18 HEIMBACH
Pulp Paper & Logistics
Fig. 22 Machine direction yarn showing internal abrasion The usual distance between
Fig. 21 Oscillation speed 3
pressure shower systems, one directed to the paper side and the other to the machine-side surface of the fabric (Figure 15). This is recommended especially when running SSB fabrics which, although they are very open designs, have a high caliper and therefore can be cleaned more efficiently through needle jet impingement from both sides. These systems are similarly available with integrated closed suction boxes to remove spray and mist.
2. Positioning and operation It is recommended that high- pressure showers should always be installed so that the distance between nozzle and fabric surface is between 25mm and 100mm. The shorter the distance, the better the laminarity of the water jet. At a distance in excess of 100mm the kinetic energy of the jets will decline and they will start to lose laminarity and break up into droplets. These two effects will reduce cleaning performance. In addition, the action of the droplets impacting on the fabric can lead to severe shower damage.
2.1 Impact angle of the water jets, deflection of the shower pipes The jets should generally be positioned in such a way that the water jets impinge onto the fabric
May 2013
surface at an angle of 90 degrees plus 10 degrees in the run direction (Figure 16). If the angle of the water jets is much greater than this it can result in reduced cleaning performance, due to a smaller difference between jet and fabric speeds. An impingement angle that is against the run direction can lead to a slow down in fabric speed, higher power consumption and increased fabric damage. It may be necessary to adjust the impact angle slightly to achieve optimum cleaning performance. Deflection of the jets must not be allowed to occur under any circumstances, as this would lead to the levels of cleaning across the fabric width being inconsistent. To achieve this, the jet nozzles need to be cleaned and maintained on a regular basis. Sagging shower pipes can be stabilised by having a triangular support welded onto them in a suitable position to attract a minimum of deposits (Figure 17).
2.2 Separation distance between the nozzles, cleaning efficiency in the area of the fabric edges One of the most important criteria for the correct functioning of a shower system is the oscillation stroke, and especially the ratio between oscillation stroke and nozzle spacing.
nozzles ranges from 50 to 100mm. The total number of nozzles – and therefore the distance between them – depends on the level of cleaning performance required taking into account pulp quality, fabric speed etc. Highest priority must be given to
an evenly spread jet coverage over the whole fabric width. Therefore the distance from the first nozzle on the front side to the last nozzle on the back side of the fabric has to be as follows: • Equal to the fabric width minus one nozzle spacing: where the oscillation stroke equals one nozzle-spacing (Figure 18) • Equal to the exact fabric width: where the oscillation stroke equals two nozzle-spacings • Equal to the fabric width plus one nozzle spacing: where the oscillation stroke equals three nozzle spacings • Equal to the fabric width plus two nozzle spacings: where the oscillation stroke equals four nozzle-spacings (Figure 19) Unless this is adhered to, there
would be the risk of reduced coverage in the edge areas of the fabric.
3. Oscillation, and the correlation between the effect of high pressure showers and cross direction moisture profiles A well maintained shower system makes a specific contribution towards finished paper quality because efficient cleaning of the fabric will help achieve balanced cross direction moisture profiles.
3.1 Oscillation stroke For the oscillation stroke to work with optimum efficiency it has to equal the exact nozzle spacing or a multiple of it. If this is not the case, streaks will occur across the width of the fabric as a result of over or under coverage (Figure 20). The schematic illustration
shows an example of an incorrect oscillation stroke of 2.33-times the distance between nozzles. This leads to wet streaks across the width of the fabric and the sheet, caused by uneven overlapping of the showers, and dry streaks at each of the edges. It is well-known that these streaks cannot be eliminated from the sheet either by the press or the dryer section.
3.2 Oscillation speed The key to achieving continuous coverage and providing comprehensive fabric cleaning action, is the oscillation speed. A jet of 1 mm width will provide coverage and fabric cleaning in strips of 1 mm width. To avoid gaps between the cleaned strips, it is crucial that the oscillation stroke per fabric cycle be the same as the diameter of one jet. In the above example, this would amount to 1 mm (Figure 21). To expand on this example, suppose the nozzle-spacing on the shower pipe is 100mm. To cover the fabric in an endless spiral without any gaps, and to clean it efficiently, the fabric would have to go through 100 cycles. In the course of these 100 cycles each jet will clean a strip of fabric in a width equal to the nozzle spacing,
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