Dielectric heating

Dielectric heating or capacitive heating is the heating of a non-conductive material by electromagnetic waves. When this effect is used in process engineering, the energy is generated at a very high frequency (in the MHz or GHz range) using powerful oscillators and 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. However, there is also undesirable dielectric heating, e.g. B. when using a mobile phone, cf. SAR value .

function

The permittivity of water (20 ° C) depends weakly on the temperature, but very strongly on the frequency. The real part is decisive for the capacity calculation of a capacitor, the imaginary part characterizes the energy absorption.

If electromagnetic waves hit an electrically conductive material such as metal, currents are induced on its surface, which heat it up. The penetration depth is usually only a few micrometers (see skin effect ).

Currents cannot flow with non-conductors, but with some materials the charge carriers of the molecules can only follow the changes in direction of the high-frequency field with some delay, which increases the internal energy in the material and thus its temperature. The suitability of a material for dielectric heating can be read from the imaginary part of the complex permittivity of a material at a given frequency. With some materials such as ceramics, this is extremely low, with water-containing substances it is very high and depends on the frequency.

With increasing material thickness, the heating is limited to material layers close to the surface, which largely absorb the electromagnetic field. At the 2.45 GHz operating frequency of a microwave oven, the penetration depth of the field into the material is only a few centimeters. If the frequency were increased to around 20 GHz (in the case of water), the radiant energy would be absorbed within the first millimeters, and material lying deeper would remain cold.

The waves used in dielectric heating are not thermal radiation because the temperature of the transmitter is not critical. The even higher frequencies of typical thermal radiation in the infrared region are generated differently and are absorbed within the first micrometers when they hit, provided the material is not transparent in this wavelength range, such as table salt .

Areas of application

In many cases, capacitive heating systems are used for gluing the wood, as the heat output is created directly inside the wood. Systems based on heat conduction would be unsuitable for this application because wood only low thermal diffusivity and thermal conductivity has. Capacitive heating systems are used to dry wood , food and similar materials.

A specialty is pest control z. B. in wood or grain. The infected material is heated with high-frequency electromagnetic fields; Since pests have a higher water content than the material to be protected, they are heated more strongly and, if they are sufficiently powerful, they are overheated and killed. Further applications are:

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

Heat input into a material volume

The power loss density p with dielectric heating based on the material volume is:

{\ displaystyle p = \ omega \ cdot \ varepsilon _ {r} '' \ cdot \ varepsilon _ {0} \ cdot E ^ {2}}

Here, ω is the angular frequency , ε _r '' the imaginary part of the complex relative permittivity , ε ₀ the permittivity of the free space and E the amount of the electric field strength (effective value; if the peak value, i.e. the amplitude, is used, the factor must be in the equation 1/2 can be added). The dielectric heating associated with the power loss , when integrated over the heating period, corresponds exactly to the internal energy of a material supplied to a material volume with electromagnetic waves , as described in thermodynamics . The imaginary part of the complex-valued, relative permittivity is a measure of the ability of a dielectric to convert electromagnetic field energy at high frequency into thermal energy. For substances or mixtures of substances that also have an electrical conductivity σ, the following applies:

{\ displaystyle p = (\ sigma + \ omega \ cdot \ varepsilon _ {r} '' \ cdot \ varepsilon _ {0}) \ cdot E ^ {2}}

.

The power loss density input via the ohmic losses takes place via the electrical conductivity σ. This proportion is not included in dielectric heating. It is independent of the frequency of the electromagnetic wave; the extent to which it is effective depends on the skin effect and thus indirectly on the material geometry.

advantages

In industrial gluing, short gluing times can be achieved by supporting the drying process of the gluing point, which can lead to a high production speed.
Often this is more efficient than conventional types of heating, since the heat is generated in the material itself and does not have to be conveyed there indirectly.

disadvantage

for large systems:

Very high voltages are required inside the device (approx. 2 to 15 kV)
High acquisition costs

literature

Arthur von Hippel, Editor: Dielectric Materials and Applications . Artech House, London, 1954, ISBN 0-89006-805-4 .
Arthur von Hippel: Dielectrics and Waves . Artech House, London, 1954, ISBN 0-89006-803-8 .
AC Metaxas, RJ Meredith: Industrial Microwave Heating (IEE Power Engineering Series) . Institution of Engineering and Technology, 1983, ISBN 0-906048-89-3 .
AC Metaxas: Foundations of Electroheat, A Unified Approach . John Wiley and Sons, 1996, ISBN 0-471-95644-9 .