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.