High Frequency and Microwave Heating

Abstract

This chapter presents the industrial applications of high frequency and microwave heating, which are based on the thermal effect produced in a dielectric material exposed to a high frequency alternating electrical field. Since dielectric materials are characterized by low electrical and thermal conductivity, dielectric heating has found wide acceptance in the industry because it represents the best way to achieve fast and uniform heating in dielectric workpieces. The first paragraphs of the chapter deal with the theory of polarization processes, the equations of the power transformed into heat in the workpiece and the influence of frequency and temperature on the properties of dielectrics. In a subsequent paragraph the distribution of the electric field depending on the shape of the workpiece and the geometry of the working capacitor are studied. A paragraph deals with the calculation of the equivalent circuit of the working capacitor with load and the transient temperature distribution in the workpiece. The last theoretical paragraph deals with the peculiarities of microwave heating and the main elements of a microwave installation (magnetron, waveguide and resonant cavity). In the second part of the chapter are described the main industrial processes based on high frequency and microwave heating. In particular: gluing of wood, welding of sheets of thermoplastic materials, preheating of thermosetting resins and rubber, and applications in the textile, paper and food sectors.