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e. Staining is less likely to occur as humidity is lowered. With these facts in mind, then, it is clear that with species that are likely to check or crack during drying, the humidity must be quite high, yet not too high so that the risk of staining and warping is increased above tolerable levels. For species that can tolerate lower humidities, then these low humidities should be used to provide brighter and flatter lumber, and faster drying. Velocity As the velocity through a load of lumber increases, several changes occur that affect drying: a. Above 40% MC, wood will dry faster as the velocity increases; b. Below 20% MC, wood will NOT dry faster as velocity increases; c. Between 40% and 20% MC, the effect of increases in velocity diminishes; and d. At higher velocities, drying is more uniform. As always, faster drying increases the risk of checking and cracking, but decreases the risk of stain and warp. When the cost of velocity changes is assessed, it is found that the cost of doubling the velocity can increase by 4 times, depending on the electric rate. So lower velocities below 20% can result in very significant cost savings. Because velocity is not well understood and is not well discussed in drying handbooks, a special detailed section The Role of Velocity in Drying is included in this chapter. Changing the Rate of Drying (or) Changing the Amount of Degrade When lumber is drying, if it is drying too fast (i.e., checks, splitting, and honeycomb are likely), the drying rate can be slowed by lowering the temperature (That's why low temperature predryers are so popular for oak drying!); by raising the relative humidity (Some equipment performs poorly and there may be a risk of staining if the RH is too high!); and/or by lowering the velocity (With long air flow paths in wet lumber, uniformity may suffer tremendously if velocity is too slow!). If the wood is drying too slowly (i.e., stain or warp is excessive), then the humidity can be lowered and the velocity increased. Generally, the temperature is not increased, due to the potential effect on the wood's strength. So, degrade control is as easy as one, two, three - temperature, RH, and velocity. It should be understood that there is not one correct set of drying conditions. Rather, for the given MC, species, and thickness of lumber and for the quality needed, there are many sets or combinations of temperature, relative humidity (or EMC), and velocity that will dry the lumber at the correct, degrade-free rate and keep the wood strong and flat. As an illustration of this fact, consider that the drying rate for a certain species is 1% per day. Previous research has, for this 26PDF Image | Principles and Practices of Drying Lumber
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