Pulp and Paper Canada

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There’s New Life in the Old Dryer Section

Older dryer sections that have been stretched beyond their original design specifications can become productivity and quality bottlenecks, aggravating runnability problems, limiting machine speeds and...


February 1, 2005
By Pulp & Paper Canada

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Older dryer sections that have been stretched beyond their original design specifications can become productivity and quality bottlenecks, aggravating runnability problems, limiting machine speeds and adversely affecting MD and CD paper quality. It’s an important issue for some of Canada’s newsprint machines where wet end improvements and uni-run felt conversions have allowed speed increases. Yet the dryer section’s ability to handle changed heat transfer dynamics and condensate removal requirements may have taken a back seat to other investments over the years.

With the assistance of Johnson Canada, Tembec’s Spruce Falls operations in Kapuskasing, ON, took a hard look at the performance of the 1967-vintage dryer section on its flagship PM5 newsprint machine. Their drying improvement programs would then zero-in on improving product quality and enhancing productivity. This was to be accomplished by improving condensate removal from the dryers, increasing heat transfer rates and achieving better MD and CD moisture control. Once improved dryer section stability was achieved, it would play an important role in increasing machine speed.

In early April 2004, new stationary syphons and Turbulator bars from Johnson were installed in 36 of the 47 heated dryer cans. After the new drying dynamics were confirmed, a more responsive and stable drying control strategy was implemented. The dryer section changes were completed in a very aggressive time frame; the turnkey installation was accomplished within 72 hours sheet to sheet.

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The results of the dryer section improvements have been impressive, both in the mill and in their customers’ pressrooms. Terry Skiffington, vice president and general manager reported, “We’ve had a significant reduction in draws, better dryer section runnability and a step change in runnability in our customers’ pressrooms.” Machine speeds are also up and poised to increase even more after the next phase of the dryer improvement program is completed in November. That rebuild includes Turbulator bars and stationary syphons in the remaining 11 dryer cans and a new low-pressure steam supply system.

Quality, the driving force

PM5’s history of improvements has been typical. The original 1967 Beloit machine, designed for 2800 fpm, was upgraded over the years with a Black Clawson top wire former and a Beloit Twinver press plus third press. The 8-cylinder first dryer section was converted to a uni-run section. But the original rotary syphons, which were state-of-the-art at the time, had not been changed. The mill recognized that with speeds at 3310 fpm, uneven condensate removal, uneven CD dryer temperature profiles, sluggish moisture control and slow recovery times after a break were major problems. These interrelated problems had a negative impact on quality and runnability.

Skiffington explained that product quality was the main driving force behind the investment in the dryer section. He said they developed a strategic plan in cooperation with their customers to improve pressroom performance of PM5’s paper. The improved quality would give Tembec the “leverage” needed to maintain a good standing with its customer base.

Unpredictable profiles, slow response

In the paper mill, there was ample evidence that improving the dryer section was key to better quality. Robert Beaulac, Superintendent of PM5, described the situation before: “We had uneven and unpredictable CD moisture profiles. After a break, the profiles would vary. We also knew that after a break the dryers were partially flooded. The sight glasses in the condensate lines ran white with condensate.”

After a break, the dryers were slow to respond to the gauging system’s moisture control setpoints. Chris Jones, process engineer, reported, “Prior to the installation of the stationary syphons, PM5 required at least 30 minutes and sometimes longer to return to the moisture target.” This caused significant off-spec production.

The mill also felt that MD stretch and strength properties could be improved by better, gentler control over the drying in the early sections. With the old rotary syphons, a higher than desirable differential pressure was required to overcome the centrifugal force created by the rotating dryer to ensure adequate condensate removal. After the short uni-run section, the second dryer section was operated at a minimum of 8.5 psi.

“We were hitting the sheet way too hard in the second section,” said Beaulac. This is where the mill felt it was losing MD stretch and strength properties. With the excessive shrinkage and the tendency of the sheet to stick to the hot dryer surface, the draws had to be maintained at a high level to avoid problematic wrinkling and dryer cuts.

The solution proposed by Johnson would solve several problems: the CD profile instability, the sluggish recovery to the moisture target and off-spec production after a break, and the excessive draws which reduced MD strength and stretch.

Quality up several notches

The Turbulator bars, installed inside each dryer can, create turbulence in the condensate boundary layer at rimming speeds above 1000 fpm. When the condensate layer is turbulent, the heat transfer to the sheet is higher and more uniform across the machine. With the Turbulator bars, the moisture profile on PM5 has been significantly improved. “Right out of the gate, the CD temperature profiles were constant,” says Beaulac. Jones reports the CD 2-sigma moisture profile numbers were reduced by about 50%.

The new stationary syphons do not have to overcome the centrifugal force encountered by the rotary syphons. As a result, a much lower differential pressure is required to evacuate condensate during normal operation and after a web break. For instance, differential pressure in the second section is now as low as 1.5 psi, compared to 10 psi with the rotary syphons.

Beaulac said the second section can be run at a much lower pressure — zero, instead of 8.5 psi — thereby drying the sheet more gently, avoiding excess shrinkage and allowing draws to be reduced. Beaulac reported that culls (dryer cuts) due to wrinkles “are a thing of the past.”

The reduced draws have had a major impact on MD stretch and strength properties. Immediately after the dryer section improvements, the mill saw a 10% increase in MD tensile and a 30% increase in MD stretch on the 45 g/m2 newsprint grade. (The mill also produces 48.8 g/m2 newsprint) A new daily record for MD tensile for the 48.8 g/m2 grade was set in early May, as shown in Figure 1.

Improvement in both MD tensile and stretch means better pressroom performance. In a pressroom runnability study completed in 2000, Paprican researchers concluded that the product of MD tensile and the square root of MD stretch correlated with the frequency of pressroom breaks. In this respect, Tembec’s PM5 has moved up several notches on the competitive scale.

A world of difference in control

A dryer section is like a massive control actuator with a high thermal inertia. If the condensate removal and thermal efficiency is not adequate, it can be very slow to respond to upsets. On PM5, the new syphons and Turbulator bars have improved the speed of response of the previously sluggish dryer section. “We now have more control [of the dryer section] and drying efficiency has improved dramatically. The dryers respond much more quickly, both in the upward and downward directions. To control moisture, a 1 psi change in dryer pressure is the same as a 5 psi change in the past,” said Beaulac. These numbers were confirmed by control bump tests.

The control of moisture was accomplished previously by adjusting the pressure setpoints to the 4th and 5th sections, 11 dryers after the former breaker stack. Because of the increased drying efficiency these dryers are not used. The quality control system now provides a setpoint to the third section, which has 18 dryer cans. “It has made a world of difference in the control,” say
s Beaulac. Jones’ numbers confirm MD moisture control 2-sigma variation numbers are 44% better. Moreover, the average time required to regain the target moisture after a web break is down by an impressive 90%. The moisture is at target within two minutes and is stable within five minutes.

Skiffington added, “The improvement in control response in the dryer section has given us the ability to tune out variations coming from the wet end that in the past would have ended up in the sheet and eventually showed up on our customers’ presses.”

As of July 2004, PM5’s speed is 3375 fpm, up from 3310 fpm before the dryer section project. The mill now plans to increase the speed to 3500 fpm and sections 4 and 5 will be re-activated. In November 2004 the remaining dryers will be equipped with stationary syphons and Turbulator bars. The steam supply system will be converted to a lower pressure 35 psi header. “Then we will see the full benefits, [from the energy saving potential],” said Skiffington.

He explained that the PM5 dryer section improvements are part of a mill-wide program to reduce peak energy consumption. While the lower differentials, lower blow-through rates and higher thermal efficiency on PM5 point to lower energy consumption, it has been difficult to measure with present steam flow instrumentation. As part of the continuing energy conservation program Tembec will install 30 new steam flow measurements to follow steam consumption more closely and pinpoint specific problems.

The gain in sheet strength properties certainly added to the energy reduction program, since it gives the mill some leverage or leeway to reduce energy in the TMP mill. “It helped to support our energy reduction program in the TMP mill,” said Skiffington.

Of course, the major benefit is in Tembec’s good relationships with its customers. “We did what we told them we would do by improving PM5,” said Skiffington. “And we did it on time and on budget,” he added.

Mark Williamson is a freelance writer from Thornhill, ON, with many years of experience in the pulp and paper industry.

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Table 1: Results of dryer section improvement program
BenefitResults
MD Tensile Test (45g/m2)Increased 10%
MD Stretch Test (45g/m2)Increased 30%
CD Moisture VariationReduced 50%
MD Moisture VariationReduced 44%
Sheet Break Recovery TimeReduced 90%
Runnability20% less breaks in dryer section
Machine SpeedIncreased 2%