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2.3.1 TYPES OF VACUUM DRYING SYSTEMS Vacuum drying heat transfer needs to be done continuously. This characteristic leaded to the development of different methods to transfer heat to wood. Vacuum drying can be separated into four types (Chen and Lamb 2003) based on the method of transferring heat to the wood: a) Convection using hot air as in cyclic systems b) Radio-frequency dielectric heating. c) Conduction by direct contact with a hot plate or electric heating blanket. d) Convection using superheated steam at high temperature. In cyclic vacuum drying, lumber is heated up and then a vacuum is drawn in the chamber. Drying occurs while there is temperature gradient and pressure difference between the ambient and inside the wood. Wood temperature drops rapidly and then the cycle is repeated. There are two distinct phases: rapid drying and then as drying slows down when the pressure inside the material approaches the ambient pressure (Chen and Lamb 2003). Radio frequency (RF) drying technology constitutes a combination of two special methods: heating by high frequency current and drying in vacuum where the boiling point of water is decreased with decreasing pressure. The intensity of the heating depends directly on the moisture content of the wood and the electric field, while the moisture movement depends on the permeability of the wood and the internal pressure gradient. According to Resch (2006), RF heat penetration can be done with a small electric field, which makes possible the heating of the entire stack of lumber. Regarding conduction by direct contact with a hot plate, the stacks of wood are laid out between aluminum heating plates. Hot water or oil flows through the plates to heat the lumber to the required temperatures (Kanagawa and Yasujima 1993). Superheated steam vacuum drying (SSV) consists of a process that uses low pressure conditions to make the superheated steam circulate through the lumber layers producing a vacuum process inside the kiln. Superheated steam has better heat transfer properties than hot air at the same temperature (Kudra 2002). However, steam under vacuum has lower heat capacity 9PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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