Dryer section gets a second wind
By Pulp & Paper Canada
The drying section is often a significant drawback for older newsprint and wood-containing specialty machines, sometimes limiting production speeds, causing profile problems and wasting energy. Tembec found this was the case on PM4 at its...
By Pulp & Paper Canada
The drying section is often a significant drawback for older newsprint and wood-containing specialty machines, sometimes limiting production speeds, causing profile problems and wasting energy. Tembec found this was the case on PM4 at its Kapuskasing, Ont., mill, which produces standard newsprint and bulky grades. The dryer section of the 220-inch trim machine dates back to 1946 and has been updated several times. The current steam and condensate system required high maintenance costs to maximize equipment reliability and performance. Furthermore, the mill staff wanted to solve production limitations, improve profiles, make the steam and condensate system easier to control and, at the same time, realize significant energy savings.
Those goals were achieved conclusively with a steam and condensate system rebuild provided by Kadant Canada. The project was concluded in January, 2012. The precise planning and execution of the project by the mill and Kadant team impressed Doug Beemer, paper mill superintendent, who said, “This is one of the best capital projects during my time at this mill. It is the model in terms of safely completing a project on-time and on-budget. Now, our speed is up and efficiency is much improved.”
Not an off-the-shelf solution
As you can imagine, a rebuild on an often-modified dryer section is not an off-the-shelf task, so the mill first enlisted the help of Kadant to do a thorough steam and condensate system audit and make recommendations as to how those limitations could be lifted and energy could be saved. Several years before, Kadant had engineered a successful rebuild of the PM5 newsprint machine steam and condensate system after an initial audit. The PM4 system audit similarly involved a detailed analysis and simulation of the drying system, including drying and condensing rates, heat transfer efficiency, and the steam and condensate handling system. This study comprised nuts and bolts issues like piping design, valves, instrumentation and system control.
The recommendations included:
• Replacing all rotary syphons with Kadant stationary syphons and PTXTM steam joints. The new syphons would allow effective condensate removal at much lower differential pressure and correspondingly lower blow-through steam rates. Dryer flooding would be eliminated.
• Installing Kadant Turbulator® bars on all dryers to generally increase heat transfer through the dryer shells, reduced cross-directional heat transfer variations and thereby improved sheet moisture profiles.
• Reconfiguring the steam and condensate system to eliminate the need for high-pressure steam by converting the existing thermocompressor system, which used 250 psig motive steam, to a three-section cascade system which now uses only 35 psig low-pressure steam.
• New instrumentation and electronic controllers to replace the pneumatic
Payback in less than one year
The project was justified by a return on investment of less than one year as calculated by mill staff. The mill hoped to run PM4 consistently at 2800 fpm or higher on all grades. This would lead to a lower production cost per ton. High bulk grades were previously limited by drying capacity and production speeds varied between 2700 and 2750 fpm.
The elimination of the high-pressure motive steam for the thermocompressor would free up extra steam energy for electricity production in the steam turbine, thereby replacing some external power purchases. Energy would also be saved by using less condensate for the moisturizing profiler as the moisture profiles improved.
The maintenance cost for the existing syphons was also a major factor since the wear and tear of many years caused numerous failures which lowered speeds, affected machine efficiency and resulted in maintenance costs of more than $100,000 per year. The experience with new syphons on PM5 has been positive since minimal maintenance has been required.
On an older machine, details are important
The project preparation work, such as piping tie-in points, was done during several scheduled machine shutdowns in 2011 and the final installation took place during a three-day maintenance shut in January 2012. Innovative and customized engineering and a high degree of teamwork were required to pull off such a complicated project on an old dryer section with limited access space for staging on the drive side. Denis Duguay, Tembec’s project manager, sums up some of the key requirements: “The project was done on a vintage machine which had a lot of changes, some of them not documented. We had to do a lot of pre-engineering and that was an awesome task. Even minute details like the length of nuts and bolts were important. Small details can slow you down and even a $20 piece of equipment can make or break the success of a project.”
Duguay credits Kadant’s innovative engineering regarding the design of a unique bracket for all dryers that fastens the new steam joints to the original dryer frame. “The bracket design was the main thing that helped us,” says Duguay. Since access and staging space was limited, components were pre-assembled for efficient in-place final installation.
Just to make sure everything fit properly and in a time-efficient way, a trial installation was planned, fitting the new components on two dryer cans. Once any bugs in the design and installation procedure were worked out, the team was ready for the final installation on the rest of the dryer cans. The mill staff gives credit to this “dress rehearsal” as an essential way to ensure a successful on-time and on-budget project.
Improved drying rate, no steam wastage
The results of the dryer section rebuild have been realized quickly after start-up. Saleable paper was made soon after. All evidence points to a more responsive drying system which makes the most of the drying energy and is free of the former drying limitations. With the new stationary syphons and cascade control system, the differential pressures are now regulated precisely to about 3 psig, compared to 8 to 10 psig with the former rotary syphons.
“The periodic flooding problem has gone away,” reports Denis Payant, PM4 superintendent. He also notes the improved stability of the drive load, indicating a lower volume of condensate in the dryer cans. The drive load is now 9% of total load capacity compared to more than 12% when dryer flooding would occur.
Less blow-through steam is indicated by the low use of cooling water in the condenser. “We are not wasting steam. The steam vent valves do not open and we have tons of vacuum now. As a result, we are able to run the first steam section at minus 5 psig,” says Payant. This allows a gentle ramp up in dryer surface temperature and avoids dryer wraps.
The drying rate for the dryer section has been improved considerably and is now closer to the industry norm for newsprint. This is achieved with a lower average steam temperature.
The machine operators are enthusiastic about the control of the new steam and condensate system and the visibility they now have into how it works. “We have learned a lot about our steam system. Now we know what is happening on the machine,” says one operator.
The energy of the high-pressure steam formerly used in the dryer section has been shifted to the turbine generator where it can produce extra electrical energy. The offset in power costs is estimated to be almost
$90,000 per year.
Improved potential for machine
The improvements in papermaking operations have also been noticed. The spread of the CD moisture profiles is less, indicating more uniform heat transfer though the dryer cans. The drying capacity restrictions of the former system have been eliminated, allowing for higher production speeds. The speed is now set at 2800 fpm for all grades and
a new record speed of 2813 fpm has been set. While the final numbers for the ROI are not in yet, it looks like a good success story from preliminary indications. Doug Beemer sums up the mill’s thoughts: “The potential of this machine is now awesome.” The former drawback is now an asset.