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Table 2. Typical Drying Times for Various Softwoods Products Using Conventional and Several Different Types of Vacuum Drying Technology Species Produ ct Initial State Drying Time ( days) Reference Conventio nal Vacuum Superheated steam Plate n High Frequency Microwa ve Softwood Fortin 1998 Hemlock-Fir Group Larch White Pine Douglas Fir Subalpine Fir Red Cedar Spruce 60-80% 70-91% Green 4/4 3/6 5/4 4/4 47% - 22 10-12 5-14 9-14 6-7 7.5-12.5 3-5 - - - - - - - - - - - - 8-9 - 18-20 - Fortin 1998 Fortin 1998 Fortin 1998 Fortin 1998 Fortin 1998 Fortin 1998 and Yamsaengsug 2005 Fortin 1998 Fortin 1998 Avramidis et al 1994 Leiker and Adamska (2004) 3/10 Green 50-55 - 8x8 Green - - 4/4 Green 5 - 2 - - 4/4 30-45% 10-14 1.5-7 - - - 2/6 62-82% 3-5 2.5-6 - - - 6/4 Green 27 - - 6 - 4/4 Green - - - - 2.5 Satho It can be seen in Tables 1 and 2 that vacuum drying rates are faster than conventional drying. For example, 4/4 red oak averaged 8 days when it is dried from the green state and 4 days when it is air dried; while conventional takes a total of 25 to 30 days from air drying, and then 8 to 9 days kiln drying, or 20 to 30 days when only kiln dried. In the case of 8/4 air dried sugar maple, it can be seen that if sugar maple is platen heated, drying rates are 83% less than conventional; while with superheated steam, drying rates are 69% less than conventional. In the case of softwoods, vacuum drying rates for 6/4 red cedar are 77% less than conventional drying when lumber is green dried. Douglas fir air dried to 30-45% takes an average of 14 days to be dried by kiln drying and on average 7 days to be dried by using superheated steam. In the case of 5/4 larch, the vacuum drying rate is 50% less than conventional. According to Tables 1 and 2, vacuum drying has the capacity of drying lumber faster than conventional drying for hardwoods and softwoods. 12PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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