Underpressure against the drying fabric have been used since early eighties to improve the cylinder dryer opening nip runnability. For a long time the runnability system underpressure level was low and could be easily increased with the rising running speed. Recent development in the paper machine running speed has led to a situation where the window of runnability, which is situated at the area of positive effective web tension below the web tensile strength, is becoming narrow. Also the fan power consumption needed by the runnability systems is becoming an environmental and economical burden. The dimensioning of the underpressure has been mainly empirical and more exact tools would be needed.
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