From Carl Hagstrom, Systems Administrator at WOODWEB
Gene - over the years you've provided extensive information regarding air drying. There is one area I'm not entirely clear on, and I'm hoping you might be able to clear things up for me. You've mentioned that the rate of (air) drying depends, in part, on the temperature, with faster drying at higher ambient air temperatures, and slower drying at lower air temperatures. I'm trying to get a sense of a relative comparison between drying rates at, say, average temperatures in the teens (dead of winter) Vs. drying rates when average temps are in the 70-80's (peak summer). I can certainly understand that other factors are at play here - relative humidity and species, but if we considered all other factors to be the same, is it possible to quantify the comparative rate of drying in those two examples? In other words, would the drying rate be twice as fast in the summer then in the winter? Half again as fast?
On another somewhat related question, in a situation where lumber is sawn and will air dry for an extended period (six to ten months and no kiln drying), would one expect the drying loss to be less if the lumber were sawn in the fall and allowed to air dry at a slower rate through the winter, rather than sawn in late spring, and allowed to dry more quickly through the summer?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
The rate of drying does indeed relate to the quality of drying. So, hot summer conditions can result in more loss due to checking than in the fall, winter and spring. On the other hand, it is not unusual to find days with low Rh’s and high wind in the spring, so there can be quality loss then too. In fact, I have seen beech lumber check in the winter. So, the answer to your second question is not a clear cut answer, but temperature is certainly a key consideration. Incidentally, rewetting partially dry lumber can make existing small checks worsen, so again, it is not 100% temperature related.
As a rough rule of thumb, for every 20 degrees F hotter, the wood will dry twice as fast. So, 50 degrees is 1/4 the rate of 90 degrees. Because the RH in the summer and the winter is the same (within a few points), the season RH change is not going to affect drying. However, in addition to the average RH, we also want to know the extremes. We often get some very dry days in the springtime, even though the average is the same as the summer and winter values. It is during the extremes that we can damage the lumber and then the average conditions the following weeks or months will not repair the damage and could even make it worse. Once checked, it is easy to make checks worse with "normal" conditions.
If all the free water is evaporated, but the bound water is present, the wood will be at 28% - 30% MC about. Some tropical wood will be as low as 22% MC. From a technical point, when a fiber has lost its free water but none of its bound water, it is said to be at the fiber saturation point. However, a fiber will seldom be at this exact point, as it will be losing both free and bound water, and not only free water as it dries. The same is true for a piece of lumber. Due to moisture gradients in the wood the outside fibers begin to lose moisture first, and they will eventually experience bound water loss even though the core has lots of free water. So a piece of lumber is never at the fiber saturation point. Individual cells may be at the fsp, but not a piece of lumber. So, anyone who talks about drying lumber reaching the fsp when the lumber is at 28% MC is incorrect. Incidentally, when bound water leaves a fiber, or cell, the cell wall shrinks.
Does this pattern hold even at temps below freezing? Is the drying rate at 20F roughly half the rate at 40F?
