l Dielectric Heating Terminology

 

 

  

Electronic heating or radio frequency heating may be classified as induction, capacitive or "macrowave," and microwave.

 

 

 

 

As the name implies, induction heating produces heat in a conductor by inducing eddy currents in a manner similar to that of a transformer when a current in the secondary windings is produced by induction from a current inthe primary windings. These currents supply the heat required due to resistance heating (I2R)losses.

One of the main advantages of induction heating is that the heating is that the heating can be confined to that part of the work pieceor material whih is directly opposite the coil inducing the current.

 

 

 

 

 

 

Induction Heating

 

 

 

 

 

 

 

 

Capacitive heating involves the heating of poor electrical conductors through

dipole polarization at frequencies between 1 and 300 MHz.  Although the phenomenon of dipole losses had been known for a long time, its application as a means of heating dates back only to just before 1940.  Capacitive heating is often referred to as dielcetric heating because it is the heating of a dielectric or nonconductor.  In practice the heating usually takes place between parallel plates such as in a capacitor or condenser.  Indeed, the term dielectric loss comes from the fact that in a capacitor some of the current is lost as heat.

The term microwave heating is sometimes used, referring to the much longer wavelengths of the capacitive heating range compared to the microwave range.

 

 

 

 

 

 

 

 

 

Low-Frequency capacitive heating

 

 

 

 

 

 

 

 

Microwave heating is also dielectric heating but refers to the heating that takes place in a nonconductor due to polarization effects at frequencies between 300 MHz and 300 GHz (wavelengths between 1 m and 1 mm).  Thus, the basic differences between capacitive and microwave heating are related first to the frequencies employed and second to the manner in which heating is carried out (in capacitive heating the material is usually placed between electrodes, whereas in microwave heating a closed cavity or oven often is used.

 

 

 

 

 

 

 

 

 

Microwave Heating

 

 

 

 

 

 

 

 

 

Dielectric

 

     l Dielectric Heating

 

 

 

The interaction of an electric field with a dielectric has its origin in the response of charge particled to the applied field.  The displacement of these charge particles from their equilibrium positions gives rise to induced dipoles which respond to the applied field. Such induced polarisation arises mainly from the displacement of electrons around the nuclei (electronic polarisation) or due to the relative displacement of atomic nuclei because of the unequal distribution of charge in molecule formation (atomic polarisation).  In addition to induced dipoles some dielectrics, known as polar dielectrics, contain permanent dipoles due to the assymmetric charge distribution of unlike charge partners in a molecule which tend to reorientate under the influence of a changing electric field, thus giving rise to orientation polarisation.  Finally, another source of polarisation arises from charge build-up in interfaces between components in heterogeneous systems, termed interfacial, space charge or Maxwell-Wagner polarisation.  Figure shows a schematic representation of Maxwell-Wagner and orientation polarisation due to an alternating electric field.

These two mechanisms, together with d.c. conductivity, are the basis of high frequency heating.

 

 

              

 

 

Allocated

 

 

     l Allocated Frequency

 

 

 

Microwave are electromagnetic energy.  Microwave energy is a nonioizing radiation that causes molecular motion by migration of ions and rotation of dipoles, but does not cause changes in molecular structure.  Microwave energy has a frequency range from 300 to 300,000 MHz.  Four frequencies are used for industrial and scientific microwave heating and drying.  These frequencies were established for industrial, scientific, and medical use by the federal communi- cations comission and conform to the international radio regulations adopted at Geneva in 1959.  Of these frequencies, 2450 MHz is the most commonly used and is the frequency used in all home microwave units.

 

1. 915 

±

25 MHz

 

2. 2450

±

50 MHz

 

3. 5800  

±

75 MHz

 

4. 24125 

±

125 MHz

 

 

The properties

 

 

     l The properties of a microwave heating

 

 

We don't heat conduction and can heat dielectric substances in short time.

Heating efficiency is good and we can save energy more than normal heating method.

We can heat complex shaped material from inside of it.

We can heat with packing case.

We can dry the moisture uniformly.

It is simple to control the heating temperature.

The adaption of sensor (temperature, moisture etc.)and automation are very simple.

Seal up heating is possible and it is easy to optional mood or vacuum condition for heating.

Environments (no noise, hot air, exhausted gas...) are good.

It is easy to control.

 

 

Microwave Penetration

 

 

     l Microwave Penetration Depth

 

 

 

The attenuation factor determines the absorption of energy within the dielectric as a function of depth from the surface of the dielectric, as a function of depth from the surface of the dielectric, as described by Lambert's law (Pz = Poe -2z) and is

inversely related to the materials penetration depth, that is, the depth from the surface at which 1/e of the power at the surface is not attenuated or absorbed.

 

 

 

 

Substance

Temperature

915MHz

2450 MHz

½ Penetration

r

tan

½ Penetration

r

tan

ICE

-12

1600

 

 

780

3.2

9×10-4

Water

25

9.0

 

 

1.3

76.7

1600×10-4

Water

55

16.0

 

300×10-4

2.3

 

700×10-4

Water

85

24.0

 

 

3.9

56.5

550×10-4

Beef

4.5

1.7

 

700×10-4

1.8

40.0

3000×10-4

Polyethylene

 

 

 

2×10-4

 

 

2×10-4

Teflon

 

 

 

2×10-4

 

 

2×10-4

Nylon

 

 

 

 

 

2.8

120×10-4

Paper

 

 

2.7

600×10-4

 

2.7

600×10-4

Glass

 

 

5.1

100×10-4

60.0

5.1

100×10-4

Ceramics

 

 

5.6

145×10-4

 

5.6

150×10-4

 

 

 

The experiences of

 

 

     l The experiences of microwave heating

 

 

 

     Using to food

 

 

 

 

 

 

        Foaming (snack...)

        Dry (vegetable, onion, meat......)

        Heat cooking (fish cake, bacon, re-heaing of food...)

        Defrost (fish, meat...)

        Bread aging

 

 

 

      Using to grain

 

 

        insecticide

        Dry (wheat, paddy...)

 

  

 

      Using to rubber industry

 

 

        Rubber vulcanization

        Foaming

 

 

 

      Dry of wood, stick , bending working

 

 

 

 

     Using to medical care

 

 

        Care of cancer

 

 

 

     Melting

 

 

        Making glass

 

 

 

 

      Making paper, dyeing of febric and dry

 

 

 

 

      Leather dry

 

 

 

Point of microwave

 

 

     l Point of microwave application

 

 

Reference Plane(Axis of Output Antenna)

 

Operating Conditions :

     Power Supply : Single Phase

     Full-Wave Rectifier Without Filter,

     Average Anode Current : 350mA

 

Wave Guide : Standard

                   Launcher

     Output Power(W)

   ...... Frequency(MHz)

 

 

 

 

 

       Load Impedance Matching Technique (Maximum Efficiency)

        Optimum design of Cavity and Wave Guide

        Impedance measurement and analysis (Network Analyzer, User

           Calibration Kit, Antenna Probe)

        Tuner adaption

        Simulation Program pre-analysis

 

       Leakage shield Technique

        Shielding Gasket

        g/4 Choke, Choke Coil

        Microwave Absorbent

 

       Uniform heating Technique

        3-Stub Tuner, Stirrer

        sub-heater

        Absorbent  Material

 

 

Typical System

 

 

     l Typical System Diagram