Combining Radio Frequency & Conventional Methods
- Faster heating times, which allow faster line speeds and shorter line lengths.
- Even heating with a consistent temperature gradient and less solids migration.
- No overheating of base material during drying. Radio frequency heating is self-limiting.
- Selective heating. Water is heated and removed with little heating of the base product.
- Moisture profiling or leveling to create more consistent product quality.
- Fast shutdowns and startups due to instant-on/instant-off heating.
- Fewer environmental issues because there are no combustion byproducts.
Conventional heating (i.e., conduction, convection, radiation) has an outside heat source and relies on transferring heat to the surface of the material and conducting that heat to the middle of the material. By contrast, radio frequency heating creates heat at the molecular level, meaning it heats uniformly from within.
Often, a conventionally heated product is hot and dry on the outside and cold and wet on the inside. The dry outer layer acts as an insulating barrier and reduces the amount of conductive heat transfer to the middle of the product. This dry outer layer also can cause quality problems such as a skin on coatings and uneven solids dispersion through wicking of emulsions. For heating applications, the surface temperature will be higher than the interior temperature, creating an uneven temperature gradient throughout the product.
With radio frequency heating, there is no hot, dry outer layer. The electric field stimulates the molecules at the surface and in the middle of the product equally, so the product heats relatively evenly and water migrates to the surface. In general, because of heat losses at the surface, products dried with radio frequency are hot and dry on the inside and cool and wet on the outside. Using both heating methods in a single process allows the radio frequency energy to heat the product's inside and move water to the surface, where conventional methods do an effective job of removing it.
How It WorksThe basic theory of radio frequency heating involves exposing dielectric materials to a high voltage, high frequency electric field. The best materials for this type of heating are those that are neither good conductors nor good insulators. Because metallic materials generally are good conductors, they are not suitable candidates for radio frequency heating.
Radio frequency heating uses two heating mechanisms: dipole rotation and ionic conduction. In dipole rotation, individual molecules rotate to align themselves with the electric field. In the radio frequency oven or dryer, the polarity of the electric field is reversed millions of times per second, causing the individual molecules to rotate millions of times per second. This molecular movement causes friction and creates heat.
Likewise, in ionic conduction, charged particles (ions) are always moving toward the oppositely charged plate. Again, because the polarity of the electric field in the oven or dryer is reversed millions of times per second, ions in the material are constantly moving and colliding with other particles. These collisions cause friction and create heat to warm the material.
The key to effective application of radio frequency energy is the right applicator or electrode design. Traditionally, heating was accomplished by creating a uniform electric field between two parallel plates. This approach is capable of heating thicker materials uniformly because a high voltage gradient can be established in the material. However, it does not work well for thin materials such as webs. To establish a high voltage gradient in a thin web material, the plates must be close together, which can cause arcing between the plates.
For thin materials, the stray field electrode design was developed. This design creates an electric field between alternating parallel rods that gives a higher voltage gradient in the web for faster heating. A variation on this electrode design for thicker webs is the staggered stray field design, which provides more uniform heating in thicker webs. As a general rule, materials less than 0.25" thick use the stray field design; materials 0.25 to 0.5" use the staggered stray field design; and materials more than 0.5" thick use the parallel plate design. In all of these electrode designs, the material can be either self-supporting or transported on a conveyor (figure 1).
The materials a product is constructed of largely determine the success of radio frequency heating. Some materials heat well, and some do not. The key measure of a material's heatability is the loss factor. This property determines how well a material absorbs radio frequency energy.
Materials with a high loss factor absorb energy quickly and thus heat quickly. Conversely, materials with a low loss factor absorb energy slowly and heat slowly. In general, polymers tend to have low loss factors and thus take longer to heat. Water, by contrast, has a high loss factor. For these reasons, radio frequency lends itself to drying. It heats water quickly but only slightly heats most polymer materials. But, because of the complexity of the interaction between a given material and the radio frequency field, laboratory testing and consultation with a radio frequency expert are critical.
Combining Radio Frequency and Convection HeatingAs noted, conventional heating methods remove moisture from the surface of materials while radio frequency heats the middle of a product and drives moisture to the surface. Combining these two technologies allows you to take advantage of the benefits each provides. The combination of radio frequency and conventional drying has been employed in several different ways, and each has resulted in significantly improved drying processes. There are four main radio frequency/conventional heating combinations: RF preheat, RF boost, RF finish and full RF/conventional.
RF Preheat. One combination of radio frequency and conventional drying uses radio frequency at the beginning of the process. With this approach, overall time in the falling rate zone is shortened because the entire product is heated. Uniform heating also avoids a dry outer layer, which can cause uneven dispersion of coatings and dyes. Radio frequency preheat also is used in curing processes. It can quickly heat a product to a consistent temperature, after which conventional methods are good at maintaining the temperature for a dwell or cure time.
RF Boost. Radio frequency energy also can be added in the middle of a process line to provide a boost to the conventional drying process. This option has been used successfully in a paperboard line. Radio frequency allowed the downstream steam cylinders to be more effective by moving moisture to the surface for them to remove. As with the other combination approaches, overall drying time is reduced.
RF Finish. Another combination of radio frequency and conventional drying involves using radio frequency for finish drying. This combination is used on materials with good thermal insulating properties where the dry insulating surface inhibits drying. It also is used for postbaking of cookies and snack crackers.
Full Radio Frequency and Conventional. The first three drying combinations use radio frequency in part of the overall process cycle to effect some significant reductions in drying times. It makes sense that another option is to use radio frequency and conventional methods simultaneously during the entire process. This offers the largest potential reduction in drying time, although it does complicate equipment design. In many cases, drying times with hybrid systems have been reduced from hours to minutes. The simultaneous use of radio frequency and convection provides an added benefit for temperature-sensitive webs. In this case, radio frequency is used to heat and evaporate water in the web, and ambient air (rather than heated air) is used to remove moisture from the surface and keep the web temperature lower. This combination offers gentle drying where fast drying is necessary but the web cannot be exposed to high temperatures.