High-moisture lumber with green leaves will dry quickly. There is also a lot of water near the wood’s surface. A significant number of water molecules only have a very short distance (diffuse) to travel to the surface.
The water waits for the air (to evaporate water) to transfer energy to it. Low moisture lumber means that the amount of water on the surface is less. The water molecules in the core must travel farther to reach the surface. The process of water diffusion toward the surface is slow. As an analogy, instead of the water waiting for air as in the case with green lumber; the air is waiting to receive the water from dry lumber.
Drying Rate
Velocity in drying lumber has two main effects: it removes moisture from the lumber’s surfaces and provides energy to the wood to allow for evaporation. Remember that drying requires energy.
It takes approximately 1000 BTUs to evaporate one pound of water. 1.5 million BTUs of air will be required to evaporate 1000 BF of lumber. This lumber typically has around 2500 pounds of water. A cubic foot of air moving through loaded lumber will only provide 0.2 BTUs of heat. You should know that it takes a lot of air to heat lumber. The actual volume of the air can be calculated if needed.
The amount of air needed to remove moisture from lumber can also be calculated. At low temperatures and high humidity, a cubic foot of air can hold less than 0.001 pounds of water and as much as 0.01 pounds at higher relative humidity and higher temperatures. Again, large volumes of air are required to transport 2500 pounds worth of water.
The lumber’s surface is very wet (the average lumber MC is more than 40%). However, a higher velocity will lead to a faster drying rate. Higher velocity means faster heat transfer to lumber, quicker removal of moisture from lumber’s surface, and more cubic feet of air passing through a sticker aperture.
Integral Effect
The integral effect is the second effect of velocity on lumber drying. The integral effect is the increase in air volume through the channels or sticker spaces as velocity increases. As the air flows through the load, if it has more volume, its conditions (that is, temperature and humidity) change slower as it enters the load.
If the volume of the load is low, the air will lose heat quickly and pick up enough moisture. This will cause it to become saturated (100% RH) shortly after entering the lumber load.
Low velocity means that only the pile’s edges will dry well; the rest of the load will remain wet and humid. It is possible to stain and cup. The higher the volume flow rate, which in practice is the greater the velocity and thickness of the sticker, the more uniform drying from the edges to the center to the exit edge will occur. To ensure uniform drying, we often reverse the flow direction.
Take this example
Volume is directly related to sticker thickness and velocity. Take the following example. The following example shows how a kiln can be loaded with 50 MBF 4/4 green lumber, stacked on 3/4 inch thick stickers, and moving at a speed of 300 fpm. The kiln’s capacity would drop from 44 MBF to 12% if the sticker thickness was increased to 1 inch. The sticker openings would be reduced by 12%, but they would have a greater area (3/4 to 1 in). This would result in a 17% increase in sticker space.
Assuming that fans deliver the same volume of air, the sticker openings would have 12% more volume (because there are 12% fewer openings), but the average velocity would decrease from 300 to 250 feet per minute. Thicker stickers at higher MCs will result in slower drying, but uniform drying across the pile because of the increased volume flow.
The slower velocity could be offset by lower relative humidity. Low MCs would have little impact on drying speed and uniformity through the pile. While thicker stickers are more expensive and can result in lower kiln capacities, they can also be a benefit when drying green lumber. However, this could lead to higher operating costs. With only 44 MBF, the heating and venting systems of the kiln will often be more matched to the load, which can lead to better performance.
Venting Effect
Venting is the third effect of velocity in conventional dryers. It occurs from the inside to the outside and in the other direction. This effect is not possible in dehumidifier dryers. Venting is a method used by conventional dryers to remove the moisture inside the kiln. The humid air is expelled and dried air is brought outside.
Properly operating vents will allow them to open when conditions are too humid 31. They will also close when humidity is at the desired level. This means excessive venting should be avoided, regardless of the velocity.
Heat Effect
Fourth, velocity can cause heat transfer from steam heating fin pipes, coils, or other heating systems to air.
The heating rate is dependent on the velocity of the coils. This would be the most significant when drying energy requirements are high. When the lumber is wettest, the greatest heat demand exists. The lumber’s heat demand drops as it dries. Lower lumber MCs and slightly lower velocity across the coils are not usually of concern. You can reduce the velocity of the heat coils by increasing the fin size, increasing the fin pipe length, or increasing steam pressure.
Electric Consumption
Electrical consumption is the fifth effect of velocity. When fan speeds (RPMs), increase, then electrical consumption also increases. Conversely, when fan speeds (RPMs), decrease, electrical consumption falls.
An increase in velocity of 20%, assuming all else is equal, will reduce energy consumption by 50%. As mentioned above, there are times when velocity can be reduced (thereby reducing energy usage by approximately 50%) without affecting quality or drying time.
However, if energy consumption is decreased by 50% over the majority of the month, then the monthly electric bill will likely be reduced by 25%. These smaller savings are due to the fact that electric utilities charge a demand fee based on the maximum usage over a 15-minute period. If peak usage is not changed, the demand charge will not change. This would occur if the fan speed is decreased for a portion of the month. The demand charge is usually one-half the bill. Therefore, the maximum predicted savings by reducing fan speed below 30% MC is 25%.
Other Concerns with Velocity
You might argue that turbulent flow would be more beneficial as it results in faster drying. It is true that lumber with MC greater than 30% will dry more quickly with turbulent flow than with laminar flow. Is faster drying desirable for all woods? For most hardwoods, faster drying means more damage – honeycomb, checks, splits, and splits. You can also lower humidity to speed up drying. This is easier than creating turbulence. You can also increase the velocity to speed up drying. For hardwood lumber drying above 30%MC, turbulence is not desirable. Below 30% MC, velocity plays a minimal or insignificant role in controlling drying rate. Therefore, any effort to create turbulently is futile – there are no benefits in terms of quality or drying speed. The velocity required to dry softwood dimension lumber in high-temperature ovens is over 1000 fpm. This causes turbulent airflow within the pile.
Diagnostic Tools
Three tools are available to measure the velocity effect and determine if velocity can be increased/ decreased.
First, we measure the moisture content variation in lumber loads. If there are more than 1% of MC variations from the edges to the middle, it is worth considering higher speeds to ensure uniform drying. Reduced velocity is possible when variations are less than 1%.
The temperatures and humidity at each load side (entering air side and exiting air side) must be measured. Next, calculate the EMC for each side. EMC differences of less than 2% can be reduced, but increases are possible if they are greater. To ensure safe drying, it is important to monitor the lumber’s drying rate at MCs higher than 30% in either of these cases.
Conclusion
High moisture content lumber generally means that the drying rate for high-moisture lumber increases with increasing air velocity. This results in a more uniform drying process and less warp and stain . Higher air velocity means greater chances of honeycomb, checking, splitting, and checking. But we in Shuowei create a solution in every problem to get the perfect dried wood to increase the revenue of your business. Just reach out to us for more details.
