How do the RF & MW technologies work?

Electromagnetic waves

Electromagnetic waves are formed by the combined and simultaneous action of electric and magnetic fields, whose intensity varies cyclically with a certain oscillation frequency. Frequency is inversely proportional to wavelength, according to the equation: v = f λ
where v is the speed of the wave, f is the frequency and λ is the wavelength.

Thanks to the interaction effects of electromagnetic waves with the matter (atoms, molecules, ions), under specific conditions it is possible to generate heat directly inside several materials. The heating mechanism will depend on the frequency of the electromagnetic waves applied, and on the specific chemical and physical characteristics of the material.

Radio Frequency and Microwave

The oscillation frequency of electromagnetic waves is measured in Hz (cycles per second). Conventionally, electromagnetic waves ranging from 30 kHz to 300 GHz are called Radio Frequency (RF) and those over 300 MHz are called specifically Microwave (MW).

Since RF and MW are widely utilised by radio communication systems, in order to avoid interferences the competent authorities have allocated specific frequency ranges (bands) to be used worldwide for ISM (Industrial, Scientific, Medical) purposes. Permitted frequencies of industrial significance for dielectric heating applications are: 13.56, 27.12 and 40.68 MHz within the RF range; 2.45 GHz (and, in some countries only, 896 or 915 MHz) within the MW range.

Dielectric heating

If we exclude the materials, in particular metals, which are good conductors of electric current, in all other materials subjected to RF or MW electromagnetic fields, heat will be generated mainly due to the so called “dielectric losses”.

Dielectric losses are caused by the vibration and rotation of polar or polarised molecules, and by the polarisation and translation movement of ionic particles present in the material, induced by the quick (several million times per second) polarity reversal of the electromagnetic field. This can be interpreted as if the electromagnetic field is absorbed and converted into thermal energy by the effect of the rapid movement of polar(ised) molecules and ions.

Water molecules are highly polar, more than all substrates in which water can normally be found, and many ionic species are usually dissolved in water. Therefore, both RF and MW electromagnetic fields can heat up very quickly wet materials. In particular, RF has the ability to evaporate water very quickly, efficiently and selectively from many substrates (textiles, agricultural commodities, baked products, etc.).

Advantages of dielectric heating

Radio Frequency and Microwave generate heat directly inside the products, in depth and instantly. On the contrary, with traditional methods first the heat is generated outside the product and is then transferred to it by means of the well known heat transfer mechanisms (conduction, convection, radiation).

The endogenous heat generation is the key that makes the RF and MW heating methods fast, efficient and ensures the best product quality while eliminating all typical drawbacks of conventional methods (slow heating, surface overheating, heat losses to the environment, etc.).

Typical structure of a Radio Frequency heating equipment

Broadly speaking, all RF heating equipment consist of two main distinct parts:

- the generator
- the applicator (or electrodes)

The generator converts the normal electricity from the mains supply into radio frequency electromagnetic energy. It is composed of a suitably designed combination of capacitors and inductances (the oscillating LC circuit) connected to a vacuum valve (the triode), complete with the high voltage DC supply unit.

The applicator receives the electromagnetic energy from the generator through simple conductive metal connectors and applies it to the product to be heated. The applicator design depends on the product shape and size, but in practice three basic configurations are sufficient to cover the majority of industrial cases, depending on the electric field direction (orientation):

vertical: for through heating on bulky products
horizontal: for surface heating on thin or layered products
inclined: for through heating on thin or layered products

Typical structure of a Microwave heating equipment

Regardless of its complexity, a microwave equipment can always be seen as made of three basic components:

- the generator, incorporating the magnetron and its power supply system, which is the actual source of MW energy;

- the waveguide, which is a metallic duct in which the microwave energy propagates and is brought from the generator to the process area (cavity);

- the (resonant) cavity, which is a box or tunnel made of microwave reflecting material (generally stainless steel), having suitable geometry and dimensions, where the heating process of the product takes place.