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6 Fig. 2. Expected steam flow patterns in kilns with different fan configurations cause large uncontrolled variations of the steam temperature. Large variations in drying temperature can be seen in vacuum kilns with on/off regulation of heat valves, especially at the end of the drying process when the power demand is limited (Behnke et al. 1995; Hedlund 1996). The oscillations of the drying climate can be increased by variations in pressure due to on/off regulation of the vacuum pump. Several vacuum kiln designs use climate control systems based on set temperature and set RH- values rather than set temperatures and set pressures. The connection between pressure, temperature and RH in combination with long response times of the temperature and pressure control systems, can lead to an increase in both the amplitude and the time period of the oscillations, creating a risk of wood damage. Time periods exceeding 2.5 h and amplitudes exceeding 5°C can be seen at the final stages of the drying process (Hedlund 1996). A common problem in vacuum kilns is an uncontrolled temperature increase in the later stages of the drying process caused by the energy released from the fan motors as the power demand of the drying process is reduced. Measurements have shown temperatures increasing up to 10°C above the set temperature (Hedlund 1996). Critical design factors to reach a controlled drying climate The design of a vacuum kiln should incorporate two main features in order to reach a controlled drying process: A. A capability to reach and maintain the set climate at the control points throughout the drying process B. To ensure a controlled steam flow pattern. In addition to maintaining the set climate, a vacuum kiln should have a conditioning system capable of remoistening the wood in order to reduce the MC gradient of the wood. Maintaining the set climate As the drying climate is defined by pressure and temperature, both are equally important to control. However, as opposed to temperature, pressure will always be more or less equal throughout the entire length of the kiln and varies only by the fan pressure. This makes it advantageous to base the climate control system on a well-defined pressure and thus the vacuum pump should be equipped with a proportional/integrating/derivating (PID) or PI control system.PDF Image | Tech Table: Vacuum bagging supplies
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