Kiln-drying of wood: technology, pros and cons


2020-03-25 07:20:18




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Wood in the form of finished lumber, plywood products and particle Board panels remains the basic consumable raw materials for many fields of industry and construction. Its environmental friendliness and flexibility in processing allow to compete even with high-tech fiberglass. Thus are the disadvantages of the material due to low strength and sensitivity to aggressive environments. Minimize special methods allow pre-processing. In particular, the kiln-drying of wood with the thermal effects is one of the most common ways of increasing its technical and physical properties.


There are different types of drying machines, the General principle of the device which involves the presence of heat transfer. From them hot flows are distributed on the surface of the target material. The basic classification distinguishes the drying chamber dielectric and convective type. Dielectric model form of thermal energy in the process of converting high-frequency currents.

chamber drying of wood

In turn, convective units pump heat fluxes to lumber through the pre-heated air, which contributes to elimination of moisture. The drying agent in such a working scheme is heated together with water vapor. After which the mixture circulates in the heat exchanger. Convection chamber for drying of wood can be used as functional elements (conductors of heat) fire-tube, steam, and induction media. Sometimes used gas chambers that emit large amounts of thermal energy. And dielectric and convective models of dryers can operate in a vacuum, that is at low pressure.


The Quality of drying depends largely on how well was prepared stacks – the so-called packages of lumber, which is specially formed to lay in the camera. For example, unevenness of drying and deformation of individual sections on the boards will just be the result of improper packaging.


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drying chamber

Typically used in two ways this kind of training:

  • Single stack;
  • Batch formation of the bookmark.

In the first case, single stack includes multiple boards, stacked in the same configuration. Batch bookmark assumes that the lumber will be stacked in several piles ready. As the basis for a stack use hard platform with a smooth surface. It can be a rail trolley, generated as separate tracks.

To kiln-drying of wood was made without leakage of the thermal agent, the underload stack is not allowed. It needs to be completely filled or planks of the same length, or lumber of different sizes vrazbezhku. Between abutting boards includes placement of one or more layers of lining.


There are three categories of drying:

  1. First category – high quality mode, carried out at a temperature of about +60+70°C. This mode allows to dry the material to ratio 6-8% moisture. Obtained in this method of processing products are characterized by high technical and operational indicators allowing to use the material in instrument and mechanical engineering, power construction, manufacturing of elements of engineering equipment, etc.
  2. The Second category provides the regime with bringing the moisture level to 8-10%. For such purposes use the dryers, operating at temperatures up to +75°C. Target areas are processed in this mode, material is furniture manufacture and the manufacture of joinery structures.
  3. The Third category of drying allows to obtain the lumber of average quality and humidity, which can reach 15%. Of such wood in the future, produce containers, mouldings and inexpensive building material.

Wood Heating

The Primary heating required to prevent tension processes the internal structure of the lumber during the main drying. Is intense but short-lived thermal effects, in which the moisture does not evaporate. In parallel with the thermal processing in the camera is saturated steam. This function is performed by a group of fans with heaters. The process of evaporation of moisture throughout the thickness of the material will begin only after the termination of heat exposure.

chamber drying of wood

Again, that kiln dried wood is not damaged its structure, at the first stage of heating is regulated by a balance between temperature and rate of fall moisture. In case of violation of this balance, there is a risk of cracking. The duration of this procedure is determined by the external conditions and characteristics of wood – an average of 1 to 2 hours.

Main stage

After the initial warm-up immediately drying. But the transition should be gradual, which is reflected in the slow opening air flow dampers, bringing the moisture. At the same period set the optimal parameters of heat flow. Temperature, in turn, is regulated by the opening level of camera shutters. If you want to increase the saturation of the agent, supplynozzles is closed.

dryer for lumber

The degree of saturation of the thermal effect can be adjusted by running the steam generator. However, when you open the hood to connect the system steam injection is impossible. In operation, kiln-drying of wood is controlled by the characteristics of the temperature and its error. In industries is usually information about these values every hour logged.

Blagodarova processing

This is also a kind of intermediate stage, for which there are special environmental conditions: it should provide high intensity blagovischennia and stable temperature. This environment is created through the same steam injection when the heaters. In the regulation can have the fans and exhaust ducts. The optimum temperature for this mode of treatment should be at 6-7°C higher than environment on the main stage of drying, but at the same time to be below +100°C.

It is Important to emphasize that blagodarova processing is not applied to all tree species. For example, the manufacturing technology of dry timber include the use of this operation for timber of cedar, pine, Linden, the thickness of which exceeds 60 mm. Thick plates can be vegetableoil treatment several times for full coverage of the internal structure.

Process Completed

After drying is performed the conditioning treatment, the purpose of which is the alignment of the indicators of humidity and structure of the material. Through a humidifying device and a heater, formed a special environment in which the dried portions of the lumber is wet and not dried – dry. At the time this treatment lasts for several hours – the exact length depends on the amount of bookmarks and settings of the material.

The moisture characteristics of wood after kiln-dried with conditioning treatment also vary from 6 to 15 % depending on the applied mode. Then after the pre cooling is done the discharge of stacks from the camera. If necessary, may be operated and the additional operation of forced cooling with ventilation.

Pros of technology

The Cameras with temperature and moisture treatment ensure the processes of drying and the environmental friendliness of the material. In most cases, these treatment methods do not require the involvement of active chemicals that increase the efficiency of the process. It is noted and high quality result. If the operator made no mistakes in the organization process, the resulting lumber will attain high strength and protective qualities.

wood moisture content after kiln drying

Many point to the high performance of chamber drying of wood. The pros and cons, however, in this feature agree. On the one hand, packaged treatment really allows for short time intervals to serve large volumes of wood. And on the other – the impossibility of processing small packets with the reduced loading limits of the scope of application of this equipment.

Cons of technology

The Main criticism of the technology of drying is reduced risk of the formation of numerous defects in lumber in the process of operation. One of the most common of them is the warping. Violation of the technology may trigger both longitudinal and transverse deformation.

technology of production of dry timber

Typically, these processes are a consequence of insufficient moisture before the main stage of drying. Often produces defects in the form of cracking at the ends. Such flaws occur when a rack of lumber worked in the organized mode during initial warm-up. The technology requires that the process, even when the minimum temperature was performed at the softening of the structure of the timber.


Simplification of management principles and maintenance of processing units contributed to the formation of entire segments of the equipment intended for home use. How justified the organization of a private line for drying lumber? This will depend on the volume of processing.

kiln-drying of wood pros and cons

And it is also important to consider the cost of kiln drying wood. The price of 1 m3 on average 2-3 thousand rubles. of Course, if you build your own house out of timber, then the benefits will be obvious. But the equipment itself is not cheap, not to mention the nuances of operation and risks of receiving the same defects without professional support.

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