- Stage 1: Initial conditioning.
- We establish the initial temperature, the initial depression (a low number = high humidity), and the duration of stage one.
- Above, the initial temperature is set to 100 degrees F., the initial depression to 6 degrees, and the duration of phase one is one day.
- The controller will maintain the initial values for the specified period of time before advancing to stage two.
- Stage 2: Raise temperature and depression slowly over time.
- During phase two, the depression is increased slowly — the humidity is reduced slowly. Slope 1 (set to 0.7, above) is the desired increase in depression per day. The controller continuously recalculates setpoints, so the depression will be changed very slowly.
- The temperature setpoint is also continuously recalculated, and is a function of the initial and terminal temperature, the depression @ terminal temperature, and the current depression. In the example above, we are telling the controller that we want it to hold a temperature of 100 degrees when the depression is 6 degrees, 135 degrees when the depression is 25 degrees, and ramp the temperature proportionally between those two points.
- Phase two continues until the depression is “stage 3 start’ — 20 degrees in the above example.
- Stage 3: Raise temperature and depression a little more aggressively.
- The controller logic in stage three is identical to that in stage two except that the wood is closer to dry and so it is less likely to be damaged. We can now increase the rate at which we increase the depression — slope 2 is 1.2 degrees per day above.
- Stage three terminates when the depression reaches the terminal value (32 degrees above).
- Stage 4: Final conditioning.
- In stage four the controller will maintain the terminal temperature but will leave the kiln vents closed causing an increase in humidity. The wood’s core is, unavoidably, somewhat wetter than the shell — the rise in humidity in the kiln helps to balance the humidity throughout the wood.
Most hardwood kilns are operated on stepped schedules i.e. run a temperature/depression until your sample is at some given moisture content. Then, increase temperature, decrease humidity, and run until your sample is at another given value. More modern control systems will control the temperature and depression based on sensors in the kiln which continuously detect the moisture content of samples or the weight of samples or the weight of the entire load. The weight decreases as the wood loses moisture.
We mainly dry mixed loads of dense, difficult to dry, and expensive exotics. They are of diverse species, thickness, specific gravities, and in various stages from green to well air-dried. Attempting to run from kiln samples, is unrealistic. Our schedules are established on the basis of our experience, and we err on the side of too slow. An extra few days costs less than damaging an entire kiln charge. The simulator allows us to play with the various values and determine what the schedule will look like and how long it will take to complete.
The kiln is loaded, the decided upon values are keyed into the controller, and we stand back and watch. The controller operates a gas-fired boiler to maintain the temperature, actuates a powered vent to decrease the humidity, and a solenoid valve on a water spray line allows it to increase the humidity. Much of our wood, by the way, gives up water so slowly that we frequently need to add moisture to the kiln. We take occasional samples to determine the moisture content of the wood and can adjust the schedule, if necessary.