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RF Drying Provides Process Savings

New systems optimize radio frequency drying for the ceramic and glass fiber industries

Reprinted from the April 1996 issue of Ceramic Industry Magazine

By: Thomas W. James
President and CEO
Radio Frequency Co., Inc.
Millis, MA

Radio frequency (RF) dryers have been utilized by the ceramics and glass fiber industries for a number of years. A growing variety of advanced ceramic products, refractory materials, and fiberglass materials are being dried today in RF (Radio Frequency) systems. Use of RF systems has provided the means for drying ceramic extrusions quickly and uniformly, eliminating surface cracking and quality problems associated with moisture in the products during firing. The drying of cellular ceramic extrusions represents a large market for RF dryers as they offer the only practical way to dry these products.

Several types of ceramic filters used by metal foundries are produced by firing parts made of plastic foam impregnated with a ceramic slurry. After impregnation, the blocks are dried by radio frequency. This method has proved to be superior to conventional means of drying and microwave drying as well.

The principle product applications for RF drying in the fiberglass industry are chopped strand, roving packages, and forming cakes. RF dryers can uniformly reduce the moisture in these products to the level desired, down to a fraction of a percent without overheating organic yarn coatings or, in the case of roving packages and forming cakes, without causing migration of solid components in the yarn treatments.

In the ceramics and glass industries, the installed base of radio frequency dryers in the United States exceeds 3 megawatts of output power. This capacity is sufficient to evaporate 3,750 tons of water per hour, which is testimony to the importance of this drying method.

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 like the way magnets would move in an alternating magnetic field. The friction of this movement causes the water in the material to rapidly heat throughout its entire mass.

Below is a depiction of 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.

Operating principle of a radio frequency dryer
Fig.1. Operating principle of a radio frequency dryer.

The amount of heat generated in the product is determined by the frequency, the square of the applied voltage, 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, the water is preferentially heated and evaporated. The reduction in loss factor as the material dries out provides a valuable safeguard against overheating. This method of drying, therefore, is ideal for applications where uniformity of product dryness is an important requirement.

What makes a product a candidate for RF drying?

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 when it comes to heating and drying. Radio frequency heats all parts of the product mass simultaneously and evaporates the water in situ at relatively low temperatures usually not exceeding 180o F. Since water moves through the product in the form of a gas rather than by capillary action, migration of solids is avoided. Warping, surface discoloration, and cracking associated with conventional drying methods are also avoided.

The Benefits of RF Drying

Precise Control of Moisture Content and Uniformity.
In ceramics production the primary cost driver is product yield. Radio frequency drying increases product yield because of the uniform level of dryness throughout the product prior to the firing process. The moisture leveling phenomenon of RF drying likewise occurs within each item being dried.

Reduction of Surface Cracking
Cracking caused by the stresses of uneven shrinkage in drying is eliminated by RF drying. This is achieved by the RF dryer's even heating throughout the product maintaining moisture uniformity from the center to the surface during the drying process. Other factors may contribute to surface cracking, however, the control of moisture uniformity achieved by RF drying has, by far, been the most significant in solving 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, but with more moisture to be removed, the RF dryer provides an energy-efficient means of achieving the desired moisture objectives. Typically, one kilowatt of RF energy will evaporate 1kg of water per hour.

Savings in Plant Space
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. The drawing below shows a typical 150kW dryer, capable of evaporating over 300 pounds of water per hour, to be 24 feet, 7 inches (7.5 meters) long.

Dimensions of a typical 150 kW Dryer
Fig.2. Dimensions of a typical 150 kW Dryer

The importance of frequency in the drying process

The radio frequencies reserved for industrial use by the Federal Communications Comission are 13.56MHz +/-.05%, 27.12MHz +/-.60%, and 40.68MHz +/-.05%. It is important that the frequency remain within tolerance or be attenuated so as not to interfere with radio communications.

The frequency chosen for RF drying is very important. Earlier generations of RF dryers operate at 10 to 27MHz. At these lower frequencies, 20% to 60% higher voltages must be applied to the product than if the equipment operated at 40MHz. These higher voltages often result in arcing which can damage the product. The past use of lower frequency systems often resulted in the rejection of product applications where extremely low moisture levels were required and arcing problems caused the application to fail.

The Optimization of Radio Frequency Drying

Modern design techniques produce RF dryers which operate at 40MHz which successfully handle drying applications previously thought to be inappropriate for RF drying because of arcing problems. An RF dryer operating at 40MHz can do the same work as a 27MHz dryer, but at 20% lower voltage. 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.

Two general types of dryers are in use today. Single high power generators are rated for 50kW to 300kW output. Shown in the photo below is a 150kW RF dryer designed for drying ceramics and glass fibers.

RF Dryer
Fig.3. The generator section is located above the RF applicator cabinet
with a programmable logic controller and system diagnostics display.
Attenuation ducts prevent radio waves from escaping from the applicator.

Multiple generators in tandem are designed for applications requiring longer dwell times. In the photo below a 150kW output system is shown comprised of six 25kW modules. This system configuration provides a much longer dwell time which allows the RF drying to be accomplished at a lower temperature and lower internal vapor pressure.

A modular system consisting of six power modules
Fig.4. A modular system consisting of six power modules.

Note the generator section is located above the RF applicator cabinet with a programmable logic controller and system diagnostics display. Attenuation ducts prevent radio waves from escaping from the applicator.

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 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.


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