SPECIAL FOCUS: Infrared Heating
Infrared HEATS UP
June 14, 2006
Infrared (IR) technology is currently used in a wide range of industrial applications such as:
- The electronics industry for drying solder resists
- Food processing for browning and sterilization
- Coating/finishing for drying and curing
- Textiles for drying and sealing
- Plastics for softening
- Printing for drying
- Manufacturing/engineering for degreasing, preheating, and shrink fitting
Infrared energy causes the surface electrons of an object to excite and oscillate, creating heat. Several factors determine the temperature, not the least of which being the wavelength of the electromagnetic radiation generated by the infrared heating source. The equipment used for the above applications utilizes medium and long wavelength IR and is typically capable of less than 800°F (427°C). Since the wavelength is inversely proportional to the temperature, the wavelength shortens as the temperature goes up .
The high temperature infrared processing discussed here primarily utilizes short wavelength T-3 tungsten-halogen lamp-based systems with unique designs to enable the systems to operate at higher temperatures in industrial environments. Infrared heating has many advantages over other heating techniques. It provides:
- An inherently clean non-contact heating method
- Rapid response energy fluxes capable of heating rates in excess of 50°C/s (material and mass dependant)
- Excellent spatial and tempered control allowing sample-only heating uni-directionally over large areas
- Rapid power level changes due to low thermal mass (halogen lamps)
- Rapid cooling rates due to the "cold wall" nature of IR
- Controllable temperature gradient processing up to flux densities of 50 W/cm2
Thus, infrared provides a versatile and flexible answer to heat transfer problems throughout the industrial spectrum .
High Temperature InfraredOak Ridge National Laboratory (ORNL) has been investigating infrared-based processing for high temperature applications [above 800 °F (427 °C)] for more than a decade. During this time, many unique applications have been developed, and several of these will be highlighted. Most of the experimental research at ORNL was conducted in two in-house fabricated infrared systems. These use tungsten halogen-based heating elements, water-cooled bodies, ceramic reflectors, and fuzzy logic controls. Figure 1 shows a 33 kW four-inch diameter cylindrical vacuum and an 88 kW - 24" °- 24" flat bed system at ORNL.
These systems are utilized in various programs to develop the "Process Science" of targeted applications. Infrared, like other heating technologies, has its niche and each potential application has to be investigated to assess the feasibility. Typically, systems need to be designed for the given application based on the data produced. Several applications, which have seen some success, are discussed and shown below. These applications/designs utilize a hybrid rapid infrared (RI) furnace with a two-zone heating approach. Zone one uses the high power densities available by the T-3 lamps to bring the load/parts near the targeted process temperature, and zone two acts more like a convection furnace to bring the load uniformly to the targeted temperature. The term "hybrid" is derived from this unique combination of heating modes .
Preheating for the Forging IndustryInsert Die Heater
The forging community in the United States presently either does not preheat their forging dies or preheats them inadequately due to the 3-4 hour length of time required. If preheating of the dies is not performed or is performed improperly, premature failure occurs due to thermal shock and improper deposition of die lubricant. A set of these dies typically costs tens of thousands of dollars and die rebuilds, thousands of dollars.
To combat these problems, an infrared insert heater was designed to simultaneously heat the upper and lower dies in a hammer forge. This system can simply plug into a standard welding outlet and be ready to preheat in seconds. Initial testing of this heater in an ORNL forge revealed that it could preheat 10-inch platens to 400°F in approximately 10-15 minutes (Fig. 2). The system converts electrical into radiant energy with efficiencies in excess of 90% and goes from cold to full power in less than one second. The technology has been successfully implemented in the preheating of die castings as well.
Basic research at ORNL has shown that through rapid preheating of aluminum billets prior to forging, fatigue properties can be greatly enhanced. Typical preheating times can be as long as four hours in traditional shop setups, while utilizing a hybrid infrared approach can reduce this time to less than 20 minutes. The hybrid infrared system used for preheating aluminum billets prior to forging is shown on the title page of this article (p. 42). The benefit of the shorter heating time is a finer grain size on 2019 Al (Fig. 3). It is this grain refinement that results in the improved fatigue properties .
Other Promising Applications
- Rapid preheating of aluminum and inter-metallic sheets prior to hot rolling for reduced process cycle times. Figure 4 shows an infrared system performing in-line preheating of aluminum metal matrix composites.
- In-line stress annealing of steel springs prior to coating
- Preferential tempering in steel tooling
- Presetting of shape memory alloys for medical applications
Figure 5 shows an industrial scale infrared/convection furnace for flattening large aluminum composites plates. The T-3 elements are utilized to rapidly bring the load to temperature, and convection is used to equalize the temperature of the load.
ConclusionsMany of the infrared systems at ORNL's Infrared Processing Center utilize tungsten-halogen lamps which contain filaments that glow at approximately 2900°C, resulting in a theoretical power density of approximately 500 W/cm2. Design considerations typically limit the power densities to approximately 40 W/cm2. This technology has been shown to be an environmentally clean heating alternative for the preferential tempering of steel tooling, flash annealing, joining, preheating, stress relieving, and other unique bio-material applications.
While the mainstay of infrared process heating still remains below 800°F, new niche applications have been identified and implemented at temperatures above and beyond.
References1. N. C. Cox and D. E. McGee, "Use of High Density IR for the Rapid Heating of Metals," Industrial Heating, 4, (1989) 46-48.
2. M. Sirotnak, "5 Misconceptions About Infrared Heating," Process Heating, 1, (2003) 25-27.
3. H. Bischof, "The Answer is Electrical Infrared," J. Microwave Power and Electron. Energy, 25(1) (1990) 47-52.
4. P. Kadolkar, H. Lu, C. Blue, T. Ando, R. Mayer, "Application of Rapid Infrared Heating to Aluminum Forgings," paper presented at 25th Forging Industry Technical Conference, 2004.
The author would like to thank Craig Blue, of ORNL's Materials Science and Technology Division, for his contributions.
For more information: The Infrared Division of IHEA (IRED) recently published a handbook that explains the benefits of infrared for industrial applications.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: infrared heating, T-3 halogen lamp, hybrid rapid infrared, short wavelength infrared