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and will close when the humidity is at or below the desired level. This means that excessive venting is avoided no matter what the velocity is. The kiln fans that determine the velocity of air through the lumber pile also are used to force air through the vents when they are open - the higher the kiln velocity, the higher the venting rate. Therefore, changes in velocity will not affect the overall or net amount of air vented; but velocity changes result in changes in the rate of venting. With fast drying woods that releases moisture quickly to the air, high venting rates to keep the air from getting too humid are required. These high rates can be achieved by using large vents or high air velocities (or both). The venting effectiveness is also increased with higher temperatures - hotter air holds more moisture. Heat Effect The fourth effect of velocity is to cause heat transfer from the steam heating fin pipes, coils, or other heating system to the air. The higher the velocity through the coils, the greater the heating rate. This effect would be most important when energy demands for drying are highest. The largest heat demand exists when the lumber is the wettest. As the lumber dries, the heat demand decreases. Therefore, somewhat lower velocities across the coils at lower lumber MCs are generally not of great concern. Poor velocity across the heat coils can be offset by increasing the fin size, increasing the amount of fin pipe, or increasing the steam pressure. Electrical Consumption The fifth effect of velocity is electrical consumption. As velocity increases when fan speeds (RPMs) are increased, electrical consumption increases; likewise, when speeds are lowered, electrical consumption decreases. A decrease in velocity of 20% (all else being kept equal) will reduce energy consumption by approximately 50%! So, as mentioned in the previous sections, there are times (low MCs) when velocities can be decreased (thereby reducing energy consumption) without a decrease in quality or an increase in drying time. It should be noted, however, that if the energy use is reduced by 50% for most of the month, the monthly electrical bill will probably only be reduced by 25%. The reason for this smaller savings is that most electric utilities have a demand charge, based on the maximum usage rate over a 15 minute period for the month. The demand charge won't change if the peak usage remains unchanged, which would happen if fan speed is reduced for only part of the month. The demand charge is often one-half of the bill; hence, the predicted money savings, by reducing fan speed when below 30% MC, is 25% maximum. Other Concerns With Velocity One might argue that it would be better if the flow were turbulent, because turbulent flow results 31PDF Image | Principles and Practices of Drying Lumber
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