Vibrating foil improves paper properties, Solutions!, Online Exclusives, October 2003

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VIBRATING FOIL IMPROVES PAPER PROPERTIES

By William Pettit, Mike Bricco, Dale Reynebeau, and Alan F. Button

Want to achieve improved sheet properties, reduced refining and drying energy, and increased machine speed? This new technology may help—if you’re willing to shake things up a bit.

Today, more than ever, it is critical for papermakers to reduce cost, improve quality and increase productivity without significant capital expenditures. It is well known that paper machine manifolds and headboxes introduce non-uniformities in the cross-direction distribution of basis weight and fiber orientation. Headbox elements such as rectifier rolls and tube bundles produce repeating patterns of fiber movement across the paper machine [1].

A new technology, the vibrating foil, takes the concept of a table drainage foil a step beyond by imposing a tunable vibration pattern on a foil specially designed for this purpose. This new, innovative and cost-effective forming aid can have a positive impact on drainage rates, couch solids, refining and drying energy, formation, sheet structure, fiber furnish efficiency, CD basis weight and moisture profiles, paper machine productivity and operating cost of a conventional fourdrinier paper machine.

Operating principle
The vibrating foil is a specially designed foil that mounts on the paper machine rails under the forming wire similar to conventional foils. Figure 1 shows the foil from the topside in a typical installation on a paper machine drainage table. The foil blade, mounted on the top of the unit, oscillates about a pivot immediately underneath the foil blade and running the length of the foil. This oscillating movement causes both the leading and trailing foil edges to alternate between pressure and vacuum pulses while the foil translates back and forth in the machine direction. This is in contrast to stationary conventional foils that typically have a pressure pulse on the leading edge followed by a vacuum on the trailing edge.


Figure 1. Top View of vibrating foil from the front-side of the paper machine.

Foil vibration frequency and height and angle (relative to the wire) are adjustable. Displacement of the foil edges is also determined by the foil blade width. The equipment that controls the foil vibration, height and angle can be seen in Fig. 2. The control system allows the foil vibration frequency to be tuned to match the natural frequency of the paper machine and automatically adjust to speed and paper grade changes.


Figure 2. View of accelerometers, vibration inducers and isolators under the foil.

The vibrating foil imparts a cycling energy pulse to the partially formed sheet to fluidize the fiber slurry. For a brief instant the energy pulse causes the fiber suspension to behave as if it was at a much lower consistency. This allows individual fibers to flow and change orientation to fill in lower basis weight areas of the sheet. As will be shown later in this paper, this fiber movement results in an improved cross-direction (CD) basis weight profile and a squarer sheet. The amount of energy required to fluidize the fiber slurry depends on the furnish characteristics and consistency at the point the energy pulse is applied.

Effect on paper machine table activity and drainage
Application of a foil vibrating at a frequency compatible with the natural frequency of the fourdrinier drainage table can significantly increase table activity (see Fig. 3.) The increased table activity results in an improved CD distribution of basis weight, a better-formed sheet and better overall sheet structure. Some of the benefits of the improvements in sheet structure are increased efficiency of the vacuum drainage units and higher solids levels going to the press and dryer sections.


Figure 3. Table activity 6 ft. from the vibrating foil with the foil off (left) and on (right).

The graph in Fig. 4, showing the increase in the solids levels down the drainage table of a fourdrinier paper machine, illustrates the impact the vibrating foils had on the efficiency of all the drainage elements. There were two vibrating foils installed on this paper machine. The first vibrating foil (VF#1) was installed ahead of the first flat box (FB#1) and the second (VF#2) was positioned between flat boxes three and four (FB#3 and FB#4, respectively). While both vibrating foils improved drainage, the second vibrating foil produced the majority of the improvements in sheet structure.


Figure 4. Solids levels at various positions on a fourdrinier table, without and with vibrating foils (VF#1and VF#2).

Effect on paper properties
As the graph in Fig. 5 illustrates, the vibrating foils significantly reduced the variation in the basis weight across this paper machine. With the vibrating foils off (VF OFF), basis weight varied from a maximum of 27.6 lbs to a minimum of 25.3 lbs. With the vibrating foils on (VF ON) the maximum decreased to 27.1 and the minimum increased to 26.0 lbs. This represented about a 50 % reduction in the level of basis weight variability.

A similar improvement was seen in the CD moisture profile. As presented in Fig. 6., with the vibrating foils off, moisture content ranged from 11.5 % to 8.3 %. The range of moisture content narrowed to a max. of 10.6 % to a min. of 9.2 % when the vibrating foils were on. This was equivalent to about a 56 % reduction in CD moisture content variability. There was a similar reduction in the variability of the CD porosity values. The reduction in CD test value variability was evident in other paper properties.



Figures 5 & 6. Cross-direction profiles of the basis weight (Fig. 5) and moisture content (Fig. 6) of a corrugating medium web.

Additional evidence for the improvement in sheet structure can be seen in Fig. 7. The photographs of 26 lb corrugating medium sheets, without and with the vibrating foils, show very obvious improvements in the uniformity of the formation. Quantification with a Quantimet 970 image analyzer indicated that formation (as coefficient of variation) improved 39.8 % with the vibrating foils on.



Figure 7. Formation of 26 lb corrugating medium with vibrating foil off (left) and on (right).

Other paper properties were impacted favorably. In a two-day trial on 26 lb corrugating medium the MD/CD tear ratio showed an increase in squareness with the vibrating foils on, going from 0.89 to 0.72 as the CD tear increased from 86 gf to 103 gf . These improvements occurred in sprite of a 9.5 % reduction in primary refiner loads.

Operating experience
Extended operating experience was obtained on a conventional fourdrinier producing corrugating medium. The paper machine parameters and paper grades are presented in Table I. The headbox was an air-padded Allis-Chalmers with rectifier rolls. The drainage elements consisted of those shown in the graph of Fig. 4.; six flat boxes, followed by four low vacuum boxes, a DuoVac and another flat box before the couch. The fiber furnish consisted of integrated semi-chemical hardwood (50 %), recovered OCC (40 %) and broke (10 %).



Table I. Paper machine specifications.

The beneficial effect that the vibrating foils had on the variability of cross-direction basis weight and moisture content profiles was discussed earlier and shown in Figs. 5 and 6. With the more uniform basis weight distribution, the table drainage elements were more effective and the web entered the press section at higher couch solids (26.3 % versus 19.1 %). An additional benefit of the improved CD profile was improved first pass retention and lower whitewater solids.

To take advantage of the faster drainage and further improve sheet formation the paper machine operators reduced headbox consistency from 1.3 % to 0.9 %, as shown in Fig. 8. These additional improvements in formation resulted in the medium being stronger than target values. To bring the medium strength levels in line with product specifications, as shown in Fig. 9, the primary refiner power levels were reduced 8.2 %, from 2450 hp down to 2250 hp.

Figure 8. Reduction in Headbox Consistency.
Figure 9. Reduction in Primary Refining Level.

With less CD variability in the corrugating medium it was possible to reduce basis weight (from 26.4 to 26.2 lbs/1000 ft2) closer to target and still meet the performance requirements for the medium. This resulted in a 0.8 % reduction basis weight and lowered fiber cost.

Among the most significant changes resulting from the vibrating foils installation was the reduction in steam used to dry the paper. As the graph in Fig. 10 shows, the drying steam usage decreased 42 %, from 1.9 lbs/lbs of paper to 1.1 lbs/lbs of paper. The combination of increased solids off the couch and out of the press section, substantial improvement in CD moisture profile, reduced refining and improved formation resulted in more uniform and efficient drying and the major reduction in steam usage. All these improvements resulted in lower cost, higher performance paper and higher paper machine productivity.


Figure 10. Reduction in Steam Used to Dry Paper.

The impact on operating cost
We can estimate the direct economic impact of the vibrating foil from the cost reductions resulting from the reductions in basis weight (0.8 %), refining (8.2 %) and drying energy (42 %). The estimated annualized value of these cost reductions is presented in Table II. These cost reduction opportunities were obtained while meeting grade test targets with paper that was more uniform and an overall better product.


Table II. Estimated Annualized Cost Reduction.

As the graph in Fig. 11 indicates, there is also an opportunity to increase the paper machine speed for the range of papers produced by an average of 14.2 %.


Figure 11. Potential Paper Machine Speed Increases.

This opportunity for increased paper machine productivity is a direct result of improvements in sheet structure and CD basis weight profiles. The increased solids and reduced drying energy make speed increases possible where drainage and drying capacity were limiting.

In conclusion, the vibrating foil provides a new and innovative technology for:

  • Improving sheet structure and formation
  • Reducing CD basis weight and moisture profile variability
  • Increasing drainage rates
  • Increasing couch and press solids
  • Reducing drying energy
  • Reducing refining requirements
  • Reducing costs and increasing paper machine productivity
  • Improving paper quality and performance
  • REFERENCES:

    1. Aidun, C., TAPPI J. 81(5):159 (1998).

    About the authors:
    William Pettit is National Sales Manager, Mike Bricco is President & CEO, and Dale Reynebeau is Vice President & COO for Vibre-Tech, LLC, in Little Chute, Wisconsin; and Alan F. Button is with Buttonwood Consulting, LLC.

    Author: Pettit, W., Bricco, M., Reynebeau, D., Button, A.F.
    Vibrating foil improves paper properties, Solutions!, Online
    Vibrating foil improves paper properties, Solutions!, Online Exclusives, October 2003
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