Key Drying Considerations

    1.    Energy Efficiency in Drying:

    •    Achieving high dryness after the press stage reduces steam consumption.

    •    Evaporating water in the dryer section is 7-10 times more expensive than removing it in the press section 

          and 60-70 times more expensive than in the wire section.

    •    The cost-effectiveness of drying varies depending on the efficiency of heat utilisation, power, and steam costs.

    2.    Dryness of the Web:

    •    Press Section Dryness Range: Typically, web dryness in the press part ranges from 15-21% to 35-55%, 

         depending on the grade of paper being manufactured and the efficiency of the wet end operation.

    •    By using higher pressure and longer nip time, it’s theoretically possible to reach up to 65% dryness, 

          though the practical limit on presses is usually 35-55% depending on the type of paper.

    3.    Limitations of Pressing:

    •    At 65% dryness, further water removal by pressing can damage the paper’s fibre structure.

    •    Approximately 35% water is necessary to maintain fibre wetting, so removing more water through pressing is impractical without compromising paper quality.

    4.    Dryer Section Considerations:

    •    Ignition Temperature: Paper ignites at 233°C, so the design surface temperature of the dryer cylinders is kept below this, typically around 200°C.

    •    This temperature is achieved with 14.5 bar steam pressure and maintained at an operating temperature of 185°C, equivalent to 10.5 bar steam pressure.

    Summary

        To optimise drying in paper production, achieving maximum dryness in the press section reduces the energy and cost burden in the dryer section. 

         However, limitations exist due to the structural needs of the paper fibres, and dryer temperatures must be managed to avoid fire hazards.