Design issues of vacuum pumps

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The water ring pump should be used as the main pump, with the Roots pump serving as the auxiliary pump; during operation, the water ring pump should be started first, followed by the Roots pump

When performing third-stage vacuum pumping, if a water ring pump and a Roots pump are used together, it is recommended to use the water ring pump as the pump in the outermost stage (refer to relevant information found online). Roths vacuum pumps (mechanically driven pumps) designed for higher vacuum levels cannot be connected directly to the atmosphere. Doing so would create an excessive pressure difference between the suction and exhaust ports of the pump, resulting in overload. Increasing the power of the pump’s motor alone would cause the pump to overheat, which in turn would lead to the tiny gaps between the rotor blades becoming blocked due to thermal expansion. To ensure that the Roots vacuum pump can achieve a high level of vacuum, it is necessary to maintain the proper gap between the rotors of the pump. Therefore, a pre-stage pump must be used when operating a Roots vacuum pump; the pre-stage pump is used to reduce the pressure in the system to a certain level before starting the Roots vacuum pump, which helps to prevent it from being overloaded. For the primary pump, vacuum pumps that can discharge directly to the atmosphere can be used, such as water (liquid) ring vacuum pumps, rotary vane vacuum pumps, slide valve vacuum pumps, and reciprocating vacuum pumps. Under normal circumstances, it is more advantageous to use a water ring pump as the pre-stage pump in rotary vane vacuum systems compared to other types of vacuum pumps. This is mainly because the water ring pump is capable of removing large amounts of condensable vapor. This advantage is particularly evident when gas-loaded oil-sealed mechanical vacuum pumps are not sufficient for removing such vapor, or when the solvents used can degrade the pump oil and affect its performance, or yet again when the vacuum system does not allow for oil contamination. The function of the water ring pump in the unit is to create the preliminary vacuum required by the Roots pump; therefore, there is a requirement for a maximum allowable exhaust pressure for this water ring pump. In other words, it is necessary to increase the ultimate vacuum capacity of the water ring pump as much as possible, while at the same time striving to raise the maximum allowable exhaust pressure of the Roots pump. Generally, single-stage water ring pumps do not achieve high ultimate vacuum levels. Since the pre-vacuum required for the Roots pumps produced in China is relatively high, single-stage water ring pumps are not used as pre-pumps for such pumps. Instead, two-stage water ring pumps, which can achieve higher ultimate vacuum levels, are employed as pre-pumps. The use of two-stage water ring pumps also helps to increase the ultimate vacuum level of the entire system. The ultimate vacuum level of a single Roots pump is relatively low; especially when it operates in combination with a water ring pump, its range of application is limited, and the ultimate vacuum level of the entire system may be even lower. However, by using two Roots pumps in series along with a water ring pump, it is possible to **increase the ultimate vacuum level of the system**. Therefore, in this type, it is common to see a unit composed of two Roots pumps connected in series, with a two-stage water ring pump serving as the pre-pump.