The quality of tissue paper and the efficiency of tissue machines at maximum speeds are strongly influenced by cross-direction moisture profiles. Irregularities in moisture distribution can be difficult to identify and correct, and many machines experience persistent issues
Laura Scek, Application Engineer / Marketing and Communications Coordinator, Valmet Corporation
These problems can stem from several factors, including basis weight variations, felt permeability issues, sharp Yankee Dryer temperature gradients near the edges, uneven coating, condensate build-up, and variations in hood impingement speed or temperature distribution.
Additionally, suction pressure rolls and shoe presses often fail to maintain uniform linear loads and press-to-roll consistency under all operating conditions and considering a wide range of tissue grades. The crown fit between the press and the Yankee is typically optimized for a narrow operating window, meaning changes in machine speed, steam pressure, or linear load can exacerbate moisture inconsistencies.
The high cost of uneven moisture profiles
Addressing the root causes of moisture inconsistencies is ideal but often difficult. Operators struggling with uneven profiles frequently resort to over-drying the paper to achieve the desired final basis weight, consuming excessive fiber and energy in the process.
Overdrying increases costs since fiber is expensive, and the energy wasted in overdrying contributes to unnecessary carbon emissions. Additionally, reduced machine speed and frequent shutdowns—often needed to regrind the Yankee Dryer due to coating inconsistencies—lead to lower productivity and revenue losses.
Current moisture profiling methods for tissue machines
Most modern tissue machines use profiling dampers to adjust the air supplied by the Yankee Hood to areas where the paper is drier than average. Some machines also employ a profiling steam box, either separately or in combination with dampers.
However, both approaches require additional energy. Hood profiling dampers can consume up to 45% of circulation fan electrical consumption, and steam boxes may use as much as 300 kg of steam per ton of paper produced.
While hood profiling dampers allow moisture increases where needed, they cannot enhance drying in wetter areas without increasing the machine’s total drying capacity. This results in overall energy waste, as operators must over-dry the paper while shutting all other profiling dampers—an inefficient solution.
A smarter approach: advantage AirCap Pro moisture profiling system
Unlike conventional profiling techniques, the new AirCap Pro system locally adjusts the temperature of air supplied by the hood. This innovative method allows increased drying intensity where the paper is wetter than average, something current dampers cannot achieve.
Simultaneously, the system reduces drying intensity in areas where the paper is drier than average, leading to a more uniform moisture profile at the reel.
The AirCap Pro features two main headers: one at high temperature and one at lower temperature. Each profiling zone is equipped with one hot and a cold profiling damper. The control system adjusts damper openings according to drying needs:
• In areas with higher-than-average moisture, the hot air valve opens more while the cold air damper closes.
• In drier areas, the hot air valve closes while the cold air damper opens.
Each profiling chamber’s hot and cold air dampers can either be mechanically linked or operated independently with linkage through the control system. Independent operation enhances profiling accuracy by allowing adjustments to both air temperature and volume, thereby doubling the profiling performance.
Efficiency gains & energy savings
The AirCap Pro enables an even moisture profile at the reel, reducing peak-to-peak variation to 1.6%, even when post-pressure roll consistency fluctuates with a peak-to-peak 2-3%.
Thanks to its dual-header setup with always-open dampers, airflow pressure drop through profiling dampers is minimized, cutting circulation fan energy consumption to just 5% of the total fan power usage—considerably less than conventional profiling methods.
Additionally, this system reduces overall thermal energy consumption. Because the average air temperature supplied to the paper is lower than that of traditional hoods, burner gas savings are significant, especially when correcting wet-edge profiles.
While inclined moisture profiles also benefit from reduced energy use, there are no savings for dry-edge profiles, as conventional hood profiling techniques are already effective in those cases.
Conclusion
The Advantage AirCap Pro optimizes moisture profiling by controlling air temperature. In cases of extreme profiling needs, it can further refine air supply distribution, much like conventional hood systems.
By doubling profiling effectiveness, this system ensures superior moisture consistency, enabling peak-to-peak moisture variations as low as 1.6% – comparable to new Valmet Tissue Machines equipped with ViscoNip technology.
Furthermore, the AirCap Pro minimizes electrical energy losses caused by profiling dampers while significantly reducing gas consumption by lowering the average air temperature used for drying. This innovative approach not only improves tissue machine efficiency but also decreases operational costs and environmental impact.