Drying is the removal of a solvent from a product. This term sometimes is used interchangeably with heating or curing. However, heating is simply raising the temperature of a product, while curing is holding a product at a temperature for a given time to complete a reaction. Methods of heat transfer used in dryers and ovens include conduction, convection, infrared/radiant, and radio frequency. Radio frequency is the transfer of energy to create heat. Each method has benefits and challenges for manufacturing processes. As a result, it is common to design combination ovens to utilize the benefits of two or more technologies together. For example, a dryer utilizing both infrared and convection improves the process by having the high rate of heat transfer of infrared with the uniform heating of convection.
Drying requires heat transfer and mass transfer. Heat transfer heats the solvent while mass transfer removes the solvent from the product. Higher temperatures mean faster drying, and higher mass transfer also means faster drying. Mass transfer is the difference between the partial pressures in the air and coating.
No single heating technology is perfect for every process. Heat processing applications may require convection, radiant/infrared and radio frequency heating technologies, or a combination of them.
ConvectionConvection dryers and oven systems utilize heated air to process a variety of products for drying and curing. Convection heating offers consistent, even heating throughout the dryer or oven. Convection heating can be direct or indirect with virtually any fuel from natural gas to coal. Typical convection applications include heating, drying and curing of water and solvent coatings for textiles, films, paper, fibers and metals.
Advantages of convection heating include:
- Provides heat transfer and mass transfer.
- Allows accurate control of temperature.
- Provides uniform heating regardless of product size, color or shape.
- Does not require contact to heat (webs can be floated with no contact).
- Achieves a lower rate of heat transfer than other heating methods.
- Requires increased product exposure time.
- Has slower startup/cool down than other heating methods.
- Airflow can disturb or contaminate the product.
- Relies on thermal conductivity of the product to heat interior of thick products.
Types of convection dryers/ovens include flotation, impingement, flow-through and tunnel. Flotation systems dry both sides of a web at high speeds without leaving marks or scratches. The web is supported by air bars that provide the flotation as well as a high heat transfer coefficient. Advantages of flotation dryers include high drying speeds and the ability to dry on both sides. These dryers are used with solvent- or water-based systems for paper, film or foil. Typical applications include flexible packaging, magnetic media, photographic films, nonwovens and printed products.
With impingement dryers, air is directed at the product through nozzle and supply systems. Spent air is recirculated between the supply nozzles to increase heat transfer and web stability. This technique produces the even side-to-side airflows required for consistent cross-web drying and curing.
Flow-through dryers heat webs throughout rather than just heating the surface. They operate by pulling heated air through the web and are particularly effective heating thick, permeable webs. While flow-through dryers generally are arranged with the heated air flowing either up or down through the product, they also can be configured with the air flowing alternately up and down through the web. Even though this arrangement is more complex, it provides the highest and most uniform heating.
Tunnel dryers utilize airflow that is parallel to the product flow. It is common for strands and cords. Tunnel dryers provide excellent uniformity.
InfraredAlso called radiant heating, infrared heating is the transfer of energy via electromagnetic radiation between a source and an object. Most materials absorb heat in the infrared wave band. Radiant heating (electric, gas, steam, oil) often is used for drying textiles, ceramics, paint (wood and metal), hardening powder paint and heat treatment of plastics. Radiant heat is most beneficial in rapid, direct heating of a product.
Advantages of infrared heating include:
- Achieves high rate of heat transfer.
- Provides quick startup.
- Requires less space than convection systems.
- Has no air movement, which means the product or coating is undisturbed.
- Temperature control can be more difficult to manage than convection.
- Potential for overheat exists.
- Heat transfer is line of sight, which means any part of the product that does not “see” the infrared energy may not be heated adequately.
- Effective infrared dryer design requires precise layout of sources.
- Heating rate depends on product characteristics.
- Infrared relies on thermal conductivity of the product to heat interior of thick products.
Radio FrequencyConventional heating (i.e., conduction, convection and radiant) has a heat source on the outside and relies on transferring the heat to the surface of the material to be heated and then conducting the heat to the middle of the material. Radio frequency heating is different. It heats at the molecular level, so it heats from within the material and heats the middle as well as the surface.
Radio frequency or dielectric heating utilizes radio frequencies between 10 and 100 MHz to dry or heat product. Radio frequency drying normally is used in products with high loss factors such as wet materials, vinyl, plastics and adhesives, and other non-electrically conductive products.
Advantages of radio frequency drying include:
- Faster drying/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 in the product, with less heating of the base material.
- Ability to achieve moisture profiling or leveling.
- No air movement, so drying does not disturb the product.
- Faster shut downs and startups because the heating is instant on and instant off.
- Radio frequency may not heat complex shapes uniformly.
- Radio frequency cannot be used on metal or conductive materials.
- Radio frequency generally is not effective with organic solvents.
- Radio frequency generally has higher capital cost.
Conventional heating methods remove moisture from the surface of materials while radio frequency heats the middle of a product and drives moisture to the surface. Combining these two technologies allows users to take advantage of the benefits each provides.
No single heating technology, whether infrared, convection or radio frequency, can alone meet every complex and changing requirement of today's manufacturing needs. Each method has capabilities and limitations. Combination dryers and ovens utilize two or more drying methods to meet the process requirements. Typically, combination dryers and ovens have independent temperature controls for the convection, infrared or radio frequency zones.
Combination dryers and ovens can increase quality and productivity; maintain hold temperatures to within a few degrees of the setpoint; and require less space. The use of two technologies delivers effective, efficient heat transfer while increasing productivity and quality.