Several problems in the design of industrial microwave equipment

Published: 12/02/25 This article has been viewed 185 times

   
    It is well known that the biggest difference between microwave ovens for industrial microwave equipment (except traveling wave heaters) and domestic microwave ovens is that the microwave power of the two microwave ovens differ greatly from the volume of the furnace chamber. Generally, the volume of a domestic microwave oven is generally between 15 and 30 liters, and that in Europe and the United States is generally between 25 and 40 liters, while the volume of an industrial microwave oven is above 500 liters, and sometimes even tens of thousands of liters. Therefore, there are many different considerations when designing such large-volume industrial microwave ovens, but they can be summarized as follows:

    1 , having a sufficiently high microwave power density;

    2 , with enough number of modes to ensure the uniformity of microwave field intensity distribution in the furnace;

    3. It has a good coupling with the microwave source to ensure a sufficiently high coupling efficiency;

    4. Avoid high frequency breakdown caused by high water vapor in the furnace, especially near the coupling port;

    5 , with a good anti-leakage device to ensure the safety of personnel in the operating position.

    Focus on 2 , 3 , 4 issues

    First, the design of multi-cavity

     Generally, due to the commonality of the microwave wavelength and the geometrical size of the object, the microwave single-mode cavity in the S -band has a small geometric size and a small volume. This small-volume cavity is not only of no use in industrial heating, even in There is no use value in household microwave ovens. This has determined that both domestic microwave ovens and industrial microwave ovens have to use over-mode multimode electromagnetic resonant cavities. From the theory of physics and electromagnetism, it is known that in any resonant cavity, in the over-mode state, there may be The number of resonant modes is proportional to the volume of the cavity. In other words, the larger the volume, the more the number of modes that may exist, and the uniformity of the distribution of the microwave electric field in the cavity is proportional to the number of modes. That is why people always want to design a cavity with a large cavity to improve the uniformity of the electric field in the cavity. However, the one-sided pursuit of increasing the volume will reduce the power density of the microwave power under certain conditions. Of course, from the law of conservation of energy, as long as the volume is increased, the metal boundary around the cavity does not increase the loss (such as using ideal or close Ideal for metal materials), even if the power density is reduced, it will not have a significant effect on the heating effect. However, in reality, the conductivity of any metal material is limited, especially stainless steel, and its loss is aluminum. 25 times, is 41 times that of copper, is 44 times that of silver, so as stainless steel as the material of the cavity, while increasing the volume has increased significantly and the loss of the whole area of cavity wall, in this case by using The bulky approach is only correct if the microwave input power is increased at the same time. This is the reason why 1kW~1.6kW magnetrons are used in large-volume household microwave ovens exported from the market. In industrial microwave ovens, the volume of the furnace chamber is usually between 500 and 20000 liters due to the volume factor of the heated material . Sometimes even bigger. In such a large cavity, such as the microwave magnetron of the 915MHz or 2450MHz band as the microwave source, it is obviously working in a large " overmodulation " state, so the number of modes that may exist therein is very large. In this case, as long as the power density is sufficient, the uniformity of the field distribution in the cavity is ideal. The no-load quality factor of the furnace chamber is very high, but once the material to be heated (especially the material with high water content) is added, the load quality factor of the furnace chamber drops sharply, and the degree of decline can reach 3 to 5 An order of magnitude, sometimes not even resonant, a typical high- Q resonant system becomes a low- Q microwave irradiation system.

     The second factor is the shape and volume of the material to be heated. The choice of furnace cavity size, especially the cross-sectional dimension, is mainly based on the volume and shape requirements of the material. The choice of the length direction should take into account the microwave power and the processing yield. Claim.

     The third factor is that the volume of the furnace chamber should be selected according to the water content of the material to be heated, especially the dimension in the length direction. The length of the water content can be shorter, otherwise it should be longer.

     The fourth factor is based on the number of feed ports of the microwave source, whether it is a single feed port or a multi feed port, whether it is a single microwave source or a plurality of microwave sources (especially a plurality of microwave sources independent of each other).

     The fifth factor is the magnitude of the power density in the cavity. Too high a microwave power density will result in the following two effects.

   ( 1 ) dielectric breakdown of air or gas-vapor mixture in the furnace;

   ( 2 ) Excessive internal stress causes damage to the material being heated.

     In summary, when designing a multi-cavity cavity, we should select reasonable dimensions in theory, especially experimentally, according to the constraints of many mutual constraints, in order to achieve high output with good uniform distribution and high efficiency for safe operation. Industrial microwave oven.

     Finally, it should be pointed out that in industrial microwave ovens, due to the large power, a three-phase full-wave rectification and non-filtering power supply system is usually used. This DC power supply greatly reduces the current ripple of the magnetron, so there is no such thing as a household microwave oven. The multi-frequency output phenomenon of the microwave source caused by the half-wave double-voltage DC power supply. On the other hand, in the industrial microwave oven, a looper is usually connected between the microwave source output and the furnace chamber, so that the load change does not generate frequency and power to the microwave source. Traction phenomenon. These two factors make the performance of the magnetron used in industrial microwave ovens relatively stable.

    Second, the coupling of the microwave source and the cavity

     The coupling of the microwave source to the furnace cavity is another important issue in the design of industrial microwave ovens. Usually the basic requirement for this coupling is;

   ( 1 ) The furnace cavity and the microwave source have a good (loading) matching, so that the power of the microwave source is fed into the cavity without reflection:

   ( 2 ) The coupling device should be able to excite a large number of electromagnetic oscillation modes to ensure the uniformity of the field distribution in the cavity.

     Such coupling devices are generally classified into two types: direct coupling and indirect coupling:

     1. Direct coupling type

     In the direct coupling mode, it can be divided into single-tube and multi-tube direct coupling. Since the continuous wave magnetron is mostly an axial coaxial antenna output structure, the direct coupling method is to directly insert the coaxial antenna of the magnetron into a suitable position in the cavity to generate surface polarization, or add at the antenna end. A helical antenna is connected to generate a circularly polarized electromagnetic wave. For a single-mode cavity, the position and coupling of the coupling port determines whether the mode can be excited, but the resonant frequency after excitation is completely determined by the boundary of the cavity. The multi-cavity case is much more complicated, because the many modes excited in the multi-mode cavity (including TEmnp mode and TMmnp mode) are determined by the number of coupling ports and the position and boundary conditions; the same boundary conditions can be excited. How many modes are determined by the coupling device, so it is possible to determine the number of coupling ports by electric field simulation or empirically, especially the specific distribution mode and mutual position, but there is a common premise that the maximum incentive should be As many modes as possible to ensure uniformity of field distribution within the furnace cavity. In principle, the number of modes in the multi-cavity cavity is not directly related to the number of coupling ports. However, in the current actual situation, due to the low price of the magnetron for household microwave ovens, several and dozens of them have been widely used in industrial microwave ovens. Even hundreds of furnaces use a magnetron to simultaneously feed energy to produce a high-power device. In this case, multi-tube direct coupling is an inevitable coupling method. Therefore, the distribution, relative position and polarization of the coupling port will affect the coupling efficiency when multi-tube coupling; the problem to be considered at this time is no longer the number of simple modes, but also the mutual coupling of multiple tubes. Coupling efficiency problems, poor design, multi-cavity will work inefficiently, and even if it can not run, it will destroy the pipe. In this case, it is necessary to consider its effect on cross-coupled oscillator * life and heating uniformity between the oscillation source.

Each oscillator is subject to reflections due to poor matching and some of the energy of other oscillators. The sum of these energies must not exceed the allowable reflected energy acceptable to the oscillator specifications, calculate the magnitude of the furnace inner wall current as determined by the power density and quality factor, and feed it separately when the power is output solely by the oscillator. current at the mouth wall effects can be compared to the estimated * cross coupling, which can be drawn effectively reflectance value of the oscillation source seen. 20 years ago, the British used on the 896 / 915MHz 36 1.5kW magnetron, established nearly 54kW of power, its coupling efficiency close to 100%, while the number of China has also been more than 80 or even 100's Multi-tube output structure, but the specific coupling efficiency is unknown.

       2 , indirect coupling type

     This coupling method means that the microwave magnetron first excites a waveguide, and then couples the waveguide to the furnace chamber in a single hole or a porous phase to excite a multimode electromagnetic field in the cavity. This coupling method is suitable for single-tube high-power magnetrons. Because then the absence of the multi-tube coupling problems * cross each other, and thus is relatively simple.

     At present, many domestic manufacturers introduce multi-slot waveguide antenna radiators into industrial microwave ovens, and two points should attract attention and attention. First, when the load in the furnace is light (ie, when the QL is high), the multi-slot antenna radiation system is actually an excitation device in the furnace cavity. It cannot be considered as a microwave irradiation system, and the design is not Well, it will produce large reflection, and the coupling efficiency is not high. Secondly, when the load is heavy and close to the material from the antenna, this is an ideal microwave irradiation system, and the surrounding metal wall only plays an anti-electromagnetic system. The radiation shielding system is no longer a multi-cavity. The volume and geometry are not critical at this time. The starting point of the design should be the radiation efficiency of the multi-slot antenna. Unfortunately, many designers directly use the design and calculation methods of multi-slot microwave in-phase antennas as a theoretical basis. We believe that the multi-slit microwave irradiation system of industrial microwave ovens is different from the in-phase antenna requirements in radar. The former requires near-area. The medium power is uniformly irradiated onto the material, and the microwave source should be well matched with the antenna, while the in-phase antenna requires a beam of a certain pattern in the far region, so the relative distance between the slots in the axial direction is different, inphase antennas, the separation distance should be a half wavelength, to ensure phase requiring mutual radiation same, while the microwave heating slit separation distance is the cross between the slits * minimum coupling and to prevent the superposition of the slit caused by reflection, so this distance should Equal to 3/4λg instead of 1/2λg in the in-phase antenna , and the distance of each slot from the centerline of the waveguide should take into account the equal power radiation of each slot. The waveguide terminal should have an adjustable piston to ensure good matching. When the design is good, the standing wave ratio can be between 1.1 and 1.4 , and the coupling efficiency can be as high as 95 % or more.

    3. High-frequency breakdown or discharge in the furnace cavity at high power density

     In actual operation, we often find that high frequency breakdown occurs during heating. There are several possible reasons for this breakdown. One is that there are some structural defects in the cavity, such as metal burrs, lap joints, etc., which cause sparks to be generated by microwave discharge. Another common situation is air or water vapor mixture in the furnace. The gas breakdown; the third is that some granular materials with a high dielectric constant cause dielectric breakdown due to concentration of field strength on the indirect contacts of the particles, causing local overheating, and finally high frequency breakdown of the superheated air. Breakdown and discharge, regardless of the cause, especially the discharge that occurs near the waveguide coupling, induces avalanche breakdown in the waveguide feed, which can cause damage or damage to the microwave source and circulator. It is crucial because the current price of high-power microwave sources and circulators is considerable, and damage to both the pipes and the circulators will result in production shutdowns, increased production costs, and impact on production and efficiency.

     For this reason, we should strive to make the inner surface of the furnace cavity smooth and smooth during the design and processing. In particular, there should be no obvious gap at the weld or joint to ensure good contact, in order to prevent the power density from being too high during the water vapor discharge process. The gas breakdown can move the waveguide or multi-slot antenna with the feed port from the top to the bottom, and the feed port is upward, so as to prevent water vapor from entering the waveguide and triggering the light in the waveguide, thereby protecting the safe operation of the circulator and the magnetron.

    Fourth, other types of industrial microwave ovens

      In addition to the above-mentioned multi-cavity industrial microwave ovens, there are many traveling wave type industrial microwave ovens, such as zigzag waveguides, V -shaped waveguides, ridge waveguides, surface wave waveguides, spiral waveguides, circular or elliptical waveguides, coaxial waveguides, and disks. ...... loaded waveguide, etc., in these heaters, the microwave traveling wave (fast wave or slow wave form) in the form of propagation along the waveguide, the electric field interacts with material having a certain polarization directions by the waveguide to achieve the purpose of heating, These traveling wave heaters are particularly suitable for heating filaments, rods, strips, sheets, etc., such as various plastic or nylon filaments, rubber strips, paper sheets, paper or cardboard, cloth, film, wood, carpet, and the like.