Famously discovered accidentally by Dr. Percy Spencer when a chocolate bar mysteriously melted in his pocket while he was working on a new device for radar applications, microwave heating has been around since World War II. Given the length of time and the speed with which many new technologies have been applied during the last 60 years, industrial microwave heating could be considered a somewhat slow developer.
Industrial uses of microwave technology initially were restricted to simple heating and drying applications, either at laboratory scale or in a production system. Food, paper, textiles, wood, rubber, chemicals, semiconductors and ceramics - all nonmetals with poor thermal conductivity - are among the materials that now commonly are processed by microwave heating equipment. But other materials, including metals, are continuously joining the list, and the temperatures and complexity of the processes also are steadily increasing.
One of the reasons for the slow start for industrial applications was perhaps the incomplete understanding of the mechanisms involved. Spencer’s chocolate bar melted when he was working with a magnetron, which is still the predominant mechanism for generating microwaves. But for many years, the way microwaves acted on a given material was not understood. Even now, research continues into some of the technology’s features, and new potential benefits still are becoming apparent.
A magnetron is an oscillator capable of converting electric power, usually in the form of high voltage DC current, into high frequency radiant energy. The polarity of the emitted radiation changes between negative and positive at high frequencies, and material within the radiation field heats up through molecular friction as the dipoles within it try to reorient themselves. By international agreement, certain microwave frequencies are reserved for industrial, scientific and medical applications, each having a specific wavelength.
The standard frequency used in domestic microwave ovens is 2,450 MHz, with the magnetrons producing 800 W or so at maximum power. This frequency also is used for industrial systems with power ratings commonly up to 20 kW and occasionally higher. Larger industrial heating systems use 896 MHz or 915 MHz magnetrons; there is some overlap of the power ratings of magnetrons at these two frequencies. Waveguides transfer the generated energy from the magnetron to the processing chamber, where - depending on the cavity design - a device known as a mode stirrer may be used to improve energy distribution.
Microwave heating has some particular characteristics and is quite different in many ways from conventional radiant heating.
First, it is volumetric, which means that energy is generated directly within the body of the material itself. (By contrast, with radiant heating, the energy is absorbed at the surface and the interior gradually heats up through conduction from the external surface.) Some materials are more susceptible than others to microwave energy and heat more readily. Volumetric heating also can result in energy being used efficiently because only the target material is heated.
Second, in many materials, heating is almost instant and takes place without the need for radiating elements to heat the air or a container.
And third, heating is highly specific, with different materials displaying different “susceptibilities” to microwave energy. Because of this susceptibility, preferential heating can take place, which can provide process advantages when optimized effectively. This differential can be used to advantage in microwave processing. For example, pharmaceuticals can be sterilized in their packaging without the plastic heating up, and wet areas of a product will take up heat more than dry areas, so moisture content will equalize.
Despite the advantages offered by microwave heating, when applied in isolation, it sometimes can be less successful at higher temperatures such as those required for firing or sintering ceramics. This is because once a sample heats up, it will generally be at a higher temperature than the surrounding atmosphere, and heat can be lost from the material’s surface. This in turn can create temperature gradients within the material, albeit the reverse of those associated with radiant heating, and the gradients increase as the component becomes hotter. This limiting factor can be particularly significant for materials requiring high structural integrity.
Another challenge when designing industrial microwave heating systems is that the optimum frequency for any given material may not be constant over the entire temperature range encountered during heating. Therefore, it is important to test the system and process design with the actual material to be heated.
In conclusion, microwave heating offers the four key advantages:
- Energy efficiency. Power is only applied to the material.
- High quality. Case-hardening and other surface damage is avoided.
- Selective heating. In many cases, this provides processing benefits.
- Direct heating of the sample body. This reduces process times.
If your product has a high water content and can be heated effectively by microwaves, consider whether technology that often heats your lunch could also drive your processing line.