Project Details
Figure 1: Schematic diagram of a tissue paper machine.
Tissue paper machines (diagram in Figure 1) often have several so-called Yankee drying cylinders. In these drying cylinders with superheated steam as the energy source, the formation and removal of condensate is an important design point, as the condensation heat of the steam dries the paper web. A diagram of this process can be seen in Figure 3. Since the amount of condensate is decisive for optimum heat transfer, its removal is an important design parameter.
Figure 2: Calculation mesh of the Yankee drying cylinder.
Figure 3: Diagram of how a Yankee drying cylinder works.
This project focused on the so-called soda straws, which are responsible for removing the condensate. Only half a segment of the entire cylinder (see Figure 2) was used for this, as extrapolation is possible with the right boundary conditions. The simulation was implemented using a two-phase flow in order to be able to map the condensate movement in the steam. This is very challenging as our engineers have to take into account the variable droplet sizes due to interactions. In addition, a different liquid zone results in the pipe flow at different Reynolds numbers, which is of particular interest in the soda straws.
Figure 4: Geometry of the current Sodastraw arrangement.
Figure 5: Heat flow for the current arrangement.
Figure 7 clearly shows that, contrary to popular belief, the liquid does not flow along the pipe wall under the flow influences of the operating point, but is transported as a liquid cone inside the pipe. For this reason, the diameters of the drum (approx. 5.5 m) and sodastraw (4 mm) also had to be taken into account (Figure 4), as these have a major influence on the Reynolds number and thus the flow pattern, as well as the resulting pressure loss.
Figure 6: Comparison of the simulation with the thermal measurement.
The condensation process itself also required a keen eye for detail, as only empirical models are available for this and no analytical solutions. Despite the high demands placed on our engineers in the implementation of the simulation, this type of plant investigation offers valuable insights into the processes inside the drying cylinder, which cannot be easily measured and documented without having to shut down the plant and equip it with expensive measuring equipment.
Figure 7: Volume fraction of liquid phase in soda straw.
Simulations can also be used to efficiently identify associated opportunities for savings by optimizing the system, especially if a validated basic simulation has already identified the critical areas of the process. To validate the simulation, the temperature distribution from a thermal image measurement was simply used (see Figure 6), which can be carried out on the running system without great effort. If the temperature distributions match well, it can be assumed that the simulated transferred heat flow in Figure 5 is also realistic. Possible optimizations to the soda traws, which were investigated as part of this project, were, for example, the distance from the condensate grooves as well as their angle of attack and the distribution over the drum.