Summary
Task
Optimization of pressure screener regarding their power input, wear reduction, lifetime increase, improvement of product quality.
Solution
Promising solutions can be found by geometry adjustment and investigations of optimal operating conditions of the pressure screener.
Benefit
CFD analysis provides insight into the sorting process and a better understanding of potential problems.
Project Details
Figure 1: Outer structure of a pressure screener with its inlet and outlet flows (left side); inner structure with stator and rotor.
Structure and Function of a Pressure Screener
The pressure screener plays a crucial role in papermaking by efficiently sorting and cleaning the feed stream, which consists of a mixture of fibrous materials and potential impurities. It consists of two main components: the stator basket and the rotor basket (Figure 1).
The stator basket, consisting of the sorting bars, forms an important basis for the separation process. It is stationary and is the outer part of the pressure screener. The rotor, on the other hand, is in motion and consists of a cylindrical basket to which foils are attached. Positioned within the stator basket, the rotor is installed with a narrow gap of some millimeters between itself and the stator.
The main task of the pressure screener is to effectively sort and clean the feed stream. During this process, the feed is divided into two streams (Figure 1):
1. The "accept stream" contains the desired fibers, which are passed on to the process run for paper production.
2. The "reject stream" consists of the impurities removed from the feed stream. These are discharged from the process to ensure the quality.
Overall, the pressure screen enables precise separation of fibers and impurities in the feed stream, which should improve the quality of the end product.
Figure 2: AVERAGE: Case without/ after foil, averaged flow in accept area; PEAK: Case near foil leading edge, pressure peak and strong flow into accept area; SUCK: negative pressure, fibers are lifted from the screen basket, moderate flow into rotor area.
The rotor creates a pressure difference between the front and the back of the sieve bars during rotation. This difference creates an overpressure on the front side and a negative pressure on the back side, which is decisive for the separation process. The fibers in the feed stream are forced through the slots in the stator basket by this pressure difference. Meanwhile, impurities and contaminants are not able to pass through due to their size and nature and remain behind. Due to the negative pressure generated, the sieve bars are cleaned and thus tend to become declogged. As the foil is moved along the stator, different areas are created, called "AVERAGE", "PEAK" and "SUCK". These areas are characterized by different pressure ratios, as shown in Figure 2.
CFD- Analysis and Optimization Options
Computational Fluid Dynamics can be used to analyze complex flow patterns and characteristics in a pressure screener. These simulations make it possible to model different rotor-stator geometries and evaluate their effects on the flow dynamics. Various geometry changes are being researched for the optimization of pressure screener. In the process, a wide range of options are available. (see Figure 3, Figure 4, Figure 5)
• Inclination of the foils on the rotor
Figure 3: Tilting the foils on the rotor; straight foils (left); tilted foils on rotor (right)
• Variation of the cone angle on the rotor
Figure 4: Tilting the cone; 0° angle on the cone (left), 15° angle on the cone (right)
• Changing the sieve bar shapes on the stator basket
Figure 5: Simulation of different sieve bar shapes
• Variation of the cone angle on the rotor
As part of the optimization considerations, changing the foil geometry, which includes modifications in length, width, and shape, is also a possibility for improvement. Another possibility is to adjust the position of the foil on the rotor. Furthermore, it is crucial to closely examine and influence the effects of the gap width between the rotor and the stator.
In addition to these geometric adjustments, it's worth noting that varying the rotational speed of the rotor is another variable that can be manipulated. This adjustment in rotational speed can effectively modulate the power input, and it can be advantageous when implementing changes in foil geometry. Thus, it offers an additional control and optimization potential for the system.
Overall Results
The application of CFD simulations basically allows the identification of optimal rotor-stator geometries for specific purposes and the evaluation of which configurations are advantageous. In this context, various objectives are pursued, including the reduction of energy consumption, the minimization of wear and the associated extension of component life without compromising the required quality.
1. Performance optimization: by adjusting the geometry of the rotor and stator, the energy consumption of the system can be reduced. This contributes to cost reduction and energy efficiency.
2. Wear minimization: Simulation can help identify critical areas where wear can occur and develop ways to reduce abrasion. This leads to longer component life and lower maintenance costs.
3. Quality preservation: While changes are made to the rotor-stator geometry, it is critical to ensure that the desired quality of the final product is maintained. The CFD simulations can help to ensure that the fluid dynamics are not adversely affected.
4. Efficiency increase: The optimal design of the geometry can improve the efficiency of the separation process by optimizing the separation of fibers and impurities. This leads to higher productivity and quality.
Overall, CFD simulations provide an efficient way to optimize rotor-stator system performance, minimize wear problems, and extend equipment life while maintaining product quality at desired levels. This enables companies to operate more cost-efficiently and sustainably.
The choice of optimization performed is closely linked to the respective initial situation and is significantly influenced by the objective to be achieved. This means that the decision on which steps to take depends on various factors, including the existing operating conditions, the desired performance targets, and the limits in the system.
