Radio frequency and microwave are sometimes used as alternatives to convective, conductive or radiant heat transfer for the proccessing of "nonmetals" [Langton (1949)]. Industries making use of these techniques include textiles, paper, food, plastic and chemicals. The applications are many and varied including drying, baking, defrosting, welding and polymerization [Jones (1987)]. Known as high frequency or dielectric heating, both are forms of electromagnetic wave energy, which share some characteristics but also have significant differences.
The perceived advantage of dielectric heating is based on the so-called "volumetric" effect arising from the fact that the energy is absorbed directly in the body of the material rather than being transferred to it via a surface. The concept of "volumetric" heating needs to be qualified since there is in reality a limiting penetration depth, which depends on the properties of the material being heated as well as on the wavelength of the energy source (Metaxas and Meridith, 1983). At the shorter wavelengths associated with microwave, penetration depth into a wet body is normally a few centimeters; at the longer wavelengths of radio frequency the depth can be a large fraction of a meter or a few centimeters depending on the ionic conductivity of the water in the material.
Dielectric process heating uses the frequency range from about 5 MHz to 5 GHz with radio frequency, RF, being normally defined as being less than 100 MHz. The definition of microwave usually is between 500 MHz and 5 GHZ. Within these ranges there are specific frequencies allocated for industrial, scientific and medical uses, the so-called ISM bands. The most common of these are 13.56 and 27.12 MHz (wavelengths 22.4 and 11.2 meters, respectively) for RF with 900 MHz and 2.45 GHz (wavelengths 0.35 and 0.13 meters, respectively) being the permitted frequencies for microwave. These particular frequencies have been chosen in order to minimize the risk of interference with telecommunications by either the fundamental or by higher harmonics and have no particular significance as far as the resonance of the water dipole is concerned. The actual frequency within the "900 band" varies from country to country, depending on local regulations.
Dielectric heat transfer depends on a number of polarization effects which take place over a broad band of frequencies. The most commonly described one is dipolar orientational polarization [Hasted (1973)].
As can be seen from Figure 1 which shows loss factor as a function of frequency, this is important at microwave frequency but of relatively little significance at the lower, radio frequencies. The dominant mode in the RF range is space charge orientation, which in turn is dependent on the ionic conductivity of the material being processed. If the dielectric properties of a particular material are known it is possible, in theory, to choose the most appropriate frequency from those available in the ISM bands. In reality unless the dielectric loss factor, is very low most products can be dried or processed by either RF or microwave. The choice can then be made on other considerations such as the engineering required to make a satisfactory heat transfer applicator compatible with the process line requirements, i.e., product width, height and shape.
The heat transferred per unit volume of product is given by
|P =||W/m, where|
|f = frequency||hertz|
|E = electric field strength||V/m|
|= loss factor or relative permittivity|
|ε0 = permittivity of free space||(8.85 × 1012 farad/m).|
Loss factor, i.e., the product of dielectric constant and loss tangent, varies with a number of parameters including frequency, moisture content and temperature. The relationships are often quite complex as, for example, in drying where as the temperature increases the moisture content falls. These relationships can be such that preferential heating and drying of the wetter areas takes place, in the right circumstances leading to "moisture profile correction".
When dealing with process heating, interpretation of published data, such as that by Von Hippel (1961) needs to be undertaken with care. For example the figures quoted for "water" often refer to deionizsd water, in this case the effect of having any level of ionic material present (as in most "real" water), will substantially increase the loss factor at RF but have relatively little effect at microwave frequency.
RF and microwave are alternatives to conventional heat transfer with several features, which need careful consideration. Internal mass transfer may be in liquid or vapor phase depending on the structure of the material, for example, in certain capillary porous materials the internal generation of heat having first reduced the viscosity and surface tension of the bulk of the free water in the body will then cause a small quantity to be evaporated, raising the internal vapor pressure sufficiently for liquid phase flow to occur. In other materials where the pore structure is coarser the more conventional mechanism of evaporation followed by vapor phase flow takes place. Perhaps the most significant aspect of this form of heating is that moisture gradients are avoided because the heat is transferred directly to the water and is not dependent on the thermal conductivity of the substrate and consequently the classic model of drying on a retreating wet front does not apply to this form of heating. Because the surface moisture content is normally at least as high as the average in the body the vapor pressure difference across the surface is such that relatively modest air flows are required.
Because the heating arises from the interaction between the high frequency field and the product there is no incidental heating of the oven metal work or the air. In processes, which involve the evaporation of water it is important that recondensation is avoided. The minimum requirement is a flow of heated air over the product to act as a means of mass transfer from the surrounding atmosphere; this same air flow is used to maintain the metal work above the dew point.
Radio frequency power supplies are usually a "class C" oscillator based on a triode valve, which has been built into a cavity type tank circuit. With new regulations in place increasing importance is being attached to the need avoid electromagnetic interference with other equipment, and therefore alternative generator types are being considered for some applications, such as the use of crystal driven linear amplifiers in conjunction with 50 ohm transmission lines.
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 2.
When used for drying an air space is required above the dielectric to allow for the movement of the product through the machine and for ventilation of the water vapor. This then means an increase in voltage between the plate 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.
For industrial heating applications using microwaves the usual power source is the magnetron. The most common form of microwave heating applicator is the multimode cavity, similar in concept to the domestic oven. When used for continuous, conveyorized processing the design of the ports to allow the passage of product is critical in order to prevent the emission of microwave energy. The limits on the dimensions of these apertures can be very restrictive, typically 20 to 30 mm for 2.45 GHz and perhaps 80 to 100 mm at 900 MHz.
Industrial microwave heating has been used extensively in the rubber industry for curing and preheating prior to moulding. In the food industry it has been used for tempering, melting, cooking and drying. Recently microwave vacuum dryers have been developed for drying expensive, high quality temperature sensitive pharmaceuticals.
Hastel, J. B. (1973) Aqueous Dielectrics, Chapman and Hall, London.
Jones, P. L. (1987) Radio frequency processing in Europe. J. Microwave Power, 22-3, 143-153.
Langton, L. L. (1949) Radio Frequency Heating Equipment, Pitmans, London.
Metaxas, A. C. and Meridith R. J. (1983) Industrial Microwave Heating , Peter Perigrinus, London.
Von Hippel, A. (1961) Dielectric Materials and Applications, MIT Press, Cambridge Mass.