Question
As you may know, there is a trend toward using plantation grown SYP in small log mills. It's a strange animal to dry - very finicky to say the least. Would anyone be able to recommend a high-temp kiln schedule that you feel might work with this type of lumber? Most of our products are 2x4’s. I realize that there are a multitude of factors that could affect choice of schedules. We have developed a schedule that is showing some promise, but needs some fine tuning. I would like to see what you come up with without giving too much info to see if our thinking is sound. I will say that our max temp is 230F and run time is about 18 hours. The kiln holds around 88,000 board feet. The main problem is in our inconsistencies - they are too inconsistent! For instance, in the middle of a given pack you may have one board that has a MC of 10, the next may be 37, the next 8. Also, can air drying lumber show signs of casehardening? We are located in east central Mississippi.
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
Drying rate and MC’s are affected by wood factors, such as permeability (when drying at HT), thickness, heartwood/sapwood, etc. and also by environmental factors (RH, temperature, and velocity). My experience with kilns shows that they induce a great deal of variability. Hence, I developed the multi-zone zone kiln for drying SYP at HT back in 1978. The zones were controlled by temperature drop and constant exit air temperature (brief description of a big item.) Contact the Southeastern Lumber Manufacturers Association for a copy of their booklet on drying southern pine.
One man I worked for years ago who had his own controls company stated that lumber that was not ruined at the beginning of the charge by attempting to remove moisture too quickly did not need a conditioning cycle at the end of the charge. He accomplished this by balancing the temperature with enough airflow to remove the moisture as it was generated. Too much airflow results in hardcasing. Too much heat without enough airflow resulted in warping and crook. Do your own testing of green lumber. You would be amazed at the change in the weight of a pack of 2x6x12 SYP cause by extended dry or wet spells! Extra moisture equals more drying time, every time!
I know of no SYP kiln that uses a stress relief period at the end of drying to remove casehardening stress. However, we did do an experiment with Union Camp where we steamed the lumber briefly at the end of drying after it had cooled and found it seemed to plane more easily. However, the time for cooling made the total drying time exceed 24 hours and so was not acceptable.
The book mentioned above on drying SYP has a great deal of info about uniformity in drying and how to reduce the variation in final MC, practical techniques and procedures. Note that if you use a very low RH in the kiln and high air flow at the end, the outside fibers will be very dry, even though the lumber averages 15% MC. These dry fibers are sometimes called casehardening as they are really hard because they are so dry. The term casehardening refers to drying stresses however and not the hardness of the wood. In my experience, SYP with compression wood in the core will shrink along the grain in strong ways and create warp. It is the wood and not the drying process that makes this happen. The longleaf pine (Pinus patula) is supposed to have much less compression wood than the common SYP here - loblolly.
