RU2530983C1 - Method of drying wood in microwave wood drying kiln with resonance method - Google Patents

Abstract

FIELD: woodworking industry.

SUBSTANCE: wood pile is placed in a closed metal cavity - wood drying kiln resonator; the dimensions of the resonator are selected much greater than the wavelength λ of the feeding microwave generator, so that the possibility is provided of excitation in an unloaded state in the cavity of a plurality of oscillations of different spatial structure, the temperature of the wood before drying and the temperature after the interaction of the electromagnetic field of many modes is measured, and according to the temperature difference the absorbed power is determined by calorimetric method; according to the function of damping of the drying chamber resonator the moisture of wood is measured, and control of drying of wood is carried out in the gradients of humidity and temperature by adjustment of the output power of the RG generator, which in the function of damping of the loaded resonator dH is a changing parameter.

EFFECT: improvement of accuracy of determining the moisture content of various types of wood, simplifying of the hardware implementation of the method.

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Description

The invention is intended to improve the microwave drying of wood by applying the standing wave resonance method, controlling the energy supplied to the resonator of the drying chamber by means of a primary measuring transducer (PIP) of the microwave drying chamber resonator, and can be used in the woodworking industry.

The known microwave method for determining the moisture content of solid and liquid materials, based on the method of free space [see Berliner M.A. Humidity measurement / M.A. Berliner. - M .: Energy, 1973. - 345 p.]. This method can be divided into two modifications:

- using a transmitted wave;

- using a reflected wave.

In both versions, the measured characteristic is the transmission coefficient or the coefficient of the reflected wave.

The disadvantage of this method is the inability to control the current moisture content of the material in the process of microwave drying of wood.

A known resonator method for controlling the humidity of the material [see Klyuev V.V. Non-Destructive Testing and Diagnostics: A Guide. - M .: Mashinostroenie, 1995. - 487 p.], In which the test sample of a certain size is placed in the cavity of a cavity resonator (OR), an electromagnetic field (EMF) of a certain spatial structure is excited. In a cylindrical OP with a wave of type E _{010, a} sample in the form of a cylindrical rod of small diameter is placed along the axis of the resonator; in a cylindrical OR with an oscillation of H _011, samples of larger diameter and with large losses, in the form of a cylinder, spools, bundles of threads, etc., are installed along the axis of the resonator, and samples in the form of thin flat disks are placed perpendicular to the axis. The output values of the primary measuring transducer (PIP) are the changes in the resonator parameters caused by the introduction into the OR cavity of the material under study: resonance frequency Δf = ff ₀ and Q factor ΔQ = QQ ₀ , where f ₀ and Q ₀ are the values of the intrinsic (unloaded) resonator parameters.

The disadvantage of this method is that to measure the moisture content of the material, it is necessary to use a sample of a strictly defined shape and size; the sample must be placed exactly in a specific place in the OP, since the field structure of a certain type is strictly defined and uneven in the spatial cavity of the resonator, it is possible to mix up the main type of vibrations with others, which causes additional measurement error, and the use of filters reduces the quality factor of the main type of vibrations and complicates the design of the primary measuring transducer; the hardware implementation of the method is quite complicated due to the presence of valves, circulators, a detector, a mixer, a quality factor meter, a frequency meter controlled by the frequency of the microwave generator to change Δf. The considered solution cannot be applied for the operational control of the current humidity of the state of a stack of lumber in a microwave drying chamber during the drying process.

The closest in technical essence to the claimed solution is a method of drying oak billets in a microwave drying chamber [patent RU No. 2125691, IPC F26B 3/347], including stacking the blanks in a stack, microwave heating with simultaneous air blowing from the microwave cooling system -generators, before drying begins, the stack is covered with a moisture-proof film and, with a microwave generator power of 15 kW, the blanks are heated to a drying temperature of 58-60 ° C, then the film is removed and the blanks are dried to a moisture of 5 kW microwave generator 30%, when the power of the microwave generator 4 kW is dried to a moisture content of 20%, when the power of the microwave generator 3 kW is dried to a moisture content of 12%, and when the power of the microwave generator 2 kW, the workpieces are dried to the required humidity. This method is adopted as a prototype.

The disadvantage of this method is the inability to continuously measure the current moisture content of wood in the microwave chamber to control the drying process by quickly changing the humidity and temperature gradients, the absence of a moisture meter based on the primary measuring transducer (PIL) of the microwave cavity of the wood drying chamber, and stepwise implementation the choice of the generator output power is not justified and does not contribute to maintaining a uniform drying mode according to the functional dependence.

The solved problem of the invention is to improve the microwave drying process by continuously measuring and controlling wood moisture in the microwave chamber while simultaneously monitoring the drying process parameters, temperature and humidity gradients, based on the energy characteristics of the cavity of the drying chamber - Q factor and attenuation d - according to the functional dependence these parameters of the resonator and the simplification of the hardware implementation of the method.

The essence of the invention lies in the fact that in the proposed method, the humidity control of the wood is carried out continuously during the drying process. The dried lumber laid in a drying stack is placed in a closed metal cavity - a resonator, which is a forest drying chamber. A microwave electromagnetic field is excited by the generator, the dimensions of the closed metal cavity are chosen to be much larger than the wavelength λ of the microwave supply generator, so that it is possible to excite in the unloaded state many vibrations of different spatial structures with a constant power of the supply generator and a fixed interaction time of the studied wet material with many fields mode (mode is the largest value of the distribution function of a random variable); measure the temperature of the material exposed to electromagnetic energy before being placed in a closed metal cavity and the temperature after exposure to the electromagnetic field of many modes, determine the absorbed power P _damp by the calorimetric method.

Based on the energy parameters of the resonator, the Q factor and attenuation d, the reciprocal of the Q factor d = 1 / Q, we find the attenuation of the resonator based on the energy ratio

$$Q={\omega }_p\frac{W}{P_{P\sum }}$$

where ω _p is the resonant angular frequency; W is the energy stored in the oscillatory system; P _PΣ is the idle loss power in the resonator; Q is the quality factor of an unloaded resonator (idle), while the load is the moisture contained in the wood.

If the resonator is loaded, then the average power given by the resonator to the load P _N = P _damp should be added to the average power of losses in the resonator Р _ПΣ = P _Г The expression for the Q factor of a loaded resonator can be written:

$$Q_H=Q\frac{\mathrm{one}}{\mathrm{one}+P_H/P_G}$$

, где P_Н=Р_погл - мощность нагрузки, расходуемая на нагрев древесины в сушильной камере, Р_Г - мощность генератора, d - затухание ненагруженного резонатора. По приведенному соотношению для затухания нагруженного резонатора d_Н с учетом того, что находят интегральное значение средней влажности материала в сушильной камере, вычисленное по формуле для поглощенной мощности Р_погл калориметрическим способом, и поскольку влажность древесины пропорциональна потерям передаточной функции, т.е. вносимому в резонатор затуханию А_ВН при отсутствии согласования в тракте передачиaccordingly, for the attenuation of a loaded resonator: $$d_n=\frac{\mathrm{one}}{Q_n}=d(\mathrm{one}+P_N/P_G)$$

where P _Н = Р _go - the load power spent on heating wood in the drying chamber, Р _Г - generator power, d - attenuation of an unloaded resonator. According to the above relation, for damping of a loaded resonator, d _Н , taking into account the fact that the integral value of the average moisture content of the material in the drying chamber is found, calculated by the formula for the absorbed power P _{by the} calorimetric method, and since the wood moisture is proportional to the loss of the transfer function, i.e. Insertion into the resonator A damping _HV without matching in the transmission path

$$A_{\mathrm{AT}N}=\frac{P_N}{P_G}$$

The value of wood moisture W is determined by the damping function of the loaded resonator d _{N by the} calibration of the meter, given that

W% = (M-M ₀ ) / M ₀ · 100%,

where M is the mass of wet wood; M ₀ - the mass of absolutely dry wood.

Since the feature of drying wood by the standing-wave resonance method is the load mismatch with the generator due to a decrease in the power of specific load losses in the drying process and, as a result, an increase in the quality factor of the drying chamber resonator, it is necessary to reduce the generator power R _g with a given time resolution in accordance with the attenuation relation loaded resonator d _N.

To control the drying process of wood, it is necessary to carry out the drying process algorithm for the PIP attenuation function by calculating the power of the microwave generator Pr and the moisture content of the material at each discrete step with the corresponding power adjustment.

Examples of the implementation of the proposed method

Example 1. For a fixed power of the microwave supply generator (P _OUT = 15 kW), the interaction time t of the dried wood with the field of many modes in the resonance chamber is set. By measuring the temperature of the test material before being placed in a closed volume T ₁ ° C, and then the temperature T ₂ ° C of the material after interaction by the temperature difference ΔT = T ₂ -T ₁ , using the calorimetric method of measuring absorbed power

$$P_{P\mathrm{about}gl}=\frac{\mathrm{from}\nu \Delta T}{0.24t}\approx 4.18\cdot {10}^3\frac{\nu }{t}\Delta T$$

where ν is the volume of the liquid, s is the specific heat, t is the time.

For the energy parameter - attenuation of the loaded resonator d _N - we obtain a graph of the attenuation calibrated in units of wood moisture.

$$d_n=\frac{\mathrm{one}}{Qn}=d(\mathrm{one}+P_N/P_G)$$

The values are substituted into the function: the Q factor of the loaded resonator Q = 1 / d of the drying chamber at idle, we can take Q = Q _xx ≈100 (the exact value can be obtained by extrapolating the function d _n ), therefore, for the supplied magnetron power equal to 15 kW

$$d_N=\frac{\mathrm{one}}{Q_N}=W=0.01(\mathrm{one}+P_N/\mathrm{fifteen})\cdot \mathrm{one\; hundred}\%$$

Here (P _N = P _sweep ) is a variable, and R _G is a parameter that decreases during the drying of the material in accordance with a change in humidity.

Calibration characteristic (PIP) for the moisture meter after calibrating the attenuation of the cavity of the drying chamber according to the formula

W% = (M-M ₀ ) / M ₀ · 100% is shown in Figure 1.

For example, with a step of measuring humidity of 10%, we obtain a characteristic when adjusting power in the process of monitoring the drying process, FIG. 2.

Example 2. With the summed up power of the generator P ₁ = 1 kW at an initial wood moisture content of 100%, using the colorimetric method for measuring the absorbed power, for the energy parameter - attenuation of the loaded resonator d _N - we obtain a graph of the attenuation calibrated in units of humidity, Fig.3. The process is identical to example 1.

$$d_n=\frac{\mathrm{one}}{Q_N}=d(\mathrm{one}+P_{P\mathrm{ABOUT}GL}/\mathrm{one})$$

Then, in the drying process at a step of measuring humidity of 10%, the parameter, which is the summed up power of the generator P ₁ , takes values of 1.0, respectively; 0.9; 0.8; 0.7; 0.6; 0.5; 0.4; 0.3; 0.2; 0.1. We obtain a characteristic of the wood drying process for the output power of the drying unit one kilowatt, calibrated at 100% humidity at maximum absorbed power, Figure 4.

Thus, the claimed technical solution allows a method of controlling the drying of wood in a microwave resonator-type microwave forest drying chamber by the resonant standing wave method and to ensure high quality of dried wood of different species.

Claims (1)

The method of drying wood in a microwave drying chamber by the resonance method, namely, that a stack of lumber is placed in a drying chamber made in the form of a closed metal cavity, which is a resonator, characterized in that the drying process is controlled by measuring the initial moisture and temperature of the wood and temperature after the interaction of the electromagnetic field of many modes, and by the temperature difference, the absorbed power P _{N is} determined by the calorimetric method; according to the functional dependence of the attenuation of the loaded resonator of the drying chamber $$d_H=\frac{\mathrm{one}}{Q_H}=d(\mathrm{one}+P_N/P_G)$$

determine the moisture content of wood, pre-calibrating the meter, taking the value of P _N as a variable, and control the drying process of wood according to moisture and temperature gradients by adjusting the output power of the generator P _G , which in the functional dependence of the attenuation of the loaded resonator d _H is a variable parameter, calculating the humidity material at each discrete step, where d _H is the attenuation of the loaded resonator, Q _H is the Q factor of the loaded resonator, d is the attenuation of the unloaded resonator, P _H is the power absorbed by the material, P _G is the power of the generator.

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