Dielectric heating is one of the most common applications for not only industrial, but also domestic microwave systems. The microwave ovens used at home are heating our food based on this principal, while industrial microwave systems also cover applications like drying of materials, sintering of ceramics, vulcanisation of rubber, waste management and others. In the following we give you an overview how dielectric heating works, what are the advantages and what are the limitations.
Per definition a dielectric material is an electrical insulating material which, when exposed to an electromagnetic field, can be polarised. Polarised in this context means that at least some of the electrical charges inside the material are slightly displaced under the influence of the external electromagnetic field, however they do not flow through the material like in an electrical conducting material. If some molecules inside the material are only weakly bound, they will not only get polarised, but align themselves to the external electromagnetic field. If this electromagnetic field is changing direction, as it is the case for a RF or microwave field, this alignment will take place at the same frequency as that of the external electromagnetic field, leading to a rapid rotational movement of the molecules inside the material. As they rotate, they collide with neighbouring molecules, transferring some of their kinetic energy to these molecules. Because temperature is nothing else but the average kinetic energy of atoms or molecules within a material, the alternating external electromagnetic field is increasing the average kinetic energy and thus the temperature of the material. This is known as dielectric heating (The actual mechanisms are a little bit more complicated, but we try to keep it simple).
One of the main advantages of dielectric heating via microwaves is that the microwave energy is able to penetrate the material and heat it from the inside, an effect known as volumetric heating. This means that in a properly designed plant the temperature of the material increases almost evenly over its cross section, unlike in conventional heating processes where the energy can only be applied to the surface and the heating of the inner sections can only take place via thermal conductivity. This means that the product can be heated fast and efficiently, without the risk of overheating the surface areas.
The most commonly known dielectric material is water. Because it is a liquid at room temperature, the molecules are weakly bound and can rapidly align to the electromagnetic field, thus absorbing a high degree of the electromagnetic field's energy and converting it into kinetic energy. This is termed as being lossy, i.e. materials which can absorb a high percentage of the electromagnetic field have a high loss factor. Because our food generally contains a high amount of water and water has a high loss factor, water is mainly responsible when it comes to heating our food in domestic microwave ovens. In industrial drying processes the high loss factor of water makes it possible to dry the products fast and efficiently using microwave energy, and, when applied as combined microwave vacuum drying, far more gentle than other drying technologies.
Other dielectric materials commonly used in industrial dielectric heating applications are polarised activated carbon powder, better known as carbon black, which is widely used in the rubber processing industry, and certain types of ceramic materials, which makes it possible to heat them to the required sintering temperatures by using microwave energy.
Unfortunately, dielectric heating is not as straight forward as it might seem from the previous paragraph. The dielectric loss factor for a given material is not a constant, but is a function of a number of different factors, e.g. temperature, frequency etc. For example, if the temperature of water is increased by 20 K, its loss factor can drop by a factor of 10, depending on the absolute temperature. At the same time, the frequency at which the maximum loss factor occurs varies over temperature, and if that would not be enough the parameters change again if the water contains salt. Another example are certain types of ceramic materials, which suffer from a sudden drop in the loss factor once they reach a certain temperature, making it almost impossible to heat them above this temperature using microwave energy alone. An additional important factor for even volumetric heating is the penetration depth, which describes how much microwave energy is absorbed by different layers of the material depending on their distance to the surface. Penetration depth depends largely on the actual loss factor and the frequency of the microwave radiation, and wrongly designed microwave equipment can lead to big differences in minimum and maximum temperatures, especially for materials with a low thermal conductivity or when rapid heating of the material is required.
It is therefore important to analyse the process at hand thoroughly before deciding on the design of a suitable microwave plant for dielectric heating. Besides our experience gained from the optimisation of existing plants and processes, we can also support you when it comes to develop new applications, as we do have access to the required research and testing facilities. So if you are looking into optimising your existing process, or need support to develop a new application, please feel free to contact us.