Category: RF Dielectric Heating

Radio Frequency

Short for radio frequency, RF consists of waves of electromagnetic energy moving through space.  All types of electromagnetic energy are grouped together in the electromagnetic spectrum.  RF waves can be characterized by a wavelength and a frequency.  The number of waves generated in one second is known as frequency and it is measured in cycles per second or Hertz (Hz).  The wavelength is the distance covered by one complete cycle of the electromagnetic wave.  The higher the frequency, the shorter is the wavelength.  RF is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kHz to 300 GHz

Often the term RF is used to indicate the presence of electromagnetic field or RF energy.  RF energy is mainly used in telecommunications, emanated by transmitting antennas or to generate heat in many industrial applications.

These frequencies make up part of the electromagnetic radiation spectrum:

• Ultra-low frequency (ULF) — 0-3 Hz
• Extremely low frequency (ELF) — 3 Hz – 3 kHz
• Very low frequency (VLF) — 3kHz – 30 kHz
• Low frequency (LF) — 30 kHz – 300 kHz
• Medium frequency (MF) — 300 kHz – 3 MHz
• High frequency (HF) — 3MHz – 30 MHz
• Very high frequency (VHF) — 30 MHz – 300 MHz
• Ultra-high frequency (UHF)– 300MHz – 3 GHz
• Super high frequency (SHF) — 3GHz – 30 GHz
• Extremely high frequency (EHF) — 30GHz – 300 GHz

Dielectric Heating

In conventional systems the transfer of heat to a body, whether by convection, radiation or conduction, singly or in combination, can only take place through its surface. Consequently the rate at which the internal body of the object heats depends primarily on its thermal conductivity. Heat transfer often has to be restricted in order to avoid damage due to overheating of the surface. This limit to the temperature gradient between the outside and the inside of a body leads to extended process times, poor equipment utilisation and in some cases batch operations instead of continuous flow.

An alternative approach which can eliminate some or all of the problems is dielectric heating, the generic name covering both radio frequency and microwave sectors of the electromagnetic spectrum. Both are widely used by industry because of their so called ‘volumetric’ heat transfer characteristics which avoid the need to heat the surface first.

The types of materials and products which can be processed using dielectric heating are basically non-metals which inevitably have poor thermal conductivity. These include food, textiles, paper, ceramics, plastics, pharmaceuticals, chemicals and timber.

The process operations with which it is associated include:

• Curing of wood working adhesives
• Drying of textiles and other bulk materials
• Tempering and defrosting of frozen products
• Curing of polymers & composites
• Pre-vulcanisation heating of rubbers
• Melting of fats and waxes
• Granulation/drying of pharmaceuticals
• Preheating in fibre board manufacture
• Moisture profile correction in paper making
• Post baking of biscuits
• Baking in a steam atmosphere

The principal advantages of dielectric process heating are based on the fact that heat is generated within the substance itself. Some materials notably water, are much more susceptible to this form of heating than are the substrates in which they are held and consequently, preferential heating takes place. This can lead to significant process advantage; for example, a more uniform rate of drying which in turn results in a better quality finished product usually associated with faster production speeds. In non-aqueous systems, such as polymer curing, the internally generated heat reduces process times and improves plant utilisation.

Whether dielectric heating is suitable for a given product will depend not only on its loss factor but also to great extent on the product dimensions and shape as well as the way in which the electric field can be applied to it.

High Frequency Electric Field

Dielectric heating arises when a high frequency electric field is established within a material which is a poor electrical conductor. The ease with which electrical energy can be transferred to a body as well as the distribution of that energy, depends on the dielectric properties of the various constituent materials which make up the substance. The main dielectric property of interest is the Loss Factor, which principally determines the amount of energy absorbed by the material for a given electric field.

It is important to realise that the dielectric loss factor is not constant but varies with a number of parameters such as frequency, moisture content and temperature. It is possible to find information in the literature giving values of loss factors for certain common materials. However, these are normally measured at ambient temperature and at equilibrium moisture content and should be regarded only as a guide, for a material at a given temperature and moisture content, there is usually a frequency (or resonance) which gives a maximum value of loss factor.

For example, water has a maximum dipolar loss factor at about 20GHz. However the loss factor at other frequencies is usually sufficient for an alternative to be satisfactory.

The power transferred to a dielectric is given by:

P = 2 π f εo εr tanδ E²

Where:

• • P is the power of density in Watts/m³
  • f is the frequency in Hertz
  • εo  is the permittivity of free space = 8.85 x 10^-12 Farad/metre
  • εr is the relative permittivity of the material to be heated
  • tanδ is the loss tangent
  • εr x tanδ = is the Loss Factor of the material being heated
  • E is the RF field strength or voltage gradient within the material, in Volts/metre

The frequencies used for dielectric heating are not chosen because of any relationship with the resonance of water or other molecules but are specified by international regulation in order to minimise the risk of interference with telecommunications.  The frequencies available include 13.56, 27.12 and 40.68 MHz in the radio frequency band and are usually referred to as “ISM” bands used exclusively for Industrial, Scientific and Medical applications.

Radio Frequency

Radio frequency process heating and drying equipment can be conveniently described as a power supply, or generator, delivering power to an applicator or work circuit, which applies an electric field to the material to be heated.

Traditional standard RF generators consist of a single valve (usually a triode) configured as a self excited ‘class C’ oscillator typically a modified Colpitts, tuned anode/tuned grid circuit.

For valves to operate efficiently, a high DC voltage must be applied to the anode. This is produced from the mains three phase supply by a step-up transformer and full wave rectification. Power is drawn by the work circuit from the oscillator circuit through mutual inductance coupling, similar to the windings of a transformer, where the tuned oscillator circuit is the primary winding and the work circuit is the secondary.

RF Generators

Typical RF generators have a conversion efficiency of about 70% between mains supply and RF power output and single generators producing over 100kW are available. How much of this is transferred to the product depends on a number of things including the design of the applicator, the degree of optimisation of the coupling and by the product presentation in the applicator. Consequently it is not necessarily easy to use a radio frequency heater as a general purpose unit.

Triode valves are either air or water cooled and are still widely available.  Brand new valves will typically last in excess of 15000 hours in a well maintained RF generator.  Solid state generators are now also available for special applications in the semiconductor industry.

Applicators

Radio frequency applicators are essentially capacitors, which contain the product requiring heating as the whole or a part of its dielectric. The simplest and most widely used is the through field or parallel plate electrode as shown in Figure 1.

Such a system would be used in a press, for example in plastics welding or wood gluing, in  which case there would be no air gap. However, when used for drying like in a textile dryer, an air space is required above the dielectric to allow for the movement of the product through the machine and for removal of the water vapour. This then means an increase in voltage between the plates in order to maintain an adequate field strength in the product. It is therefore important to consider the relative dimensions of the dielectric and air space capacitors to give the desired heating effect without an electrical discharge taking place. For very thin materials such as paper it may be necessary to use an alternative electrode configuration.

Several other configurations are available for when the product dimensions or other factors make the use of the through-field system undesirable. These include the fringe field, also known as the stray field array shown in Figure 2 which is used in the processing of thin sheets and webs typically in the paper industry.

Another variant is the staggered through-field, also shown in Figure 2 in which the rods are arranged above and below the product. The best known application of this arrangement is in the post baking of biscuits.

These latter two arrays require that the product is transported through the applicator on a conveyor belt in order to achieve uniform heating.

Selective Heating

Radio frequency dielectric heating will heat water in preference to most substrates with which it is associated. This means not only rapid heating but, in the case of drying, equalisation of moisture content in a product as the wetter areas may absorb more heat than the drier ones.

Volumetric Heating

Since energy is transferred by the interaction between the high frequency electric field and the responsive components in the substance, heat is generated throughout the volume of the body.

Efficient Use of Energy

In a RF system, it is possible to design the system so that power is drawn in the proportion to the amount of responsive material in the applicator, hence there is no energy used when the equipment is empty. When carrying out drying, baking or similar operations the energy output will rise and fall proportionally with the quantity of water present in the unit, resulting in high efficiencies.

Improved Product Quality

Because it can avoid case hardening and other surface damage because of the volumetric nature of the heating, the quality of a whole range of products, from textiles, to glass fibre, ceramics, polymers wood, paper and food, can be greatly enhanced.