Exploratory evaluation of oscillation drying for thick hemlock timbers

The study explores the effect of dry-bulb (Tdb ) and wet-bulb (Twb ) 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 Twb =53°C, Tdb =60°C for the first drying phase and increased to Twb =65°C, Tdb =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 Tdb 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 Twb oscillated schedules. Core and shell moisture content variation also slightly decreased in the low amplitude Twb oscillated schedules. High amplitude oscillated Tdb also showed 5% less casehardening than the control, whilst its low amplitude counterpart increased. The low amplitude Twb oscillated schedule had the least surface checks and splits.