Kiln overheat when conditioning lumber

With large, well insulated lumber dry kilns, the dry-bulb temperature often rises above the set point when moisture is added with a steam spray to condition the lumber. Desuperheaters are widely recommended as the cure for the problem. But this analysis shows that less than 5 percent of the heat is produced when the hot steam is sprayed into the kiln. More than 95 percent is generated at the board surfaces when the vapor enters the wood. If all of the heat produced by a 1 percent moisture content (MC) increase is used to heat the lumber, the stack temperature will rise by 24? to 33?F for normal ranges of temperature and MC. But some of the heat will be lost from the kiln by conduction. A steam spray containing sufficient water droplets can use the remainder of this heat to vaporize the water droplets, thus preventing overheat. Desuperheaters may be effective to the extent that they introduce sufficient water droplets into the spray. Techniques for conditioning with or without overheat are analyzed with a simple equation using tabular data and also with a computerized sorption simulation. The risk in controlling the dry-bulb temperature from the hotter of dual dry-bulbs is indicated.

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