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of the board are reduced since a boiling point temperature is achieved in every part of the board. Therefore, the final moisture content is more uniform (Oliveira & Mackay, 1996). This kiln type reduces drying times up to 50% of the time needed with conventional kilns without a decrease in the quality (Oliveira et al., 2001). However, several disadvantages of super heated steam vacuum kilns are reported. Uneven drying can occur and optimised schedules must be developed since adjustment of existing schedules is not sufficient (Pang & Pearson, 2002). This type of kiln is limited to cases where flexibility and short drying times are necessary. Also, higher investment costs (compared to conventional kilns), presence of important drying defects when using sensitive species, and an increased need for maintenance are noted in describing these types of kilns (Welling, 1994). 2.5. KILN CONTROL A systematic way to control the quality of the kiln dried material is to monitor the entire process before, during and after drying. This should be part of an overall quality control approach that helps maximize the value of lumber and ensures that high quality products are manufactured. Such a control system can identify maintenance needs (e.g., by revealing problems that occur in regular patterns) which should be addressed in a systematic way and opportunities to improve the process (Rice, 1976). Quality procedures in every manufacturing step, from harvesting lumber to delivering the final product, are required to implement quality control. Furthermore, quality control is based on methodical data collection which can be achieved effectively through statistical process control (Denig & Hanover, 1990). Statistical process control, which is a mathematical procedure that provides feedback on quality parameters, can be used to monitor the kiln process. With statistical process control, a company can estimate average values of important drying variables (such as range of moisture content or amount of stain) by taking samples of boards, set limits within which these variables are considered to be under control, monitor daily performance, and finally, determine if a kiln load is to be accepted or rejected depending on the sample results (Wengert, 1994a). 14PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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