Research has recently focused on various novel timber-drying methods, such as vacuum and radio-frequency/microwave drying. Even so, most timber in the industry is still dried in forced-circulation convection kilns at atmospheric pressure. Hot and humid air sweeps through the sticker space between stacked timber boards.
On its way through, the air transfers heat to the wood surface and picks up evaporated water while being cooled down and humidified in the process. Fans, which are usually positioned overhead, generate the desired flow rate by providing the pressure difference to compensate for pressure losses through the stack and kiln components. The vents on the ceiling permit the elimination of humid air and the circulation of fresh, dry air. Heating coils re-establish the necessary gas temperature. Steam and water are injected during specific drying stages to control the relative humidity in the kiln.
In large kilns, the airflow is reversed regularly to reduce moisture variations along the flow path, which result from the decreasing drying capacity of the air as it is cooled through the stack. An efficient operation of a convection kiln aims at drying timber quickly and uniformly while minimizing energy consumption. This is usually achieved only to a certain extent, and in practice, different drying kilns can exhibit different efficiencies.
Despite the numerous recommendations for improving the uniformity of the airflow in kilns, there is no general conclusion for the optimum kiln design. But after a long period of innovation in the kiln drying method Shouwei, developed a high-frequency vacuum kiln that can dry wood faster and maintain its color and more durable dried lumber.
There are five major topics that I want to discuss with you regarding the airflow in the kilns.
Observed airflow phenomena in a kiln
The box shape of a typical wood drying kiln offers a rather confined path for the drying air, which results in various flow resistances both outside and inside the piled timber. In addition to the three-dimensionality and unsteadiness of the airflow, this leads to a complex flow configuration in the kiln. The flow in a kiln is, therefore, inherently non-uniform. A variation coefficient of air velocity can express the degree of this non-uniformity.
Regarding the spatial distribution of the flow, two distinct sections can be distinguished: the region outside the timber stack and the spaces between the boards or packs. These will be discussed in the two sections that follow.
Kiln-wide flow phenomena
The fans generate the airflow in the kiln. The flow capacity of the fans depends on the pressure drop created by the stack and various kiln parts such as bends, baffles, heating coils, and screens. The pressure drop of kiln parts is related to the velocity via the pressure-loss coefficient.
To calculate the flow through industrial equipment, it is common practice to follow the path of a streamline along which, for an incompressible steady flow, an extended form of Bernoulli’s equation can be used. It contains the expressions for kinetic and potential specific energy, static pressure, as well as additional terms for pressure loss and pressure rise.
With the help of the incompressible steady-state continuity equation, the flow in duct networks can be readily quantified. In an overhead-fan timber kiln, the airflow is turned after the fan battery, passing through a 90° bend.
Typical velocity distribution at the stack inlet over the plenum height. For identical plenums on both sides of the stack, inertial effects are balanced against each other, and hence frictional effects determine the flow distribution across the stack height.
Stack flow phenomena
The flow down the plenum chamber resembles the flow in a manifold with the fillet spaces as its branches. The board layers form channel walls, which exert a drag on the fluid. The resulting wall shear stress for a Newtonian fluid is proportional to the gradient of the velocity normal to this wall. The dimensionless shear stress in internal flow is called friction factor and can be derived from a force balance for a rectangular duct.
The characteristic length for the force balance is defined as the ratio of the cross-sectional area A and the wetted perimeter U. The model of a duct flow is just an idealization of the absolute configuration in the stack since the stack layers are made up of single boards, which do not form smooth channels.
As illustrated, the flow is disturbed by the blunt edge at the inlet and subsequent gaps and steps within the stack. As a result, flow separation occurs, accompanied by a sharp local increase of the shear stress and intensified turbulence.
Measurement of the flow field
An experimental investigation is helpful in analyzing the flow field in a timber kiln and in eliminating possible sources of flow maldistribution. The measurement can occur directly within the industrial kiln and sometimes be incorporated into the control system.
However, for a more detailed examination, measuring the flow patterns in small-scale models or water channels is often necessary. The results can be transferred to large-scale kilns using the principles of flow analogy, as characterized by the Reynolds number.
Typical Measuring Equipment
Fluid mechanics research has seen considerable advancements in recent years. New optical measuring principles, as well as automation of signal processing and growing computing power, are the main reasons.
However, the essential task remains the same: the experimental acquisition of all necessary parameters that characterize the flow of a fluid. This can involve measuring the flow field itself (velocity field with direction and magnitude, temperature field, turbulence parameters) and additionally measuring the flow domain boundary conditions like pressure, wall shear stress, and heat transfer.
Selected measuring techniques for airflow in kilns
The choice of a suitable measuring device depends on the flow parameters and the size of the region to be investigated. From the numerous techniques mentioned above, pressure tubes, hot-wire anemometry, and Laser-Doppler anemometry has thus far been applied to airflow measurement in various kiln geometries.
Pressure probes: Pressure measurement plays a central role in experimental fluid dynamics. According to the equation, the total force at any location is static, dynamic, and hydrostatic.
Airflow simulation
The experimental methods are essential to obtain information about the flow field in complex domains. However, because of the large dimensions of typical industrial applications, detailed measurements have to be restricted to selected locations, and results are only valid for the given configuration of kiln and stack.
Optimization of the airflow in a kiln and the analysis of stacking strategies will therefore require a simulation tool to perform case studies in a more cost-effective way. Airflow simulation uses computer-based numerical methods to solve the governing equations of fluid flow for more general cases and thus complements experimental techniques.
Conservation equations
The basic equations for fluid movement are derived from conservation laws for a liquid element’s mass, momentum, and energy. The derivation of these equations is omitted here for reasons of brevity and is covered exhaustively in the standard fluid mechanics literature. The equations will be given in index notation and Cartesian coordinates for the case of an incompressible flow of a pure fluid, which is a valid approximation for the conditions in timber kilns.
Airflow analysis
While the previous sections introduced experimental and theoretical methods for studying flow fields in a drying kiln, the following areas will present the results of their application in analyzing the main flow features inside and outside the stack.
Whereas the first section will focus on the details of the flow inside the stack, the second section will deal with the kiln-wide flow situation providing some engineering guidelines to assist in the proper design of drying kilns to help in the creation of drying.
Airflow in the stack
Several phenomena characterize fluid dynamics in the stack. At the stack entrance, the behavior is similar to the flow around blunt plates. Further downstream, the upper and lower boundary layers are thickening and, finally, grow together, establishing a channel flow profile. To describe the situation correctly, discontinuities along each airflow channel, which result from variations in board thickness and gaps between boards, and board surface roughness effects should also be addressed.
Influence of airflow on wood moisture distribution
Thus far, the airflow analysis has concentrated on various features within a kiln that affects the uniformity of airflow across the stack, which in turn affects the even drying of timber. However, in the drying process, the air is cooled down as it carries away the evaporated moisture from the wood surface, which also influences the uniform drying of the timber boards. On its way through the stack, the drying potential of the air decreases.
Even in perfect airflow distribution across the stack, this decreasing drying potential leads to uneven drying across the stack width. Airflow reversals can reduce but not eliminate this effect. The following sections will address these phenomena and consider the impact of nonuniform external drying conditions on local drying rates in the stack.
Airflow-induced drying non-uniformities
There is a direct correlation between the magnitude of the air velocity in the stack and the magnitude of the heat- and mass transfer coefficients. This correlation is often described by so-called Nusselt relations. Examples of these Nusselt relations are covered in the chapter “External heat and mass transfer.” As a consequence, local variations in the airflow will have an influence on the drying rate and on the spread of the final moisture content of the boards.
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