Industrial microwave equipment design several issues
As we all know, industrial microwave heating equipment microwave (except for traveling wave heater) and the biggest difference is that both domestic microwave oven microwave oven cavity power and volume vary greatly. Typically the volume of the microwave oven is generally between 15 and 30 liters, the U.S. and Europe in general between 25 and 40 liters, the volume of the industrial microwave oven in 500 liters or more, and sometimes even up to thousands of liters. Therefore, the design of such a large volume of industrial microwave oven, there are many different considerations principle, but can also be categorized into the following main factors:
1, with the microwave power density high enough;
2, a number of patterns sufficient to guarantee the furnace microwave field intensity uniformity;
3, with the microwave source with good coupling to ensure a sufficiently high coupling efficiency;
4, to avoid coupling especially near the mouth of the furnace produces water vapor generated by high-frequency breakdown;
5, has a good anti-leakage device, ensure that the operator position on the safety of personnel.
First, the design of multi-cavity
Usually because the microwave wavelength spend the object geometry, and therefore in the S-band single-mode microwave cavity geometry is very small, the volume is not large, such a small volume of the furnace chamber, not only in industrial microwave heating no use value, even if In a domestic microwave oven no use value. This determines whether the microwave oven or industrial microwave ovens have to use over multimode electromagnetic mold cavity, from the electromagnetic theory of physics and know that any one of the resonant cavity, the over-molded state, wherein the possible the number of resonant modes is proportional to the volume of the cavity, in other words, the greater the volume, the number of patterns which may exist in more and more, and the cavity of the microwave electric field distribution proportional to the number and the pattern, which that is why people always want to design a larger volume cavity cavity to improve the uniformity of the oven cavity field.
However, increasing the volume is one-sided pursuit of microwave power under certain circumstances, cause the power density decreases, of course, starting from the energy conservation law, as long as the increase in volume at the same time, the cavity around the metal border without increasing losses (such as the use or close to ideal ideal metal material), even if the power density of the heating effect will not decrease a significant effect, but in practical cases, the conductivity of any metal material is limited, in particular stainless steel, its loss is aluminum 25 times, 41 times that of copper, silver, 44 times, so as stainless steel as the material of the cavity, while the increase in volume is greatly increased the size of the cavity wall and loss, in this case using the increase The only way a large volume while increasing the microwave power input is correct conditions.
This is currently on the market a large part of the export volume used in household microwave oven magnetron 1kW ~ 1.6kW reasons, industrial microwave oven, since the volume of material to be heated considerations, the furnace chamber volume is usually between 500 to 20,000 liters , sometimes even larger. In such a large cavity of 915MHz or 2450MHz bands such as the use of the microwave magnetron as the microwave source, which apparently is working in a big "over-molded" state, in which the number of patterns that may exist very much. In this case, if the power density is sufficient, the field distribution in the oven cavity is ideal uniformity. The quality factor of the oven cavity is very high load, but when added to the material to be heated (especially high moisture content of the material), the oven cavity quality factor load sharp drop, this drop of up to 3 to 5 order of magnitude of the spectrum, and sometimes not even resonance, and a typical high-Q resonance system into a low Q microwave irradiation system.
The second factor is the shape of the material to be heated and the volume size, the furnace chamber size cross-sectional dimensions in particular the choice of the material mainly on account of the volume size and shape requirements, consideration should be given the choice of the longitudinal direction of the microwave output power and processing requirements.
The third factor is the moisture content of the material should be heated to choose the level of the volume of the cavity, especially the length size of the low water content can be shorter in length, or should be longer.
The fourth factor is based on the microwave source to the mouth of the number of feed, feed port is a single or multiple feed ports, a single microwave source or a plurality of microwave sources (in particular, a plurality of microwave sources independent of each other).
The fifth factor is the size of the cavity power density, high power density of the microwave will cause the following two effects.
(1), the furnace air or gas - vapor mixture dielectric breakdown;
(2), internal stress caused by excessive damage to the material to be heated.
In summary, we design a multi-cavity mutual restraint should be based on several factors, especially the theory experimentally choose a reasonable size, in order to achieve a good uniform, efficient safe operation of high-volume industrial microwave.
It should finally be noted that, because of the industrial microwave power is larger, it is often three-phase full-wave rectification of the non-filtered power supply system, the DC power supply so that the current pulse magnetron greatly reduced, so is not a phenomenon due to household microwave oven half-wave voltage doubler microwave source DC power supply causes the output multi-frequency phenomena, on the other hand, industrial microwave ovens usually in the microwave cavity of the source output and the circulator indirectly, so that the load does not change the frequency of the microwave source and power generation traction phenomenon. These two factors make the use of industrial microwave oven magnetron performance is more stable.

Second, the microwave source and the coupling cavity
Microwave source with the coupling cavity in the industrial microwave oven design is another important issue. Usually the basic requirements of this coupling is;
(1), the oven cavity and a good microwave source (load case) matching the power of the microwave source to feed non-reflective cavity:
(2), the coupling device should be able to maximize the number of electromagnetic vibration mode excited in order to ensure uniformity of distribution cavity.
This coupling device can generally be divided into direct coupling and indirect coupling of two types:
1, the direct coupling type
In the direct coupling method can be divided into single-tube and multi-tube direct coupling. Since continuous wave magnetron current multi-axial antenna output coaxial structure, so that direct coupling coaxial magnetron antenna directly into a suitable position in the cavity to produce a surface polarization, or added at the antenna pick a helical antenna to produce circularly polarized electromagnetic waves. Single-mode cavity, the coupling port position and determines the mode coupling can be energized up, but after excitation of the resonant frequency is entirely determined by the boundary of the cavity. The multi-cavity situation is much more complicated, because the multi-cavity mode is excited many (including TEmnp mold and TMmnp mold) is determined by the number and location of the coupling port and boundary conditions jointly determined; same boundary conditions can inspire since the number of patterns is determined by the coupling means, so the simulation by the electric field coupling or empirically determine how much of the mouth, especially the specific distribution pattern and the mutual position, but there is a common premise that should maximize incentives as much as possible from the model to the oven cavity to ensure uniformity of the distribution of the field.
In principle, the more the number of patterns in the mold cavity and the connection port number is not directly related, but in the current situation, since the home inexpensive microwave magnetron in the industry have been widely used in microwave few, dozens even hundreds oven magnetron can be fed simultaneously to produce high-power devices, in this case, multi-pipe direct coupling is an inevitable coupling. Therefore coupling port distribution, relative position and polarization will affect the multi-tube coupling coupling efficiency; this time to consider the issue is no longer a simple pattern as the number, but also consider the mutual coupling that multi-tube coupling efficiency, poorly designed, multi-cavity will work inefficiently, or even destroy the tubes will not run. In this case, you need to take into account the cross-coupling between the oscillator and the oscillator life and the impact of heating uniformity.
Each oscillator will be generated because the match is not ideal and reflection as well as other part of the energy of the oscillator. The sum of these energies must not exceed acceptable specifications by the oscillator allowed reflected energy calculated by the power density and the quality factor of the determined size of the current furnace wall, and with only a single output from the oscillator when the feed the mouth of the wall can be compared with the current estimate of pay * coupling, whereby the source of the oscillation can be drawn that the effective reflection coefficient.
20 years ago, Britain 896/915MHz used on 36 1.5kW magnetron, established nearly 54kW of power, the coupling efficiency of nearly 100%, while the number present, China has also been up to 80 or even 100 or more multi-tube output structure, but the specific coupling efficiency is unknown.
2, indirect coupling type
This coupling method is to go to inspire a microwave magnetron waveguide, the waveguide and the oven cavity and then to single or multi-phase excitation cavity coupled to the electromagnetic field generated multimode. This coupling method is applicable to single-tube high-power magnetron case. Because then there is no mutual cross * multi-tube coupling problems, which is relatively simple.
At present, many manufacturers have multi-slot waveguide antenna radiator into industrial microwave oven, two points should be given attention and attention. First, the furnace load is light (i.e. higher QL case), this multi-slot antenna system is in fact a pumping means in the furnace chamber, not that it is a microwave irradiation system, designed not to good, will have a greater reflection, coupling efficiency is not high, the second, when the load is heavy and relatively close proximity to the antenna away from the material, this is an ideal microwave irradiation system, the metal walls surrounding electromagnetic only acts as a radiation shielding system, it is no longer a multi-cavity, and then the volume and geometry are not critical, the design starting point should be a multi-slot antenna radiation efficiency.
Unfortunately, many designers directly using multi-aperture phase microwave antenna design and calculation method as a theoretical basis, we believe that industrial microwave microwave irradiation system with multi-aperture radar antenna in the same phase requirements are different, the former requirement in the near area uniformly irradiated to the power on the material, and the microwave source and the antenna should be a good match, while the noninverting antenna requires a certain direction in the far zone beam pattern, so in these slit relative distance in the axial direction of the requirements are different for phase antenna, the separation distance should be half wavelength, in order to ensure mutual radiation-phase requirements, and microwave heating of the slit separation distance is to make the gaps between the pay * coupling is minimized and to prevent gaps caused by reflections superimposed on each other, so this distance should be equal 3/4λg, rather than phase antennas 1/2λg, and each slot waveguide centerline distance from the aperture should be considered of equal power radiation. Waveguide terminals should have an adjustable piston in order to ensure a good match, the design is good SWR 1.1 to 1.4, the coupling efficiency of up to 95%.

Third, the high power density in the furnace chamber breakdown or discharge frequency
In actual operation, we often find in the heating process will produce high breakdown phenomenon. This breakdown There are several possible reasons, one oven cavity some structural defects, such as metal burrs, lap gaps as a result of microwave discharge sparks; Another common situation is that the furnace air or water vapor mixture gas breakdown; third is the number of particles having a high dielectric constant material shaped indirect contacts due to particle concentration on the cause dielectric breakdown field strength, causing local overheating, leading to overheating of the air-frequency breakdown.
No matter what causes the breakdown and discharge, especially near the mouth occurred in the waveguide coupler waveguide feeder discharge will cause the avalanche breakdown, then it will cause the microwave source and circulators damage or injury, and this is crucial, because the current high-power microwave source and the ring's price is substantial, whether pipe or circulator shutdown will result in damage to be repaired, increasing production costs, affecting the yield and efficiency.
To this end, we design and processing should seek to smooth the surface of the oven cavity, especially welds or seams to be no obvious gaps, to ensure good contact, in order to prevent the discharge process due to moisture caused by high power density breakdown of the gas can be fed with the mouth of the waveguide to move from the top or bottom of the slot antenna, feeder mouth upwards, thus avoiding the mouth caused by moisture entering the waveguide waveguide lights to protect the circulator and the safe operation of the magnetron.

Fourth, other types of industrial microwave
In addition to the multi-cavity structure of industrial microwave ovens, the current wave surviving in many industrial microwave ovens, such as folded waveguide, V-shaped waveguide, ridge waveguide, the waveguide surface waves, spiral waveguide, circular or elliptical waveguide, coaxial waveguide plate loaded waveguide ......, etc., in these heaters, the microwave to the traveling wave (fast or slow wave form) form along the waveguide, the waveguide having a certain use of the polarization direction of the electric field interaction with the material to achieve the purpose of heating, These lines are especially suitable for wave heater wire or rod, strip, sheet and other materials, heating, such as all kinds of plastic or nylon wire, rubber bands, paper, paper or cardboard, cloth, film, wood, carpet.

 

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