The key to effective lumber drying is to remove the water from the surface of the wood only as quickly as it will flow from the inside of the wood. Water can only be evaporated and transported by the air from the surface of the wood. If the water is removed from this surface faster than it is able to flow from the interior of the wood several problems occur.
A. The flow of water to the surface is broken.
B. The surface fibers shrink because they are being dried making it difficult for any water behind the surface layer to reach the surface. It could be viewed as `closing a tap’. This is called case hardening.
C. Depending on the depth of case hardening, the shrinking of the surface fibers causes stress on the wood resulting in warp, twist, checking & shake.
D. A non-linear moisture gradient with dry wood at the surface and climbing to wet wood (>22%) very quickly near the surface.
E. Drying times become very long because of A & B.
F. Quality decreases because of C & D.
Our process is different because we pay very close attention to the heat up of the wood. Based on our research, case hardening can already occur at temperatures as low as 100 F.
The operator sets a basic heat up rate, after which our process controller adjusts the heat up rate to achieve 2 basic principles.
A. A correct temperature gradient within the wood. Water will flow from the relatively cool interior towards the warmer surface. If you create too large of a gradient, the center of the wood does not get warm enough to provide a good flow. Often the small flow of water created will break. Too small of a gradient and there is no incentive for the water to flow.
B. Optimum ambient humidity levels. We monitor the humidity as an indication of a proper heat up. We also operate with a much different humidity profile than typical schedule controllers. We use higher humidity levels during the drying cycle to provide for better energy transport to the wood, and minimize degrade.
As with any controller, the actuators we operate are the vents and the amount of heat. The heat control must be proportional, and the vent control can be either proportional or open/close. The circulation fan direction is controlled. With gas fired kilns the heat fan, combustion air blower, and power to the Gas safety line is sequenced. We decide on how to adjust these by continuously monitoring the temperatures in the kiln. A typical kiln will have 12 dry bulbs & 1 or 2 wet bulbs. The dry bulbs are distributed along each side of the kiln (4 bulbs/side) and also down the center of a 2-track kiln. We monitor the temperatures in the kiln, humidity level, amount of heat being used, rate of temperature climb, temperature drop, and outside ambient temperature, and then continuously adjust the desired set point temperature. The humidity level is controlled by monitoring the wet bulbs, and opening / closing the vents. The fan direction, for the majority of species, is only changed twice during the drying cycle. Once we establish a flow of water we do not want to break that flow by changing fan direction. Typically we will perform the preheat in the forward direction, then complete the remaining heat up in the reverse direction, until the free water is removed, and then back to forward to complete the drying cycle. The 1st reversal point is fixed at 110°F, and the 2nd reversal point is adjustable to achieve even track-track moisture content. The operator can adjust up to 59 different parameters to control the drying process. Typically each species & size of wood has its own process setup. There are various features of the control process that can be turned on / off to provide for difficult to dry species such as Cedar, Balsam, and Hemlock, provide MSR mode & sealing control. Additionally there are another 2 sets of parameters to match our control to your specific kiln type, heat source(s), and actuator types. One unique feature of our control is being able to determine the moisture content of the wood being dried. If we achieve optimum heat up rate, then the initial MC can be estimated based on the amount of heat required to raise the kiln temperature from 110°F to 125°F. Removing more water will slow down the heat-up and depending on the BTU of the burner will use max heat. During the drying cycle, the average kiln temperature, temperature drop, humidity level, & amount of heat required are integrated thru a formula to calculate the MC of the lumber. The process can then adjust the 2nd fan reversal & kiln shutdown based on this calculation. A safety time limit is used as a backup. The formulas are adjusted with parameters to make up for different heat losses, heat capacity, kiln airflow, of your specific kiln and also the reaction of different species. Once setup, the different initial moisture contents from charge to charge can be compensated for resulting in the optimum drying time & shutdown point for each charge.
Example: 2 pieces of wood – 1 green, 1 semi-dry with half as much water content as the green piece. Same kiln temperature applied to both pieces.
1. Water will flow from a cool environment to a warmer environment.
2. Only place we can apply the heat is to the surface of the wood.
3. The heat capacity of water is much higher than the wood fiber meaning water will absorb much more energy than wood fiber.
Each piece will end up with a different temperature gradient, resulting in a different rate of water flow. The green piece will absorb more energy because of its higher water content therefore the heat will take longer to penetrate to the center, resulting in a larger temperature gradient from the center to the surface of the wood. As the water starts to flow in the green piece the larger flow of water limits the amount of energy penetration because cooler water from the center is replacing the warmed water leaving. The semi-dry piece, in comparison has less water and therefore the heat is able to penetrate further into the center of the wood than the green piece. The semi-dry piece will have a smaller temperature gradient. Water wants to flow towards the surface where the heat source is being applied. The greater the difference in temperature the more incentive there is for the water too flow limited by the cell / fiber structure of the species being dried. If the moisture is not evaporated off the surface faster than it can flow to the surface, the green piece of wood will have a greater flow of water resulting in a faster rate of drying than the drier piece. The drier piece with its smaller temperature gradient has created an environment with less incentive for water flow hence its rate of drying will be slower. If we heat the wood too slowly it is possible to obtain lower MC in the green wood than the dry wood, because the drier wood has an interior temperature almost the same as its surface and there is no incentive for the water to flow. The resulting high humidity levels from the green wood create an atmosphere whereby the maximum equilibrium MC based on the RH % prevents over drying of the semi-dry wood. This process works well with woods of the same species. Mixing different species of wood with different initial MC is much more difficult to dry evenly because of the large difference in water volumes being removed. Species have different flow rates based on their fiber & cell composition.
Drying the wood in a higher humidity atmosphere, avoiding case hardening and establishing a good water flow we will typically only require 1500 BTU / lb of water removed. Factors such as heat distribution, kiln condition, and outside temperature will vary the amount of BTU required, but this compares with 2000 BTU / lb of water using the schedule method of poor heat-up control, changing fan directions often and venting during the last half of the cycle in an effort to lower the wood’s MC. Our customer’s largest gain is in the consistent quality of wood and the time saved, allowing more throughputs with their kilns, and less fuel consumed per charge. If you have any additional questions please don’t hesitate to call Frank Controls (1986) Ltd.