An Investigation on Drying Process: An Experimental Approach - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 168

14.6 CONCLUSION
Although many methods of drying timber have been tried over the years only a few
of these enable drying to be carried out at a reasonable cost and with minimal damage
to the timber. The most common method of drying is to extract moisture in the form
of water vapor. To do this, heat must be supplied to the wood to provide the latent
heat of vaporization. The temperature of a piece of wood and of the air surrounding
it will also affect the rate of water evaporation from the wood surface. With kiln dry-
ing, warm or hot air is passed over the timber and at the start of the drying process the
temperature differential between the air and the wet wood will usually be large. As a
result, heat energy will be transferred from the air to the wood surface where it will
raise the temperature of both the wood and the water it contains. Water, in the form of
vapor, will then be lost from the wood surfaces, provided the surrounding air is not al-
ready saturated with moisture. This results in the development of a MC gradient from
the inside to the outside of the wood. As the temperature is raised this increases not
only the steepness of this moisture gradient, but also the rate of moisture movement
along the gradient and the rate of loss of water vapor from the surface of the wood. At
a given temperature the rate of evaporation is dependent on the vapor pressure differ-
ence between the air close to the wood and that of the more mobile air above this zone.

FIGURE 13 Tension test.

Unfortunately, the considerable bene¿ ts obtainable by raising the drying tempera-
ture cannot always be fully exploited because there are limits to the drying rates which
various wood species will tolerate without degrade. In contrast to hot air drying a
modern radiation drying provides temperature control and a steady, adequate À ow of