Fundamentals of Microwave Technology

How Industrial Microwaves Heat...

When microwave heating can be used - and is used properly - almost certainly you will process more products in less time at lower cost. Throughout history there has been one way to heat materials: apply heat to its surface. About thirty years ago, industrial engineers began developing microwave-heating techniques that avoid some limitations of conventional heating. Results often are remarkable. With microwaves, no heat is applied per se. Instead a form of radio waves (neither nuclear nor ionising radiation pass through the material. The molecules in the material then act like miniature magnets attempting to align themselves with the electrical field. Under the influence of this high frequency alternating electrical field, the particles oscillate about their axes creating intermolecular friction, which manifests itself as heat.

What are the advantages

In conventional heating the heat source causes the molecules to react from the surface toward the centre so that successive layers of molecules heat in turn. The product surfaces may be in danger of over heating by the time heat penetrates the material. Microwaves, however, produce a volume heating effect. All molecules are set in action at the same time. It also evens temperature gradients and offers other important benefits.

How does an industrial microwave system differ from a home microwave oven

The home/restaurant batch-type ovens available to date employ a relatively low power source as compared to the industrial power source. This makes producing products beyond pilot scale levels very labour intensive. In addition, the typical home oven is considerably less efficient, up to 25% less, in converting electrical energy into microwave energy. By comparison the industrial oven can be designed as either a batch or as a continuous system with much higher power and greatly increased throughputs, reducing the amount of handling required. In addition, a forced air system can also be employed, which greatly improves moisture removal in drying applications.

Electromagnetic waves with frequencies between 300MHz and 300GHz are called microwaves, by definition. Their respective wavelengths are between 1mm and 1m. Only a few frequency bands are allocated for microwave heating applications, the most common being 2450 MHz50 MHz or 'S' Band.


Generation of Microwave Energy

Microwave power output for heating processes is produced by a special tube called a Magnetron. Structurally, a magnetron is a high vacuum electronic valve consisting of a hollow copper anode incorporating a series of resonance cavities, at the center of which is an electron emitting cathode.

To gain a rough idea of how a magnetron generates microwaves, one can resort to a simple acoustic analogy. You know that if you blow air across the mouth of a bottle, a tone, or oscillation, will be generated which has a wavelength proportional to the size of the bottle. If you fill the bottle partially with water, the frequency of the acoustical tone generated increases. In the case of a magnetron tube, the electron cloud generated by the cathode is the 'air' and the resonant cavities of the anode surrounding the cathode are the 'bottles.' As electrons leave the central anode they are induced into circular rotation by a magnetic field, passing many resonator 'bottles' before finally being captured by the anode. The result is microwave energy generated within a very narrow frequency bandwidth.

Typical modern magnetron tubes operate at electrical efficiencies greater than 70% and have lives on the order of 5000 hours.


Interaction of Fluids with Microwave Energy

There are several mechanisms by which matter absorbs microwaves, however, the most common are dipolar rotation and ionic conduction.

Dipolar rotation accounts for nearly 80% of the heating contribution at 2450MHz. Dipolar molecules (e.g.:H2O) exist in nature normally in a random orientation. When imposing an electrical field (such as microwave energy), dipolar molecules tend to become ordered due to their asymmetric distribution of unlike charge partners. As the electrical field dies down, the dipoles return to their random (disoriented) orientation. The net result is a conversion of energy from electric field energy to stored potential energy in the material and then to stored random kinetic or thermal energy in the material. Typical microwave energy fields are able to pull into alignment only as few as 1 molecule in every 103 - 105 molecules.

Ionic conduction accounts for the bulk of the remaining heating contribution at 2450MHz. Ions exist in many fluids naturally, however, the ionic content of a fluid such as water can be increased with addition of a salt such as sodium chloride (NaCl). The application of an electrical field to an ion causes an acceleration of that particle towards its like charge. This acceleration causes an increase in collisions with unionized molecules. The net result is electric field energy converted into ordered kinetic energy and then disordered kinetic energy or heat.

The effect of dipolar rotation is strongly temperature and frequency dependent. Ionic conduction does not depend on temperature or frequency.

Are Microwaves Dangerous?

No, our microwave systems are even safer than the magnetrons people use at home in their kitchen.
The microwave shall not be mixed up with x-ray radiation or radioactivity, which is also called ionizing radiation. Microwave is an electromagnetic wave, generated in a high frequency generator, which is tuned at a fixed oscillating frequency. The microwave is only present as long the generator is being operated.
Naturally radioactivity cannot be switched off.

The limit for the maximal permitted energy density of microwave to which a human body can be exposed without any detectable harm, is set after many tests at 5 mW/cm2. Per definition the maximal microwave leakage energy of a microwave generator shall not be greater than the 5 mW/cm2 at a distance of 5 cm from any possible radiating location at the surface of a microwave generator. Because non focused microwave behaves in its expansion similar to the light from a light bulb, the energy density is decreasing at a level, which is reversed proportional to the square of the distance from the source.