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(due to lower density) and lower drying rates than with hot moist air used in conventional drying. However, this can be compensated by circulating air at high speeds, of about 10 m/s and with frequent fan reversals (Pang and Pearson, 2004). 2.4 Comparison of Vacuum and Conventional Drying Rates Any drying technology should meet the following objectives: (1) provide the shortest drying time possible, (2) consume minimum amount of energy, (3) have the lowest drying costs possible, and (4) result in adequate drying quality (Ressel 2002; Wengert, 1988). Vacuum drying has been shown to be from 3 to 17 times faster than conventional drying (Chan and Lamb 2007). Vacuum drying rates are greatly influenced by permeability, thermal conductivity, and method used for heating the wood. This section focuses on drying times for vacuum drying using radio frequency, microwave and superheated steam. Drying times using radio frequency heating are significantly shorter than conventional drying, since the temperatures are higher in the lumber core than on the surface. Moyne and Martin (1982) resolved in their study that the acceleration of the drying rate was product of the pressure flow that occurs in vacuum conditions. Tables 1 and 2 show a brief summary of drying times for various hardwoods and softwoods products using conventional and several different types of vacuum drying technology. The data was gathered from different authors: Fortin (1998), Yamsaengsug (2005), Avramidis et al (1994), and Mottonen (2006), Welling and Riehl (1999), and Harris and Taras (1984), Leiker and Adamska (2004), and Redman (2011). 10PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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