Radio frequency heating pre-treatment of sub-alpine fir to improve kiln drying

The objective of the study was to reveal the effect of RF heating at different power densities and time combinations as pre-conventional kiln treatment on the drying characteristics and quality of sub-alpine fir lumber. As a consequence of this objective, the study hypothesis was formulated as: “if RF heating improves the permeability of sub-alpine fir, then upon kiln drying, final moisture content variability between and within lumbers as well as drying defects will decrease”. In this research, thirteen groups of one meter long, 51 x 102 mm sub-alpine fir lumbers were RF heated with two power densities (27 and 72 kW/m³) and eight time combinations (15, 30, 45, 60, 75, 90, 105 and 120 minutes), but all to the same final temperature level (100°C) before kiln drying in a two-phase experimental design. Two groups (one for each phase) served as controls. Permeability tests were also performed in the second phase of the study. The effect of RF heating on permeability, drying rates, moisture content gradient, final moisture content variability between pieces and drying defects was evaluated and analyzed. Treatment effect on total energy consumption (sum of RF heating and kiln drying energies) was also assessed to ascertain the feasibility of applying this technology in industrial setting. Data analysis revealed in phase 1 that, not all treatments reduced moisture content variability within and between specimens and improved drying rates above and below fiber saturation point, compared to the control. Defect appearance also did not significantly reduce, and all except two treatments increased total energy consumption. In phase 2 however, permeability improved in all except one treatment but was found not to be statistically significant. Treatments also improved moisture gradient as well as drying rates above and below fiber saturation point but the moisture gradient was found not to be statistically significant. Compared to the control, not all treatments reduced moisture content variability between wood samples. Defects did not significantly improve between treatments and control, and total energy consumption was relatively higher in treatments. Results obtained within the limitations of this study led to the rejection of the hypothesis.Forestry, Faculty ofGraduat