专业外语讲稿第3课Drying

3Drying
Wood products should be dried to the correct moisture content for best results.
This is closely related to the utilization of a correct frying procedure that is carried out under proper conditions.
Water is found free in cellular cavities of the tree as well as in a diffused state in cell walls.
During drying, the free water is lost first.
The fibers stay saturated and swollen until a humidity level slightly below of approximately 30% is reached.
Beyond this point, drying caused a step-wise shrinkage.
As wood dries. it changes shape based on its cross section.
Drying Methods
Drying may be accomplished either in the open air or in a drying kiln.
However, natural drying (drying in air ) requires long time periods and difficult conditions.
V alues of humidity below 20% cannot be attained by this procedure.
To reach humidity levels below 20%, kiln-drying is particularly required.
There are various methods of kiln drying, but the basic priciple is the same in all of them, requiring that the extent of drying be controlled so that a proper level of moisture is maitained.
This is a condition that must be met in natural drying, as well as in artificial procedures.
An additional important advantage of kiln drying is that it causes the expulsion of the resins in the wood.
This reduces the problem of resin staining in the painting process.
There is no method for instantaneous accomplishment of drying.
A certain time span is required for proper drying which, generally , is in the order of weeks.
(1)Air Drying
In air drying, lumber is stacked into packages generally around 1.25 m wide by 1.25 m high by variable length.
These units are then placed as many as four high in the air yard.
Each layer of lumber within the package is seperated by stickers ranging from 12 to 40 mm in thickness, placed every 0.6 ~ 1.25 m perpendicular to the long length of the lumber,
The use of stickers permits air to pass through the unit.
The thinner the sticker is the less the air flow, and the slower the drying rate.
(2)Kiln Drying
With kiln drying, lumber is placed in a dry kiln.
A modern-day dry kiln is nothing more than a large chamber in which the air movement, temperature and relative humidity are controlled.
This control permits the drying rate to be both regulated and accelerated.
The lumber can be dried to any desired final moisture content.
The physical layout and primary components of a steam-heated dry kiln are shown in Figure 3-1.
Commercial kilns in capacity from 24 m3 to as large as 350 m3.
A dry kiln has five primary components:
(1) Housing----this is usually a prefabricated outer shell made of aluminium nowadays, although concrete structures are still common; brick or wooden design are less common.
(2) Heating----the lumber in a kiln is dried in air that has been heated by steam-heat-exchanger coils or by the addition of combustion gases from a wood residue, natural gas or oil burner.
Conventional kilns use air temperatures from ambient up to 95 ℃, high-temperature kilns, used when more rapid drying can be tolerated, operate at temperatures above the boiling point of water up to about 115 ℃.
(3)Humidity----this is controlled through two different mechanisms.
When the humidity is too high, vents located on the roof open, enabling the hot moist kiln air to be exhausted.
At the same time cool dry air is sucked into the kiln, thereby reducing the relative humidity.
If the humidity is too low, steam is sprayed into the kiln to raise it to the desired level.
(4)Air circulation----the air or conbustion gases present in the kiln serve
several purposes.
The primary purpose is to deliver heat to the lumber to evaporate the water in it and to carry away the evaporated moisture.
Using constant static pressure in the side plenums, the gases assist in maintaining the uniform temperature and relative humidity conditions needed for quality control.
Air velocities typically range from 1 to 3 m/s.
(5)Sensing-control----the coditions within the kiln are monitored and
recorded using dry-and wet-bulb, liquid or vapor filled thermometers connected normally to a recorder-controller, which is set manually.
Multiple dty-bulb thermometers are usually employed whereas a single wet-bulb sensor is standard practice.
Drying in conventional kilns is accomplished using a series of temperature and relative humidity conditions designed to be mildest when the lumber is the wettest.
The series of combinations of desired air temperature and relative humidity is known as the kiln schedule.
As the moisture content changes the schedule is made progressively more
severe.
Once all of the free water has been evaporated the lumber becomes less defect-prone and kiln conditions are further harshened to increase the rate of drying.
Softwood lumber is usually dried using a time schedule and the conditions are changed at predetermined times, usually every 12 to 24 hours.
Hardwood and thicker softwood lumber are dried using moisture-content schedules.
These schedules are based on an actual measurement of the moisture content of boards within the charge being dried.
Both types of schedule have been derived empirically by trial and error over a number of years.
The Care of Dried Wood
Wood dried to correct level of moisture required for proper usage should
be stored under conditions which minimize the change of its moisture content.
Wood stored in an enclosed, dry setting can be kept with minimun change of moisture content for one or two weeks.
However, the extent to which water is absobed varies with the species.
Low-humidity wood used on floors, carpentry, furniture or similar kind of word shoukd be put in practice after the building has itself dried completely.。