Press fabric selection & performance improvement.

Machines producing all major grades of paper have benefited from press fabric design upgrades. Some machines have achieved annual economic improvement of more than US$5 million after changing press fabric designs.

Stories of improvements realized on some machines were presented to paper mill superintendents who attended a "Mill VIP" session on optimizing press fabric design at the TAPPI Paper Summit 2004 in Atlanta. Examples include:

* One fine paper mill with four presses was able to meet copy paper bulk, smoothness, and stiffness specifications at lower average basis weight by replacing laminated three-layer fabrics with multi-axial press fabrics with flat fiber batt. The sheet out of the press section was smoother and bulkier, allowing for a 1.7% reduction in average basis weight, because less calendering was required to meet the Sheffield smoothness specification. The press fabric changes also permitted a 5% increase in machine speed, a reduction in dryer section steam consumption, and a 25% reduction in number of breaks per day.

* A three-layer laminated fabric with multiaxial bottom base fabrics and a porous polymer top layer was installed on the third press position on a kraft paper machine to replace an integrally woven fabric. Surface batt denier was changed from 30 to 15. The trial fabric doubled average life and could be run at higher press loading, which increased tensile strength and allowed the mill to produce lower basis weight paper on the machine. Also, the sheet was drier entering the dryers; however, the fabric was more difficult to install and fabric cost was higher.

* A lightweight linerboard machine with two double-felted presses was experiencing problems with high Emveco smoothness (rough sheet), resulting in marginal printability. The second press top fabric was a seamed, three-layer laminated fabric with a cabled monofilament MD top base and 24-denier top surface batt. The seamed, replacement fabric had a three-layer monofilament base with a porous polymeric top layer and 15-denier surface batt. The trial fabric provided up to a 100-point reduction in Emveco smoothness numbers, improved seam flap integrity, and reduced seam mark.

In his presentation, Daniel Hedou of AstenJohnson focused on press fabric design and press loading concepts to improve paper characteristics. Three key factors are to provide a uniform fabric surface, provide good sheet support and low water flow resistance, and provide a graduated press loading schedule. Benefits of optimized web building using these concepts include the following:

* Maximum water removal.

* Good hydrogen bonding.

* High caliper.

* Improved strength.

* Better smoothness.

* Reduced linting.

Initial field trials indicate that Concora and other strength tests can be improved by following these concepts. Hedou offered the following practical observations on web building concepts:

* More vacuum may be required on early press uhle boxes to shift water removal to uhle boxes.

* Smooth fabric surfaces are more sensitive to plugging and wear.

* Crushing is possible in latter presses on heavy-weight grades.

* Web building is more critical as basis weight increases.

Joe DiCesare of Albany International said advanced press fabric technologies include super-stratified structures and nonwoven MD membrane material platforms. Advanced construction platforms include seam designs for conventional and multi-axial press fabrics. The following qualities characterize the latest press fabric technology:

* Ultra-thin, high-permeability lightweight membranes.

* Super-stratified structures, with super fine paper surface and extra coarse interior structure.

* Proprietary needling processes.

* Single, monofilament multi-axial bases.

Benefits of new technology fabrics include faster start-ups, cleaner running, improved sheet smoothness, maximum dewatering capacity, reduced energy costs, and consistent performance and reliability.

Doug Sweet, of Doug Sweet and Associates, Inc., said effective fabric conditioning systems typically combine physical (uhle boxes, showers, and vacuum) and chemical processes. He noted that press sections on many paper machines operate without adequate fabric conditioning, that solutions to vacuum system problems are not difficult to resolve, that measurable results are possible, and that solutions are cost effective. A few of Sweet's tips for fabric conditioning systems include the following:

1. Dwell time for each uhle box should be 2-4 milliseconds, with minimum slot width of 0.5 in. Consider double slots if total width exceeds 1 in. Water removal should be equal for single or double slots.

2. Uhle box diameter is based on airflow in cubic feet per minute, with typical velocities of 3,500-4,500 feet/min. One uhle box per fabric is usually adequate on machines running less than 2,000 feet/min. For critical fabrics, use two uhle boxes.

3. All uhle boxes require lubrication showers (some mills use oscillating lube showers). Adjust high pressure oscillating showers to achieve complete coverage and to minimize streaking. Typical needle shower pressures are 200-250 psi; shower water temperature should be 120[degrees]F-140[degrees]F.

4. Each fabric should have a separate vacuum source. Vacuum piping velocities should be 3,500-4,000 feet/min with wet air and 5,500-6,000 feet/min with dry air. Install vacuum separators ahead of all vacuum pumps.

5. Use proper solvents to remove soluble materials. Use continuous and intermittent cleaning. Cleaning shower position is critical. It should be ahead of an outside felt roll, if possible. Consider using oscillating chemical addition showers, also.

Ed Norris of HelioJET Cleaning Technologies suggested that press fabric performance might be optimized by using pressurized hot water at 150[degrees]F-185[degrees]F for cleaning. Benefits of using a fixed flow pressure amplifier condenser to deliver pressurized hot water at pressures up to 600 psi include:

* Increased volume of water removed from the sheet in the press section (for faster machine speeds and reduced dryer steam consumption).

* Improved contaminant removal from press fabrics.

* Improved CD moisture profiles of press fabrics and the sheet.

* Increased press fabric life and extended time between machine shutdowns.

* Increased cleaning chemical effectiveness.

Questions and answers presented during the sessions included the following:

1. How can you determine the difference between embossing and hydraulic flow effects in paper? Image analysis can be used to identify embossing sheet marking patterns caused by base fabric yarns or batt fiber. Flow effects usually do not fit the frequency of yarns or match the diameter of batt fibers.

2. What can be done with fabric design to reduce press section blowing problems? Blowing reduction generally requires improving contact between the fabric and the web. This requires proper needling and use of relatively fine batt surface fiber or treatments to optimize surface contact. Sometimes fabric base structure plays a role in allowing proper functioning of vacuum elements. The center plane of fabrics must allow air flow provided by suction rolls and uhle boxes.

3. What percentage of press fabrics is currently seamed? Overall percentage is around 60%. The percentages are around 70% for fine paper fabrics and 10% for tissue fabrics.

4. Is it cost-effective to oscillate uhle box lubrication showers? Some mills have found that oscillating lube showers reduces fabric streaking and improves CD moisture profiles.

5. How can low pressure steam showers over uhle boxes improve fabric conditioning? Low pressure steam showers are installed on several corrugating medium and linerboard machines. One recycled liner machine with a profiling steam shower installed over a suction transfer roll experienced problems with waxes and stickies building up in the second press top fabric when the steam shower was used. Adding a low pressure steam shower over the uhle box to heat the second press top fabric eliminated the fabric filling problem. Low pressure steam showers over uhle boxes have helped some machines run faster, also.

IN THIS ARTICLE, YOU WILL LEARN:

* The potential benefits of new press fabric designs.

* Tips for effective press fabric cleaning.

* Components of efficient press fabric systems.

ADDITIONAL RESOURCES:

* Learn more about machine fabric innovations and maintenance at the company web sites of Albany International (www.albint.com), Asten-Johnson (www.astenjohnson.com), and Helio JET Cleaning Technologies (www.heliojet.com).

ABOUT THE AUTHOR

Dick Reese served as moderator for this TAPPI Paper Summit 2004 Mill VIP session. He heads the consulting firm, Dick Reese and Associates Inc., in Norcross, Georgia, USA. Contact him by by phone at [phone], or by email at [e-mail].

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