Inside view on deep heat

The radio frequency oven could soon replace the microwave – and can also help plastic forming, too. By coupling more deeply within the material to heat it from the inside, time is saved by not having to wait for heat to diffuse, while heating is also more even. Radio frequency induction heating has been around for decades, but using radio frequencies for domestic and catering cooking – and other potential applications such as pre-heating plastics for forming – is the brainchild of C-Tech Innovation. According to Michael Harrison, operations director at C-Tech, the frequencies it mainly uses are 13.56 and 27.12 MHz, as opposed to the 2450 MHz used by domestic microwave ovens. These, he says, induce currents – instead of the rotation of water dipoles in a microwave heater – which means they will heat ordinary water but not distilled water, which has very low electrical conductivity. Their advantage over microwaves is that they penetrate more deeply. “Microwaves penetrate only one or two centimetres, so there tend to be hot and cold spots,” says Harrison. Another use of radio frequency heating is in defrosting, which microwaves perform poorly. “Microwaves do not couple well with ice [as the water molecules cannot rotate],” he adds. “Radio frequencies do not couple much better with ice, but they couple well with impurities.” Microwaves came out of wartime radar developments that have led to the availability of very cheap magnetrons. They were chosen in the 1960s as a means of domestic cooking. “I’m not sure why,” says Harrison. “The ideal frequencies to use are lower. More products are susceptible to microwaves, but radio frequencies penetrate to greater depth and RF is the option we tend to prefer.” Apart from other considerations, radio frequencies can be more easily screened than microwaves, because the hole size in screens can be larger. Generations of radio frequencies used to depend on the use of expensive glass vacuum tubes, but Harrison says that this can now be accomplished using solid state crystal controlled oscillators that are more robust and less expensive, and matched into standard 50 ohm coaxial power cable, allowing easy transfer to applications. Specific developments at C-Tech include radio frequency food ovens that combine a standard convection oven suitable for the catering trade or a science laboratory with up to 600W of RF power. C-Tech claims this defrosts up to 10 times faster and, typically, cooks twice as fast as a conventional product. It handles foods that cannot be processed in a microwave oven, and improves food quality and safety, meaning that it does not, for example, produce a chicken that appears fully cooked on the outside, while still frozen in the middle. C-Tech’s prototype is a 2.4kW fan oven and runs at 27.12MHz. Evaluation units are available for hire or sale. A 13.56 MHz version is also available. As well as food, radio frequencies couple with many plastics. Conventional plastic heating processes involve surface heating – conductive, convective or infrared. These lead to the same time delays and processing problems as heating bulk food items. C-Tech has been investigating RF heating for thermoplastic plasticising and extrusion, rapid moulding of thermoplastics, thermoforming of plastics and the manufacture of structural composites. In a novel plastic-forming process developed at C-Tech, 300W of RF power enabled heating in less than 15 seconds, as opposed to 20 minutes in a conventional oven. RF forming could also have potential for blow moulding or advanced welding. Pointers * Radio frequencies penetrate into the bulk of materials, much more so than either surface heating or microwaves * The technology increases defrosting speed by up to 10 times, cooks food twice as fast and throughout and can, in some cases, speed the heating of plastics for forming by a factor of 80