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Company A and B do not quantify the quality of the lumber, but visually inspect the lumber before and after drying. Table 11 lists a brief description of drying degrade observed by company A and B for 4/4 red oak. Table 11. Drying Degrade for Vacuum Drying According to Company A and B for 4/4 Red Oak Comp any Not A affecte d Not B affecte d Drying degrade for vacuum drying Internal, surface, end checks Not presented Neither any better nor any worse than conventional Drying stresses Not present Color Warp No difference compared to conventional No difference compared to conventional Moisture Content Uniformity MC deviation +/- 1% Problems with both Not conventional and vacuum present According to both Company A and B, vacuum drying does not affect the color of wood, and the wood comes out of the kiln looking as it did when it went in because of the absence of oxygen in the drying process. For example, Moldrup (1992) established that with vacuum drying, lumber is similar in color as it was before drying. Company A and B stated that drying stresses are not present in their vacuum drying processes since as vapor accumulates in the chamber, the chamber pressure rises. The boiling point of water goes up with the chamber pressure. Evaporation from the wood slows. Water migrates from the wetter areas to the dryer areas in the wood. The wood warms. When the top end of the band is reached, the vacuum system begins to lower chamber pressure. The water in the wood boils even though the temperature is less than 105oF. This process is repeated several times per hour, and stress can be non-existent. Also, since no stresses were observed, no surface checks were presented. Warp can occur in boards that have tension wood; also, poorly manufactured boards containing excessive wane will also warp, as the sapwood of the tree cut in the warm afternoon tends to have a higher MC. It can be said that vacuum technology is slightly better than conventional drying (air drying + kiln drying) in terms of shrinkage. In vacuum drying, shrinkage can vary from 4.1% to 7.1%, 65PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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