For Heat, Tune to
915 or 2450 Megacycles 
© Litton Industries 1965

A Discussion of Microwave Energy for

Industrial and Food Processing,

Agricultural and Medical Applications.

CONTENTS

Heat in a Cool Oven

Microwave Heating... Fast, Efficient

Microwaves Offer Advantages

What is Microwave Heat?

How Microwaves Work

Microwave Industrial Frequencies

Can You Use Microwave Heating

Microwaves Many Potentials

How Microwave Heat is Used Today

Microwave Heating Technicalities

Scientific, Engineering Proficiency Offered

Imagine a Raw Potato Changing from Cold to
Baking Hot in a Few Seconds.. in a Cool Oven.

Or think of water swiftly transformed from cold to boiling hot in a cool paper cup. Or picture solvent rapidly boiling off of a strip of plastic recording tape without damaging the heat-sensitive tape. Or visualize paint drying seconds after application with no hot lamps or furnaces nearby.

What's happening in each of these instances? Certainly there's no conformity here with traditional methods of baking, boiling and drying. Where is the flame, the electric heat radiator, the roaring furnace? Why does the air surrounding these objects remain cool? What kind of heat is this that so remarkably focuses on and affects only the object you wish to heat?

The answer is microwave heating. And if you now use heat to prepare or process your product, this exciting medium for supplying heat may offer you substantial savings in time, cost, labor, space and equipment. Consider for a moment the total amount of heat you now generate, the relatively large amount dissipated in building up and transferring the heat, and the little bit that actually goes into the end product. Think, further, how much more efficiently, economically and faster you could perform the same task if you could take only .the small amount of heat your product requires and place just that much inside the product alone. If the thought and the attendant economics and advantages intrigue you, read on. For this is exactly what microwave heating does.

Conventional Heating Methods are Slow and Wasteful, Microwave Heating is Extremely Fast and Efficient.

Take the simple matter of baking a potato. You start out with a very hot heat source a gas burner or a set of electric coils. You heat up the inside walls of the oven, the grill, the pan and all of the air trapped inside. Just to bring the oven up to the required 400 degrees takes 4 to 5 minutes. You then overwhelm the potato with a massive onslaught of heat from all sides. The potato, heating from surface inward through conduction, requires an hour to bake. Cooling down time for the oven is an additional 3 to 4 hours.

On the other hand, the microwave oven starts instantly -like a radio -from a cold condition. The minute you turn it on, the potato begins to absorb heat throughout its entire bulk. It's baked ready-to-serve in 4 or 5 minutes -the same time it required to bring the conventional oven up to working temperature. And when you turn off the microwave oven, it's cool. There's no wasted-heat hangover.

These, then, are the Advantages Microwave Heating Offers You.

 - Elimination of a hot heat source

No external source of hot heat involving flames, burners, blazing banks of electric lamps and coils. Microwave energy stays cold until absorbed by the object.

-Elimination of insufficient, cumbersome heat transfer equipment

No extra-long drying chambers or voluminous masses of hot air to carry heat from hot source to the object. No heavy, metallic, heat-absorbing fixtures. Use paper, plastic or other materials transparent to microwave energy.

-Elimination of slow, conductive, surface-to-interior heating

No need to worry about too much heat too fast for fear of scorching the object or applying heat slowly to give it time to sink in gradually. Microwave energy affects all parts of the object virtually simultaneously, swiftly, evenly.

-Far higher efficiency

No heat loss at heat source or in heat transfer equipment and fixtures. Microwave energy releases heat only upon contact with object and then only to the object itself.

-Considerably greater speed

No waiting for temperature to build up and for heat to reach the object. Microwave energy reaches the object directly and instantly, immediately starts to impart heat.

-Precise on-off control of heat

No lack of control over exact start-stop times, no inadvertent heat carryover or insufficiency. Microwave heat can be measured out with extreme precision, permitting confident tie-in with fastest-response automatic servo systems.

What is Microwave Heat?

Microwave heat is produced by microwaves which are short radio waves. They belong to the same family of electromagnetic waves that contain the energy waves already widely used in radar, television and communications. They vary greatly in length and certain lengths have been found to be practical heat generators.

How Does It Work?

Microwaves do not, in themselves, contain heat. But they are capable of generating heat when they pass through various types of matter. In other words, microwaves do not give up their energy until they contact certain types of matter. You simply shoot the microwaves into the object you wish to heat and it alone directly converts the energy to heat.

As already pointed out, microwaves are just like radio and television waves. A television station transmits microwave energy of a selected frequency in all directions. When you tune your television set to that station's frequency, it picks up a minute part of the transmitted energy and processes it into sound and picture. In microwave heating, the energy instead of being dispersed in all directions, is focused and concentrated on the object. The object, if it is of the desired molecular structure, is tuned by nature to receive the microwave energy in the form of heat instead of sound and picture.

What are the Microwave Heating Channels?

Just as the Federal Communications Commission controls and allocates radio frequencies used by the broadcasting industry, they regulate the microwave frequencies for heating applications. The FCC has allotted a number of microwave frequencies for use in Industrial, Scientific and Medical applications. They are called the ISM frequencies.

FCC-Assigned ISM Microwave Frequencies

915 Megacycles

2,450 Megacycles

5,800 Megacycles

22,125 Megacycles

Is Microwave Processing for You?

Microwave processing has already effectively replaced or supplemented conventional equipment. It has greatly simplified and reduced heat-conveying apparatus in others, led to the invention of new processes and procedures, and greatly increased production yield in still others. It has also made possible reformulation of products and the creation of entirely new ones. Whether or not microwave processing is. suitable for your purposes can ultimately be determined only by you working with a qualified industrial microwave engineer. The significant point is that microwave processing is now a fact of industrial life In the years ahead, it is quite conceivable that microwave processing may determine the ability of a company to maintain a competitive position.

At this moment, we are conducting studies on the possible application of microwave energy to many processes and products. In our laboratories, we are analyzing materials for clients to determine their ability to react to microwave energy. Finally, we manufacture and install microwave equipment and engineer complete production systems for our customers.

Microwave energy will undoubtedly influence major changes in industrial processes, procedures, equipment, plant layout and products. Complete familiarity with its present capabilities, its potential uses and, above all, its applicability to your processes or products becomes increasingly vital.

What are Some Applications?

The potential uses of microwave heating are unlimited. Not every application now requiring heat, however, is amenable to microwave heating. For example. metals reflect microwaves. Others, like most plastics, are transparent to microwave heat energy. Microwave heating is presently being studied in the following areas:

Agriculture Baking Ceramics Graphic Arts Lumber Metalworking Chemical Electrical Electronics Food Processing Foundry Medical Paper Pharmaceutical Plastics Rubber Textile

 Typical applications being studied include:

Contamination Control Fusing Heat-treating Laminating Melting Pasteurization Polymerization Pre-heating Sealing Softening Sterilization Baking Bonding  Blanching Broiling Cooking Dehydrating Disinfestation Drying Freeze-Drying

Owing to the newness of microwave processing, anyone considering its use must do so with more than the idea of simply replacing a conventional system with a microwave system if he is to exploit the new technology fully. In many cases, for example, it may be necessary to reformulate the product to render it susceptible to microwave energy at the frequencies at which power can most economically be generated. Again, changing the configuration of the present system to incorporate microwave equipment can lead to a better product, improve yield, reduce raw material costs, increase production capacity, reduce associated tooling costs or permit the creation of completely new products.

Rather than attempt to force microwave energy into conventional or existing processes and systems, the potential microwave user must start with a thorough understanding of the nature of microwaves and its basic advantages and then imaginatively and freely apply them to his requirements. Above all, he must not let preconceptions formed through long association with conventional heating equipment and methods straight-jacket his investigation and analysis of microwave heating.

Examples of Intelligent and Highly Beneficial Application of Microwave Heating.

...Solvent Removal in Magnetic Tape Production.

The conventional method of manufacturing magnetic tape is to deposit iron oxide powder on a sheet of Mylar in a binder system containing a solvent which must be evaporated. Removal of the solvent cures the binding system and makes the iron oxide particles adhere uniformly to the Mylar base. Evaporation of the solvent is achieved by drawing the tape through a hot air drying system. The drying chamber is extremely long because the heat sensitive Mylar tape requires use of a safe temperature level. Since a super-clean environment must be maintained, extensive precautions are taken to make sure that impurities do not enter the drying chamber during the process of generating the heat which is blown through the long tunnel at high velocity. To attain fast production, moreover, the solvent used is necessarily selected for its ease of evaporation. This, in turn, limits the choice of a binding system to one that is compatible with the solvent. In practice, binding systems found to be compatible with solvents of highest volatility are not those that produce the highest attainable quality in the finished tape.

Replacing the conventional heating-blowing-drying system with microwave heating brings about many marked improvements. Because microwaves do not transfer heat energy to Mylar or iron oxide, the solvent can be very rapidly evaporated without heat-damaging the tape in a self limiting fashion. The drying chamber can therefore be 'shortened to nearly 1/20th of its conventional counterpart. Removal of the heat source and blower greatly minimizes cleanliness problems and equipment requirements. Also, the binder system can now be reformulated to produce a better product. In addition to greater yield, increased productivity, reduction of the size and cost of processing equipment, simplification and reduction of peripheral equipment, and an appreciable product improvement, the inspection costs and line power energy demand for drying are measurably reduced.

...Moisture Removal in Potato Chip Processing

Conventional potato chip manufacturing entails the use of high-temperature oil baths for the purpose of removing moisture from raw potatoes. With

ordinary equipment, potato chips tend to discolor as the last few percentages of moisture are removed. By using microwaves to finish off moisture removal, the discoloration of chips is eliminated. This is particularly important when top quality chipping potatoes are unavailable.

Consequently, incorporation of microwave heating substantially increases yield (fewer culls) and allows far greater latitude in the selection of the raw material. An additional advantage experienced as a result of using microwave heating is increased shelf life of the finished product.

...Sterilization, Disinfestation, Dormancy Control

The medical-biological-botanical benefits and uses of microwave energy are numerous and promising. At first, this area might appear to be foreign to the subject of heating. Yet, many of the conventional, industrial and commercial heating operations and processes directly or indirectly include sterilization pasteurization objectives. Also, because of the possibility of non-thermal effects of microwave energy in organism control, the very same microwave energy used for heating can frequently lead to multiple-benefit applications.

Examples of present and projected uses of microwave energy in this area include: Pasteurization of dairy products, Sterilization of soil, which is now accomplished with highly toxic chemicals, or steam, at a cost of hundreds of dollars per acre, Sterilization of non-metallic medical equipment, Destruction of disease-causing organisms in plants. Control and acceleration of the dormancy cycles of flower bulbs.

A Technical Discussion of Microwave Heating.

In somewhat simplified form, we have described in the previous pages the concept of microwave heating, its differences from conventional or non-electronic methods of transferring heat and the possible areas of application. We have explained that, unlike conventional systems, microwave heating does not involve heat transfer from a hot source to the load or work and that thermal energy is not conducted from the surface inward.

Microwave heating is the result of a direct transfer of energy from an electromagnetic field to the work. The transfer takes place directly without the necessity of an intermediate medium. Moreover, energy transfer OCC'urs wherever the field penetrates. Because of this phenomenon, no limit is imposed on the speed of heating of the microwave energy or its intensity of concentration on the work.

Microwave heating offers far higher efficiencies because: (a) it eliminates the inherent inefficiency of generating thermal energy at an outside source;

(b) it eliminates the inefficiency of transferring heat from an external source to the work; and (c) it reduces re-radiation loss from the surface of the work while the interior is being brought up to temperature.

The power developed in the work by a microwave electromagnetic field is governed by the basic power equation:

P_v = 1.41 E₂ f E_r tan d X 10^-12 watts/in³

Where

P_v = power dissipated in the work per unit volume in watts

E = electric field strength in the work in volts per inch

f = frequency in cycles per second

E_r = relative dielectric constant of the work

tan d= loss tangent of the work

The uniformity of energy transfer throughout the bulk of the work material is related to the dimensions of the work and the loss characteristics of the material. A standard way of reviewing this characteristic is to know the half power depth for a given material at a given frequency. The half power depth is that depth in the work where a molecule receives 1/2 of the energy that a molecule at the surface receives. 

Half power depth equation

                    3λ_o            
^{x =}     8.686 p √E_r tan d

Where λo = wavelength in free space

The amount of heat developed in the work material is a function of the intensity and frequency of the electromagnetic field and the electrical properties of the material. In general. tan S increases in proportion to the frequency. Consequently, power transfer occurs at the lowest field strength when the frequency is highest. However, generating very high frequency power for heating purposes is extremely costly and penetration difficulties are encountered. In microwave energy transfer for heating, the molecular characteristics of the work material are significant. Since the molecules are placed within an alternating field, they will exhibit a tendency to align themselves with the electric field generated by the microwaves particularly if they are dipolar. Each time the field reverses, the molecules tend to realign accordingly. The resulting inter-molecular friction acts to convert the electromagnetic energy to thermal energy. The nature of the electric dipole in the molecule and the rotation time of the molecule determine the magnitude of the loss tangent of the material. The loss tangent value and the dielectric constant value of any given material is a function of the frequency.

In determining the suitability of a product for microwave heating, the effective loss tangent of the material at the various microwave frequencies is measured. The physical size of the material is then related to the half power depth of the optimum frequency. The final step is the choice of the frequency which proves best for the required process. If several frequencies are equally effective, the choice is governed by the cost of the equipment.

As stated earlier, microwave frequencies presently allotted by the FCC for Industrial, Scientific and Medical uses are: 915 Mc, 2450 Mc, 5800 Mc and 22,125 Mc. The two higher frequencies are seldom considered other than for limited laboratory and research applications because of the very high cost of power generation at these frequencies.

ATHERTON-Specialists in Research, Development and Production of Microwave Heating Devices and Systems for Industry.

The Atherton Division of Litton Industries combines the scientific and engineering proficiency in microwave technology found at the most advanced levels of aero-space activity with commensurate capabilities in industrial process engineering, chemical research, biomedical research, microbiological research, food processing engineering and materials research. Its singular aim is to perform investigations and design equipment which will result in maximum exploitation of microwave energy for industrial heating and processing applications.

Atherton is fully staffed and equipped to: Determine the microwave properties of any given material under laboratory conditions. Provide equipment to subject quantities of materials to pilot runs to determine their practicability for microwave processing. Conduct investigatory and feasibility studies on the adaptability of conventional equipment, processes and products to microwave energy and devices. Design and produce microwave equipment of the highest profitability in terms of user requirements.

Our microbiology laboratory is unique in industry for studying the inter-relation of biology, chemistry and microwave energy. It provides incomparable capability to investigate microwave effects in food preservation, disinfestation, plant pathology and basic research.

If You Wish Additional Information on Microwave Heating

Or desire an evaluation of the suitability of your product to microwave heating, write 974 Commercial Street, Palo Alto, Calif. Telephone (415) 321-7440.