Vacuum Drying of Wood—State of the Art

In this paper, we review the literature published on vacuum drying of wood. Vacuum drying is not a new technology, and its use for drying wood has been suggested since the early 1900s. Technologies for vacuum drying of wood can be classified by the heating method used. In this paper, we define vacuum-drying methods in four groups: conductive heating vacuum, cyclic vacuum, superheated steam vacuum, and dielectric vacuum. Advantages of drying wood below atmospheric pressure are the ability to dry at lower temperatures (and thus lower the probability of developing some drying defects), greatly reduced drying times, color preservation, greater energy efficiency, better control of volatile organic compound emissions, and the ability to dry very large cross sections. Some characteristics that differentiate vacuum from conventional drying are that in vacuum the primary driving force is the total pressure difference, the prevailing moisture transfer mechanism is water vapor bulk flow, and there is greater water migration in the longitudinal direction. While past research has focused on increasing the understanding of the fundamental mechanisms for vacuum drying and applications to specific industries and species, more recent efforts have concentrated on improving existing methods, for example, by improving moisture control and the use of pretreatments to improve drying quality.