RU2319915C1 - Method for drying of lumbers - Google Patents

Abstract

FIELD: wood-working industry.

SUBSTANCE: heating of lumbers is accomplished by hot air in an aerodynamic drying chamber in four-stepped duties with a parabolic-varying hardness with observance of condition (t₁, φ_1, P₁=1) > (t₂, φ_2, P₂=1.3) < (t₃, φ₃, P₃=1.5) < (t₄, φ₄, P₄=1) where t_i - the temperature of the drying agent by steps; °C;φ_i - the saturation of the drying agent by steps; P_i - the drying agent pressure caused by injection of hot air depending on the degree of closing of the exhaust gates, atm.

EFFECT: provided an intensified drying process, reduced consumption of electric powder.

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Description

The invention relates to techniques for drying wood materials using the heat of hot air and can be used in the woodworking industry for accelerated aerodynamic drying of lumber.

A known method of drying wood by heating it to a temperature that ensures a safe state of the material, followed by cooling it to obtain a uniform distribution of temperature and moisture content inside it, further heating to a temperature higher than the temperature that provides a safe state, and isothermal exposure at this temperature with subsequent cycles cooling and heating [RF Patent 2006765 C1, MKI ³ 5F26B3 / 02. The method of drying wood [Text] /I. B. Kozoderov; - No. 5006582/06; declared 10/18/91; publ. 01/30/94; bull. No. 2.-3 p.].

As a prototype, a method of drying wood by heating lumber with hot superheated steam, followed by exposure, with the removal of moisture and cooling, was selected. In this case, heating and aging are carried out at a temperature of 110-150 ° C and a pressure of sharp superheated steam of 1 - 2 atm, moreover, aging is carried out for 1 -2 hours, and cooling is carried out with compressed air to ambient temperature [A.S. USSR 346558, MKI ³ F26B7 / 00. Method for drying materials [Text] / S.G. Romanovsky, M.N. Nikulin; Applicant Institute for Heat and Mass Transfer, Belorussian SSR Academy of Sciences. - No. 164718 8 / 29-33; declared 04/14/71; publ. 07/28/72; bull. No. 23.-2 p.].

A disadvantage of the known methods is both prohibitively hard operating parameters that exceed the safe state, leading to a sharp and uncontrolled boiling of all the moisture contained in the material and the loss of its integrity, as well as the long drying time associated with the complete uniform distribution of temperature and moisture content inside the wood. Studies conducted by the authors with industrial aerodynamic chambers found that the uniformity of heating does not significantly affect the process. To ensure the highest intensity and quality of the drying process as a whole, it is necessary to strive to create an uneven distribution of temperature and moisture content inside the material directed to the surface, as well as to bring closer the evaporation rate of moisture from the surface and its movement from the central zones of the material to peripheral ones.

The invention solves the problem of ensuring high quality of the dried material, intensifying the drying process and reducing the consumption of electric energy for drying.

This is achieved by the fact that in the method of drying lumber by heating them in a temperature-humidity environment, according to the invention, the heating is carried out with hot air in an aerodynamic drying chamber in 4-step modes with parabolic variation in stiffness, subject to the condition ((ti, φ ₁ , P ₁ = 1)> (t ₂ , φ ₂ , P ₂ ≤1,3) <(t ₃ , φ ₃ , P ₃ ≤1,5) <(t ₄ , φ ₄ , P ₄ = 1), where t ₁ - temperature of the drying agent in steps, ° C; φ ₁ - saturation of the drying agent in steps; P ₁ - pressure of the drying agent from the injection of hot air, at.

(где τ - продолжительность сушки) при той же текущей влажности древесины. Интенсивность испарения влаги при сушке в среде заданного состояния характеризуется жесткостью режима. Жесткость режима возрастает с увеличением психрометрической разности. При одинаковой степени насыщенности сушильного агента (насыщенность φ пара в воздухе характеризуется отношением плотности перегретого пара ρ_n к его плотности в насыщенном состоянии ρ_n при той же температуре - хотя чаще применяется термин - степень насыщенности) более жестким будет режим с повышенной температурой, а при одинаковой температуре - режим с меньшей степенью насыщенности.In order to compare different modes, they use the concept of “rigidity of the drying mode”, which means the value of the psychrometric difference Δt times

(where τ is the drying time) at the same current wood moisture. The moisture evaporation rate during drying in a medium of a given state is characterized by the rigidity of the regime. The stiffness of the regime increases with an increase in the psychrometric difference. At the same degree of saturation of the drying agent (vapor saturation φ in air is characterized by the ratio of the density of superheated steam ρ _n to its density in the saturated state ρ _n at the same temperature - although the term is used more often - the degree of saturation), the more severe the regime will be, and at at the same temperature - a mode with a lower degree of saturation.

When drying lumber, modes with increasing rigidity during the process are used. In the initial stage, at a given temperature, a high degree of saturation is maintained, then when the moisture content of the wood decreases, the temperature rises and the degree of saturation decreases. This nature of the change in the parameters of the drying agent during drying is due to the peculiarities of the development of internal stresses in the wood and the requirement to maintain the integrity of the dried boards and blanks.

The increase in the stiffness of the mode in the proposed method is carried out automatically using a computer through the humidifiers and heaters included in the camera device.

The stiffness parabolic during the process in the proposed method consists in the fact that the main operating parameters in the first stage are quite high, in the second they decrease, in the third they increase again and can reach the values of the first stage, and in the fourth they are much higher.

The method is as follows.

At the first stage, when the surface layers are intensively dried, and the humidity of the inner layers is very high, drying is carried out at a temperature of 75-85 ° C, φ = 0.5-0.7 and open gates. At the second stage, when the equilibrium moisture content of the material approaches 50-40%, drying is carried out at a lower temperature, a higher degree of saturation of the drying agent with covered exhaust gates, which somewhat increases the pressure inside the chamber. During this period, moisture is removed at a constant rate, but with a decrease in the moisture core, the internal moisture transfer is increasingly lagging behind the external moisture exchange. Therefore, to create a positive temperature gradient and a gradient of moisture content from the inside to the surface, the temperature of the drying agent is reduced, the outer layers of the wood also cool due to evaporation of moisture and a decrease in ambient temperature. Due to the high heat capacity of the water, the inner moist layers remain more heated at this time. To reduce external moisture exchange, they cover the gates and increase the external air intake, which leads to a certain increase in the pressure of the drying agent.

This mode is maintained until the saturation point of the fiber reaches the wood's moisture content, when chemically bound moisture remains in the material, which is more difficult to dry. To remove such moisture, drying is carried out with a stepwise increasing temperature and a decreasing degree of saturation of the drying agent. To equalize the rate of moisture evaporation from the surface and move it from the inner layers to the outer ones, the pressure of the medium is slightly increased and the exhaust gates are almost completely closed until the material reaches equilibrium humidity of 20-15%. After that, the gates are fully open, significantly increase the temperature and reduce the degree of saturation until the material reaches the final moisture content.

The invention is illustrated by the following examples.

Example 1. Pine, 52 × 180 × 6000 mm, the initial humidity of 85%, the desired final humidity of 8%.

1 stage: temperature of drying agent 74 ° С, degree of saturation 0.65, pressure 1 at. Humidity of wood will be 60%; 2 stage: temperature of drying agent 64 ° С, degree of saturation 0.83, pressure 1.2 at. Humidity will be 30%; 3 stage: temperature of drying agent 84 ° С, degree of saturation 0.59, pressure 1.4 at. Humidity will be 20%; Stage 4: drying agent temperature 95 ° С, saturation degree 0.32, pressure 1 at. Humidity will be 8%.

Example 2. Oak, 42 × 120 × 4000 mm, the initial humidity of 75%, the desired final humidity of 8%.

1 stage: temperature of drying agent 62 ° С, degree of saturation 0.7, pressure 1 at. The moisture content of wood will be 60%; 2 stage: temperature of drying agent 52 ° С, degree of saturation 0.88, pressure 1.3 at. Humidity will be 30%; 3 stage: temperature of drying agent 56 ° С, degree of saturation 0.68, pressure 1.5 at. Humidity will be 20%; Stage 4: drying agent temperature 74 ° С, saturation degree 0.32, pressure 1 at. Humidity will be 8%.

Thus, the invention allows in an industrial environment without the use of special tools, devices and energy carriers to reduce the drying time of wood, to increase its quality, which leads to a reduction in energy costs.

Claims (1)

A method of drying lumber by heating them in a temperature-humidity environment, characterized in that the heating is carried out with hot air in an aerodynamic drying chamber according to 4-step modes with parabolically varying stiffness subject to the conditions (t ₁ , φ ₁ , P ₁ = 1)> ( t ₂ , φ ₂ , P ₂ ≤1,3) <(t ₃ , φ ₃ , P ₃ ≤1,5) <(t ₄ , φ ₄ , P ₄ = 1), where t ₁ is the temperature of the drying agent steps, ° C; φ ₁ is the saturation of the drying agent in steps; P ₁ - pressure of the drying agent from the injection of hot air, at.