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Where Xb is the bottom specimen surface MC (%) and Xt is the top specimen surface MC (%). 3.4.5 Shrinkage The shrinkage of wood during drying is one of the most important properties as it is responsible for deformations during drying (twist, spring, bow and cupping), dimensional variations in situ, and is the driving force for stress. Shrinkage induced drying stress is one of the main reasons for poor quality of dried products. Due to this project’s timeframe and model complexities, a mechanical modelling component was not included; heat and mass transfer only. An accurate heat and mass transfer model is however necessary to develop an accurate drying mechanics model. Even though the shrinkage values derived were not required for the heat and mass transfer model, the measurements were necessary to determine sorption isotherms and hysteresis for each species for accurate board MC predictions. Salin (2010) proposes that traditional models that use an average MC sorption curve (derived across a range of species) are problematic because different species have different sorption isotherms of which drying models are very sensitive. The work presented in this section aims to characterise the wood-water relations of each in particular shrinkage and sorption isotherms. We performed tests on small micro-samples using specialised precision apparatus designed and built to determine the drying behaviour of small micro-samples. By using such small samples, the stress level during testing is not high enough to produce checks and, more importantly, the sample can be considered relatively homogeneous particularly in terms of moisture content variation. As far as the author is aware, there are no other methods available to continuously follow the evolution of wood samples as a function of external conditions. The highly accurate results produced by this method are imperative as input parameters for theoretical modelling. Test apparatus The apparatus used for these tests was designed and built specifically to accurately determine the wood-water relations on micro-samples, by members of the ENGREF team. Plate 17 and Figure 10 present the experimental set-up. The dimensions of micro-samples are measured using a non-contact laser system while continuously measuring the MC of samples. The system incorporates a highly sensitive electronic microbalance (MC2, Sartorius) with a capacity of 2 g and typical sensitivity of 0.1 μg. Two high-speed laser micrometers (Keyance, LS-5000 series) are incorporated to measure changes in the width and height samples without contact. The laser micrometers can measure within a range of 0.2 – 40 mm at a scanning rate of 1200 scans/s, resolution of 0.05 μm and accuracy of 2 μm. The outputs from both devices are fed back to a PC. 33 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final reportPDF Image | Evaluation of super-heated steam vacuum drying
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