Mechanisms Responsible For The Effect of Wet Bulb Depression on Heat Sterilization of Slash Pine Lumber

Authors

  • William T. Simpson

Keywords:

Heat sterilization, international trade, kiln-drying

Abstract

Heat sterilization is often required to prevent spread of insects and pathogens in wood products in international trade. Heat sterilization requires estimating the time necessary for the center of the wood configuration to reach the temperature required to kill insects or pathogens. In these experiments on 1.0- and 1.8-in.- (25- and 46-mm-) thick slash pine, heating time at 160&deg:F (71&deg:C) increased exponentially with increase in wet bulb depression. The time required for the center of green 1.0-in.- (25-mm-) thick boards to reach 133&deg:F (56&deg:C) varied from 15 min at a low wet bulb depression near saturation to 438 min at the dry conditions of 50&deg:F (27.8&deg:C) wet bulb depression. For green 1.8-in.- (46-mm-) thick boards, the range was 38 to 198 min, and for air-dried 1.0-in.- (25-mm-) thick boards, the range was 9 to 23 min. When the wet bulb temperature in the kiln was below the desired target center temperature of 133&deg:F (56&deg:C), heating times were extended far beyond the times when it was not, which caused problems in attempts to estimate heating times. Surface temperatures during heating were found to decrease from evaporative cooling as wet bulb depression increased. A finite difference solution to the heat flow equations, solved for the boundary condition of a time-dependent change in surface temperature during heating, offers a good estimate of heating time when the wet bulb temperature of the heating air is above the desired target center temperature. An analysis was developed to estimate the surface transfer coefficient that describes this boundary condition.

References

Carslaw, H. S., and J. C. Jaeger. 1986. Conduction of heat in solids, 2nd ed. Clarendon Press, Oxford, UK.nCrank, J. 1975. The mathematics of diffusion, 2nd ed. Oxford University Press, Oxford, UK.nFleischer, H. O. 1953. Veneer drying rates and factors affecting them. Forest Prod. J. 3(3):27-32.nSimpson, W. T. 2001. Heating times for round and rectangular cross sections of wood in steam. Gen. Tech. Rep. FPL-GTR-130. USDA Forest Serv., Forest Prod. Lab., Madison, WI. 103 pp.nSimpson, W. T. 2002. Effect of wet bulb depression on heat sterilization time of slash pine lumber. Res. Pap. FPL-RP- 604. USDA Forest Serv., Forest Prod. Lab., Madison, WI. 6 pp.nSimpson, W. T, and J. Y. Liu. 1991. Dependence of the water vapor diffusion coefficient of aspen (Populus sp.) on moisture content. Wood Sci. Technol.26:9-21. 187-200n

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Published

2007-06-05

Issue

Number 2 / April 2003

Section

Research Contributions

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