Features of Softwood Lumber Drying Under Reduced Medium Pressure

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UDС

674.047.38

DOI:

10.17238/issn0536-1036.2016.5.132

Abstract

The paper considers the convective drying of softwood lumber at the continuous circulation of a drying agent under low pressure. A noticeable rate reduction of the drying agent circu-lation takes place in the kiln at a pressure of 50 kPa and less. The water-vapor saturation pressure in the drying medium is uniquely and logarithmically dependent on a temperature. That is why the steam point reduces with decrease of a medium pressure. Moreover the dif-fusion and self-diffusion coefficients of water vapor and the moisture content of the drying medium increase. In connection with these phenomena, heat- and mass transfer under re-duced medium pressure is determined. The diffusion coefficients under reduced pressure increase. The intensity of the vacuum drying process depends on the internal diffusion of moisture to the wood surface. The moisture content of the drying agent determined by the partial pressure of water vapor in a vacuum kiln increases with decreasing pressure. The temperature of wood should be equal to the boiling temperature or higher to intensify the evaporation of moisture from wood. The evaporating moisture cools the wood surface, and the internal diffusion of moisture to the wood surface increases. An overpressure occurs inside the wood, which relaxes depending on the hydraulic resistance of wood. After the relaxation of pressure in wood the saturation temperature is below of the moisture tempera-ture, and moisture boils throughout the wood volume. A drying process is carried out in a vacuum drying kiln – an autoclave. The autoclave casing is doubled for water-cooling for condensation of water vapor from the drying agent. The condensed moisture is collected in a graduated container with a water meter. In the initial lumber moisture we can determine the current moisture content of wood by a condensate volume. The autoclave single occupancy is up to 22 m3 of sawn timber. Three autoclaves work at the same time. The use of a medium low-pressure in the process of convective drying of softwood lumber reduces its duration by 13...26 hours. The drying process is controlled by the remote control using the wood tem-perature and medium condition in a vacuum drying kiln.

Authors

A.A. Goryaev, Candidate of Engineering Sciences, Associate Professor V.I. Melekhov, Doctor of Engineering Sciences, Professor N.B. Balantseva, Candidate of Engineering Sciences, Associate Professor O.A. Kalinicheva, Candidate of Engineering Sciences, Associate Professor

Affiliation

Northern (Arctic) Federal University named after M.V. Lomonosov, Naberezhnaya Severnoy Dviny, 17, Arkhangelsk, 163002, Russian Federation; е-mail: ark16111936@gmail.com

Keywords

drying, timber, vacuum, degree of saturation, drying agent, superheated steam, equilibrium moisture content, diffusion, heat conduction, circulation, pressure.

References

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Received on May 26, 2016