Valid Measurement Conditions for Roller Wedge and Direct View Measurements:
All infrared technologies produce accurate measures of temperature under ideal conditions. In the continuous annealing furnace, there are two issues that complicate the temperature measurement process. The emissivity of the steel strip varies and hot furnace walls create a reflected interference source. There are two popular measurement techniques applied to the continuous annealing furnace that effectively address these two important application issues. It is important for any user of infrared thermometers for the continuous annealing process to understand the capabilities and limitations of both of these measurement techniques.
When properly applied, the roller wedge measurement technique eliminates emissivity variation and background reflection errors through the thoughtful installation geometry. In this case, the pyrometer views a reflected image of the roll rather than a reflected image of the hot furnace wall, and a multi-reflective technique increases the apparent emissivity of the strip to near-blackbody conditions. This installation technique produces exceptionally accurate temperature values when properly applied and when measurement conditions are valid. Errors result when the installation geometry is less than ideal and when there is a temperature difference between the strip and the roll.
The roller wedge technique will provide accurate results when the following conditions are met:
- Mounted at an appropriate viewing angle typically 10 degrees.
- The pyrometer is correctly aligned to the sweet spot where the roller wedge geometry enhances the apparent emissivity to near blackbody conditions. Emissivity needs to be above 0.9 for accurate use. DW sensor can provide real-time emissivity feedback to confirm.
- Assuming that the pyrometer is viewing the centerline of the strip from the side of the process, the strip needs to be sufficiently wide to permit multiple reflections between the roll and the strip.
- The roll and the strip need to be the same temperature.
Because the strip has a low thermal mass and the roll has a high thermal mass, even when the installation geometry is valid and the pyrometer is properly aligned, significant temperature errors exist when using the low-angle roller wedge measurement technique during startup and during set point temperature changes.
The most appropriate process position to apply the roller wedge measurement technique is in the soaking zone. In the soaking zone, the strip temperature and the roller temperature are nearly identical during steady state operating conditions. For plants that do not change temperature set points frequently, the roller wedge measurement technique can and does produce exceptionally accurate and repeatable temperature readings.
When properly applied, the direct view measurement technique eliminates errors associated with emissivity variation through the use of a multi-wavelength algorithm, and hot background reflections are eliminated through the use of cooled viewing tubes, although cooled viewing tubes are not required in zones where the furnace wall is cooler than the strip. As we will see, the direct view measurement technique can produce accurate measurement results during process conditions where the roller wedge measurement technique is inappropriate.
The direct view measurement will produce accurate results when the following conditions are met:
- The IR Pyrometer correctly characterizes the emissive properties of the strip.
- The hot wall reflections are eliminated. Multi-wavelength pyrometers are used to characterize the emissive properties of the strip and, when the background is hotter than the strip, a cooled viewing tube is used to eliminate background reflections.
No cooled viewing tube is required when the background temperature is cooler than the temperature of the strip. When these two conditions are met, then the direct view measurement technique produces a robust and accurate measure of strip temperature. The direct view measurement technique is most appropriate for plants that run a wide range of products (varying emissivity values) and that make frequent process temperature set point changes. For these plants, experience has proven that significant process efficiencies are gained by controlling to the true strip temperature during startup and set point changes and in zones where the wedge measurement technique is inaccurate.
The multi-wavelength direct view measurement technique is commonly applied in the soaking zone, the cooling zone, the bridle roll area, and the snout. The multi-wavelength pyrometer may be applied to the heating zone, but a cooled sight tube is essential in that area. In the soaking zone, the temperature reading is highly repeatable (if slightly offset from the absolute temperature) without a cooled viewing tube, but a cooling tube is required in the soaking zone in order to produce an absolutely accurate measure of both temperature and emissivity. In the cooling zone and the snout, no cooling tube is required. Plants that have changed to a multi-wavelength direct view pyrometer have experienced significant quality and productivity gains as a result of the more accurate temperature values.
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