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comparison, in conventional drying, red oak dried from green to 7.8% MC, had shrinkage values of 5.6% and 10.3% in the radial and tangential direction respectively. In comparison with dehumidification drying and conventional kiln drying, lumber dried by RFV has lower compressive strength, bending stiffness, bending strength, shear strength, and hardness. However these differences are not statistically significant except for compressive strength (Taniguchi and Nishio 1991, Lee and Harris 1984). There was no difference in hardness or toughness between oak lumber dried by RFV and by conventional kiln drying according to Wengert and Lamb (1982). Regarding stresses, Harris and Taras (1984) found that drying stresses in vacuum drying are similar to those in conventional kiln drying. They also stated that smaller residual stresses and less degrade were found for lumber that was cyclic vacuum dried compared to lumber that was conventionally dried. However, Trofatter et al (1986) reported that conventional kiln drying maintains a lower moisture gradient through the board thickness than RFV does. The variation among boards ranges from 2 to 28% MC for RFV compared to 7 to 11% MC for conventional drying. 2.7. COMPARISON OF METHODS TO DETERMINE DRYING LUMBER COSTS One of the goals of this project is to estimate conventional and vacuum drying costs for drying 4/4 red oak lumber. To be able to achieve this comparison, a methodology for calculating the costs associated with each method must be developed. Literature describing how to determine the cost of lumber drying has been analyzed and is presented in this section. There have been many published methods for determining the costs associated with of drying lumber, which range from basic cost accounting methods to more in-depth feasibility analyses. The cost accounting approach is used to collect, analyze and evaluate a process and to provide detailed cost information that can be used to evaluate the current state and various alternatives. In cost accounting methods, costs are usually divided into two groups: 1) fixed, and 2) variable, where fixed costs do not differ with the volume of production and variable 16PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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