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Figure 1. Conventional steam-heated dry kilns. There are large doors for loading the lumber in the front of the kiln, and the kiln is constructed from corrosion-resistant materials. Notice the number of vents in the kiln roof. These are used to vent water vapor from the drying lumber. Whenever possible the steam for conventional kilns is produced on site by burning wood waste. Because of their size, these kilns are the most expensive to construct and they are expensive to operate if oil or gas is used for boiler fuel, but they offer the greatest flexibility in possible drying conditions. The steam that provides heat for drying is also be used to relieve lumber stresses when drying is finished. HOT WATER-HEATED DRY KILNS. Hot water-heated dry kilns are relatively new to the North American market, though they have been available in Europe. They are similar in many respects to conventional steam kilns, but they are heated by circulating hot water or oil. Sawdust-fired boilers heat the water which is then injected into heating coils as needed to control the kiln temperature. Kilns are custom-fabricated, but sales literature indicates that they are available in sizes from approximately 4000 board feet to 60,000 board feet. DIRECT-FIRED DRY KILNS. Direct-fired dry kilns dry wood by the introduction of heated air into the kiln chamber. Often the heat is generated by the firing of natural gas, but wood waste is also used for heat at some locations. Direct-fired kilns work quite well and are sometimes favored for fast-drying species (such as pine). To avoid hot spots, direct-fired kilns should not be operated with partial loads. Kilns offered by one company range from 9000 to 40,000 board feet capacity. Hot water and steam heating may be offered as options. If hot water or steam are not used, additional humidification equipment will likely be needed to relieve drying stresses for hardwood lumber. VACUUM KILNS. Vacuum kilns work by reducing the atmospheric pressure, thereby enabling water to vaporize at lower temperatures. They’re used to dry specialty products such as baseball bat billets and guitar bodies, but because they’re expensive this is not appropriate technology for a small operation drying grade lumber. Capacities range from 500 board feet to over 10,000 board feet. Heat won’t transfer by convection in a vacuum, so as seen in Figure 2 heated blankets are used to directly transfer heat to the wood to be dried. 9PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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