RU2162581C2 - Method of drying wood material - Google Patents

Abstract

FIELD: wood-working industry; drying various wood species in form of lumber and wood blanks. SUBSTANCE: according to proposed method, various wood species in form of lumber and/or blanks are loaded into drying chamber and are kept there at atmospheric pressure and high relative humidity of air (about 90%) beginning from temperature of material of 30 C to 55 C during several hours, after which successive alternating stages are performed: heating the material in circulating air medium at atmospheric pressure; material is kept at reduced pressure during period of time at which temperature in center of material reaches the level below boiling point of water at given degree of vacuum. Successive alternating stages are combined in cycles; each cycle includes four and more alternating stages; each cycle is characterized by constant heating temperature at heating stages and by constant relative humidity of air; heating temperature rises fractionally from cycle to cycle and relative humidity of air decreases fractionally from cycle to cycle. EFFECT: high productivity at perfect quality of material. 2 dwg

Description

 The proposed method relates to the woodworking industry and can be used for drying material from wood of various species in the form of both lumber and wood blanks.

 There are various methods of drying material from wood of various species, which are implemented using known drying chambers (1, 2, 3).

 These methods allow the drying of material from wood of various species, however, some of the methods are difficult to implement, others have low productivity to achieve the required quality of the dried material.

 The closest set of essential features to the proposed method is a known method of drying a material from wood, which is implemented using the known device "convective-vacuum dryer", and which the authors selected as a prototype (4).

The known method includes loading material from wood and into the known drying chamber - "convective-vacuum dryer" and the implementation of sequentially alternating stages:
heating the material at atmospheric pressure in a circulating air environment, during this period the temperature of the material rises, which is accompanied by evaporation of moisture from its surface and leads to its decrease in the surface layers, and as a result, moisture begins to move inside the material under the influence of a humidity gradient, and then heating material at reduced pressure to a temperature exceeding the boiling point of water, at this stage of evacuation, intense evaporation of moisture from the surface immediately begins, which at This leads to a rapid decrease in the temperature of the surface of the material, reaching the boiling point of water, and then the temperature of the surface of the material continues to fall, at this time water boils in the cell cavities, and the resulting water vapor moves to the surface under excessive pressure, the water vapor that leaves the material forms around the medium is almost pure saturated steam, as a result of this moisture is removed at a sufficiently high humidity of the surface of the material and therefore, in its small difference in the thickness of the material, This avoids significant drying stresses and permanent deformations.

 Evacuation stops after the temperature in the center of the material drops below the boiling point of water at a given vacuum depth.

 During the exposure of the material in vacuum with the removal of free moisture, its movement takes place under the influence of gradients of pressure, humidity and temperature.

 The number of successively alternating stages determines the required final moisture content of the material.

 The inventors of the invention recalculated the drying time according to the known method for conditional material, and the drying time of the conditional material was ≈ 120 hours.

 The disadvantage of this method is its low productivity.

 The technical task of the proposed method is to increase productivity with high quality drying of wood material, in particular lumber and / or wood blanks.

 The problem is solved by the proposed method of drying a material from wood by performing cycles loaded into the chamber material consisting of the steps of heating the material in a circulating air medium at atmospheric pressure and holding the material under reduced pressure for a time at which the temperature in the center of the material reaches a value below the temperature boiling water, according to the invention, which is preliminarily before sequentially alternating cycles carry out the exposure of the material heated to t temperatures, significantly lower boiling points of water, at high relative humidity, at atmospheric pressure for several hours, and each of the cycles includes four or more successively alternating stages, each cycle being characterized at the heating stages by a constant heating temperature and a fractional increase from cycle to cycle and constant relative humidity in the chamber at the stages in the cycle with a fractional decrease from cycle to cycle.

Preferably, the preliminary exposure of the material is carried out at a relative humidity of ≈ 90%, starting from a material temperature of 30 ^o C to 55 ^o C.

 The fact that prior to the sequential alternation of cycles the material is heated to a temperature significantly lower than the boiling point of water at high relative humidity at atmospheric pressure for several hours, and each of the cycles includes four or more successively alternating stages, each cycle characterized at the heating stages by a constant heating temperature and its fractional increase from cycle to cycle and constant relative humidity in Amer at stages in its cycle at fractional decrease from cycle to cycle, allows to obtain a material of high-quality wood in sawmills for 100 hours.

 Analysis of scientific, technical and patent documentation showed that the proposed method is new and meets the criterion of "inventive step".

 The proposed method is as follows.

The wood material in the form of lumber and / or in the form of blanks is loaded into the drying chamber and maintained at atmospheric pressure and high relative humidity ≈ 90%, starting from a material temperature of 30 ^o C to 55 ^o C for several hours, and then carry out sequentially alternating stages:
heating the material in a medium of circulating air at atmospheric pressure and keeping the material at a reduced pressure of at least 2 kPa for a time at which the temperature in the center of the material reaches a value below the boiling point of water at a given vacuum depth.

 Sequentially alternating stages are combined into cycles, each of which contains 4 or more alternating stages, and each cycle is characterized by a constant temperature of heating the air at the heating stages and constant relative humidity, the heating temperature increasing fractionally from cycle to cycle, and the relative air humidity fractionally decreasing from cycle to cycle.

 The number of cycles and sequentially alternating stages included in the cycle depends on the final required moisture content of the material.

 The proposed method, the required quality of conventional lumber can be obtained in 100 hours.

 An example of a specific implementation of the proposed method.

10 m ^{3 of} sawn timber from edged pine, with a board size of 40x150x6500 mm, was placed in a vacuum convective forest drying chamber, and previously, to equalize the initial moisture of the material, it was kept for 4 hours at atmospheric pressure and relative humidity of 86% and material temperature from 30 ^o C to 55 ^o C, while achieving an average moisture content of the entire material ≈ 60%, after which 5 cycles of successively alternating stages of heating and aging in vacuum were carried out.

The 1st cycle consisted of 18 alternating stages, with the heating stage being carried out at an air temperature of 60 ^° C, at atmospheric pressure, air circulation in the chamber and relative humidity of 73%, and the aging stage was carried out at a pressure of 2 kPa, as in all subsequent cycles.

2 cycle consisted of 19 alternating stages, the heating stage was carried out at an air temperature of 65 ^o C and a relative humidity of 67%.

The 3rd cycle consisted of 7 alternating stages, the heating stage was carried out at an air temperature of 70 ^o C and a relative humidity of 58%.

4 cycle consisting of 6 alternating stages, the heating stage was carried out at an air temperature of 75 ^o C and a relative humidity of 46%.

The 5th cycle consisted of 4 alternating stages, the heating stage was carried out at an air temperature of 80 ^o C and a relative humidity of 37%.

 In this case, constant monitoring of the moisture content of the material was carried out.

 The result was a material with a moisture content of 12% in 100 hours.

 As can be seen from the obtained results, the proposed method has high performance with high quality material.

Sources of information
1. Prospectus of the company MASPELL (Italy) "Vacuum forest kilns Lo-La and TANDEMLO-LA".

 2. A. I. Rasev. Wood drying. - M .: "Higher School", 1990, p. 45-52, p. 74-85.

 3. E. S. Bogdanov. Drying lumber. - M .: "Forest industry", 1988, p. 55-69, p. 122-141, p. 162-166.

 4. Prototype / Rasev A.I. etc. Art. Convective vacuum dryer for lumber. g. Woodworking industry, 1993, 4, p. 9-10.

Claims (2)

 1. A method of drying a material from wood by carrying out cycles with material loaded into the chamber, consisting of the steps of heating the material in a medium of circulating air at atmospheric pressure and holding the material under reduced pressure for a time at which the temperature in the center of the material reaches below the boiling point of water characterized in that prior to the sequential alternation of cycles, the material is heated to a temperature significantly lower than the boiling point water, at high relative humidity, at atmospheric pressure for several hours, and each of the cycles includes four or more successively alternating stages, with each cycle characterized by a constant heating temperature and a fractional increase from cycle to cycle and constant relative humidity in the chamber at the stages in the cycle with a fractional decrease from cycle to cycle. 2. The method according to claim 1, characterized in that the preliminary exposure of the material is carried out at a relative humidity of ≈ 90%, starting from a material temperature of 30 to 55 ^o C.