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EQUALIZING. To equalize the lumber, set an EMC condition within the kiln that is the same as the driest kiln sample (5% in this example). This will prevent the driest boards from getting any drier, and it will continue to encourage the wetter boards to reach the target MC range. (It’s ok to dry some of the load below the target MC range at this point, because you’ll add a little water back in during conditioning). Continue equalizing the lumber until the wettest sample board reaches the target MC - not the upper limit of the moisture range including the MC tolerance, but the target MC itself. To continue the previous example, if the target MC is 7% with a ±1% MC tolerance (i.e., 6%–8% MC range for all the lumber being dried), continue to equalize the lumber until the wettest sample board reaches 7% (not 8%). You now have a load of lumber which – if the sample boards were chosen well to begin with – contains lumber which is all within the range of 5%–7%. That’s 1% lower than you ultimately want, but it’s perfectly ok at this point. Different parts of your kiln sometimes dry at different rates, and boards in the center of the pack often dry more slowly than the boards around the edges. The air entering the packs becomes less dry (perhaps even saturated) after flowing over several boards, and the drying for the interior boards will be delayed or slowed. This might be of particular concern if you are using a kiln where the air has to flow through several stacks of lumber, and variability in drying across the load will be more noticeable in kilns with low fan speeds. If this description resembles your kiln, consider using a longer equalizing period than would be indicated by the MCs of your sample boards. If you only have kiln samples in convenient retrieval locations you might misjudge the length of equalizing needed, no matter how carefully you selected the kiln samples. If you have a way to monitor the moisture content of boards in the interior of your packs (using electronic sensors, perhaps), that would be a good set of observations to pair with your sample boards. CONDITIONING. I mentioned earlier that when the average moisture content reaches the target value there is tensile stress in the outer layer and compression stress in the core. This residual tension stress in the lumber core after drying is called casehardening.10 Casehardening results in dried wood pinching a sawblade when it’s machined, so it’s very desirable to relieve these stresses before lumber is sold. Conditioning is a method used to relieve these drying stresses by adding a very small amount of moisture back to the lumber under controlled conditions. Conditioning raises the average moisture content slightly, which is why equalization target MCs are set a little lower than the final target moisture content (ex., 5% MC instead of 7% MC). Adding steam to the kiln is a very effective way to reduce casehardening; the heat and water combine to plasticize the wood shell, relieving residual drying stresses. Water mist systems can also be effective, and are particularly popular with companies don’t have a boiler on the premises (like most DH kiln operations). The length of time that lumber is conditioned depends upon the amount of drying stress and the kiln temperature among other things – higher temperatures result in shorter conditioning times. Lower density species condition more quickly than higher-density species as well, and thicker lumber takes more time than thinner lumber. In many respects, air-drying conditions somewhat replicate the cyclic temperature and RH conditions experienced during solar drying, and air-dried lumber typically has less drying stress than green lumber with a similar moisture content that’s been dried in a kiln. Each load of drying lumber may have a different drying history (and the species 10 Casehardening is a term from the metals industry that implies that the wood somehow harder on the surface – and this is not the case – but it’s a historical term that has been used in our industry for well over 100 years. 51PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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