Mixed-Species Loads and Moisture Equalization

Kiln-drying Spruce and Fir together creates difficulty, because they dry at radically different rates and because there is moisture transfer between boards. June 13, 2014

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.

Forum Responses
(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.

From contributor B:
Drying baslam fir with spruce is an age old problem, and the only options I know of that make sense are:
a) separate the species prior to drying, and both will dry better, or
b) dry it all according to the fir MC, which will take a lot longer and probably do horrible things to your production (because the spruce can be dried so much faster, you'll be waiting for the fir to dry when you could've already dried the spruce 2 or 3 times).

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.

From contributor J:
From your description it sounds like you are mixing dry balsam (5%) with green SPF. Is this true?

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.

From Professor Gene Wengert, forum technical advisor:
I agree that this is an age-old problem and there is no easy solution. Varying the kiln schedule cannot fix the problem. I have seen some very effective green moisture meters that can separate the two groups of woods and make drying so much better.

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.

From the original questioner:
Thanks. I forgot to ask what the average chimney width is used when loading a double track kiln. I was told years ago 4 to 6 inches. Now most kiln experts advise a wider chimney, since air flow could be an issue with the wets inside the package after drying 2x8, 2x10 loads.

From Professor Gene Wengert, forum technical advisor:
In a kiln with forced air flow, there is no chimney required. Chimneys were used only with natural draft kilns and for 8' wide packs in air drying. However, with adjacent packages, edge to edge, 4 inches is a good spacing to allow the air flow to adjust slightly when going from one pack to the next.

From the original questioner:
If you had a 2x8 balsam or alpine fir board and on either side of it were lodge pole pine boards, could the wetter board make the surrounding boards wetter in the load? We are debating whether wetter boards are responsible for the lodge pole pine remaining wet after drying.

From Professor Gene Wengert, forum technical advisor:
That would not happen to any significant practical amount at all.

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.

From contributor T:
It is possible to dry 33%, 40%, 70%, etc. balsam down to equal moisture at around 15% even while drying in the same kiln.

From contributor K:
Yes, it is possible, but maybe trying to dry that broad of a MC spread is not the best solution. If you do try to dry lumber at these MCs, you will need to run an equalization step towards the end of drying. Search the archives for information regarding equalization.

From contributor R:
We are drying lumber parts inside dehumidifier between 7% to 8% MC prior to assembly. Our maintenance manager insists these parts be sprayed with water to improve the dehumidifying process. Is this a fact or a fallacy?

From Professor Gene Wengert, forum technical advisor:

Not true and can indeed hurt the lumber. (I am assuming that the lumber is sprayed prior to drying. Is this what you mean?)