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Air drying times will vary tremendously according to the species, the thickness, the time of year the wood is stickered and the location, and there’s no magic formula to tell you how long the lumber should air dry. The USDA Forest Service Forest Products Laboratory has a publication called Air Drying of Lumber that you may find helpful.1 DRY KILNS. Kilns can increase the rate at which lumber dries compared to ambient conditions. In their simplest forms, dry kilns are basically heated and insulated boxes with fans and vents. More sophisticated kilns, like those used in most commercial operations, add the ability to control the temperature and relative humidity for better control of the drying environment. Choosing the right type of kiln is important to becoming a profitable operation. There are essentially six types of dry kilns: 1) Conventional (steam-heated) dry kilns 2) Hot water-heated dry kilns 3) Direct-fired dry kilns 4) Vacuum kilns 5) Dehumidification (DH) dry kilns 6) Solar dry kilns CONVENTIONAL (STEAM) KILNS. In conventional steam-heated dry kilns, water removal is induced by heat and mechanical ventilation. Many large kilns are heated by steam, often with the steam coming from biomass boilers consuming on-site wood residues. Steam kilns are heated by steam coils with the steam coming from a nearby boiler. These kilns are typically built from corrosion-resistant metals with large doors for convenient fork truck access (as shown in Figure 1). Steam coils are positioned inside the kiln to heat the air, the fans often reverse to make moisture removal more uniform, and vents are used to allow water vapor to escape. These kilns are commonly quite large; hardwood kilns often have a 40,000 board foot capacity or more, while steam kilns used for softwoods can have capacities over 100,000 board feet. Hardwood kilns typically operate at temperatures below 180°F to minimize drying defects and color darkening, but kilns drying southern pine often operate at 240°F using high-temperature steam. See Figure 1 for an example of a conventional dry kiln. 1 Forest Products Laboratory. 1999. Air Drying of Lumber. Gen. Tech. Rep. FPL–GTR–117. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 62 p. Available for download at http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf 8PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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