Roots vacuum pumps and water ring vacuum pumps

We should understand this by considering the working principle of rotary vane pump–water ring pump vacuum systems: Rotary vane vacuum pumps (mechanically driven pumps) used for achieving high levels of vacuum cannot be connected directly to the atmosphere. Doing so would create a large pressure difference between the suction and exhaust ports of the rotary vane pump, resulting in overload of the pump. Increasing the power of the motor driving the rotary vane pump would cause the pump to overheat, which in turn would lead to the tiny gaps between the pump’s rotors becoming blocked due to thermal expansion. To ensure that the Roots vacuum pump can achieve a high vacuum level, it is necessary to maintain the proper gap between the rotors of the pump. Therefore, a forepump must be used when operating a Roots vacuum pump; the forepump 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. In general, for Roots vacuum systems, it is more advantageous to use a water ring pump as the pre-pump rather than 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 encore when the vacuum system does not allow for oil contamination.

The water ring vacuum pump should be placed in front of the Roots vacuum pump. When starting, the Roots vacuum pump should be turned on only after the vacuum level achieved by the water ring vacuum pump has reached a certain level, in order to prevent the Roots vacuum pump from being overloaded.

In general, for Roots vacuum systems, it is more advantageous to use a water ring pump as the pre-pump 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 thus affect its performance, or in cases where the vacuum system does not allow for oil contamination. 　　Roths pump-water ring pump units are widely used in vacuum systems for applications such as the chemical industry, food sublimation drying, and high-altitude simulation tests. There are roughly the following types of such combined units. 　　(1): The function of the water ring pump in the unit is to create the preliminary vacuum required by the Roots pump. Therefore, the Roots pump and the water ring pump both have requirements regarding the maximum allowable exhaust pressure of this water ring pump. In other words, it is necessary to maximize the ultimate vacuum achieved by the water ring pump on one hand, and at the same time, to increase the maximum allowable exhaust pressure of the Roots pump as well. 　　Generally speaking, the ultimate vacuum level achieved by single-stage water ring pumps is not very high. Meanwhile, the pre-vacuum required for the Roots pumps produced in China is relatively high. Therefore, single-stage water ring pumps are not actually used as pre-pumps for Roots pumps; instead, two-stage water ring pumps, which can achieve a higher ultimate vacuum level, are employed as pre-pumps. The use of two-stage water ring pumps also allows for an increase in the ultimate vacuum level of the entire system. 　　The ultimate vacuum level of a single Roots pump is relatively low; especially when it is used in combination with a water ring pump, its range of application is limited. 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 two Roots pumps connected in series, with a two-stage water ring pump serving as the pre-pump (the unit configuration in Figure 1). 　　(2) Roots pump – water ring pump – atmospheric pump unit: Even when a two-stage water ring pump is used, the increase in ultimate vacuum is limited to a certain extent due to the constraint imposed by the saturated vapor pressure of water. The theoretical ultimate pressure of a water ring pump is equal to the saturated vapor pressure of water. Taking into account factors such as gas backflow, the actual ultimate pressure of a water ring pump is significantly higher than this saturated vapor pressure at that temperature. To improve the ultimate vacuum of the preceding pump, it is also possible to use a combination of a water ring pump and an atmospheric pump. 　　 　　In this case, the ultimate vacuum level achievable with one stage of atmospheric pump in series can reach 20–30 Torr; if a water ring pump is combined with a second stage of atmospheric pump, the ultimate vacuum can reach 2–10 Torr. 　　(III) Rotary vane pump – water ring pump parallel mechanical vacuum pump: This type of unit is mainly used in vacuuming systems that require the handling of large amounts of water vapor and have high demands regarding ultimate vacuum levels, such as in vacuum drying applications. 　　In vacuum systems that require the handling of large amounts of water vapor, water ring pumps are a suitable choice; however, due to their limited ability to achieve high vacuum levels, the overall vacuum level of the system remains low. In vacuum systems that demand higher vacuum levels, mechanical vacuum pumps with higher vacuum capabilities are needed as pre-pumps. But because water ring pumps consume a lot of electricity, have low efficiency, and generate significant noise, it is not economical to use them as pre-pumps for rotary vane pumps in systems that require long-term vacuum drying. 　　Under the above conditions, a gas-jet mechanical vacuum pump can be used in parallel with a water ring pump as a pre-pump for a Roots pump. During vacuum drying, the water ring pump is used first for preliminary evacuation; once the amount of water vapor has been significantly reduced, the gas-jet mechanical vacuum pump is activated and the water ring pump is turned off. In cases where a longer time is required to complete the drying process, less cooling water and power are needed. 　　 　　Operation of the Roots pump–water ring pump unit: A condenser (1) is installed in front of the unit. To minimize the size of the unit, it is possible to cause the steam to be pumped to condense before it reaches the pump unit; what remains are non-condensable gases and trace amounts of residual steam. As the gas cools down, its volume also decreases at constant pressure. Therefore, the amount of gas that needs to be pumped out after condensation is reduced, and accordingly, a smaller pump can be used. 　　Which method is more economical? It depends on the specific circumstances; for example: There are two ways to condense steam: one is to install a cooling device, and the other is to place a condenser in the high-pressure stage of the unit so that it can be cooled with ordinary water. 　　Its system needs to remove 50 kg of water vapor per hour, with a volumetric flow rate of 50,000 m3/h at an inlet pressure of 1 Torr. 　　1) To extract the aforementioned amount of water vapor, a unit consisting of three Roots pumps connected in series along with a water ring pump as a pre-stage is required; the total power of this unit is 90 kW. 　　II) In order to cause the steam to condense before it reaches the vacuum pump, a condenser and a cooling unit with a capacity of 30,000 kcal/h must be installed at point A, as shown in Figure 4. At an intake pressure of 1 Torr, the condensation temperature of water vapor is -19°C; to ensure continuous operation, the condensation temperature of the cooling unit should be set at -25°C, and two condensers should be installed in parallel. Based on the composition of the non-condensable gases, the pumping capacity of the vacuum pump can be reduced to 1,000–2,000 m³/h, with the total power consumption of the entire system (including that of the condensers) remaining at 90 kW. 　　III) First, water vapor is pumped out using a Roots pump, and then condensed at a pressure of 45 Torr; at this pressure, the condensation temperature is around 36°C to 35°C. Thus, it is possible to keep the condensation temperature at 30°C, and ordinary cooling water can be used for cooling. The condenser is located at point B. The total power consumption in this case is approximately 75 kW. 　　By comparing the above three approaches, it can be seen that the third option is the best, as it reduces power consumption by 15 kW. 　　In summary, after water vapor is cooled, only non-condensable gases remain. At very low pressures, the specific volume of water vapor is quite high; once these condensable vapors condense, the amount of gas that needs to be pumped out is significantly reduced. Additionally, whether or not the vapor condenses, the volumetric flow rate decreases as the gas temperature drops at the same pressure. For example, in chemical processes, it is common to find gases at temperatures of 200–300°C. If such gases are cooled from 300°C to 50°C, the volume of dry air decreases by about 45%, allowing for the use of a vacuum pump system with a smaller capacity. 　　(2) Operation sequence of the unit: 1) When there is no bypass valve in the unit, the water ring pump should be started first; the gas in the system to be evacuated enters the water ring pump via the Roots pump (the gas drives the rotor of the Roots pump to rotate on its own, similar to how a flow meter works), and then is discharged into the atmosphere. The Roots pump is started only when the suction pressure of the water ring pump – or, if an atmospheric pump is connected in series, the suction pressure of that atmospheric pump – reaches the predetermined value for the Roots pump (i.e., the allowable exhaust pressure); at that point, the unit begins to operate properly. 　　II) When there is a bypass valve in the unit, as shown in Figure 5, the water ring pump is started first, followed by the Roots pump. At this point, the pressure difference between the inlet and outlet of the Roots pump is relatively large, so the bypass valve opens automatically. Part of the gas inside the container to be evacuated enters the water ring pump through the bypass valve, while another part enters the water ring pump via the Roots pump itself. Obviously, the evacuation rate increases, and thus the pre-vacuum state of the Roots pump is reached quickly; the pressure difference between the inlet and outlet becomes smaller, and the valve closes automatically (or manually), at which point the unit begins to operate properly. This method can **reduce the pre-evacuation time, but the equipment required is more complex. 　　(III) Performance relationship between the unit, the Roots pump, and the pre-pump The performance of the unit is closely related to that of the Roots pump, and the performance of the Roots pump varies depending on the type of pre-pump used. 　　1) Due to the gaps between the rotors of a Roots pump, as well as between the rotors and the casing, backflow occurs. This backflow is influenced by the inlet pressure and outlet pressure; even for the same Roots pump, using different pre-pumps will result in varying pumping rates. 　　The pumping speed of a Roots pump can be determined by the following formula: (=δ0δP2 / P1 / K). 　　In the formula: δ0 is the designed pumping rate. 　　P1 – Inlet pressure. 　　P2 – Outlet pressure. 　　K is an inherent constant, determined by the shape and clearance of the pump rotor, the circumferential speed of the rotor, and the outlet pressure. 　　As can be seen from the above equation, the pumping volume is affected by the ratio of the outlet pressure to the inlet pressure; in other words, if the pumping rate of the preceding pump is increased, then the pumping rate of the Roots pump will also increase. 　　(2) The ultimate pressure is determined by the pump’s pumping rate, the backflow volume in each gap, the leakage from the pump body, and the gas release volume on the high-vacuum side. That is: Section 2: Ultimate pressure = (+)/δ + Equation 3; where P0 represents the ultimate pressure. 　　δ-pumping rate. 　　Q1-Return flow. 　　Q2-Leakage volume. 　　Q3-Air release volume. 　　Among these parameters, Q1 is greatly influenced by the exhaust pressure, that is, the limit pressure of the pump preceding it. When a water ring pump is used as the preceding pump, the limit pressure of the Roots pump varies depending on the saturated vapor pressure of the water ring. 　　Figure 6 shows a performance comparison using the same Roots pump with different pre-pumps. 　　 　　The higher the ultimate vacuum level of the pre-pump, the higher the ultimate vacuum level of the entire unit as well. Using two Roots pumps in series can increase the ultimate vacuum level of the unit; in essence, the first Roots pump acts as a pre-pump for the second one). Moreover, the performance curve expands more gradually, meaning that the range of operation increases (as can be seen from the comparison between curves 1 and 2, and curves 3 and 5). The curves for units 1 and 2 are roughly similar, and similarly, the curves for units 3, 4, and 5 also share similarities. However, the curves for units 1 and 2 are completely different from those of units 3, 4, and 5. This shows that when different pre-pumps are used with the same Roots pump, there are substantial differences in the performance curves of the units. It is thus evident that the pre-pump has a significant impact on the performance of the unit. 　　(IV) Selection of water ring pumps A water ring pump is one that uses water as the liquid ring. Such pumps are readily available, and using water as the liquid ring has many advantages, such as low cost and no environmental pollution. However, it also has a significant drawback: due to the high saturated vapor pressure of water, the suction pressure of the water ring pump is high as well. If a liquid with a lower saturated vapor pressure is used as the liquid ring, the ultimate vacuum level achievable by the pump can be increased. If the maximum allowable exhaust pressure for a Roots pump in a certain system is 10 Torr, then an air pump must be used as a pre-stage pump when water is used as the liquid ring; whereas if mineral oil is used as the liquid ring, no additional air pump is needed to serve as the pre-stage pump, which simplifies the setup. 　　(V) Unit performance and the allowable discharge pressure of the Roots pump The performance of the unit depends to a large extent on the allowable discharge pressure of the Roots pump. The lower this allowable value is, the less likely it is that a water ring pump can be used as a pump preceding the Roots pump. If the allowable value for the Roots pump is 1–10 Torr, then the maximum pressure of single-stage or multi-stage water ring pumps is higher than this value; therefore, such water ring pumps cannot be used in combination with the Roots pump alone, and an additional atmospheric pump is required. If the allowable discharge pressure of the Roots pump is above 100 Torr, then a water ring pump can serve as a pump preceding the pump that precedes the Roots pump, thereby greatly expanding the range of applications for the pump preceding the Roots pump. 　　(VI) Application Examples The production process of a certain fiber product is as follows: low-molecular-weight stage – high-molecular-weight stage – formation into strips – slicing – drying (using rotary vane pumps and water ring pump systems for vacuum drying) – spinning – drawing, heating – weaving – finished product. 　　One of the important steps is to dry the slices measuring 5××5 millimeters for 5 hours in order to enable fiber drawing. The ideal condition during this process is to achieve a water content of zero; however, since this goal cannot be fully achieved, the required water content is no more than 0.02%. If the water content exceeds this limit, fiber drawing must be carried out under high temperature and pressure, which can cause the polymers to decompose and affect the strength of the product. 　　The vacuum pumping system used in the drying process is a Roots pump-water ring pump unit. The technical parameters of each pump in this unit are shown in the table below. 　　The functions of the main equipment are as follows: (1) The vacuum valve closes, stopping the operation of the unit and helping to maintain a certain level of vacuum in the drying system. 　　(2) The automatic pneumatic safety valve is used to prevent water in the water ring pump system from flowing back into the Roots pump and the vacuum drying system in the event of a sudden shutdown of the unit. 　　(III) When the differential pressure valve unit starts operating, the water ring pump is activated first, thereby creating a differential pressure across the two ends of the valve. Once this pressure reaches a certain value, the valve opens automatically, allowing most of the gas in the system to flow into the water ring pump through this valve. As air gradually flows into the water ring pump via the air pump, the pressure across the two ends of the valve decreases again, causing the valve to close. At that point, the air pump begins to operate normally. The function of the differential pressure valve is to reduce the pre-pumping time required before the air pump can start working properly. 　　(IV) The water level gauge acts as a check valve to prevent the water from the water ring pump from flowing back. 　　The sequence of operations is as follows: First, start the water ring pump. The gas in the system enters the water ring pump via the Roots pump (which drives the rotation of the Roots pump rotor), and then it is discharged into the atmosphere through the gas-liquid separator. When the absolute pressure reaches 50 Torr, start Roots pump 2; when the absolute pressure drops to 20 Torr, start Roots pump 1. Ultimately, the system pressure can reach 0.088 Torr, and it can generally be maintained below 1 Torr.

It is sufficient to provide schematic diagrams of the working principles for the two types of pumps.

There is a difference; rotary vane pumps generally can only be started when a certain negative pressure is reached. It is started directly; however, due to the relatively large pressure difference, this can cause damage to the Roots pump, which is why the Roots pump is usually placed in the back circuit. Start the water ring pump first, and then start the Roots pump once the pressure reaches a certain level.

Pay attention to the drainage issue of water ring pumps, which is also an environmental concern.

We are currently using a soda-water series vacuum pump in combination with a 2-stage Roots pump; this setup allows us to achieve a vacuum level of –0.099. The Roots pump starts to operate only after a certain vacuum level is reached.

What everyone is saying is that there is a water ring vacuum pump before the Roots vacuum pump. What’s wrong with using the Roots vacuum pump as the primary stage and the water ring vacuum pump as the secondary stage?

It is suggested that those upstairs review the remarks made by the experts above again. It’s explained very clearly inside. . .

Reply to 5# lingl: The primary water ring pump creates a certain level of vacuum for the Roots pump; the Roots pump cannot extract gas at atmospheric pressure, as this would result in excessive load on the motor and could easily damage the pump