Optimization of Life Cycle Costs for Scroll Pumps
Realization of temperature management
Dry-running scroll pumps are predestined for processes that require oil-free pumping. They are an adequate replacement for the widely used rotary vane pumps, whose pumping speed is not inferior. However, the dry seal required for this is subject to progressive wear, for which the temperature conditions in the pump play a major role. By reducing operating temperatures by 15 %, maintenance intervals are extended and higher performance is achieved at higher suction pressures. At the same time, redundancy of the measured operating parameters increases operational reliability.
To optimize the temperatures, the Finite Element Method (FEM) was used to calculate the temperature field in the pump for three operating conditions and different design variants. This made it possible to identify points with the highest thermally induced wear. As a result, an intelligent fan control system was developed based on the temperature readings from a sensor in conjunction with the power consumption of the motor.
Illustration of an overall thermal system of a scroll pump
The new development of a scroll pump offers the possibility to use numerical simulation techniques to predict and optimize the temperature distribution within the entire pump. This also allows temperatures to be determined at inaccessible points and hotspots to be identified. In the course of product development, the Finite Element Method (FEM) is used to calculate the temperature field in the pump's components for three operating states and for different design variants.
The problem with mapping an overall thermal system is that not all physical phenomena are completely solved and thus not all boundary conditions are known. These include:
- Heat conduction in the moving fluid within the chamber volumes
- Heat conduction through the rotating bearings in which lubricant is also circulated
- Heat transfer coefficient due to forced convection at the outer surfaces of the housing, keyword impingement flow and deflection by hood
Figure 1: Mean value of abrasion of two plastics as a function of temperature, measurements by Pfeiffer Vacuum.
In order to determine the unknown parameters, an engineering approach is used to fit them to temperature readings from a real prototype. Figure 1 illustrates this fitting process. First, the uncertain parameters are assumed based on empirical values, the FEM model is calculated and then compared with the available measurement points. Then the parameters are adjusted and calculated again until the agreement between measurement and simulation is satisfactory. In practice, an agreement between the measured and simulated values of > 94 % has proven to be satisfactory. This process is carried out for three different operating states. Only when the deviation between measurement and calculation is small for all states, the determination of the parameters can be considered reasonable. With the numerical model and the boundary conditions found, it is then possible to compare and evaluate further design variants.
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