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The Role of Velocity in Drying Drying Rate The first effect of velocity in drying lumber is to remove moisture from the lumber's surface, while at the same time providing energy to the wood so that evaporation can take place. It should be remembered that drying always requires energy - about 1000 BTUs of energy to evaporate a pound of water. Therefore, 1000 BF of lumber, which typically contains about 2500 pounds of water that needs to be evaporated, will require 2.5 million BTUs of energy from the air. If a cubic foot of air blowing through a load of lumber changes temperature by 10°F, the air will have supplied only 0.2 BTUs of heat to the wood. It should be clear that a lot of cubic feet of air are required to carry the heat to the lumber - the actual volume can be calculated if necessary. Likewise, the amount of air required to carry the moisture away from the lumber can be calculated. A cubic foot of air can carry less than 0.001 pounds of water at low temperatures and high humidities, and up to 0.01 pounds at low relative humidities and higher temperatures. To carry 2500 pounds of water will require, again, large air volumes. While the lumber's surface is quite wet (the average MC of the lumber is over 40%), an increase in velocity will result in an increase in the rate of drying. Higher velocities mean faster heat transfer to the lumber, faster removal of moisture from the lumber's surface, and more cubic feet of air passing through a sticker opening. However, as the outer shell of the lumber becomes drier (average MC of the lumber is between 40% to 20%), then the effect of increasing velocity begins to diminish. Finally, as the lumber surface becomes quite dry (average MC of the lumber is under 20%), changes in the velocity of the air have almost no effect on the drying rate. Let's look at this more closely. With wet wood, the molecules are at or close to the surface of the lumber, so that they don't wiggle very far to be evaporated into the air. In essence, they are waiting for energy from the air and/or for the air to scrub them off the surface. What changes as the wood dries to lower MCs, is that the major resistance to drying switches from the resistance that the air provides to the resistance within the wood itself. In other words, as the MC drops, we begin waiting for the water molecules to wiggle their way out of the inside of the wood up to the surface, rather than for the air to scrub the molecules off the surface or for the air to provide energy to the wood. This effect can be shown graphically (Figure 2), where at 60% MC, it can be seen that the drying rate increases with increasing velocity, while at 20% MC the rate is constant for any velocity. Questions may arise concerning the roughness of the lumber. Surprisingly, variations in lumber roughness do not result in major variations in drying. The air flow through a load of lumber begins as laminar flow, which is several times less effective in transferring heat and removing moisture than turbulent flow. Air flow becomes turbulent after about 4 feet of travel into the pile, the distance depending on the velocity and whether the individual pieces of lumber in the 29PDF Image | Principles and Practices of Drying Lumber
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