Infrared ovens, including gas- and electric-operated units, are utilized in a range of industrial production processes such as curing of coatings, heating of plastic prior to forming and processing of glass. In these applications, an infrared (IR) system can be regulated instantly to changing conditions as part of a closed-loop control system (figure 1).
Within an infrared heating system, infrared heaters, or emitters, are set at a higher temperature than a manufactured part will attain. The heaters transfer energy to the part through electromagnetic radiation. Molecules within the product are excited by the infrared radiation and vibrate. Consequently, energy levels are raised, resulting in increases in temperature, changes in state (liquid to gas), and/or polymerization and curing.
In a typical production application, final part temperature is determined by the dwell time of the part in the infrared oven. Radiant energy transfer is increased as the difference between the heater and part temperatures becomes greater. No contact or medium between the two bodies is needed for this process to occur.
For practical purposes, gas infrared heaters provide energy via combustion or a catalytic reaction, through a metal and/or ceramic medium, or an electrically heated filament or coil as the emitting body.
If an electrically operated infrared heater is used, a heat-resistant quartz glass tube or flat panel quartz window may protect the filament or coil. Exposed electric element heaters can provide rapid heater warmup and cooldown. Quartz tubular infrared heaters sometimes use a gold coating inside the tube or an internal ceramic fiber reflector to reflect the infrared radiation and direct it towards the product to be heated. Consequently, the infrared radiation impinging on the product is increased over a heater without a reflector. Flat panel heaters will perform in a similar manner.
When to Use an Infrared OvenInfrared ovens provide an efficient heating solution for many manufacturing operations due to their ability to control the penetration of infrared energy into a product. Unlike convection ovens, which are controlled by air temperature and airspeed only, infrared emitters offer the flexibility of using different energy densities and wavelengths to achieve the proper heatup rate and temperature.
In a convection oven, the product spends a significant portion of the total dwell time just reaching the process temperature. Of course, because a product in a convection oven can never get hotter than its environment, there is no danger of overheating the product as it dwells in the hot air.
By contrast, infrared will raise the product temperature much more rapidly than the convection oven due to its higher energy transfer rate. For instance long wavelength infrared emits energy between 4.0 and 6.0 microns, with energy densities of 5 to 15 W/in2. Medium wavelength infrared emits energy between 2.4 and 4.0 microns with energy densities of 15 to 60 W/in2. Short wavelength infrared emits energy between 1.0 and 1.2 microns with energy densities between 100 and 200 W/in2. When high energy densities are possible, it is possible to heat the product much more quickly, but it also is possible to overheat the product.
When using infrared to heat a coating applied on a continuous coil, the coating can be cured without fully heating through the thickness of the metal. Infrared heaters allow for discrete zone control and are suited for precise temperature profiles such as moisture control across a continuous web of paper or textile.
Another advantage of infrared technology is the rapid startup time to reach process conditions. The oven can cause the product to reach the process temperature desired in minutes or sometimes even seconds.
Process Control RequirementsIn industrial plants, temperature plays an important role as an indicator of the condition of a process, product, or piece of equipment. Precise temperature monitoring improves product quality and increases throughput. It also minimizes downtime because production processes can proceed uninterrupted under optimal conditions.
Precise temperature monitoring and control are critical in applications such as furnaces, bulk glass, melters, regenerators, refiners, molds, float lines and annealing lehrs, as well as at cooling and coating areas (figure 2). In the glass industry, for example, careful monitoring from the molten state through the cooling process ensures that glass retains its desired properties (e.g., thickness, shape, color, texture, etc.).
Temperature control also is a vital aspect of thermoforming operations. The core temperature of the plastic sheet, its thickness and the temperature of the manufacturing environment all affect how plastic polymer chains flow into a moldable state and reform into a semi-crystalline polymer structure. The final frozen molecular structure determines the physical characteristics of the material as well as the performance of the final product.
Temperature is a key variable when coatings are dried on paper, film and foil. Infrared dryers are used to raise the coating and base material to the desired dry/cure temperature. Precise process control requires closed-loop feedback from web temperature sensors providing operators with detailed temperature profiles.
Most importantly, end users need a temperature measurement solution able to withstand rigorous production environments. This sometimes means putting temperature instruments in insulating jackets or providing air- or water-cooling for the device. The use of an air cushion may even be required to thermally isolate the temperature sensor from harsh process conditions.
Temperature Measurement OptionsManufacturers of infrared ovens, dryers and other process heating systems design their equipment with accurate, reliable temperature control capabilities. During production, oven operators must have a precise reading of the actual product temperature; otherwise, process quality is diminished, and high scrap rates reduce the plant’s profitability.
Traditionally, the choice of a temperature measurement solution has included either contact or noncontact sensors. Contact-type instruments such as thermocouples are impractical in applications where the temperature sensor might touch the product before it is fully cooled, resulting in damage to coatings or the surface of the product itself. Thermocouples also may be too slow to keep up with rapid temperature changes during heat cycles.
Oven manufacturers have found that noncontact infrared thermometers are useful for measuring temperature under circumstances in which thermocouples or other probe-type sensors cannot be utilized (figure 3). Infrared devices enable precise temperature control in process heating ovens used to bake, cure, bond, preheat, thermoform, cook, fuse, shrink, laminate and dry a range of products.
To realize the benefits of infrared thermometers, it is important to understand how they function. All objects emit infrared energy. The hotter an object is, the more active its molecules are, and the more infrared energy it emits. Optics located inside an infrared thermometer collect the infrared energy emitted by an object and focus the energy onto a detector. The detector then converts the energy into an electrical signal, which is amplified and displayed as a temperature reading (figure 4).
Infrared thermometers can be used to monitor temperatures of dynamic processes quickly and efficiently. Unlike other measurement techniques, they measure the temperature of the process directly, allowing users to quickly adjust process parameters to optimize product quality. Infrared instruments also increase production efficiency and improve yields by enabling smaller units of measurement and a greater accumulation of temperature data. Temperature measurements can be made of a large area or a small spot.
The most sophisticated noncontact infrared sensors take temperature measurement a step further, providing multiple extended temperature ranges, laser sighting and high-resolution optics. Simultaneous analog and digital outputs allow temperature data to be integrated into a closed-loop control system and simultaneously output for remote temperature monitoring and analysis.
Modern, miniaturized infrared sensors are being applied diverse applications in thermoforming and drying processes, paper mills, printing, paint booths, food and tobacco processing. The sensors incorporate enhanced measurement resolution and can withstand high ambient temperatures without requiring external cooling. This allows accurate measurement of extremely small spot sizes in confined spaces and difficult ambient conditions.
Combining infrared temperature measurement and data-acquisition software, infrared sensor systems control a setpoint temperature. The temperature of each process target is read and recorded by the sensor as the software produces an accurate production record. Upon reaching the setpoint temperature, the heat source can be shut off automatically, maintaining the correct temperature (figure 5).
Infrared linescanners, which provide a "picture" of surface temperatures across a moving process, are an effective way to measure edge-to-edge temperatures for control of product uniformity. These systems provide multiple data points per scan in a 90° field-of-view. When paired with the latest system software, they enable operators to view two-dimensional real-time and saved thermal images, and correct process irregularities before they become problems.
End User BenefitsInfrared technology is not a new phenomenon. It has been utilized successfully in industrial and research settings for decades, but recent innovations have reduced costs, increased reliability, and resulted in noncontact infrared temperature sensors offering smaller units of measurement. This has made infrared an area of interest for new kinds of users and applications.
Infrared thermometers offer flexibility, ease of use, and improved performance over thermocouples and other contact probe systems in some applications. With specialized sensors for applications ranging from measuring glass coatings to monitoring temperatures in hazardous locations, infrared technology is well equipped to handle surface temperature measurement challenges.
For oven manufacturers, infrared thermometry offers several advantages over other types of temperature measurement. These include:
- Sub-second response.