Rubbing a piece of plastic with wool together develops a static charge that can attract or repel small bits of paper. We can assume then that the electrically charged plastic can develop electric field changes in the surrounding space. If electric charges are present in a specific space, this space is disturbed by the presence of these charges. In this space resides an electric force field. These charges create an electric field. A magnetic field is a field produced by moving electric charges or by electric fields that vary in time; electric and magnetic field together produce the electromagnetic field. Radio frequency (Rf) and microwave (Mo) techniques are “endogenous” or “volumetric” heating techniques, i.e. there is a uniform volumetric heat generation throughout the product volume. Unlike conventional thermal treatment systems used in food applications, based on the transfer of heat from the outer to the inner surface by means of conduction, convention or radiation, Mo or Rf heating is a mechanism by which energy in the form of electromagnetic radiation is converted directly to heat energy in matter. As already mentioned, microwave ovens use a device called magnetron that oscillates an electron beam at very high frequency; it emits electromagnetic waves whose wavelength is typically about 12 cm long with a frequency of 2450 MHz. In Mo applications this frequency is used, because water absorbs more electromagnetic energy at full capacity at 2450 MHz, so that food containing water heats up quickly. Molecules are caused to rapidly align themselves with the electric field and oscillate around their axis. Oscillating particles create a considerable intermolecular friction, which results in the generation of heat. An empirical rule states that non-ionizing electromagnetic waves (like RF and Mo) can penetrate solid surfaces up to a depth corresponding to approx. 1/10 of their wavelength. It results that Mo are more suitable for processing products with a thickness of little more than one centimeter. Hence, the possibility of heating thicker bodies with Mo depends from the capacity of transmitting heat from the outer layers to the core by means of conduction and convention systems, typical of conventional heating systems. To obviate the limited penetration depth and the impossibility of heating quickly and uniformly bodies measuring more than a few centimeters, sometimes lower operating frequencies are used, as for instance 915 MHz (wavelength of approx. 33 cm and penetration depth of approx. 3-5 cm); however, in many countries these frequencies can be used only after total shielding of microwave radiation. This is possible only on semi-continuous or discontinuous systems, entirely closed, or further to special authorization when incompatible with existing international regulations.
Energy transfer and even heating
Apart from the non-uniformity typical of Mo thermal treatment due to the limited penetration depth, another reason for the uneven heating of treated product is the mechanism for the transfer of energy from the source: the generator. In industrial and household appliances, Mo waves are generally produced by magnetrons and then transported – through “waveguides” – to the so-called cavity, where the product to be treated is placed. The energy conducted to the cavity can be “punctual” and is absorbed by the product where it hits it, either directly or indirectly (by the waves bouncing on the reflective metal cavity). Mo energy virtually hits the product randomly, depending on its position in the cavity and its geometry. To obviate this inconvenience, usually the product is kept moving (by means of rotary rests or conveyors) and wave stirrers are used. In spite of this, energy transfer cannot be controlled with statistical accuracy, with uneven treatment results and penalizing temperatures.
Generator power output
The megatrons implemented to produce microwaves are characterized by a power output of a few kW (usually between 1 and 3 kW at 2450 MHz). Hence, although microwave technology is extremely competitive for small household appliances thanks to the low cost of a single magnetron and the economies of scale that can be attained for mass production, the use of this technology at industrial level is not profitable in terms of plant construction and costs since it requires extremely high powers. Very demanding and burdensome is also the combined management of a large number of magnetrons, especially considering what is required for the accurate adjustment of the delivered power (On-Off at variable times or modulating power feeding devices).
Efficiency and energy consumptions
Although theoretical energy performances of Rf generators and magnetrons are similar (ranging between 65-70%), due to energy transfer methods and energy reflection systems in microwave equipment , their total energy performance range is approximately 50-55%.
Electromagnetic field emissions and safety
Due to their reduced wavelength, microwaves are particularly “aggressive” and difficult to shield: openings larger than approx. 1/10 of the wavelength, if not adequately protected, create electromagnetic field emissions that do not comply with international safety limits. For this reason, microwave appliances must be entirely enclosed in metal, and even the inspection windows must be adequately shielded by inserting a fine metal mesh. In the case of continuous industrial processes, the plant must be equipped with double gates that close alternatively or equivalent shielding systems with metal mesh, to prevent direct communication between external environment and microwave cavity via the product feeding and exit doors. As an alternative, very long shielding openings can be used with microwave energy reflecting and absorbing systems, so as to reduce emissions to acceptable levels; their effectiveness, however, cannot be previously foreseen for all operation conditions.