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• • • • These schedules, especially for oak, were developed for high quality wood. Today, the quality of our wood is often not as good as in the past. Therefore, many firms have modified the final conditions of the schedule so that the temperature never exceeds 160°F and the depression never exceeds a 40° or 45° depression. Sampling Moisture Content The key to using the U.S. FPL schedules is knowing the moisture content of the wetter pieces of lumber. This is done with the samples mentioned in the previous section. Only the wettest half of the samples are used in determining the average MC, which is then in turn used to establish the correct temperature and humidity levels. The U.S. FPL schedules are based on the average MC of each sample board and not just the shell or just the core MC. As a general rule, there should be at least one sample for every 5000 BF of lumber in the dryer. This will assure that there will be an adequate cross-section of MCs representing the variety of MCs in the kiln. Again, it is essential that the samples represent the wetter lumber, as it is this lumber that has the greatest risk of degrade. At the same time, a drier piece or two must be included to avoid over drying and to indicate when to begin equalization. It should be noted that the U.S. FPL schedules assume a velocity of 375 fpm. Higher velocities would result in faster drying than desired and thereby increase the risk of checking; lower velocities could slow drying too much and increase the risk of stain or warp. The recommended schedule for 4/4 and 5/4 red oak is T4-D2. The actual conditions specified for this schedule are presented in Table 7. All codes begin with the letter "T". The first number is the temperature number, indicating how hot the conditions will be, especially at the higher MCs (Stages I and II). The numbers range from 1 to 14. With a number "1" or "2" temperature schedule, the initial temperature for MCs above 30% is 100°F; for numbers "3" and "4," 110°F; and so on. The third character indicates the MC at which 1/3 of the green MC has been lost and therefore, Stage II begins. When Stage II begins, the relative humidity can be lowered slightly. The letter "A" is the lowest MC condition, making the first change in RH at 30% MC; an "F" schedule begins to change RH at 70% MC. The final character is a number that indicates the humidity level. A "1" is a high humidity schedule, while an "8" is a low humidity schedule. The lower numbers would be used for check prone lumber. 41PDF Image | Principles and Practices of Drying Lumber
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