Overview of the concept and selection methods for Roots vacuum pumps

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The function of a Roots vacuum pump is to remove gas molecules from the vacuum chamber, reducing the gas pressure inside it so as to achieve the desired level of vacuum. In general, there is a wide range from the atmosphere to ultra-high vacuum, and to date no vacuum system has been able to cover this entire range. Therefore, to meet the requirements regarding process parameters, work efficiency, and equipment service life for different products, different vacuum system configurations must be selected for various vacuum sections. 　 To achieve the optimal configuration, the following points should be considered when selecting a vacuum system: 1. Type of gas to be evacuated and the amount of evacuation required. Determine the type of gas that needs to be evacuated and the volume of gas that must be removed, based on the process requirements. Because if the gas being pumped reacts with the liquid inside the pump, the Roots vacuum pump system will become contaminated. At the same time, it is necessary to consider determining an appropriate exhaust time and the amount of gas generated during the pumping process. 　　II. Vacuum level The vacuum level determines the structure to be used for the product; it is expressed in terms of both gauge pressure and absolute pressure. 　　Absolute pressure indicates that the reading is an absolute value; the closer the reading is to ‘0’, the higher the degree of vacuum. On the other hand, regarding gauge pressure, the closer it is to 760 mmHg, the higher the level of vacuum. If the absolute pressure (ultimate vacuum) required is close to ‘0’, then only a vacuum pump can meet this requirement; but if the desired pressure is close to 1 bar (i.e., atmospheric pressure), then rotary blowers or centrifugal fans can be used to satisfy this need. 　　III. Determining the operating range of the Roots vacuum pump It is necessary to determine the vacuum level required for each process. Since each process has its own suitable range of vacuum levels, it is necessary to carefully study and determine what kind of vacuum is appropriate for that process. 　　IV. Vacuum volume inspection: Determining the time required to achieve the desired level of vacuum, the flow resistance of the vacuum pipes, and any leaks. Consider the pumping rate required to maintain the vacuum under certain process conditions after achieving the desired vacuum level. 　　V. Determining the ultimate vacuum level The ultimate vacuum level of the Roots vacuum pump system should be checked, based on the vacuum level required by the process; as it is the ultimate vacuum level that determines the optimal operating vacuum level for the system. Generally speaking, the ultimate vacuum of the system is 20% lower than the operating vacuum of the system, and 50% lower than the ultimate vacuum of the pre-pump. 　　Of course, the equation above is only a result of theoretical calculations; there are several variable factors that have not been taken into account, such as pipe flow resistance, leakage, filter flow resistance, and the temperature of the gas being pumped. In fact, the safety factor should also be taken into account.