RU2228497C2 - Method for drying of lumbers by microwave radiation - Google Patents

Abstract

FIELD: procedure of drying, in particular, methods for drying of lumbers by microwave radiation, applicable in wood-working industry. SUBSTANCE: wood drying consists in local brinding of sources of microwave radiation to the lumber to be dried with a subsequent displacement of them. The sources of microwave radiation are brought up to the lumber or lumber pile on the side of the opposite ends and displaced along the lumber being dried from the ends to their middle part in length. The speed of displacement of the radiators and the rate of filtrate flow of water vapor for the preset species of wood are determined from relations set forth in the claim. EFFECT: facilitated procedure. 2 cl, 2 dwg

Description

The invention relates to a drying technique, in particular to methods for drying lumber with microwave energy, and can be used in the woodworking industry.

When drying wood materials with microwave energy, microwave radiation generated by microwave generators is fed into the drying chamber with a stack of sawn timber formed in it.

The drying process is carried out when both microwave radiation and heat carrier are supplied to the chamber. Stack is irradiated with microwave energy evenly and mainly from its opposite sides, while various technological operations and drying process control modes are used to increase the drying efficiency and quality of dried lumber. In the technical solution (see Pat. RU No. 2099656, class F 26 B 3/34, publ. 12/20/97), a heat carrier is introduced when drying the sawn timber, the flow direction of which relative to the stack being dried is changed to the opposite no less than 3-5 times.

In the solution (see Pat. RU No. 20706943, class F 26 V 3/347, publ. 20.12.96), the sawn timber is dried in the mode of dosed supply of microwave power, taking into account the power of the heater, which supplies the coolant to the chamber.

Drying of wood in accordance with the technical solution according to US Pat. RU No. 2116588, class F 26 V 3/347 is produced in the modes of moisture and heat treatment of lumber in the chamber, taking into account the supply of the last microwave radiation and coolant.

There are also known technological processes of drying lumber microwave energy, according to which the supply of microwave radiation to the chamber is produced from the opposite ends of the formed stack of lumber. In this case, the stack is formed by a certain arrangement of lumber (see US Pat. RU No. 2101631, class F 26 B 3/347, publ. 10.01.98).

However, the efficiency of the known technological processes for drying lumber with microwave energy is provided by increasing the cost of these processes due to the introduction of additional energy-intensive and material-intensive technological operations necessary for microwave drying of wood without violating its physical and mechanical properties.

In the implementation of the known processes of wood drying with microwave energy, physical phenomena are used in which moisture in the wood moves from zones warmer to places colder in the direction of the temperature gradient ΔТ, as well as from places wetter to places drier (moisture diffusion) in direction of the negative moisture content gradient ΔV, which requires creating conditions in the drying chamber that would ensure uniform heating of the dried lumber to eliminate the likelihood of formation in the dried wood INE internal and external damage. The need for uniform heating of the dried lumber increases the number of microwave emitters in the chamber leads to an increase in the power of the used microwave generators. The need to comply with the conditions for maintaining the dried lumber in the drying process in an equilibrium physical state in temperature and humidity, as follows from the prior art, increases energy and material costs.

There is also a method of drying lumber (see US Pat. 2061935, F 26 B 3/34, publ. 10.06.96), which is selected as the closest analogue of the claimed invention. In the decision of US Pat. 2061935 drying is carried out by local supply of microwave radiation sources to the dried lumber with their subsequent movement. The radiation sources are supplied to the middle part of the lumber being dried, followed by periodic displacement of the sources from the middle of the lumber to its ends, which implies the creation of the moisture transfer effect during drying, both under the influence of the temperature gradient ΔT and under the influence of an overpressure gradient ΔР, at which moisture moves in the direction of decreasing pressure, i.e. from the zone of increased pressure of water vapor created in the internal volume of the material when it is locally heated, to the zone of reduced pressure. When implementing this technical solution, energy and material costs are reduced compared with the above technical solutions.

However, when carrying out this drying process, the intensity and efficiency are technologically limited by the parameters of the dried lumber along their length and cross-section, which, in particular, is confirmed by the examples of the method described in the description of the invention, according to which this drying process is suitable for standardized small-sized lumber. These circumstances are explained by the fact that when drying lengthy lumber, the length of the capillary system of the wood fibers increases, and therefore the volume of moisture contained in them. As a result, during the implementation of the known process, the rate of movement of more viscous (compared to the viscosity of water vapor) moisture along the wood fibers in the direction from the medium-long zone lumber to its ends under the influence of water vapor pressure is reduced. The possibility of intensifying the process due to the temperature mode specified by the method of heating to 160 ° C is impractical, because at this temperature, destruction of the dried wood is possible.

The technical task of the invention was to increase the drying efficiency of lumber due to the intensification of the process and improve the quality of the products with local exposure to microwave radiation on lumber and regardless of the standardized lengths of lumber.

To solve the technical problem, a method of drying wood is proposed, according to which microwave radiation sources are locally brought to the dried lumber or a stack of lumber from the opposite ends, the radiation sources are moved along the dried material from the ends to their middle part in length and at a speed determined by the ratio:

where V _f.d. - water vapor filtration rate permissible for a given wood species, cm / s;

W _n - a given initial moisture content of wood,%;

W _to - a given final moisture content of wood,%;

ρ _b - a given basic density of wood, g / cm ³ ;

ν is the specific volume of steam, cm ³ / g.

the rate of filtration of water vapor for a given wood species is determined from the ratio:

.

.

where V _{max. lam} - the maximum rate of water vapor filtration in a given wood species with its laminar flow, cm / s;

k is the permeability coefficient of wood, cm ² ;

P is the pressure of saturated water vapor at a given maximum drying temperature, Pa;

μ is the dynamic viscosity of the vapor, Pa · s;

l is the distance from the end to the emitter, see

According to the invention, the speed of movement of the microwave emitters along the dried lumber is regulated.

When implementing the invention, the drying process is intensified, the drying quality is improved regardless of the standardized length of the dried lumber, its initial moisture content and cross section, which is ensured by the creation of a fluidized layer of moisture moving from the ends of the dried lumber in the direction of their center along the length of the zone. The condition for moving the fluidized bed of moisture is achieved by the claimed location of the sources of local microwave radiation and a preset speed regime for their movement, taking into account the initial and final humidity of the dried sawn timber. The direction of motion of the fluidized bed of moisture arising during the drying process from the ends to the central along the length of the zone of the dried lumber creates optimal pressure and temperature conditions in the zone of the internal volume of the lumber. At the same time, with increasing water vapor pressure in the local sawn timber heating zone, their volumetric evaporation through the capillary system of wood also increases, and mainly in the direction of the ends of the sawn timber. Removing part of the moisture in the form of steam through the moisture-free capillary system of wood ensures its “steaming”, i.e. moisture-heat treatment mode, in which the removal of residual stresses occurs, and therefore, the manifestation of internal and external damage in the lumber is reduced, the quality of the resulting product is improved.

In the analysis of the prior art, no technical solutions with a combination of features similar to the claimed method of drying lumber with microwave energy were revealed, which indicates the presence of novelty and inventive step in the claimed method that meet the criteria of the invention. The inventive method can be implemented using traditional technological equipment used in industry for microwave drying of wood for its implementation.

In FIG. 1 shows a flow diagram of a microwave drying process; FIG. 2 is a graph for determining an acceptable vapor filtration rate.

The method of drying lumber is carried out using for these purposes the following technical means:

- a drying chamber designed for microwave drying and placing in it a stack of lumber or a brush formed using traditional methods using dielectric gaskets (wood) or a drying chamber for individually dried lumber (the latter taking into account the specified chamber dimensions and cross-sectional parameters of the dried lumber) ;

- microwave emitters, which are equipped with a drying chamber. The connection of microwave emitters to a source of microwave energy (microwave generators) is carried out using appropriate means, in particular waveguides and matching devices. The presence of the latter provides the ability to move microwave emitters, for example, by means of an electric drive along guide tracks along the chamber (not shown).

In the initial position, microwave emitters are placed in the end zones of the chamber in accordance with the orientation of the end surfaces of the lumber in the chamber. The longitudinal axes of the emitters are predominantly oriented at an acute angle to the longitudinal axis of the lumber with the apex of the angle between them directed towards the central zone along the length of the lumber, which contributes to the formation of a fluidized bed in the local internal volume of the lumber parallel to the end surfaces.

To ensure moisture removal from the end surfaces of lumber, moisture removal, the camera has a traditional moisture removal system, including a fan and slots made in the chamber for moisture to escape.

The drive for moving microwave emitters is electrically coupled to a system of in-line moisture meters, for example, microwave micrometers of the Mikroradar-113 brand (manufacturer of the NPMP Mikroradar, Minsk). These moisture meters are placed mainly along the entire length of the chamber with the implementation of their electrical connection with the drive of the emitters according to traditional methods of process automation.

The formed stack or brush of lumber is fed into the chamber and unloaded from it by, for example, movable trolleys.

When microwave energy is supplied through emitters, local heating of the internal volume of lumber is provided. Microwave emitters located on the side of the opposite ends of the lumber are counter-moved in the direction from the ends to the central zone along the length of the lumber and at a speed determined by the ratio:

the rate of filtration of water vapor for a given wood species is determined from the ratio:

.

.

When moving microwave emitters in the indicated directions in the local internal volumes of lumber, a fluidized layer of moisture is created that maintains an excess stable (compared to atmospheric) pressure of the vapor-air mixture in the internal volume of lumber. The excess pressure of the vapor-air mixture provides a stable flow of water vapor in the direction of the capillary system of wood and mainly along its fibers. In this case, water vapor is removed through the moisture-free capillary system of wood, moisture and heat treatment of the dried internal volumes of wood is provided, which stabilizes its equilibrium physical state in temperature and humidity and reduces the decrease in strength characteristics.

When selecting and selecting the technological regime of the high-speed movement of microwave emitters according to the relations (1) and (2), the following points were known in the theory of wood drying:

- according to the calculation of the mass (M) of moisture removed during drying from 1 m ^{3 of} wood, taking into account W _n - the initial initial moisture content of the wood (before drying); W _to - the specified final moisture content of the wood (after drying) and the average density (ρ _b ) of the wood (with an estimated initial moisture content), taking into account the dependence:

M = ρ _b (W _n -W _k ) / 100 (kg / m ³ ),

where W _n - the initial initial moisture content of the wood, is set according to the reference data for this dried wood species,%;

W _to - the final moisture content of the wood, is set taking into account the technological requirements for dried products,%;

ρ _b is the basic density of wood, is set according to reference data for the corresponding wood species or is calculated according to the dependence ρ _b = G / V (kg / m ³ ) known in the theory of drying,

where G is the mass of wood (sample) in an absolutely dry state;

V is the volume of the wood sample at a moisture content of more than 30%;

- by assessing the influence of the drying process of wood on its mechanical properties (relative strength indicators for compression and tension along the fibers) depending on the temperature (° C) of heating and drying time (hour). It was taken into account (see, for example, Prince IV Krechetov, “Drying and Wood Protection”, Moscow: Forestry, 1975, p. 78, Fig. 31), that the strength indices (indicated above) of wood at temperature exposure to it up to 120 ° С and drying time within no more than 6 hours correspond to the technological norms of drying for various wood species;

- by assessing the ability of a porous medium to pass liquid, which characterizes the permeability of wood and according to the law A. Darcy is determined from the dependence Q = kS Δp / μl (3), where Δp is the pressure difference at the ends of the sample; S is the cross-sectional area of the sample; μ is the viscosity of the liquid; k is the coefficient of permeability of the sample; l is the length of the sample. Under appropriate transformations, relation (3) has the form: U = Q / S, from which it follows that the intensity of the moisture flow in a porous medium is characterized by its consumption per unit cross-sectional area of the sample or the filtration rate. From a number of works it is known (see, for example, Prince Osnach N.F. “Permeability and conductivity of wood”, M .: Forest industry, 1964, p. 182) that the filtration flow rate along the wood fibers is within 0 , 5 ... 8 cm / s, which corresponds to the maximum rate of water vapor filtration in a given wood species during its laminar flow (V _{max. Lam} );

- to determine the required specific power of microwave energy spent on drying lumber from the initial initial moisture content to the final specified, taking into account the set drying temperature, specific density of wood and the feed rate (V _under ) of microwave radiation. In order to provide this drying speed, the specific microwave power (W / cm ² ) is determined by the formula:

,

,

where C _d - the heat capacity of absolutely dry wood, J / g · deg;

C _in - heat capacity of water, J / g · deg;

r is the latent heat of vaporization, J / g;

t _c - set temperature of wood drying, ° С;

t _n - the initial temperature of wet wood, ° C.

To confirm the optimality of the ratios (1 and 2), taking into account the claimed directions of movement of the microwave emitters, an experimental check was carried out on drying the lumber, respectively, with a length of 1 to 6 m.

During the research, pine lumber, the most traditional for the manufacture of various lumber products, was selected for drying.

Based on the selected wood species, the following key indicators were determined from the reference data:

ρ _b - a given basic density of wood, 0.42 g / cm ³ ;

ν - specific volume of steam - 1675 cm ³ / g;

C _d - the heat capacity of absolutely dry wood (not more than 10% humidity) - 1.6 J / g · deg;

C _in - heat capacity of water - 4.18 J / g · deg;

r is the latent heat of vaporization - 2260 J / g.

Based on the lumber selected for the research on drying, the following data were determined and set:

W _n - initial wood moisture - 80%;

W _to - a given final moisture content of wood - 10%;

t _n - the initial temperature of wet wood - 20 ° C;

t _c - wood drying temperature, not less than 100 ° С;

V _f.d. - the rate of water vapor filtration, permissible for a given wood species, is equal to the maximum rate of water vapor filtration in a given wood species (pine) with a laminar flow (V _{max. lam} ). For this breed (pine) of wood V _{max. lamas} - 8 cm / s.

To obtain more accurate results on determining the rate of water vapor filtration, acceptable for a particular (according to the anatomical structure of the wood fibers, its physical and mechanical state) of the microwave drying of the lumber, use the methodology for determining the permeability of wood (see book. Haruk EV “ Permeability of wood of some coniferous species ”, Krasnoyarsk, 1969, p. 10-16). When using this technique, using a standardized device (GOST 6336-52) determine the coefficient (k) of permeability specified for the length (1) of the sample for a given wood species. Taking into account the selected wood sample and its permeability coefficient, the maximum filtration rate of water vapor during laminar flow for a given drying temperature is determined by relation (2). Studies have established that for wood species, pine V _max.lam depends on both the length of the lumber being dried and the maximum set drying temperature, see Fig. 2. From the FIG. 2 graphs to determine the permissible rate of steam filtration follows that at the maximum specified drying temperature, for example 120 ° C, V _{max. Lam} for lumber up to 5.8 m long corresponds to 8 cm / s (point A). As the length of the dried material increases, V _{max. Llam} should be reduced along a curve corresponding to the maximum set drying temperature. Studies, in particular, found that for hardwood (oak) V _{max. lam} = 0.5-1 cm / s.

After conducting research on drying lumber according to the claimed method with a length of 1, 3 and 6 m, an assessment was carried out according to the following indicators:

- the effectiveness of removing moisture from wood. The weighted method for measuring lumber before drying and after drying was used, taking into account the comparison of the results obtained with the known position of the calculation for moisture removal according to the formula: M = ρ _b (W _n -W _k ) / 100 (kg / m ³ ). As a result of research, it was found that the deviation between the calculated and experimental data (according to the claimed method) for all dried lumber, regardless of their lengths, does not exceed 2-5%, which indicates the effectiveness of the claimed process of drying lumber;

- the effect of the drying process on the mechanical properties of wood (relative strength indicators for compression and tension along the fibers). The assessment was carried out using traditional methods of mechanical testing of wood samples before drying and after drying and in accordance with the methodology for determining the strength indices of compressive and tensile forces on test specimens (see book A. Perelygin AM “Wood science”, M .: Forest industry, 1964, p. 123-128). The results obtained by determining the strength parameters of wood were compared with reference data of similar indicators for a given wood species (pine), respectively, with wood moisture content of more than 30% and dried wood (moisture content of not more than 15%) (see the book “Wood Science”, p. 126, table. .23 (tensile strength of wood along the fibers) and p. 129, Table 124 (compressive strength of wood)). The research results found that the deviation between the calculated and experimental data for all dried lumber (according to the present method), regardless of their lengths, does not exceed 5-8%, which indicates the effectiveness of the claimed process of drying lumber in terms of compliance with the requirements for the quality of the resulting product, taking into account local exposure to microwave radiation on lumber and specified according to the claimed method of technological modes.

For illustrative purposes of the proposed method, example 1 for its implementation, also example 2 (comparative) is given. When carrying out the drying process in accordance with examples 1 and 2, pine lumber 6 m long with an initial humidity of 80% and an initial temperature of 20 ° C were used.

According to the initial data set above, the speed of movement of the microwave emitters was determined:

Taking into account the length of the dried lumber, the drying time was determined:

and specific microwave power

The drying process of lumber in accordance with examples 1 and 2 was carried out taking into account the above-mentioned high-speed regime of movement of microwave emitters, drying time and supplied specific microwave power.

When drying lumber in accordance with Example 2, the direction of movement of the microwave emitters was carried out from the central zone along the length of the lumber to their ends, i.e. in accordance with the decision of US Pat. 2061935.

As a result of drying lumber in examples 1 and 2, the following indicators were obtained:

The final moisture content of the dried material:

Example 1 - 9.5-11.3% (over the entire length).

Example 2 - 25 ... 35% (in areas with a length of 1.0 ... 1.5 m from the ends).

Mechanical strength (sample 20 × 20 × 20 (mm) - example 1) tensile - 960 kg / cm ² , compression - 390 kg / cm ² . These indicators differ from the reference by 5-8%.

The mechanical strength of the wood according to example 2, taking into account its moisture content, was not evaluated.

Drying time (according to examples 1 and 2) is calculated, i.e. satisfies the requirements of technological norms of drying (no more than 6 hours).

When visual inspection of wood dried according to example 1, no external damage was found.

Thus, from the results it follows that when implementing the proposed method of drying wood with microwave energy, the process is intensified with the required quality of the resulting product and regardless of the length of the dried lumber.

When implementing the invention provides a mode of automatic control of the speed parameters of the movement of microwave emitters along the dried materials, taking into account the properties of the latter.

Claims (2)

1. The method of drying wood, which consists in the local supply of microwave radiation sources to the dried lumber with their subsequent movement, characterized in that the microwave radiation sources are fed to the dried lumber or a stack of lumber from their opposite ends and move along the dried material from the ends to their middle part in length at a speed determined by the ratio

where V _f.d. - water vapor filtration rate permissible for a given wood species, cm / s; W _n - a given initial moisture content of wood,%; W _to - a given final moisture content of wood,%; ρ _b - a given basic density of wood, g / cm ³ ; v is the specific volume of steam, cm ³ / g, the rate of water vapor filtration for a given wood species is determined from the ratio

where V _{max. lamas.} - the maximum rate of water vapor filtration in a given wood species with a laminar flow, cm / s; k is the permeability coefficient of wood, cm ² ; P is the pressure of saturated water vapor at a given maximum drying temperature, Pa; μ is the dynamic viscosity of the vapor, Pa · s; 1 - the distance from the end of the lumber to the emitter, see 2. The method according to claim 1, characterized in that the speed of movement of the microwave emitters along the dried lumber is regulated.

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