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conditioning treatment due to the absence of steam, and require longer drying times (Boone & Simpson, 1997a). 2.4.5.6. Heating Plate Vacuum In heating plate vacuum kilns, metal plates or special blankets are used to transfer heat to the wood. These kilns have slow drying rates and can only dry small volumes of lumber (Avramidis, personal communication). Corrosion is also a major concern in this kiln type. However, because of the relatively low temperatures used, drying defects (such as discolourations) are minimized. These kilns are more suitable for drying hardwoods (Keey et al., 2000). 2.4.5.7. Radio Frequency Vacuum A combination of a radio frequency field with a vacuum is used in radio frequency vacuum kilns. With these kilns, installation is easy (because there is no steam), no stickers are necessary, boards of varying length can be dried, drying times are shortened, high quality material is produced, final moisture content has low variation, and drying costs are low. However, capital investment costs are high, and this type of kiln is not suitable for small operations or for drying commodity products (Avramidis, 2003). Radio frequency vacuum kilns are more energy efficient than conventional or dehumidification kilns (Zwick & Avramidis, 1996). Smith et al. (1994) mention that older design radio frequency vacuum kilns presented problems like wide variations in final moisture content or high energy costs. However, recent models provide more uniform drying, while reducing energy requirements and drying times. 2.4.5.8. Super Heated Steam Vacuum In super heated steam vacuum kilns, the wood is dried above the water boiling point under a certain pressure. Since drying occurs in a vacuum, the boiling point is below 100°C. The mild temperatures assist in reducing drying defects. Furthermore, the differences in drying rates between the inner and outer part 13PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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