Not all annealing lines have the same accuracy requirements. Some alloys require tighter temperature control than others. Plants that run high-strength steel or dual/complex-phase steel require precise control of steel temperature. Plants that run low-alloy steels for non-transportation and non-appliance applications often can produce a high-quality product with looser control of the steel temperature; however, these plants also benefit from the improved speed and energy efficiency associated with precise control of steel temperature. Regardless of alloy, effective temperature control dictates the use of accurate steel infrared temperature sensors. Better temperature control allows plants to perfect their recipes which improves efficiency and lowers operating costs. Moreover, precise temperature control ensures final product quality and repeatability. Infrared pyrometers are the best solution for continuous annealing lines because of their ability to make accurate non-contact temperature measurements. However, not all pyrometers offer the same level of performance and different wavelength technologies further validate this.
Pyrometers are typically installed on a continuous annealing furnace using one of two mounting techniques.
1.) Roller Wedge
The roller wedge measurement technique, also known as the nip measurement technique or as the low-angle multi-reflective measurement technique, relies on the unique geometry associated with the roller nip combined with a low-angle of alignment to artificially enhance the apparent emissivity of the strip and to eliminate the influence of hot background reflections. Roller wedge measurement conditions are valid and the technique is appropriate only when certain conditions are met: (1) the roll and the strip are the same temperature and (2) the pyrometer must be mounted at the sweet spot—an appropriate shallow angle to produce a significant number of reflections.(*)
2.) Direct View
The direct view measurement technique is used when the roller wedge measurement technique is not available or is inappropriate. In this case, the pyrometer views the strip directly. For measurement locations where the furnace wall is hot (heating and soaking zones), a cooled viewing tube extends into the furnace to shield the point of measurement from hot wall reflections.
In the spirit of the 2016 Summer Olympic Games bronze, silver and gold medals can be awarded to a range of infrared technologies following our good, better best approach to monitor and substantially improve temperature control on the annealing line.
The bronze medal approach would be using a single-wavelength pyrometer with the roller wedge technique. A single-wavelength pyrometer will provide an accurate measure of temperature as long as the wedge conditions are appropriate (*). The single-wavelength pyrometer assumes that the strip and the roll are the same temperature and that the pyrometer is correctly aligned to the sweet spot from an appropriate mounting angle. This is the preferred measurement technique for plants that do not have frequent temperature set point changes, and do not require real-time feedback or confirmation of a clean window and alignment to the sweet spot. This technology receives a bronze medal because it does not provide a real-time measurement of emissivity and relies on the assumption that the viewing conditions will always be valid. A single-wavelength pyrometer at the roller wedge is a good approach but there are two other technologies which can provide much more accurate, reliable and repeatable measurements.
Dual-wavelength pyrometers provide a measure of both temperature and emissivity. This technology would be a better or silver medal approach because of this additional capability. By monitoring the measured emissivity value, a dual-wavelength pyrometer may be used to confirm proper alignment and identify conditions when the roller wedge reading is inaccurate, assuring that the reading can be trusted as true. The dual wavelength pyrometer technology is more forgiving of misalignment to the sweet spot—in fact, it self-aligns to the sweet spot, better tolerates optical obstructions, and self-corrects for the true emissivity of the roller wedge. This is a more fool-proof technology and it provides a built-in assurance policy that things are less likely to go wrong, and when they do go wrong there is an immediate notification of that fact. This technology is a much better option than the single-wavelength alternative but alas this method would not receive the highest score from a panel of temperature experts/judges because it is only applicable when the necessary roller wedge conditions are valid (*).
The best method or a gold medal approach would be using a multi-wavelength pyrometer with a direct view of the strip. The multi-wavelength possesses all the qualities of the dual-wavelength but can be used to measure both the emissivity and actual temperature of the strip directly as opposed to viewing the wedge. This is possible because multi-wavelength technology automatically corrects for non-greybody emissivity variation, allowing it to accurately measure all steel alloys including high strength steels. Further, this is the best technology because a multi-wavelength pyrometer can report true strip temperature even during times of transition; whereas a dual or single-wavelength with the roller wedge measurement technique cannot because the strip changes temperature quicker than the roll.
Plants that have frequent process changes benefit from a direct view measurement with a multi-wavelength even if the roller wedge measurement technique is otherwise appropriate and valid (*). Plants with frequent process changes report more consistent product properties and higher line speeds when controlling off the direct view multi-wavelength pyrometer reading as opposed to the roller wedge pyrometer reading. In addition, multi-wavelength technology is 20 times less sensitive to warm background reflections compared to single-wavelength and two-color technologies, and, therefore, the influence of a warm background change has twenty times less impact on the multi-wavelength compared to these other technologies.
Temperature control is paramount in order to produce the desired customer-specific mechanical structure in the steel annealing process. A multi-wavelength pyrometer is the best tool for monitoring and controlling the strip temperature because it can make a direct view reading of both temperature and emissivity. This facilitates precise temperature control and superior line optimization in comparison to single and dual-wavelength pyrometers. These attributes are what makes the multi-wavelength the clear front-runner and undisputed gold medal winner for continuous annealing furnace applications—no photo finish needed here.