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When the pressure is defined, climate control can be based on temperature control. The temperature control system must be divided into individually controlled climatic zones along the kiln in order to maintain the set temperature along the entire length of the kiln. Variations in heat losses through the kiln walls as well as variations in drying characteristics of the wood can only be controlled by using climate control zones. The number of zones depends on the dryer length and the expected variations in drying characteristics of the wood to be dried. A minimum of three zones is needed to allow for one zone in the centre section and one zone at each end of the kiln. Each climate control zone needs to incorporate individual temperature 7 measurements and individually controlled heating coils. As the energy demand is greatly reduced at the end of the drying process, a simple on/off regulation of the heat valves is insufficient. The heat valves of each climate control zone should be equipped with a PI or PID control system. The energy input from the fan motors can cause an uncontrolled temperature increase at the end of the drying process so it is necessary either to cool the steam or to reduce the fan speed. The kiln and the wood cart should be de- signed to reduce the risk of a cooler drying climate at the bottom of the stack. This could be achieved by means of an insulated cart bottom or double bottom plates where a limited amount of steam is allowed to pass below the stack in order to heat the cart. Controlled steam flow pattern The only possible method to ensure a controlled steam flow pattern is to have a well-defined pressure side and suction side of the stack. The steam must pass from one side of the stack to the other. In a very short kiln this can be achieved with the fans placed at the ends of the stack, but in a kiln of normal industrial size, the fans should be placed above the stack. A fan configuration with the fans beside the wood stack will always result in an uncontrolled drying process, since the steam will follow the easiest route from the pressure side to the suction side of the fans, resulting in dry areas and wet areas of the cross section. The steam should pass through the wood stack with a well-defined velocity. Thus no steam should be allowed to pass around the stack at the ends, above or below. This makes it necessary to use flaps above the stack and at the stack ends. Weights, in the form of plates resting on the stack, have the double advantage that they eliminate the risk of steam passing above the stack and reduce deformations of the wood. In a similar fashion, plates at the ends of the stack eliminate the risk of steam passing around the stack, reduce end checking, and reduce the risk of colder steam at the ends due to the relatively greater heat losses through the kiln ends. The direction of steam flow should be reversed as in conventional kilns in order to reduce the effect of the temperature drop across the stack. Conditioning system In order to accomplish an efficient conditioning phase it is necessary to add moisture to the wood. It is of vital importance, therefore, to incorporate a water spray or steaming system to the kiln design. Such a system also benefits the initial heating phase. Steaming systems are incorporated in, for instance, Brunner- Hildebrand and Maspell vacuum kilns (Ressel 1993) and water spray systems inPDF Image | Tech Table: Vacuum bagging supplies
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