Mead Paper's mill in Escanaba, Mich., is one of the largest coated paper mills in North America, with capacity to produce 695,000 tpy of coated groundwood and coated free-sheet. Typical of many, if not all of North America's coated paper mills, the facility consists of three older paper machines, with the newest machine installed in the early 1980s.
To address these issues, the mill recently completed projects on to two important components of the E- 4 system: the off-machine coater and two supercalenders. The goal was to improve operating efficiency and lowering production costs while improving quality. The project also set the groundwork for possible future upgrades. The coater and supercalenders are part of an ongoing process than included a new winder installed in early 1999 to replace an old two-drum winder with a single drum unit.
The latest E-4 project included the first North American application of two technologies. The first production scale Valmet OptiCoat Jet coating stations were installed on the off-machine coater. The Nos. 41 and 42 supercalenders were converted to an OptiLoad configuration, the first retro-fit of an existing stack using this Valmet system anywhere. The new equipment was brought online in February and March 2000, and the results were seen almost immediately. Coater operation improved, and the glossing power of the new stacks allowed adjustment of the coating formulation for major cost savings.
The Escanaba mill, located in Michigan's upper peninsula, not far from the Wisconsin border, is part of Mead Paper, which includes three coated mills: Escanaba; Rumford, Maine; and Chillicothe, Ohio. Escanaba produces a wide range of grades, including No. 1 to No. 4 quality coated papers in sheet and web form, on three machines. The No. 4 paper machine line-the Spirit of Escanaba-produces coated free-sheet and coated groundwood papers ranging from 45 lb to 60 lb/3,300 ft2. The E-1 line produces heavy weight coated grades, and a third machine, E-3, also produces coated papers.
New coater heads for PM 4¹s (the Spirit of Escanaba) off-machine coater improved the operation of one of the fastest coaters in North America.
COATER PROJECT. The E-4 coater at Escanaba is one of the fastest-possibly the fastest-operating coater in North America. However, the mill identified several key efficiency issues on the coater to be addressed in the project, including, operational cleanliness, edge deposit buildup, and reduction of coating color losses.
The key issue was that the coater had reached the operational limits of applicator roll technology. With the original system, splashing occurred, making cleanliness a major issue for both the operators and in terms of operational efficiency. Deposits would also build up on the edge of the sheet, forming dried flakes of coating that would break off and cause sheet breaks. Another problem with the original coater was coating skip if the coating became too thin.
Applicator roll technology, installed on the coater in 1989, was replaced with two Valmet OptiCoat Jet coating stations, the first such units running at a North American paper mill. Mead personnel, as part of the selection process, went to see a jet system operating at Metsa-Serla's Kirkniemi mill in Finland. The company has experience with jet applicator technology, running a system on one of Rumford's machines.
The jet application system replaced the messy transfer of coating from the applicator roll to the sheet with a well-controlled jet application. The new jet stations were built around the existing backup rolls, and the entire head including the beam was replaced with new equipment that was set in place on the old sole plates. Piping for the coater was also changed, and new Ronningen-Petter coating screens were added as well. The coater heads have a OptiAir de-aeration system supplied by Valmet-Raisio.
Several key features made the OptiJet the mill's choice for the project. "One key feature was an internal filter for screening coating color in the head, which minimizes jet splits that could be caused by particles lodging in the jet nozzles," says Michael J. Nelson, technical service engineer at Escanaba. "The splits cause coating skips and can permanently damage the coater metering blade."
In addition, a built-in laser beam straightness measurement system was an important consideration in equipment selection. This system ensures that sheet cross-direction (CD) profile controls are operating from a "flat" starting point. A bellows system within the head can be charged with air to flex the head to keep it straight.
The system also eliminated edge buildup problems. "At higher speeds, a lot of splashing was occurring, so operational cleanliness was an issue. In addition to improving coater runability, the operating yield of the coater improved, and the new coating stations led to a major reduction in coating losses," according to Paul DeHaan, E-4 coater superintendent.
Other important components in this equipment include an automatic blade wear compensation system and a cooling system for the jet lips that alleviates nozzle buildups of dried coating.
Results from the new coater heads were immediate. The process runs cleaner, wet edges are gone, and the mill has seen a decrease in sheet breaks. Quality has also improved due to factors such as a reduction in streaks from applicator roll ribbing.
One negative factor observed is that jet split losses, where the coating jet splits into a "V" due to debris in the nozzle, were higher recently. However, the mill replaced the original screen in the head of one of the units and this solved the problem, indicating that the old screen might have been damaged during installation.
A carbon steel plate frame wraps around the existing supercalenders frame to attach the new lever arm roll mounting and load relief system.
SUPERCALENDER PROJECT. Improving the operation of the coater was one step, but additional work in the finishing area was also a key part of the overall project, centering the operation of E-4's two supercalenders. The Bruderhaus open face units (as opposed to an A-frame) were originally installed in 1982 and had nine-roll stacks with bottom roll lifting and top roll loading.
The stacks were typical of supercalender design at the time. They had already been modified from the original design to improve operation. For example, in 1990 six polymer rolls were put in service to reduce roll change frequency, and polymer covers were used in the bottom two positions. However, the older configuration had high maintenance costs and produced sheet two-sidedness.
- The supercalender project had several goals including:
- Reduce the use of costly additives (cut plastic pigment use significantly)
- Reduce maintenance costs
- Improve log mechanical condition
- Reduce sheet two-sidedness and produce even coat weight.
Mill staff examined a number of options and chose to rebuild the existing supercalenders using Valmet OptiLoad lever technology. The mill considered replacing the old units with completely new equipment. However, the rebuild method was considerably less expensive. To accomplish these goals, mill staff identified a project scope that included converting to all-polymer rolls, converting to higher water temperature, and adding a reversing nip.
The decision to use all-polymer rolls meant that a number of issues had to be resolved. When using filled rolls, a significant diameter loss occurs as the rolls wear, and thus a spindle system was required on the original supercalenders. These were a major maintenance cost due to the inherent design, with lots of moving parts and pieces. To modernize the stack and reduce these costs, it was important to eliminate this spindle system.
Polymer rolls, however, would require less frequent grinding, and a much smaller amount of diameter loss would occur compared with cotton rolls. Therefore, polymer rolls would require a different means of roll mounting. One alternative considered, and then implemented, was to use lever arms as found in the OptiLoad calender stack. The limitation off these arms is that they can accommodate only narrow diameter ranges in the compliant roll.
The downside to using all-polymer rolls versus a traditional mixture of polymer and cotton rolls is a reduction in gloss. However, trials indicated that losses in gloss could be counteracted by the new lever arm loading concept, an increase in steel roll temperate, and an addition of a tenth roll in the stack.
The rebuilt supercalenders provide operators with increased flexibility and control over calendering, allowing the mill to optimize the coating and calendering processes to reduce the cost of the coating but still maintain gloss. The power to produce gloss and minimize two-sidedness comes from several factors. These include new steel rolls with higher temperatures up to 320 degrees F; a smooth tungsten carbide coating on the steel rolls; and the top loading flexibility offered by the OptiLoad design. The main result was that the mill was able to modify its coating formulation and eliminate the use of costly plastic pigments, which had been added to achieve desired sheet gloss. This cost saving was the biggest, most over-riding reason that justified the project.
The original heated intermediate rolls installed on the supercalenders were displacement body rolls. These typically are thick rolls, and thus the resistance to heat transfer is greater than a thinner shell. An improved heat transfer roll-a tri-pass design-passes the heating fluid through holes in the exterior of the shell. The result of using these new rolls was a net temperature increase at the steel roll surface of 25 degrees to 30 degrees F.
Simply increasing the fluid temperature could further increase gloss as well. The hot water system previously used could heat liquid up to 240 degrees F. Thus, the mill was faced with the question of what heating method and fluid to employ and how hot could the liquid be? A hot oil system was considered, but hot water was chosen since it did not have the drawback of oil (housekeeping concerns). In addition, the higher temperature possible with oil was not needed.
Features such as internal screens in the head and lower coating color volume have led to improved runnability, cleanliness and efficiency at Escanaba.
ROLL MOUNTING SYSTEM. The roll mounting and movement methods as seen in a lever arm system are fairly simple and straightforward. The hydraulic cylinder integral with each roll mounting arm removes all or a portion of the roll weight from the stack. Thus, the resulting stack loading is different from a traditional supercalender, where the load increases as you progress down through the stack. Instead, the lever arms remove the dead weight of the roll out of the loading picture.
The most difficult aspect of the rebuild was determining how to attach the lever arm components to the existing frame. The frame is cast iron, and the ways are part of that casting. Therefore, the new components could not be welded to or easily bolted in place of the way. The solution was to build a carbon steel plate frame around the existing cast iron frame. The existing ways were used as the master for positioning the new frame. The new carbon steel frame pieces were bolted onto existing machined surfaces on the inside of the frames.
"The real challenge was to make everything fit using field measurements," according to Jeff Nelson, senior project engineer. "Of anything that could be an issue, that was it. Nevertheless, it worked out well. Some problems were encountered in the first stack rebuild, but as we learned, we improved. The shutdown duration went from 11 days on the first to only 6 days on the second stack."
To improve two-sidedness that had existed since the startup of the original stack, a tenth roll was added as part of the rebuild. This added a reversing nip. Before this, the wire side saw all steel rolls, and the mill had to bias the coating to compensate. Other aspects of the rebuild included installation of a paper carrying roll to allow for a changed sheet path that now went over the top of the stack. The existing king and queen rolls were reused as part of the rebuild, but with some changes. The queen roll was no longer moveable for stack loading and was fixed. The king roll doctor was mounted to the king roll housing and moves with the roll.
The nip guards were also changed. Instead of having nip guards at each of the roll nip entrance points, moveable face guard assemblies mounted on the elevator platforms were installed. This arrangement served to remove a troublesome component that led to roll damage. It makes the stack safer by keeping people completely away from the stack whenever a nip is present.
One of the biggest concerns with the rebuild was stack threading. The project raised the queen roll, requiring that the sheet be threaded over the top of the stack, making it more cumbersome and time consuming. To address this the threading platform limits were raised and an air-threading pan was installed to carry a sheet tail over the stack. Also, with addition of a tenth roll, an additional fly roll was needed.
Work began in Fall 1999 to prepare for the rebuilds scheduled in February and March 2000. The pre-work done included installation of the roll heating units, nip guards, hydraulic tubing, piping, and a significant amount of wiring. The roll-changing crane was also raised ten inches to accommodate the taller stack.
RESULTS. At startup it was apparent that the gloss-producing power of the stack had been significantly increased. The higher liquid temperatures, coupled with all-new steel rolls with a hard surface finish and the added roll in the stack, led to significant glossing capability. This allowed the coating color to be modified by eliminating the plastic pigment without losing sheet gloss. The performance of the roll heating system and rolls was clearly the driver for this gloss improvement. The new roll body (tri-pass rolls versus displacement body) and the higher temperatures were the keys. The heated roll surfaces have maintained smoothness since startup, and no sheet sticking occurred due to the higher temperatures.
One operating concern had been that the higher surface temperatures on the steel rolls might lead to higher differential temperature at the polymer roll edges. To avoid any problems with this, air-cooling showers were added after the rebuild to reduce the temperature differential.
A goal of the project was to improve log mechanical condition by improving caliper profiles. Sheet caliper profiles exiting the stack did not improve as a result of the rebuild. Despite the use of all-polymer covered rolls and lever arm weight relieving, no improvement was seen.
Throughout the project, the mill had discussed the possibility of pumping temperature-controlled water through the polymer rolls to reduce "thermal crowning." The downside of doing this was that it would require rotary joints on each roll and this would be a problem for every roll change.
Based on past experience, the mill tried partially filling each roll with a water and glycol mixture and simply capping off the roll. The idea was that this fluid would have the effect of flattening out any high temperature buildups. It was tried mainly as a means of protecting the covers. However, the change had the surprising effect of leading to a significant improvement in sheet caliper. Therefore, all the polymer-covered rolls in both stacks were similarly filled. It had been discovered earlier by an operator using an infrared camera that high temperatures in the stack correlated with the caliper problems. The high temperatures correlated with bearings in the fly rolls, which heat up during operation. Apparently, the glycol evened out this temperature gradient. Eventually, the mill would like to replace the old fly rolls with new ones having staggered bearings.
The mill has not seen a large benefit from having the ability to vary nip loading. They are using load relief, but the benefit has been mechanical-i.e., less wear on the bottom rolls. But they haven't seen a sheet benefit of being able to apply additional load from the top down.
One key to the success of the whole project was the close involvement of the operators. They had critical input to the design of the operating system, including the screen interfaces. A full buy-in by the operating staff is an important aspect to any successful project.
With the completion of this project, the E-4 line is now near the top in terms of world class coated paper machines.
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