Items
of Interest
Ecology
in Belize | Belize
Rain Forests | Kiln Drying Methods
Below is a
view of our Windows program which allows us to visualize the effect of
modifying the various parameters which control the dry kiln.
- 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 -- 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,
etc. 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 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 blowing up wood.
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.
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©2006 Hibdon Hardwood, Inc.