Save energy by optimizing paper machine clothing.

High energy costs have marginalized several paper machines and led to shut downs of non-competitive mills. Clothing optimization offers non-capital, low cost alternatives for reducing energy consumption in most mills. Some progressive papermakers have achieved significant improvements in machine speeds and operating efficiencies and reductions in energy consumption by installing modern clothing designs. Papermakers that are not taking advantage of these developments are missing some excellent opportunities. This article summarizes several case histories on use of modern clothing designs to reduce energy consumption.

A recent trade journal article indicted that paper machine clothing suppliers have experienced greater change in the last five years than at any time since the industry converted from metal to synthetic forming fabrics. (1) Paper machine clothing development has been excellent in recent years. Some articles providing an overview of paper machine clothing development have previously been published in Solutions! magazine (2), (3), (4)

DRIVING DOWN ENERGY COSTS

Modern forming fabric designs can reduce energy consumption by increasing sheet consistency entering the press section, reducing drive loads, and reducing steam addition to whitewater by providing better drainage. Press fabrics can reduce rewet and increase sheet consistency entering the dryers. Modern dryer fabric designs provide energy benefits by improving sheet-to-dryer contact, improving air movement in dryer pockets, and providing resistance to fabric filling. Installation of modern fabric designs has produced higher overall paper machine efficiency on several machines.

Papermakers have historically measured fabric performance by cost per ton and days life. Fabric cost is relatively low compared to energy savings achieved with modern clothing designs. Clothing costs vary from about US$ 6 to $16 per ton on various grades of paper. A rule of thumb is that clothing cost is approximately 2% of operating cost for different grades. New and well-maintained machines have lower clothing costs than older machines due to less fabric damage and wear. Energy costs typically range from 15 to 30% of operating costs. These percentages vary widely from mill to mill depending on grade, fiber costs, fuels used, integrated vs. non-integrated and other factors.

Installation of modern clothing designs can provide significant energy savings on most paper machines. Figure 1 indicates that clothing cost averages 2% (US$ 8.80 per ton) of operating costs on a typical newsprint machine while energy cost is 31.8% ($139.28 per ton). Using the numbers in Figure 1, if installation of modern clothing designs increases total clothing cost by 25% ($2.20 per ton), and energy consumption is reduced by 5% ($6.96 per ton), the payback period would be three to four months. Payback period can be lower or higher depending on operating parameters on each machine, including fabric cleaning, vacuum application, machine limitations, and other factors. Strong focus on clothing budgets can result in significantly higher overall operating cost if papermakers do not take advantage of energy cost reductions available with modern fabric designs.

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Paper machine energy efficiency and production rate (on dryer-limited machines) should be monitored throughout fabric life to document performance. Many mills do not have adequate energy monitoring and it is difficult to manage variables that are not measured.

PRESS FABRICS

Cost of removing water after the press section is very high so maximizing press section performance is critical to minimizing paper machine energy consumption. Figure 2 shows the relative cost of removing water on the forming, pressing, and drying sections. Optimizing press section dewatering is critical to minimizing dryer section steam cost. There have been significant improvements in press fabric designs in recent years. Some current fabric designs that perform well include laminated, multi-axial, seamed, fabrics with special surface treatments, and anti-rewet designs. Schematics of some modern clothing designs are shown in Figure 3. Laminated fabrics are made with a combination of two or three base weaves and are used to improve fabric performance on the most demanding press sections. Yarns in multi-axial fabrics are oriented in at least four directions. Advantages include better pressure uniformity, improved sheet moisture profiles, and better compaction resistance, which provide good dewatering throughout fabric life.

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Press section exit solids improvements of 0.5 to 1.5 percentage points are common with modern press fabric designs. These gains permit a 2 to 6% reduction in main section dryer steam consumption at the same production rates. Smoother sheet surfaces can also improve heat transfer rates in the dryer section. Many modern fabric designs maintain good water removal throughout their life with proper fabric conditioning.

Press fabric variables related to water removal are pressure uniformity, compressibility, and flow resistance. Pressure uniformity is affected by fabric base structure and batt application. Large scale pressure uniformity is related to base fabric mesh count and yarn size. Small scale pressure uniformity is related to batt structure. Fabric compressive properties affect nip width, peak pressure achieved in press nips, and fabric void volume. Compressibility of press fabrics decreases as fabrics compact and become filled with contaminants. Flow resistance is the rate at which water can enter and exit the fabric. Low flow resistance allows water to move easily into and out of the fabric.

Press Fabric Case History 1: A fine paper machine producing copy paper grades with a tri-nip press plus a fourth press increased overall paper machine economics by over US$ 4 million by upgrading press fabric designs. Laminated triple-layer fabrics with 15 denier batt were replaced by multi-axial fabrics with 9 denier flat fiber batt. Basis weight was run at 20.3 lb/1300 f[t.sup.2] to meet bulk and smoothness specifications with the original fabrics. The sheet was bulkier and smoother out of the press section with the multi-axial fabrics. The sheet required less calendering so the basis weight could be reduced to 20.0 lb. The fabric changes also permitted a 5% increase in machine speed, reduction in main dryer section steam consumption, and a 25% reduction in number of breaks per day.

Press Fabric Case History 2: Results from installing a conventionally woven high technology press fabric on a corrugating medium machine are shown in Figure 4. The paper machine was able to run 3.5% faster while using 10% less steam. Annual benefits were $1.9 million.

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Press Fabric Case History 3: A southern linerboard machine operating at 1800 fpm has a double-felted extended nip press operating at 5500 pli. The mill installed multi-axial fabrics on the top and bottom positions. Production remained constant due to speed limitations. Emveco smoothness improved from 241 to 187 (lower is better). Steam consumption went down by 390 K lb per day. Annual savings were US$ 927,000.

Press Fabric Case History 4: A light-weight coated (LWC) paper machine operating at 4300 fpm installed multi-axial press fabrics on all four positions on their tri-nip plus fourth press section. The resulting 1.4 percentage point increase in press exit sheet solids provided the following benefits:

* Steam consumption dropped from 1.34 lb steam/lb paper to 1.25 lb/lb at the same production rates (878 tons/day),

* Annual energy savings were US$ 332,000.

Press Fabric Case History 5: A Crescent Former tissue machine achieved total annual savings of US$ 259,000 after installing a modern forming/press fabric design. The machine produces 9 to 14 lb per 3000 ft (2) tissue grades at speeds up to 6200 fpm. Overall energy consumption decreased by 4.4% including:

* 400 ft (3) per ton of gas.

* 15.6 kWh per ton of electricity.

DRYER FABRICS

Dryer fabrics influence drying by openness and contact with the paper web. Fabric permeability has a large impact on ventilation of dryer pockets and evaporation rates. Some modern dryer fabric designs have high resistance to contaminant buildup so fabric permeability can be maintained over time. Contact pressure caused by fabric tension and surface structure influence heat transfer from dryer cylinders to the sheet. The combination of heat transfer, mass transfer, and stable transport of the sheet affect drying efficiency.

The first question to ask is whether you are running the highest dryer fabric permeabilities that will not cause sheet instability. Fabric permeability has a large impact on drying rate. Opening up dryer fabrics increases the quantity of hot, dry air introduced into dryer pockets and the driving force for drying. Papermakers also should make sure that hood exhaust capacities are adequate for good water removal.

Mills should be running dryer fabric designs that slow down build-up of contaminants and are easy to clean. Fabrics that run clean throughout life result in higher drying rates without drop-off near the end of life. Clothing suppliers have developed an effective array of contaminant release products with impregnated fluoropolymers and unique physical constructions, including flat cross-direction filaments and grooved filaments.

Recent developments in spiral and woven dryer fabric designs have provided energy savings on some paper machines. Spiral dryer fabrics are an endless construction formed by joining helical formed loops of monofilament yarn. They provide higher permeability ranges than woven designs as well as potential for higher drying rates. Spiral fabrics have high contact area surfaces and have no weak link since they do not have traditional seams like woven fabrics. Recent processing and material improvements have resulted in spiral fabrics having no application limitations. They run on some of the fastest publication grade machines in the world.

Recent woven dryer fabric innovations include designs capable of moving more air in dryer pockets at the same permeability as standard woven designs. This is achieved through unique construction that causes air to pulse from one side of the fabric to the other.

Dryer Fabric Case Study 1: A southern linerboard machine was converted from running all woven dryer fabrics to spiral fabrics. No other changes were made on the machine. Benefits from installing spiral fabrics included:

* Machine speed on 42 lb increased from 1880 to 1940 fpm.

* Average steam pressure dropped from 113 psig to 104 psig.

* TAPPI drying rate increased from 6.07 to 6.22 lb water evaporated per hour per sq ft of dryer surface.

* Production increased by 8.05 tons/day for an annual profit increase of US$ 564,000.

* Alternately, steam consumption could have been reduced by 3.2%.

Dryer Fabric Case Study 2: Another southern linerboard machine was converted from running all woven multifilament to all spiral designs. No other changes were made to the machine. Benefits included:

* Sheet temperatures decreased indicating that additional evaporation was occurring.

* Pocket humidity levels stayed low showing that that higher fabric void volumes increased air flows and increased drying.

* Drying rates increased from 4.49 to 4.69 lb/hr/f[t.sup.2].

* Production increased from 886 to 939 tons/day.

* Average dryer steam pressures dropped from 119 psig to 76 psig.

* Incremental annual profit improvement from the production increase was US$ 3.68 million.

* Annual steam savings were $122,000.

Dryer Fabric Case Study 3: A coated groundwood mill in the Midwest installed high contact, contaminant resistant dryer fabrics on the first section top and bottom positions. Conventional weave fabrics were previously run and no other fabrics on the machine were changed. The trial was designed to use the improved drying characteristics of the new design to reduce steam consumption.

Steam flow to the dryers was reduced by 3000 lb per hour while running 2750 fpm on 38 lb LWC. The mill had previously conducted speed trials and determined that the steam savings would permit a 200+ fpm speed increase. However the mill chose to keep speed constant and reduce steam consumption. Annual steam savings were US$ 73,000. An added benefit was that the new fabrics had low air carrying characteristics so there was less sheet flutter.

Dryer Fabric Case Studies 4 and 5: A dryer-limited uncoated freesheet machine in Europe installed a woven "pulsating" fabric design to increase drying rates by reducing dryer pocket humidities. This change provided a 40 to 65% reduction in pocket humidities. Fabric permeabilities of the original and pulsating design fabrics were the same. Machine speed was increased by 20 m/min (66 feet/min) and steam consumption went down by 2%. Total productivity and energy cost savings were over US$ 850,000 per year.

A European machine producing carbonless grades used the same air pulsating technology to reduce pocket humidities by 16%. Steam consumption went down by 6% and machine speed was increased by 17 m/min (56 fpm). Total annual savings were US$ 520,000.

FORMING FABRICS

Forming fabrics have a greater effect on final paper properties than press and dryer fabrics but less effect on energy consumption. Application of modern forming fabric designs can provide the following energy-related benefits:

* Some triple-layer designs reduce drag load and provide energy savings in forming section drive loads.

* Installation of fabrics with better drainage characteristics can permit reducing whitewater temperature on paper machines that have forming section drainage limits. Heating of whitewater is very inefficient--especially on machines with high water consumption per ton.

* Some forming fabric designs have improved sheet dewatering so that the web is drier leaving the forming section. This can result in fewer sheet breaks and permit higher press loads on some paper machines.

Forming Fabric Case Study 1: A newsprint machine with a Symformer top-wire forming unit operating at 4150 fpm was able to reduce forming section drive load by replacing the base fourdrinier double-layer fabric with a triple-layer fabric. Drive load went down by 212 kW and provided US$ 71,000 annual savings.

Forming Fabric Case Study 2: A 100% recycled liner machine in the Southwest with two BelBond units reduced forming section drive load after installing a triple-layer base fabric. Forming section drive load went down by 30% and provided a 352 reduction in horsepower and annual savings of US$ 706,000.

Forming Fabric Case Study 3: A linerboard machine on the West Coast saw good benefits after installing an intrinsic weft, triple-layer design on the base four-drinier position. The machine makes 35 to 90 lb per 1000 f[t.sup.2] brown and white top liner in a speed range of 1430 to 2000 fpm. The machine has two top-wire dewatering units. The objectives of the trial were to improve fabric drainage, reduce drive loads, improve sheet tests, and increase fabric life. Results from the trial included:

* Improved sheet CD profiles.

* Improved retention and formation on white top.

* Increased solids off the couch from 22 to 24%.

* Reduced retention aid addition by 20% while maintaining 94% retention.

* Forming section drive loads were reduced by 8%.

* Test specifications were easier to obtain.

* Improved paper machine efficiency.

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SUMMARY

Good application of modern clothing fabric designs can produce significant energy savings on paper machines. The case histories include results from installation of high technology fabrics on several machines producing different grades. Payback from installing more sophisticated fabric designs often is a few months. Papermakers that are not running modern fabric designs are missing excellent opportunities to reduce energy consumption and total operating cost.

Figure 1.

Clothing Impact on Energy Consumption

Total Cash Cost per Ton

Grade - Newsprint

Total Cash Cost - $438

Chemicals        5.7%
Fiber           34.3%
Fuel             9.7%
Hourly Labor    11.0%
Materials       11.3%
Clothing         2.0%
Power           22.1%
Salaried Labor   3.8%

Paper Machine Clothing averages 2% of operating costs of a publication
grade machine but has a significant impact on Energy, Fiber, and
Chemicals

* Source: Fisher Pulp and Paper Worldwide 2005 (Fisher International
Inc.)

Note: Table made from pie chart.

Figure 2.

Relative Cost of Water Removal

Water Removal kg/kg fibers

Specific Cost of Water Removal

Former          Presses      Dryers

$0.014/ton      $0.019/ton   $22.00/ton
Vaccum
Fabric Tension  Compression  Evaporation
Gravity         Mechanical
124              2.25          1.25

* Source "Pressing Principles" Dr. Ivan Pikulik Paprican 2002

Note: Table made from bar graph.

REFERENCES

(1.) "Forming the Best Sheet", Pulp and Paper International, December 2005, pp 25-27.

(2.) Reese, R. A., "The Basics: What You Need to Know About Forming Fabrics", Solutions!, August 2005, pp 33-36.

(3.) Reese, R. A., "The Basics: What You Need to Know About Dryer Fabrics", Solutions!, October 2005, pp 31-34.

(4.) Reese, R. A., "The Basics: What You Need to Know About Press Fabrics", Solutions!, January 2006, pp 49-52.

(5.) Reese, Dick, "Press Fabric Selection & Performance Improvement", Solutions!, October 2004, pp 42-43.

(6.) McVey, David, "Benefits of Paper machine Clothing Application on Energy Consumption", TAPPI Papermakers Conference, Atlanta, April 25, 2006.

(7.) Hedou, Daniel, "Saving Energy with Modern Clothing Designs", TAPPI Papermakers Conference, Atlanta, April 26, 2006.

(8.) Maggard, Jim, "Mill Energy and Production Impacting Products", TAPPI Papermakers Conference, Atlanta, April 26, 2006.

(9.) Reese, R. A., "Paper Machine Energy Considerations", TAPPI Papermakers Conference, Atlanta, April 25, 2006.

(10.) Farly, Luc, "TAPPI Energy Discussion", TAPPI Papermakers Conference, Atlanta, April 26, 2006.

RELATED ARTICLE: WHAT YOU WILL LEARN

* Why papermakers should take advantage of new fabric designs that can reduce energy usage

* Several case histories in forming, pressing and drying fabrics that illustrate the benefits of using advanced clothing, including energy cost reductions

ADDITIONAL RESOURCES

* See references at the end of this article

ABOUT THE AUTHOR

Dick Reese has worked on paper machines for more than 40 years in various production and engineering/technical support roles. He has been a TAPPI member since 1973 and is an active member of the Papermaking and Water Removal committees. He was named a TAPPI Fellow in 1992, received the Manufacturing Division Technical Award in 1997, and the Engineering Division Technical Award in 1999. He is currently an independent papermaking consultant in Norcross, Georgia, USA. He can be reached by phone at [phone] or email at [e-mail].