Exploratory evaluation of oscillation drying for thick hemlock timbers

Abstract

The study explores the effect of dry-bulb (T_db) and wet-bulb (T_wb) temperature oscillation in a conventional kiln on the drying characteristics of 105-mm-square Pacific coast hemlock. Five kiln-charges were dried using four oscillating drying schedules and a control. The base drying temperatures for the control were T_wb=53°C, T_db=60°C for the first drying phase and increased to T_wb=65°C, T_db=80°C in the second phase. These temperatures were oscillated at two amplitude and frequency combinations, namely, 3°C for 4 h (low) and 6°C for 8 h (high). The results indicated that the total impact of oscillating schedules on the drying rate was more pronounced at the early stages of the drying process. Oscillating T_db at higher amplitudes increased drying rate by 12%, whereas the low amplitude counterpart reduced kiln residence time by 10–14%. The final moisture content variability decreased in both T_wb oscillated schedules. Core and shell moisture content variation also slightly decreased in the low amplitude T_wb oscillated schedules. High amplitude oscillated T_db also showed 5% less casehardening than the control, whilst its low amplitude counterpart increased. The low amplitude T_wb oscillated schedule had the least surface checks and splits.