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In more-permanent structures, some means of humidity control may be used to ensure dry air, as well. It is possible for 1-inch red oak lumber to be dried to 20 percent MC in less than 30 days in a predryer. Proper stacking of the lumber is necessary to achieve optimal drying conditions for any species in any situation. Conventional kiln drying More than 75 percent of the existing hardwood dry kilns in the United States are steam heated, are humidity controlled and have reversible fans inside the kiln. These kilns will typically hold several thousand board foot of lumber. Temperatures inside the kiln range from 120 to 190 degrees F, depending on the stage of drying. Although there are kilns in which temperatures reach up to 240 degrees F, they are usually limited to softwoods that dry very easily and low-value hardwoods. Both temperature and humidity are controlled by either fully automated or semi-automated systems. These systems contain devices that monitor conditions in the kiln and lumber, and adjust interior conditions accordingly. The rate at which the lumber dries is controlled to minimize or eliminate defects caused by drying. Defects include discoloration, warping, cracking, splitting, and surface checking (small openings on the faces of the boards). In extreme cases, drying causes honeycomb (internal splits that cannot be seen on the surface) or collapse. Other forms of heat include direct-fired hot air, electricity, solar, and hot water or oil. Direct-fired hot air kilns usually are limited to softwoods and relatively low-value hardwoods because precise humidity control is difficult — if not impossible — to obtain in the kilns. Usually, electric heat is limited to either dehumidifica- tion kilns or to one of the vacuum-drying processes. Solar heat can be used as supplemental heat in some areas. Some successful designs of solar kilns intended for hobby use are available at 500 to 1,000 board foot capacity. Usually, commercial solar kilns are restricted to tropical areas, to areas where conventional energy sources are not available, or to small, home-based businesses. Neither hot water nor hot oil systems find much use in commercial kilns because of low heat-transfer efficiency. Air movement in the kiln is controlled by reversible fans and baffles. Air movement is necessary to carry moist air away from the surface of the boards, and heat to the boards. The air flow is periodically reversed automatically to ensure even drying across the width of the kiln. Baffles are used to force the air through the lumber packages across the surface of the boards. Place baffles at the top, bottom and ends of the kiln. Adjust the baffles to accommodate size variations in lumber packages in the kiln and to account for the lumber shrinkage that occurs during the drying process. Specific details on humidity and temperature control systems are beyond the scope of this publication, but most systems rely on measuring the dry-bulb and wet-bulb temperatures in one or more places inside the kiln. Use a standard thermometer to measure the dry-bulb temperature. Measure the wet-bulb temperature by placing a wet cotton wick over a conventional thermometer. Water evaporating from the cotton wick cools the thermometer. The wet-bulb temperature will always be lower than the dry-bulb temperature. The difference between these readings is called wet-bulb depression. It is a measure of the drying force applied to the lumber. The relative humidity inside the kiln — or anywhere — is calculated using these two measurements and a conversion chart. Accurate humidity control in the kiln is mandatory for properly drying hardwood lumber. Periodically sampling the actual moisture content of the lumber in the kiln ensures that changes in temperature and humidity settings keep the lumber drying at the maximum safe rate schedule. The U.S. Department of Agriculture (USDA) report Dry Kiln Schedules for Commercial Woods contain several typical schedules for various species and thicknesses of lumber. Measurement can be done either by weighing predetermined sample boards or by using a hand-held, battery-powered moisture meter. In either case, several measurements must be taken throughout the kiln to get reliable readings. Special drying methods Dehumidification kilns A dehumidification kiln captures the energy needed to dry wood from the water in the wood itself. Warm, dry air is circulated through the lumber package and absorbs the water from the lumber. The warm, humid air passes over condenser coils in a dehumidifier. The process of condensing water vapor into liquid allows the recovery of the energy required to evaporate the water from the wood in the first place. The process is essentially identical to an air conditioner. Typically, this kiln is more energy efficient. Also, the construction costs are somewhat lower, since conventional well-insulated, wood-framed buildings can be used for the structure. The disadvantages include the cost of power. Usually, the kiln is electrically powered. Also, the maximum temperature is somewhat lower and the water condensate may contain unwanted chemicals. Generally, this process takes longer to dry lumber than does a conventional kiln. Predryers Essentially, predryers are large, low-temperature (below 120 degrees F) drying sheds used instead of conventional air-drying to get the initial MC of the wood to 25 percent or less. Limited temperature, humidity and air movement controls are used in the building. g5507 page 2 University of Missouri ExtensionPDF Image | Considerations in Drying Hardwood Lumber
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