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Forced-Air Air-Drying (or Fan Shed Drying) Forced-air air-drying is a drying system where the lumber is stacked conventionally and then placed under outside sheds for drying. The sheds have a large number of fans that pull outside air through the lumber pile. The roof (with an adequate overhang) keeps rain and sun off the lumber thereby providing very fast drying with minimal degrade losses. As a rough rule of thumb, air velocity is about 700 fpm and maximum air flow path is 20 feet. Typically, there is 1 hp of fan power for every 6 MBF in the dryer. This drying method is especially attractive for fast drying, low-degrade-risk species and thicknesses. Low Temperature Warehouse Predrying A warehouse predryer is a large building, holding as much as 1.5 million BF of stickered lumber. Fans circulate air through the lumber piles. Heat is provided typically with steam heating coils. Venting is provided with a fan powered exhaust system. Usually there is no humidification system other than moisture added by the drying lumber. This method of drying is most popular for oak and other hard to dry species. Low Temperature Dehumidification Drying Dehumidification drying is a form of kiln drying. With a dehumidifier, moisture is removed from the dryer by condensing the moisture on cold coils. When the water condenses, heat is released which is then put back into the dryer and used to evaporate more water from the lumber. As a result of this recycling, the dryer is very energy efficient. The heat pump is electrically operated. The pump is typically sized to provide 1 hp per MBF of lumber; smaller sizes are used for slower drying species and larger sizes for easier drying species. The building is usually well insulated, to avoid wasting electrically generated heat. The building may be wood frame or prefabricated aluminum. Normal Temperature or Conventional Temperature Kiln Drying This is the tried and tested method of drying lumber; it has been used for decades for both hardwoods and softwoods. Quality is usually outstanding, if the equipment is designed and operated correctly. Lumber is stacked for drying and placed in a kiln. Fans circulate air through the load, with heat provided by steam coils. Excess moisture is exhausted through vents. Moisture may be added with steam spray. Operating techniques are well covered in USDA Agriculture Handbook No. 188, Dry Kiln Operator's Manual. Today most new kilns are aluminum prefab, with a masonry structure sometimes being seen. In either case, at least two inches of insulation are worthwhile to conserve energy and avoid uneven temperatures in the dryer. 9PDF Image | Principles and Practices of Drying Lumber
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