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Drying Schedules The combination of temperature, relative humidity (or EMC), and velocity - that is, the schedule - that is correct is the combination that dries the lumber at the correct, safe rate without causing degrade. As mentioned earlier, there are many combinations that will work well with a given species and thickness. So there is no one correct schedule. However, the U.S. Forest Products Laboratory in the 1950's developed a systematic approach to schedule development. This systematic approach has many advantages, when compared to a random schedule development process. The advantage today from their approach is that every recommended schedule has been used for over thirty years to dry millions, if not billions, of board feet of lumber successfully. There is something comforting about that, especially when a kiln has $50,000 (or more) worth of lumber in it. U.S. Forest Products Laboratory Schedules The philosophy of the U.S. FPL schedules is based on the four Stages of drying mentioned earlier. • • • • • The schedules also were developed with the assumption that there may be slight variations in equipment and conditions. This necessitated building in a safety factor. Therefore, almost all of the schedules are conservative. With better, more accurate and more reliable controls possible today, this conservatism is no longer necessary. In other words, the old schedules can be accelerated safely with today's modern equipment. All the U.S. FPL schedules have a four character code, such as T4-D2. A master code listing is given in the book Kiln Schedules for Commercial Woods, available from the U.S. Forest Products Laboratory at no cost. The coding system works as follows: Keep an initial mild condition for the loss of the first 1/3 of the moisture, the degree of mildness depending on the likelihood of surface checking. Slowly lower the relative humidity, beginning after the first 1/3 of the green moisture is lost. Keep the temperature low until average MC is below 30%, the actual temperature depending on the species and its degrade risk. Slowly raise the temperature after the average MC is below 30%. Equalize and condition at the end of drying. 40PDF Image | Principles and Practices of Drying Lumber
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