SHOE PRESSES - THE BREAKTHROUGH FOR BOARD
Alexander Wasserman and Rudolf Estermann
After many years in which the board industry hesitated to use them, shoe presses have experienced a breakthrough in recent years, manifested by several units successfully put into operation.
The present article explains why the initial hesitation had been justified, but could be overcome due to further development of the shoe press, persistent research work and the paper mills' courage to make innovations and striving for technical leadership.
First, a brief review of the history of presses for paper and board is given, and then fundamental technical details of the Voith Sulzer NipcoFlex shoe press are explained, which are crucial for the use in board machines. Afterwards, proven press concepts for various board grades are presented and their efficiency in terms of mechanical drainage is outlined. Finally, results from trials and practical applications are discussed.
Technical developments in the press section
Quantum leaps of mechanical web drainage
The static press was suited to dewater sheets in the batch mode, pressing them between woolen felts at a constant pressure for a certain time.
From the time when the continuous manufacturing process was introduced, the roll press was standard. Single- or double-felted presses allowed to impart different surface properties to the web. Compared with the static press, the shorter nip dwell time turned out to be a disadvantage with increasing speed. Larger roll diameters were not much of a success, and this was the beginning of the shoe press age.
Voith Sulzer put the first closed shoe press into operation in 1984 and is the market leader today, with installations in the field of graphic papers, tissue, pulp dewatering, packaging papers and board.
Development of shoe presses for board
Initially, the board producers hesitated to use shoe presses. Although the first shoe press had been installed ten years ago, only four shoe presses were in operation after seven years. Two years ago, a run on the shoe press technology started, which is still continuing.
Only recently, the 200th Voith Sulzer shoe press has been sold for a board machine.
Hesitation about introducing the shoe press technology for board
Although the shoe press technology had already proven a success for different paper grades, there seem to be three main reasons for the sluggish introduction of this technology in the production of board:
Technical details of NipcoFlex press for board production
Disadvantages of plateaus in the pressure curve
Figure 1 shows the typical pressure curve of the hydrostatic press shoe used in the beginning as a function of the nip dwell time. The lubrication pockets in the shoe cause a pressure plateau acting similarly to the static press namely at constant pressure over a period of time.
The introduction of the hydrodynamic shoe allowed a smaller maximum pressure in the nip with identical line force and shoe length, contributing essentially to the densification of the web. The advantage for board is quite obvious; the hydrodynamic shoe causes a lower reduction in specific bulk and bending stiffness than the hydrostatic shoe.
Pressing operation and product properties
When comparing the pressing operations in a conventional roll nip or hydrostatic shoe nip with those in a hydrodynamic shoe nip, the differences are as follows:
During drainage, steep pressure gradients result in high flow velocities, possibly causing fibre displacements in the form of crushing.
In a shoe nip with a gradually ascending pressure profile, the web structure in z-direction shows more uniform densification over the web thickness. The resulting advantages for specific bulk and porosity will be explained in more detail later on when talking about the results.
Comparison of pressure profiles of roll presses and shoe press
From a diagram showing the nip pressure versus the shoe length (see figure 2), it becomes evident that despite low line forces, maximum nip pressures of more than 60 bar can occur in a board machine press section commonly used 10 years ago. Nevertheless, several nips are required to achieve a sufficient drainage capacity.
In a single double-felted shoe press of 300 mm shoe length, however, the maximum pressure of 40 bar with a line force as high as 750 N/mm is more than 30% lower than in roll presses. Naturally, the drainage capacity of a single shoe press is often too high for the water absorbency of the press felts available today; therefore, three nips with five felts on an average are still used.
NipcoFlex press with solid mating roll
In press sections without rolls compensating for deflection, as commonly used today, the rolls must be designed with a certain crown (see figure 3), which, however, is never suited for the entire basis weight range. The NipcoFlex shoe press, whose cross section is shown here, ensures a constantly straight nip at any line forces due to an oil pressure identical for all shoe loading elements. A lower pressure is only used for the edge elements in order to increase the lifetime of the rotating sleeve subjected to highest loads at the edges.
The solid mating roll, as shown in this transparency, is sufficient for board machines in most cases, since higher demands are made neither on the machine widths nor on the maximum line forces.
Press concepts used for the production of board
Press section for coated duplex and triplex board
A typical press section for white lined chipboard is equipped with a double-felted suction press installed in the first nip today often designed with closed web run from the wire. A double-felted shoe press is installed in the second nip and a single-felted roll press in the last nip to achieve the required smoothness on the topliner. The stated dry contents and amounts of water removed make it clear that the highest amounts of water are still discharged in the first nip and not in the shoe press. The final dryness, however, is already achieved ahead of the last nip.
For a closed web run (see figure 4), as already used in the first high-speed board machines today to reduce the transfer times to a minimum, a second pick-up suction roll is recommended to be arranged between the first and second nips. An inverted shoe press has the only purpose of ensuring an optimum web run without influencing the board quality.
Press section for uncoated duplex board
Heavier grades are still produced on cylinder mould or suction former machines. In such a case, two shoe presses first press double-felted and second press single-felted - are recommended to be installed after the couch press. This arrangement permits achieving considerable drainage even in the last nip and increasing the dry content.
For chipboard or cardboard grades where surface smoothness is no longer important, maximum dry contents can be achieved by an additional felt in the last nip (see figure 5).
Results from trials and practical applications
Dryness versus line force
Figure 6 shows test results from the pilot facilities where the drainage capacity of two double-felted shoe presses was examined. Board samples of 400 g/mē were run into the press in as moist a condition as possible. The development of the dryness as a function of the sum of the line forces of both presses was shown in a diagram. It is quite amazing that even the highest line forces of 800 N/mm in the first nip and 1000 N/mm in the second nip did not cause any crushing of the web. The dryness was enhanced from 18% to more than 40% in the first nip.
Specific bulk versus line force
With the basis weight shown in figure 7, an increase of the sum of line forces from 600 to 1200 N/mm which means doubling the total line force did not result in any further reduction of the specific bulk.
Dryness after press section
Figure 8 shows results achieved after the rebuild of a board machine for coated duplex and triplex board. The dry contents in all press positions before and after the rebuild are shown. The dryness increase of about 3%, as shown here, met the customer's expectations. It goes without saying that the target values of the bending stiffness were achieved.
Dryness versus porosity
Tests for sack paper and gypsum board have already shown that the shoe press has a very positive influence on porosity. In the case of white lined chipboard, the effect on the product itself is less important, whereas the effects on the specific evaporation rates are definitely to be considered.
Figure 9 (missing) summarizes results of porosity tests carried out in the Voith Sulzer pilot facilities. The four sections show the dryness and porosity at different line forces of the two press nips used. The section on the far left shows that in roll presses a higher line force in the second nip causes an increase in dryness of about 1.5%, while increasing the Gurley value by approximately 4 s. The second section shows that a shoe press alone achieves considerably lower Gurley values with equal parameters. Even when doubling the dryness, the Gurley value does not exceed the value achieved in roll presses.
As expected, a single-felted roll press installed after the shoe press does not increase the dryness either. The Gurley values are identical to those in roll presses.
If two shoe presses are used, the highest dry contents will undoubtedly be achieved, but even with an increase of as much as 4% the Gurley value will not rise by more than 2 s. This means that only in shoe presses the gradient of porosity deterioration is lower than in all combinations with roll presses.
Dryness versus roughness
The roughness of the base board is, among other things, defined by the effect of a smooth press roll in the last nip.
Figure 10 shows roughness and dryness versus the line forces of two nips under the same conditions as before.
Again on the far left, it can be seen how the Bendtsen roughness is reduced with different loading of the smooth nip. The second concept with shoe and roll press installed downstream produces a different result. First, an increase in dryness reduces the roughness. If the web, however, gets too dry, smoothing by the roll nip will no longer be possible. Two shoe presses produce a quite different result. Compared to the other concepts, the roughness is significantly reduced due to the long dwell time on the smooth press roll. In order to further reduce the roughness even with maximum dry contents, the line force in the last nip has to be increased.
This advantage should also be mentioned in connection with Yankee machines where the roughness of the base board leaving the press is of particular importance, since a low roughness ensures that the board sticks uniformly to the dryer owing to its higher contact surface.
Roughness versus line force
Another question asked frequently is how the structure of the felt influences the surface roughness at the line forces used.
Figure 11 shows the development of the roughness versus the sum of the line forces of two shoe presses, once with single-felted and once with double-felted last nip. It should be noted that instead of board, liner of 120 g/mē was used.
It is again obvious how single felting influences the roughness level; on the other hand, this transparency shows that only a minor increase in roughness is to be expected with a high increase in the sum of line forces.
Dryness versus bending stiffness
When comparing once again a combination of double-felted shoe press and single-felted roll press with two shoe presses with identical clothing and considering the influence of different line forces on the dryness and bending stiffness, a clear picture will again be given (see figure 12):
In the first case, the dryness is about 3.5% lower than in the second case at equal bending stiffness. It should be noted that with increasing line forces in the shoe press the bending stiffness curve flattens out. The reason is that compared to the rise in maximum pressure, the pressing impulse is much more increased than in a roll press. Practical experience has also shown that at a line force of approximately 400 N/mm and more, no further densification can be determined in white lined chipboard.
To sum up, the advantages of the shoe press technology in board machines can be described as follows:
Voith Sulzer offers the ideal press concept for the various board grades. As already mentioned, the 200th NipcoFlex shoe press has been ordered; this is why we can call it a proven and advanced technology. Due to the number of references, the confidence in the technological advantages will continue to grow.