A combination radio frequency/convection oven like this one is suitable for water-based sensitive adhesives.

Infrared. Also called radiant heating, infrared heating involves an exterior heat source directing its energy at the product to be heated. Infrared heating results from the absorption of radiant energy by an object. Infrared relies mainly on line-of-sight heat transfer, which means that a radiant source only heats surfaces it can see; it does not heat the air. Once the product is heated, some heat transfer occurs within the product due to conduction. This type of heating is suitable for transferring large amounts of thermal energy to rapidly heat a product. A good example is a toaster.

High rates of heat transfer without air movement are possible with infrared heating. Varying intensities can be selected based on the process requirements. Infrared technology provides several process advantages, including:

• Fast production startup.

• High heating efficiency.

• No air movement.

• Easy and effective zoning of heating sources.

• High temperature capabilities.

So, when you need to transfer large amounts of thermal energy to heat a product quickly, there are definite advantages to be had using infrared.

Infrared also has limitations. Precise control of product temperature is difficult with infrared heating and can require sophisticated sensors and temperature controllers. This can be a problem because infrared radiation heats a product to a transient temperature, not the equilibrium or "soak" temperature a product would reach if heated for a long period of time. Because infrared source temperatures range from 400^oF to 4,000^oF (204 to 2,204^oC), the equilibrium temperature of a product heated solely by infrared energy could exceed an acceptable product temperature. Surface heating relies on conduction, so it may not be as good for thick products -- especially if they have low thermal conductivity.

Convection. This heating method involves the indirect transfer of thermal energy by the circulation of a heated carrier such as air. The greater the air velocity and turbulence, the faster the heat transfer. A common example of convection heating is a hand-held hair dryer.

Convection heating supplies heated air at a temperature above the desired product temperature; in effect, the entire product is heated to an equilibrium temperature, usually between 250 to 600^oF (121 to 316^oC). Convection ovens direct air at the product to do the work of drying, curing or heating.

Convection is suitable for uniformly transferring heat to a product. Its advantages include:

• Accurate control of temperature.

• Simplified control systems.

• Uniform, heating regardless of product size or shape.

• Moisture removal.

When a uniform process heating temperature is required, convection heating is a good choice.

Convection heating also has some limitations. The primary disadvantage of convection heating is its low rate of heat transfer and inherent requirement for air movement. These characteristics can result in a lower startup to temperature, increased exposure time and the possibility of disturbing or contaminating the product from air movement.

Radio Frequency. The theory of radio frequency heating is that dielectric materials are heated when placed in a high voltage, high frequency electric field. The best materials for radio frequency heating are those that are neither good conductors nor good insulators (i.e., dielectrics). This particularly applies to water and explains why radio frequency is a good drying method.

Radio frequency heating offers several advantages, including:

• Faster drying or heating times.

• More uniform drying with a consistent temperature gradient, especially in thicker products.

• Lower temperature drying because the radio frequency energy heats the water contained in the product, with less heating of the base material.

• Moisture profiling or leveling to create a more consistent quality product.

• Faster shut downs and start ups because the heating is instant on and instant off.

When you need to heat a dielectric, radio frequency is a good method.

Radio frequency heating also has some limitations. It is not useful on irregularly shaped products because it might not heat complex shapes uniformly. Also, radio frequency cannot be used on metal or conductive materials, and it generally is not effective with organic solvents.

Combining Technology

Combination heating, drying and curing joins convection with infrared or radio frequency technologies together in a single oven -- productively, precisely and flexibly.

An inside look at an infrared oven shows that high rates of heat transfer are possible without air movement. Varying intensities can be selected based on process requirements.

Combination Infrared/Convection

Concurrent Combination Ovens. Typical objectives of a concurrent combination oven are to rapidly transfer heat to a product and/or coating with infrared and to simultaneously move air over the product and/or coating with convection in the same zone.

Concurrent combination ovens are suitable for applications where temperature-sensitive products or coatings require high rates of mass transfer. Mass transfer of water or solvents requires effective air movement. Drying applications are the most common. Other applications include paper drying, adhesive drying, porcelain frit drying and many constant- or falling-rate drying applications.

Advantages of concurrent combination ovens include faster line speeds, accurate control, consistent heating, removal of moisture, and dilution of solvents.

Consecutive Combination Ovens. Typical objectives of a consecutive combination oven are to rapidly bring a product and/or coating to a transient temperature in the infrared section and to hold the product and/or coating at a precise equilibrium temperature for a specified time (cure time) in the convection section.

Consecutive combination ovens are suitable for applications where a high initial rate of heat transfer is followed by an accurate and uniform convective heat transfer. Applications include powder coating curing and many falling-rate drying applications. Advantages of consecutive combination ovens include faster line speeds, accurate control, consistent heating and undisturbed products or coatings.

Radio Frequency/Convection Combinations

A typical drying curve shows the high initial drying rate (when moisture is near the surface) and the long falling-rate zone. This falling-rate zone typically is 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 radio frequency drying curve is essentially the same but is compressed because radio frequency heats throughout the whole product.

The combination of radio frequency and convection is based on the proven benefits of radio frequency drying from within and convection air movements enhancing heat transfer.

Radio Frequency Preheat. This method combines radio frequency and convectional drying by placing radio frequency heat at the beginning of a process. This approach heats the material quickly and evenly and helps move moisture to the product's surface. The overall drying 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 radio frequency preheat is curing processes. The radio frequency quickly heats the product to a consistent temperature, after which convection heating maintains the temperature for a dwell or cure time.

Radio Frequency Boost. Radio frequency energy can be added in the middle of a process line to give a radio frequency "boost" to the convection drying process. In these applications, the radio frequency heats the inside of the product and drives moisture to the surface, where convection is effective. This approach has been used in a paperboard line, and the radio frequency made the downstream steam cylinders (conduction) more effective by moving moisture to the paperboard's surface, where the cylinders contacted a wet product surface rather than a dry, insulating product surface. The overall drying time was reduced with the use of the radio frequency boost.

Radio Frequency Finish. Another combination of radio frequency and convectional drying is using radio frequency to perform finish drying. This approach is used with materials that have good thermal insulating properties, so the dry isolating surface inhibits the drying process.

Full Radio Frequency and Conventional. The first three combinations of radio frequency and convectional drying use radio frequency as a part of the overall process cycle to reduce drying times. Another option is to use radio frequency and convection simultaneously. This approach offers the largest potential reduction in drying time of all the methods. 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 indicates the significant reduction in drying time with the addition of radio frequency to the standard process using 356^oF (180^oC) air. Drying times with other hybrid systems have been reduced from hours to minutes in many cases.

The simultaneous use of radio frequency and convection has an interesting application for temperature-sensitive products. In these ovens, the radio frequency 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.

Obviously, neither infrared, radio frequency nor convection heating alone can meet all process heating requirements. But, when joined together in a synergistic oven design that capitalizes on the strengths of convection with infrared or radio frequency and minimizes their limitations, an optimum oven can be achieved. The results of maintaining optimum control and process flexibility are higher productivity and consistent, higher quality parts at lower costs.

Links

• C.A. Litzler Co. Inc.