An Investigation on Drying Process: An Experimental Approach - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 158

Change in moisture content (MC) results in strain and strain induced stresses, the
magnitudes of which are suf¿ ciently large to produce con¿ gurational strain known as
warp and fracture. We dry wood for several reasons. Among the most important are
to minimize changes in dimension and to improve strength properties. Loss of water
results provide changes in many of the properties of wood, such as strength, both ther-
mal, and electrical conductivity. Perhaps greater importance is the fact that moisture
loss from the cell walls results for shrinkage.
When wood is dried during manufacture, all the liquid water in the cell lumen is
removed. The cell lumen always contains some water vapor, however. The amount
of water remaining in the cell walls of a ¿ nished product depends upon the extent of
drying during manufacture and the environment into which the product is later placed.
After once being removed by drying, water will recur in the lumen only if the product
is exposed to liquid water.
The porous materials such as wood have microscopic capillaries and pores which
cause a mixture of transfer mechanisms to occur simultaneously when subjected to
heating [1]. Transfer of vapor and liquids occurs in porous bodies in the form of dif-
fusion [2]. In essence, transfer of liquids can occur by means of diffusion arising from
hydrostatic pressure gradient.
Heat and mass transfer in porous media is a complicated phenomenon and a typi-
cal case is the drying of moist porous materials. Scheidegger [3] claimed 47 years ago
that the structure of porous media is too complex to be described either in macroscale
or microscale, not to mention the combination of water with matrix. To date, there is
no credible work proving that Scheidegger was wrong.
The convective drying is usually encountered in wood industry. The study of this
type of drying has attracted the attention of several authors. Among the works relating
to this question it cite the works of Plumb et al [4] and Basilico and Martin [5]. Con-
vective drying of timber is one of the oldest and time-consuming methods to prepare
the wood for painting and chemical treatments. The drying method can obviously have
signi¿ cant effect on the mechanical properties of wood. Major disadvantages of hot air
drying are low energy ef¿ ciency and lengthy drying time during the falling rate period.
The desired to achieve fast thermal processing has resulted in the increasing use of
radiation heating [6-8]. In this case, not only the removal of moisture is accelerated
but also a smaller À oor space is required, as compared to conventional heating and
drying equipment. In the drying of many species, especially, medium density, heavy
hardwoods, shrinkage, and accompanying distortion may increase as the temperature
is raised. So with species which are prone to distort it is normal to use comparatively
low kiln temperatures.
It has also been recognized that dielectric heating could perform a useful function
in drying of porous materials in the leveling out moisture pro¿ les across wet sample
[9-11]. This is not surprising because water is more reactive than any other material
to dielectric heating so that water removal is accelerated [12-18]. This leads to giv-
ing a temperature gradient inside the wood sample with opposite directions to that in
conventional drying processes.
The objective of any drying process is to produce a dried product of desired quality
at minimum cost and maximum throughput possible. High temperature and long dry-