An Investigation on Drying Process: An Experimental Approach - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 166

As the outer layers dry below the ¿ ber saturation point their tendency to start
shrinking is resisted by the wetter interior so that stresses develop. If these stresses
become large they can lead to a number of drying defects. The various methods used
to dry lumber can be divided into two categories: the commonly used procedures of
air-drying and kiln-drying, and the specialized techniques. Although the primary ob-
jective of all drying methods is to remove water from wood, the selection of a particu-
lar procedure will depend on several other factors such as capital investment, energy
sources, production capacity, drying ef¿ ciency, and end product.
14.3 EXPERIMENTAL
Fifty cylindrical green wood samples of Spruce were derived from Guilan province.
The diameter and height of the specimens were approximately 300 mm and 21 mm
respectively. A programmable domestic microwave oven (Daewoo, KOC-1B4K), with
a maximum power output of 1000 W at 2450 MHz was used. The oven has the facility
to adjust power (Wattage) supply and the time of processing. The hot air drying ex-
periments were performed in a pilot tray dryer consisted a temperature controller. Air
was drawn into the duct through a mesh guard by a motor driven axial flow fan impel-
ler whose speed can be controlled in the duct. The infrared dryer was equipped with
eight red glass lamps (Philips) with power 175 W, each emitting radiation with peak
wavelength 1200 nm. Radiators were arranged in three rows, with three lamps in each
row. Dryer was equipped with measuring devices, which made it possible to control
air parameters. The amount of water in a piece of wood is known as its MC. All the
fifty dried samples were tested on a universal tension test machine model (Hounsfield
HS100KS), with a loading capacity of 100 kN. During the tensile testing, the stress-
strain curves as well as the peak load were recorded.
14.4 RESULTS
The conventional hot air drying is one of the most frequently used operations. The
drying curves for conventional hot air drying of wood samples are shown in Figures
1-5. It can be observed that the drying usually take place in the falling rate period. In
essence, air in the oven is saturated, by time, and forms a thick film around the wood
sample. That prevents effective separation of the evaporated moisture from the wood.
This may be the reason for existence of constant rate period in this study.
The microwave drying is an alternative drying method, which is recently used in
different industries. The effect of changing power output in the microwave oven on the
MC is shown in the Figures 6-10. At all power levels, drying curves were tended to
end at about the same time. The observed initial acceleration of drying may be caused
by allowing rapid evaporation and transport of water.
Infrared radiation is transmitted through water at short wavelength it is absorbed
on the surface. Infrared radiation has some advantages over convective heating. Heat
transfer coef¿ cients are high, the process time is short and the cost of energy is low. In
this study, the drying time was reduced by nearly 34% compare to hot air drying. The
drying curves were plotted in Figures 11 and 12. In contrast to the hot air drying curves
which had a short constant rate period followed by a falling rate period, Figures 11 and
12 indicates that the infrared had only a falling rate period.