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layer are the same size and therefore are smooth from piece to piece, or whether there are gaps between each piece. Trying to encourage turbulent flow could result in faster drying at high MCs, but lowering humidities and increasing velocities can do the same thing. Integral Effect The second effect of velocity on lumber drying is termed the integral effect. As velocity increases, there is an increase of volume flow of air through the sticker spaces or channels. This means that as the air moves through the load, if there is more volume, the conditions of the air (that is, its temperature and humidity) will change more slowly as it enters the load and passes through the lumber pile. When volume is very low, the air will give up its heat and will pick up enough moisture rather quickly; this means in turn that it will become saturated (100% RH) soon after it enters the load of lumber. With low velocity, then, only the edge of the pile will dry very much; the rest of the load will be subjected to very humid air and just won't dry much. (Staining and cupping are likely too.) So, the higher the volume flow rate (which in practice means the higher the velocity and the thicker the sticker), the more uniform drying will be from the edge of the pile to the center and then to the exit edge. Of course, we often reverse the direction of flow to also provide more uniform drying from side to side. In the previous paragraphs, the role of volume has been mentioned. Volume is related both to velocity and sticker thickness. Consider the following example. A kiln is loaded with 50 MBF of 4/4 green lumber stacked on 3/4-inch thick stickers and with a velocity of 300 fpm through the load. If the sticker size is increased to 1-inch thickness, the kiln capacity would be decreased 12% to 44 MBF. There would also be 12% fewer sticker openings, but the area of sticker opening would increase by 33% (3/4 to 1 inch). The net effect is an increase in sticker spaces of 17%. Now if we assume that the volume of air delivered by the fans would remain constant, there would be 12% more volume per sticker opening (because there are 12% fewer openings), but the average velocity in each opening would drop from 300 fpm to 250 fpm. The net result of thicker stickers at high MCs would be slower drying due to the velocity drop, but more uniform drying throughout the pile due to the increase in volume flow. A little lower relative humidity could offset the slower velocity. On the other hand, at low MCs, there would be little effect on drying rate and uniformity of drying through the pile. So, thicker stickers can be advantageous when drying green lumber, although the price is decreased kiln capacity and therefore potentially higher operating costs. Note that with only 44 MBF, often the kiln's heating and venting systems will be better matched to the load as well, giving better performance. Venting Effect The third effect of velocity in conventional dryers is to cause venting from the inside of the kiln to the outside and from the outside inward. (Dehumidifier dryers would not have vents, so this effect would not be present.) Venting is the method that conventional dryers remove moisture from inside the kiln - humid air is exhausted and drier outside air is brought into the dryer. If the vents are properly operating, they will open whenever the conditions in the kiln are too humid 30PDF Image | Principles and Practices of Drying Lumber
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