AIRFLOW BEHAVIOUR IN TIMBER (LUMBER) KILNS

ABSTRACT

Even drying of stacked timber boards in a kiln depends, amongst other things, on having a uniform airflow through the pile. A study of possible airflow maldistribution in existing kilns has been carried out using a Perspex model of a kiln, with water as the fluid, which has been designed to be geometrically similar to an actual kiln and have dynamically similar flows. Measurements of local velocities were possible by tracing the flow of fine gas bubbles. The standard kiln arrangements result in significant recirculation zones before and after the stack, with a non-uniform incident velocity profile. Different design alternatives for the geometry of the plenum spaces on either side of the stack have been investigated.     

AIRFLOW DISTRIBUTIONS IN THE FILLET SPACES OF A TIMBER STACK

An important requirement for producing evenly dried timber (lumber) in a kiln is that the airflow is uniformly distributed throughout the timber stack. The airflow distribution within the timber stack of a semi-scale kiln, which has multiple fans, was investigated to determine the possible causes of any airflow non-uniformity. Airflow measurements along the length of the timber stack illustrated the importance of ensuring that each fan generates the same flowrate, and that end-baffles are placed on either side of the timber stack to reduce bypass around these edges. Airflow measurements down the height of the timber stack demonstrated the effects of a separation zone near the air-inlet face of the stack, which partially reduced the airflow through the upper portion of the stack, and of non-aligned boards, which generated a saw-toothed velocity profile. Once the data had been smoothed to account for these effects, the down-stack velocity profile took on the expected form, with the maximum and minimum velocities at the top and bottom of the stack, respectively. Comparison of the velocity profiles at the air-inlet and outlet faces of the stack indicated a significant redistribution of the flow through the stack. The prediction of a one-dimensional model of airflow in a kiln was shown to be in good agreement with the smoothed experimental data at the air outlet of the stack.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

ABSTRACT

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.     

The Application of Mathematical Models to the Commercial High-Temperature Drying of Softwood Lumber

High temperature drying of softwood is used because it provides much faster drying rate than is possible at lower temperatures. However, the occurrence of some drying defects limits its use where the quality is critical. In order to understand the drying phenomena and to describe the drying processes, numcrous mathematical models have been developed in the past two decades. The diffusion model is the earliest attempt to describe wood drying processes and is relatively simple in form, so it is often used for stress analysis. However.further substantial work is still required before it is possible to apply the stress model to kiln control. Recently. transpon-based mathematical models have been receiving attention in modelling studies. This review discusses one of these models, a physiological-transport-based model, which has been further applied to the drying of mixed sap/heartwood boards and the drying of a kiln-wide stack. The mixed boards with a thin heanwood layer parallel to the flat surface are considered to have added difficulty in drying. In the analysis of the timber stack drying, a kiln-wide model is proposed in which the above physiological-transpon-based model is used to generate the characteristic drying curves. Airflow reversal is essential in kiln     

The Application of Mathematical Models to the Commercial High-Temperature Drying of Softwood Lumber

ABSTRACT

High temperature drying of softwood is used because it provides much faster drying rate than is possible at lower temperatures. However, the occurrence of some drying defects limits its use where the quality is critical. In order to understand the drying phenomena and to describe the drying processes, numcrous mathematical models have been developed in the past two decades. The diffusion model is the earliest attempt to describe wood drying processes and is relatively simple in form, so it is often used for stress analysis. However.further substantial work is still required before it is possible to apply the stress model to kiln control. Recently. transpon-based mathematical models have been receiving attention in modelling studies. This review discusses one of these models, a physiological-transport-based model, which has been further applied to the drying of mixed sap/heartwood boards and the drying of a kiln-wide stack. The mixed boards with a thin heanwood layer parallel to the flat surface are considered to have added difficulty in drying. In the analysis of the timber stack drying, a kiln-wide model is proposed in which the above physiological-transpon-based model is used to generate the characteristic drying curves. Airflow reversal is essential in kiln     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high final moisture content.     

浅谈料垛码法对倒焰式燃气窑内气流分布及温度均匀性的影响

通过料垛码法形成的气流通道大小对倒焰式燃气窑内气流分布以及温度均匀性的影响进行了分析,认为在一定的条件下,通道大小与气体流动速度和流量成正比,根据这一结论,可对倒焰燃气窑内料垛码法起到一定的指导作用。     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

ABSTRACT

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high final moisture content.     

石油焦煅烧回转窑内多场耦合数值仿真与操作参数的优化

石油焦烧损严重是石油焦煅烧回转窑存在缺陷。为研究降低石油焦烧损的方法，应用计算流体力学软件，对窑内流场、温度场、浓度场、燃烧释热场进行耦合计算，并对二次风与三次风的位置与方向对窑内各种场的影响进行了研究。结果表明，改变二次风的位置与方向对煅烧带氧气浓度分布的影响较大，适当调整其位置，有利于降低该区域氧气浓度，减少石油焦的烧损；改变三次风的位置与方向，对挥发分的燃烧影响较大，其合理布置可加速挥发分的燃烧，降低能耗。     

THE INFLUENCE OF KILN GEOMETRY ON FLOW MALDISTRIBUTION ACROSS TIMBER STACKS IN KILNS

A one-dimensional mathematical model is evaluated for flow through a timber-drying kiln. This model is used to develop recommendations for the design of the plenum chambers on either side of the timber stack. The variation of the vertical air velocity was explored for a sticker spacing of 20 mm and a board thickness of 50 mm. When the width of the plenum chambers is at least equal to the sum of the thickness of the stickers, flow maldistribution is substantially reduced in single-track kilns. The width of the plenum chambers of double-track kilns may be reduced to three-quarters of the sum of the thickness of the stickers, because the increase in timber-stack resistance to airflow mitigates the effect of pressure variations down the length of the plenum chambers.     

THE INFLUENCE OF KILN GEOMETRY ON FLOW MALDISTRIBUTION ACROSS TIMBER STACKS IN KILNS

ABSTRACT

A one-dimensional mathematical model is evaluated for flow through a timber-drying kiln. This model is used to develop recommendations for the design of the plenum chambers on either side of the timber stack. The variation of the vertical air velocity was explored for a sticker spacing of 20 mm and a board thickness of 50 mm. When the width of the plenum chambers is at least equal to the sum of the thickness of the stickers, flow maldistribution is substantially reduced in single-track kilns. The width of the plenum chambers of double-track kilns may be reduced to three-quarters of the sum of the thickness of the stickers, because the increase in timber-stack resistance to airflow mitigates the effect of pressure variations down the length of the plenum chambers.     

STACK-WIDE EFFECTS IN THE MODELING OF SOLAR KILNS FOR DRYING TIMBER

This study examines the stack-wide effects due to the humidification and cooling of air as it passes through a 6 m wide stack of Australian ironbark timber for conditions that are representative of those for solar drying (dry and wet-bulb temperatures of 60 and 50°C, respectively). A solar kiln model for a greenhouse-type design has been modified to account for the drying of timber boards and the possibility of stack-wide effects, in terms of moisture-content differences in the streamwise direction of air flow through the stack. The maximum difference between the moisture contents of the leading and trailing boards is predicted to be 0.011 kg kg^-1 for these conditions, compared with timber moisture contents of 0.15-0.35 kg kg^-1. Hence, the stack-wide effect is insignificant for these conditions in this greenhouse kiln design and may be ignored, reducing the simulation time by over 50%. In addition, 14 elements within a finite-difference model for the drying of the timber boards (25 mm thick) gives predictions of the drying time that are acceptably accurate, while minimizing the computational time.     

陶瓷辊道窑宽断面需解决的问题

本文分析了辊道窑的窑宽对产量,窑内气体流动阻力等的影响以及由于断面的增大而带来的温差问题,并提出了解决宽断面窑炉温差的办法。     

陶瓷辊道窑宽断面需解决的问题

分析了辊道窑的窑宽对产量、窑内气体流动阻力等的影响以及由于断面的增大而带来的温差问题 ,并提出了解决宽断面窑炉温差的办法     

带内翅片管回转脱硫石膏煅烧机的研究

脱硫石膏的煅烧工艺直接决定其产品的质量。本文对脱硫石膏的产生过程和特性进行了叙述,对常用煅烧工艺的优缺点进行了分析,选择带内翅片管回转脱硫石膏煅烧工艺进行研究。对该煅烧机的结构组成及工作流程进行了分析,结果表明该工艺有以下优点:避免了脱硫石膏煅烧过程中杂质的混入;内翅片管可在筒体出料端实现自由伸缩,可保护筒体不受热胀冷缩而变形;内翅片管使气流由层流变成紊流且增加了换热面积,提高了换热效果。总之,带内翅片管回转脱硫石膏煅烧工艺是一种性能好的煅烧工艺,可以应用于工业现场,并具有广泛的发展前景。     

Performance Comparison of Two Solar Kiln Designs for Wood Drying Using a Numerical Simulation

The predicted performances of the two different timber-drying solar kiln designs (Oxford and Boral kilns) have been compared by using the climatic and geographical conditions of Brisbane (27.46°S) in Australia. The dimensions of the original Boral kiln have been scaled down in order to compare the kilns’ performances on the basis of same load capacity. The comparison was made in terms of the key performance indicators, including drying rates, drying qualities (i.e., the level of strains developed within the timber boards during drying), energy gains, and major elements of the energy losses (i.e., the convection and radiation losses) while drying timber from an average initial moisture content of 53% (dry basis) to a final moisture content of 15% (dry basis) in both kilns. The relative performances for the two designs were discussed in more detail by using the typical climatic conditions (spring season 2013), when the days and nights are of approximately equal length. Finally, an analysis of the seasonal effects on the kiln performances was also carried out in this article. More effective orientation of the solar absorbers with respect to sun, together with faster heat transfer rates between the circulating air and the timber stack for the Oxford design, were likely to give consistently better predicted performance for the Oxford design than for the Boral kiln throughout the year.     

EFFECTS OF SAWING PATTERN ON LUMBER DRYING: MODEL SIMULATION AND EXPERIMENTAL INVESTIGATION

The sawing pattern of lumber affects the drying rate due to transverse permeability differences. These permeability differences are considered in a single board drying model which is able to investigate the drying rates for boards with varying growth ring angles. For the drying of Pinus radiata lumber, the model predicts that the quartersawn boards need longer drying time than the flatsawn boards. The drying time difference was 10-15% of the total drying time for conventional temperature (CT) drying and accelerated conventional temperature (ACT) drying, but was less significant for high temperature (HT) drying. In the simulation of a kiln stack drying, a sawing pattern factor was introduced to the relative drying rate function, which reflected the effects of the growth ring angle and the drying temperatures. The modified kiln wide drying model was used to predict the drying rates for a kiln stack consisting of entirely flatsawn boards and a kiln stack consisting of entirely quartersawn boards. Drying tests were conducted using stacks of mixed flatsawn and quartersawn Pinus radiata sapwood boards. In the tests, three drying schedules were used which included CT, ACT and HT drying. The experimental results agree closely with the model predictions and thus, after further validation, the drying models can be used to predict commercial kiln drying of boards with different sawing patterns.     

EFFECTS OF SAWING PATTERN ON LUMBER DRYING: MODEL SIMULATION AND EXPERIMENTAL INVESTIGATION

ABSTRACT

The sawing pattern of lumber affects the drying rate due to transverse permeability differences. These permeability differences are considered in a single board drying model which is able to investigate the drying rates for boards with varying growth ring angles. For the drying of Pinus radiata lumber, the model predicts that the quartersawn boards need longer drying time than the flatsawn boards. The drying time difference was 10–15% of the total drying time for conventional temperature (CT) drying and accelerated conventional temperature (ACT) drying, but was less significant for high temperature (HT) drying. In the simulation of a kiln stack drying, a sawing pattern factor was introduced to the relative drying rate function, which reflected the effects of the growth ring angle and the drying temperatures. The modified kiln wide drying model was used to predict the drying rates for a kiln stack consisting of entirely flatsawn boards and a kiln stack consisting of entirely quartersawn boards. Drying tests were conducted using stacks of mixed flatsawn and quartersawn Pinus radiata sapwood boards. In the tests, three drying schedules were used which included CT, ACT and HT drying. The experimental results agree closely with the model predictions and thus, after further validation, the drying models can be used to predict commercial kiln drying of boards with different sawing patterns.     

STABILITY OF A KILN STACK*

Mathematical proof is presented that the wind pressure on a round stack is two-thirds that of a square stack with the wind square to one side. A reconsideration is given of a stack section¹ where tension occurred, and a correction is presented. A convenient method and its application is also given for calculating the stability of a kiln stack upon the soil. The application of the “kern” theorem is discussed, using the kern for the worst condition, viz., with wind blowing diagonally against a square stack.