An Investigation on Drying Process: An Experimental Approach - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 167

All dried samples were tested on a Houns¿ eld universal tension test machine with
a loading capacity of 100 kN (Figure 13). The results of tensile loading of dried sam-
ples are presented in Figures 14-17. It is clear that the microwave dried spruce speci-
men with failure strength of 49.6 MPa has made a signi¿ cant property improvement
(Figure 14). The normal stiffness of infrared dried sample is reported as 35.0 MPa
(Figure 15) whereas the oven dried sample showed strength of about 44.5 MPa (Figure
16). From Figure 17 it is revealed that the natural convection dried specimens are the
strongest of about 50 MPa. In practice the drying time for this can take up months and
years. In Figure 18 the strength of dried samples are compared for a better judgment.
14.5 APPLICATION OF RESULTS
In wood drying process we should note that the wood can hold moisture in the cell lu-
men (cavity) as liquid or “free” water, or as adsorbed or “bound” water attached to the
cellulose molecules in the cell wall. Meanwhile, the occurrence of the free water does
not affect the properties of wood other than its weight. Bound water, however, does
affect many properties of wood, and is more difficult to remove in the drying process.
Microscopically, the dimensional change with MC is anisotropic (referring to the fact
that wood has very different properties parallel to the fact grain versus the transverse
direction). As the MC decreases, wood shrinks conversely, as the MC increases, wood
swells or grows larger. The process of drying focuses on producing wood with an MC
about the same as the equilibrium value for the intended service environment.
For the design of dryers it is necessary to carry out drying experiments at vari-
ous drying conditions. Experimentally determined drying times, transition points, and
constant rate regime temperature can be used as a base case for the analytical results.
Based on the information from the experimental trials, runs with lower amounts of
moisture to evaporate, and higher dryer temperatures, should be expected to dry faster
and reach transition point more rapidly. After an initial increase or decrease of the rate
of drying, the drying process enters the constant rate period. This initial change of the
rate of drying is caused by a variation of the surface temperature which in turn results
into a change of vapor density.
It can be noted that time interval of drying process is solely determined by external
conditions. Once the drying process has entered the falling rate period, the external
conditions become relatively unimportant compared to the internal parameters.
By comparing runs with the same initial moisture, we see that as oven tempera-
ture increases, the transition points are reached more quickly and total drying times
are shorter. Sample temperatures are higher because they are exposed to higher heat
transfer rates, giving rise to higher mass transfer rates during the constant rate regime.
The experimental study suggests that the humidity of the free stream should be as
low as possible. Partial recirculation, 100% fresh air intake, or dehumidi¿ cations are
some of the possible ways to accomplish this task, but a cost analysis is imperative
before deciding on any option.
Reduction of the drying time in microwave heater seems to be a motivating cost
saving factor for industries. In this case a moderate mechanical property is obtained
(Table 1). To minimize directional variations in use, wood needs to be dry enough to
match the service environment.