Pre-Sorting Methods

[AliciaS]

When presorting lumber prior to going into the kiln what method do you recommend: density separation, species separation or moisture separation? If you recommend more than one which one should be taken into consideration first?

(Posted on behalf of industry mill workers.)

[StavrosA]

When a green lumber load goes into a kiln, it contains many pieces with physical characteristics that vary from each other often quite significantly. For example, in BC we dry SPF (spruce-pine-fir) or hemfir mixed species as one. Even when you have one species only, lumber pieces come from different parts of the tree (juvenile vs. mature wood), thus having different densities or, from sapwood vs. heartwood, consequently having different moisture contents, just to name a few.
 
When this spread (or high standard deviation) of those green moisture contents or densities goes into a kiln as one load, it is expected that a high spread on KD moisture content will show up on the exit side of the kiln. This spread can also happen when we mix species. If you cut the population in two groups by pre-drying sorting, we have proven through research over the last 25 years, that you benefit your KD standard deviation, you may decrease drying times and improve lumber quality.
 
Separation based on species, density or moisture content means having the space to store the various sorts, have a good logistics system to move them through your kilns and more importantly, having the technology to carry out the sorting. The latter is the bottleneck. Species separation can be done swiftly and with high accuracy using near infrared (NIR) technology however we are still working on developing a robust system. We are now working to take the “F” out of the “SPF” and the “hem” out of the “hemfir”.
 
Density split can be done based on pure lumber weight, but this requires very homogeneous moisture content distribution among lumber pieces - something not always realistic. NIR might also be the option for that.
 
Moisture content is something that a sawmill with inline moisture meters can utilize to implement an MC-based pre-sorting program. For that, you need a little experimentation, namely, to figure out what is the “engineering value” your MC-meter gives you that corresponds to the MC you want to use as your separation point. You can work on that with the meter’s manufacturer, but once you decide of a value, say 60%, and you figure out a ball-park engineering value, you can then program your sorting system to move lumber pieces to respective bins based on that value. This way, you’ll have a pretty good split of your population to high and low MC groups. Adjustment of your kiln schedule to accommodate the drying of the two sorts might be required.

[Craig Jensen]

"you may decrease drying times and improve lumber quality"
In our mill, we used to dry Engleman Spruce, Lodgepole pine, and Alpine Fir together in a product we call ESLPAF.  We also used to dry our Douglas Fir and Larch together.

We found that when we separated the larch and the DF, we were able to optimize the drying schedules and reduce drying time on both species.  We continued to use an elevated temperature schedule when drying DF, while going to a high temperature schedule on the Larch.  Rather than the combined schedules taking 20-24 hours, we were able to reduce both schedules to an average of 14-16 hours.

We then decided to separate the AF from the ESLP, with the same result.  Those schedules used to take 24 - 28 hours and now our average drying time on both is 18 to 20 hours.

My theory (unproven) is that AF and Larch both have much more bound moisture which is more easily removed over the boiling point.  However, our kilns cannot get above the boiling point until a significant amount of the free moisture is removed.  I believe that the Engleman Spruce, Lodgepole, and DF all have more free moisture, so they were holding the AF and Larch back and not allowing drying to really start until later in the schedule.