Single-nip shoe press: the single choice: whether you call it evolution or revolution, the single-nip shoe press is changing papermaking operations and economics.

We often say that change in paper machinery is an evolutionary process rather than a revolutionary one--and slow evolution at that. A more accurate view is that paper machines change very little until something big comes along and then things change in a major way. Call it punctuated evolution. A good example would be the change from table rolls to foils back in the 1960s.

Now it is time for a major change again, this time in the press section of paper machines that make lighter weight grades, such as fine papers. The next step in the evolution of press sections is the single-nip shoe press (Fig. 1).

[FIGURE 1 OMITTED]

Shoe presses are nothing new. They have been the press of choice for heavier weight grades such as linerboard for two decades now, and have expanded to almost all other grades in recent years. Shoe he presses replace a conventional roll with a hydraulically loaded curved shoe, which is typically around 250-mm or 10-in. wide. Typical pressures in a shoe press are up to for 1400 kN/m or 13000 pli, but since the pressure is spread over such a wide nip, the force per unit area that the sheet sees is gentler than in a conventional roll press. Shoe presses, therefore, can remove a large amount of water without crushing the sheet. Mills have commonly reported sheet solids after shoe presses of 50% or better.

SHOE PRESSES AND FINE PAPERS

In 1998, Willamette Industries (now Weyerhaeuser) started up the first fine paper machine using two double-felted (tandem) shoe presses on its K-2 machine in Hawesville, Kentucky, USA. Experience with this press section gave Willamette the confidence to replace a conventional three-nip cluster press with a single-nip shoe press on the No. 5 fine paper machine in Johnsonburg, Pennsylvania. In August, a second single-nip shoe press will be started up in Weyerhaeuser's Kingsport, Tennessee, mill.

Trials on pilot research machines and real world experience with tandem and single-nip shoe presses have shown that a single-nip, double-felted shoe press can give about the same amount of dewatering as two shoe presses on lighter weight grades of paper.

How can one shoe press work as well as two? For lightweight sheets, the controlling mechanism is probably rewetting. The second press takes out additional water, but since the sheet is held against the bottom felt, a large amount of that water re-enters the sheet. Sheet rewetting can be overcome to some degree by using a nonpermeable polyurethane transfer belt in the bottom position of the second shoe press, but this results in some increase in sheet two-sidedness.

If one nip works as well as two nips, why bother with the second press? Remember that two felts now have to do the work of four. And sheet roughness after a single-nip shoe press can be worse than after a tandem shoe press. Why? With a tandem press, the impression from the second set of felts tends to break up the pattern from the first set of felts.

Felt life can also be a problem for a single-nip shoe press. Initial experience at Johnsonburg showed that felt life dropped in half compared with the old four roll cluster press. Felt design for a single-nip shoe press is quite difficult because of the competing functions of smoothness, water-carrying capability, and felt life.

To impart good sheet smoothness. the felt surface or cap must be very smooth--that is, with small denier yarns--and the felt base also needs to be fine enough that its pattern does not show through. On the other hand, a Free cap and base structure tends to mat down easier, reducing the void volume of the felt; felt life suffers as well. Another problem with a line denier cap is more re-wetting. In fact, the main development work in single-nip shoe presses has been in felt design, not in the design of the press itself.

WHAT ARE THE REWARDS?

Several major benefits are driving the trend for single-nip press sections to be the standard design for lightweight grades:

Benefit 1: In real-world experience, at a given loading there is very little difference in the solids content of the sheet after a single-nip press compared with a tandem nip press. A transfer belt can improve the difference somewhat, but the difference will only be 1%-2% at best. You can expect 46-48% sheet solids after a single nip shoe press, depending on the furnish and the application.

Benefit 2: Single-nip shoe presses are less expensive. Although a single-nip press is really one-half of a tandem shoe press, capital costs will be greater than one-half. Engineering and manufacturing costs for two identical presses are not twice what they are for one press. Also, spares will be nearly the same for a single-nip shoe press versus a tandem press due to shared spares, still, a single-nip shoe press will cost a lot less than a tandem press.

Benefit 3: other associated project costs will be lower: Installation costs will likely be much lower for a single-nip press, and the associated machine outage will be shorter. For existing paper machines, a real benefit is the space requirements for a single-nip press over other designs. This is particularly true for the drive side of the paper machine, which is normally cluttered with concrete pillars, drive shafts, and motors.

Benefit 4: A smaller press section leaves more room for other things. Since a single-nip press is approximately half as long in the machine direction as a tandem press, more space will be available for dryers and thus the possibility of even greater production increases.

Benefit 5: Operating costs will be much lower. A tandem double-felted shoe press has twice as many felt as a single-nip double felted press. Although felt life is currently shorter for a single-nip press, that gap will close once clothing suppliers optimize felt designs, if we assume a felt cost of US$ 30,000 and a felt life of 30 days for a tandem press with four felts, then clothing costs would be US$1.44 million year. Felt costs for a single-nip press with two felts and a felt life of 25 days would be US$ 876,00. That is a savings of US$ 564,000.

Another part of operating costs energy consumption. A single-nip press has one-half the connected horsepower, both in drives and in vacuum requirements For an average paper machine of around 200 in. width, total connected horsepower for a single-nip press might be 2800 hp (2100kW). Assuming a cost of US$ 0.07/kW.h, 350 operating days, and 90% uptime, total energy cost for a single-nip press would be US$ 1.1 million. A tandem press would need twice the connected horsepower and, therefore, twice the annual energy cost, or US$ 2.2 million. If you add savings on clothing to the energy savings, the yearly total is more than US$1.66 million.

SINGLE-NIP FUTURE FOR FINE PAPER

In many cases, the press configuration of the future will be a single-nip shoe press, especially for fine paper machines. A single-nip shoe press makes good sense as a production upgrade for existing paper machines. In some cases, the production increase on one machine will allow another marginal machine to be shut down, with resulting overall improvements in cost/ton for the mill.

The development of the single-nip shoe press is just beginning. While clothing designs have not completely caught up with the demands of this type of press, they will in the near future. Evolution or revolution--take your pick. In either case, expect to see a lot of new single-nip shoe presses in the future.

Acknowledgements

My grateful appreciation to Metso Paper, Voith Paper; and Willamette Industries for providing me the information needed for this article.

IN THIS ARTICLE, YOU WILL LEARN:

* Why the single-nip shoe press is the next step in press section evolution

* How one shoe press can work as well as two

* The benefits driving the trend for single-nip press sections

* Financial comparison of two-nip vs. single-nip configuration

ADDITIONAL RESOURCES:

* To view previous articles on shoe presses referenced in this article, visit the Solutions! archives at www.tappi.org (click on S! logo)

References

Bottiglieri, J., "Shoe Presses: Dressing for Success", Solutions!, November 2001, pp. 34-38.

Kivimaa, J., "A 'shoe in' for printing papers", Solutions!, September 2001, pp. 77-78.

About the author. Jim Atkins is president of Atkins. Inc., a consulting firm, and serves on the TAPPI Editorial Board.

Contact him by email at [e-mail], or by phone at + 1908 806-8689.