Q: After we are done kiln drying a load of lumber, our kiln operator always leaves the lumber in the kiln for another 12 hours to cool. Is this really necessary? Obviously, if you read between the lines, we need the lumber ASAP or earlier, sometimes.
A: I prefer to wait at least 12 hours before pulling the load. The reason is that at the end of drying, the lumber is steamed briefly to relieve the drying stresses. The typical procedure means that the lumber's surface will be at 180F and 11 percent MC when the drying cycle is completed. When this hot lumber is exposed to the outside air, the conditions at the surface of the lumber would be approximately 180 F and 2 percent RH. (If you know about wet-bulbs, the conditions are a 120-degree wet-bulb depression.) Such dry conditions can potentially damage the lumber, creating some cracks on the surface.
If you are in a hurry, then manually open the kiln vents and keep the fans running. This will cool the load more gently than opening the doors. But the best idea is to wait 12 hours.
Incidentally, if you process "hot" lumber, the high surface moisture means that there will be some drying of the wood in the plant. This drying means some shrinkage (size change and warp) will occur after planing or other machining. Further, gluing is also not the best with such MC gradients. If quality manufacturing is important, then wait another 2-1/2 days after the lumber is unloaded from the kiln before processing. If you do not mind making pieces that will be rejected, then go ahead and process "hot" lumber.
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