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In your search for improved process heating methods, you may have experienced promising test results using a home or lab size microwave oven. However, as you proceed to scale up, it will be helpful to recognize some important differences between microwave and RF equipment.

Most industrial process heating applications, originally thought as being microwave applications, are found to be more ideally suited to lower frequency RF equipment such as RFC's Macrowave™ systems.

The advantages of RFC Macrowave Systems over microwave are:

  • greater uniformity of heating
  • improved power control
  • greater depth of penetration
  • lower operating cost
  • modular design considerations
  • lower equipment cost
  • greater processing versatility

There are numerous cases where microwave dryers in industrial production have been replaced by RF dryers for these fundamental reasons. The excerpts below, from a technical paper published on the subject, provide a detailed explanation of scale-up differences between microwave and RF.


EXCERPTS FROM:

ENGINEERING AND SCALE-UP CONSIDERATIONS
FOR MICROWAVE INDUCED REACTIONS

M. Mehdizadeh
E. I. DuPont de Nemours & Co., Central Research and Development, P.O. Box 80357, Wilmington, DE 19880, U.S.A.
Published in Res. Chem. Intermed Vol. 20 No. 1 pp79-84 (1994)

INTRODUCTION

In most cases when a small scale experiment on a microwave induced reaction is intended, the sample is simply placed within a microwave oven or a waveguide connected to a microwave source. Here the method of applying the energy is quite convenient and is similar to more conventional energy sources such as light or radiant heat. The home microwave oven is certainly developed to work in this fashion, otherwise it would have been impossible to mass market such an appliance. The user in such instances can afford to lose a great deal of energy efficiency, field intensity, and choice of material for gaining such convenience. In large scale microwave processing, as well as in highly demanding experiments, however, such simplicity is not possible. In such cases, requirements for energy efficiency, uniformity, and control would dictate a design where the subject material is an integral part of the system, and its electrical and mechanical properties play a critical role in the design and operation of the processes. Furthermore, in large scale processing the spatial uniformity of the process is often critical.

FIELD UNIFORMITY/PENETRATION CONSIDERATIONS

Dealing with spatial nonuniformities in microwave processes have been an essential part of microwave engineering practice. Mode mixers and turntables (mechanical methods) in home microwave ovens have been designed to reduce the nonuniformity effects. In microwave heating, the nonuniformities can be of particular interest due to the fact that the correct temperature over the whole mass may be critical to effectiveness of the process.

The microwave heating process suffers from two distinct forms of nonuniformity. The first, is the fundamental "standing wave" effect. This is a repeated pattern of field intensity variation within a microwave applicator which, for the most part, follows a half-sine pattern. In a distance of one quarter of the operating wavelength the field intensity can change from a maximum to zero, or in a one-tenth of a wavelength the intensity can change by 60%. Uniformity considerations can also play a role in the choice of frequency. The lower the frequency, the larger is the uniform volume.

The second type of nonuniformity is the penetration depth problem. The microwave fields attenuate within the bulk of conductive materials and materials with high dielectric loss. This is particularly troublesome for larger scale processes. Both types of nonuniformities described above are frequency dependent and become less severe as frequency is lowered.

CHOICE OF FREQUENCY

Microwaves are without any doubt a natural choice for laboratory scale investigations of the feasibility of using electromagnetic energy for process of materials. Microwave ovens, sources, and waveguide equipment are highly available, and can be readily adopted to the processing of samples. Radio frequency heating, which is essentially the same concept, but at a much lower frequency, has thrived as an industry alongside microwaves over the decades. For the same electric field, the higher the frequency, the higher the amount of power into the material. This is the reason why microwaves are a conceptually more effective means of heating. However, RF equipment has several advantages which workers in the field of microwave processing may find more suitable for scale-up of some processes. RF equipment is available commercially at much higher power levels than microwave sources. While commercial microwave sources are available only below 75kW, RF equipment at hundreds of kW are very common. At these high levels, the price per watt of RF equipment is much cheaper than microwaves.

In addition to higher power and lower cost, RF equipment has another important advantage. Because of much longer wavelengths, they have better uniformity. Also, the depth of penetration is much higher. So, in cases where uniformity is a critical issue they may be a better choice.

Yet another advantage of RF equipment over microwaves is in the control area. In high power RF systems, the source and the load are commonly locked together in a feedback circuit. Therefore, variations in the load can be followed by the source without external controls.

There is a major difference between the way RF and microwave equipment are available in the market. Unlike microwave sources, one cannot purchase an RF high power source. Due to the high impedance nature of RF coupling, the RF source and applicator normally need to be designed and built together. Manufacturers of RF equipment develop the whole system, rather than only the power source. Therefore, developments in RF processing must involve the commercial RF manufacturers. The most common commercial RF frequencies are 13MHz, 27MHz, and 40MHz.

– The full text of this paper is available upon request. -- Contact RFC

– For applications which can benefit from lower frequency RF Macrowave™ heating equipment, click on Applications.


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