Dielectric heating

When dielectric heating or capacitive heating, the heat in the non-conductive material itself, the energy required will be generated at a very high frequency is generated ( in the MHz or GHz range ) by means of high-performance oscillator and is transmitted by radio waves. The material to be heated, the dielectric, is located for example between two plates that form the electrodes of the capacitor.

Function

Meet electromagnetic waves on an electrically conductive material such as metal are induced on its surface currents which heat them. The penetration depth is usually only a few microns (see skin effect).

For non-conductors, no currents can flow, but in some materials, the carrier of the molecules able to follow the changes in direction of the high-frequency field, only with some delay, so that the internal energy of the material and hence its temperature rises. The suitability of a material for dielectric heating can be seen at a given frequency on the imaginary part of the complex permittivity of a material. This is extremely low for some materials such as ceramics, in water-containing substances very large and frequency-dependent.

With increasing material thickness, the heating is limited to near-surface layers of material, which largely absorb the electromagnetic field. Thus, when the operating frequency is 2.45 GHz, a microwave oven, the penetration depth of the field in the material only a few centimeters. If the frequency increases to about 20 GHz ( for water), the radiant energy would be absorbed in the first few millimeters deeper material kept cold.

The waves used in dielectric heating are no heat radiation, because the temperature of the transmitter is not decisive. The even higher frequencies typical thermal radiation in the infrared region are produced differently and upon impact within the first micrometer absorbed, unless the material is transparent, such as saline in this wavelength range.

Areas of application

In many cases, capacitive heating equipment for gluing wood to be used as the heat output is produced directly inside the wood. Systems based on the heat pipe would be unsuitable for this application, since wood has only a low thermal diffusivity and thermal conductivity. Capacitive heating systems are used for drying of wood, foodstuffs and related materials.

A specialty is the pest control eg in wood or grain. In this case, the infected material is heated by high-frequency electromagnetic fields; because pests have a higher water content than the material to be protected, they are heated more and with sufficient power to overheat and killed. Other applications are:

  • The drying of wood, food, or other non-conductive materials,
  • The drying of glue spots (mainly in the beam gluing )
  • Disinfestation of pests in the wood,
  • Drying interspersed with harmful liquids soil,
  • Microwave ovens,
  • Diathermy as a medical application for therapeutic heating of tissues.

Heat power input into a volume of material

The power loss density p is obtained from dielectric heating on the volume of material:

Therein are ω is the angular frequency, εr '' is the imaginary part of complex relative permittivity ε0 is the permittivity of free space, and E, the amount of electric field intensity (effective value, is the peak value, which is the amplitude used, must be in the equation of the factor of 1 / are 2 supplements ). The dielectric heating is connected to the power loss corresponds to integration over the heating period of an exact volume of material with the electromagnetic waves supplied to the inner energy of a material, as described in thermodynamics. The imaginary part of the complex, relative permittivity is a measure of ability of a dielectric to convert electromagnetic field energy in high frequency heat energy. For substances or mixtures of substances which have an electrical conductivity is in addition σ:

The power loss density of the Listing ohmic losses via the electric conductivity σ. This proportion is not attributed to the dielectric heating. It is independent of the frequency of the electromagnetic wave; to what extent it is effective, but depends on the skin effect and thus indirectly by the material geometry.

Benefits

  • In industrial glues short Verleimungszeiten can be achieved by supporting the drying process of Leimstelle, thereby possibly attain a high production speed.
  • An often better compared to conventional heating types, since the heat generated in the material itself and does not need to be transported indirectly there.

Disadvantages

For large systems:

  • Within the device are very high voltages required (approximately 2 to 15 kV )
  • High cost
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