It is a common process in wood production mills and effectively reduces the moisture content of green timber to “workable” ranges, which prevents the variety of issues that can result from excessive moisture levels in wood.

Kiln schedules are used to determine the temperature and relative humidity needed in the kiln to dry specific wood products at a satisfactory rate without causing objectionable drying defects. A typical kiln schedule is a series of temperatures and relative humidity, which are applied at various stages of drying.

The wood drying process is divided into the following steps:

1, Warm-up/ Heat through stage: the purpose is to heat the wood, so that the humidity inside and outside the wood tends to be consistent.

2, Drying stage: according to the basic standard of control, the moisture content of wood gradually decreases. At this stage, the temperature and humidity should be adjusted, and the speed of temperature rise and fall should be paid special attention to, and the error of exceeding the set value of temperature should not occur.

3, Conditioning stage: the purpose is to stop the evaporation of water on the wood surface too fast, make the surface of wood contain moisture and adjust the distribution of water on the surface and inside of wood, reduce the continuous difference of moisture content, and ease the residual stress of the wood. The frequency of treatment and processing should be set according to the thickness of the wood species, drying quality and actual wood resistance.

4, The final processing and cooling down stage: the purpose is to spray the wood and adjust its humidity, so that the moisture content is balanced, the remaining residual stress disappears, and its stability is improved.

Typical drying schedule

Equalize and condition as necessary

In general, the schedule should be developed so that drying stresses do not exceed the strength of the wood at any given temperature and moisture content. Schedules vary by species, thickness, grade and intended final use of the material.

For example, a typical hardwood schedule would begin at 110 to 120oF and 70- to 80-percent relative humidity when lumber is green. Temperatures could reach up to 170 to 180oF by the time lumber has a moisture content of 10 to 15 percent.

Softwood kiln schedules differ from hardwood schedules because the kiln temperature and relative humidity are decided at predetermined times rather than at moisture content levels.