Microwave Processing of Materials (1994) / Chapter Skim
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MICROWAVE SYSTEM INTEGRATION
MICROWAVE SYSTEM INTEGRATION
Pages 39-66
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From page 39... ...
While the basic components of a microwave processing system generator, applicator, and control systems are simple, the interaction of materials with microwave fields and changes in fundamental material properties during processing make design and development of microwave processes very complex. This complexity may be dealt with using an integrated approach with a process design team consisting of the materials ant} process engineer, the microwave equipment manufacturer, and an electromagnetic specialist.
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From page 40... ...
They include uniformity of heating, required microwave power, applicator size, leakage suppression, and required performance characteristics. Heating Uniformity Uniform heating is difficult to obtain in a multimode oven.
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From page 41... ...
Required Microwave Power Required power is usually calculated based on an initial assessment of the proposed process and verified through actual testing once an initial oven concept, layout, and size have been established. Key parameters to be verified include heating-rate sensitivity, temperature uniformity, and process efficiency.
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From page 42... ...
web materials processed in industrial microwave ovens. Reactive chokes are ineffective when the height of the opening is greater than about half a wavelength.
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From page 43... ...
(c) "Maze" Leakage Suppression RF Choke TON Product Flow Product Flow (overhead conveyor or floor mounted chain/dog)
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From page 44... ...
heating time curves for multimode applicator with constant microwave power, sample weight and sample geometry. Figure 3-4 illustrates how the efficiency of a multimode cavity is affected by the amount of matenal in the cavity, and the clielectric loss properties of the material.
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From page 45... ...
45 FIGURE 3-4 Schematic of multimode cavity efficiency vs. load size with constant power and variable sample weight.
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From page 46... ...
Periodic S1;ruct;ures Periodic structures are microwave transmission lines that have been distorted or had discontinuities inserted into them at regular intervals. When microwave energy is fed through these structures, there are reflections caused by the discontinuities which result in the formation of pass bands and stop bands.
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From page 47... ...
Microwave System Integration SINGLE MODE APPLICATOR r (a)
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From page 48... ...
ELECTRIC FIELD STRENGTH \ (b) DISTANCE FROM SURFACE OF APPLICATION GIRECTION OF ____; PROPAGATION ~ ~
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From page 49... ...
In microwave processing. Temperature Measurement in a Microwave Envirorunent Temperature measurements in a microwave environment present several difficulties: · Temperature measurement must be made directly within the sample and not in its vicinity.
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From page 50... ...
The presence of a metallic temperature probe in a microwave environment can cause electromagnetic interference problems, causing distortion of the electrical field or affecting the electronics used for temperature measurement, as well as errors due to self heating; heat conduction; shielding; and excessive localized heating, particularly at the tip of the probe. Optical measurement techniques such as pyrometers and optical fiber probes assume knowledge of emissivity.
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From page 51... ...
Therefore, serious distortion of the electric field in the microwave environment will occur when a thermocouple is placed at a location where the electric field is parallel to the conductor. Figure 3-8a, shows results from numerical simulations, where it may be seen that placement of a conductor parallel to electric-field vectors caused significant distortion in the field distribution in a multimode cavity.
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From page 52... ...
5448 1 ? 632 ,881B FIGURE 3-8a Electric-f~eld intensity distribution in a vertical plane through a multimode cavity and a ceramic base plate.
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Plot (a) is the empty cavity, and plot (b)
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From page 54... ...
Figure 3-10 shows the temperaturemeasurement error based on percent error in emissivity as a function of temperature. Figure 3-10, part a shows the case when temperature measurements were made at ~ = 0.95 ,um, while Figure 3-10, part b shows the expected errors when the measurements are ma(le at ~ = 3.5 ,um,
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From page 55... ...
Additional error may result from high sample transparency (emittance) , which causes the temperature measurements using pyrometers or lightpipe to be sensitive to the cooler environment surrounding the heated sample.
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From page 56... ...
(b) 200 Microwave Processing of Materials 0.95 Am Wavelength 100 50 L11 _.
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From page 57... ...
Summary Based on a brief review of available temperature measurement techniques and the complications that may result when measuring temperature in a microwave environment, the committee made the following observations: ~ Temperature measurement in a microwave environment is a nontrivial procedure. Maintaining good thermal contact with the object being heated is crucial when heating using microwaves, and it is important that temperature probes produce minimum perturbation to the existing fields in the microwave heating chamber.
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From page 58... ...
These procedures shouict therefore be clearly described when reporting temperature measurements. · When optical pyrometers are used in surface-temperature measurements, the transmission properties of the viewing windows and their variation with the wavelength and during the heating process must also be quantified, and their impact on the accuracy of the temperature measurement must be reported.
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From page 59... ...
Calculations may also provide Q of the empty and loaded cavities, and the electromagnetic fields and power distributions at the true resonant frequency (Iskander et al., 1993; Chapman et al., 19921. Finite-difference time-domain modeling of multimode cavities to simulate heating processes, including hybrid heating, in multimode cavities can provide information about field distributions in the cavity and about the effects of sample insertion, insulations, susceptors, etc.
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From page 60... ...
it. FIGURE 3-14 Cross-sectional view of the field distribution in the cylindrical cavity and the waveguide feed: (a)
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From page 61... ...
Results from numerical simulations need to be validated to show correlation with actual processes, including mapping of electric-field and power-density distributions in microwave processing systems. As described earlier, one of the most important challenges facing the validation procedures is related to the accuracy of temperature measurements in a microwave environment.
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From page 62... ...
used for simulating continuous microwave heating in a traveling-wavetype applicator (Metaxas anci Meredith, 1983~.
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From page 63... ...
1 250 O g9O 0.925 .O.gOO 0.875 0.850 0825 ~ ,~ .k ~ _ ~ 0 200 400 600 800 FIELD STRENGTH Eo(V/CM) FIGURE 3-18 Equilibrium temperature at center of alumina sphere (Barmatz and Jackson, 1992)
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From page 64... ...
modeling techniques and software that are discussed in the previous section, application of numerical simulations is hampered by incomplete or unreliable characterization of clielectric properties of materials and their variation of dielectric properties with temperature and frequency. Measurement of complex permittivity over a broac!
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From page 65... ...
Cavity Perturbation Method Cavity perturbation methods have been widely used to measure the complex permittivity of materials at microwave frequencies. The basic assumption of this method is that the sample must be very small compared with the cavity itself, so that a frequency shift that is small compared with the resonant frequency of the empty cavity is produced by the insertion of the sample (Altschuler, 19631.
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From page 66... ...
Samples are positioned at a given angle in the path of the incident beam, and the transmission and reflection coefficients are measured by two identical receiver horns suitably aligned with respect to the incident beam and the sample. The dielectric properties are then determined from the observed transmission and reflection coefficients (Ho, 19881.
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