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By Pekka Kivioja, Global Technology Manager, Services Business Line, Metso

Cost elements in felt conditioning

Felt conditioning costs stem from many sources, so you first need to understand the magnitude and savings potential on each particular cost element.

In felt conditioning, most costs come from:
• Vacuum creation (and associated efficiency losses)

• Shower water use
• Add-on friction from Uhle boxes
• Felt costs

More than one-fifth of the total energy consumed by the press section goes to vacuum generation; the remaining four-fifths is used by sectional drives.

Figure 1
Figure 1 Press section electricity use distribution.

In addition to the above, properly functioning felt conditioning also affects such other cost elements as press runnability and felt washing and change related costs.

Nip dewatering vs. felt dewatering

Many paper machines have been optimizing their processes for nip dewatering in recent years, especially at the last couple of felt loops. However, many board machines running heavier basis weights still use felt dewatering where the majority of water removal occurs at Uhle boxes. A new, more effective felt conditioning method has been developed for those machines.

New strategy for felt conditioning

One recent trend in felt conditioning has been the reduction of vacuum levels to lower air flows and reduce energy consumption. Maintaining the desired dewatering rate in this environment requires longer dwell times to give Uhle boxes enough time to remove the necessary volume of water. In Figure 2, maintaining the same relative dewatering rate while at the same time reducing the vacuum level from 45 kPa to 20 kPa means that the dwell time has to be increased from 3.5 ms to 11 ms.

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