Forest Products Notes on...

Factors controlling the Drying of 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 certain factors which influence these processes  - some are controllable and some not. An understanding of these factors which control the drying of wood is vital if undesirable financial losses are to be avoided.

The rate at which the surface water is replaced by depends upon the permeability of the wood.

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

· tortuous and vary between species

·   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>>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.

The desirable movement of water when drying depends on having a MOISTURE GRADIENT (a reduction in moisture from the centre to the surfaces) - 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 gradient can lead to drying too slowly (because of pit aspiration might occur) AND PROBABLY MORE LIKELY MANY DRYING DEFECTS 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. Thus the rate of loss 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:


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. Thus:-

· In the “open air” it is important to ensure that appropriate air movement can occur through the stacks of timber.

· Fans may be used to enhance air replacement (about 2 m/s is the usual requirement).

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

The final factor controlling the drying of wood (and hence the quality of stock produced) is the structure of wood. So on the NEXT PAGE Read about the structure of softwoods and hardwoods and how their structure affects their rate of drying...

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For a good appreciation of how wood can be optimally dried without serious financial loss it is important to understand some of the theory about the subject.  So on the  PREVIOUS PAGE read about Drying Timber: Fundamentals: Concepts and Definitions...

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