Project Details
Figure 1: Simulation result of the perforated roll headbox.
Turbulence or air inclusions in the headbox are the main reasons for a systematic fluctuation in basis weight depending on the cross position in the paper web. In a perforated roll headbox, the pulp suspension is mixed again after the cross-flow distributor by one or more perforated rolls in order to achieve a paper web that is as uniform as possible.
Figure 2: Geometry of the perforated roller headbox.
As part of this project, the perforated rolls were to be optimized given the available space in order to reduce the formation of streaks in the paper. For this purpose, the two perforated rolls including the cross-flow distributor in front of them were simulated, see the geometry used in Figure 2. A shear rate-dependent viscosity was used for the simulation itself, and the boundary conditions of the mass flows were taken from the operating point of the system. As can be seen in the velocity streamlines (see Figure 1 and Figure 3), the second perforated roll creates the relevant turbulence in the paper in the form of streaking.
Figure 3: Simulation results for the pressure (cross-flow distributor) and the streamlines of the velocity.
The turbulence intensity is also assumed to be the decisive factor in determining whether the change in geometry of the second perforated roll has brought about an improvement. In addition, the expected profile quality can be represented by the volume flow rate over time (see Figure 5).
Figure 4: Speed distribution in cross-section, once from the driver's side (left) and from the traction side (right).
Figure 5: Simulation results of the volume flow deviation.
This type of evaluation is based on the profile quality investigation of finished paper webs, where samples of the paper web are taken according to Figure 6 and Figure 7 and examined for their basis weight variation. The results of this examination are then entered into a matrix from which the profile quality can be read. If a paper machine produces excessive streaks in the paper web, the operator basically has two options - after clarifying whether pulp composition or operating point are responsible: On the one hand, to replace the existing headbox, which is ultimately responsible for the profile quality, at the machine manufacturer, or to carry out an internal optimization of the components.
Figure 6: Schematic representation of the theory of profile quality analysis.
Internal optimization offers a number of advantages, both financially and in terms of results: First and foremost, internal optimization guarantees that the quality is reliably better in the end, whereas, depending on the extent of the conversion, a completely new headbox does not necessarily ensure an ideal result on every machine. If the entire machine or at least the headbox is examined using flow simulation, not only can the real cause of the streaks in the paper be identified, but it can also be clarified within the scope of the structural possibilities whether an improved geometry of individual components will lead to the desired result.
Figure 7: Structure of the evaluation matrix for profile quality.
In the case of a perforated roll headbox, for example, an improved perforated roll geometry can be easily implemented, thus improving the paper quality at low cost. If several identical systems are also operated, the knowledge gained can be implemented several times, which is not the case if the entire system component is purchased new.
