Steady State Pressing II
Rob Hansen, Geschmay
Modern paper machines have reached speeds exceeding 1.900 MPM in publication grades, 1.500 MPM in packaging grades, and 2.000 MPM in Tissue. The increasing demands placed upon paper machine clothing are also increasing at an exponential rate as well, and many developments have occurred in the area of press fabric construction, which further enables the paper maker to achieve these fantastic goals.
In previous publications, we have defined the behavior of press fabrics in a macro sense, and have described their behaviour over time compared with different performance parameters, such as fabric caliper, fabric void volume when compressed, fabric air permeability, or fabric water permeability, for example. Their behaviour over time can be observed in Figure 1.
Note that most fabrics change over time in an exponential fashion: they experience a condition during start-up, which may indeed lie outside of the traditional range of acceptable performance. This is most often deemed the 'start-up phase', and the typical press fabric will experience varying degrees of this start-up phase, depending mainly upon the mid-nip water handling capacity of the fabric, the density of the fabric, and of course, the compressibility of the fabric. Further examination of this curve also reveals that there is a certain time in any press fabrics usable lifetime at which the press fabric ceases to function properly, and causes one problem or another, albeit press vibration, sheet crushing, fibre shedding, etc. It is at this point of 'minimum acceptable performance' where the press fabric needs to be removed.
An example of this phenomenon is clearly visible when comparing the new and used photographs of a typical tandem press arrangement on a modern high-speed paper machine producing newsprint (Figure 2).
Changes in press fabric design can, and do, affect this relationship to a great degree. A design change can significantly flatten this curve, minimizing the rate of change over time. However, a change in the slope of this curve can also imply negative impacts regarding start-up, for instance. In fact, this is quite often the case, as in many cases heavier, higher void volume press fabrics are provided in order to lengthen the product's usable lifetime, only resulting in a drastic lengthening of the start-up curve! This is of course particularly true for pressure controlled paper sheets, such as newsprint or LWC.
Note that an ideal press fabric would exhibit a very fast start-up and perform without any significant change in any of the aforementioned performance parameters, and thus operate in perfect Steady-StateTM Performance. This is obviously the ideal that we strive for, and many of our internal studies have focused on the various ways to achieve this ideal. Of course, the press type, paper grade, press loads, temperatures, pressures, speeds are all involved, and many different solutions can be applicable, depending upon the specific application. However, throughout our product development, a distinctive trend was becoming obvious: the inclusion of single monofilament base weaves in press fabrics had a profound and significant effect on the performance qualities of press fabrics, both in the pressure controlled paper grades as well as the flow controlled sheets, such as linerboard, corrugating medium, or pulp.
Initially, these single monofilament structures were utilized primarily in seamed press fabrics. The reason was that the single monofilament yarn was used to form a loop within the press fabric structure, allowing for a pintle to be inserted through these loops, thus seaming (i.e., closing and connecting) the fabric directly on the paper machine. This not only added convenience and time saving opportunities for the papermaker, but it also increased the safety of the crews during press fabric replacement.
It is extremely interesting to note, however, that the performance of these seamed press fabrics was exceptional on many positions, frequently outperforming their 'conventional' endless competitors which utilized the traditional twisted monofilaments or twisted multifilaments in the base weaves. The dewatering characteristics, for instance, in many positions resulted in clearly improved dewatering performance on a variety of positions with a variety of paper grades.
All of these benefits resulted in a drastic rise in the utilization of seamed press fabrics, which can be observed in Figure 3, which depicts the expansion of single monofilament technology within the Albany International family of companies (the inventor of seamed press fabrics).
Clearly, after the invention of the technology in Sweden, it expanded rapidly in North America, and has now extended itself back to Europe and Asia.
Of course, the clear benefits of using single monofilaments in modern press fabrics became a reality due to improvements in raw materials, tension control in the weaving process, and improved process knowledge in order to produce fabrics from these relatively unforgiving materials. Previously, problems in processing were magnified due to the lack of forgiveness, or elongation, of these materials, thus necessitating the need for utilizing twisted yarns in base fabric weaves. However, the most modern machines available, typical of those used in Geschmay, Goeppingen, an Albany International Company, for instance, can now handle these materials in an effective and productive way.
But why do these single monofilament base fabric designs improve the performance on the paper machine?
A primary reason for the improved dewatering can be seen in Figure 4.
This graph depicts the difference in air permeability between many common base fabrics, typically used in modern press fabric constructions. Clearly, the single monofilament base fabrics are significantly more open than base fabrics utilizing twisted yarns, and this provides significantly lower flow resistance within the felt structure. This reduced flow resistance aids in sheet dewatering significantly, as can be seen in Figure 5.
Note that this technology has also shown increased resistance to compaction over time. Profiles are also more uniform, showing less change over time, as can be seen in the following figure:
As you can see from the curves, this particular fabric ran on the pickup position of a high-speed newsprint machine, and the profiles showed very little change over a 5-week run!
This technology has proven to be a revelation at the firm Geschmay, which now offers a variety of single monofilament constructions to suit all press types and paper grades. The following is a short description of the different varieties of single monofilament press fabrics available from Geschmay:
These integrally woven press fabrics are woven utilizing 100% single monofilament yarns in the base fabric, and can be delivered in 1.5, 2, 3 and 4 layer constructions. Note that these fabrics are of course available with and without seam! An example of a typical construction can be seen in the diagram above. (See diagram) These fabrics have proven themselves in a variety of applications, including High Speed Publication Grade paper machines in the typical 'cluster' arrangement (i.e., Beloit Tri-Nip, Metso SymPress II, Voith Duo-Centri III, etc.), in the new Metso Optipress II machines, and the Voith Tandem Duo-Nipcoflex machines. They are also superb performers in the flow controlled paper grades, in both roll press and shoe press applications.
This family of single monofilament press fabrics distinguish themselves from their HydroProTM cousins in that these fabrics are multi-axial press fabrics. The multi-axial press fabrics differ from their endless woven cousins in the fact that their yarn systems are not woven in a perpendicular fashion, but are in fact at an angle to one another, providing yarn systems in 4 different directions. An example of this technology can be seen in the diagram.
The multi-axial nature of this product lends an even higher degree of compaction resistance than is evident with integrally woven products; this being due to fact that conventional woven products tend to collapse over time, while the multi-axial nature of the HydroMax2 TM prevents this collapse! This results in superior compaction resistance, excellent pressure distribution, and excellent dewatering characteristics. An example of the dewatering potential of this design can be seen in Figure 7, which depicts the water removal of a HydroMax2 TM fabric compared to a conventional II+I laminated press fabric on the pickup of a German high speed paper machine (1.650 MPM) producing SC grades. The superior water removal of the HydroMax2 TM is clearly visible.
Of course, this technology is available from Geschmay with or without seam!
The superior dewatering characteristics of the single monofilament HydroProTM designs can be combined with the superior compression properties of our HydroDuctTM in order to deliver a free draining, compaction resistant press fabric that improves on the vibration resistance and long running times of our standard single monofilament integrally woven designs. The knuckle-free HydroDuctTM layer provides improved strength and longevity, while also adding dampening to the structure – ideal for vibration prone press positions! This design has run on some of the world's fastest and most demanding paper machines! Also available with and without a seam!
Lastly, the FusionTM line of press fabrics provides exactly what it says -the fusion of multi-axial technology with our proven HydroDuctTM technology within a single press fabric. The combination of single monofilament multi -axial construction with the superior pressure distribution and dampening capabilities of our HydroDuctTM allows an extremely versatile press fabric, suitable for many press section applications, ranging from blind-drilled roll jumbo (Large diameter roll) presses to High Speed Publication third and fourth presses. The combination of these technologies can be utilized to provide a very thin and uniform fabric, resulting in fast start-ups and Steady-StateTM operation throughout life.
In summation, it is clear that the obvious advantages of single monofilament press fabrics have resulted in a variety of new modern press fabric designs capable of flattening the curve of improved press fabric performance. The various combinations of structures all lend themselves to different applications, and must be custom designed with the help of a trained press fabric design engineer.
Tel.: +49 (0)7161 / 604-134