No.59
Microwave
Istok Magnetrons

What Are Microwaves?

Microwaves are electromagnetic waves of the same class as radio and light. Like radio and light waves, microwaves can be sent through space and received or absorbed at a distance. The space may be a vacuum or may be filled with certain gases, liquids, or solids.

Materials that do not block the radiation are said (in analogy to visible light) to be transparent to microwaves. Most materials have some degree of absorption so that a significant portion of microwave energy is consumed as it passes through. The material is then heated by this absorbed energy.

The frequencies at which microwaves are generated are quite high in terms of normal human comprehension. For example, 3.15 GHz, which is midway between the two middle frequencies assigned for industrial use, represents a number of pulses per second equal to the number of seconds in a century. The second highest frequency reserved for industrial use, 5.8 GHz, represents a number roughly equal to the total of human heartbeats occurring on the entire earth in one second.

Why Microwaves Make Things Hot

Most molecules, though electrically neutral, have an asymmetrical distribution of electrons; they may be electrically positive on one end, negative on the other. As a result, in a changing electrical field these polar molecules behave much as a weathervane does in a shifting wind. A burst of microwave energy sets up an electric field which drives the molecdules toward a specific alignment. Then when the field is removed, elastic recovery pulls the molecule back toward its initial alignment. These energy bursts occur many million times each second in a microwave field, so that the molecules are driven into and out of alignment at an extremely rapid rate. It is this molecular agitation that generates heat. This effect is illustrated schematically in the diagram, where a molecule of water is located between two plates connected to the terminals of a battery. When the plates are charged, the molecule is pulled into alignment. This effect is called dipole rotation.

Microwave energy also creates heat by another mechanism called ionic conduction. In this effect, which occurs primarily in liquids, any free ions present are alternately driven back and forth in response to the oscillating field. As the ions are rammed back and forth, they collide with un-ionized molecules, and these collisions generate heat.

What Does It Cost?

The power levels of a microwave generator in commercial use generally range from a few kilowatts to 100 kW. Typical systems include one or more generators and may run up to many megawatts. Future systems are expected to be in the 1,000 to 10,000 kw range. The energy available from a typical 60 kw microwave generator can boil away about 200 lb. of water an hour. The power required for a specific application is found from:

p = r c (delta) t/3400

where p = power, kw; r = rate of material processed, lb/hr; c = specific heat, Btu/lb-deg F; (delta)t =temperature increase, deg F. The cost of a microwave generator is approximately $1,000 to $1,500 per kw. Operating costs are about 15 cents per kw-hr.

Microwave Tubes

Two tubes, the magnetron and klystron, may be used for generating industrial microwaves. The magnetron is the preferred tube because of cost. In the klystron, a powerful electron beam is focused by axial magnetic fields through a series of resonant cavities. Electron bunching induces microwave energy into the last cavity. Operation of the magnetron is similar, except that the electron beam is directed into a spiral by a crossed magnetic field, and the resonant cavities are arranged in a circle.

The klystron usually lasts longer and can be made more powerful than the magnetron, but the magnetron is more efficient and costs less.

Microwave Plumbing

The devices used to “pipe” microwave energy from a microwave tube to the workpiece are called waveguides. They are simply rectangular pipes (about 2 in. by 4 in. for 2450 MHz; 5 in. by 10 in. for 915 MHz) bolted together with flanges much like those of ordinary steam fittings. Typical waveguides are shown in the photo.

The electric field is strongest at the center of the waveguide as shown in the drawing. A slot can be cut in the waveguide parallel to wave propagation without inducing radiation from the guide. This arrangement is used for heating sheet material, and typically is expanded into the serpentine configuration shown. The particular structure used to transmit energy from the waveguide to the workpiece is called an applicator. Thus, the slotted guide shown here would be termed an applicator.

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