Wood in the form of finished lumber, plywood products and particle boards is still the basic consumable for many industries and construction. Its environmental cleanliness and flexibility in processing allow you to compete even with high-tech fiberglass. At the same time, there are also disadvantages of the material due to low strength and sensitivity to aggressive environments. Minimize them allow special methods of pre-processing. In particular, chamber drying of wood with heat exposure is one of the most common ways to increase its technical and physical properties.
Applied Equipment
There are different types of drying units, the general principle of which provides for the presence of coolants. From them, hot flows are distributed over the surface of the target material. The basic classification distinguishes dielectric and convective drying chambers. Dielectric models generate thermal energy during the conversion of high-frequency currents.
In turn, convective units pump heat fluxes to the lumber through preheated air, which helps to remove moisture. The drying agent in this working scheme is heated together with water vapor. After which the mixture circulates in the heat exchanger. A convection chamber for drying wood can use fire tube, steam-water, and induction carriers as functional elements (heat conductors). Sometimes gas chambers that emit large amounts of thermal energy are also used. Both dielectric and convective models of dryers can work in vacuum, that is, at low pressure.
Material preparation
The quality of drying largely depends on how correctly the stacks were prepared - the so-called packages with lumber, which are specially formed for laying in the chamber. For example, the unevenness of drying and deformation of individual sections on the boards will be the result of improper packaging.
Usually two methods of this kind of preparation are used:
- single stack
- batch bookmarking.
In the first case, one stack includes several boards folded in the same configuration. Batch bookmarking assumes that the lumber will be stacked with several ready-made stacks. A rigid platform with a flat surface is used as the basis for the stack. It can be a rail car formed by separate tracks.
To chamber drying of wood was carried out without leakage of a thermal agent, underloading of the stack is not allowed. It should be completely filled with either boards of the same length, or lumber of different sizes apart. Between the joined boards provides for the placement of one or more layers of the lining.
Modes
There are three categories of drying:
- The first category is a high-quality mode, carried out at a temperature of the order of + 60 + 70 ° . This mode allows you to dry the material to a moisture coefficient of 6-8%. The products obtained in this way of processing are distinguished by high technical and operational indicators, which make it possible to use the material in instrument and mechanical engineering, in power building structures, in the production of elements of engineering equipment, etc.
- The second category provides a mode with a moisture level of 8-10%. For such purposes, use drying chambers operating at temperatures up to + 75 ° C. The target area of use of materials processed in this mode is furniture production, as well as the manufacture of carpentry and building structures.
- The third category of drying allows you to get lumber of medium quality, the moisture content of which can reach 15%. Such wood is subsequently used for packaging, moldings and inexpensive building materials.
Wood heating
Primary heating is required to prevent stress processes of the internal structure of the lumber during the main drying. An intense, but short-term thermal effect is produced, during which the moisture does not evaporate. In parallel with the heat treatment, saturated steam is sent to the chamber. This function is performed by a group of fans with air heaters. The process of evaporation of moisture throughout the thickness of the material will begin only after the cessation of heat exposure.
Again, so that the chamber drying of wood does not damage its structure, already at the first stage of heating the balance between the temperature regime and the rate of humidity drop is regulated. If this balance is violated, there is a risk of cracking. The duration of this procedure is determined by the external conditions and characteristics of the wood - an average of 1 to 2 hours.
Main stage
After the initial warm-up, drying follows directly. But the transition should be gradual, which is reflected in the slow opening of the air exchange dampers that remove moisture. At the same interval, the optimal parameters of heat fluxes are established. The temperature regime, in turn, is controlled by the level of opening of the camera blinds. If it is required to increase the saturation of the agent, the supply pipes are completely closed.
Also, the degree of saturation of the thermal effect can be adjusted by starting the steam generator. However, with an open exhaust hood, steam injection systems cannot be connected. During the operation, chamber drying of wood is controlled by the characteristics of the temperature and its error. In factories, information about these quantities is usually logged every hour.
Moisture and heat treatment
This is also a kind of intermediate stage for which special environmental conditions are created: it should provide a high rate of moisture removal and a stable temperature regime. Such an environment is created by the same steam injection with the heaters on. The regulation may involve fans and exhaust ducts. The optimum temperature for this processing mode should be 6-7 ° C higher than the medium at the main stage of drying, but at the same time be lower than + 100 ° C.
It is important to emphasize that thermal treatment does not apply to all tree species. For example, dry timber manufacturing technologies include the use of this operation for cedar, pine, linden timber , the thickness of which exceeds 60 mm. Thick preforms can be subjected to moisture and heat treatment several times to fully cover the internal structure.
Process completion
After drying, conditioning treatment is carried out, the purpose of which is to equalize the moisture indicators and the structure of the material. By means of humidifying devices and heaters, a special environment is formed in which the overdried sections of the lumber are moistened, and the insufficiently dried ones dry out. In time, such processing lasts several hours - the specific duration depends on the volume of the bookmark and the parameters of the material.
The moisture content of wood after chamber drying with conditioning treatment also varies from 6 to 15%, depending on the mode applied. Then, after preliminary cooling, the stacks are unloaded from the chamber. If necessary, an additional forced cooling operation with ventilation can also be used.
Technology advantages
Chambers with temperature and humidity treatment allow drying processes to be fully preserved with the environmentally friendly material. In most cases, such processing methods do not require the use of active chemicals that increase the efficiency of the process. High quality results are also noted. If the operator made no mistakes in organizing the process, then the resulting beam will gain high strength and protective qualities.
Many point to the high productivity of chamber drying of wood. Pros and cons, however, converge in this characteristic. On the one hand, batch processing really allows you to serve large volumes of wood in short time intervals. And on the other hand, the inability to process small packets with a reduced load limits the scope of such equipment.
Cons of technology
The main criticism of this drying technology comes down to the risk of the formation of numerous lumber defects during the operation. One of the most common of these is warpage. Violation of the technology can provoke both longitudinal and transverse deformations.
Typically, such processes are the result of insufficient removal of moisture before the main stage of drying. Often the material gets defects in the form of cracking at the ends. Similar flaws arise if the lumber dryer worked in the formed mode at the initial heating stage. The technology requires that this process, even at the lowest temperature indices, be carried out under conditions of softening the structure of lumber.
Conclusion
Simplification of the principles of management and maintenance of processing units contributed to the formation of entire segments of equipment intended for domestic use. How justified is the organization of its own line for drying lumber? This will depend on the amount of processing.
And it is also important to consider the cost of chamber drying of wood. The price of 1 m 3 on average is 2-3 thousand rubles. Of course, if you build your own house out of timber, the benefits will be obvious. But the equipment itself is not cheap, not to mention the nuances of operation and the risks of obtaining the same defects without professional support.