Porous Materials Such As Wood Biology Essay

such as physical or chemical transmutations, which in bend, may do alterations in merchandise 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 belongingss have been studied for many stuffs, but wood merchandises have rarely been the topic. Spruce wood samples ( length = 140 millimeter and diameter = 21.mm ) were dried utilizing the following drying governments: convective ( 100 & A ; Ugrave ; ‘ C and 40 0C. ) , microwave ( 480, 790, and1000 W power ) and infrared ( 100 % power ) . The dried samples were so subjected to tensile lading in order to measure the mechanical belongingss.

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It is concluded that the microwave drying can better the strength ofdried samples significantly.

Keywords: Thermal drying, wood, mechanical belongingss


Porous stuffs such as wood have microscopic capillaries and pores which cause a mixture of transportation mechanisms to happen at the same time when subjected to warming ( Haghi, 2005 ) . Transportation of vapour and liquids occurs in porous organic structures in the signifier of diffusion ( Haghi et al, 2005. In kernel, transportation of liquidscan happen by agencies of diffusion originating from hydrostatic force per unit area gradient.Heat and mass transportation inporous media is a complicated phenomenon and a typical instance is the drying of moist porous stuffs. Scheidegger ( 1958 ) claimed 50 old ages ago that the construction of porous media is excessively complex to be described exactly either in macro-scale or micro-scale, non to advert the combination of H2O with matrix. To day of the month, there is no believable work turn outing that Scheidegger was incorrect.

Convective drying is normally encountered in wood industry. The survey of this type of drying has attracted the attending of several writers. Among the plants associating to this inquiry we cite the plants of Plumb et Al ( 1958 ) and Basilico and Martin ( 1984 ) . Convective drying of lumber is one of the oldest and time- consuming methods to fix the wood for picture and chemical interventions. The drying method can evidently hold important consequence on the mechanical belongingss of wood. Major disadvantages of hot air drying are low energy efficiency and drawn-out drying clip during the falling rate period.

The desired to accomplish fast thermic processing has resulted in the increasing usage of radiation warming. In this instance, non merely the remotion of wet is accelerated but besides a smaller floor infinite is required, as compared to conventional warming and drying equipment.It has besides been recognized that dielectric warming could execute a utile map in drying of porous stuffs in the grading out wet profiles across wet sample. This is non surprising because H2O is more reactive than any other stuff to dielectric heating so that H2O remotion is accelerated ( Haghi,2003, 2004a, B ) . This leads to giving a temperature gradient inside the wood sample with opposite waies to that in conventional drying procedures.

The aim of any drying procedure is to bring forth a dried merchandise of coveted quality at minimal cost and upper limit throughput possible. High temperature and long drying times required removing H2O from lumber in conventional hot air drying. Microwave and infrared drying could be rapid, more unvarying and- 1energy efficient compared to conventional hot air drying. The chief intent of this survey is to look into 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 state.

The diameter and tallness of the specimens were about 300mm and 21mm severally. A programmable domestic microwave oven ( Deawoo, KOC-1B4K ) , with a maximal power end product of 1000 W at 2450 MHz was used. The oven has the installation to set power ( Wattage ) supply and the clip of processing.

The hot air drying experiments were performed in a pilot tray drier consisted a temperature accountant. Air was drawn into the canal through a mesh guard by a motor driven axial flow fan impeller whose velocity can be controlled in the canal. The infrared drier was equipped with eight ruddy glass lamps ( Philips ) with power175 W, each breathing radiation with extremum wavelength 1200 nanometer. Radiators were arranged in three rows, with three lamps in each row. Dryer was equipped with mensurating devices, which made it possible tocontrol air parametric quantities. The sum of H2O in a piece of wood is known as its wet content. Becausethis is expressed as a per centum of the dry weight of the piece, non of the entire weight, it is possible to hold wet of contents good over 100 % .

A really of import factor which affects the strength is the wet content. The wet content of green wood varies greatly from one species to another. Moisture content can change between seemingly similar pieces of the same species and in add-on there may be differences, between and within species, in the rates at which wet is lost from lumber during drying. These built-in differences in lumber mean that it is of import during the drying procedure to be able to supervise wet content and look into that the drying procedure is continuing right.All the 50 dried samples were tested on a cosmopolitan Tension Test machine theoretical account ( Hounsfield HS100KS ) , with a loading capacity of 100 KN. During the tensile testing, the stress-strain curves every bit good as the extremum burden were recorded.


As the samples were collected at different times, it is obvious that the initial wet content of all samples was non the same. In order to normalise the drying curves, the information affecting per centum dry footing called as wet ratio versus clip. Typical features of drying curves of wood samplesduring different drying operation will be discussed in the undermentioned subdivisions.

Hot air drying

Conventional hot air drying is one of the most often 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 normally take topographic point in the falling rate period. In kernel, air in the oven is saturated, by clip, and forms a thick movie around thewood sample. That prevents effectual separation of the evaporated wet from the wood.

This may bethe ground for being of changeless rate period in this survey.

Microwave drying

Microwave drying is an alternate drying method, which is late used in different industries. The consequence of altering power end product in the microwave oven on the wet content is shown in the Figures 4 and 5. At all power degrees, drying curves were tended to stop at about the same clip. The ascertained initialacceleration of drying may be caused by leting rapid vaporization and conveyance of H2O.

Infrared drying

Infrared radiation is transmitted through H2O at short wavelength, it is absorbed on the surface. Infrared radiation has some advantages over convective warming. Heat transportation coefficients are high, the procedureclip is short and the cost of energy is low.

In this survey, the drying clip was reduced by about 34 %comparison 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 changeless rate period followed by a falling rate period, Figures 6 and 7 indicates that the infrared had merely a falling rate period.

Stress-strain curves

The consequences of tensile burden of dried samples are presented in Figures 8-10.

It is clear that the microwave dried dapper specimen with failure strength of 49.6 Mpa has made a important- 2belongings betterment ( 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 remotion of wet depends on the conditions of the hot air, the belongingss of the wood and the design of the drier. In drying, it is obvious that the H2O that is slackly held will beremoved most easy. Thus it would be expected that the drying rates would diminish as wet content lessenings, with the staying H2O being bound more and more strongly as its measure lessenings.

Thealteration from changeless drying rate occurs at different wet contents for different forests. Another pointof importance is that many forests do non demo a changeless drying periods. They do, nevertheless, frequently show rather a crisp interruption after a easy and steady worsening drying-rate period and the construct of changeless rate is still a utile estimate. The terminal of the constant-rate period, at the interruption point of drying rate curves, signifies that the H2O has ceased to act as if it were at a free surface and that factors other than vapor-pressure differences are act uponing the rate of drying. Thereafter the drying rate lessenings 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 lumber, and upon the altering energy-binding form of the H2O molecules. Very small theoretical information is available for drying of forests in this part and experimental drying curves are the lone equal usher to plan.


Although many methods of drying lumber have been tried over the old ages merely a few of these enable drying to be carried out at a sensible cost and with minimum harm to the lumber. The most common method of drying is to pull out wet in the signifier of H2O vapour. To make this, heat must be supplied to the wood to supply the latent heat of vaporisation.

The temperature of a piece of wood and of the air environing it will besides impact the rate of H2O vaporization from the wood surface. With kiln drying, warm or hot air is passed over the lumber and at the start of the drying procedure the temperature derived function between the air and the moisture wood will normally be big. As a consequence, heat energy will be transferred from the air to the wood surface where it will raise the temperature of both the wood and the H2O it contains. Water, in the signifier of vapour, will so be lost from the wood surfaces, provided the environing air is non already saturated with wet. This consequences in the development of a wet content gradient from the interior to the exterior of the wood. As the temperature is raised this increases non merely the abruptness of this wet gradient, but besides the rate of wet motion along the gradient and the rate of loss of H2O vapour from the surface of the wood. At a given temperature the rate of vaporization is dependent on the vapor force per unit area difference between the air near to the wood and that of the more nomadic air above this zone. Unfortunately the considerable benefits gettable by raising the drying temperature can non ever be to the full exploited because there are bounds to the drying rates which assorted wood species will digest without degrade.

In the drying of many species, particularly average denseness and heavy hardwoods, shrinking and attach toing deformation may increase as the temperature is raised. So with species which are prone to falsify it is normal to utilize relatively low kiln temperatures.In contrast to air drying a modern radiation drying provides temperature control and a steady and equal flow of air over the timber surface. The air flow rate and way is controlled by fans and the temperature and comparative humidness of the air can be adjusted to accommodate the species and sizes of lumber being dried.

It is therefore possible to do full usage of the addition in drying rate which can be achieved by raising the temperature to the maximal value which a peculiar lumber species can digest without inordinate degrade.The experiments shows that in microwave warming, the drying clip is significantly reduced while the strength were comparatively improved in comparing to the strength obtained in conventional drying. It is besides noted that the infrared drying can cut down the strength of the spruce forests significantly. The above treatment suggests farther probe for future work on different specimens.- 3


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