.
· margos become encrusted with extractives.
Thus the permeability of heartwood is greatly reduced.
Hardwoods have complex and varied structures.
Flow is:
· Longitudinally: high in sapwood because of the occurrence of long vessels
· Radially: via rays
· Tangentially: complex -
Flow through hardwoods is much less dependent on pits interconnecting the cells and so drying has less effect on hardwood permeability than on softwoods.
Flow in heartwood:
· Gums, resins and tyloses (remains of dead cell contents) reduce the permeability of the wood.
Temperatures, Relative Humidities and Air Flow can, to some extent be controlled. The internal structure of softwoods and hardwood drying stock cannot be controlled, however it is important to appreciate that this factor is of critical influence on the optimal drying of timber.
Reasons to Dry Timber: An Introduction to Timber Drying
Timber Drying -
Factors controlling the Drying of Wood
The Structures of Softwoods and Hardwoods and their effect on Wood Drying
An Introduction to the Air Seasoning of Timber
Layout of a Timber Drying Yard
Design of Stacks in the Timber Drying Yard
Types of Kiln Drying Equipment
Benefits of Kiln Dried Timber production compared to Air Seasoning Timber
Using a Dehumidifier to Dry Wood
High Temperature Timber Drying
Avoiding Case Hardening by Monitoring the Drying of Timber. Also Collapse & Staining of Timber