New tissue pressing technology: soft on the Yankee

A new approach to pressing, which uses a flexible nip instead of rigid steel to deliver the pressure, will give a uniform nip load across the Yankee over a wide range of linear loads. Here we take a brief look at the new ViscoNip and the possible benefits for tissue makers.

Perini Journal

A new tissue pressing method that could have some very interesting benefits for tissue makers has recently been patented and launched by Metso Paper. Following a three year research and development program aimed at reducing energy consumption, while also offering the possibility for improved tissue quality, the company has introduced the Advantage ViscoNip press. The key feature of the technology is a multi-chamber pillow which effectively delivers the nip pressure against the Yankee dryer (see Fig. 1).

Metso says that extensive pilot machine trials at its Karlstad, Sweden, tissue technology center confirm that the new press design can give higher dryness, better bulk and improved runnability compared with either a suction press or a shoe press. One of the most important benefits, it says, can be considerable savings in drying energy consumed, on the order of 20-30%. In addition, fiber savings of up to 10% are possible if the tissue maker chooses higher bulk.

“THE KEY THING WITH THE VISCONIP,” says Ingvar Klerelid, Vice President of Product Management at Metso Paper Karlstad, “is that it conforms to the shape of the Yankee so that we get a uniform nip load over a very wide range of linear loads, from 80-160 kN/m. Instead of using a rigid roll or shoe against the Yankee we have chosen something much more accommodating that delivers the pressure where it is needed in a uniform manner. This is truly new.”

The technology is based on a simple but effective “pillow press” arrangement. The press consists of hydraulically-loaded polyurethane chambers which are positioned parallel to each other in the cross-machine direction. The chambers apply pressure to a belt which in turn presses the sheet. (see Fig. 2). A smoothing sheet is placed between the polyurethane chambers and the belt to reduce friction on the pillow.

COMPLEX SCIENCE OF YANKEE DEFORMATION. “The ViscoNip press arrangement,” explains Magnus Hultcrantz, R&D Project Manager, “is a very conforming counterforce against the Yankee. This is extremely important because of the deformation of the shell. The shell, which is relatively thin and flexible, is being subjected to deformation due to three main variables: the internal pressure, the surface temperature variations and the linear load applied by the press roll or shoe. The outer edges of the shell near the heads are especially troublesome (see Fig. 3) because there is very little buckling taking place there, due to the support from the heads.”

“The edges”, says Hultcrantz, “have always been the place where the most problems occur when using a shoe press or a suction roll, as these press rolls have a hard time adjusting to the change from the buckling in the center to the rigid edges.

Crowning, tapering and edge relief systems are used to minimize these effects but still they persist. The new ViscoNip, on the other hand, conforms to the Yankee surface geometry independent of running conditions, meaning that the issues of crowning and edge effects become much less important.” The design also gives much greater flexibility to make adjustments (see Fig. 4) on the run says Hultcrantz. “Since each chamber can be loaded individually with a different pressure, the operator can custom-tailor the pressure curve that best suits his needs for product quality or higher dryness. In addition, this pressure curve can be changed on the run which gives enormous flexibility.”

HIGH DRYNESS SAVES ENERGY. Tests at Metso show that linear pressures of up to 160 kN/m or even higher can be applied with the ViscoNip, which means that after-press dryness can be raised accordingly. With energy prices near all time highs, the energy saving feature of the new concept is extremely advantageous. “If you can increase the dryness by 5%, the energy for drying is reduced by more than 25% and for a high speed 5.3 m wide tissue machine in central Europe the cost saving is about 600,000 Euro per year,” comments Klerelid. “That is a significant cost saving. Of course the Yankee shell must be able to take the load but the pillow press allows incredible flexibility in distributing the load. It adapts to where the pressure is needed.”

ESPECIALLY SUITED FOR REBUILDS. Papermakers now have a choice between a suction press, a shoe press, or the new ViscoNip. Klerelid says the shoe press can be a good solution for a new machine when the Yankee can be designed to handle the loads from the press. “This has also been proven by the success of the SymBelt TIS press running on the new machine at Al Sindian in Egypt, but for rebuilds the ViscoNip is far superior due to the self-profiling ability. We have been very skeptical about using the shoe press with an existing Yankee. Due to deformation complexities, we are certainly not promoting our shoe press solution for rebuilds. If it is a new machine where the Yankee is designed to run against a shoe press, it can work very well. But for rebuilds we feel there is a risk a shoe press won’t work. The ViscoNip is clearly superior for a rebuild.”

FLATTER PROFILES FOR FASTER SPEEDS. An added benefit that has been proven in pilot trials, says Tord Gustavsson, Metso’s R&D Project Manager who came up with the ViscoNip idea and has led the project, is a much better CD dryness profile across the sheet. So not only is runnability through the press improved but also through the rest of the machine and it will most likely result in better runnability through the converting process. Final product quality is also enhanced by the flatter dryness profile. “The uniform nip pressure gives a much flatter dryness profile across the sheet. The coating will be more uniform which means the creping is more uniform which all lead to a flatter profile, which is the key to faster speeds and better productivity. It is all related to the uniform nip pressure.” •

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