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FOLLOWING AND CREATING DRYING SCHEDULES As the lumber dries in the air drying yard or kiln, the moisture content is tracked using the sample boards you prepared. There are a lot of details yet to be discussed, but in general, running a dry kiln is a lot like following a baking recipe where you start the oven at a low temperature, subsequently increase it, wait awhile and then check for doneness. For lumber, you start up the kiln at one temperature and relative humidity (a starting EMC condition), and when certain conditions are reached (a specified lower MC as determined from your wettest sample boards), the EMC is lowered (by raising the temperature and/or lowering the RH). This cycle is repeated several times. Defect prevention and drying rate are intertwined, which is why people have created lumber drying schedules to dry different species successfully. If you kiln dry wood too quickly or too slowly you can create various defects (especially if you’re drying green lumber), so both kiln companies and the USDA Forest Products Laboratory have published schedules to help. Most hardwood dry kiln operators use a moisture content-based schedule to dry their lumber, though time-based schedules are often used in the softwood lumber drying industry. Table 7 shows what a hardwood drying schedule looks like. The following example is a schedule for 4/4 hard maple from Drying Hardwood Lumber (Table 7.8). Table 7. Drying schedule for 4/4 hard maple Moisture Content (%) at Start of Step Dry Bulb (°F) Wet Bulb (°F) Depression (°F) RH (%) EMC (%) Over 40% 40 35 30 25 20 15 130 125 130 123 130 119 140 121 150 115 160 115 160 115 5 86 16 7 81 14.0 11 71 11.5 19 56 8.4 35 35 5.1 45 25 3.7 45 25 3.7 Note that the temperature does not get very high until the end of the kiln run. Higher temperatures cause wet wood to change color and lose brightness, and brightness can be an important factor affecting the value of hard 86PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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