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and perhaps the thicknesses may be different as well), so each load should be checked for drying stresses and treated for casehardening as needed; there’s no “magic recipe” that can be automatically repeated at the end of drying. CONDITIONING IN CONVENTIONAL (STEAM) KILNS. To condition hardwood lumber when steam is available, a conventional procedure is to set the dry bulb to a temperature about 10°F higher than the equalizing temperature. Before raising the dry bulb temperature, though, set the wet bulb to a value that would give you an EMC of 4% higher than the target MC – steam contains a lot of heat and the dry bulb temperature in the kiln will increase from the steam alone. (For softwood lumber, set an EMC of 3% higher than the target MC.) To minimize the chances of exceeding the set point, turn off the heating coils or temporarily set a low dry bulb temperature to achieve the same result. Maintaining a set temperature might be difficult even with venting, however. Besides venting, another way to address the problem of steam overheating the kiln is to purposely cool the kiln before starting to condition the load. When the kiln is turned back on, the operator still has to ensure that the target EMC has been reached, but the problem of steam overheating the kiln past the DB setting is lessened. For the 7% target MC example, you’d set the wet bulb to give an EMC of 11%, and after the wet bulb temperature was reached you would then increase the dry bulb temperature. The most important point isn’t that you have to be able to set the prescribed dry bulb and wet bulb temperatures – the most important point is that you have to be able to actually reach the correct EMC. CONDITIONING IN OLDER STEAM KILNS. For various reasons, some older kilns may not be capable of reaching the desired conditioning set points. One common problem is leaks around the doors, vents, between panels and so forth. Fixing leaks will help you to achieve your set conditions throughout the kiln cycle, not just during conditioning. Leaks can sometimes be detected by steam plumes in extreme cases, but another way to discover leaks is to examine the kiln with an IR camera. If you can’t borrow one from your local university or town engineer, you can buy one online starting at a couple of hundred dollars (as of 2017). If you have to live with a leaky kiln in the meantime, try for the highest dry bulb/wet bulb temperature combination that is both appropriate for the desired EMC and attainable. CONDITIONING IN DEHUMIDIFICATION KILNS. DH kilns typically don’t have boilers or access to steam, so traditional conditioning methods aren’t available–which is why water mist systems are used. Steam conditioning proceeds more quickly, but fine water mists also work well and are relatively inexpensive to install and operate. Large DH kilns might use pressurized water at about 1000 psi (±200 psi), and they will spray somewhere between five pounds and ten pounds of water per hour for each 1000 board feet of lumber. (The water needs to be filtered to help prevent the fine nozzles from clogging.) Smaller kilns with less urgent time constraints might use lower pressures (around 100–125 psi) with slightly larger nozzle sizes.11 The heat in the kilns will evaporate the water mist and increase the RH, thereby helping to relieve casehardening. Mist systems appear to work well, but some people prefer steam because water mists don’t release heat to the lumber like steam does. Of course, steam might also be preferred because dry kilns have used steam for decades and mist systems are relatively new to North America. People tend to prefer what they’re familiar with. 11 Phone conversation with Don Lewis, Nyle Systems, January 2017. 52PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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