The temperature is closely related to the drying rate of the lumber and is one of the three controlable factors we can utilize to either slow or increase the drying rate.
An increase in kiln temperature will dry the lumber faster. This is accomplished because of two factors.
- The moisture inside the lumber moves faster from the wetter core to the dryer surface.
- The hotter air can carry more moisture away at the same relative humidity and air flow.
Rule of thumb:A 20F or 10C increase in temperature can dry lumber up to twice as fast (with the same RH% and air velocity).
The above rule would suggest that higher drying temperatures will always be an advantage; however, there are noticeable drawbacks to using higher temperatures . . .especially in the beginning of the drying process (when removing free water). Some species, such as maple and beach, will turn darker when higher temperatures are used. On oak, degrade (checking, honeycombing, warping) is significantly increased when temperatures above 37-46C (100-110F) are used on green oak. In addition, it is generally true that lumber dried at lower temperatures is stronger, consequently, lumber dried at lower temperatures is able to withstand more stress before checking than lumber dried at higher temperatures.
Over the last several decades, the average drying temperature has decreased in order to improve both the quality and appearance of the lumber. Wooddryer System is a strong advocate of low temperature drying and believes that one of the major obstacles in using lower temperatures efficiently is poor and/or outdated kiln design and specifications.
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