- Radio frequency heating
Radio frequency heating is the heating of materials by radio frequency (otherwise called electromagnetic) energy. This can be divided into 3 general categories as below. The term "radio frequency" is misleading - electromagnetic energy of any frequency is absorbed (and reflected) to a greater or lesser degree by all materials. The frequency used for any particular purpose will depend on many things and this is shown below.
In general, any material may accept electromagnetic energy but the degree to which that happens is dependent on;
•Frequency of the electromagnetic energy,
•Intensity of the electromagnetic energy,
•Proximity to the source of the electromagnetic energy,
•Conducting or non conducting material,
•Nature of the material (ie how lossy).
Induction heatinginvolves the heating of electrically conducting materials by electromagnetic induction. Currents are induced in the material and these currents cause heating. The frequency used may vary from as low as mains frequency (50/60 Hz) to more than 10 MHz. Heating also occurs by hysteresis loss if the material has significant relative permeability(eg. Steel).Induction heating is generally a non-contact process and usually consists of a coil in close proximity to but not touching the material to be heated (usually a metal).
Dielectric heatinginvolves the heating of electrically insulating materials by dielectric loss. Voltage across the material causes energy to be dissipated as the molecules attempt to line up with the continuously changing electric field. A common perception is that the molecules rub together, with the friction causing heat. This is not so. Friction is a macroscopic process and does not exist at the molecular level. The heat is generated solely by the inability of the molecules to line up with the electric field.Frequencies in the range of 10-100 MHz are necessary to perform dielectric heating.Dielectric heating is generally a contact process and usually consists of the material to be heated (usually a non-metal) sandwiched between metal plates forming a capacitor.
Microwave heatingis actually a sub-category of dielectric heating in that insulating materials are heated primarily by dielectric loss. The difference is that of frequency. At frequencies above 100 MHz an electromagnetic wave can be launched from a small dimension emitter and conveyed through space. The material to be heated (a non-metal) can therefore be simply placed in the path of the waves and heating takes place. It is a non-contact process. Typical domestic microwave ovens operate at 2.45 GHz.
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