Microwave frequencies lies between radio frequencies and infrared radiation with the 300 MHz to 300 GHz on the electromagnetic spectrum. The corresponding wavelengths of the microwave are between 1 m to 1 mm as shown in Figure 1.
With these frequency range, the microwaves could be used in two major application: transmission of information (telecommunication)or transmission of energy. The extensive application area of the microwaves are the telecommunication and the industrial, medical, scientific, and mili- tary applications. Due to these wide range of applications, the allocated frequencies for transmission of en- ergy with microwaves is limited by the ISM (Industrial Scientific Medical) in order to minimize parasite effects and supply the information security. The ISM allows 915 MHz and 2450 MHz for transmission of energy applications. Nearly all household applications for microwave heating use 2450 MHz, some industrial microwave applica- tions use 915 MHz too. The selec- tion of the frequency depends on the heating purpose.
Microwave heating systems consist of basically three components as shown in Figure 2.
1) Microwave power source (gen-erator): Microwave power source in industrial applications are generally magnetrons which works at 2450 MHz or 915 MHz.
2) Transmission line (waveguide): Waveguides are using in order to deliver microwaves from the gen- erator into an applicator.
3) Microwave applicator (cavity): The cavity is the heating chamber where the materials are put in.
Microwave heating is supplied by both electric and magnetic field. Depending on the magnetron pow- er, waveguide and cavity dimen- sion and the electromagnetic and thermophysical properties of the heating material, the field strength varies inside the material and the cavity. In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz, the field oscillates 4.9 x 109 times/sec), and the strong agitation, provided by cy- clic reorientation of molecules, can result in an intense internal heating that can lead to heating rates in ex- cess of 10 °C/sec when microwave radiation of a kilowatt-capacity source is used.
To learn more about Hybrid Microwave Furnaces, check out our magazine: Industrial Furnaces 3. Issue