Combination dryers using both radio frequency (RF) technology and convection technology can solve this problem. These dryers allow ceramic manufacturers to change from older batch processes to lean, continuous processes, in some cases reducing drying times from 24 hours to 90 minutes and from 12 hours to 30 minutes. Combination RF-convection dryers also offer the added benefits of more uniform temperature gradients and moisture levels, less solids migration of sizing and additives, lower drying temperatures, and smaller equipment.
RF vs. Conventional HeatingFigure 1 illustrates the differences between conventional and RF heating. Conventional heating (also called conduction, convection and radiant heating) heats products from the outside in. The heat is first transferred to the surface of the material and is then conducted to the middle of the material. Radio frequency heating, on the other hand, heats at the molecular level. It starts heating from within the material and heats the middle as well as the surface.
With RF drying, the heating is from within, so there is no hot, dry outer layer. The product is heated throughout—as the water in the middle is heated, it moves 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. For this reason, combining these two technologies—using RF heating to heat the inside and move the water to the surface where it is effectively removed with a well-designed convection system—offers significant potential benefits.
The Theory of RF HeatingThe basic theory of RF heating is that dielectric materials are heated when placed in a high voltage, high frequency electric field. The best materials for RF heating are those that are neither good conductors nor good insulators (i.e., dielectrics). Because RF heating heats at the molecular level, heating takes place throughout the whole material, from the middle to the surface. The material is heated through two heating mechanisms: dipole rotation and ionic conduction.
In dipole rotation, the individual molecules rotate to align themselves with the electric field. Since the electric field changes polarity millions of times per second, the molecules rotate millions of times per second, which causes friction and heat. The power put into the material by dipole rotation is based on the voltage gradient in the material, the frequency of the electric field and the loss factor of the material. The relationship among these variables is complicated by the fact that the loss factor varies with both frequency and temperature.
In ionic conduction, charged particles (ions) are always moving toward the opposite charged plate. Since the polarity is changing millions of times per second, these ions are constantly moving and colliding with other particles, similar to billiard balls. These collisions create friction and heat that warms the material. The power put into the material by ionic conduction is based on the voltage gradient in the material and on the conductivity of the material. In ionic conduction, frequency has no effect on the power.
The Effect of Materials on RF HeatingMaterials have a major effect on the success of RF heating. Some materials heat very well, while others do not heat well at all. The key measure of “heatability” is the loss factor of the material—a material property that determines how well the material absorbs the RF 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 and ceramics 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 RF lends itself to drying so well—it heats the water quickly but does not heat most polymer and ceramic materials.
It is important to remember that every material reacts differently, and loss factors can change with frequency and temperature. A material that does not absorb RF energy at room temperature might absorb the energy at higher temperatures. Likewise, the loss factor can vary with frequency (water is a good example). Due to the complexity of the interaction between materials and the RF field, it is crucial to consult with an expert in RF drying and conduct trials on any given product before investing in any new equipment.
Benefits of RF DryingBecause RF drying differs from conventional drying methods, it provides some significant advantages in drying materials such as ceramics. These include:
- Faster drying times and reduced labor costs with continuous versus batch processing. Because the energy is absorbed directly by the water throughout the product, heating and drying are faster than convection, conduction and infrared methods. This means faster cycle times and lower labor costs with a continuous process.
- A more uniform temperature gradient through the product due to heating from within, resulting in more consistent product quality. This uniform heating prevents a dry outer layer and provides a more uniform dispersion of sizing and additives throughout the product. This is especially important in drying fiberglass packages.
- No overheating of the base ceramic material, since most materials are self-limiting, i.e., they won’t heat once the material is dry. This prevents scrap product and improves product quality.
- Selective heating, which allows water to be heated with less heating of the base ceramic material. This improves the quality of the product through lower drying temperature. Product temperatures do not normally exceed 212∞F.
- Moisture leveling in the product, resulting in more consistent product quality and lower cost by not over-drying some areas to meet required moisture levels in others.
- Instant on and off power and heating, reducing costs by eliminating warm up and cool down cycles associated with conventional systems.
- Efficient energy usage, because the energy used is proportional to the amount of work being done (i.e. water being evaporated). This reduces costs as lower production volume will have reduced operating costs.
- Fewer environmental issues, as there are no combustion by-products to exhaust. This improves quality by keeping the dryer cleaner and reduces costs by reducing permits and approvals for process emissions.
Combining RF and Convection DryingAs described earlier, convection heating technology works very well at removing moisture from the surface of materials, and RF works very well at heating the middle of a product and driving the moisture to the surface. It is only natural to then look at combining these two technologies to take advantage of the benefits each provides. However, simply adding hot air to a RF dryer does not typically provide an adequate solution. An effective combination system requires a thorough knowledge of convective drying, RF drying, and how to effectively integrate the two.
Figure 3 shows a typical conventional drying curve with the high initial drying rate (when moisture is near the surface) and the long falling rate zone. This falling rate zone is typically a result of the formation of a dry, insulating layer at the surface of the material that impedes the heating of the middle of the product. The RF drying curve is essentially the same but is compressed due to the fact RF heats throughout the whole product.
Types of RF-Convection DryersSeveral different types of RF-convection combinations are possible:
- RF Preheat. One possible combination of RF and convection drying is using RF at the beginning of a process. This heats the material quickly, evenly and helps move the moisture to the surface. The drying time is shortened in the falling rate zone because the whole product has been heated, not just the surface. Another application for RF preheat is curing processes. The RF is very good at quickly heating the product to a consistent temperature, after which convection is very good at maintaining the temperature for a dwell or cure time.
- RF Boost. RF energy can be added in the middle of a process line to give an RF “boost” to the convection drying process. In this case, convection first removes the surface moisture, and RF then heats the inside of the product and drives the moisture to the surface, where convection finishes the drying process.
- RF Finish. Another combination of RF and convection drying is using RF to do the finish drying. This can be used on insulating materials such as ceramic fiber mat to remove the last remaining moisture from the middle of the material.
- Full RF and Convection. The first three combinations of RF and convection drying use RF in part of the overall process cycle with some significant reductions in drying times. It makes sense that another option is to use RF and convection simultaneously during the whole process. This offers the largest potential reduction in drying time of all the methods.