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.
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.
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.
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.
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.