Juha Leimu
Turku University of Applied Sciences, Leimu Engineering Oy, Turku, Finland
The runnability of cylinder dryers is critical to the productivity of paper and board machines. Although paper
and board machines are often considered separately, the runnability problems in their drying sections are
technically the same. However, most of the runnability technologies used today were developed over two decades
ago for specific types of production, with only minor updates to the technology and equipment.
The most common runnability problems in cylinder dryers can be attributed to the equilibrium of four forces,
namely the opening nip underpressure, adhesion between the web and the cylinder, web tension, and the
underpressure created by the runnability systems. To address these issues, a computational model was developed,
which combines these forces to provide a theoretical basis for dimensioning. Although this model was originally
designed for fast and wide machines, it can be adjusted to suit different production scenarios by varying the
parameters.
Most runnability systems were developed for high running speeds, low adhesion, and high web tensions.
However, today these systems are often used with sticky masses, low web tensions, and low running speeds.
Nevertheless, the solution remains the same: achieving the equilibrium of the four forces. Therefore, the
dimensioning of the necessary underpressure and the methods for producing it in the current production situation
are discussed.
Keywords: runnability, cylinder dryers, paper web, web tension, underpressure, model
