RU2613307C1 - Method of permanent humidity control of timber in drying chamber - Google Patents

Abstract

FIELD: heating; drying.

SUBSTANCE: piled timber on a pile cart with strain gauges is placed into a hermetical drying chamber between electrodes connected to a high-frequency generator. The gauges are connected to a weight measuring system. Values of the set parameters of timber and required residual pressure are input into a computing unit. Before vacuuming, the initial pressure in the chamber is measured, then, a vacuum pump is turned on and the mass of removed air is measured. After reaching the required residual pressure, the pump is turned off and the temperature and humidity of the medium are measured. Following the measured magnitudes, the computing unit defines the volume of timber at the bone-dry humidity. After that, the high-frequency generator is turned on and the process of drying is intensified by means of the dielectric heating of timber. During the process of drying, the current pile mass is measured constantly or frequently and the current humidity of timber is defined. Before the loading into the chamber, samples are taken from the pile; these samples are used for definition of properties of timber according to the prescribed standards; the pile is stored on the premises, where the chamber is installed, at the temperature of 10-20°C and relative air humidity of 70-90%. Directly before the loading, the initial humidity is measured, and after the loading, the previously measured properties of timber, values of the initial humidity and required residual pressure are input into the computing unit. Whereupon, the pressure inside the chamber is reduced down to 8-10 kPa and the dielectric heating is performed reaching the temperature of 45-60°C, which is kept constant by means of adjusting of the power of the high-frequency generator during the entire process of drying.

EFFECT: increasing of measurement accuracy of the current humidity of timber, as well as increasing of effectiveness of humidity control during the process of drying.

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Description

The invention is based on weighted methods for determining moisture and can be used for continuous monitoring of wood moisture during vacuum dielectric drying.

A known method of controlling the moisture content of wood during the drying process, in which, before starting drying, sections of moisture are cut out from control samples in a certain way, which are immediately weighed on a technical balance. The weighted sections are placed in an oven and dried at a temperature of 100-105 ° C, controlling their weight. The first weighing is carried out 5-6 hours after the start of drying, the subsequent weighing every 2 hours. If the last weight coincides with the previous one or differs by no more than 0.02 g, then it is taken as the weight of the absolutely dry section, and the drying is completed . The initial humidity W _H ,%, sections are calculated by the formula

where G _beg - the initial weight of the section, g;

G _cyx - absolutely dry section weight, g.

The average value of the humidity of the sections is taken as the initial humidity of the control samples from which the sections were cut. Control samples are weighed and stacked, which is then loaded into a drying chamber. During the drying process, the samples are periodically removed from the stack and weighed. The moisture content of the samples at the moment of drying is found by the formula

where G _t is the weight of the control sample at the current moment of drying, g;

G _suh.obr - absolutely dry weight of the reference sample, g;

where G _beg - the initial weight of the control sample, g;

W _beg - the initial moisture content of the control sample,%.

Control samples are weighed every day at the expected drying time of 5-10 days and every other day at a longer duration (Peych N.N., Tsarev B.S. Wood drying. A textbook for vocational schools and training workers in production. Edition. 2 -e, revised. - and additional - Moscow: Higher School, 1971, p. 128-130).

The disadvantages of this method are the need to interrupt the drying process to remove control samples, the inability to use with vacuum drying methods, the inability to automate the drying process. The impossibility of using this method of humidity control in vacuum drying methods is due to the fact that they are characterized by a short duration of the drying process, therefore, frequent depressurization of the chamber for removing control samples for weighing violates the technological regime of drying.

A known method of controlling the moisture content of wood in a drying chamber, in which the moisture is determined directly during the drying process by analyzing the change in time of the weight of the wood stack. The wood stacked in a stack in advance is loaded into the drying chamber, the weight sensors mounted on the stacker truck are connected to the weighing system located outside the chamber. Then, according to the specified mode, the drying process is carried out, during which the temperature and the degree of saturation of air humidity in the chamber and the readings of the weight sensors are monitored. The weight of the stack is determined at three to four points corresponding to different points in time of drying. The time period between measurements can be 1-10 hours. Based on the measurements, the approximate dependence of the stack weight on the drying time is calculated, the drying speed equation is determined, the final drying time and the calculated value of the absolute dry weight of the stack are calculated, humidity W _t ,%, wood at the moment time is determined by the formula

where G _t - the current weight of the stack, kg;

G _con - the calculated value of the absolutely dry weight of the stack, kg;

W _equal (t, ϕ) is the equilibrium moisture content of the wood at the given parameters of the drying agent,%,

(RF patent 2037755; IPC F26B 25/22, G01N 9/36, publ. 1995).

The disadvantage of this method is the long duration of determining the absolutely dry weight of the stack, which makes it impossible to control the moisture content of the stack at the beginning of the drying process.

There is a method in which the wood to be dried, laid in a stack, is loaded into a sealed chamber on a stacker carriage equipped with strain gauges connected to a weight measuring system, then the initial wood moisture content, density in absolutely dry state, drying coefficient and value of the required residual pressure in the chamber. Before the stage of evacuation, convective heating of the wood is carried out, after its completion, the initial values of temperature and pressure in the chamber are measured using a temperature sensor and a pressure sensor. The vacuum pump is then turned on and the mass of air removed from the chamber is measured by means of a mass flow meter. The pump is turned off when the desired residual pressure is reached in the chamber. After evacuation, the temperature and humidity of the medium in the chamber are measured. Based on the measured values in the computing unit calculate the residual mass m _OST , kg, air

where T ₁ , T ₂ - temperature of the medium before and after evacuation, K;

m _beats - mass of removed air, kg;

P ₁ - medium pressure before evacuation, Pa;

P ₂ - the required residual pressure, Pa.

The volume of loaded wood V _M , m ^{3 is} calculated by the formula

where V is the chamber volume, m ³ ;

m _ost - the mass of the remaining air, kg;

ρ _in - the density of air at a pressure of P ₂ , temperature T ₂ and humidity d, kg / m ³ .

During the drying process, the current weight of the stack is determined continuously using strain gauge sensors and a weight measuring system, continuously or at specified intervals. If the initial humidity is greater than the hygroscopicity limit (W _pg = 30%), then the current humidity W _t ,%, is determined by the formula:

where G _t - the current weight of the stack, kg;

V _M - the volume of loaded lumber, m ³ ;

ρ ₀ - density in absolutely dry condition, kg / m ³ .

k _y - volumetric shrinkage coefficient of wood, rel. units

If the initial humidity is below the hygroscopicity limit, then the current humidity W _t ,%, is determined by solving the equation

(Kachanov A.N. Korenkov D.A. Moisture control of capillary-porous building materials during vacuum processing. // Fundamental and applied problems of engineering and technology. - 2014. - No. 3. - P. 154-160).

The disadvantage of this method is the significant error due to the tabular values of the coefficient of shrinkage and density in a completely dry state, determined for a particular wood species. The noted properties for the same breed vary depending on many factors, for example, the place of growth, the age of the tree. The error also introduces the value of the hygroscopicity limit, which is assumed to be 30%, but which is also not the same for different lots of wood.

The prototype adopted a method in which the wood is stacked on a stacking trolley equipped with strain gauges connected to a weight measuring system, and placed in a sealed chamber between the electrodes connected to a high-frequency generator, then the density values are set in the absolutely dry state in the computing unit, drying coefficient and the value of the required residual pressure in the chamber. Before the evacuation step, the initial pressure in the chamber is measured by means of a pressure sensor. Then turn on the vacuum pump and using a mass flow meter measure the mass of air removed from the chamber. The pump is turned off when the desired residual pressure is reached in the chamber. After evacuation, the temperature and humidity of the medium in the chamber are measured. Based on the measured values in the computing unit calculate the residual mass m _OST , kg, air

where m _beats - mass of removed air, kg;

P ₁ - medium pressure before evacuation, Pa;

P ₂ - the required residual pressure, Pa.

Calculate the volume of loaded wood V _M , m ³ according to the formula (2)

where V is the chamber volume, m ³ ;

m _ost - the mass of the remaining air, kg;

ρ _in - the density of air, kg / m ³ .

P ₂ - medium pressure after evacuation, Pa;

T is the temperature of the medium after evacuation, ° C;

d is the relative humidity after evacuation,%.

Calculate the volume of wood in absolutely dry condition, kg / m ³ , according to the formula

where V _M is the volume of loaded wood, m ³ ;

k _y - volumetric shrinkage coefficient of wood, rel. units

After that, a high-frequency generator is turned on, the wood is heated in a dielectric manner due to relaxation-polarization phenomena, moisture begins to evaporate from the surface of the wood. In the drying process, by means of strain gauges and a weight measuring system, the current stack weight is determined continuously or at certain time intervals, and the current moisture content W _t ,%, is determined by the formula

where G _t - the current weight of the stack, kg;

V ₀ - the volume of wood in absolutely dry condition, m ³ ;

ρ ₀ - density in absolutely dry condition, kg / m ³ ,

(Kachanov A.N., Korenkov D.A. Control of wood moisture in vacuum drying chambers // Polzunovsky Bulletin. - No. 4. - Barnaul: ALSTU named after Polzunov, 2014. - 197-202 p.).

The disadvantage of this method is also a significant error due to the tabular values of the drying coefficient, the density of the wood in a completely dry state and the hygroscopicity limit, taken equal to 30%.

The objective of the method is to increase the efficiency of controlling the moisture content of wood in drying chambers by increasing the accuracy of measuring current humidity during the drying process.

This problem is solved due to the fact that in the method of continuous monitoring of wood moisture in the drying chamber, in which the wood is stacked on a stacker trolley equipped with strain gauges connected to a weight measuring system, and placed in a sealed chamber between the electrodes connected to the high-frequency generator , set in the computing unit the density values in a completely dry state, the drying coefficient and the required residual pressure in the chamber, before the stage of evacuation measure the initial value of the pressure in the chamber, then turn on the vacuum pump and measure the mass of air removed from the chamber, when the desired residual pressure is reached, the pump is turned off and the temperature and humidity of the medium in the chamber are measured, based on the measured values in the computing unit, the volume of wood in an absolutely dry state is determined, after which they turn on the high-frequency generator and begin dielectric heating of the wood to intensify the drying process, during which the current ma stacks and determine the current moisture content of the wood, according to the invention, samples are taken from the wood stack before being loaded into the drying chamber, according to the established standards, the density is absolutely dry for 4 days, the drying coefficient and the hygroscopicity limit, while the wood stack is kept in indoors at an air temperature of 10-20 ° C and a relative humidity of 70-90%, immediately before loading with a manual electrometer, measure the initial moisture content hyesins, and after loading the stack into the computing unit, in addition to the density values in an absolutely dry state, the drying coefficient and the required residual pressure, the hygroscopicity limit and initial humidity are also introduced, while the pressure in the chamber is reduced to 8-10 kPa, and dielectric heating is performed to a temperature of 45-60 ° C, which is maintained constant throughout the entire drying process by adjusting the power of the high-frequency generator.

The technical result of the proposed method is to increase the accuracy of measuring the current moisture content of the wood and the effectiveness of controlling moisture during the drying process by taking into account the physical properties of the wood, such as density in a completely dry state, drying coefficient and hygroscopicity limit of each batch fed to drying.

The drawing shows a device for implementing the method.

It includes a sealed drying chamber 1, rails 2, a stacker carriage 3. A stack of wood 4 is stacked on a stacker carriage 3. Inside the chamber 1, a stack of wood 4 is placed between two electrodes 5, one of which is connected through a high-frequency feeder 6 to a high-frequency generator 7, the other is grounded. The high frequency generator is also grounded. The stacking trolley 3 is equipped with strain gauges 8, which are connected via a plug 9 to a weight measuring system 10 located outside the chamber 1. The drying chamber 1 is connected through a check valve 11 to a vacuum pump 12, to the outlet of which a mass flow meter is connected 13. The outlet of the flow meter 13 has a direct relationship with the atmosphere. Inside the chamber 1 are a pressure sensor 14, a temperature sensor 15, an air humidity sensor 16, which are connected to a computing unit 17 located outside the chamber 1. The weight measuring system 10 and the mass flow meter 13 are also connected to the computing unit 17.

To measure the weight of the stack 4 can be used, for example, strain gauges M50 (strain gauges made of stainless steel membrane type. Brochure No. 608. Http://www.tenso-m.ru). As the weight measuring system 10, for example, a weight transducer TV-006C can be used (Catalog “Weight transducers”, 2nd edition, brochure No. 700, 2014, http://www.tenso-m.ru). To reduce the pressure in the chamber 1, for example, a VALUE VE 2100 N vacuum pump can be used (Catalog "Vane pumps", http://evtec.ru). To measure the mass of the removed air, for example, a TMU mass cryolis flowmeter can be used (Catalog "Cryolis mass flowmeters", http://www.rizur.ru). To measure the humidity of the medium in chamber 1, for example, a HIH-4000-001 humidity sensor (Humidity sensors line guide, http://sensing.honeywell.com) can be used. To measure the temperature of the medium in chamber 1, for example, a temperature sensor 700-101BAA-B00 (700 Series Platinum RTDs Temperature Sensors, http://z.compel.ru/item-pdf/ca6bd7cc7b0f8d5f5532dbba3370ed49/ps/honey~700s can be used. pdf). To measure the pressure of the medium in the chamber 1, for example, the MT100M pressure sensor (TU 4212-010-00226218-2010) can be used. A high-frequency generator can be used, for example, of type VChG8-60 / 13 (TU 16-682.140-86).

The method is implemented as follows.

Samples are taken in advance from the wood batch and the coefficient k_at volumetric shrinkage according to GOST 16483.37-88, density ρ₀ in absolutely dry condition according to GOST 16483.1-84 and limit W_pg hygroscopicity according to GOST 16483.32-77. To determine the hygroscopicity limit according to GOST 16483.32-77, 4 days are required. During this time, the wood is stacked in a stack 4 on a stacking trolley 3 and kept in the room where the chamber 1 is installed. This ensures that the wood temperature and the temperature of the medium are equal at the time of evacuation and, as a result, the temperature of the medium in chamber 1 is constant before and after evacuation due to lack of heat transfer or heat absorption on the wood surface. This simplifies the determination of the mass of the remaining air. According to the operating conditions of the high-frequency generator 7 and the condition for preventing excessive drying of the surface layers of wood, the room temperature should be in the range of 10-20 ° C, and humidity in the range of 70-90%.

After determining the drying coefficient k _y , the density ρ _{0 of the} wood in an absolutely dry state and the hygroscopicity limit W _{pg, the} initial moisture content W _{n of the} wood is measured in a stack 4 using a handheld moisture meter, then the stacking trolley 3 along the rails 2 is rolled into the chamber 1. A plug connector 9 connect the strain gauges 8 and the weight measurement system 10, after which the chamber 1 is hermetically closed. In the computing unit 17 enter the measured values of W _n , k _y , ρ ₀ , W _pg , as well as regulated by the technological mode, the value of the residual pressure P ₂ . According to general recommendations for vacuum dielectric drying, the residual pressure is selected in the range of 8-10 kPa, which corresponds to a boiling point of water of 45-60 ° C. Next, using the computing unit 17 start the measurement process. Before the evacuation of the chamber 1, the pressure P _{1 of the} medium is measured by means of a pressure sensor 14. A signal proportional to the measured pressure enters the measuring unit 17. The vacuum pump 12 is turned on, and during its operation, the mass m _beats of air removed from the chamber 1 is measured by means of a mass flow meter 13. After reaching the required residual pressure P ₂ in the chamber 1, the vacuum pump 12 is turned off and the temperature T and the humidity d of the medium in the chamber 1 are measured using a temperature sensor 15 and a humidity sensor 16 after evacuation. Signals proportional to the measured values of m _beats , T and d, are received in the computing unit 17. In block 17 calculate the mass of air remaining in the chamber 1 m _ost , kg, by the formula (3)

where P ₁ is the pressure of the medium before evacuation, Pa;

P ₂ - the required residual pressure, Pa;

m _beats - mass of removed air, kg.

The use of formula (3) instead of (1) is possible due to exposure of a stack of 4 wood in one room with an installed drying chamber 1. Next, calculate the volume of loaded lumber V _M , m ³ , by the formula (2)

where V is the volume of the vacuum chamber, m ³ ;

m _ost - the mass of the remaining air, kg;

ρ _in - the density of air at a pressure of P ₂ , temperature T and humidity d, kg / m ³ .

If the initial moisture content W _n lumber above the limit of hygroscopicity W _pg , calculate the volume of the stack in a completely dry state V ₀ , m ³ according to the formula

where V _M is the volume of loaded lumber, m;

W _{P. g} - hygroscopicity limit,%.

k _y - volumetric shrinkage coefficient of wood, rel. units

If the initial humidity W _{n is} below the hygroscopicity limit W _pg , then the volume of the stack in a completely dry state V ₀ , m ³ is calculated by the formula:

where V _M is the volume of loaded lumber, m ³ ;

k _y - volumetric shrinkage coefficient of wood, rel. units

W _n - the initial moisture content of lumber,%.

After determining the volume in a completely dry state, dielectric heating of the wood is carried out. For this, a high-frequency generator 7 is turned on and high-frequency voltage is supplied to the electrodes 5 via a high-frequency feeder 6. In stack 4, an alternating electromagnetic field is formed, which, due to relaxation-polarization phenomena, heats the wood and intensifies the drying process. The wood is heated to a temperature of 45-60 ° C, corresponding to the boiling of water at a given residual pressure. The temperature is maintained the same throughout the drying by adjusting the power of the high-frequency generator 7. During the drying process, using the weight measuring system 10, the current stack mass m _{t is} continuously measured. The moisture content of wood W,%, at the moment of the drying process is calculated by the formula

where m _t is the current mass of the stack, kg;

V ₀ - the volume of the stack in a completely dry state, m ³ ;

ρ ₀ is the density of wood in an absolutely dry state, kg / m ³ .

Claims (1)

A method of continuously monitoring the moisture in a wood in a drying chamber, in which the wood is stacked on a stacker carriage equipped with strain gauges connected to a weight measuring system, and placed in a sealed chamber between the electrodes connected to a high-frequency generator, set the density of the wood to absolutely dry state, drying coefficient and the value of the required residual pressure in the chamber, before the stage of evacuation measure the initial pressure in Hereinafter, the vacuum pump is turned on and the mass of air removed from the chamber is measured, when the required residual pressure is reached, the pump is turned off and the temperature and humidity of the medium in the chamber are measured, based on the measured values in the computing unit, the volume of wood in absolutely dry state is determined, and then the high-frequency generator is turned on and dielectric heating of the wood is started to intensify the drying process, during which the current mass of the stack is continuously or periodically measured and the current humidity is determined l wood, characterized in that before loading into the drying chamber, samples are taken from the wood stack, according to which, according to the established standards, the density in absolutely dry state, drying coefficient and hygroscopicity limit are determined for 4 days, at the same time, the wood stack is kept in a room at a temperature of air 10-20 ° C and relative humidity of 70-90%, immediately before loading with a manual electric moisture meter, measure the initial moisture of the wood, and after loading the stack into the dryer the chamber in the computing unit, in addition to the density values in an absolutely dry state, the drying coefficient and the required residual pressure, also the hygroscopicity limit and initial humidity are introduced, while the pressure in the chamber is reduced to 8-10 kPa, and dielectric heating is carried out to a temperature of 45-60 ° C, which is maintained constant throughout the drying process by adjusting the power of the high-frequency generator.

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