Advantages of Using Microwave Energy for Process Heating

It is surprising to realize that microwave heating has been in use for quite some time. Microwave heating technology originated in the early 1800s when Michael Faraday, an English scientist presented the first theories of electromagnetism. Over the next century other notable scientists, including James Clerk Maxwell and Heinrich Hertz, experimented on a variety of industrial, scientific and medical applications with their new “electronic heat” or radio frequency energy source.
    Drying technology expert Arun S. Mujumdar, professor of mechanical engineering at the National University of Singapore and author of the Handbook of Industrial Drying, 3rd edition, states that for many engineers these are new forms of heating when in fact practical microwave applications began during World War II. Currently the annual worldwide sales of industrial microwave heating systems probably amounts to less than $100 million, but the sales of the home microwave ovens in the U.S. is of the order of $1.5 to $2 billion. It’s estimated that 90% of all households in America own a microwave.
    The reasons for the relatively small size of the industrial markets are several, but two stand out: first, the heating mechanisms are not familiar to most engineers; second, they often represent a radical departure from conventional systems and there is generally a tendency to resist real innovation in most industries. (Mujumdar, Handbook of Industrial Drying, 3rd ed., 286.)
    In the past few years, there has been a surge of interest in the applications of microwave heating for industrial purposes. This is primarily due to: 1 - the desire for improved sanitation; 2 - improved quality control by dynamic temperature control.
    The unique heating mechanisms of microwaves permit dramatic energy savings in many instances, as well as providing other benefits which will be discussed further in this article.

Technology
Microwaves are electromagnetic waves with frequencies that lie between 300 MHz and 300 GHz, with wavelengths that vary from 1 mm to 1 m. The radio frequencies that may be used for heating are those allocated by the Federal Communications Commission. These are the so called ISM (industrial, scientific, and medical) frequencies, which have been set aside for applications in these specific areas.
    Microwaves are not forms of heat but rather forms of energy that appear as heat through their interaction with materials. Microwaves initially excite the outer layers of molecules.
    The mechanism for drying with microwave energy is quite different from that of conventional drying. The drying system consists of a transmitter, waveguide, vessel, air supply, and controls, as shown in the image at right. Microwave systems are more compact, thus requiring a smaller equipment footprint. The transmitter may be located remotely in a dry, safe area. The waveguide may extend up to several hundred feet.
    Microwave drying works fast. Instead of applying energy only to the outside of the product, microwaves work directly to dry material from the inside out. Most of the moisture is vaporized before leaving the material. This creates a sort of pumping action forcing liquid to the surface. Heat is added uniformly. This is known as volumetric heating.
Most conventional heating and drying methods approach material from the surface, applying heat only to the outside edges. This technique quickly removes surface moisture, but it is highly inefficient when it comes to removing liquid trapped inside the material. If external temperatures are kept high enough, as in an oven, the material’s inner moisture will diffuse to the surface and evaporate, but this is a passive and lengthy process.
    “Whereas conventional methods depend upon the slow march of heat from the surface of the material to the interior as determined by differential in temperature from a hot outside to a cool inside, heating with microwave energy is, in effect, bulk heating in which the electromagnetic field interacts with the material as a whole,” said Stating, Dr. Mujumdar. The heating occurs nearly instantaneously and can be very fast, although it does not have to be. However, the speed of heating can be an advantage, and is often possible to accomplish in seconds and minutes what could take minutes, hours, and even days with conventional heating methods. The fastest industrial heating system of which I am aware, heats fine plastic threads at a rate of about 30,000° C/s (the material was actually heated about 100°C in about 3 ms.)

Advantages
Because volumetric heating is not dependent on heat transfer by conduction or convection, it is possible to use microwave heating for applications where conventional heat transfer is inadequate. One example is in heterogeneous fluids where the identical heating of solids and liquids is required to minimize over-processing. Another is for obtaining very low final moisture levels for product without over-drying.

Eight Things You Should Know

1. Microwaves generate higher power densities, enabling increased production speeds and decreased production costs.
2. Microwave systems are more compact, requiring a smaller equipment footprint.
3. Microwave energy is precisely controllable and can be turned on and off instantly, eliminating the need for warm-up and cool-down. Lack of high-temperature heating surfaces greatly reduces the amount of product that is burned or overheated.
4. Microwaves reduce production run times and reduce both cleaning times and chemical costs.
5. Microwaves are a non-contact drying technology.
6. Microwave energy provides uniform energy distribution. This results in more uniform temperature and moisture profiles, improved yields, and enhanced product performance. This makes it possible to eliminate such disadvantages of convective drying as case-hardening, surface cracking, and local overheating.
7. The use of industrial microwave systems avoids combustible gaseous by-products, eliminating the need for environmental permits and improving working conditions.
8. Compared to conventional heating, microwave heated food products tend to retain a higher percentage of flavors and nutrients.



    Historically, the main drawback towards using microwave energy for industrial processing has been its inability to create uniform energy distribution. If uniform energy distribution does not occur, wet regions of the material are underexposed, and other regions are overexposed. However, recent technology advances have overcome this obstacle. For example, the microwave mixer, features gentle agitation with paddle-style agitator that stirs the material for superior, uniform heat distribution.

Energy Savings
A common misconception is that microwave heating is always more expensive than heating by conventional techniques. This will actually depend on the application and utility costs. However, in some cases, microwaves can be 50% or more efficient than conventional systems, resulting in major savings in energy consumption and cost.
    The offsets to the current cost of electricity include: increased speed of drying, greater yields, and better quality products. The microwave transmitter assembly provides a dependable source for industrial microwave power. The transmitter can be used as a single, self-contained microwave power unit, where it typically delivers power levels from 5 to 75 kWs continuous-rated duty. Or it can be used in combination with other transmitters to provide “networked” microwave power for any processes requiring higher power levels.
    A main feature of the transmitter is the magnetron, a low-cost, efficient, cross-field microwave oscillator. 915 MHz transmitters can provide up to 100 kW from a single magnetron.

Maintenance
In addition to downtime for cleaning and inspection, conventional dryers and heat exchangers need periodic servicing with an expensive inventory of parts and a highly trained labor force. Typically, the only part that requires maintenance is the magnetron. Magnetrons, like all electron tubes, have a finite life and should be considered a consumable item. There are, however, a number of factors that contribute to the life of a magnetron.
    For example, adequate cooling should be provided to the magnetron. Most magnetrons rated below 3 kW of output power only require air cooling. Most magnetrons rated 3 kW and above require water cooling for adequate heat dissipation.
    The operating life of a 915 MHz commercial magnetron is usually around 6000-7000 hours or greater. This translates to a maintenance cost of about $1 per operating hour. Low power, 2450 MHz magnetrons cannot be repaired, but larger units usually can be. A typical operating life for magnetrons at this frequency is 6000 hours, although some vendors limit their warranty to six months or 500 hours.

Applications
Microwave technology is an extremely efficient but under-recognized energy source for process drying. Applications where microwave processes prove beneficial include: dehydration, sterilization, pasteurization, tempering (thawing), blanching, and cooking. In dehydration, the main purpose is to remove water. With pasteurization and sterilization, microwave systems are designed to raise the product temperature to a certain level to destroy pathogens while maintaining product integrity. Other applications include blanching where the product is heated then cooled rapidly or when product is maintained at an elevated temperature, as in cooking or tempering.
    Uniform energy distribution minimizes fouling in even the most viscous products. This is particularly important with thermally sensitive materials such as chemical polymers, food ingredients, nutraceuticals, biotech products, and pharmaceuticals. Other successful applications using microwave drying include lumber, industrial coatings, ceramics, and a wide variety of powder bulk materials. The list is growing, though: since the technology has only fairly recently been applied to the industrial sphere, there are many additional applications and uses to be found.
    It’s worth remembering, too, that microwave technology can work well as a partner process. That is, it can be a highly effective supplement to a conventional process. A hybrid system utilizing a traditional convection oven and microwave can capitalize on the efficiencies of both technologies.

Three hybrid combinations that are the most common include:

Preheating: Microwave energy is applied first thing to bring all moisture content to the surface of the material. The material is then sent to a conventional gas- or oil-heated air dryer or oven, which flashes the moisture off by performing three functions, namely:
1. Removing residual moisture
2. Preheating moisture to the evaporative temperature
3. Equalizing the moisture level of product to the conventional dryer

Booster Drying: Microwave energy is added when all surface moisture has been removed and the drying rate of conventional systems begin to taper off—about two-thirds of the way through the process. The microwave is used to bring remaining moisture to the surface and remove it.

Finish Drying: Microwave energy is applied at the last stage of the drying process, as the material leaves the conventional oven. In finish drying, the microwave takes over just as the conventional system’s efficiency plummets and its own efficiency is greatest. Finish drying is the most popular of the hybrid styles. A primary benefit it can offer is sanitization of the material: if the appropriate temperatures are used, the product in its final stage will be pasteurized as well as dry.

Economics
A home microwave oven operates at 2450 MHz. Industrial/commercial microwave systems typically operate in the 915 MHz frequency. The lower-frequency range allows more efficient penetration of the microwave through the material, typically about three times as great as that of the 2450 MHz microwave. With their higher total system efficiencies, 915 MHz heaters and dryers tend to have lower running costs than comparable 2450 MHz units. For example, one 100 kW 915 MHz transmitter will be about 50% cheaper to operate than seven 15 kW 2450 MHz units.
    The low-power 2450 MHz magnetrons are more ideal for flow capacity R&D applications. The size of the magnetrons and wave-guides for a 2450 MHz system is considerably smaller than those used in 915 MHz units. This makes them suitable for small-scale installations. Although the cost is similar, the largest commercial 2450 MHz units available use 30kW magnetrons. However, the 2450 MHz units cost 4-5 times as much per watt compared to the 915 MHz units.
    With current energy costs, the return on capital invested may typically vary from 12 to 24 months. This may be the most economical solution where minimal equipment floor space or footprint is available for a new application, or when expansion of existing production facilities would require building modifications to accommodate a conventional drying system.
    In the case of liquid heating, the production cost of providing sensible heat transfer from microwave energy is approximately one third higher than using steam in a conventional heat exchanger. However, this is offset by several factors, including:

1. The reduced capital investment in steam boilers, steam trains, condensate collection and water treatment
2.  The ability to use high-power densities enables microwave heaters to substantially increase production rates
3. With volumetric heating of multiphase products, solids loadings of 70% or higher can be processed since the carrier fluid is not used as the primary heat delivery medium. The shorter residence times achievable with microwave heating improve product quality.

CONCLUSION
Microwave technology is now being seriously considered as a viable energy source in process heating. It’s been a slow development process over the last two centuries. However, the timing is right to achieve significant economic benefits for those who are now considering a change. The myths have been dispelled and various safety issues have been overcome.
    Industrial microwave processing is not for everyone. Some products – especially those with reflective tendencies – do not perform as well. The best way to find out is with testing. Some vendors perform elaborate tests, usually in well-equipped test labs, to provide crucial data that will determine success. Once you have found a microwave equipment manufacturer that makes sense, be sure to schedule a test. It may be one of the best decisions you ever make.
    Scott Jones is marketing manager, Marion Mixers (Marion, IA). For more information, call 800-397-6371, or visit www.marionmixers.com.