(1) Field of Invention
The present invention related generally to improvements in the design of forced circulation kilns or chambers for drying and, in particular, to kilns for conditioning or seasoning lumber or similar material.
(2) Description of the Prior Art
Newly cut lumber contains varying amounts of moisture which may range from 30% to more than 200% by weight depending on the species of the wood and other factors. The dimensions of the cut wood, as well as its strength, will vary as a function of the moisture content. Thus, in order to ensure greater dimensional stability and strength, as well as lower shipping costs, it is necessary to reduce the moisture content of the cut wood.
Removal of the moisture from the cut wood is usually referred to as curing. The two commonly used methods of curing are known as the kiln-dry method and the air-dry plus kiln-dry method. The primary difference between the two methods is the air-dry plus kiln-dry method includes the additional steps of first arranging the lumber on elevated racks in open yards to take advantage of the moisture removing capacity of the natural atmosphere. Depending on the type of wood, method of stacking, and other environmental conditions, between 40 and 300 days may be necessary to reduce the moisture content to 20%.
The kiln-dry method may be used with or without the air-dry pre-drying step. The lumber is tiered upon racks or kiln trucks and placed in the kiln where the dry bulb temperature and the wet bulb temperature of the surrounding air are maintained at various predetermined conditions and according to published schedules to reduce the moisture content to an amount near that desired in final use. Typically, the desired moisture content is between 6 and 10%.
While kiln-drying is significantly more rapid and more controllable than simple air-drying, it adds significantly to the cost of the cured lumber. A conventional lumber drying kiln is both costly to construct and to operate. First, the conventional lumber-drying kiln must be large enough to accomodate large amounts of stacked lumber. Second, the walls of the kiln must be insulated in order to prevent excessive heat loss. Finally, electricity for the large capacity fans, steam for heating the air in the kiln, and high maintenance costs due to the moisture-laden atmosphere of the kiln result in high operating costs.
One example of such a well-known conventional kiln is shown in U.S. Pat. No. 3,131,034 to Marsh. Such kilns are usually provided with two or more longitudinal rows of vents which serve to alternately admit air to the kiln and to exhaust the kiln atmosphere depending on the direction of the fan rotation within the kiln. Each of these vents usually includes a body member aligned to a complementary metal opening in the top of the kiln. The upper portion of the body member is hingedly connected to a vent cap having operating arms which are actuated in response to the direction of the fan operation. Thus, in operation, outside air is brought in on the inlet side of the fan assembly and blown through the heating elements, then a portion of the heated air is exhausted out of the exhaust vent while the larger portion of the air passes around and through the stacked lumber.
Certain disadvantages become apparent with such a design. Firstly, a significant amount of treated air passes through the steam heaters and is then exhausted through the outlet vents. Secondly, a significant portion of the total fan airflow goes out through the vents before passing through the stacked lumber. Finally, the large number of vents located in the roof adds considerably to the cost of construction.
Because of the high temperature, high humidity in the interior of the drying kiln, it has also been the practice in the past to locate the drive motor outside of the kiln proper (see Marsh esp. FIG. 1). Because the fans and the associated motors are conventionally located above the lumber stacks, it has also become the practice to construct a separate room, isolated from the drying chamber, which may house the motor and associated controls. One example of such a separate control room is shown in U.S. Pat. No. 4,098,008, issued to Schuette et al (see esp. FIG. 4).
Certain disadvantages become apparent with such a design. First, a completely separate structure must be constructed adjacent to the drying kiln which adds significantly to the cost of the drying kiln. Second, the drive shaft between the fans and motor must be significantly lengthened to reach the secondary building. This adds to the construction cost, maintenance, and probability of damage due to the higher vibration of the longer drive shaft. As a result, this design has not been well received by the industry.
Finally, in the past during the construction of kilns, each kiln has been custom designed and fabricated. As such, once the construction of the housing of the kiln has progressed to a certain stage, it has been the conventional practice for the various fans, heat exchangers, humidifiers, baffles, etc. to be individually stalled prior to completing construction of the kiln. This results in a tedious and prolonged construction of the kiln such that the normal time for construction is approximately eight weeks. This has been due, in a large part, to the complicated roof designs of the previous kilns.
It has thus become desirable to develop a lumber drying kiln which will more effectively utilize the energy necessary for drying lumber. It has also become desirable to develop a lumber kiln structure which will better facilitate access to the fan motors without unnecessary costs and maintenance. Finally, it has become desirable to develop a lumber kiln structure which is particularly adaptable to prefabrication, thereby eliminating the long lead time necessary to construct present day lumber kilns.