Understanding extended drying process

The kiln operator is likely to be blamed for any damage, degrade, stain, and other devaluation that occurs from the time the logs are sawn to the time it ships out or are used for a finished product. Therefore he should concern himself with all the intermittent steps from fresh sawn lumber to the final use.



Example of the general steps in the extended drying process:

 Initial stacking of fresh sawn lumber.
In most cases the lumber is dead packed and then moved to an automatic stacker, which places the lumber on sticks. On certain species it is important to insure the lumber is only dead packed for a short period, less than 24 hours to avoid stain and mold. In other cases the lumber may be placed on sticks right at the green chain. Here it is essential to make sure that the lumber packs are properly sticked to minimize later drying degrade.

Handling the lumber from the sawmill to the kilns.
Since lumber is a valuable material it should be handled with care to maximize the final quality and prevent waste through poor handling. Poor handling often results in physical damage to the lumber rendering it useless as does the more hidden dangers, such as embedded stones into the bottom board, which can damage the lumber machinery, such as moulders, planners, etc.

Chemical treatment.
It is very common to treat the lumber with a chemical solution in the warmer climates to avoid stain and mold which degrades the lumber before the actual kiln drying begins. The kiln operator should make sure the chemical solution performs well by checking the concentration and its performance.

Air drying and storage before entering the kilns.
Some lumber operations make extended use of air drying while others prefer to start the kiln drying process as soon as possible. Learn more about air drying versus kiln drying from green. In either case the operator must insure that the conditions for storage will prevent as much degrade as possible. The required storage conditions will depend on the species and thickness. For example 26mm (4/4) pine will require as much air flow as possible to prevent stain, mold, and mildew; whereas 38mm (6/4) oak must be protected from excess air flow to prevent checking. Keeping the lumber stacks away from too much rain and direct sun is important. This can be accomplished by placing the lumber in sheds or through the use of portable roof tops placed on top of the lumber bundles.

Predrying or fanshed drying.
This can also be considered as accelerated air drying in a fully (predryer) or semi (fanshed) controlled environment. If either fansheds or predryers are used they should be considered part of the closely supervised drying process with daily recordkeeping and moisture measurements.

Kiln drying.
This includes more than just operating the kiln. It is important to make sure the kiln is loaded correctly and that the kiln is maintained well. The kiln operator should pay very close attention to the lumber in the kilns to minimize the degrade and to maximize the kiln drying quality.

Taking the lumber off sticks.
This is normally not part of the kiln operators responsibility, but he should use this step to aquire as much information about the lumber and its drying quality. By comparing the grades and quality of the lumber from the sawmill to after the kiln drying process can help the operator increase the quality of the kiln drying.

Storage of dried lumber before shipping or usage.
The main problem in this stage is when lumber may be stored for weeks or months before shipped or used. When lumber is stored it may begin to pick up moisture from the surrounding air, which can result in the moisture content increasing to unacceptable levels. This can be prevented by heating the storage building or installing some type of a dehumidification unit.

We hope the above information was helpful in explaining the reasons a kiln operator should broaden his field of responsibility in order to:

-- help prevent degrade before the actual kiln drying process,
-- collect data to evaluate the drying results, and
-- prevent moisture gain after the kiln process.

The example above is only a sample and your situation may have more steps which need attention to provide your company with the best possible kiln drying result.
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