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Exploratory study of the effect of oscillation drying on thick hemlock timbers Sackey, Emmanuel Kuuku

Abstract

This study was aimed at exploring the effect of oscillating the drying temperatures of a drying schedule on the drying characteristics of thick western hemlock timbers. Pacific coast hemlock which is comprised of about 70% western hemlock (Tsuga heterophylla (Raf.) Sarg.) is one of the best quality and most abundant softwood species along the coast of British Columbia. The squares of 105 x 105mm of this species group is widely used for housing construction in Japan, and mostly imported from British Columbia. In recent years the overall demand has decreased and there is also a great market shift in demand from green to kiln-dried lumber. However, conventional kiln-drying of thick hemlock timbers has difficulties of long drying times, uneven final moisture content distribution within and between lumber, honeycombs, checking and splitting. Since most industries still use conventional dry kilns (also known as "heat-and-vent" kilns), it is imperative to explore alternative drying schedules to improve the quality of 105mm square dried Pacific coast hemlock. In this study, thick industrial size square Pacific coast hemlock of dimensions 115 x 115 x 920 mm were dried in a laboratory conventional kiln, using a control schedule and four oscillating drying schedules with drying temperatures (wet bulb –T[sub wb]=53°C, dry bulb-T[sub db]=60°C) oscillating at two amplitude and frequency combinations (3°C/4hours and 6°C/8 hours). The research results indicated that, the total effect of oscillating schedule on drying rate was more pronounced at the early stages of the drying process. Oscillating dry bulb temperatures at higher amplitudes increased drying rate by 12%, whereas the lower amplitude counterpart reduced kiln residence time by 14%. All schedules were also found to reduce moisture content variability between lumber. Core and shell moisture content variation slightly decreased when the wet bulb temperature was oscillated at low amplitudes. Climate oscillations also affect the drying stresses of thick hemlock lumber. Drying stresses in the lumber increased when oscillating the dry bulb temperature at lower amplitudes. Width shrinkage in the baby square hemlock lumber reduced in the low amplitude dry bulb oscillated schedule.

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