Conventional heating (i.e., conduction, convection and radiant) has a heat source on the outside and relies on transferring the heat to the surface of the material to be heated and then conducting the heat to the middle of the material. Radio frequency heating is different. It heats at the molecular level, so it heats from within the material and heats the middle as well as the surface (figure 1).
A conventionally dried product is hot and dry on the outside and cold and wet on the inside. Unfortunately, this is not efficient because the dry outer layer acts as an insulating barrier and reduces the conduction heat transfer to the middle of the product. This dry outer layer also can cause quality problems such as surface cracking, a skin on coatings, and uneven solids dispersion through wicking of sizing and additives from the middle to the surface.
With radio frequency drying, the heating is from within, so there is no hot, dry outer layer. The product is heated throughout, so the water in the middle will be heated and will move to the surface. In general, because of the heat losses at the surface, radio frequency dried products are hot and dry on the inside and cooler and wetter on the outside. The combination of two technologies -- radio frequency and convection -- offers some great potential benefits. Radio frequency can be used to heat the inside and move the water to the surface, where conventional methods are effective at removing it.
Materials have a major effect on the success of radio frequency heating. Some materials heat very well, and some do not heat well at all. The key measure of “heatability” is the loss factor of the material. The loss factor is a material property that determines how well the material absorbs radio frequency energy. If the material has a high loss factor, it absorbs energy quickly and thus heats quickly. If a material has a low loss factor, it absorbs energy slowly and thus heats slowly. In general, polymers tend to have low loss factors and thus do not heat well. Water, on the other hand, has a high loss factor so it heats rapidly. This is why radio frequency lends itself to drying so well: it heats the water quickly but does not heat most base materials.
It is important to remember every material reacts differently and loss factors (the ability to absorb radio frequency energy) can change with frequency and temperature. A material that does not absorb radio frequency energy at room temperature might absorb the energy at higher temperatures. This is especially important in a composite product with a high loss factor material (radio frequency heats rapidly) and low loss factor material (radio frequency heats slowly). As the high loss factor material is heated by the radio frequency energy, it will heat up the low loss factor material through normal conduction. If this heat raises the temperature of the low loss factor material to where it now absorbs radio frequency energy, both products are heated and could be overheated. In rare cases, this can lead to a runaway situation where as the temperature increases, it absorbs more energy, which increases the temperature, which increases the energy absorbed, and it continues until the material overheats.
In most cases, the product can be heated faster than the solvent can be removed, so the heating rate must be scaled back to get the right balance of heat transfer and mass transfer. If the heat transfer rate is too high, steam will be generated, which can damage the product. The complexity of the interaction between materials and the radio frequency field is why it is critical to consult with an expert in radio frequency drying and conduct trials on your product. PH
Want to learn more? Visit www.process-heating.com and search for “Ovens” using our Google-powered Linx search engine.