Microwave heating technology are used in the industry more and more often. Microwave systems are used not only for thawing, but also for cooking, drying, and pasteurization. The unlimited application possibilities translate into optimized production, efficiency, and effectiveness of processes – starting from the chemical industry, through the food and pharmaceutical industries, to mining and metallurgy. The list of applications keeps growing.
Microwave heating is more beneficial economically than conventional heat treatment. One of the most important advantages of microwave heating is its speed; it reduces the duration of processes that normally (using conventional methods) take days or hours to just minutes and seconds! An example is production of aspirin by the pharmaceutical industry: use of microwave driers makes it possible to shorten the heating process from over 2 hours to just 1 minute! Microwave systems also enable great energy savings, higher efficiency, and higher quality of the end product.
Microwaves are electromagnetic waves in the frequency band of 300 MHz to 300 GHz and wavelength in the range of 1 mm to 1 m. Microwave heating consists in transformation of the energy of a variable electromagnetic field into heat as a result of the so-called microwave dielectric heating effect. Radiation in the microwave band causes uniform alignment of dipole molecules (such as water particles) and ions in the created electric field. Because the field is variable, both dipole particles and ions try to become aligned in accordance with the field and oscillate and rub against one another and against other particles, thus transforming the energy supplied as a result of the rubbing into heat.
Most conventional heating and drying methods supply heat from a surface. As a result, moisture is removed from the surface very quickly but removal of liquids from the interior of a product is very ineffective. If the outside temperature is maintained on a sufficiently high level (as it is in an oven), moisture inside the material diffuses to its surface and evaporates. However, this is a passive and long process. In the process of microwave heating, the electromagnetic field acts on the entire material. Heating takes place nearly at the same time in all parts of a heated object (so-called volumetric heating).
Microwave heating systems consist of several elements. The “engine” of the machine is a generator with a microwave oscillator, called magnetron, located inside; the magnetron is responsible for the power of the machine. Microwave generators can be used as single microwave supply sources that usually supply 5-100 kW of energy during continuous operation. In the case of processes that require higher power, they can be used in combination with other generators. Microwave heating systems also have a heating chamber, a waveguide that transports microwaves from the generator to the heating chamber, and a control panel.
In industrial practice, microwave systems working with the frequency of 915 MHz (most industrial systems) and 2,450 MHz (home microwave systems) are used most often. Lower frequency allows for more efficient penetration of microwaves through the material, which translates into lower operating costs of such systems.