Dielectric heating - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Dielectric_heating

Dielectric heating, also known as electronic heating, RF heating, high-frequency heating is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.

RF dielectric heating at intermediate[clarification needed] frequencies, due to its greater penetration over microwave heating, shows greater promise than microwave systems as a method of very rapidly heating and uniformly preparing certain food items, and also killing parasites and pests in certain harvested crops.[1]

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Thermopedia - A-Z Index

http://www.thermopedia.com/content/850/

Radio frequency and microwave are sometimes used as alternatives to convective, conductive or radiant heat transfer for the proccessing of "nonmetals" [Langton (1949)]. Industries making use of these techniques include textiles, paper, food, plastic and chemicals. The applications are many and varied including drying, baking, defrosting, welding and polymerization [Jones (1987)]. Known as high frequency or dielectric heating, both are forms of electromagnetic wave energy, which share some characteristics but also have significant differences.

The perceived advantage of dielectric heating is based on the so-called "volumetric" effect arising from the fact that the energy is absorbed directly in the body of the material rather than being transferred to it via a surface. The concept of "volumetric" heating needs to be qualified since there is in reality a limiting penetration depth, which depends on the properties of the material being heated as well as on the wavelength of the energy source (Metaxas and Meridith, 1983). At the shorter wavelengths associated with microwave, penetration depth into a wet body is normally a few centimeters; at the longer wavelengths of radio frequency the depth can be a large fraction of a meter or a few centimeters depending on the ionic conductivity of the water in the material.

Dielectric process heating uses the frequency range from about 5 MHz to 5 GHz with radio frequency, RF, being normally defined as being less than 100 MHz. The definition of microwave usually is between 500 MHz and 5 GHZ. Within these ranges there are specific frequencies allocated for industrial, scientific and medical uses, the so-called ISM bands. The most common of these are 13.56 and 27.12 MHz (wavelengths 22.4 and 11.2 meters, respectively) for RF with 900 MHz and 2.45 GHz (wavelengths 0.35 and 0.13 meters, respectively) being the permitted frequencies for microwave. These particular frequencies have been chosen in order to minimize the risk of interference with telecommunications by either the fundamental or by higher harmonics and have no particular significance as far as the resonance of the water dipole is concerned. The actual frequency within the "900 band" varies from country to country, depending on local regulations.

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Radio Frequency Heating - A Different Tool

http://www.stanleyzinn.com/induction-heating/different-tool.html

Abstract

The need for radio frequency (RF) induction operations are increasing as thinner metals and shallower case requirements occur. While the theoretic basis of induction heating is the same for all frequencies, the power supplies and their operations are considerably different for RF. This paper explores those differences, and provides information on how to properly select RF equipment for various applications.

The accepted definition of Radio Frequency heating encompasses those frequencies above 50 kHz. The frequency most people associate with the Radio Frequency (RF) range is 450 kHz. This was the center of the high frequency band initially allotted for industrial use by the FCC.

Almost all tube type RF power supplies used the common designation of 450 kHz as their operating frequency. In fact, power supplies operated in a range between 200 and 750 kHz. In tube-type power supplies the design of the coil and the shape and mass of the part changed the tuned inductance and the power supply frequency changed accordingly. In contrast, the solid state power supply searches for the optimal frequency.

Today, solid state power supplies operate in the range of 50 - 750 kHz. There are supplies operating above this frequency in nominal ranges of 2-3 Megahertz (MHz) and up to and including 60-70 MHz, frequencies which have been usually associated with dielectric heating. High frequencies are selected to provide a specific shallow depth of case or for heating thin materials.

At low and medium frequencies, the deep penetration of the magnetic field provides are utilized for through heating of materials. Typical applications are forging and melting. Frequencies to 10 kHz are also used for these applications. However, frequencies above 8 kHz find major application in heat treating.

At the lower frequencies, most loads are uniform in cross section. This permits the user or manufacturer to utilize computer programs for coil design with a measure of certainty. Usually, they do not require further modification when installed on the equipment.

Where the part configurations are not symmetrical, computerized coil information can generally only be used for an approximation of the shape of the magnetic field. Modification of coil designs is a necessary part of this process. With RF power supplies, the modifications that must be made are generally more complicated and must be verified by actual laboratory tests.

In order to fully utilize the available power of Radio Frequency equipment, tuning of the coil and load to the power supply is extremely important. What are the factors that affect this tuning of the coil and load to the frequency range of the power supply?

One major factor is the Hysteresis loss in magnetic materials. Hysteresis losses are caused by reversal of the magnetic field due to frequency. As the field reverses it causes the molecular structure of the metal to realign to match the polarity of the field. The molecules however, being of a greater mass cannot respond as quickly to the changes in the field and thus friction generates a loss we call Hysteresis.

As the frequency increases, these magnetic reversals increase in speed and due to the friction generated Hysteresis losses are greater. Accordingly, Hysteresis affects RF applications to a much greater degree than lower frequencies.

At approximately 1340°F magnetic materials lose their magnetic properties and react as non-magnetic materials. This effect is known as the "Curie Point" of the material. The change from magnetic to non-magnetic state produces a significant shift in frequency. The reason for this is the change in the coil/ part inductance as Hysteresis occurs.

The combination of the coil, the part and the capacitance set the Resonant Frequency for the application. This combination of components is referred to as the "Tank Circuit". To achieve maximum energy transfer to the part, the tank circuit resonant frequency must be within the frequency capabilities of the equipment. When the Curie Point is reached, then tank circuit frequency will shift radically.

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Radio frequency heating of foods: principles, applications and related properties--a review. - PubMed - NCBI

http://www.ncbi.nlm.nih.gov/pubmed/14669879

Abstract

Radio frequency (RF) heating is a promising technology for food applications because of the associated rapid and uniform heat distribution, large penetration depth and lower energy consumption. Radio frequency heating has been successfully applied for drying, baking and thawing of frozen meat and in meat processing. However, its use in continuous pasteurization and sterilization of foods is rather limited. During RF heating, heat is generated within the product due to molecular friction resulting from oscillating molecules and ions caused by the applied alternating electric field. RF heating is influenced principally by the dielectric properties of the product when other conditions are kept constant. This review deals with the current status of RF heating applications in food processing, as well as product and system specific factors that influence the RF heating. It is evident that frequency level, temperature and properties of food, such as viscosity, water content and chemical composition affect the dielectric properties and thus the RF heating of foods. Therefore, these parameters should be taken into account when designing a radio frequency heating system for foods.