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4.1 COMPARISON BETWEEN VACUUM AND CONVENTIONAL DRYING QUALITY Quality is an important aspect to be evaluated during the wood drying process since poor quality or degrade can cause a loss of value or reduce the ability to utilize the raw material; however, many lumber manufactures do not quantitative or qualitatively evaluate this aspect. Conventional drying (air drying + kiln drying) defects were analyzed to compare the differences between vacuum and conventional drying. Since air drying is typically the process with the least amount of control over drying process, defects can be severe. When air drying is used to reduce the moisture content to 20-25% prior to kiln drying, most defects are started during the air-drying phase. For example, surface check and end checks are formed during the first 1/3 loss of moisture content. Air drying rate can be affected by different factors like species, thickness, grain pattern, yard characteristics, pile characteristics, and weather conditions (Dening et. al 2000, Reeb and Brown 2007). Hart et. al (1992) and Wengert (1990) determined that the number of checks increased in the lumber that has been dried or partially air dried after moisture has been added. The open checks may cause yield losses from the lumber that might be used for furniture or saw mills. In a study done by Gammon (1971), the author found a 13% potential loss in 4/4 red oak after it was air dried. This percentage was mostly due to shrinkage, and most of the lumber degrade can be due to seasoning defects. The author mentioned that defects like warp, checks and splits can be caused to a lack of uniformity in the structure of the wood. In a study developed by the Forest Products Laboratory (1999), it was found that the tendency of checks in air dried oak was high; while the tendency of warp was intermediate. Stains and discolorations are defects that can be presented in lumber been air dried. These types of stains are called chemical stains, which can darken the wood from gray to brownish colors. The most susceptible species to present chemical stains are white pine, hard maple, western hemlock, ash, birch, hickory, and others. Stains can also be cause by fungi that use wood as food. The typical stains are called “blue stains”, which usually grows in the sapwood of heartwoods and softwoods (Forest Products Laboratory 1999). 62PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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