Drying Juvenile SYP
From contributor E:
SYP, especially juvenile, or plantation pine is not as easy to dry as most people would have you think. I am ready to retire so I care less what people think. My experience with steam kilns, slope-grate green fuel, fluidized bed, and suspension shavings heated kilns all lead to one thing. The longer the lumber is dried the more even the MC distribution. To get below 19% as I have to with 2x8 lumber takes even longer as it is more difficult to get the bound water removed. You do need to get an airflow study done to see if you have even end-to-end air velocity through the lumber. I have found that most times if you have even airflow, then board to board variation within a pack is caused by something that you probably cannot control. That being a variation board to board of the green wood going into the kiln. One way to control this is to let the lumber set at least five days before planing. Another is to use a conditioning cycle at the end of the charge. This is not feasible for most companies due to drying capacity versus sawmill output.
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!
To contributor E: This is more or less the approach that we are taking. We have successfully dried (Pinus patula) juvenile core material 50mm x 76 mm cut form logs too small to cut anything bigger, without using strainers or weights and without a conditioning or equalizing phase. Drying times were less than 40 hours running at a DB temp of 85 deg C and a constant RH of 38% from start to finish.
From Gene Wengert, forum technical advisor:
I agree with all the observations that Contributor E made regarding drying of SYP. We know that low RH, which can only be obtained with high air flow, creates wood fibers on the outside that are twice as strong as wet fibers and so they resist warping. The opposite is that with poor air flow the RH within the pile will increase and that means more warp. The hotter that wood is, the easier it is to bend, so with non-perfect stacking warp will be more with higher temperatures.
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.
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