Microwave Processing of Materials (1994) / Chapter Skim
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APPLICATION CRITERIA
APPLICATION CRITERIA
Pages 67-78
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From page 67... ...
Some of the key characteristics of microwaves' interactions with materials are penetrating radiation; controllable electric-field distributions; rapid heating; selective heating (differential absorption) of materials; ant]
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From page 68... ...
Microwave Processing of Materials ·~ cot v, 2 3 so o Ct i .
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From page 69... ...
Thus, the reverse thermal gradients in microwave heating provide several unique benefits (Table 4-~) , which include rapist volumetric heating without overheating the surface, especially in materials with low thermal conductivity; reduced surface degradation during the drying of wet materials; and removal of binders or gases from the interior of porous materials without cracking, or conversely, penetration of reactive gases (during chemical vapor infiltration)
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From page 70... ...
As discussed in Chapter 2, this rapid increase in dielectric loss can lead to uneven heating and thermal runaway. If thermal runaway is controlled using hybrid heating or insulation, this phenomenon can allow the processing of low-Ioss ceramic materials, where both rapid heating and very high processing temperatures are desired.
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From page 71... ...
The economic feasibility is a function of local variations in energy costs, environmental laws, and labor costs balanced with the properties of finished materials or parts, improvements in yield or productivity, and the markets for the products. This report therefore seeks only to discuss the various cost aspects, provide guidelines for what must be considered, provide costs or savings as appropriate or available, and give examples as available.
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From page 72... ...
The cost of microwave equipment depends on size, power rating, frequency, applicator design, gas control system, peripherals, manufacturer, and the size of the market for that particular equipment. Because of these dependencies, capital costs vary widely depending on the applications.
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From page 73... ...
Energy savings arising from the use of microwave energy should be considered on the basis of the energy cost for the entire process. As discussed elsewhere in this report, hybrid systems that combine conventional and microwave heating are beneficial for many applications.
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From page 74... ...
According to these estimates, the use of microwave drying and firing could save as much as 80 percent of the energy used in conventional processes. This figure correlates reasonably well with calculation of energy savings made in a study of sintering of alumina (Patterson, et al., 19911.
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From page 75... ...
FIGURE 4-1 Energy required to sinter alumina (A16) as a function of load size (Patterson et ad., 1991)
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From page 76... ...
TABLE 4-6 Energy Savings Reported in Processing of Various Ceramics Material Process Energy Saving Performing Source Compared with Organization Conventional Process Alumina Sintering 90% Alcan Patterson etal.,1991 Ceramics Drying 50% Canadian Sheppard, Ministry of 1988 Energy Ceramics Firing 90% Canadian Sheppard, Ministry of 1988 Energy Steel ladle Drying 80% Nippon Steel Sutton, 1992 Refractories Boron Sintering i8% Los Alamos Katz et al., Carbide ~9~ PZT Sintering 95 % Honeywell Sheppard, Ceramic 1988 Center High Alumina Drying 20-30% Special Sheppard, Castables Metals Corp. 1988 Savings From Processing Changes Although the energy savings quoted in the previous section appear to be substantial, it should be remembered that energy costs are only a small part of the total cost of an advancer!
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From page 77... ...
TABLE 4-7 Time-Savings/Productivity Improvements Material Process Time savings Productivity Performing Source Improvement Organization High Drying50% increased yield/ Special Sheppard, Alumina improved Metals Corp 1988 Castables properties Whiteware Slip Casting66% (60 min immediate mold MBM Sheppard, to 20 min) recycling ceramics 1988 Whiteware Drying24 h to MBM Sheppard, 8 min ceramics 1988 Whiteware Overall70%(7days 6.25 MBM Sheppard, Processto 2 days)
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From page 78... ...
The use of microwave processing is inhibited by the high capital costs of microwave systems and the inherent inefficiency of electric power. In most successful industrial uses of microwaves, factors other than energy account for savings realized from microwave processing; improvements in productivity and material properties, and savings in time, space, and capital equipment, are probably the best bases for selecting microwaves over conventional processes.
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