Characteristics of commercial-scale radio-frequency/ vacuum (RF/V) drying for hardwood lumber

Abstract

Two runs of commercial-scale radio-frequency/vacuum (RF/V) drying for maple hardwood were performed to explore the practical technology and its drying characteristics. The results revealed that the power density was a prerequisite for the drying schedule development. The drying time and in-process moisture content (MC) were evaluated by the calculated amount of dehydration at 1% MC removal. The drying defects, such as checks, bowing, and twist, and MC variation met the requirements of GB/T 6491 (2012). The drying rate of run 2 increased 22% after the drying schedule modification. The dehydration capacity was affected by the temperature, which first increased fast as the wood temperature increased to the boiling point and then increased more slowly after that point. The dehydration capacity was also associated with the initial lumber MC. Approximately 30% of the total energy maintained the chamber vacuum and approximately 70% was used for RF heating for both runs. The overall specific energy for water removal during RF/V drying had a competitive advantage compared with conventional kiln drying. The energy conversion efficiency of both runs was low at 50% during the warming stage, and increased to 80% and 90% for Run 1 and Run 2, respectively, during the drying stage.