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ABSTRACT
The uniformity of drying and the quality of the wood in a kiln are strongly influenced by the airflow. A numerical model that simulates the airflow distribution is described and applied to solve the flow distribution in a kiln. The influence of the plenum geometry, the sticker thickness, and the roof design on the velocity distribution in the wood packages is investigated numerically. Results provide useful information for the selection of sticker thickness and the determination of possible plenum and roof designs. This paper is the first part of a larger effort to develop computer models for predicting the airflow, heat transfer, and drying rate in wood kilns. The model will constitute a powerful tool to optimize kiln design and to help operators improve kiln operations without entailing major new costs.
Aschematic diagram of a wood kiln is shown in Figure 1. A wood kiln provides control over the temperature, humidity, and air circulation to reduce water inside the wood in the least amount of time to a desired value while avoiding creation of wood defects occurring during rapid drying. Kiln operators must increase profitability of the operation by balancing costs in improving the process versus costs associated with loss of grade. They must try to achieve uniform drying between loads, within a load, and within a lumber piece. The uniformity of drying in wood kilns is governed, in part, by the uniformity of airflow, especially in the first and second stage of drying where air convection governs the removal of the water within the wood (Fig. 2). Figure 3 shows a typical moisture content distribution of one piece of lumber placed in a kiln. A detailed description of the wood-- drying model and the influence of airflow on the drying can be found in another report (3).
Airflow in a kiln is influenced by such parameters as the geometry of the kiln plenum, air ducts, roof design, the proper stacking of each wood package, and lumber dimensional control. All these factors can cause airflow variations that translate into different drying rates in a kiln. Minimization of airflow variations requires a proper understanding of the mechanisms that contribute to airflow non-uniformities. Kiln operators can control airflow by regulating the fan speed, changing...