Radiant heat has become a preferred process heating method for many applications in the process industries due to the unique properties of the infrared heating process. Infrared heaters create radiant energy to directly heat the product. By contrast, a convection oven depends on air circulation as the heat transfer medium. The direct heating provided by infrared heaters results in more efficient energy usage and can lower operating costs. By eliminating the air medium, radiant heating also reduces the losses associated with hot air ovens. In addition, infrared-friendly products can be heated more rapidly with radiant heat than they would be with convection. Due to shorter product heating cycles using radiant heat, less oven length is required to do the job. This can allow oven designers to reduce oven lengths by 30 to 50 percent compared to convection oven designs.
Infrared heating also provides other benefits. Infrared heaters do not create dirty or contaminating products of combustion. Because infrared does not depend on air as the heat transfer method, the air circulation in an infrared oven is kept at a minimum. Powder coatings and lightweight materials can be heated rapidly due to low air volume and low velocities.
Infrared heat can be controlled and directed easily. Electric infrared heaters can be zoned to provide a high heating zone in one area and a lower heating rate in another. They are responsive to control changes and can provide consistent product temperature control to within close tolerances.
For these reasons, it makes sense to consider whether infrared heaters might suit your process applications.
Infrared OverviewRadiation is the process by which energy is transmitted through space. Radiant energy is transferred from the source (emitter) to a receiver (absorber) in the form of electromagnetic waves. Heat is the result of absorption of this radiant energy by the receiver.
Radiation differs from convective and conductive heat transfer because it does not require the presence of a medium (solid, liquid or gas) to transmit energy from the source to its final destination. By eliminating the heat transfer medium, radiant heating also avoids the losses associated with other methods, allowing radiant heating to provide maximum efficiency.
The electromagnetic spectrum spans the range of wavelengths of radiant energy. The infrared portion of the spectrum (0.72 to 1,000 micron) includes those wavelengths that will produce heat upon being absorbed by an object (figure 1). The radiant energy, or wavelength, of an infrared element depends on its temperature: the higher the temperature, the shorter the peak wavelength. Infrared wavelengths are longer than visible light but shorter than microwaves. The energy output of a radiant source depends upon the absolute temperature of the source, raised to the fourth power. As source temperature increases, heating intensity becomes greater. The useful wavelengths for industrial applications are from 1 to 10 micron.
Infrared radiation is similar to visible light:
- It travels through space at the speed of light.
- It moves in a straight line.
- It can be focused by optical reflectors.
- It will travel through a vacuum.
- It is absorbed, transmitted or reflected by objects or materials.
Pros and ConsConvection and radiation are capable of transferring energy from a source to the work material without contact. They are naturally considered together when contact-free heating must be performed (figure 2).
Forced convection of heated air directed at the work assists in breaking up the boundary film that occurs at the products’ surface. However, the heating method has the disadvantages of requiring an enclosure and air-handling equipment. If not recirculated, the spent heating medium will be discharged -- with consequent heat loss. The desire for faster heating often means that high airflow velocities are used, which can lead to oven heat losses, damage to delicate surfaces, or contamination of the work by airborne dust.
One factor promoting the efficiency of radiant heating is that radiation falling on an opaque surface is immediately absorbed and transformed into heat. The surface -- and, by thermal conduction, the internal body -- frequently is heated above the surrounding ambient temperature. Where exhaust ventilation must be provided to remove volatiles, noxious fumes or moisture, lower ambient temperatures reduce the amount of heat carried away by the exhaust air and the necessity for extensive oven insulation.
When infrared is the method deemed most suitable for an application, factors such as heater size, response time, efficiency and even pricing must be considered. There are several types of elements to choose from as well, including sheath, quartz tube, quartz lamp, ceramic, quartz panel and ceramic composite panel types. Each has its own unique advantages, depending on the application. An experienced supplier of process infrared heaters can help select the elements, fixtures and controls that are most suitable for a given application.
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