In the timber industry, the most widespread drying technology
is the conventional kiln. This comprises a specialpurpose
room, the drying chamber, fitted with overhead
fans for circulating the air and heating coils for maintaining
the air (and thus the wood) temperature at the set levels.
The moisture in the air is controlled by means of vents in
the kiln’s roof, which governs the amount of moist air that
returns to the fans to be mixed with fresh air and
re-circulated.[2] Humidification of the air is sometimes
required, particularly at the beginning of drying process,
and is performed by water droplet or steam injection
through nozzles.
The lumber is stacked externally in rectangular piles with
rows of boards separated by wooden stickers to provide
space between the boards for the kiln air circulation. Trolleys
circulating on tracks are a common solution to help
stack handling. Kilns may be single-tracked with a 2.4m
wide stack, or twin-tracked, with two stacks side by side
to yield a double-width stack of 4.8m. Figure 1 illustrates
a vertical cross-section through a single-tracked batch kiln.
Usually, the airflow in the duct-like space between the
rows of boards is in the range of 2 to 5 m.s1, althoughhigher
velocities are required for fast-drying species. To attain these
values, the required electrical power of the fans can be calculated
on the basis of 0.15 to 0.20 kWper cubic meter of wood
for hardwoods and 0.30 kWper cubic meter for softwoods.[3]
The set of constraints resulting from the design of conventional
kilns is important in evaluating the ratio between the
lumber volume and the kiln volume. Depending on the kiln
and the board characteristics, the filling factor varies between
0.15 and 0.35.[4] The filling factor works better for thick
boards, as the relative volume of the space provided by the
stickers decreases with the board thickness.