4. Determine Drying RH Settings

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4. Determine Drying RH Settings

Developing a drying schedule based on a drying gradient.

Developing drying programs/schedules is an ongoing process of trying to perfect the drying quality and the efficiency of the kilns. On some occasions the same specie will require a little different drying schedule if it comes from a different region. One example is the difference between oak grown in warmer climates and oak grown in colder climates.

Finding the right relative humidity schedule (wet bulb) based on the drying gradient.

The relative humidity (and temperature) is closely related to the equilibrium moisture content on the air. The equilibrium moisture content of the air is a measure of the moisture content the lumber will react to if placed in a certain climate. This is due to the fact that lumber is a hygroscopical material, which permits it to absorb and retain moisture depending on the outside climate (temp and relative humidity). The relative humidity setting is very important since the wrong setting can cause either stain (to high RH%) or surface checking (to low RH%). We suggest you calculate the RH% setting by using the drying gradient.

Table with different species divided into 4 groups used to determine drying gradient in table.

Guidelines for approximate drying gradient for different groups of species and different thicknesses

Example on determining the RH% setting using drying gradient:

28mm (4/4") white oak.
Temperature set point	45 C or 115 F
Current MC% in the core	21% (This is not by weight method.)
Oak belongs to specie group	4
Suggested drying gradient for 28mm (4/4) group 4 species	1.5 to 2.0 (1.5 for careful drying) (2.0 for faster drying)
Drying gradient	1.7 Example.
Calculated EMC% of the air	12.4 EMC% (21 MC% / 1.7)
Use EMC graph to determine	approx. 72% RH (12.4 EMC% at 45 C)
The relative humidity setting should be	72%

If the MC% of the wood is above 28% you must use 28% in your calculation. When drying lumber, the shrinkage starts around 28%, this is the reason why MC% above 28% is considered 28% for purposes of calculating the RH settings. We are trying to prevent the excess shrinkage of the shell (in comparison to the core) to be stretched beyond the breaking point when surface checking starts.

Kiln-direct believes this is the best method to determine the relative humidity or depression schedule for species and thicknesses that you cannot find good and accurate existing data. Using this method does require a thorough understanding of kiln theory and very close attention during the first drying cycles to insure that drying defect and other degrade becomes excessive.

We hope this information meets with your expectation and approval. We always welcome your opinions and suggestions. You can email us at techsupport@kiln-direct.com. Thank you for your continued interest.

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