FEATURE: Pressing & Drying
Single NipcoFlex – breaking out to new paper grades
It is now more than six years since the first Single NipcoFlex press was put into operation. It quickly developed into a bestseller. Then, as now, the low investment and energy costs were especially appealing. Today seven Single NipcoFlex presses are being used to produce wood-free copy, writing, and printer paper. In the future, the presses will also be able to be considered as alternatives for coated and wood-containing paper.
These days, budgets for new machines and rebuilds are being cut massively, leading to the obvious desire for a low-cost but high-performance press section, which should then also be characterized by low operating and especially low energy costs. The requirements list sounds like an unattainable dream. However, Voith believes that the installation of a Single NipcoFlex press can make this dream come true.
Apart from investment and energy savings, maintenance and clothing savings are also possible.
A valuable treasure trove of experience
There are currently seven Single NipcoFlex presses in operation. In August 2006, PM 1 in Docelles, France, was converted to a Single NipcoFlex. This represented a technological challenge. For one thing, very demanding qualities, in part with high smoothness values, are made on this paper machine. For another thing, a multitude of grades are produced, meaning that speed, furnish, and additives as well as basis weight must be changed frequently. The latter, especially, was classified as critical in advance because a real test of the process stability with frequent grade changes cannot be realized on the pilot paper machine.
Through the combination of the single press with two Softnip calenders, the quality specifications could be achieved. The machine speed and stability improved step-by-step, with the use of Voith press felts especially contributing to the progress. The moisture profile was significantly improved. This, in turn, enabled the moisture after the pre-drying section
to be increased. The drying capacity thereby gained was then immediately converted to an additional increase in speed.
With the experience from the single presses that was now available, start-up of the next two new machines with Single NipcoFlex presses was nearly child’s play.
In April 2008, PM 1 in Khon Kaen, Thailand, was put into operation, and in February 2009 the PM 1 in Tres Lagoas, Brazil, was started up. The stable, high solids contents of both machines made it possible to increase the speed significantly in the shortest amount of time. Thanks to the furnish, a very good bulk and a high smoothness can be achieved in Khon Kaen with the Single NipcoFlex press at extremely high solids contents. Even with the load on the press eased considerably, press solids above 50% are achieved on this machine.
Good news can also be reported from the six-year-old single press being used on PM 18 in Ruzomberok, Slovakia. It is continuously improving its production speed. By now copy paper has already been produced at over 1,600 m/min. This is an impressive achievement for this paper grade and simultaneously a world record for single-nip presses. PM 18 was originally designed for a maximum speed of 1,400 m/min.
In contrast, PM 1 in Ledesma, Argentinia, is the first machine with a Single NipcoFlex that is being used in part to produce high-quality coated grades. This demonstrates the potential of this design.
Single NipcoFlex – what’s next?
Due to the success of copy and wood-free uncoated paper, the question naturally arose as to whether or not a Single NipcoFlex could also be used for projects with other paper grades. Within the framework of an extensive development project, Voith investigated the possibilities and limitations of the Single NipcoFlex for other paper grades.
The challenges are manifold. Production of wood-free coated paper arises as the next application case. Purely in terms of the dewatering behavior and the expected pollutant load, the furnish qualities used are very similar to those used for copy paper. In extensive process trials with subsequent print tests, the quality differences in comparison with conventional press designs were investigated. The quality of the paper from the Single NipcoFlex was definitely convincing. Fears that the surface quality in particular could suffer from the hefty pressing in a single press nip were clearly crushed.
As expected, papers from various press systems differ in terms of parameters such as bulk, porosity, and internal bond strength. For this reason, for every single customer project, the advantages and disadvantages of the various design alternatives must be weighed. The single press can now also be viewed as an alternative to the tandem press in particular, especially if the overall machine design provides a pre-calender and a starch application.
Greater obstacles must be overcome in the use of the Single NipcoFlex with wood-containing grades. Considerably higher peak pressures in the press nip are necessary in order to achieve the required solids content for fast machines. Besides this, the furnish used contaminates the felts more than it does with wood-free paper. This occurs, for example, with recycled paper-containing furnish with high stickies content. Particular attention must hence be paid to the combination of clothing used and suitable machine construction during development. Voith is ideally equipped for these development types in particular. Clothing and mechanical engineering experts work hand-in-hand so that this goal can be promptly achieved.
Trials with surprising results
How high can the press solids actually be for wood-containing furnish with the use of the correspondingly adapted clothing? The answer even surprised the Voith experts.
With just a single press, newspaper could be produced on a trial scale at a speed of 2,000 m/min, with a solids content after the press of much higher than 50%. The basic requirement that a press design must fulfill for high-speed machines was therefore met. How, though, are these high solids contents possible after a single shoe press nip?
Even in classic press systems (considerably lower) and even in the two shoe press nips of the tandem press together (barely higher), dewatering performance is achieved. Two factors are decisive for this.
The Single NipcoFlex uses a shoe length that achieves a considerably higher dwell time in the press nip than in the sum of three or four roll nips in series. In addition, the fiber web is dewatered on both sides along the entire press length. The two felts in the Single NipcoFlex have a difficult task to accomplish. They have to handle the total water and pollutant load in the press section. In doing so, they must work stably to ensure that process fluctuations have practically no effect on the dryness after the press.
The compensating effect of several press nips in series does not exist in the single press. Hence, it will still take some time before the Single NipcoFlex press also becomes a standard design for wood-containing stocks and extremely high speeds.
Despite this, in the future the Single NipcoFlex press design will increasingly come into consideration as an alternative for projects in the wood-containing area. It is simply unbeatable in terms of investment and operating costs.
