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RF drying has several advantages over conventional drying, like no wood extractive staining, reduced surface checking, no internal stresses, and good MC uniformity. Degrade is about half of that of conventional drying, and shrinkage is also reduced (Avramidis, 1999). In a laboratory- scale experience, thick squares (4 and 10 inches thick) of western red cedar and hemlock were dried in 24 and 32 hours, respectively. Drying quality was good, measured by shell and core MC differences (around 0.65 percent, compared with 8-12 percent in conventional drying) total shrinkage about half compared to conventional drying, no internal checking, and no discoloration (Avramidis and Zwick, 1992). 2.6 PROPERTY CHANGES IN VACUUM DRYING Wood is an anisotropic material, which means that its dimensions change differently in three directions: tangentially, radially, and longitudinally. Dimensional changes, shrinkage, and swelling in wood take place below the fiber saturation point (FSP) where all of the water exists only within the cell wall. The method of how water is removed between vacuum and conventional drying can lead to differences in the amount of shrinkage that takes place, thus there are also differences in the type and amount of drying stresses. Wood shrinks and swells differently in each plane of reference. Longitudinal shrinkage occurs because of the orientation of the micro fibrils in the S-2, so as the wood dries there is a measurable shortening of the cell. Longitudinal shrinkage is considerably larger in compression wood than in normal wood. Differences with the transverse plane are due to the alternation of late wood and early wood increments within the annual ring, influence of wood rays on the radial direction, and the chemical composition of the middle lamella. Meanwhile, tangential shrinkage is greater than radial shrinkage due to the presence of ray tissue, frequent pitting on radial walls, domination of summerwood in the tangential direction, and differences in the amount of cell wall material radially versus tangentially (Simpson 1991). The shrinkage of red oak lumber dried by RFV was measured by Harris and Taras (1984) to be approximately 30% less than that dried by conventional kiln drying. For RFV, red oak was dried from green to 5.5% MC with an average shrinkage of 4.1% radially and 7.5% tangentially. By 15PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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