In addition to stand-alone RF drying, RF can be combined with conventional drying methods such as convection and conduction to gain even greater improvements in line speed and product quality.  There is great synergy with these combination systems where RF excels at heating evenly within the material and pushing the moisture to the surface and conventional methods
excel at removing moisture from the surface.  Whether the RF is used as a pre-heat, boost, or finish, or a full RF application, drying can be much more effective with these systems.

At the beginning of a process RF heats the material quickly, evenly and helps move the moisture to the surface. The overall time is shortened in the falling rate zone because the whole product has been heated, not just the surface. The even heating avoids a dry outer layer that causes uneven dispersion of sizing and additives in the product. Another application for RF preheat is curing processes. The RF is very good at quickly heating the product to a consistent temperature, after which conventional methods are very good at maintaining the temperature for a dwell or cure time.

DRYING CURVE, RF PREHEAT

RF energy can be added in the middle of a process line to give an RF “boost” to the conventional drying process. In this case, the RF heats the inside of the product and drives the moisture to the surface where conventional methods are effective. This option has been used in a paperboard line and the RF made the downstream steam cylinders more effective by moving the moisture to the surface where the cylinders now contacted a wet product surface rather than a dry, insulating product surface. As with the other combinations of RF and conventional drying, the overall drying time is reduced with the use of the RF boost.

DRYING CURVE, RF BOOST

Another combination of RF and conventional drying is using RF to do the finish drying. This is used with good thermal insulating materials such as ceramic fiber mat where the dry isolating surface inhibits the drying process. The graph below shows how an RF finish can be used to reduce the drying time of the product.

DRYING CURVE, RF FINISH

The first three combinations of RF and conventional 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 conventional methods simultaneously during the whole process. This offers the largest potential reduction in drying time of all the methods, although it does complicate equipment design. Greenbank Engineering and the Electricity Council of the United Kingdom did a notable study in this area. The results of their work on Air, Radio Frequency Assisted (ARFA) drying is shown below. It shows the significant reduction in drying time with the addition of RF to the standard process using 180°C air. Drying times with other hybrid systems have been reduced from hours to minutes in many cases.

DRYING CURVE, FULL RF + CONVENTIONAL DRYING
The simultaneous use of RF and convection has another interesting application for temperature sensitive products. In this case, the RF is used to heat and evaporate the water in the product and ambient air (rather than heated air) is used to remove the moisture from the surface and keep the product temperature lower. This combination offers a “gentle” drying where fast drying is needed but the product cannot be exposed to high temperatures.