Tuning In To Better Drying

April 1, 2001
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Advances in radio frequency drying technology are increasing the efficiency and quality of the drying process.

Radio frequency (RF) dryers have been used by the ceramics and glass fiber industries since the early 1970s. Today, an increasing variety of advanced ceramic products, refractories and fiberglass materials are being processed in a new generation of advanced radio frequency drying systems appropriate for the modern factory environment. Reliable and user-friendly RF systems are being manufactured with state-of-the-art process control systems, readily enabling their integration into today’s sophisticated production facilities.

Figure 1. Operating principle of a radio frequency dryer.

Principle of Operation

In a radio frequency drying system, the RF generator creates an alternating electric field between two electrodes. The material to be dried is conveyed between the electrodes, where the alternating energy causes polar molecules in the water to continuously re-orient themselves to face opposite poles—much in the same way magnets move in an alternating magnetic field. The friction of this movement causes the water in the material to rapidly heat throughout the material’s entire mass.

Figure 1 depicts a radio frequency drying system with a product between the electrodes. Polar molecules within the product are represented by the spheres, with “+” and “-” signs connected by bars.

The amount of heat generated in the product is determined by the frequency, the square of the applied voltage, the dimensions of the product and the dielectric “loss factor” of the material, which is essentially a measure of the ease with which the material can be heated by this method. Because water is far more receptive than other materials usually found in glass or ceramics, it is preferentially heated and evaporated in situ. The reduction in loss factor or receptivity to RF energy as the material dries provides a valuable safeguard against overheating. This method of drying is therefore ideal for applications where uniformity of product dryness is an important requirement.

The Importance of Frequency

The radio frequencies reserved for industrial use by the Federal Communications Commission are 13.56 MHz +/-.05%, 27.12 MHz +/-.60% and 40.68 MHz +/-.05%. It is important that the frequency remain within tolerance or be attenuated so as not to interfere with radio communications—however, not all frequencies are equal when it comes to RF drying. A high moisture content product offers a good “load” to an RF dryer since sufficient water molecules exist within the product to absorb the RF energy. But as a product approaches a completely dry state, fewer water molecules are available to absorb the RF energy. Earlier generations of RF dryers operated at the lower frequency levels of 13 to 27 MHz, and these lower frequencies required that high voltages be applied to the product. These systems had a tendency to become unstable at low moisture contents and were prone to random arcing problems, making the lower frequency systems unpopular for applications where extremely low moisture levels were required.

Modern design techniques produce RF dryers that operate at 40 MHz. These higher frequency systems do the same drying work as 27 or 13 MHz dryers, but at 20 to 60% lower voltage, respectively. This permits the RF drying system to effectively process materials at very low moisture levels without the arcing problems encountered with dryers at lower frequencies.

Benefits of RF Drying

RF drying offers numerous benefits to ceramic and glass manufacturers, including moisture control and uniformity; reduction in surface cracking; and savings in energy, drying time and plant space.

Precise Control of Moisture Content and Uniformity. Heating in an RF dryer occurs selectively in those areas where heat is needed because water is much more responsive to RF energy than most other dielectric materials. Since wetter areas absorb more RF power than dryer areas, more water is automatically removed from wet areas, resulting in a more uniform moisture distribution.

Reduction of Surface Cracking. Surface cracking caused by the stresses of uneven shrinkage in a conventional drying process is eliminated by RF drying. This is achieved by the RF dryer’s even heating throughout the product thickness, which maintains a moisture uniformity from the center to the surface during the drying process. While other factors such as mechanical handling issues may also contribute to surface cracking, the moisture uniformity achieved by RF drying has often solved such problems.

Energy Savings. The efficiency of a convection dryer drops significantly as lower moisture levels are reached and the dried product surface becomes a greater thermal insulator. At this point, the RF dryer provides an energy-efficient means of achieving the desired moisture objectives. Typically, 1 kW of RF energy will evaporate 1 kg of water per hour. Additionally, because RF is a “direct” form of applying heat, no heat is wasted in the drying process.

Savings in Plant Space/Production Time. The factory space required for an RF drying system is one-fifth to one-eighth the space required for a conventional hot air type of dryer. Additionally, since heating begins instantaneously throughout the product, the dwell time in an RF dryer is far less than in a conventional dryer. This translates into significant savings in floor space and overhead cost per part.

Conventional Applications

Use of RF drying systems has provided a reliable full-scale production method for drying ceramic catalytic converter extrusions quickly and uniformly. In this application, RF drying has eliminated surface cracking and quality problems associated with a non-uniform moisture content within the substrates during the shaping and firing process.

Additionally, several types of ceramic filters used by metal foundries are produced by firing parts made of plastic foam impregnated with ceramic slurry. After impregnation, the blocks are dried by radio frequency energy. This method has proved to be superior to conventional and microwave drying because it uniformly applies energy throughout the product thickness, preventing product warpage and discoloration while also attaining rapid drying times.

In the fiberglass industry, the principle product applications for RF drying are chopped strand, roving packages and forming cakes, as well as specialty glass fabrics. RF dryers can uniformly reduce the moisture in these products to the desired level—down to a fraction of a percent—without overheating organic yarn coatings. They also prevent the migration of solid components in the yarn treatments during the drying process, which provides a higher-quality end product.

What Products Are Good Candidates?

The more difficult an item is to dry with convection heating, the more likely it is to be a good candidate for RF drying. Materials with poor heat transfer characteristics, such as ceramics and glass fibers, have traditionally been problem materials with conventional heating and drying. Radio frequency energy heats all parts of the product mass simultaneously/volumetrically, and evaporates the water in situ at relatively low temperatures, usually not exceeding 180ºF. Since water moves through the product in the form of a gas rather than by capillary action, the solids are unable to migrate, and the warping, surface discoloration and cracking associated with conventional drying methods are avoided.

The evaluation of an RF drying application usually begins with a feasibility study in the dryer manufacturer’s laboratory, where receptivity, dwell time and power requirements are determined and economic factors are investigated. If a material qualifies in a feasibility test, a scale-up test is usually done with a leased unit at the customer’s plant.

An advanced hybrid RF/convection heating system* is often used to evaluate new applications and define the necessary parameters to heat or dry materials under production conditions. It is a fully instrumented system that can apply both RF and convection heating under a wide variety of conditions. Through the use of this advanced hybrid RF heating system, a company can accurately determine the scale-up requirements necessary to meet production goals.

In the new millennium, manufacturers are faced with finding ever more efficient means of production to offset the increasing costs of raw materials and labor. Today’s RF systems are playing a steadily increasing role in maximizing production profits while assuring product quality.

For More Information

For more information about radio frequency drying, contact Radio Frequency, Inc., 150 Dover Road, Millis, MA 02054; (508) 376-9555; fax (508) 376-9944; e-mail RFC@radiofrequency.com; or visit http://www.radiofrequency.com.

*The OmniTherm™ Simulator, supplied by Radio Frequency Co., Inc.

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