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Any papermaker will tell you that the control of cross-direction (CD) moisture streaks is one of the keys to good papermaking. Overall profile shape is important, but small streaks contribute to built-in sheet tensions that affect sheet stability and runnability in a paper mill's sheet finishing and winding operations. Those built-in tensions then ultimately affect performance on customers' presses. Also, streaks have an adverse influence on sheet quality, particularly gloss, on online or offline finishing equipment.
Blandin Paper of Grand Rapids, Minn., is now effectively controlling those small moisture streaks on its No. 6 production line off-machine coater (OMC) with new IRT Webmate high-resolution electric in-frared (IR) profilers. The No. 6 line produces lightweight coated papers with basis weights from 30 to 38 lb/3,300 ft2. The No. 6 OMC is a 307-in. trim Valmet short-dwell coater that was installed in 1989.
The new-generation profilers were part of a drying equipment upgrade project that eliminated a production bottleneck on the OMC and allowed it to set a new North American speed record of 5,371 fpm, averaged over a 24-hour period. The drying upgrade project, completed in February 2000, included a replacement non-profiling, gas-fired IR dryer supplied by Solaronics and mechanical upgrades to improve OMC efficiency.
TOP PRIORITY: PROFILE CORRECTION. According to Tom Newman, senior project engineer, profile correction capability was the most important selection criteria for the moisture profilers: "We evaluated the profile correcting capability of these systems by comparing the width of the zones and the intensity of the power applied within those zones. The parabolic reflector design of the IRT equipment concentrates 86% of its energy within the target profiling zone." The profile zone width is 2.8 in. Newman says that lamp life, control system technology, and energy efficiency were also evaluated in the selection process.
George Shields, OMC No. 6 production specialist, describes the projected advantages from the papermaker's point of view: "We expected to reduce burst breaks and associated waste in our finishing and winding operations and also to reduce waste in our customers' press rooms. Burst breaks on the supercalenders and winders are caused by built-in tensions from uneven moisture profiles, among other things. Wet spots also cause problems with caliper uniformity."
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On Blandin's OMC No. 6, a double bank of profilers is installed after the second coating station, improving parent reel structure and gloss profiles on the supercalenders.
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MORE CONCENTRATED ENERGY. Since the mid-1980s, the cross-direction resolution of online quality measurement systems and profiling actuators systems has im-proved considerably. The 6-in. "slice zone" resolution of 15 years ago is not sufficient to effectively control many CD streaks. Many profiling systems like slice position controllers, headbox dilution controllers, water sprays, and caliper profilers have been improved so that a 3-in. profile zone or less is considered state of the art.
The resolution of the new generation of infrared profilers is now better than 3 in. However, infrared drying profile control has much different dynamic range requirements compared with other CD actuators. The responses of other CD actuator controls are more or less linear—whether it is bending a metal slice, diluting stock flow, applying water to the sheet surface, or causing metal rolls to change their diameter. However, drying is a very non-linear process, especially in the so-called "falling rate zone" where moisture profile correction is done. As the sheet is dried, more and more energy intensity is required. For more precise profile control the control zones need to be smaller, of course. But, the energy required to correct moisture streaks needs to be highly concentrated within the target zone.
The high-resolution profile correcting capability of the Webmate profilers is made possible by focusing the drying energy on the sheet using special parabolic reflectors that surround the infrared lamps. Previous generations of infrared profilers used flat reflectors that generated a wider, less intense radiation profile.
Independent research institute studies show that the focused parabolic reflectors provided a much narrower, more concentrated profile of infrared drying power as compared to flat reflectors. Eighty-six percent of the applied energy falls within the 72-mm target zone. Minimal control zone overlap and better profile control can therefore be expected. The studies also showed that the profile correcting capability of the focused parabolic module was about 9% moisture at a full power of 8 kW per module compared to less than 7% moisture using the previous generation system at a full power of 12 kW per module—50% more power than the parabolic modules.
TWO-SIDED DRYING CONTROL. The profilers installed at Blandin Paper use two different sources of infrared radiation for each side of the sheet to achieve the desired profile correction capability as efficiently as possible (Figure 1).
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Figure 1. The infrared profilers on OMC No. 6 combine two types of infrared heating—high resolution 2.8-in. profilers on the uncoated side of the sheet and active "coradiators" with 10-in. profiling resolution on the coated side of the sheet.
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Short‚wave IR profilers on the uncoated side of the sheet provide the high-resolution profiling capability. The short-wave radiation is readily absorbed by the base sheet, so it heats up quickly and provides the driving force for drying the small coating streaks.
When the profilers are also used to provide the main source of energy for drying the coating, the "bottom up" type of moisture transfer reduces the tendency for water penetration in the base sheet and keeps coating binders near the coating surface. Also, with this type of evaporation mechanism, drying is not impeded by a blocking layer on the coating's surface.
The short-wave radiation that passes through the sheet is absorbed by "coradiators" on the coated side of the sheet. In addition to the heating provided by the transmitted radiation, the coradiators are provided with supplementary heating to maintain a surface temperature that is lower than the high-resolution profiling radiators. Like gas IR dryers, the coradiator emits a medium‚wave radiation. The coradiators have a coarse profiling capability, with 10-in. resolution.
The absorption of the short-wave radiation that passes through the sheet and its re-transmission as medium-wave radiation improves overall drying efficiency by 20%, according to the supplier. The distribution of the drying power in the sheet's Z-direction can be adjusted for the best coating quality and drying efficiency.
TWO PROFILING LOCATIONS. On OMC No. 6, a single bank of profilers is located after the first coating station between the air-foil and the cylinder dryers. An even profile here ensures more even coating pickup at the second coating station. After the second coating station, a double bank of profilers controls the quality of the final product to achieve good parent reel structure and good gloss profiles on the supercalenders.
The profiling system interfaces via serial links to an existing ABB 1180 quality measurement system and to a Fisher Provox distributed control system (DCS). The supplier offers interfaces to control systems using Siemens or Allen Bradley programmable logic controllers (PLCs) or through a direct Profibus DP interface. The internal communications within the profiling system are based on the Profibus standard.
The new system uses a duty cycle power control rather than the previous phase angle control. This design ensures that there are no disturbances created on the mill's electrical network that might affect computer equipment. Newman reports that the system's power factor is one, meaning there are no penalties to pay for excess power draw, and transformers can be sized for lower power use.
The energy efficiency of the system has been complemented by employing the hot air used for lamp cooling as building makeup air. The air is cleaned by filters before being used to carry heat away from the lamps, so no heat exchange is required.
The profilers are equipped with a fire suppression system that fills the surrounding air with a fine water spray after a break, displacing air, moisturizing the surroundings, and absorbing the burner's energy to extinguish any flames.
SPEED RECORD. With the improved profile control, many CD 2-sigma variations have been less than 0.2%, with some results as low as 0.12%. These results have been achieved even though there is no online, automatic control for the basis weight profile of the base sheet.
Since the drying improvements, including the gas infrared dryers and the profilers, the mill has set the 5,371 fpm speed record on OMC No. 6. "We are no longer dryer limited and the profilers definitely helped us to increase coater speeds by reducing the moisture streaks," describes Shields. "Moisture streaks are reduced, so less drying has to be done by the air foils." In addition, Newman notes that "the profilers also give us some extra, emergency drying capacity if the air foils or the gas infrared dryers have problems."
Shields also reports that the improved profiles have had "positive effects" in the pressroom.
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