Process heating technologies supply heat to nearly all manufacturing processes. Because the use of process heating systems represents a significant percentage of overall U.S. industrial energy consumption, process heating technologies also represent a significant opportunity to improve industrial productivity and energy efficiency. Faced with rising energy prices and competitive pressures, today’s industries are considering technologies that can complete process heating more quickly though the use of electrotechnologies, or heating through electromagnetic radiation.
Electromagnetic radiation is composed of particles (photons) or waves. Its properties depend on its wavelength: longer waves are less energetic than shorter waves. Electromagnetic radiation usually is described as bands of radiation of similar wavelength, e.g., infrared, radio waves, or microwaves. Only a small portion of the full range of electromagnetic radiation can be detected by the human eye; that portion is called visible light. The electromagnetic spectrum includes wavelengths longer than those of visible light such as those of infrared light, microwaves or radio waves, as well as those wavelengths shorter than visible light: ultraviolet light, X-rays, and gamma rays.
Among the technologies being considered for industrial process heating are infrared, microwave, ultraviolet and electron beam. This brief overview of common technologies will provide a starting point for processors who want to take another look at these technologies and others.
InfraredMany industrial heating processes cure a coating or remove moisture. If this heating is limited to the surface, then infrared may be the most economical method to achieve this heating. Industrial infrared elements work like those in a toaster and are designed to emit at certain wavelengths depending on the application. Infrared typically is much faster than hot air convection.
Infrared energy is made up of electromagnetic waves and behaves in much the same way as light energy -- both can be reflected, absorbed and transmitted in the same manner. Infrared is a line-of-sight technology; that is, an object must be in sight of the emitter to be heated. This attribute makes infrared particularly good in applications where an object needs to be heated only on a surface location (for example, curing a coating).
The infrared spectrum ranges from 0.76 to 10 micron and is divided into three subranges: short wave, medium wave and long wave. Each wavelength possesses its own heat transfer qualities. This range of capabilities makes using infrared technology an excellent choice for many applications, but it also requires careful planning to ensure that the right wavelengths and configurations are implemented.
Applications for infrared heating include:
- Ink curing.
- Powder coating.
- Drying of parts.
- Fine soldering.
- Silk screening.
- Latex and adhesive drying.
- Annealing/curing of rubber.
- Shrink wrapping.
- Molding plastics by blowing, vacuuming, rotamolding or squeezing the
plastic between calendar rolls.
- Drying textiles and paper.
- Quick response time.
- Ability to heat only the product’s surface.
- Ability to achieve high energy intensity.
- Short-wave emitters reach desired temperature in less than 1 sec and
cool almost as quickly.
- The high density of heating that can be achieved requires less floor
space and quicker processing of most products.
- An ordinary convection oven may be 30 percent efficient, where
infrared can be 50 percent (long wave) to 70 percent (short wave)
- Low maintenance.
MicrowaveMicrowave heating is a heat transfer technology using microwaves -- electromagnetic waves between 100 cm and 1 cm in wavelength -- to heat an object. Microwaves are able to penetrate objects, enabling a rapid, high-intensity heat transfer throughout an object to be heated. The volumetric heating capability of microwaves is well-suited for heating bulky products with a high volume-to-surface ratio.
Microwave radiation is suitable for removing moisture from bulky products and is able to penetrate more deeply than infrared; therefore, microwave drying allows much faster line speeds than typical hot air convection.
Applications for industrial microwave heating include:
- Tempering frozen meat to enable effective cutting.
- Curing of seamless rubber gaskets and moldings.
- Quicker heating of food.
- Curing of adhesives for plywood and construction lumber.
- Bonding of composite sheets.
- Quick response time for both startup time and time to reach
- Small temperature gradient, resulting in heat consistency throughout
- Quicker moisture removal while maintaining lower temperatures.
- Low maintenance.
- No package deformation.
Radio FrequencyRadio frequency energy and microwave energy are both dielectric heating technologies, where high-frequency electromagnetic radiation generates heat to dry moisture in nonmetallic materials. Radio frequency waves are longer than microwaves, enabling them to penetrate larger objects better than microwave energy.
During process heating, the material to be dried is placed in a high-frequency electric field created between a set of parallel plates or bars. Water molecules in the material are heated until they become steam. Air circulating through the drying chamber removes the steam and prevents condensation.
Radio frequency drying has been successfully used in the textile and furniture industries for more than 30 years and its use has progressively grown in other industries such as food processing and paper manufacturing.
Applications for radio frequency heating include:
- Drying natural and synthetic textiles.
- Drying water-based adhesives, emulsions and coatings at high
- Post-bake drying and moisture control in food.
- Preheating fiber mat board.
- Instantaneous glue-setting in furniture manufacturing.
- Moisture removal from glass fibers in both roving and bale form.
- Quick response time.
- Accurate final moisture control/moisture leveling.
- Environmentally friendly.
- Moisture removal at low temperatures.
- Low maintenance.
- Energy efficient.
Electron BeamElectron beam is used for curing and high temperature heating. In electron beam curing, a liquid is chemically transformed to a solid on the work surface by a stream of directed electrons. In electron beam heating, metals are heated at intense temperatures when a directed beam of electrons is focused against the work surface.
Electron beam curing generally is used to cure a thicker, more heavily pigmented coating than ultraviolet curing. It is used widely in film lamination and magnetic tape manufacture, with limited use in the wood finishing and automotive industries.
Electron beam heating is used extensively in many high-production-volume applications for welding, especially in the automotive industry. Using electron beams for heat treating applications is relatively new, with the primary application being in the automotive industry for local surface hardening of high-wear components.
Benefits to processors using electron beam curing systems include:
- Reduced requirements for floor space and operating labor.
- Higher productivity levels.
- Reduced curing time, from minutes to a second or less.
- Environmental benefits because they eliminate the use of
- The amount of indoor heat produced is insignificant.
Ultraviolet CuringIn the ultraviolet (UV) curing process, special chemicals (photoinitiators) are added to coatings, adhesives and inks. When these chemicals react with certain wavelengths of ultraviolet light, molecular linking occurs, creating a durable finish, superior adhesion and better-quality products.
Applications for ultraviolet heating include:
- Curing headlight and taillights lenses as well as
- Applying stains, dyes and scratch resistant coatings.
- Curing magnetic media.
- Printing applications such as aluminum cans and business cards.
- Instantaneous results.
- Space saving.
- Environmentally friendly; solvent-free.
- Cost-effective; simple implementation with little maintenance.
- Low energy.
- High utilization rate (no solvent carrier to evaporate).
InductionInduction heating is a heat transfer technology where an electromagnetic current is caused to flow through the material or its container. Induction heating produces high local temperatures very quickly -- in less than one second of residence time -- especially in conductive materials such as metals.
Induction heating is a noncontact method. Heat is generated only in the part, not in the surrounding area. The location to be heated can be specifically defined and achieves accurate, consistent results.
Induction heating frequently is used in industrial situations where reduced residence time and localized heating accelerate the production process and produce higher quality goods.
ResistanceResistance heating is a heat transfer technology where an electric current is applied directly to a conductive material such as metal. Resistance heat also is effective at heating air and fluids. With its versatility, controllability and quick heatup qualities, resistance heating is suitable for melting metals, glass and plastics before forming; welding; curing coatings and cement; and for selective surface heat treatments.
Compared to steam heating of the same surfaces, resistance heating may offer installation and operational cost efficiencies as well as improved control and reliability.
This article is adapted from materials provided by the Industrial Process Heating Team of Advanced Energy, Raleigh, N.C. Advanced Energy focuses on industrial process technologies, motors and drives testing, and applied building science. For more information about electrotechnologies from Advanced Energy, call (800) 869-8001 or visit www.advancedenergy.org.