Lumber which has recently been cut and machined contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried. Drying removes a large amount of water from the lumber and significantly reduces the lumber is warping or cracking. Acceptable water content will vary with the application as well as the type of wood, however, in many circumstances, a moisture content of 19% or less is an acceptable water content.
Although lumber may be air dried, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. A typical kiln chamber is a generally rectangular building which can be sealed to control the introduction and exhaust of air. Further, such kiln chambers typically have reversible fans for circulating the air through the chamber.
The charge of lumber placed in the kiln generally consists of a number of rectangular solid stacks of lumber. Each stack of lumber, in turn, typically consists of a number of vertically stacked, horizontal rows of lumber that form a rectangular solid. The horizontal rows are spaced apart for air to pass between the rows using wooden boards referred to as "stickers" that have a relatively small lateral cross-sectional area in relation to the lateral cross-sectional area of the lumber forming the stack. The stickers are generally spaced apart between each horizontal row to allow air to flow between the rows.
Typically, the stacks of lumber are placed on separate wheeled, flat bed cars which are mounted for movement on railroad-type tracks. Kilns may have any desired number of tracks. Multi-track kilns may therefore accept several stacks of lumber during each drying cycle.
In operation, a charge of green lumber is initially placed in a kiln chamber. After sealing the kiln chamber, the air within the kiln is heated to facilitate drying. The air may be heated in a number of ways such as by heat transfer from pipes extending through the kiln chamber in which steam flows. Alternatively, heated air may be introduced such as from a furnace. Kilns which utilize the introduction of heated air are typically referred to as direct fired kilns.
Fans generally positioned in upper portions of the kiln and above the stacked lumber circulate the heated air through the kiln chamber, including the stacks of lumber. Because the stickers provide spacing between the horizontal rows of lumber, the heated air passes between the rows of lumber and is in direct contact with both the upper and lower surfaces of the individual pieces of lumber. The fans continually recirculate the air through the kiln and the lumber to further dry the lumber. Periodically, a portion of the circulating air is exhausted from the kiln and additional air is introduced into the kiln. Typically, the additional air is heated in the kiln chamber, such as by heat transfer from the steam pipes. This periodic exhaust and replacement process allows circulating air which has absorbed a large amount of moisture from the green lumber to be removed, while drier air is introduced to accelerate the lumber's drying.
Within such kilns, the circulating air flows in a generally circular pattern. More particularly, fans above the stacks of lumber direct air laterally over the top of the lumber in a first direction. When the air contacts a first sidewall of the kiln chamber, the bulk of the air is forced downward by the fans and the ceiling of the kiln chamber. The circulating air subsequently flows through the spaces between the horizontal rows of lumber established by the stickers in a second, lateral direction opposite the first lateral direction. Upon contact with a second sidewall of the kiln chamber, the air rises and is recirculated by the fans through the lumber. Periodically, the fans are reversed such that the air flows in the opposite direction to provide generally consistent drying of the lumber.
The amount of drying of each piece of lumber in a given time period (or the time required to dry lumber to a desired moisture content) primarily depends upon the amount of air which moves across its surface, and the extent to which such air can extract moisture from the lumber. Because of its location, significantly more air flows over the top row of lumber than through the spaces between the other rows. Accordingly, the top row of lumber in each stack generally dries more than the other rows of lumber. As a result, if the majority of the wood in a stack is dried to a desired moisture content, the top row of lumber will typically dry much faster, and thus warp or crook. The warped wood is generally either sold as an inferior grade of lumber or discarded as scrap material. Although the wasted wood is just a small fraction of the entire stack, the high volume nature of the lumber business turns such small fractional losses into large gross losses.
In an attempt to prevent excessive warping of the top row of each stack of lumber, it is known to those skilled in the art to place weights on the top row of lumber. Such weights are typically concrete blocks or other heavy objects which prevent the lumber from warping. Weights, such as concrete blocks, however, occupy relatively large amounts of space in the kiln chamber that could otherwise be filled with additional lumber to be dried. Furthermore, objects in direct contact with the lumber cut off air circulation to the top surface all together. Accordingly, the drying capacity and efficiency of the kiln chamber is significantly diminished for each load, leading again to large gross losses and other inefficiencies.