For Heat, Tune to
915 or 2450 Megacycles
© Litton Industries 1965
A Discussion of Microwave Energy
Industrial and Food Processing,
Agricultural and Medical
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
Imagine a Raw Potato Changing from
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,
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
-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
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
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
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
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
Examples of Intelligent and Highly
Beneficial Application of Microwave Heating.
...Solvent Removal in Magnetic Tape
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
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.
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
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
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:
Pv = 1.41 E2 f Er
tan d X 10-12
Pv = 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
Er = 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
Half power depth equation
x = 8.686 p
√Er 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
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
ATHERTON-Specialists in Research,
Development and Production of Microwave Heating Devices and Systems for
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
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
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.