Evolving Temperature Measurements in the Steel Industry
For furnace measurement applications, a radiation thermometer measures the actual product’s surface and not the furnace atmosphere. Noncontact radiation thermometers outlast contact sensors by many times, and with platinum prices at record highs, infrared thermometers are very price competitive.
For some applications, a single-spot thermometer provides enough information for process control. Figure 1 shows a typical installation of a single-spot radiation thermometer, which measures a stripe along the strip.
In many cases, edge-to-edge temperature variations can cause product problems, which require more than one radiation spot thermometer at a location to give more complete coverage. Prices increase once you start to use multiple sensors across the product, and you will still have unmeasured gaps between each thermometer spot.
Single-point sensors all have an accuracy specification. So, one may be accurate and measuring 0.49% high and another may be similarly accurate and reading 0.49% low. In that example there’s almost a 1% difference between two sensors that both pass the manufacturer’s +/-0.5% accuracy specification.
Since the 1980s, a new breed of radiation thermometer has evolved that is specifically designed to measure temperatures from one edge of a moving product all the way to the other edge. These are commonly known as line scanners, and we’ll call them scanners for the rest of this article.
Scanners commonly take the form of a radiation thermometer sensor with a very fast response speed that is placed in a small enclosure and “looks” at the end of an inclined mirror. That mirror is on the end of a rotating high-stability motor. In this way, the view of the radiation thermometer is scanned through a viewing angle, allowing it to “see” and measure objects within that scanned line. This is very similar to the way a checkout scanner works in your local supermarket.
A rugged protective window is placed on the viewing side of the scanner’s enclosure so that all components are protected from the mill environment (Fig. 2). In a fraction of a second, the scanner can scan through 80 or 90 angular degrees and sample 1,000 temperature points. This is like having 1,000 radiation spot thermometers across your process.
Because a single sensor makes all of those measurements, there are no differential accuracy errors like there would be with separate spot sensors. Therefore, if the scanner indicates one edge is 3 degrees hotter than the other edge, it really is.
Typical scanners operate at speeds of 100 or 150 scans per second and sample 1,000 data points in each scan. This is very useful for fast-moving items like in a hot-strip mill. Each scan produces a temperature profile across the product. These profiles are then stacked together to produce a thermal image of the product that is moving past the scanner. This high-density thermal image of the process is extremely useful in understanding the thermal properties of your process.
Figure 3 shows a typical scanner installation measuring complete temperature profiles across the strip just before the downcoiler. Scanners typically talk to a PC that provides displays of thermal profiles and images. In addition, scanners have optional outputs that can be used for edge-to-edge temperature control.
Another example in a hot-strip mill would be at the entry to the finishing stands. A typical single-spot thermometer will provide a measurement stripe of temperature along the strip. This is OK to indicate the temperature of the centerline of the strip, but it tells you nothing about the temperature variations across the strip’s width. A spot thermometer may show you a centerline temperature of 1550˚F, but it would not sense the edge temperatures. The drive side of the strip may be hotter, and the operator side may be colder. If this is the case, the next mill stand will reduce the strip thickness unevenly and cause a camber in the strip. Measuring the strip temperature profile with a scanner will immediately reveal the uneven temperature profile and enable you to correct for it. Scanners provide many temperature-signal outputs related to user-positioned zones across the product width. These outputs are interfaced to control systems that modulate process actuators like water sprays, edge heaters or edge-masking dampers.
Many mills now have multiple scanners along the length of a process line. In this way, you can track product from the exit of a reheat furnace, past the descaler, into the roughing mill and finishing mill, and then to the coiler. This provides a complete knowledge of the process and any areas in need of improvement. In some cases, problems at the coiler are caused by a temperature imbalance of the slab as it exits the reheat furnace. Single-point thermometers are incapable of revealing such edge-to-edge temperature variances.
Figure 5 shows a thermal image from a scanner positioned just before a coiler. Notice the operator side of the strip is colder than the center and drive side. The temperature profile at the last finishing stand was good, and it was determined that the problem was caused in the final cooling section. Scanner zone temperature outputs across the width of the process provide the necessary signals to allow edge-masking controls in the cooling section to correct this.
Another common measurement position for a scanner is at the end of a continuous annealing line just before the strip enters the snout on its way to being dipped in the zinc pot.
A single-point thermometer can provide a centerline temperature, but it can’t provide cross-profile data. At the same location, a scanner provides detailed edge-to-edge thermal-profile information. Scanner zone outputs are used as inputs to a control system to adjust heaters or dampers necessary to achieve the correct temperature profile. Once a flat temperature profile is achieved, the zinc will naturally coat the whole surface of the strip more uniformly. This results in a quality galvanized strip with even coating thickness, which could then be galvannealed and used in high-quality products like auto-body panels. Sheets with even thickness can be formed with seams that are even along their entire length.
Steel strip can suffer from uneven grain size across a strip, which is caused by uneven cooling. Camber problems can result from a hot or cold edge. Sometimes lower-quality products can be cropped to remove nonconforming heads, tails or edges, but in the worst case, a whole coil may have to be scrapped.
By producing steel with the required characteristics from edge to edge, you produce less scrap and use less energy because of reduced rework. With higher-quality steel, your customer base is larger and you can command better market prices.
Can I afford it?
Scanners typically cost the equivalent of three single-spot radiation thermometers, but they have far better features, capabilities and accuracy. For the price of three single sensors, you get the capabilities of 1,000. Scanners use just one connection cable that carries both power and Ethernet signals, so installation costs are minimal.
In today’s world, markets require higher quality, and companies expect improved profitability. Complete temperature measurement with line scanners provides a total picture of the product’s entire surface temperature. With this information, more precise process control is possible, resulting in improved quality, increased yields and greater profitability. IH
For more information: Contact Richard Gagg, product specialist, AMETEK Process & Analytical Instruments, 150 Freeport Road, Pittsburgh, PA 15238; tel: 412-826-4462; fax: 412-826-4460; e-mail: firstname.lastname@example.org; web: www.ametek-land.com