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COMBINATION HEATING
The benefits provided by combining radio frequency with conventional heating and drying methods make it an attractive alternative heating method for many industries.
Today's heating and drying customers demand high quality at a low cost. Radio frequency (RF) heating and drying can provide a high quality/low cost combination by providing a thermal profile that conventional heating and drying methods alone are not able to achieve. Benefits of radio frequency heating include:
* 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...