14 HEIMBACH
Pulp Paper & Logistics
Figure 3: The oscillation velocity in the cross direction shows a higher amplitude at 12.56 Hz
For now, let’s just say that the damage to the CC roll was causing a lot of vibration and this was the cause of the barring. It would have to be changed as soon as possible.
‘Sauna’ in the pre-drying section To find the cause of the reduced machine speed a hood balance was carried out. Was the evaporated water from the paper web being removed quickly and efficiently? Were the supply and exhaust air flows sufficiently high? Is the quality and temperature of
the supply air in order? The contradictory measurement
results were again irritating. Even repeated measurements failed to bring any clarity. The results were simply not logical. So, the TASK team took a closer look, and noticed that one of the two exhaust fans in the pre-dryer section was facing in the wrong direction. How could that be? The PLS showed a motor in running order and machine personnel obviously assumed the fan was therefore functioning properly. However, the drive belts
from the second exhaust fan were broken and as a result the exhaust air from the first fan was transported back into the hood across the second fan. Instead of being transported outside, exhaust air was actually returned into the hood, and hot, moist air was circulated rather than removed. This effect was reinforced by the fact that the construction of the exhaust air duct impaired the air extractor. As a result, the heat exchangers no longer worked efficiently, and the hot air supply became increasingly colder.
Photo 3: Position of the accelerometer on the second press top roll
The consequence was, of
course, was that the paper was drying increasingly slowly and the customer had to continually reduce the speed of his machine. And at the same time using more energy.
Simple solutions So finally, the error was found. The fan was the second problem area, the one that had led to a significant reduction in machine speed. And here also the repair of the hood exhaust fan was able to remedy the situation quickly. Together with the replacement of the CC roll, the machine ran smoothly once more and the customer was again able to meet production targets as speed returned to normal. This case shows once more:
Figure 4: Supply and exhaust air system in the pre- and post-drying section with defective fan May/June 2019
Trust is good – control is better. Put your paper machine under the microscope. In an ideal world, proactively. Problems that are detected and corrected early – or even better avoided completely – are of the most benefit.
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