Fundamentals of Electroheat

The study of Electro-heat requires a multidisciplinary scientific approach and knowledge in different fields such as heat transfer, electromagnetic fields, electrical engineering, materials science, etc. In this book, before the treatment of the heating processes most applied in industry, there are two introductory chapters with hints concerning heat transfer problems in stationary or transient regime and particular electromagnetic phenomena that occur in applications. After a short theoretical outline of different modes of heat transfer (conduction, radiation and convection), the solution of a number of practical problems is discussed. In particular, the first part of the chapter deals with steady conduction heat transfer problems in bodies of different geometries, with or without internal heat generation. Then, in the second part, the transient heating and cooling temperature distributions in bodies of different geometries are analyzed with various initial and boundary conditions. In this chapter numerical examples are included to allow the student to become familiar with thermal units and the orders of magnitude typical in these applications.

This chapter is the second introductory chapter of the book. It deals with the basic laws of electromagnetic fields and describes some specific phenomena occurring in electro-heat technologies. In particular, the electromagnetic wave diffusion in a conductive half-space is presented and the basic quantities that characterize the thermal processes based on internal heating sources are given. The penetration depth of the electromagnetic wave is a parameter that characterizes all the relevant phenomena in these applications. A qualitative description is provided of several effects influencing the distribution of the current density and, as a consequence, the heating sources in the heated workpieces: the proximity effect (that occurs between two conductors that carry electrical current), the ring effect (that occurs in bended conductors), the slot effect (that occurs in conductors placed in the slot of a magnetic yoke), and the end and edge effects (that occur due to the finite length of inductor and load and their relative position).

This chapter deals with Electric Arc Furnaces (EAFs) installations used for melting processes based on the heat produced by one or more arcs burning between the ends of one or more electrodes and the charge. The first part of the chapter deals with AC direct arc furnaces for production of steel, considering in particular the power supply, the furnace transformer, the secondary high-current circuit, the electrodes and the furnace vessel. Then the equivalent circuits and the operating characteristics of the installation are presented, making reference either to the approximation with sinusoidal quantities or the non-linear characteristic of the arc. The analysis of these characteristics allows the selection of the optimum operating point. Finally, the evolution of the steel production cycle and the main innovations introduced in modern furnaces in the last 40 years, like foaming slag practice, scrap preheating, CO post-combustion, intensive usage of oxygen and carbon, bottom stirring are dealt with. At the end of the first part the voltage fluctuations produced by EAFs on the supply network (the “Flicker” phenomenon) and the development of DC EAFs are shortly discussed. The second part of the chapter is devoted to Submerged Arc Furnaces used for production of calcium carbide, ferroalloys, other non-ferrous alloys and phosphorus by chemical processes of reduction of one or more oxides of the ore by a reducing agent loaded with the ore. The design and energetic characteristics and the special type of self-baking electrodes used in these furnaces are described.

Resistance furnaces are heating installations that use the heat generated by Joule effect in appropriate heating elements (resistors) located on the walls of the furnace chamber, and transmitted to the workpiece to be heated mainly by radiation and convection. The electrical energy transformed into heat in the resistors is used in part to raise the temperature of the charge and in part to heat the walls of the chamber and to compensate for the furnace heat losses. After a description of the different constructive types of furnaces (batch, continuous, for low, medium or high temperature, with protective atmosphere or in vacuum), heating cycle, criteria and materials for wall design and test methods of this type of furnace are analyzed. In the final paragraphs, we will describe the different types of resistors, characteristics of materials used for their construction and design criteria of resistors for classical radiation or convection furnaces. At the end, furnaces with high specific power resistors are briefly discussed, making also reference to the energy balance of this type of furnaces.

This chapter deals with Direct Resistance Heating (DRH) installations used for heating electrical conductive materials, metallic or non-metallic, by means of an electrical current flowing directly in the workpiece to be heated. According to the process technological requirements, the installation can be with still or moving workpieces and DC or AC supply at convenient frequency (mostly 50 Hz). The chapter is subdivided into four paragraphs. The first paragraph deals with installations with DC supply: the basic equations are first given, then the influence of variations material characteristics with temperature and the heating transient in the workpiece are analyzed. In the second paragraph installations with AC supply are considered, analyzing current and power density distributions in the workpiece, heating transients in non-magnetic and magnetic materials, efficiency and energy consumption. The third paragraph deals with the equivalent circuit of DRH installations and the calculation through numerical models of the influence of the installation design on the transient temperature distribution in steel bars. In the fourth paragraph data for DRH of tubes, bars with rectangular or square cross-section and continuous heaters for metal wires and strips are presented.

Induction heating uses the heat produced by currents induced within a conducting body exposed to the alternating magnetic field produced by AC current flowing in an inductor coil. The main advantages of this process are transmission of electromagnetic energy from the inductor to the workpiece without direct contact, as well as fast and selective heating in defined regions of the workpiece. The first part of the chapter deals with the distributions of induced current and power density within a cylindrical body in longitudinal magnetic field, the equivalent impedance, the electrical efficiency and the quality and power factors of the inductor-load system. In the second paragraph, we will study the transient temperature pattern and the influence of variations of material characteristics during the heating of magnetic or non-magnetic workpieces. The third part deals with the calculation of inductors with approximate, analytical and numerical methods. Numerical examples are included to illustrate the calculation procedures. In the last part of the chapter the main industrial applications of induction heating are presented: in particular induction crucible furnaces, channel induction furnaces, mass through heating prior hot working of metals and heat treatments for induction surface hardening.

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.