Does water migrate from one board to another in a package of lumber during kiln drying? We have 5% balsam mixed in with SPF, causing drying difficulties. The outer boards dry to 14% and the inner boards of the package are 17 to 22%. I know the logs should be sorted before entering the mill. Does anyone know of a drying schedule that may be used? We have high temp gas kilns.
(Commercial Kiln Drying Forum)
From contributor N:
Moisture can move from one piece to another. It is the step called equalization. Only lumber below about 22% can pick up moisture. The best way to solve your problem would probably be to add time in an equalization step and that usually is a tough sell for people with high temp kilns.
I have seen people sort out the fir and let it air dry for a while to significantly reduce the kiln time. I've also seen them leave it mixed together, and let the wetter fir boards in the dried load make up the allowable wet percentage of the load. All in all, separation is the best way to go if you can do it in your mill.
It also sounds like there is confusion between outer boards drying faster than boards in the middle of the stack with the idea of some boards giving other boards moisture. I think the problem is that the air flow may be too low, not drying the load long enough or the stacks are too wide.
I also agree that your wet interiors are more likely a result of low air flow. Hopefully you are using 7/8" or 1" stickers and your pile widths on each track are no more than six feet. Note however that the edge pieces have both faces and one edge exposed to the drying air, while the interior pieces have only two faces exposed. Hence the edge pieces always dry more. There is also a problem in final MC distribution when going from the top to the bottom of a stack; again this is an air flow non-uniformity problem. With gas HT kilns, we also have problems with the uniformity of air in the down-risers (the openings where the hot gases enter the kiln atmosphere).
I believe you would find it helpful to have someone (a consultant) familiar with HT gas kilns from the Southern pine region visit you and provide some advice about the kilns.
You might also contact some of the other companies in your area and see what they are doing. I saw one operation at St. Leonard, NS run by JD Irving that was outstanding. I do not know if they allow visitors or competitors to visit.
If you want to understand the process in more detail, I and Julian Beckwith wrote a booklet some years ago about high temperature drying of SYP. The thoughts in that booklet regarding high temperature drying and effects of air flow, incoming MC, sticker thickness, etc. would apply to your case with SPF. Contact the Southeastern Lumber Manufacturers Association in Georgia and see if they still have copies for sale.
To clarify, the above is true for final MCs. Certainly at the beginning of drying, there is an opportunity, if the dry piece is under 30% MC at the surface, for the dry piece to gain some moisture at the surface, but this moisture will quickly be gone as drying continues.