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end when final MC is determined.) To achieve check free conditions when drying typical red or white oak lumber, the recommended Stage I kiln conditions are 87% RH, 350 feet per minute, and 110°F. If the velocity is lowered and/or temperature is lowered, then the checking risk is lowered, which can mean that the RH can be safely lowered. In a warehouse predryer, conditions of 85°F with 65% RH and 125 feet per minute are satisfactory. In essence, there is no one correct set of conditions for drying oak in Stage I; rather all three - temperature, RH, and velocity - must be considered together. The key variable to monitor in Stage I, then, in order to determine whether the conditions are satisfactory, is the daily rate of MC loss from correctly prepared samples. (Selecting and preparing samples is discussed in Operating a Lumber Dryer.) Each piece of lumber has a safe drying speed limit (Table 5). The limit depends on species, thickness, and outside factors such as the presence of bacteria. Exceeding the safe limit greatly increases the risk of checking. Operating much below the limit extends drying time and increases the risk of staining. It is important to state that the limit is the maximum limit for every piece of wood, not just the average of most of the pieces. Further it is the daily loss, not the average loss over several days. It must also be mentioned that the safe rates are based on normal wood. If anything has happened to the wood, such as being infected with bacteria, being cut with a dull saw at the sawmill, being dried too fast on a preceding day, being subjected to high temperatures prior to drying, or having small surface checks from previous drying, then the rates quoted would not apply. (I am not sure what rates would apply for these cases; the best advice is to avoid these problems at all costs.) Also note that with presurfaced lumber, that is lumber that was planed green to remove the rough surfaces which are often the source of checks (planed lumber had 18 times fewer checks than rough lumber in one study by McMillen), the drying rate can be safely increased. When studying tension stresses in drying, as the stress level increases the wood becomes plastic. This plastic level is reached before the wood fails (checks). When in this plastic range, the wood cells are permanently stretched and are larger than they would have been if they had been free to shrink without being restrained by the wet core. This phenomena is called tension set or casehardening. (Note that the case or shell is not harder, however.) 21PDF Image | Principles and Practices of Drying Lumber
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