共查询到19条相似文献,搜索用时 0 毫秒
1.
James Fuller 《Drying Technology》2013,31(4-5):1023-1031
ABSTRACT This paper emphasizes the established need for a stress based kiln control system. It refers to I) the work which verified that the initial shrinkage rate changes within a board are the result of reduced stress levels and 2) the new statistical method short/long-term slope comparison method used to detect slope changes. The paper continues by giving the explanation for the additional shrinkage slope changes as the equilibrium moisture content changes during fan reversal and kiln schedule advancements. The kiln schedule advancements can cause both shrinkage and swelling. Reduced drying rate resulting in reduced shrinkage rate explains other changes. A stress based automated kiln control system is conceptually possible with 1) shrinkage rate changes understood and 2) a production useful statistical method to detect these changes. 相似文献
2.
James Fuller 《Drying Technology》2000,18(1):261-278
The conclusions drawn by researchers on the source of changing shrinkage rates are discussed. The objective of this study was to determine the source of changing shrinkage rates early in drying and whether shrinkage would make a useful parameter for an automated kiln control system. In this study, four loads of red oak and one of maple were kiln dried using established schedules Shrinkage of the lumber was continuously recorded, and moisture content and released elastic strain were periodically recorded. The moisture content and strain data showed that the changing shrinkage rates early in drying boards were caused by reduced internal stress levels, not the occurrence of fiber saturation point or temperature as reported by some. Results of this study indicate it may be possible to develop an automated kiln system using shrinkage as the controlling parameter. This would reduce the drying time while avoiding additional drying defects. 相似文献
3.
The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics. 相似文献
4.
James Fuller 《Drying Technology》2013,31(1-2):261-278
ABSTRACT The conclusions drawn by researchers on the source of changing shrinkage rates are discussed. The objective of this study was to determine the source of changing shrinkage rates early in drying and whether shrinkage would make a useful parameter for an automated kiln control system. In this study, four loads of red oak and one of maple were kiln dried using established schedules Shrinkage of the lumber was continuously recorded, and moisture content and released elastic strain were periodically recorded. The moisture content and strain data showed that the changing shrinkage rates early in drying boards were caused by reduced internal stress levels, not the occurrence of fiber saturation point or temperature as reported by some. Results of this study indicate it may be possible to develop an automated kiln system using shrinkage as the controlling parameter. This would reduce the drying time while avoiding additional drying defects. 相似文献
5.
S. Pang 《Drying Technology》2013,31(8):1677-1696
ABSTRACT A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified. Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available. 相似文献
6.
DRYING MODEL WITH NON-ISOTROPIC SHRINKAGE DEFORMATION UNDERGOING SIMULTANEOUS HEAT AND MASS TRANSFER
《Drying Technology》2013,31(7):1441-1460
A model of simultaneous heat and moisture transfer in a cylindrical sample was coupled with the virtual work principle applicable to a body undergoing shrinkage deformation in two dimensions. Non-constant physical and thermal properties were also incorporated in the model. Governing equations and boundary conditions were solved numerically using Galerkin's finite element method. To check the mathematical model drying experiments were carried out. A cylindrical potato was used as a drying sample. Experimental conditions were as follows: a drying temperature of 333 K, relative humidity of 5.4%, and air velocity of 1.6 m/s. We obtained the central temperature of the sample, average moisture content, and the shrinkage change in the axial and radial directions during drying. It was observed that the shrinkage coefficients in the axial and the radial directions were significantly different during air-drying. Comparison between predicted and experimental results provides satisfactory agreement. 相似文献
7.
SHRINKAGE CHARACTERISTICS OF STRAWBERRIES OSMOTICALLY DEHYDRATED IN COMBINATION WITH MICROWAVE DRYING 总被引:1,自引:0,他引:1
The subject of this paper is to study the shrinkage of strawberries as a function of the moisture ratio during microwave-convective drying. Strawberries were pre-treated and osmotically dehydrated and dried at power levels 0.1 and 0.2W/g, based on initial mass. The main results showed that the shrinkage has a linear relation with moisture ratio; the equivalent diameter of the strawberry has a reciprocal logarithmic function with moisture ratio; the change in volume was bigger for the fruits osmotically dehydrated than for the not osmotically dehydrated ones, power level exerts influence on shrinkage and equivalent diameter for pre-treated and osmotically dehydrated samples. 相似文献
8.
ABSTRACT Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards. The results indicate a receding evaporation front behaviour for sapwood above approximately 40-50% MC when the moisture flux is heat transfer controlled. After that we finally reach a period where bound water diffusion is assumed to control the drying rate. The heat transfer from the circulating air to the evaporation front controls the migration flux. In many industrial kilns the heating coils therefore have too small heat transfer rates for batches of thin boards and boards with high sapwood content. 相似文献
9.
In the present study, an improved numerical heat transfer model has been developed for a rotary kiln used for drying and preheating of wet iron ore. The present model includes radiation exchange among hot gas, refractory wall and the solid surface, transient conduction in the refractory wall, and mass and energy balances of the hot gas and the solids. The contribution of gas convection has also been taken into account in terms of a fraction of the radiative heat transfer to the inner refractory wall and the solid surface. The computer results show that the present model can predict the length of the kiln as well as axial solid and gas temperature distributions with reasonably good accuracy. A detailed parametric study reveals that a good design of a rotary kiln requires medium gas flow rate, small angle of inclination and low rotational speed of the kiln. 相似文献
10.
Continuous measurement and recording of the core temperature in wood samples during convective drying in an airstream reveals that the temperature difference between the airstream and the core of the wood reflects almost quantitatively the influence of the external conditions and the characteristic features of the drying wood material on the course of the drying velocity. Conclusions concerning the drying practice of timber kiln-drying which arise from the experimental findings are discussed. 相似文献
11.
ABSTRACT Continuous measurement and recording of the core temperature in wood samples during convective drying in an airstream reveals that the temperature difference between the airstream and the core of the wood reflects almost quantitatively the influence of the external conditions and the characteristic features of the drying wood material on the course of the drying velocity. Conclusions concerning the drying practice of timber kiln-drying which arise from the experimental findings are discussed. 相似文献
12.
油烧辊道窑预干燥带干燥过程微观数学模型的研究 总被引:1,自引:0,他引:1
本文分析了辊道窑内预干燥带传热及干燥过程的特点,引入Morgan的蒸发面后退模型,建立了陶瓷制品干燥过程中温度场分布及湿含量计算的微观数学模型。 相似文献
13.
Abstract Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air. The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower. 相似文献
14.
Abstract Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with an imposed hot air flow and a conductive heat flux at the wall. The model equations are numerically solved using a finite volume method. The numerical simulation gives the time and space evolution of temperature when the lateral area of the cylinder is heated by a constant density flux and a constant temperature. The influence of different parameters (essentially the ratio of heat flux to the heat capacity of flow, and the dryer geometry) on the relative moisture content and the drying time is examined. 相似文献
15.
ABSTRACT In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low. 相似文献
16.
Abstract In this study, the critical moisture content ( CMC. ) and the internal moisture content profiles during the constant rate period were determined experimentally for several conditions of convective drying of plaster. The experimental CMC values obtained allow us to validate the theoretical model of prediction of this parameter presented by KEEY [1] and SUZUKI [2] for drying rates higher than 3×10?4 kg/(m2.s) and to determine the equivalent moisture difliisivity at the external surface of plaster which was found to be equal to 3.9 × 10?9 m2/s On the other hand, expressions of the internal humidity profiles were determined without solving the second FICK's law and were found to be same that the ones obtained by KEEY [1] for thick products and KRJSCHER and KROLL [3] for thin products by solving this law. The moisture diffusivity at the surface previously obtained was used in these equations to predict the internal humidity profiles during the constant rate period Comparison between theoretical predictions and experimental humidity profiles obtained on plaster shows a quite good agreement. 相似文献
17.
Drying of shredded coconut is usually carried out commercially in order to facilitate storage over reasonable periods of time and to obtain advantages of reduction of weight and volume in transport and packaging. Fluidised bed drying of materials is generally accepted to be an efficient method of drying. Experiments were carried out to investigate the behaviour of fluidisation of shreddet coconut at various moisture contents. The pressure drop was measured across random packings of shredded coconut. It is seen that shredded coconut does not fluidise easily at moisture contents greater than 0.55 (55 weight percent moisture). It is also seen that fluidisation can be easily achieved by drying shredded coconut to moisture contents between approximately 0.25 to 0.55. The pressure drop characteristics within this region is seen to closely resemble the theoretical behaviour of a fluidised bed. It is also seen that particles of shredded coconut in beds of moisture contents less than 0.25 tend to undergo pneumatic transport if efforts are made to fluidise such beds. 相似文献
18.
《Drying Technology》2013,31(1):127-135
The ratio of wood temperature to wet-bulb temperature (WT/ WBT) with the wood moisture, aiming at its use in the control of the drying process in drying kilns, were correlated. Slash pine boards measuring 25 mm thick and 750 mm length were dried in an electric forced convection kiln, using four temperatures and two air velocities. The process was controlled through a computerized system comprising a microcomputer and data acquisition unit. The analyses indicated a good correlation (0.97 < r < 0.99) between the coefficient WT/WBT and the moisture content of the wood. The adjustment of polynomial models was satisfactory in the range from green condition to 10% moisture content, at the temperature range and air velocity studied. These results, variables, together with the ease and speed of measurement of the suggest that the coefficient WT/WBT can be used as an alternative for the control of the drying process. However, the practical application of this system still depends on further research and development. 相似文献
19.
Kamyar Haghighi 《Drying Technology》2013,31(3):451-464
The objective of this study is the formulation of a finite element model that could be used to analyze the stress crack formation in a viscoelastic sphere resulting from temperature and moisture gradients during the drying process. Numerical solutions to the simultaneous moisture and heat diffusion equations describing moisture removal and heat intake process for the sphere are obtained. The distribution and gradients of temperature and moisture developed inside the sphere during drying are established. The calculated temperature and moisture gradients are used in a finite element analysis of the thermo-hydro viscoelastic boundary value problem to simulate the stresses in the body. The model is used to solve a sample problem of drying a soybean kernel. The simulated drying curve for the soybean model is obtained and compared favorably with the experimental results reported in the literature. Tangential stress, as a criteria for failure, is shown to change from compressive to tensile stress as it approaches the surface. It reaches its peak value at the surface in one hour and then decays slowly. The effect of different drying conditions is studied and the results are discussed. 相似文献