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How Wood Dries - Environmental Effects (RH, Temperature, Velocity) There are three environmental variables that control that rate at which wood dries. And if they control the rate, they also control degrade development. These three variables are temperature (dry-bulb), relative humidity (or wet-bulb or EMC), and velocity. Whenever drying is unsatisfactory - takes too long or degrade is excessive - then these three variables must be modified. The phases of the moon, time of year, or a horoscope do not affect how wood dries. Temperature As the temperature increases, five changes occur that affect drying: a. Water moves faster in wood at higher temperatures; b. Drying is more uniform in wood at higher temperatures; c. The relative humidity of the air will decrease, unless water is added to the air; d. Wood is weaker at higher temperature; and e. Warp is usually worse at higher temperatures (except in high temperature drying - over 212°F). With these principles in mind, then it should be clear that the temperature in drying should be kept quite low during Stage I to minimize the weakening of wood (and subsequent degrade such as checking or honeycomb). The schedules (see Drying Schedules on Page 40) provide maximum temperatures recommended. Often, such as when wood quality is not as high as desired or expected, lower temperatures will be used to prevent or lower the risk of degrade. Relative Humidity As the relative humidity is lowered, five changes occur that affect drying: a. Water moves faster as the humidity is lowered; b. Drying is more uniform at lower humidities; c. Warp is less at lower humidities; d. Checking is more likely at lower humidities; and 25PDF Image | Principles and Practices of Drying Lumber
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