[Home] [APPW 2004] [Journal papers]




Reinhard Leigraf


Voith Paper


copy paper, shoe press, single shoe press, gapformer, roughness, two-sidedness, bulk






The single nip shoe press has been developed to further improve machine productivity and efficiency. This new technology has been successfully installed in a fast running copy paper machine in Slovakia. First results show a high after press dryness, sufficient bulk, and excellent roughness two-sidedness. The press is equipped with longer felts and an advanced felt conditioning system to handle the high nip loads and dewatering demands. As a result, felt life times of more than 25 days have been achieved. A steam box has been installed in front of the nip to facilitate better dewatering and a traversing scanner is positioned under the first drying cylinder to monitor the moisture profile.

This concept is very economical for the following reasons:

  • lower power consumption for the vacuum and the drives
  • reduced shut down time
  • lower costs for sleeves and clothing.

Because of these benefits Mondi has decided to purchase its second single nip shoe press for installation in their new copy paper producing machine in Merebank, ZA.

This space saving press concept is an attractive concept for rebuilds as well. In the near future more single shoe presses will be installed for a wider range of grades and applications.

1. Introduction

Copy paper is the most important sub-grade of the wood-free uncoated papers market. For this reason a number of wide and fast running new machines have arisen on the market. The total world wide consumption of wood-free uncoated paper in 2003 was 44 billion tons while the total consumption of copy paper was 30 billion tons (Figure 1, below). The highest consumption is observed in North America and Western Europe. Europe and North America combined account for half of the world's consumption. China and Other Asia follow with 14% each; Japan with 8%; Africa and Latin America consume 5 and 6 %, respectively.

Figure 1


The growth figures of 2003 show another notable aspect (Figure 2, below). Some indication of saturation can be noted for the regions with the greatest consumption. The market growth in the regions with highest consumption is slightly negative. African market growth was 0,8 %. Latin America, Japan and other Asia displayed similar market growth. The highest growth figures were observed in China with 4,8% and Eastern Europe with 2,8 %.

Figure 2

The growth of the Eastern Europe market is related to the changes in the political situation over the last several years. The fall of the Iron Curtain set into motion political and economic changes the results of which are that 8 of the 10 new European Union countries are from Eastern Europe (Figure 3, below). The most recently completed EU expansion will generate a boost to the local economies of these countries as they gain access to new markets. Owing to these developments, the Austrian company Neusiedler AG (100% subsidiary of Mondi) purchased a 50% share in Slovakia's Ruzomberok pulp and paper mill. This mill operates 3 paper machines and a pulp plant.

Figure 3


2. A reference machine for copy paper production

In order to capitalize on the expanding markets, Mondi/Neusiedler decided to increase the production of copy paper on PM 18 from190.000 t/y to 290.000 t/y. This was achieved by increasing the maximum operating speed of this 7,3 meter machine from 900 m/min to 1400 m/min.

The components selected are as follows:

  • The wire section was rebuilt to a horizontal gapformer in order to re-use existing rolls.
  • The headbox selected was a state-of-the-art MasterJet G Headbox with the ModuleJet dilution water system for basis weight control. The MasterJet G with ModuleJet dilution control enables the papermaker to adjust the fibre orientation with the spindles while at the same time not adversely affecting the basis weight profile.
  • The press section was rebuilt to a single nip shoe press, the first of its kind in Europe. Due to the fact that this PM is a "single grade machine", it was expected that optimisation would be much faster and easier. The first results and the advantages will be discussed later in this article.
  • The drying section of the machine was rebuilt to handle the higher production speeds.
  • The hard nip calender was equipped with a new zone controlled roll for optimum caliper profiling.
  • The SpeedSizer and the pope reel were still suitable for this high speed.

Please see figure 4, below.

Figure 4

3. Key properties for copy paper

Many of the key paper properties are influenced by furnish. Paper properties are further influenced by the different machine sections. For example, the former removes the water from the suspension to build a sheet and therefore influences the formation. The task of the press section is to remove as much water as possible from the sheet. In addition, the press influences the key properties of stiffness, bulk, roughness, and two-sidedness.

4. The Single nip shoe press (Figure 5, below)

The single nip shoe press consists of a stack with 2 rolls. In the bottom position there is a NipcoP-Roll. In the top position there is the NipcoFlex shoe press roll with the grooved sleeve . The rolls are connected with anchor bars. The maximum line load of this press is 1400 kN/m. The stack is inclined so that splash water is more easily collected. A steam box is installed in front of the shoe press to heat up the water and as a tool for moisture profile improvement.

Because of the extremely high demands on press felts, the press felt life was a major concern. Therefore the 2 press felts are longer than those in a typical design to extend life time. Proper felt conditioning was imperative as was the speed and ease of felt changes. Both felts can be changed in less than 3,5 hours even though the old building does not allow for an optimum procedure.

Felt separation occurs directly after the nip and is aided by a suction box. The paper is transported with the bottom felt and picked up by a suction roll for the transfer into the drying section. Underneath the first bottom cylinder there is an EnviroScan measurement positioned which measures the moisture profile after the press.

Figure 5

5. Technological Results

The most critical issue was the dryness after the press section. According to previous trials on the trial paper machine, a minimum dryness of 49% was expected. The results achieved, many of them without steam boxes, exceeded the expectations. Depending on furnish, felt life, and measuring accuracy, the mean dryness value was 53 %. (Figure 6, below).

Figure 6

Because of the high dryness levels achieved, the machine speed could be raised very quickly from 1140 m/min to 1260 m/min. It was at this speed that the main optimisation work had been done. Today 1400 m/min was achieved for continuous operation. Because of the run-in-time of the felts following a felt change, the resulting average production speed is somewhat lower (Figure 7, below).

Figure 7

The second most critical issue was bulk. Bulk is directly proportional to after press dryness. A bulk of 1,32 cm/g is achieved at the target roughness of 220 ml/min Bendtsen in Ruzomberok. These are very good results when considering the after press dryness of up to 53% (Figure 8, below).

Figure 8

The relationship between dryness, bulk, and roughness is shown in Figure 9 (below). The data was collected from a pilot trial. The single nip press was loaded with three different line loads resulting in dryness values between 46 and 50,5 %. The corresponding bulk values showed a decrease from 1,61 to 1,53 cm/g over that range. The calender further decreases roughness and bulk to the same extent. Roughness and bulk curves were parallel. After calendering to the target of 200 ml/min-m, the bulk differences were found again. These results are typical and independent of the calender type. So the message is: Bulk is a function of dryness after press.

Figure 9

The next issue of attention is the roughness two-sidedness (Figure 10, below). Roughness two-sidedness is determined mainly in the press section. To be more specific, the last press nip in combination with line load is most influential. The worst press situation with respect to roughness two-sidedness is a press concept such as the 3nip press. The topside of the paper contacts the felt three times whereas the bottom side has contact with a felt only in the first nip. Two-sidedness values between 25 80% are achieved in a typical 3-nip press. A straight through press with a felt on bottom side following the previous press stacks can improve the situation. Here the two-sidedness is a little better and in the range of 10 30% . When using the straight through press after a shoe press, the last press can be used only for two-sidedness adjustment and not for further dewatering.

Figure 10

With a single NipcoFlex press, the last and only nip is clothed with two similar felts. Therefore, the two-sidedness is minimal. The results in figure 10 are measurements from Ruzomberok PM 18. The roughness varies from 220 ml/min Bendtsen by approximately -15 to +15 % with an absolute difference of 30 ml/min. The observed variation in roughness varies according to the age of the felts.

An added advantage of the low roughness two-sidedness after the press is that an economic hard nip calender can be used at the end of the machine instead of two soft nip calenders.

6. Economic Benefits

Felt life is the most important economic issue concerning this press concept (Figure 11, below). Based on previous experience with shoe presses as well as trial experience, a successful felt design was installed from the beginning. A felt life of approximately 20 days is currently routine. There is always room for further improvements in felt design. In the future, a minimum felt life of 30 days should be a realistic.

Figure 11

Compared to other press concepts the felt costs are lower than those of a two nip shoe press, but higher than the felt costs of a 3-nip press (Figure 12, below). Sleeve costs are nearly the same or lower.

Figure 12

The main benefits of this concept are costs - both initial investment and operating costs. A single nip press means less rolls, less framing, and less conditioning. The costs are 20 to 60 % lower than of traditional concepts. Less vacuum rolls and uhle boxes means there is a reduction in vacuum requirements. Less press rolls mean a reduction of installed drive capacity. Less equipment also means reduced down time for press maintenance. These numbers are calculated for a typical case. Regional differences in costs may differ. Due to the economic and technical benefits Mondi selected this concept for it's number 31 machine in Merebank.

7. New PM 31 at Mondi Merebank

The new PM 31 (Figure 13) replaces the PM1 and is designed for a maximum operation speed of 1300 m/min and a paper width of 5800 mm at the reel. The PM is equipped with a headbox and dilution water system. The forming section will be a new vertical gapformer bringing with it advantages in quality, operation, and maintenance. As in Ruzomberok, a single nip shoe press follows the wire section. For optimal runability at the high speed a single tier pre-dryer was selected. Following the pre-dryer section is a metering size press. The after dryer section is a combination of single tier and double tier for curl control.

Figure 13

A hard nip calender at the end is equipped with a Nipcorect roll. This calender is prepared for a soft calender concept in case of future higher surface quality requirements. At the end of the machine a MasterReel will be installed. The MasterReel is a pope reel capable of sensitive pressure measurements of the real load. The start up of this machine will be in autumn 2005.

8. Summary and Outlook

The single nip press concept can be used for wood-free copy paper production. This concept is released for a basis weight range of 60 - 120 g/m and a maximum speed of 1400 m/min. As discussed in this article, the benefits are high dryness and good paper quality combined with economic operation costs. The single nip press's low space requirement is conducive to rebuilds.

The next developmental step will be the extension of the basis weight range. Especially challenging for this concept will be higher basis weights produced at high speeds. Another machine with a single nip shoe press is already in operation for a basis weight range of 57142 g/m. Further improvements in surface quality are also targeted.

The single nip press has already been shown to be a success. In the future there will be more single nip presses installed for a wider range of applications.