Dryer clothing for Perlen PM 4

Marcus Hahn

Today, paper machine dryer clothing is expected to fulfil many more functions than simply guiding the sheet safely through the drying section of a paper machine.

In view of the fact that in state-of-the-art paper machines, such as the new PM 4 in Perlen, increasing speed requirements play a major role, the tasks to be met by dryer screens have also risen enormously.

In view of selecting the correct design and suitable materials for dryer screens, the various machine parameters such as dryer configuration, cylinder heating, air balance in the hood, etc. are very important too.

But these machine parameters are frequently dictated to the manufacturers of paper machine clothing, resulting in close interaction between the respective application and the technical requirements for the dryer screen.

To ensure that the optimum drying capacity is obtained from a drying section, the permeability, material and design of the dryer screens have to be adapted to the particular machine status.

The main selection criteria for the Perlen PM 4 dryer clothing included suitable permeability for reliable sheet guidance when used in all TopDuoRun drying groups with their DuoStabilizers, non-marking designs for the very highest quality standards for Newsprint, LWC and SCB, together with uniform/stable screen edges for reliable transfer of the sheet tail in the transfer zone.

1. Measuring method applied in the drying section Perlen PM 4

The aim is to get to know the operating status of the machine in order to reach conclusions for optimum clothing use and to discover any irregularities and/or performance potential.

1.1 Conducting the measurement
The measurements are carried out under operating conditions, i.e. with closed hood without influencing on-going production. An air-cooled protection suit is worn when entering the drying section. The own adapters provided, pressure reducers, pressure regulators and a three-stage filter are connected to a mill compressed air supply with minimum 5 bar working pressure.

Measuring activities are carried out in full compliance with the accident prevention regulations using the instructions provided and according to the safety rules of the Professional Association of the Paper Making Industry.

1.2 Measuring principle
Figure 1Fig. 1 shows the positioning of the measuring sensors in the operating paper machine. The sensors are, one after another, held manually in the measuring position using the telescopic rod until stable values are obtained and recorded.

Cylinder temperatures are measured according to the contact measurement principle. Clearance of approx. 2/10 mm between sensor and cylinder surface prevents measuring faults caused by frictional heat.

 

Infrared sensors measure the heat radiated from the paper sheet, which is directly related to the sheet temperature. The degree of emission of the sheet is known; ambient radiated heat can be neglected on account of the low differences in temperature to the measured material.

Figure 2Top-quality psychrometric moisture meters are used to measure the pocket climate. A wet and dry bulb NTC temperature sensor is used at known air pressure to measure the partial pressure of the water vapour, providing data to calculate all necessary moisture parameters.

An impeller measuring head and NTC sensor are used to examine the ventilation devices. When the machine is at a standstill, it is also possible to measure air permeability of the dryer clothing.

This provides information about the degree of contamination and service life reserves.

2. The drying section Perlen PM 4

2.1 Drying section concept
TopDuoRun  & DuoStabilizer by Voith Paper
The TopDuoRun drying system by Voith is a single-tier design with heated cylinders at the top and drilled, nonheated cylinders at the bottom. PM 4 has altogether 5 pre-drying groups and one short after-drying configuration. (Fig. 3)

Figure 3

Fig. 2 shows the measuring device

 

Together with the drilled fabric guide roll, the DuoStabilizer (Fig. 4) shares the task of sucking the paper web to the dryer screen and keeping it free of wrinkles for safe sheet transfer from cylinder to cylinder.

Figure 4The vacuum in the gap between fabric and stabilizer and in the drilled roll is generated by the DuoStabilizer which is connected to a vacuum system. The gap is closed at the top of the DuoStabilizer perpendicular to the sheet with a felt seal, and laterally with air knives. This felt seal can be swivelled out when changing the dryer screen. On the operator side, a 500 mm long transfer zone is installed in the DuoStabilizer and in the drilled guide roll. During the sheet transfer process, only the transfer zone is subject to suction. In a DuoStabilizer, the volume of air to be extracted is relatively small compared to a suction guide roll, as the felt seal doctors off the air carried by the screen.

The vacuum connection is not restricted by the bearing diameter as is the case in a suction guide roll, so it is designed for lower pressure loss. This means that smaller, cheaper ventilators can be used.

A comparison shows that the combination DuoStabilizer/drilled guide roll results in a decrease in electrical power required of approx. 70% compared to a suction guide roll and approx. 60% lower than in the combination sheet stabilizer/grooved guide roll. The following vacuum levels were measured at DuoStabilizers in production machines:

  • outgoing gap between screen and box 70 - 250 Pa
  • in the drilled dryer screen guide roll 250 - 700 Pa
  • ingoing gap between screen and box 10 - 30 Pa

Source: Voith Paper

By using DuoStabilizers it is possible to extend the evaporation zone between two cylinders by up to 30% with an unchanged guide roll diameter.

2.2 Measurement results for the drying section Perlen PM 4
Fig. 3 shows a diagram of the drying section of PM 4. During the measurements taken on December 12, 2000, the machine was producing 45 g/m2 LWC base paper (finished weight: 62 g/m2 ) at a speed of 1150 m/min. The specific steam consumption was 22.9 tons/h with a steam pressure of 2.39 bar.

Figure 5The air balance in the PM 4 drying section can be said to be good. The dew point temperature did not exceed or come even close to the paper sheet temperature in any of the drying groups, which under certain circumstances would cause re-wetting of the paper sheet.

The heating curve (cylinder temperature) shows a uniform increase and development along the whole drying section (fig. 5).

 

Figure 6In groups 46, the drying section revealed considerable reserves, which will be needed for the targeted speed of 1500 m/min. If the drying capacity were to be increased for this paper grade, this would not be possible by increasing the cylinder temperature. On the contrary, a higher feed air temperature and greater exhaust air quantity in the first 3 groups would have a more favourable effect on the drying capacity.

 

A change in the air temperature (dry) in the drying section by only 1oC, could change the relative humidity by up to 6%. (Figs. 6 & 7).

Figure 7The basic rule applies: the relative humidity increases with decreasing air temperature. Relative humidity decreases when the air temperature increases. Fig. 7 confirms somewhat higher relative humidity in the first three drying sections.

In the atmospheric air moisture is always present in the form of water vapour. The level of water vapour can differ, however. It depends on the temperature; for every temperature there is a maximum quantity of water vapour which can be contained in a certain quantity of air.

Absolute air humidity or water vapour level is understood to refer to the weight of the water vapour contained in 1m3 air/water vapour mixture. As 1m3 can hold differing air masses depending on pressure and temperature, it is in many cases simpler to base the absolute moisture on 1 kg dry air. This variable is called the mixture ratio.

 

Figure 8Fig. 8 shows the air water load in the drying groups 1-6. The highest air water content is measured in the 3rd and 4th drying sections. Here the highest quantity of water is released from the paper sheet so that good aeration and ventilation of the pockets and hood is particularly important in this area. The extremely uniform and low water load in the air of PM 4 is remarkable in comparison to traditional double-felted drying sections.

Voith's DuoStabilizers and the SpiralTop  dryer screen by Geschmay produce an outstanding performance in the whole drying section. The permeability of the dryer screen must always be suited for its particular use and should be kept as constant as possible during the fabrics life.

3. Modern dryer screen requirements

3.1 Simple and quick seaming
Time-limited stoppages demand quick and easy seaming of dryer screens. The installation aids facilitate the closing procedure. Today there are various different techniques employing Velcro or zip fasteners for a seam to be closed in just a few minutes. Ease of closure of the seam depends greatly on the particular design of the screen. There are clear differences in the fitting procedure between Geschmay spiral screens and woven screens. Once the seam is closed, it must be possible to remove the closing aids easily and simply.

3.2 No seam mark or wear
Studies have shown that one reason why woven dryer screens are changed frequently, is wear in the seam area and in the seam itself. Damage and wear in the seam area may cause marking in the paper, especially in the first drying groups , also originating from differing permeability in the seam area. Geschmay's spiral screens rule out any risk of seam marking and wear, as there is no seam.

3.3 Optimum dimensional stability (crossmachine stability)
Figure 9Woven fabrics may be distorted in the weaving process already (mesh distortion from fabric centre to the edges). In woven dryer screens this is noticeably in the "lead" when installing the screen. This property does not apply to non-woven systems, because the manufacturing process differs completely. (Fig. 9)

The distortion changes the dimensions of the woven dryer screen (Fig. 10).

The resulting differences in permeability influence the drying procedure and thus the moisture profile of the paper sheet.

 

Figure 10In the PM 4 in Perlen, the SpiralTop by Geschmay with its high crossmachine stability has a positive influence on the electric power consumption of the DuoStabilizers/drilled guide rolls.

3.4 Homogeneous permeability
As already indicated under 3.3, spiral screens have lower fluctuations in air permeability across the width compared to woven screens.

Extensive measurements have confirmed that the contamination of spiral screens remains low even after longer operating periods. This is important in the DuoStabilizers of PM 4, because the extracted air volume is relatively small compared to a suction guide roll, and adjustments not only affect the safe sheet guidance but also the drying capacity and the moisture profile of the sheet.

3.5 Low contamination tendency
Many types of dryer clothing are today available in "anti-pitch" Teflon design. These are used at extreme contamination levels or immediately after the coating stations. The monofilament structure of the spiral screen and the on-going dynamics / interaction of spirals and filling monofilaments during operation, means that such clothing systems are less vulnerable to contamination and deposits. This becomes noticeable particularly when working with waste paper.

Spiral screen products have proven particularly effective with periodic cleaning, using extremely high-pressure systems with rotating nozzles. The dynamic action of the spiral links assists the in-process cleaning procedure. Measurements have shown spiral screen products to achieve up to 90% of the original permeability with homogeneous profiles after cleaning.

Figure 113.6 Uniform screen tension
For uniform drying and the thermal transition process it is extremely important for the dryer fabric to have uniform tension across the entire machine width. In addition, dryer screens with uniform CMD screen tension are easier to guide. Fig. 11 shows Geschmay's SpiralTop in a TopDuoRun system on a 10.5 m wide fine paper machine.

3.7 Aerodynamic behaviour
Extreme air turbulences can cause the sheet to flutter in machines with speeds exceeding 750 m/min, particularly in the open-draw sections. Turbulence is caused by the air carried by the dryer screens and the paper sheet; the increase in air carried with increasing speed does not show a linear progression. This causes complications in high-speed machines, particularly on lighter paper grammages, caused by pressure differences around the ingoing and outgoing dryer cylinder pockets. (Fig. 12) .

 

Figure 12The paper sheet tends to follow the drying cylinder and briefly leaves the screen. This produces a "bubble". Remedies are sought from so-called runnability components, such as blow boxes, which use the injector effect to create a vacuum and to attract the paper sheet to the screen. Drilled or grooved guide rolls support this principle. If these elements are not present in the traditional single-tier groups, particular attention must be paid to the permeability of the dryer screen: it may not be too open or too close.

These "phenomena" do not occur on Perlen PM 4, because throughout the entire drying process, the sheet is held safely on the dryer screens, firstly by the vacuum generated in the gap between screen and box, and secondly by the vacuum generated by the DuoStabilizer  in the screen guide roll.

It is mainly the permeability of a dryer screen which is responsible for carrying air. The different surface properties of the dryer screen have a relatively negligible influence on the quantity of air carried. These are more important when it comes to non-marking properties and the contact area between paper sheet and dryer screen.

3.8 Good drying performance
Together with their sheet support function, dryer screens should also absorb the sheet moisture inside the cavities of the screen without the material itself absorbing any moisture. But this moisture has to be released again in the free draws and pockets. Compared to woven dryer screens, spiral screens can have up to 70% more void volume, with a corresponding positive effect on drying capacity

Today, dryer screens with a permeability of approx. 100-120 cfm are indispensable for single-tier dryer sections.

In the transition zone of the sheet from the press into the drying section on PM 4 in Perlen, a permeability as low as 80 cfm is necessary because of the low vacuum at the suction guide roll.

Such low permeability dryers can also be produced on weaving looms using different monofilament profiles and weave patterns. The fabric thickness here is between 1.3 and 1.7 mm.

With Geschmays SpiralFlex technology today, these permeabilities are achieved using a highly developed heat-setting process and by careful selection of specific filling monofilaments.

When flat spiral monofilaments are used, these state-of-the-art dryer screens reach a thickness of only 1.8 mm (e.g. SpiralTop by Geschmay). These dryer screens are used all over the world in many modern paper machines, for many different paper grades.

3.9 Service life
The service life depends greatly on the wear of the screen and/or seam. The latter does not apply to non-woven technology at all. Similarly, loss of permeability i.e. contamination from deposits also influences service life.

A decrease in permeability results in a considerable reduction in the screen's drying capacity. It then has to be replaced in order to achieve the best possible drying capacity. Up to now, in the relatively short operating period of PM 4, it has not been possible to reach any conclusions about the maximum service life of the SpiralTop screens. In comparable TopDuoRun systems, these screens perform successfully for 12 months and longer.

3.10 Repairs
Non-woven dryer screens can be repaired. When damage has occurred, the damaged portion can be removed and the screen ends can be joined together again. This is only possible because this technology has no fixed seam. EN PM 4

For further information:

Markus Hahn, D. Geschmay GmbH & Co. KG, Im Pfingstwasen, Germany, 73035 Gppingen

Markus_Hahn@albint.com ERLEN PM 4