20 HEIMBACH


Many practical benefits are achieved from one source: optimum pocket ventilation. A further example demonstrates why – as so often – uniformity is crucial.


Moisture: a most important detail A customer asked Heimbach to measure and evaluate the status of its pocket ventilation. Figure 5 shows the TASK log including all relevant parameters. On this paper machine blow boxes were installed only in the pockets under cylinders 30, 33, 36, and 38. This gave an uneven result in


terms of ventilation: In places where the pockets are ventilated the absolute moisture is lowered by 38 per cent – a significant reduction. Looked at the other way, the


pockets that are not ventilated contain much more moisture. In conclusion: the blow boxes make it possible for significantly more water to be absorbed and transported away in these pockets. This is as obvious as important when comparing Figure 5 with a different machine configuration.


Uniformity as a benefit The dryer group in Figure 6 shows blow boxes in all pockets.


Pulp Paper & Logistics


based on a process that always starts with measurements, since the appearance of moisture streaks can have more than one cause. TASK always recommends assessing the situations by means of measurements, and only then should optimisation and restructuring be considered. This creates certainty, and simplifies assessment and evaluation.


Figure 5: Uneven pocket ventilation equals uneven moisture removal


At first glance, everything is even with both the dry air and the dew point temperatures very regular. As a result, this also affects the water absorption capacity of the air in the pocket. This means in practice that air flow is even from the middle of the machine in all pockets towards both edges. Furthermore, the lower


moisture content of the air in all pockets leads to higher water take-up capacity: “In summary, very effective removal of the evaporated water from all pockets,” Fischer states. “The target was achieved in this case too, because an optimum CD moisture profile is, as


everybody knows, a key quality characteristic across all paper grades.”


The process is crucial Unwelcome wet streaks can be all but eliminated with the use of the right technology and appropriate adjustment of the machine components. The examples mentioned are


certainly convincing as practical achievements, but they are all


Air doctors as an option Similarly, in the third case, a customer opted for air doctor technology, a space-saving alternative that involves a combination of doctor beam and blow box (Figure 7). Air doctors are very well suited


to both conventional and slalom positions. Besides optimum use of space in the pockets, air doctors have the advantage of directing the supply air directly into the wedge between the exiting sheet and the cylinder – leading to an evening out of the vacuum created. This also applies to conventional groups.


Figure 6: Even pocket ventilation – regularity across the board January/February 2019


Figure 7: Air doctors in cross section


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