Microwave industry

J. Thunry, Microwaves Industrial, Scientific and Medical Applications, Artech House, Boston, Mass., 1992. 

J. Thuery in Microwaves Industrial, Scientific and Medicinal Applications, Ar-tech House, 1992. 

J. Thuery and E. H. Grant (eds). Microwaves Industrial, Scientific, and Medical Applications. The Artech House microwave library. 1992, Artech House Boston, xviii, 670. 

J. Thueey, Microwaves industrial, scientific and medicinal applications, Artech House, 1992. 

Anon. Gas-fired convective-microwave industrial dryer. Tech Profile. Chicago Gas Research Institute, 1995b. 

LTCC technology is used extensively in the microwave industry primarily because of three reasons ... 

Thuery, J. Microwaves Industrial, Scientitic, and Medical Applications, Adech House Boston, MA, London, 1992. 

The discovery of ferrimagnetism in yttrium iron garnet has attracted attention to the use of these materials for microwave device applications. To date garnets are the most useful materials in the microwave industry because of their magnetization, linewidth, Curie temperature and g-factor properties. With the advent of solid-state lasers, yttrium aluminum garnet was soon found to be an excellent laser host material for room temperature and high power applications. 

Proper design of microwave Industrial systems requires a knowledge of the dielectric properties of the ceramics to be processed. As was stated above, the property which describes the behaviour of the dielectric under the... 

Table 2 shows the dielectric properties of a range of ceramic materials under various conditions and near the two frequencies for which Industrial equipment can be readily purchased. It is evident that the effective losses of various ceramics depend upon the material density and the temperature, frequency and field orientation, giving a range of effective loss factors from above 70 for silicon carbide to 3 x 10 for boron nitride. This latter cereimic can be considered as transparent to microwave energy and may be used as an insulating material in microwave Industrial equipment or as a microwave window in waveguides. In fact, any material with an effective loss factor... 

Frequency Allocations. Under ideal conditions, an optimum frequency or frequency band should be selected for each appHcation of microwave power. Historically, however, development of the radio spectmm has been predominantly for communications and information processing purposes, eg, radar or radio location. Thus within each country and to some degree through international agreements, a complex Hst of frequency allocations and regulations on permitted radiated or conducted signals has been generated. Frequency allocations developed later on a much smaller scale for industrial, scientific, and medical (ISM) appHcations. 

The frequency bands at 915 and 2450 MHz (2375 MHz in the past in eastern Europe) are the most developed bands for microwave power apphcations. Microwave ovens are almost all at 2450 MHz. Many industrial heating appHcations are at 915 MHz. After the 1979 WARC, eastern Europe adopted 2450 MHz in place of 2375. 

The use of inverter-type power suppHes (63) in place of doubler-type 6O-H2 suppHes results in significant reduction in the weight of microwave ovens. Disadvantages are primarily cost for the consumer market in addition to somewhat less efficiency and increased noise. Their use in commercial or industrial equipment is more attractive. 

Fig. 4. Top and front views of typical high power conveyor-type industrial microwave equipment.

The nature of potential exposure ha2ards of low level microwave energy continues to be investigated (116—118). In the United States, leakage emission from microwave ovens is regulated to the stringent limit of 5 mW/cm at 5 cm (119). There is no federal limit on emission from industrial systems but the IMPI has set a voluntary standard which specifies 10 mW/cm at 5 cm (120). Emission values are equivalent to personnel exposures at several meters, well below limits that had previously prevailed in eastern Europe. This conclusion, derived for microwave ovens, should be vaUd for all microwave systems (121). 

Food. The most successful appHcation of microwave power is that of food processing (qv), cooking, and reheating. The consumer industry surpasses all other microwave power appHcations. Essentially all microwave ovens operate at 2450 MH2 except for a few U.S. combination range models that operate at 915 MH2. The success of this appHance resulted from the development of low cost magnetrons producing over 700 W for oven powers of 500-800 W (Table 3). 

Other chemical apphcations being studied include the use of microwaves in the petroleum (qv) industry (175), chemical synthesis (176,177), preparation of semiconductor materials (178), and the processing of polymers (179). 

R. A. Metaxas and R. Meredith, Industrial Microwave Heating, Peter Peregrinus Ltd., London, 1983. 

IMPI Peformance Standard on Eeakage from Industrial Microwave Systems, IMPI Pubhcation IS-1, IMPI, Manassas, Va., Aug. 1973. 

Polysulfones also offer desirable properties for cookware appHcations, eg, microwave transparency and environmental resistance to most common detergents. Resistance to various sterilizing media (eg, steam, disinfectants, and gamma radiation) makes polysulfones the resin family of choice for many medical devices. Uses in the electrical and electronic industry include printed circuit boards, circuit breaker components, connectors, sockets, and business machine parts, to mention a few. The good clarity of PSF makes it attractive for food service and food processing uses. Examples of appHcations in this area include coffee decanters and automated dairy processing components. 

Hot air, steam, and hot water vulcanisation is widely used in the latex industry, and fluid-bed heat transfer and electronic microwave curing has also been used. Cross-linking by electron radiation has been experimentally used, but has not yet been developed commercially. 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

---