such as physical or chemical transformations, which in turn, may cause changes in product quality. This article surveys the impacts of convective, infrared and microwave drying on the strength of dried wood. The effects of drying methods on mechanical properties have been studied for many materials, but wood products have seldom been the subject. Spruce wood samples (length = 140 mm and diameter = 21.mm) were dried using the following drying regimes: convective (100 Ù' C and 40 0C.), microwave (480, 790, and1000 W power) and infrared (100% power). The dried samples were then subjected to tensile loading in order to evaluate the mechanical properties. It is concluded that the microwave drying can improve the strength of
dried samples significantly.
Keywords: Thermal drying, wood, mechanical properties
Porous materials such as wood have microscopic capillaries and pores which cause a mixture of transfer mechanisms to occur simultaneously when subjected to heating (Haghi, 2005). Transfer of vapor and liquids occurs in porous bodies in the form of diffusion (Haghi et al, 2005. In essence, transfer of liquids
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can occur by means of diffusion arising from hydrostatic pressure gradient.Heat and mass transfer in
porous media is a complicated phenomenon and a typical case is the drying of moist porous materials. Scheidegger (1958) claimed 50 years ago that the structure of porous media is too complex to be described precisely either in macro-scale or micro-scale, not to mention the combination of water with matrix. To date, there is no credible work proving that Scheidegger was wrong.
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 we cite the works of Plumb et al (1958) and Basilico and Martin (1984). Convective 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 significant effect on the mechanical properties of wood. Major disadvantages of hot air drying are low energy efficiency 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. In this case, not only the removal of moisture is accelerated but also a smaller floor space is required, as compared to conventional heating and drying equipment.
It has also been recognized that dielectric heating could perform a useful function in drying of porous materials in the leveling out moisture profiles across wet sample. This is not surprising because water is more reactive than any other material to dielectric heating so that water removal is accelerated (Haghi,
2003, 2004a,b). This leads to giving 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 drying times required removing water from timber in conventional hot air drying. Microwave and infrared drying could be rapid, more uniform and
energy efficient compared to conventional hot air drying. The main purpose of this study is to investigate the impacts of convective, infrared and microwave drying on the strength of dried wood.
Fifty cylindrical green wood samples of Spruce were obtained from Guilan province. The diameter and height of the specimens were approximately 300mm and 21mm respectively. A programmable domestic microwave oven (Deawoo,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 experiments 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 impeller 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
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control air parameters. The amount of water in a piece of wood is known as its moisture content. Because
this is expressed as a percentage of the dry weight of the piece, not of the total weight, it is possible to have moisture of contents well over 100%.
A very important factor which affects the strength is the moisture content. The moisture content of green wood varies greatly from one species to another. Moisture content can vary between apparently similar pieces of the same species and in addition there may be differences, between and within species, in the rates at which moisture is lost from timber during drying. These inherent differences in timber mean that it is important during the drying process to be able to monitor moisture content and check that the drying process is proceeding correctly.
All the 50 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.
As the samples were collected at different times, it is obvious that the initial moisture content of all samples was not the same. In order to normalize the drying curves, the data involving percentage dry basis called as moisture ratio versus time. Typical characteristics of drying curves of wood samples
during different drying operation will be discussed in the following sections.
Hot air drying
Conventional hot air drying is one of the most frequently used operations. The drying curves for hot air drying of wood samples are shown in Figures 1-3. 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.
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 moisture content is shown in the Figures 4 and 5. 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 coefficients 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 6 and 7. In contrast to the hot air drying curves which had a short constant rate period followed by a falling rate period, Figures 6 and 7 indicates that the infrared had only a falling rate period.
The results of tensile loading of dried samples are presented in Figures 8-10.It is clear that the microwave dried spruce specimen with failure strength of 49.6 Mpa has made a significant
property improvement (Figure 8). The normal stiffness of infrared dried sample is reported as 35.0 Mpa
(Figure 9) whereas the oven dried sample showed strength of about 44.5 Mpa (Figure 10).
In general, the rate of removal of moisture depends on the conditions of the hot air, the properties of the wood and the design of the dryer. In drying, it is obvious that the water that is loosely held will be
removed most easily. Thus it would be expected that the drying rates would decrease as moisture content decreases, with the remaining water being bound more and more strongly as its quantity decreases. The
change from constant drying rate occurs at different moisture contents for different woods. Another point
of importance is that many woods do not show a constant drying periods. They do, however, often show quite a sharp break after a slowly and steady declining drying-rate period and the concept of constant rate is still a useful approximation. The end of the constant-rate period, at the break point of drying rate curves, signifies that the water has ceased to behave as if it were at a free surface and that factors other than vapor-pressure differences are influencing the rate of drying. Thereafter the drying rate decreases and this is called the falling-rate period of drying. The rate-controlling factors in the falling-rate period are complex, depending upon diffusion through the timber, and upon the changing energy-binding pattern of the water molecules. Very little theoretical information is available for drying of woods in this region and experimental drying curves are the only adequate guide to design.
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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 drying, 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 already saturated with moisture. This results in the development of a moisture content 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 difference between the air close to the wood and that of the more mobile air above this zone. Unfortunately the considerable benefits obtainable by raising the drying temperature cannot always be fully exploited because there are limits to the drying rates which various wood species will tolerate without degrade. In the drying of many species, especially medium density and 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.
In contrast to air drying a modern radiation drying provides temperature control and a steady and adequate flow of air over the timber surface. The air flow rate and direction is controlled by fans and the temperature and relative humidity of the air can be adjusted to suit the species and sizes of timber being dried. It is thus possible to make full use of the increase in drying rate which can be achieved by raising the temperature to the maximum value which a particular timber species can tolerate without excessive degrade.
The experiments shows that in microwave heating, the drying time is significantly reduced while the strength were relatively improved in comparison to the strength obtained in conventional drying. It is also noted that the infrared drying can reduce the strength of the spruce woods significantly. The above discussion suggests further investigation for future work on different specimens.