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in faster drying. Indeed, it is true that lumber at MCs greater than approximately 30% will dry faster with turbulent flow rather than laminar flow. But is faster drying a desirable and economically beneficial event for all woods? Certainly for most hardwoods, faster drying will result in more degrade - checks, splits, and honeycomb. Further, to achieve faster drying, the humidity can be lowered without much effort, which can be easier than creating turbulence. Further faster drying can be achieved by increasing velocity. So turbulence is not a desirable objective for hardwood lumber drying above 30% MC. Below 30% MC, the velocity has only a minor or insignificant role in controlling the drying rate, so certainly any effort to create turbulence is not worthwhile - that is, there is no benefit in quality and or drying rate. For drying softwood dimension lumber in today's high temperature kilns, the velocity is so high (over 1000 fpm) that air flow is turbulent soon after the air enters the pile. With today's standard kiln schedules for hardwood lumber based on laminar flow at 375 fpm, any change in air flow would have to be accompanied by a change in schedule (primarily humidity). For example, in an experimental kiln for hardwoods with 1000 fpm, the drying rate was so high that the wet-bulb had to be raised so that the relative humidity was over 95%, thereby slowing the drying rate to acceptable levels. Even then, surface checking in oak was excessive. So what air flow is best? The answer is that for a given relative humidity and temperature, the best air flow is that velocity that results in drying the wood at a rate close to (but not exceeding) its safe drying rate. For hardwoods, the schedules used throughout the U.S. are based on experiences with drying at 375 fpm. Therefore, this should be the general target for most hardwood lumber kiln operations today. For drying green hardwoods in a warehouse predryer, most work has been done at 125 fpm (or thereabouts). For most softwoods and easy drying hardwoods, velocities are often over 1000 fpm to assure uniform drying, as much as to produce fast drying. In any case, once the wood is under 30% MC, then the best flow rate is that velocity that provides good control of kiln conditions with a drop in EMC across the load of less than 2%. Diagnostic Tools There are three tools available for measuring the effect of velocity and assessing whether velocity can be increased or decreased. First is measuring the final moisture content variation across the load of lumber. When variations exceed 1% MC from the edges to the middle, higher velocities to achieve more uniform drying, especially at MCs greater than 30%, should be considered. When variations are less than 1%, then reductions in velocity can be considered. Second, the temperatures and humidity on each side of the load (entering air side and exit air side) should be measured and then the EMC calculated for each side. When EMC difference are less than 2%, reductions in velocity can be considered; when greater, then increases may be justified. In either of these first two cases, the drying rate of the lumber at MCs greater than 30% must also be monitored to assure safe drying rates. Finally, the current draw for each fan motor can be measured. Lower than normal current can indicate a lack of velocity which can be caused by a variety of reasons, including loose belts, 32PDF Image | Principles and Practices of Drying Lumber
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