No.58
Microwave
Istok Magnetrons

The household radar oven has a cousin in industry that can cure plastics, dry wood, bake coatings, or handle many other processes requiring heat. Though these industrial microwave ovens are costly, they need only minutes to do jobs that ordinarily take hours or days, and often do jobs far better than conventional heat. You can buy a radar oven that cooks a roast or bakes a cake in a few minutes. Engineers designing industrial heating equipment can make use of the same technology to perform industrial processes by microwave in a small fraction of the time normally required.

A foundry cures resin-bonded sand cores by microwave in from 3 to 10 minutes where conventional baking takes 45 to 60 minutes. Watersoaked logs are dried in a few hours where days or perhaps months would ordinarily be required. One of the major automakers rapidly processes plastic materials by microwave in a mass-production application.

In the realities of business, however, speed in itself is not important. The crucial question is: How much does a process cost? Here, microwaves suffer a handicap because microwave equipment is expensive. For every five applications where microwave power is technically attractive, only one application is feasible from a cost standpoint. Nevertheless, microwave heating can do things that simply cannot be done with conventional heating.

Microwaves generate heat directly inside the workpiece, with or without heat gradients, whichever the process designer desires. Microwave heating has no "flywheel" effect; heat input can be changed instantly without the thermal lags that occur in virtually all other processes. And it can apply heat selectively since some materials respond to microwaves while others do not.

Background on Microwaves

Microwave heating equipment uses the same type of microwave generating equipment used in military and commercial radar and communications. Microwaves can be generated by various types of electron tubes, namely, klystrons, magnetrons, crossed-field amplifiers, traveling wave tubes, and backward-wave oscillators. Of these, only the klystron and magnetron are economical enough for commercial industrial heating. The others are used in radar and communications.

The principle behind microwave heating is relatively simple. Briefly, any molecule with an electrical polarity tends to align itself with an electric field, then revert to its original position when the field is removed. When the field is applied and removed rapidly, the molecules vibrate rapidly, thereby generating heat.

Radar technology is now several decades old, but it has been only over the past few years that microwave equipment has become reliable enough and economical enough for industrial use. The predecessor to microwave heating is dielectric heating and its medical counterpart, diathermy, which came into use in the late 1930s and are based on radio rather than radar technology. Dielectric heating was used to dry glue during construction of wooden PT boats and the all-wood Mosquito bomber during World War II. The principle behind dielectric heating is the same as that of microwave heating, except that much lower frequencies are used with dielectric heating. The following list shows how the frequencies of these two industrial heating bands compare with the familiar commercial broadcasting frequencies:

1 MHz Commercial AM Radio

6-100 MHz Dielectric Heating

88-108 MHz Commercial FM Radio

54-216 MHz VHF Television

470-890 MHz UHF Television

900-30,000 MHz Microwave Heating

The frequencies shown for microwave heating encompass the entire range classified as microwaves. Actually, only four specific frequency bands are used for industrial heating applications. These four bands were allocated by the Federal Communications Commission and are called the Industrial-Scientific-Medical or ISM frequencies. These bands are at frequencies of 915 MHz, 2450 MHz, 5800 MHz, and 24,125 MHz. Users of industrial microwave equipment are allowed to generate unlimited power on these four bands, chosen so that they do not interfere with radar and communications.

Although dielectric and microwave heating are based on the same principle, the widely different frequencies used create a significant difference in their characteristics. The reason lies in the relationship defined by the basic equation for electronic heating:

p = k e^2 f

where p = heating power generated in the material, k = dielectric loss factor of the heated material, e = strength of the electric field, and f = frequency of the field.

In simple terms, this equation says that the amount of heat generated in a given material by an electric field of given voltage is proportional to the frequency of the field. Since microwave frequencies are from 10 to 200 times greater than those of dielectric heating, the microwave process can pour from 10 to 200 times as much heat into a material for a given electric field strength.

Perhaps the greater significance of this relationship is that to put a specified amount of heat into a material, equipment operating at microwave frequencies requires considerably less voltage than equipment operating at lower frequencies. In dielectric heating, a major limitation is maximum permissible field strength, which must be kept at a low level so that there is no destructive arcing within the material or over its surface. This limitation thus restricts the rate of heat input. Since field strength can be substantially lower with microwave heating, arcing is no longer a practical limitation on heat input.

Of the four frequencies set aside for ISM applications, only the 915 MHz and 2450 MHz frequencies are used industrially, since they are the only ones for which economically practical microwave tubes are available. It would in some cases be desirable to use somewhat higher frequencies for the reasons just explained, and some laboratory work on heating is being done at 5800 MHz and 24,125 MHz. But at this time these frequencies require equipment that is so expensive that its use for high-power industrial purposes is currently out of the question.

In addition to the difference in frequencies, there are other distinctions between dielectric and microwave heating that are important. The position of the electrode with respect to the product in dielectric heating is quite critical. Thus, each installation must be tailored to the specific product at hand, and then only that one product can be heated. Such care is not necessary with microwaves.

The equipment for dielectric heating, however, is much cheaper than microwave equipment. But dielectric heating works well only with materials that are quite "lossy," a term that comes from the communications field and refers to materials which exhibit high dielectric loss, and thus readily soak up radio energy. Transite, asbestos, and carbon are examples of highly lossy materials. In short, dielectric heating is cheaper, but can be used on fewer materials and is often slower than microwaves.

Heat from the Inside Out

The basic attraction of microwave heating is that it generates heat within a material. Anyone heating an object by conventional means first has to make the outside hot before the heat can be conducted to the interior. This means that the outside always must be hotter than the inside, and therein lies a major limitation of conventional heating. There is always a temperature gradient, and if the interior must be heated rapidly, the outside has to be substantially hotter than the inside. In many cases the speed of the process is limited by surface scorching, burning, or melting.

With microwave heating, there may or may not be a heat gradient at the election of the system designer. If he wants the inside hotter than the outside, he heats with microwaves while bathing the object in a flow of cooling air. This type of heat gradient is highly desirable in many drying applications where gases must be driven out of an object through a surface that would develop a skin and become impermeable with conventional heating.

If the designer does not want a temperature gradient, he simply heats with microwaves and bathes the surface of the object with air of the same temperature as that created within the workpiece. Or if he wants to use microwaves while retaining the normal outside-in heat flow, he can bathe the object in air heated beyond the microwave-induced temperature. The important point is that a tailor-made gradient can be set up without the outside-hotter-than-inside dictate of conventional heating.

Some Materials Heat, Some Don't

Only dielectric or semiconductor materials can be heated by microwaves; conductive materials do not respond except in special cases. This means that metals do not respond, except when in the form of thin wire or thin strip (in which case the heating effect is similar to induction heating). You could not, for example, build a microwave billet heater or alloy-melting furnace, but you can build microwave heaters for thin wire and strip.

Plastics as a class respond exceedingly well to microwaves, but a few plastics do not, the most notable of which are Teflon, polypropylene, and polyethylene. Other organic materials such as food, wood, paper, and glues respond well.

The food industry represents one of the largest areas of potential application for microwave heating. But prospective applications in general industrial heating are just as numerous.

Microwaves At Work

Microwaves are a practical means for heating plastics in many of the various ways that plastics are processed. Microwave energy can be used for preheating bulk material prior to processing as, for example, in the preheating of thermosetting preforms prior to compression molding, or the preheating of pre-pregs for molding or winding. Microwave heating of thermoplastics for injection molding or extrusion is practical, both for preheating and moisture removal. Small thick sheets of thermoplastic materials such as acrylics can be heated and stretched into large thin sheets at the fabricator's plant to save the cost of shipping large sheets between acrylic processor and fabricator.

Probably the area of most interest in plastics processing is the curing of thermosetting resins such as polyesters and epoxies. There is also considerable interest in the curing of urethane foam, where microwaves overcome drawbacks associated with the poor thermal conductivity typical of mold materials.

Ultrafast Drying Another large class of applications concerns drying operations, principally in woodworking, papermaking, and fiber industries. In water removal by microwave, less power is absorbed by the workpiece as the material dries. Thus, there is less tendency toward overheating. Since water is a good absorber of microwave energy, heat tends to concentrate in the wetter areas of the workpiece. This provides a leveling effect that is particularly beneficial in drying materials to a uniform moisture content.

In general, microwave heating is quite useful in any operation involving web processing; that is, where materials in long continuous sheets are wound onto rolls. In such cases, microwaves almost instantly cure coatings or dry inks or other wet agents so that the sheet can be fed through application machinery onto the web at high speed.

Cost

Microwave heating is clearly the most economical and functionally the most desirable process in many manufacturing operations. But microwave heat costs about five times as much as conventional heat, and there must be an overriding technical justification for use of microwaves if the application is to be practical. The rule is: Use conventional heating wherever you can; and use microwave where it does some economically worthwhile function that conventional heat cannot do

More often than not, microwave installation turn out to be hybrid operations where some of the heating is done conventionally, and some is done by microwave. The task cut out for the system designer is to pick the right combination of the two.

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