RU2210874C2 - Plant for heating wood and other dielectric materials in the field of high-frequency currents (alternatives) - Google Patents

Abstract

FIELD: heating wood and other dielectric materials in high-frequency field. SUBSTANCE: novelty is that proposed plant that may be used in various processes such as heating billets of small sectional area at binding ends, heating joints when assembling parts, and the like and does not produce radio noise beyond operating frequency has its electrodes which are not part of generator resonance system. Plant has standard output resistance generator, standing-wave ratio meter, coaxial transmission line, and one or more matching devices each incorporating respective electrodes. Generator has fixed frequency whose value is calculated for each particular plant. Standing-wave ratio meter enables matching check-up at all sections of circuit. Matching device has parallel oscillatory circuit affording desired voltage level across electrodes and inductance coil which forms low-frequency filter together with electrode capacitors. Low-frequency filter functions to compensate for reduction in peak frequency clipping capacitance of filter at the same time maintaining resonant frequency of tuned circuit and ensures matching of varying load resistance due to raising its Q-factor. EFFECT: reduced cost and maintenance charges of plant in proportion to generator power; enlarged functional capabilities; enhanced efficiency and frequency stability. 3 cl, 8 dwg

Description

 The invention relates to devices for dielectric heating of wood parts or materials similar in electrical characteristics when gluing, drying, heating to increase ductility during bending of workpieces and other technological operations.

 The prior art.

Existing installations consist of a high-frequency generator G, a single-wire or symmetric feeder F and electrodes (working capacitor) Crab, between which the heated medium is located. In installations with an unbalanced output, a low-potential electrode is usually connected to the body (mass) of the installation. The general installation diagram is shown in figure 1. Generator circuits may differ, however, the device design for various purposes (drying, heating adhesive joints, heating to increase ductility, etc.) does not have fundamental differences. Existing installations and processes occurring in the field of high-frequency currents, which are analogues of the proposed installation, are described in the following sources:
1. Biryukov V.A. "Processes of dielectric heating and drying of wood." Goslesbumizdat, 1961

 2. Vostrov V. N. "Electrotechnology in woodworking". Moscow, Forestry, 1981

 3. Yu.G. Doronin, S.N. Miroshnichenko, M.M. Svitkina "Synthetic resins in woodworking." Moscow, Forestry, 1987

 4. A.M. Borovikov, B.N. Ugolev "Handbook of wood". Moscow, Forestry, 1989

 5. I.V. Krechetov "Drying of wood". Moscow, Forestry, 1980

 6. Patent RU 2073314 C1.

 Generators are built on powerful triodes according to a self-excitation scheme.

 To describe the operation of the devices, the load on the circuit is conditionally divided into electrical elements Crab and Rn, but it is a single structural element.

 Regardless of the circuit, the electrodes between which the heated medium is located are the Srab capacitor, which is part of the resonant system of the generator. The feeder connecting the generator to the electrodes has a length much shorter than the wavelength, since the generator is usually located in close proximity to the electrodes, and it does not affect the operation of the circuits.

 Currently, the circuit of switching on the resonant circuits and the coupling with the load, shown in Fig. 2, is used more often than others, which is a P-circuit that allows you to automatically transform Rg into Zg depending on the ratio of reactance Sk and Crab and does not require restructuring of communication with load when changing Crab, in contrast to other schemes described in [1]. Other circuits do not have this property. The excitation elements of the generator are not of fundamental importance and are replaced on the circuits by a conventional circuit.

 The active load of the generator Rн is the sum of losses in the capacitor Crab.

 Such schemes make it possible to obtain resonance in the anode circuit irrespective of the capacitance of the Srab electrodes due to a change in the frequency of the generator and the maximum possible RF voltage at the electrodes, however, they have a number of fundamental disadvantages regardless of the specific generator circuit.

 Installations have a low coefficient of performance (COP), due to the following reasons.

To obtain the maximum efficiency of the generator, it is required that it be loaded at the optimum resistance Rn equal to the output resistance of the lamp Rg, taking into account the transformation coefficient of the anode circuit. The output resistance of the generator lamp is determined by the formula
Rv = k ₀ Ua / Ia (1)
where Rv is the output resistance of the generator lamp;
Ua is the anode voltage at the output stage;
Ia is the current of the output stage;
k ₀ = 0.5-0.9 coefficient of use of the anode current, depending on the circuit of the generator.

 Generator lamps used in the described installations have an output resistance of Rv 0.5-2 kilo-ohms depending on the power. In all the circuits shown in FIG. 2a-c, the output circuit of the generator transforms the output resistance of the lamp, while Crab due to the large area of the electrodes, as a rule, is much larger than Ck, and the output resistance of the generator Zg is much lower than Rv and is a random variable. Zg can be from several tens of ohms to 200-300 ohms.

 The load resistance Rn is complex and, as a rule, significantly exceeds the output resistance of the generator.

 When heating the adhesive joints, the load is two components - the electrical conductivity of the adhesive layer and wood, due to the movement of free charge carriers, and dielectric losses due to the energy spent on changing the polarization of the molecules of the substance.

The power loss Re associated with electrical conductivity and released in the load is practically independent of frequency and is equal to
Pe = U ² / Re (2)
where U is the RF voltage at the electrodes;
Re is the resistance of the medium between the electrodes.

The dielectric loss, the value of which is determined by the power loss Rd, is
Rd = 2πfε ₀ ε _r StgδU ² / d (3)
where f is the current frequency;
ε ₀ is the dielectric constant;
ε _r is the dielectric constant of the material;
tanδ is the dielectric loss tangent;
S is the area of the electrodes;
d is the distance between the electrodes;
U - RF voltage at the electrodes.

 The main load is adhesive layers, which have increased electrical conductivity and dielectric loss compared to wood, since dry wood has a very high resistance and low dielectric loss. When heated, the adhesive resistance increases very quickly and becomes comparable to the electrical conductivity and dielectric loss in wood. The resistance measurements of adhesive joints based on urea-formaldehyde resins of the KFZh and KF-NFP brands, which are widely used for gluing wood, were carried out.

где k₀ и Ua - то же, что и в формуле 1;
К_тр - коэффициент трансформации анодного контура.The nature of the change in the resistance Re of the adhesive joint samples heated in the HDTV field, depending on the heating time, is shown in FIG. 3. The resistance of the adhesive joint increases several tens of times. Since the losses depend on the square of the RF voltage at the electrodes (see files 2, 3), they tend to increase it. HF voltage depends on the anode and is equal to

where k ₀ and Ua is the same as in formula 1;
To _Tr - the transformation coefficient of the anode circuit.

Since Ktr and k _{0 are} less than unity, the RF voltage at the rated lamp current does not exceed 0.5-0.6 of the anode one, high-power lamps with a high voltage of the anode supply, often more than 10 kV, are used in installations, regardless of the power in the load. The RF voltage at the electrodes cannot be higher than the breakdown voltage, so it is impossible to increase losses in the medium only by increasing the voltage. Permissible field strength is 0.2-1.2 kV / cm and depends on the electrical properties of the adhesive.

 The power of dielectric losses Rd in dry wood at frequencies below 20 MHz is small even when the voltage at the electrodes is close to breakdown. It begins to affect heating only at frequencies above 30 MHz. The dielectric loss in the adhesive layer during heating also decreases mainly due to a decrease in the dielectric constant during the evaporation of water. It is possible to increase them, as follows from formula 3, by increasing the frequency f, however, in existing installations, the electrodes have a large intrinsic capacitance, which reaches several thousand picofarads. Powerful electron tubes also have large interelectrode capacitances. Since these capacitances are elements of the frequency-setting chain of generators, the frequency is low and, as a rule, does not exceed 13.56 megahertz.

 The total loss resistance Rn = Re + Rd increases during the curing of the adhesive 10-100 times depending on the properties of the adhesive and frequency. Even if the total loss resistance Rn in the glued product in the cold state is close to the output resistance of the generator Zg, then within a few seconds even before the glue has set, it increases tens of times and the power absorbed in the medium drops sharply, which leads to a decrease in the heating rate of the adhesive joint and with the balance of absorbed power and heat loss to the cessation of heating (see [3] p. 114, Fig. 56).

In wood drying plants, when the humidity changes from natural (60-80%) to operational (6-12%), the resistance increases by several orders of magnitude, the dielectric constant ε _r due to water removal decreases by 20 or more times, which leads to a corresponding decrease power absorbed in heated wood (see f-l 2 and 3).

 In the installations used, the output resistance of the generator Zg is not regulated during the cycle when the load changes over large limits, which leads to an increase in SWR and a decrease in the efficiency of the installation, since the power consumption and output of the generator remains constant. The energy that is not absorbed in the heated medium is reflected and dissipated at the anode of the lamp, which explains the extremely low efficiency of existing RF installations and their unreasonably high powers. They do not even have the simplest instruments for monitoring the coordination of resistances (SWR meters), common, for example, in radio transmitting devices.

Thermal calculations of heating adhesive joints and experiments with the proposed installation show that the required power for heating the adhesive joint before setting does not exceed 1-1.5 W / cm ² , the actual specific power of RF installations is several times (often tens of times) higher than the given values . The efficiency of existing HF installations for heating does not exceed 5%, taking into account the efficiency of the generator.

 Another serious disadvantage is the instability of the frequency and the high level of radio interference. For industrial use, a number of frequencies are assigned with a maximum deviation of not more than ± 1%. Since the capacitance of the Crab electrodes and the load resistance Rн are elements of the frequency-setting circuit of the generators, a frequency change occurs over large limits. For this reason, no existing RF installation meets the requirements of frequency stability standards.

 Generator lamps operate in non-linear mode due to load mismatch and high grid currents. The simplest circuit of the resonant circuits of the generators does not provide filtering of harmonics. Since the power of the units is tens and hundreds of kilowatts, they create a high level of radio noise in a wide range of frequencies, despite special measures (shielding generators, placing units in shielded rooms, etc.), which creates serious problems for radio communications that may currently have power transmitting units of a watt unit, television, other electronic equipment sensitive to electromagnetic radiation.

 SUMMARY OF THE INVENTION

 A variant of the proposed installation for heating, free from the above disadvantages, is shown in Fig.4.

Since the design of the generator does not matter in the proposed installation, the closest analogue (prototype) is the total amount of installations for high-frequency heating of dielectrics, described in:
Biryukov V.A. "Processes of dielectric heating and drying of wood." Roslesbumizdat Publishing House, 1961, p. 31, Fig. 12., A simplified functional analogue of which is shown in Fig. 1, and to analyze the operation of the device, the generator circuit shown in p. 37, Fig. 17, whose functional analogue is used depicted in figure 2.

The main differences of the installation from analogues are:
- the installation consists of a generator G with a fixed frequency, the value of which is calculated for a particular device depending on the capacitance of the electrodes and the load resistance, and is not random; generator power is determined by the necessary heat power;
- the generator has a standard output impedance (50 or 75 Ohms) for matching with a coaxial transmission line;
- to control the coordination of the load with the generator contains a standing wave coefficient meter (SWR meter), which is connected between the generator and the transmission line;
- high-frequency energy from the generator to the electrodes is supplied using a coaxial transmission line of arbitrary length and a matching device consisting of a communication circuit Lсв Ссв, a parallel oscillatory circuit Lк Ск, a low-pass filter formed by a coil Lfnch and a capacitance of the working capacitor (electrodes) Crab, the load of which is the sum of losses in the heated medium;
- with large sizes of parts or for heating several sections remote from each other, the installation contains several matching devices, each of which has its own electrodes. Power matching devices is produced from a single generator using the agreed transmission lines. If it is necessary to use several electrodes, analogues use separate generators for each electrodes.

 Installation works as follows.

 The required level of RF voltage is achieved using a parallel oscillating circuit Lк Sk with high quality factor tuned to the operating frequency of the generator. Coordination of the circuit with the transmission line is carried out using the communication circuit Lсв Ссв. At resonance, the voltage across the capacitor Ck will be maximum, while the higher the quality factor of the circuit, the higher the voltage. Due to this, the RF voltage in the circuit depends on its quality factor and on the power of the generator, and not on the voltage of the anode and the design of the generator, as in analogues.

 The circuit is a transformer of resistances in a wide range (from zero at the “cold” end to tens of kilo-ohms at the “hot”). This makes it possible to find point A on the loop coil Lк, where the wave resistance is equal to the load resistance Rн at the beginning of the cycle, i.e. coordinate the output impedance of the generator or transmission line with the load Rн. Resistance matching is monitored using an SWR meter. When matching the communication loop with the transmission line and the parallel oscillatory circuit with the load resistance, all the generator energy will be absorbed by the load and there will be no standing wave, i.e. SWR will be equal to one.

 Since during heating there is a change in the capacitance of Crab and an increase in Rн, when the electrodes are directly connected to the circuit, its resonant frequency will change and there will be a mismatch with the circuit. To maintain the resonant frequency, the working electrodes are connected to the circuit through the inductance Lfnch, which, with the capacitance of the electrodes Srab, forms a low-pass filter (LPF), in which the cutoff frequency is equal to or slightly higher than the frequency of the generator.

 As is known, the input impedance of the low-pass filter is equal to the load resistance, i.e., the voltage transfer coefficient is unity at any frequency from zero to the filter cutoff frequency. The cutoff frequency of the filter is selected so that when the capacitance of the working capacitor changes due to heating or resizing of parts, the cutoff of the filter is always at or above the generator frequency. This allows you to save the circuit setting and the transfer coefficient when changing the capacitance of the electrodes Srab. In analogs, a change in Crab leads to a change in the frequency of the generator or to a detuning of the circuit of the output stage.

The second function of the low-pass filter is to maintain coordination of the loop resistance with the load Rн when it increases during the cycle. The low-pass filter is a sequential circuit in which the quality factor is equal to unity when the load resistance is equal to the reactance of the filter elements. With an increase in load resistance Rн, the quality factor of the circuit Lfnch Srab increases and due to this, the voltage at the electrodes increases while maintaining the power dissipated in the medium. The input filter resistance at point A does not change, since the reactance X _{L of the} inductance remains constant. This allows you to maintain coordination with the circuit and give all the generator power to the load during the cycle with increasing load resistance.

Since the resistance Rн and the capacitance of the Crab electrodes can be measured for a particular installation, from the known relations for calculating the low-pass filter
Rн = Х _с = X _L , Х _с = 1 / 2πfС, X _L = 27πfL (5)
where X _s , X _Ll is the reactance of the capacitance and inductance of the filter;
C, L - capacitance and inductance of the filter elements, respectively;
f is the cutoff frequency of the filter.

где R_кон, R_нач - сопротивление нагрузки в конце и начале цикла нагрева соответственно.The generator frequency f is calculated at which the load is fully coordinated with the circuit at the beginning of the heating cycle:
f = 1 / 2πSrabRn (6)
and inductance Lfnch matching device
Lfnch = Rн / 2πf (7)
An increase in voltage at the electrodes with an increase in Rn is possible with the Q factor of the circuit Q equal to

where R _con , R _beg - load resistance at the end and beginning of the heating cycle, respectively.

The principle of operation of the matching device in various phases of the cycle is shown in figure 5
Curve 1. At the moment the generator is turned on, the load resistance is minimal. The circuit Lfnch Srab together with the load resistance has a quality factor and a voltage transfer coefficient equal to unity.

 Curve 2. When the adhesive joint is heated, the loss resistance due to heating, polymerisation of the resin, etc. increases, which leads to an increase in the quality factor of the circuit Lfnch Srab, the voltage transfer coefficient at the operating frequency increases, while the power dissipated in the load remains constant and equal oscillatory power of the generator.

 Curve 3. With a further increase in the resistance Rн, the quality factor increases even more and so on until the end of the process (complete polymerization of the adhesive).

 Curve 4. With a decrease in the capacitance of the working capacitor due to evaporation of water and heating, the resonant frequency of the series circuit increases slightly at the end of the cycle, but the voltage level at the electrodes decreases slightly due to the fact that the operating point is on the gentle left slope of the frequency response. Since at the end of the glue polymerization process the heat losses associated with the evaporation of moisture are significantly reduced, a slight decrease in the absorbed power no longer affects the process.

 Since a number of frequencies are reserved for industrial purposes, the one closest to the calculated one is selected. After selecting the standard frequency of the generator, Lfnch is recalculated according to formula 7 and the circuit elements Lk Sk are calculated.

 When using the proposed installation for heating parts of a small cross-section, such as bars, the electric field near the low-potential electrode, which is usually used as the installation casing ("mass"), will be much smaller than that of the high-potential due to scattering, which leads to uneven heating. In order for the RF field in the working area to be uniform, the symmetric circuit shown in Fig.6 is used. This also allows to halve the voltage relative to the housing and significantly reduce the level of even harmonics of the generator.

 Schemes 4 and 6 do not have fundamental differences in the operation; they differ only in structural elements and the position of the point of zero potential. In the symmetric version of the matching device, the coupling loop coil is located in the middle of the loop coil, a differential capacitor of variable capacitance (butterfly) is used, and the low-pass filter coil consists of two identical parts, the total inductance of which is equal to the inductance of the filter coil in an asymmetric version. LPF coils are connected to the oscillatory circuit symmetrically with respect to the point with zero potential of the coil.

 With a small area of electrodes (for example, with butt gluing of bars), the capacitance of Crab can be only a few picofarads. This allows the use of frequencies above 80 MHz. At these frequencies, it is difficult to perform a circuit with high quality factor on lumped elements due to their small size, and a decrease in frequency will lead to a decrease in the efficiency of the installation.

 In this case, instead of the circuit Lk, Ck, a resonator is used on a quarter-wavelength short-circuited segment of the coaxial line for the circuit of Fig. 4 or a symmetrical line for the circuit of Fig. 6, which are analogs of a parallel oscillatory circuit, but have significantly large dimensions and a very high Q factor, which allows to the voltage on the electrodes is 2000-4000 volts with a generator power of only a few tens of watts.

 A diagram of the matching devices on the resonators is shown in Fig.7.

All variants of the design of the matching device are full functional analogues and provide the same technical result for different conditions of application of the installation and contain the same functional elements, namely:
- a communication loop, which ensures coordination of the parallel oscillatory circuit with the transmission line;
- a parallel oscillatory circuit with autotransformer coupling, which serves to obtain the required level of high-frequency voltage and to coordinate the resistance to losses in the load;
- low-pass filter, which ensures the conservation of the resonant frequency of the circuit and its output impedance when changing load parameters.

 With the sizes of the working electrodes comparable to 1/8 of the wavelength, due to wave processes, the voltage on them will be uneven. Power supply of the load using a coaxial transmission line allows reducing the size of the electrodes n times, using the inclusion of several matching devices from one generator, each of which has its own electrodes. Since coaxial transmission lines have small losses, their length can be significant. This makes it possible, in addition to heating large-sized parts, to use one generator to heat several joints located at a considerable distance from each other, for example, when assembling frame structures or to increase the heating zone when continuously moving long material. Coordination of branched transmission lines with a common feeder (or directly with the generator) is carried out using any known resistance transformers and resonant power lines. Since there is no need to coordinate phases on separate electrodes in heating plants, the length of the transmission lines can be different.

 Since the area of the electrodes and the capacitance Crab of a separate matching device decreases several times, this allows a proportional increase in the frequency of the generator. Calculation of the elements of the matching device is made for one section. On Fig shows a variant of the circuit with four matching devices and electrodes using a quarter-wave resistance transformer in the form of two parallel sections of the same coaxial cable as the transmission line.

Significant differences from the analogue [6], figure 4, are:
- powered by parallel coaxial lines of individual matching devices and individual electrodes, and not one common;
- coordination of wave impedances of transmission lines at branch points with a common line using resistance transformers or (and) the use of resonant power lines;
- an arbitrary number of branches connected to one point, determined only by the convenience of matching the wave impedances of the power lines;
- transmission lines to individual matching devices may be of arbitrary length.

 Since in the proposed devices the working capacitor Srab is not an element of the generator, its parameters (frequency, output resistance) are independent of the load, unlike analogues, and it works with maximum efficiency throughout the cycle, and matching the resistances in all parts of the circuit allows you to use the whole generator power to heat the product. This allows you to increase the overall efficiency of the installation up to 50-60% and reduce the required capacity of the installations compared with analogues by several times.

 Since the cost of HF installations and their operating costs are proportional to the installed capacity, reducing the cost can significantly expand their use in technological operations where they were not previously used due to economic indicators - bonding of blanks of a small cross section along the length, heating in the joints of parts during assembly, installation bonding, etc.

 The introduction into the matching device of a circuit with high Q and a low-pass filter, in addition to the resonant system of the generator itself, the operation of the generator in optimal mode, a fixed frequency and a significant decrease in the power of the generators almost completely eliminate radio interference outside the operating frequency even without taking special measures to protect against radio interference.

 The list of figures drawings.

 Figure 1 General diagram of the installation for heating dielectrics.

 FIG. 2 Installation and generator circuit using a P-loop in the anode circuit.

 Figure 3 Change in the resistance of the sample of the adhesive joint, depending on the heating time.

 FIG. 4 Schematic diagram of the proposed installation for cyclic heating of dielectrics.

 FIG. 5 The amplitude-frequency characteristics of the matching device in various phases of heating the adhesive joint.

 FIG. 6 Schematic diagram of a matching device for symmetrical inclusion of electrodes.

 FIG. 7 Resonant matching systems on line segments for high frequencies.

 FIG. 8 Schematic diagram of the installation with the parallel inclusion of four matching devices for large sizes of heated parts.

 The possibility of carrying out the invention.

 The implementation of the invention does not cause difficulties. The generator and matching devices are assembled from commercially available industry radio components and materials. Reducing the power of plants greatly simplifies the design and reduces the size and cost of devices. Powerful generator lamps used in existing installations and resonant systems, as a rule, have water or evaporative and air cooling. In most cases, when the power of the generators is up to 20 kW, the proposed devices have enough air forced or free cooling, which is much simpler. Generators used in installations for heating small cross-section of compounds with power up to 1 kW can be performed on modern semiconductor devices, the dimensions and cost of which are much lower than that of tube devices.

 The experimental setup made according to the symmetrical scheme (Fig. 6), designed to heat the stud joint when gluing the bars cross section 60 • 100 mm in length, fully confirmed the calculations. The installation has the following characteristics: oscillatory power of the generator 250-300 watts. SWR, measured between the generator (output impedance of 50 Ohms) and the matching device, not more than 1.5 throughout the cycle. The overall efficiency of the installation is more than 50%. The curing time of the adhesive joint is not more than 15 s, which is significantly less than in installations of a similar purpose. The specific power required for heating the adhesive layer before curing for the CFF resin is 1.6-1.8 W / sq.cm versus 12-20 for analogues. The scheme allows, due to a certain decrease in efficiency, to use the installation while reducing the dimensions of the workpieces by section to 2 times without changing the structural elements and settings, in addition to adjusting the generator power depending on the bonding area and heating rate. Excessive power input leads to overheating of the adhesive joint and to breakdown, since even with a slight generator power the voltage on the electrodes reaches several thousand volts. For the same installation, a variant of a generator using high-power transistors KT970A with a collector supply voltage of only 40 volts was tested. Similar results are obtained.

Claims (3)

 1. Installation for heating parts of wood and other dielectrics in the electric field of high-frequency currents, containing a generator with a constant standard output impedance, consistent with the wave impedance of the coaxial transmission line, and a fixed frequency, the value of which is determined by the capacitance of the electrodes and the resistance to load losses, different the fact that the generator output is connected through a standing wave coefficient meter and a coaxial transmission line with a matching device consisting of a loop ides formed by a coupling coil and a series capacitor of variable capacitance for adjusting a coupling circuit inductively coupled to a coupling circuit of an asymmetric or symmetrical parallel oscillatory circuit consisting of a coil and a capacitor of variable capacitance connected to a coil coil circuit for an asymmetrical circuit, to the second end of which is connected high potential electrode, and the second electrode is connected to a common wire, or two coils for a symmetrical circuit, to the second ends of the cat ryh connected working electrodes, the capacitance of which together with the inductance coil form a lowpass filter.  2. Installation for heating parts made of wood and other dielectrics in the electric field of high-frequency currents, containing a generator with a constant standard output impedance, consistent with the wave impedance of the coaxial transmission line, and a fixed frequency, the value of which is determined by the capacitance of the electrodes and the resistance to load losses, different the fact that the generator output is connected through a standing wave coefficient meter and a coaxial transmission line with a matching device consisting of a communication line and a series-connected capacitor of variable capacitance for tuning a communication line inductively coupled to a communication line of a short-circuited quarter-wave resonator on a segment of a coaxial or symmetric line, with a capacitor of variable capacitance for tuning a resonator connected to a resonator on a segment of a coaxial line of a coil, to the second end of which a high-potential electrode is connected , and the second electrode is connected to a common wire, or two coils for the resonator on a symmetrical line, to the second ends of which x connected working electrodes, the capacitance of which together with the inductance coil form a lowpass filter.  3. Installation according to claim 1 or 2 for heating long products or for simultaneously heating several zones located at a distance from each other, characterized in that it contains several matching devices, each of which serves to heat a separate section, the coaxial transmission lines of which are connected parallel and consistent with the impedance of the common transmission line using a resistance transformer on quarter-wave segments of the coaxial line.

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