Radio Frequency Heating: Tuned-In for Next Gen Hybrid ApplicationsRadio Frequency Heating: Tuned-In for Next Gen Hybrid Applications

Virtually all manufactured items utilize heat somewhere in the manufacturing process and there are obviously many heating technologies to choose from. The heat requirement may be for forming, bonding, curing, pasteurization, or a myriad of other needs. For the sake of this short article, we will focus primarily on drying, as alone represents approximately 43% of gas usage in the manufacturing sector’s¹ 10.52 trillion cu ft of gas per annum² and a good area to look for enhanced efficiencies. Also, a synopsis of where radio frequency drying has fit into the mix of other drying technologies historically, and what the future looks like given a changing manufacturing environment that is demanding increased energy efficiencies and lower greenhouse gas emissions.

Radio frequency (RF) heating, by-in-large, has been a niche technology employed only where its unique heating and drying characteristics are advantageous to the process. RF is a form of electromagnetic energy that will instantaneously, and preferentially, apply heat energy wherever polar water molecules are present. However, due to its comparatively high upfront capital cost, it has been most effectively deployed where other lower cost conventional means of heating either cannot effectively dry a given material or can do so only with minimal efficiencies.

Related:Ensuring Consistent Spray Dryer Operation

Polar water molecules (Radio Frequency Company, Inc.)

Such applications include insulative materials processed in a thickness that prohibits the penetration of conventional heat sources, which apply heat from the exterior such as hot air or infra-red, nonwoven battings, ceramic extrusions, construction products and hydrophilic foams are a few examples of these materials with highly insulative properties. Because RF drying occurs at relatively low temperatures, (100˚C) temperature-sensitive materials, which may discolor or degrade with prolonged exposures to high temperatures, are also good candidates for RF drying. Items such as certain polymeric materials, which must be dried to very low moisture contents measured in the “parts-per-million,” some filter cake materials, and also food stuffs where nutritional values must be maintained during the drying process are again great opportunities for RF. Accordingly, pasteurization treatments have also been historically successful applications for RF heating systems in the areas of cereal grains and dry ingredients as the process is organic and does not negatively affect protein functionality.

Non-woven ceramic battings dryer (Radio Frequency Company, Inc.)

As a 35-year veteran in the RF heating and drying business, I can tell you from handling inquiries over all those years, they’ve mostly boiled down to one question: Can it be done satisfactorily with a less expensive form of conventional heating? If the answer was “yes,” it was wise to move along to the next inquiry as comparative capital cost has always been the ultimate hurdle. However, today manufacturers are increasingly aware of the true cost of inefficient, time-honored (that’s the way we’ve always done it), drying methods as energy costs continue to rise at the same time gas emissions need to be cut to the bare necessity in order to keep pace with corporate and global objectives. Because RF energy is “instant on/off,” requires no warmup time, is a direct form of heating, i.e. heats only the material rather than copious amounts of hot-air and the oven structures themselves, and only uses energy in proportion to the load, an increasing realm of new opportunities are emerging for a technology that has been available for the last 75 years.

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Another increasingly important aspect of RF drying is that it produces zero greenhouse gas emissions which helps manufacturers meet their sustainability goals and move towards corporate net zero initiatives. However, because RF remains a higher cost technology, from a capital standpoint, and conventional drying technologies can be efficient during certain parts of the overall drying process, the correct “fit” for RF often involves a hybridized approach by removing inefficiencies and eliminating bottlenecks from the overall drying process. In other words, only where it’s needed and where it can meet or exceed the capital ROI criteria.

Related:Scaling Up from Laboratory Drying Tests to Industrial Applications

Generally speaking, in more recent hybrid applications, conventional drying, which forces heat from the exterior to the interior of a mass, becomes increasingly inefficient towards the end of the drying process. This happens primarily because water is a good thermal conductor and as it is removed, conventional heat transfer becomes less and less efficient. This is not the case with RF, as it penetrates the load volumetrically and the water molecules will vaporize in-situ regardless of their location. For instance, if the outside surfaces can be dried with good efficiencies with conventional drying means, the remaining internal moisture content can then be specifically targeted and removed by RF very rapidly and with excellent efficiency.

This reduction in heat transfer efficiency is very clearly shown in the baking industry where RF has been running in a hybridized fashion for decades. The below case study was made on a snack cracker line for a major producer of biscuit and crackers for human consumption. However, the logic holds true for pet foods as well as many other industrial materials and finished products. This type of hybrid arrangement is referred to as “sequential hybridization” as the conventional oven is immediately proceeded by the RF drying system (see diagram).

Convection/direct gas fired (dgf) followed by RF post-baking dryer (Radio Frequency Company, Inc.)

Case Study: Baking Line Increases Production & Reduces Carbon Emissions 22% by Adding an RF Post-Baking Dryer

Environmental studies show that carbon emissions from food production alone could use up all our world’s CO2 budget to stay within the goal of a 1.5°C or 2°C global temperature rise. However, in the baking industry, there are a range of new opportunities to avoid this paradigm benchmark and move one step closer to, “net zero.”

Conventional baking ovens (those which apply heat to the outside surface of a product) are notoriously inefficient when it comes to removing moisture in the final stages of baking. This happens because, as a product develops loft and crumb structure, it becomes less of a thermal conductor and more of a thermal insulator which inhibits heat penetration toward the center of a product. Aggravating this problem is the amount of external heat applied to a product, in the final stages, must be limited to avoid scorching and over-coloration. This final stage of baking/drying often requires as much as one-third of the oven length or one-third of the total bake-time. By adding an RF post-baking dryer, band speeds can be increased up to 30% on average when this final drying requirement is removed from the conventional oven.

Because water is a polar molecule, it is extremely receptive to radio frequency (RF) heating. When exposing a product with an uneven moisture content (moist interior/drier exterior) to a rapidly oscillating RF field, the energy will preferentially target and remove moisture from the moist interior without overheating or discoloring the exterior surfaces. Also, because RF is a direct form of heating, the drying is 100% efficient, requires no warm-up time, and significantly reduces bake time for most products. By eliminating this final drying requirement from a 300-ft-long DGF/convection oven, the addition of an RF post-baking dryer achieved a 22% increase in productivity as well as a corresponding reduction in gas emissions.

Combining Conventional Heating and Radio Frequency Heating/Drying in the production of a popular snack cracker

Before the addition of the RF post-baking dryer:

Natural gas consumption of a 300-ft-long DGF/convection oven: 6,300 cu ft of gas to produce 6,250 lb of baked product per hour. 6,300 / 6,250 = 1.008 cu ft of gas/lb of product produced.

After the addition of the RF post-baking dryer:

Natural gas consumption of a 300-ft-long DGF/convection oven: 6,300 cu ft of gas, plus a final zone of RF, now is able to produce 8,000 lb of product per hour (plus 22%) 6,250 cu ft of gas / 8,000 pounds per hour = .78 cu ft of gas/lb of product produced. A reduction of 22%.

The increase in band speed provided by the efficiencies of RF drying, placed only where the inefficiencies of conventional heating/drying were apparent, The ROI for the above case history was a slam-dunk. Consider that the additional throughput per hour was an added 1,700 lb/hr. Over a 6,000-hour, three-shift year, is an added 10,200,000 lb per year with a corresponding reduction in gas emissions on a per pound basis. Depending, of course, on the margins of any particular product, but certainly a sufficient increase in productivity for a rapid payback on the equipment.

In addition to the above example of sequential hybridization, hot air followed by RF, much focus is now being applied to the continued development of combined hybridization where both hot air and RF are performing together simultaneously. Combined in one chamber or multiple chambers in series, each respective technology working together efficiently to minimize dwell times, energy consumption, and floor space requirements. One such system is the new Macrowave Zephyr-Series that was introduced in early 2024.

New Zephyr Series dryer (Radio Frequency Company, Inc.)

Radio frequency (RF) dryers have long been utilized for the drying of bedded materials due to their volumetric heating and ability to penetrate deep bed depths. Further, RF energy will selectively heat materials where the moisture content is greatest making it ideal when drying the moist core of a product becomes problematic or inefficient with conventional drying technologies.

RF energy will vaporize moisture in-situ and in past RF systems, the water vapor would exit the bed of material through natural convective currents. The Zephyr-Series employs a pressurized lower electrode array that forces hot air up through an open-weave conveyor belt to actively, yet gently, force the water vapor away from the bed where it is then exhausted from the system. Factory trials have demonstrated a 28% increase in drying efficiencies with this new approach.

Zephyr Series lab simulator (Radio Frequency Company, Inc.)

2025 will remain a time when greater manufacturing efficiencies are being sought in order to save on production costs, and a time when new ideas are pushing away some of the more traditional approaches to drying, radio frequency heating technology will certainly play an ever-increasing role in assisting manufacturers in reducing drying costs and gas emissions.

Timothy D. Clark is president & CEO, Radio Frequency Company, Inc. (Millis, MA). For more information, call 508-376-9555 or visit radiofrequency.com.

References

¹ www.naturalgas.org

² en.macromicro.me