Moisture content in wood is a volatile feature, continuously changing under influence of the oscillating ambient climate. Unquestionably an essential quality trait for the timber, the MC and other drying quality aspects must be appropriate according to customer specification at the time of delivery and final use. The objective of this presentation is to describe sorption relations in commercial timber boards for the period after kiln drying. Joint information from several semi-industrial experiments was used to establish MC prediction models for industrial applications. Kiln temperature was found to be of minor, however noticeable, significance to the equilibrium MC for timber dried in ordinary kilns operated below 100°C. Spruce board samples of varying length from small, clear specimens up to commercial length boards were observed under varying climatic situations to analyse the nature of hysteresis. The hysteresis fades for smaller specimens and in dry settings. A differential (dynamic) model for MC sorption under steady-state climate at varying temperatures was estimated and analysed numerically. Sorption speed increases with temperature, but the actual MC nevertheless remains well off the tabulated equilibrium MC even after several weeks, if not months, in a constant climate. The model further indicates that temperature plays a minor role for the sorption, unless the boards are kept in the same climate for a prolonged time. In a naturally fluctuating climate, the boards might be in a ‘dynamic equilibrium’, but never in ‘steady equilibrium’. The influence of the daily climatic fluctuation is insignificant for commercial boards. For the weekly changes, a practical algorithm for estimating the MC in boards kept out-doors on stickers is outlined.
Keywords: EMC, HT drying, hysteresis, MC dynamics, sawn timber
Authors
Gjerdrum P.
Department of Wood Technology, Norwegian Forest and Landscape Institute, Ås
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