From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
This step, which occurs right at the end of drying, will achieve uniform moisture contents (MCs) both within a single piece (end to end and shell to core) and from piece to piece.
You (and/or your boss) determine how uniform you want the MCs to be; that is, you determine the goal. Equalization requires time, so it is expensive. (If the incoming MCs vary a lot, then equalization will be longer; save time and money by loading the kiln with uniform lumber, including uniform initial MCs.)
You can only achieve this goal all the time if you have correct kiln samples (the wettest and the driest perfectly for the load). This is where many folks make an error, as they do not know the driest MCs. As a result, equalization is not perfect and cupping increases, and machining quality deteriorates.
Finally, you must follow the correct equalizing procedures and achieve the actual desired kiln conditions.
1. Start when the driest piece of wood is 2% MC below the target MC. For softwoods, and lower grade hardwoods, 3% MC below is sometimes used.
2. Use the temperature in the kiln schedule, or if at the end of the schedule, use 10 degrees F hotter than the final dry-bulb temperature. For a steam spray system, initially use the heat in the steam to get this extra 10 degrees F rather than the heat in the pipes.
3. Set the EMC in the kiln at 2% below (or 3% for softwoods) the target % MC.
4. Hold this EMC condition until the wettest sample reaches the target MC. (If not at the end of the schedule, raise the DB according to the schedule but maintain a constant EMC.)
At this point, if you have done equalizing correct, there will be no pieces under (target - 2%) and no pieces higher than the target MC. This is a standard deviation of 0.3, which may be better than needed.
This step, which occurs right at the end of drying, will achieve lumber that is free of drying stresses. Drying stresses that are left in the lumber will cause immediate warp when machining. Lumber with stress is sometimes called casehardened, but nothing is harder than anything else.
There are two types of drying stress... across the grain or transverse (causes cupping when machining mainly), and lengthwise or longitudinal (causes lengthwise warp when machining mainly).
Drying stresses occurred when the outside fibers tried to shrink early in drying, but were unable to shrink as much as they wanted because the wet core was unmovable. So, the outside fibers dried to a larger shape than they desired. This occurs at very high MCs... 50% MC plus. To remove drying stresses, we will apply moisture to the surface so that now the surface tries to swell, but the dry core is unmovable... the reverse condition of what caused the stress.
Moisture addition must be rapid. In fact, the more rapid, the faster the stress is relieved. Higher temperatures (170 or 180 degrees F) make the process go faster.
In air drying, the high RH in the early morning does relieve some stress naturally, so air-dried lumber has little stress. In predrying or kiln-drying-green, there will be a lot more stress.
Transverse stress is measured with the U-shaped prongs. Longitudinal stress is measured by ripping an 8” x 24” piece into two 4” pieces.
Conditioning time varies with MC... drier is faster. So, for uniform stress relief, the lumber needs to be well equalized prior to conditioning.
Set the kiln EMC to target MC + 4%. Continue this EMC until finished. For steam spray systems, the heat in the steam will often increase the dry-bulb. So, condition at 180 degrees F, but use this heat in the steam to reach this temperature rather than heat in the pipes.
Consider cooling for a few hours (open doors, vents open, fans on, all else off) prior to conditioning in order to cool the surface and thereby get faster moisture gain at the surface.