Nonisothermal Radiofrequency Drying of Red Oak

Abstract

The combination of nonisothermal moisture movement and radiofrequency heating has a potential application in lumber drying as it affords rapid heating of wood and provides an additional driving force for the removal of water. This paper describes the experimental setup and presents the results of the nonisothermal radiofrequency (NIRF) drying of red oak (Quercus spp.). NIRF drying is implemented by bulk heating the lumber at a preset temperature using radiofrequency energy while continuously circulating air conditioned at a lower temperature over the wood surface. The temperature at mid-thickness of the lumber is maintained at the dry bulb temperature required by the appropriate kiln-drying schedule, while the air temperature is maintained so as to establish a temperature gradient of 3°C/cm from the mid-thickness to the surface of the material. Excessive checking was observed when green lumber was dried using the NIRF method. The drying defect is due mainly to the high drying rate and steep surface moisture content gradient during the early stages of drying. Nonisothermal radiofrequency drying was successfully implemented when a pre-drying step was incorporated at the start of the process. When red oak was pre-dried to 40% MC, the modified NIRF method required a total of 275 h to dry 25-mm-thick boards from 85% to 12% MC. For lumber pre-dried to 60% MC, the total drying time for the modified NIRF method was 160 h. These drying times are significantly shorter than the 530 h needed to dry red oak over the same moisture content range using the conventional kiln-drying method.