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tissue, suggested to be constant for all wood species, the value of which is reported to be approximately 1500 kg/m3 (Siau, 1984). The following equation was used to calculate density: ρcalc =ρwt(1−φf ) Where ρcalc is the calculated density, ρwt is the density of wood tissue (1500 kg/m3) and φf is the fibre porosity. 3.4.3 Gas permeability Permeability is the property of a material that indicates how freely fluids flow in response to a pressure gradient. Permeability of wood to liquids and gases plays an important role in a number of technical processes such as wood treatment with preservative chemicals, the pulping process and wood drying. The more permeable the wood the more easily it can be processed or treated. Drying time and conditions are driven by the permeability making it a crucial factor for the resulting product quality (Hansmann et al., 2002). Wood permeability in hardwoods is affected mainly by anatomical features such as vessel size and obstructions (tyloses), pit aspiration, and the presence of extractives in heartwood.(Milota et al., 1995). For Australian eucalyptus species, as previous researchers suggest, water movement during conventional drying is restricted predominantly to diffusion, (Innes, 1996, Blakemore and Langrish, 2008). Sampling procedure For each species, we prepared one quartersawn and one backsawn board with dimensions 300 x 100 x 8 mm using boards from the vacuum drying trials of this project. Three 74 mm diameter samples were cut using an 80 mm diameter hole-saw for each board producing three radial and three tangential (in thickness direction) samples per species. For each species, one 200 x 100 x 28 mm thick sample was prepared. From those boards we cut three 19 mm diameter, 40 mm long cylinders using a 24 mm hole-saw in the same longitudinal symmetry plane (Figure 5). We used these samples to determine longitudinal permeability. Due to the impermeability of these species, the longitudinal samples were cross-cut in half to produce 20 mm long samples. We coated the side surfaces of all specimens with epoxy resin to guarantee the air tightness of the lateral surfaces during measurement. Figure 5. Sample preparation for permeability tests: Left – radial and tangential permeability sampling. Right – longitudinal permeability sampling 28 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report (37)PDF Image | Evaluation of super-heated steam vacuum drying
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