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11/22/2021 Conventional Drying • The heat source (steam) is passed through heat exchangers to heat the air. • Fans circulate the warm air through the kiln and heats the lumber by convection • Lumber releases moisture that is vented from the kiln drying the lumber Vacuum Drying • Ambient pressure is lowered which creates total pressure differences inside the wood • Water boils and changes into water vapor during vacuum drying. • Under the total pressure difference, water vapor is removed from the wood. • Under the pressure difference, most moisture moves in the longitudinal direction. 78 9 10 11 12 Vacuum Drying 1.0 atm 0.5 atm 0.2 atm 0.13 atm 52 oC (125 oF) 0.1 atm 32 oC (90oF) 100oC (212oF) 82 oC (180oF) 60 oC (140oF) Espinoza 2005 Moisture Movement Conventional Drying Diffusion Diffusion FWBF Diffusion MC Below FSP Free Water Bulk Flow MC Above FSP Surface Evaporation Water is removed by two mechanisms bulk flow and diffusion FWBF Adapted from Chen 1997 During vacuum drying, total pressure difference rather than diffusion is the primary driving force. Adapted from Chen 1997 A boiling front exists inside the wood. Drying times Conventional Vacuum Green Condition Dependent on Technology Red Oak 4/4 25-35 days 3-9 days Hard Maple 4/4 10-12 1-7 Ash 4/4 10-12 1-7 Black Walnut 4/4 12 1-7 Hard Maple 8/4 30-35 5-12 Bond and Brenes 2017 2 Surface Evaporation FWBF DiffusionPDF Image | Understanding Vacuum Drying Technologies
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