RF technology turns the tables on the traditional methods and avoids this problem. “RF is volumetric heating—it heats uniformly throughout the whole thickness of the product,” explains Ben Wilson, PSC’s sales manager. “It’s heating from within, rather than from the outside in. With RF, we typically heat the middle and push the moisture to the surface.”
This is where the benefits of a combination system come into play. Once the RF technology has heated the product and moved the moisture to the outside of the product, convection is used to remove it from the surface. “For ceramics, typically RF and convection is the best combination system,” says Wilson. “Convection does very well at the surface, but not very well from the inside of the product. RF does very well from the inside, and then pushes the water to the surface where convection can take the surface moisture off. It’s a terrific combination.”
Another benefit involves binder dispersion. “As the product is dried, many times there will be a wicking effect, where the moisture will move to the surface and carry the solids and the binder with it. You get more binder on the outside and less on the inside,” explains Wilson. “Because RF heating is uniform and the water is moving through the product as a vapor, the solids don’t get carried to the surface and the binder dispersion is more uniform.”
For some applications, such as drying or curing coatings, a system that also incorporates infrared technology will provide the best results. While it’s not typically used with RF, infrared can be combined with convection as a pre-heat or temperature boost in the convection zone. “Infrared is a very good technology for high-heat input into the product,” says Wilson. “It’s a surface heating, so it’s typically used on thinner materials and for drying coatings.”
In some cases, though, the most economical drying solution may be a single technology. Infrared can be used effectively for surface heating, convection can be used for effective drying and precise control, and RF can be used for selective heating and for heating from within.
Whether it’s a single technology system, a combination RF-convection system or one that incorporates infrared technology, PSC tailors each project to suit the customer’s needs. Customers can then either purchase the equipment or lease it to test on their process at their own facility. PSC also supplies spare parts and provides phone and field service assistance.
Additionally, new fiber optic temperature sensing allows for real-time temperature monitoring. “The RF field is not compatible with metals, so you can’t use a typical thermocouple,” explains Wilson. “You can use infrared, but that only measures the surface temperature. The fiber optic temperature sensor measures the internal temperature of the product during the heating and drying. The unit sends a light signal through a fiber optic cable, and the tip of the probe reflects that light back. As the temperature of the product changes, it changes the properties of the tip, and the unit measures the change in reflection of the light.”
Several types of infrared heaters were also added, and a combination convection and infrared unit allows the company to assess the feasibility of this combination.
As the needs of its customers evolve, PSC continually works to meet their diverse demands. “The challenge is, there’s no single perfect technology,” says Wilson. “We look at the benefits and drawbacks of each of the different heating and drying technologies and then combine them to optimize the benefits for each customer.”