Technologies / Radio frequency

What is RF Heating?
Radio Frequency (RF) heating is a type of electro-heat technique designed to heat electrically-insulating (dielectric) materials. The principle behind these techniques is that portions of the electromagnetic spectrum are utilised as the primary energy source to heat a material. The application of electromagnetic radiation may be direct (straight into the material) or indirect (via a heated appliance). This differs from conventional heating which is based on heat moving through the material.

Fundamentals of RF Heating
There are two principal mechanisms by which a dielectric material can be heated by an RF electromagnetic-field:

  1. Electrical conduction.
  2. Dipole rotation.

Heating through electrical conduction is where small currents are induced within the dielectric material by the oscillating electric-field dissipating power as heat in a process known as Joule heating.

Many dielectric materials have dipolar molecules. A common dipolar molecule in RF heating applications, for example in the food or drying industries is water. The formulation and dipolar moment for the water molecule is shown below.

Dipolar molecules within a dielectric material couple themselves electro-statically to the applied electromagnetic-field and tend to align themselves mechanically with the field polarisation. The applied electromagnetic-field is alternated in time and the dipoles attempt to realign them selves with the oscillating field, resulting in molecules which are in a state of mechanical oscillation at the applied frequency. The successive rotations generate heat through friction at the molecular level. When the applied field is removed, the dipolar molecules relax back into their original equilibrium state:

Practicalities of RF Heating
RF heating typically refers to heating in the 1-500MHz frequency range. In order to manage the electromagnetic spectrum for a wide range of commercial and industrial applications the ISM (Industrial, Scientific and Medical) RFR bands classify the portions of the spectrum for RF dielectric heating applications in the UK at 13.56 MHz (+/- 0.05%), 27.12 MHz (+/- 0.6%), and 40.68 MHz (+/- 0.05 %).

In RF heating applications the material is placed between a set of electrodes to which a high-voltage, typically in the kilovolt (kV) range, is applied at the appropriate frequency. This structure creates a capacitor where the material is part of the dielectric, and a charge and discharge current flows between the electrodes creating an alternating electric-field across the material. Although presented below as a parallel-plate electrode arrangement, this can be any number of configurations.

Typical applications of RF heating include:

  • Forming and welding of plastics

  • Seasoning and gluing of woods

  • Drying of textiles, paper, boarding, and ceramics

  • Heating, drying and defrosting of food

RF heating can also be augmented with conventional heating techniques, such as hot-air and steam, to obtain the heating benefits of multiple heating methods such as moisture containment, surface drying, and reductions in external evaporation.

Advantages of RF Heating
The advantages of RF heating for industrial processes are:

  • Volumetric Heating – energy is transferred through the surface and into the material electromagnetically. The electromagnetic-field and material interact throughout the whole product, which greatly enhances heating uniformity. In contrast to conventional heating where the heat travels from the surface inwards.

  • Rapid Volumetric Heating – the ability to dissipate extremely high power densities within a material (up to 20W/cm_) resulting in very fast heating times. This is distributed evenly across the material (not confined to the edges as with conventional heating), preventing product damage.

  • Selective Heating – the electric-field is stronger in areas of the material which can’t convert this to heat as easily (e.g. wetter parts) This gives even moisture levels throughout a product as they balance out over time.

  • Controllability – instantly controllable, and the power supplied can be regulated very accurately. This allows safe and precise control of an applicator even when applying large power or rapid heating rates.

  • Efficiency – up to 80% of the total power consumed can be transferred electromagnetically (compared to less than 25% for conventional heating) because heat losses from the equipment are dramatically reduced.

Back to top of page