I recently boughtFoundations of Electroheat: A Unified Approachby A.C. Metaxas. For anyone interested in the many ways of delivering electroheat, this book is well worth a look. It is based on courses on electroheat given in the engineering department of Cambridge University. In his book, Metaxas combines mathematical treatment of the principles of electroheat with descriptions and sketches of a variety of industrial applications. He graciously acknowledges his debt to the many colleagues who have devoted their careers to the subject and quotes sources both in the text and in a five-page reference list.
Metaxas provides an analytical treatment of the modes of heat generation and distribution in materials when subjected to electromagnetic fields. The materials can be conductors, insulators or anything in between. Metaxas demonstrates that one compact mathematics and physics toolbox can bring understanding to many applications.
In the section on properties of materials, Metaxas begins, "The prime purpose of utilizing electrical energy in the processes considered in this book is either to elevate the temperature to effect annealing, curing, sterilizing or melting, to supply the necessary energy for drying as in dielectric or infrared heating techniques, to provide sufficient energy for welding or cutting materials or to promote plasmas for a number of important industrial applications."
He describes and gives values for the properties of materials relevant to the heating process. These include electrical and thermal conductivity, specific heat, permittivity, dielectric loss and moisture content. Metaxas shows many worked examples and calculations of induction heating in metals and dielectric heating in nonmetals.
He also includes a section on applicators for electromagnetic heating such as:
- Clamps for connecting AC power directly onto a metal rod.
- The three-phase electrode setup for ohmic heating in an electrode boiler or molten glass tank.
- Various shapes of copper coils for induction heating of metal parts and continuous profile.
- The magnetic yoke arrangement for transverse flux, improving coupling for thin strip.
- Parallel plate arrangements for applying high frequency electric fields to lossy dielectric materials.
- Clamping applicators for high frequency welding of PVC.
- Microwave traveling wave applicator for sheet material.
- Microwave horn applicator for heating material in a conveyer tunnel.
The same section covers power sources. It is a given that the ultimate source is the 50 or 60 Hz public supply, so this section concerns conversion and control.
Chapter 5 covers the ionized state of a plasma, which is described by Metaxas as "a state of gaseous material which contains a fair number of ionized particles resulting from the application of an external energy source" É "may be in the form of DC or AC voltages of frequencies up to the microwave region and beyond."
Three regions of the plasma's voltage/current curve are defined, with their relevance to industrial processing:
Normal Glow: about 0.1 to some 50 mA; energy source for the CO2 laser; plasma chemistry and surface treatments such as etching, assisted chemical and physical vapor depositions, ion implantation, oxidation and sputtering.
Abnormal Glow: about 0.5 to 50 A; glow discharge carburizing and nitriding for surface treatment such as case hardening of ferrous and other components.
Arc Discharge: 1 to 10,000 A; energy source for the Nd:YAG and Ruby lasers; plasma chemistry; plasma arc for cutting, welding or spraying; plasma furnaces; arc furnaces for metal melting; arc welding.
A detailed treatment with diagrams and circuits follows, covering lasers, arcs and electron beams.
Chapter 6 covers heat pumps and energy recovery, infrared heating, electrochemical processes and ovens and furnaces. Other chapters deal with heat and mass transfer as well as the ever-growing use of computers and numerical techniques in electroheat calculations and processing. Some 70 pages of industrial applications and case studies follow, and Metaxas concludes with ten appendices on mathematical topics and material properties.
For me, the strength of this book is in the variety of industrially proven techniques revealed and explained to the processor and developer.