Dielectric Heating - Working Principle, Advantages & Applications

Dielectric heating is a high frequency alternating electric field or radio-frequency or microwave electromagnetic radiation heating caused by dielectric loss. The process of dielectric heating can be used to heat non-metallic materials that do not conduct electricity. By this method, uniform heating of non-conducting material can be achieved. The process of dielectric heating is also called high frequency or capacitive heating.

Principle of Dielectric Heating :

The principle of dielectric heating is based on dielectric loss. A dielectric material is basically a poor conductor or insulator such as wood, plastic, glass, ceramics, etc. We know that each and every material is made up of molecules whose elemental particle is an atom. When the dielectric material is not present in the electric field, the polar molecules in the material are randomly arranged as shown below.

When these molecules are subjected to the influence of the alternating electric field, the molecules in the material undergo polarization. Due to the formation of dipoles on polar molecules, the dipole moments of the polar molecules get properly oriented as shown above.

The orientation of the polar molecules will be according to the direction of the impressed alternating electric field. Since the electric field applied is alternating, the orientation of the polar molecules will be changed for every half cycle of the supply. The speed at which the orientation of the polar molecules changes will depend upon the frequency of the supply.

In doing so, atoms in the material get stressed due to inner atomic friction for which some energy will be wasted as the dielectric loss. This dielectric loss in dielectric material when subjected to an alternating electric field will results in the production of heat thereby increasing the temperature of the material.

The increase in temperature is due to the dielectric loss which is converted into heat. This loss is due to the reversal of magnetism or magneto molecular friction and is similar to hysteresis loss in ferromagnetic material.

Circuit Diagram and Working of Dielectric Heating :

The below figure shows the schematic arrangement of dielectric heating. It consists of two electrodes or metal plates in between a dielectric material that is to be heated is placed. An ac supply of high frequency is applied across the two electrodes.

Since there is no perfect conductor and insulator, the above capacitor when connected to the supply. The current drawn will never lead the voltage exactly by 90°. Due to the presence of some resistance in the capacitor, there exists some in-phase relationship between the voltage and current.

This causes some power loss in the dielectric material between the electrodes, thereby producing heat in the dielectric material as a dielectric loss. In order to obtain sufficient heating of the material i.e., to obtain required dielectric loss in the material, the frequency of the supply should be very large.

The below shows the equivalent circuit and phasor diagram for the above shown schematic arrangement of dielectric heating. The current consists of two components I_C and I_R, where I_C is the capacitive current leading the voltage by 90° and I_R is the in-phase current with voltage due to resistance.

Let,
V = Supply voltage,
f = Supply frequency in Hz,
A = Area of electrode or plate,
t = Thickness of dielectric,
δ = Loss angle in radian,
C = capacitance of condenser.
Therefore dielectric loss is given by,

For any material, C and δ are constant. Thus, the heat produced is proportional to V² and f. Therefore, for obtaining adequate heating, a voltage in the range of 15-20kV and a frequency of about 10-40MHz are usually employed. But, there are various factors that decide the frequency and voltage of dielectric heating.

Factors Affecting Frequency :

The impedance of the workpiece changes with a change in frequency. Since for maximum power transfer source impedance must be equal to the load impedance.
The mass of the workpiece, as the optimum frequency, decreases with an increase in mass.
Formation of standing waves, due to which the material will not be uniformly heated so, a suitable frequency must be selected.
The difficulty of constructing a tuning circuit.

Factors Deciding Voltage :

Insulation problem at high voltages.
Personal safety, at high voltages, breakdown may occur due to ionized air if an air gap is left, hence voltage of 60V to 3kV are more common.

Advantages of Dielectric Heating :

As the heat required is generated within the dielectric medium itself, this method of heating is useful in heating materials with poor thermal conductivities.
In dielectric heating, heat is produced throughout the material. Hence, uniform heating is obtained.
In this method, the materials are heated at a faster rate, and with the increase in frequency, the heating becomes faster.
The materials heated by this method are combustible and they cannot be heated by flame.
This is the only method that is available for heating bad conductors of heat.
Dielectric heating can be stopped as and when required by controlling input voltage and frequency.
In this method, the flame is not visible. Hence, inflammable materials such as plastics and wooden products can be heated safely.

Disadvantages of Dielectric Heating :

The efficiency of dielectric heating is about 50% only.
This method requires high frequencies due to which the cost of the equipment is quite high.
Due to the requirement of high frequency, it is difficult to tune inductance in order to resonate with the charge capacitance.
At higher frequencies, it is difficult to obtain uniform distribution of voltage.
This method causes interference in communication apparatus such as radio station services at high frequencies. This method should be employed only when other methods are impractical or slow.

Applications of Dielectric Heating :

Dielectric heating is used for drying purposes such as in the textile industry, drying of paper, drying of wood, etc.
The sterilization of bandages, absorbent cotton, etc can be done by dielectric heating.
For the treatment of certain pains and diseases, tissues and bones in the body are to be heated which can be done by dielectric heating.
Heating general processing such as coffee roasting, chocolate industry, etc.
Cooking seafood such as oysters without removing the outer shell.
Dehydration of milk, eggs, fruits, etc.
Defrosting of frozen foods such as meat and vegetables.
For control of bacterial growth and production of germicidal reactions, the food products are heated, and to prevent the product from losing their flavor, they are dielectric heated.
For the processing of chemicals, rubber, synthetic material, etc.