Optimized design method for rotoary vacuum pumps equipped with pre-pumps

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Generally, in the selection and design of a forepump for a Roots vacuum pump system, the concept of empirical compression ratio is often used to choose the forepump. However, in an optimized selection and design process, the compression ratio is merely a result derived from the pressure difference, and it is not universal. In an earlier article published on this official account, titled “Effects of Different Pre-Pump Configurations on the Performance of Roots Vacuum Pumps”, a brief introduction was given to the preliminary design of configuring Roots pumps with water ring vacuum pumps, as well as the impact of various configurations on the performance of Roots vacuum pumps. Examples were provided to illustrate that the matching of pumping speeds between Roots vacuum pumps and pre-pumps must adhere to the principle of conservation of mass. However, for Roots vacuum pumps, the conservation of mass during pumping is merely one of the necessary conditions. Given their wide range of applications in various industrial processes and their product design characteristics, optimizing the inherent design capabilities of Roots vacuum pumps can bring greater economic value to users. On this basis, when considering only rotary vane vacuum pumps, energy conservation is another principle that must be adhered to. From the initial design stage, the technical specifications and intended applications of a rotary vane vacuum pump determine its maximum allowable reliability levels. For example, Ciscoi defines certain specifications for its newly designed latest generation of advanced rotary vane vacuum pumps: the maximum pressure difference tolerance is increased by about 20%, and its reliability remains above 97% even after 16,000 hours of continuous operation. The economical version offers a time savings of approximately 10% compared to traditional rotary vane vacuum pumps with the same pumping capacity, while the high-performance version provides a time savings of around 20%. The intelligent version adds adaptive adjustment capabilities on top of those of the high-performance version. The time savings associated with the high-performance version have been demonstrated in various professional studies, so no further details are needed here. The definition of design parameters is not based on arbitrarily assigned values; rather, it refers to performance characteristics that can be achieved within the scope of basic research data. The data curves obtained through actual measurements indicate the specific ranges for these parameters. For rotary vane vacuum pumps, the maximum value within this range is determined by the law of conservation of energy – that is, under any pressure difference, for the same gas medium, pVγ = C (a constant). Unlike what is stated in academic papers, this constant C is provided by the designers of the rotary vane vacuum pump, as the result of actual measurements under standardized test conditions, rather than being a calculated value. For the latest generation of rotary vane vacuum pumps designed by Ciscoi, we will specify this value in the manual, as well as provide data on the relationship between compression ratio and volumetric efficiency for use by regional partners – all of this is done with the aim of creating greater value for users in an economical manner. There is a significant difference between professional academic research findings and engineering design studies, but engineering design studies must be based on those academic findings—the most fundamental physical phenomena serve as the foundation for such engineering designs. Just as the law of conservation of mass must be followed in the operation of rotary vacuum pumps, as mentioned earlier, the same principle applies to energy conservation. In the process of optimizing the design of rotary vacuum pumps by incorporating pre-stage pumps, the performance characteristics of these pumps are key factors; aspects such as their heat transfer properties, design accuracy and manufacturing precision, material properties, and load design determine the maximum allowable temperature rise and reliability levels. Under these conditions, our test results provide specific numerical values. For example, assuming a rotary vacuum pump with a geometric pumping speed of 1000 m3/h, when used with a suitable pre-stage pump and at an exhaust pressure of 2000 Pa.a, the inlet pressure of the rotary vacuum pump can operate reliably at 235 Pa.a. This means that the compression ratio appears to be 1:8, and the volumetric efficiency of the pump still remains at 80% under these conditions. If this level of performance represents the maximum value allowed to ensure reliability over the desired lifespan, then at pressures higher than this range, it is the constant C that determines the allowable pressure difference and compression ratio, rather than relying on so-called empirical rules. At the same time, the constant C also determines the design of the starting pressure. For example, under a working pressure of 235 Pa·a, in a design without an overflow valve, the pump ahead must meet a second condition based on the conservation of mass flow rate; this condition involves the product of p1, the compression ratio k, the geometric pumping speed sth of the Roots vacuum pump, and the volumetric efficiency η. Thus, the second condition for the pumping speed s of the pump ahead is s = p1·k·sth·η = p·v. Under the conditions of the starting pressure, this highlights the importance of the overflow valve. However, built-in overflow valves have many disadvantages. For users in the field of high vacuum, we offer an externally mounted component with an integrated design that provides additional useful functions, including an overflow function carefully designed based on pressure and Hooke’s law, as well as a function to accelerate startup. Compared to the Roots vacuum pumps commonly available in China that feature built-in overflow valves, this precisely designed solution enables short-term overload according to the defined value of the constant C, thereby further improving the startup process. Of course, this is also limited by the overload capacity permitted by the high-performance motor used. For the system design of an advanced and highly reliable Roots vacuum pump unit, the design performance of the main pump and the pre-pump is fundamental, while the optimized design between the two is key to achieving maximum performance.