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Vanicek (Vanicek and Bittner 1992) and Mu ̈hlbo ̈ck vacuum kilns (Mu ̈hlbo ̈ck, personal communication, 1993). The suggestions above are presented in Fig. 3. Discussion The demands on temperature control in vacuum kilns in order to fulfil the demands of the EDG Exclusive class are close to the theoretical limits of an industrial drying kiln. The only possible method to maintain the temperature throughout an industrial kiln with sufficient accuracy is to use independent cli- mate control zones equipped with a PI or PID control system. Furthermore, it is only possible to create a predictable temperature profile across the wood batch if 8 the steam flow is well defined. This is impossible without a well-defined pressure side and suction side of the batch meaning it is necessary to place the fans above the wood in large industrial kilns. None of the industrial kilns in which mea- surements were made can control the climate with acceptable accuracy to reach the EDG Exclusive drying class except at very low target MC levels. Conclusions The temperature control system should be divided into independent zones along the kiln in order to maintain the set climate and the heat valves of each climate control zone should be controlled by a PI or PID regulation system. Steam flow patterns must be well defined in order to reach a controlled drying process. Fans should be placed above the wood in order to accomplish such controlled steam flow, with the possible exception of very short kilns. The fans should be reversed at regular intervals and be equipped with speed control. The vacuum pump should be equipped with a PI-regulation system or equivalent. The kiln should be equipped with a conditioning system with the ability to add moisture to the wood. With proper climate control, the final drying quality will be improved since the drying schedule can be controlled, making it possible to dry wood to higher final MC as the MC variation will be reduced. Fig. 3. Suggestions to reach a controlled drying climate in a vacuum kilnPDF Image | Tech Table: Vacuum bagging supplies
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