Equipment & Systems
Experiences With Various Shoe Press Applications
By Pulp & Paper Canada
Shoe pressing has proved its worth and the process is a part of almost all new machines and rebuilds. However, there are pitfalls that are possible when shoe pressing grades such as newsprint, wood-fr...
By Pulp & Paper Canada
Shoe pressing has proved its worth and the process is a part of almost all new machines and rebuilds. However, there are pitfalls that are possible when shoe pressing grades such as newsprint, wood-free papers, white lined chipboard, and laminating board. If the fundamentals of shoe pressing are not completely understood and the press sections are configured incorrectly, the resulting sheet properties may be deemed unacceptable. Several new state-of-the-art presses have been completely rebuilt because the fundamentals of shoe press dewatering were not understood.
A quarter of a century ago, the first shoe press started up making linerboard. The thinking at that time was that the return on investment would come entirely from the steam savings that resulted from the dryness improvement. As it turned out, the profits from increased production and the reduction in furnish costs far outweighed the steam savings per ton. The shoe press created a 5-point dryness improvement over the roll presses it replaced, and this allowed the dryer-limited machine to increase production by 20%. The linerboard strength improvement made it possible to substitute recycled fibres for virgin Kraft pulp, decrease refining energy, and eliminate wet-end starch. This same basic scenario describes hundreds of shoe press installations on linerboard and fluting that have taken place since then and similar results have been realized in the rest of the paper and board grades as well.
The vast majority of press rebuilds and new machines include one or more shoe presses to take advantage of one of the two fundamental benefits of shoe presses:
1. Shoe presses are able to press the sheet to very high dryness levels. Typically this means 4 to 7 points of dryness higher than the roll press it replaces. This results in production increases on existing drying-limited machines and shortened dryer sections and machine halls on new machines.
2. Shoe presses are able to preserve sheet bulk while pressing to high dryness values. Depending on the grade and press configuration, a shoe press can create dryness values 2 to 5 points higher than typical roll press configurations while maintaining bulk.
Shoe pressing for all grades
One way to think of tissue, paper and board is to regroup them into three classifications — density grades, bulk grades, and mutually exclusive grades. Linerboard and corrugating medium make up the density grades because any pressing process that increases the sheet density increases all of the valuable physical properties. These properties include burst, internal bond, short-span compression and ring crush. At nip loads of 700 to 1400 kN/m, the shoe-pressed sheet will have higher dryness and density and hence better sheet properties than a roll-pressed sheet. Since most liner and medium machines run dryer-limited, any press rebuild and all new machines should include shoe pressing for maximum sheet strength, production, and minimum drying energy.
The bulk grades include fine paper, folding boxboard, white lined chip board and others. These grades require bulk to maximize opacity and bending stiffness. As a general rule, increased pressing decreases bulk and therefore one would think that the high nip loading capability of a shoe press would be inappropriate. However, shoe presses have the unique feature of being able to shift the sheet bulk vs. press dryness relationship relative to roll presses, which is illustrated in Figure 1. How much the shift in the press dryness versus bulk curve is, depends on the basis weight, furnish, and press section configuration.
This shift in the dryness verses bulk curve creates several ways to turn a shoe press installation into revenue. The shoe press section can be loaded to create high bulk and equal dryness compared to a roll press section. This bulk can then be used to reduce basis weight and therefore fibre savings. If the shoe press section is loaded further, it will create 2 to 5 points of dryness improvement over a roll press section and still have the same bulk. Typically this higher dryness is used to increase production or shorten dryer sections on new machines.
The mutually exclusive grades include tissue, news, SC, and LWC. These grades all have some kind of strength or porosity requirement which is enhanced through press densification, and some kind of absorption, tear or opacity requirement that is maintained by minimizing press densification. Fortunately, the furnish for these grades consists of high percentages of mechanical pulp and filler, neither of which densifies much in the press section regardless of how much the sheet is pressed. For news, SC, and LWC, the high nip load of the shoe press may cause the sheet to be denser than its roll pressed counterpart, but the difference is so slight, that the small losses in tear and opacity are deemed acceptable in light of the tensile, burst, porosity and, of course, high dryness benefits.
Fortunately, the vast majority of shoe presses are installed in correct configurations and are operated correctly. However, every so often, an opportunity to further optimize a pressing situation presents itself. These usually occur when there is too little understanding of the pressing fundamentals at the time of installation or press clothing selection.
One area of pressing that needs more understanding is the concept of water handling capacity in the nip. Clothing suppliers commonly claim ‘sufficient water handling capacity’ in their felt and shoe press belt designs and later papermakers find out otherwise. An example of this is shown in the shoe press data in Figure 2.
In this example, a single felted shoe press replaced an existing roll press in the third press of a folding box board machine. After the start-up, an on-machine trial was conducted to determine the dryness benefit of new shoe press produced. The shoe press nip load was varied from 350 to 700 kN/m, creating a 3.5% dryness advantage at equal sheet bulk. This was the expected dryness benefit and the project justification was fulfilled. A surprise came at the very high nip loads, however. When the sheet dryness data was plotted against the nip parameter Press Impulse, the non-logarithmic result indicated some fault in the nip. The data in figure 2 should have fallen into straight lines for each basis weight, but this was not the case at loads above 600 kN/m. At the highest nip loads, the rate of dryness increase per nip load decreased. This indicates that a saturation condition had formed in the nip. For whatever reason, there wasn’t enough functional void volume in the felt and/or shoe press belt to accept all of the water the sheet wanted to give up.
Figure 3 shows the dryness verses finished sheet bulk. These data points should also form straight lines if the nip has sufficient usable void volume. The fact that the rate of bulk loss increases with press dryness indicates there is some form of crushing occurring. The fact that no portion of the 250 and 300 g/m2 data is linear suggests that excessive densification can occur long before ‘nip saturation.’ The conventional wisdom is that there is some precise value at which nip saturation occurs, and up to that point there is no fault in the nip. This dryness verses bulk data is evidence that saturation is not a single point, but a continuum.
Table I shows the coated basis weight of the sheet, the dryness before and after the single felted shoe press, and the water removed in the shoe press nip.
It is interesting to note that this single felted nip includes a blind drilled shoe press belt with ~400 g/m2 of theoretical void volume. The water given up by the sheet is significantly less than this void volume and yet the data shows there is somehow too much water in the nip based on the dryness relationship and even more so on the sheet bulk relationship. Additionally, the saturation effect on dryness and excessive bulk loss increases with lighter basis weights, even though there is much less she
et water removed in the nip.
The reason for the dryness and bulk problem is not so much related to the lighter basis weight as it is to the higher machine speed that goes with the lighter weight. In the speed range this machine operates, increasing machine speed means more water remains in the felt and in the blind drilled holes in the belt. The increased speed of the 250g/m2 grade means the dwell time over the felt vacuum boxes is reduced, so the average felt moisture goes up. It also means there is less time for water to wick out of the blind-drilled holes and back into the felt. At the higher machine speeds, felt water that goes into the blind drilled holes of the belt remains in the holes and causes the belt to function as if there were no holes at all. Both of these contribute to the loss of usable void volume in the nip, which in turn creates nip saturation. This water that never leaves the felt or blind drilled holes is responsible for the excessive bulk loss in the sheet and loss of maximum dryness potential. Although there are several interventions that can be taken to optimize this situation, the point is, the need to eliminate nip saturation would not even have been known had it not been for this on-machine trial and the subsequent evaluation of the data.
Figure 4 shows the Dryness verses Bulk relationship for a nip load matrix trial conducted on a machine making 435 g/m2 laminating board. The press section includes a double felted roll press in the first press position and a double felted shoe press in the second press. One of the fundamentals that came out of this trial was that shoe pressing causes the least bulk loss per unit of dryness improvement. The steeper slopes of the roll press loading curve data confirms that the excessive bulk loss is attributable to roll pressing. This excessive bulk loss is due to the high peak pressure and rate of compression of the roll press, and not the nip saturation concept mentioned above. There will always be bulk loss with wet pressing, but roll pressing causes excessive bulk loss compared to shoe pressing. Therefore, if the press section contains both roll and shoe presses, it is important to use the appropriate combination of loadings if bulk optimization is the objective.
Sheet smoothness optimization
The need to create sheet surface smoothness also enters into the pressing picture. As a rule, felts create roughness in the sheet surface, while plain rolls or belts create smoothness. The more dewatering that takes place in a nip, the more the sheet surface replicates the surface it is pressed against. The sheet surface roughness is also a function of formation and the ability of the furnish to replicate the topography that is presented to it. Bleached Kraft is a conformable pulp and easily replicates whatever it is pressed against, while mechanical pulps and unrefined waste pulps of all kinds do not replicate press nip surfaces so easily.
A caveat to the “felts make roughness” rule is that in multiple nip press sections, the next felt smoothes out the high spots from the preceding felt (or former) and then, depending on the nip load or pressure, imparts its own roughness. This is the reason for the parabolic shape to the Dryness versus Bendtsen lines in Figure 5. The result of this 435 g/m2 laminating board trial is that for any given press dryness, the sheet surface roughness can vary as much as 200 ml/min depending on the nip load sequence through the press section.
As we saw in the dryness verses bulk example, obtaining the best possible smoothness is a matter of discovering the optimum press load sequence, and the smoothest sheet comes from the shoe press nip where, even though the total dewatering may be high, the dewatering rate and peak pressure are quite low.
Need for more applied research
Several press sections on new high-speed news and SC machines have recently been completely rebuilt because shoe press dewatering was not totally understood. These press sections contained shoe presses in configurations that have been used for containerboard for years. However, applying these configurations to these paper grades resulted in sheets with unacceptable printability issues. Faults such as dusting and micro striations forced the complete rebuilding of the press sections to the more traditional three nip cluster presses with shoe nips in the third position and conventional roll presses in the fourth positions. There are several schools of thought regarding the mechanisms responsible for the printability issues, but to date, there is nothing authoritatively known that might have prevented this. This suggests there is plenty of opportunity for closer ties between the machine builders, paper companies, and the various research organizations for applied research to keep experiences such as these from occurring again.
Shoe presses have their place in the production of grades from tissue to market pulp. Their ability to press the sheet to high dryness, preserve bulk in some grades and create density and strength in others justifies their inclusion in almost all press rebuilds and new machines. The high dewatering capacity of shoe presses means they can also cause huge changes in sheet strength, caliper, optical and surface properties. Understanding the ways in which shoe presses are fundamentally the same and also fundamentally different from roll presses is the key to realizing their benefits and not experiencing their pitfalls.
David Lange is the product manager – presses and Robert Meitner is an engineer for Ksters Paper Machinery.