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Working principle of rotary vane pump – water ring pump vacuum systems: 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 thus affect its performance, or when the vacuum system does not allow for oil contamination. Roths pump–water ring pump units are widely used in vacuum systems for chemical industries, food sublimation drying, high-altitude simulation tests, and more. There are roughly the following types of such combined units. (1) Roots pump – water ring pump: In the system, the function of the water ring pump is to create the preliminary vacuum required by the Roots pump; therefore, it is necessary to determine the maximum allowable exhaust pressure for this water ring pump. In other words, on one hand, it is important to increase the ultimate vacuum achievable by the water ring pump, and on the other hand, it is also necessary 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. http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910312734.gif The ultimate vacuum level achievable by 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, a unit is commonly constructed by connecting two Roots pumps in series and using a two-stage water ring pump as the pre-pump (Figure 1). (2) Roots pump – water ring pump – atmospheric pump unit: Even with a two-stage water ring pump, the improvement in ultimate vacuum is limited to a certain extent due to the constraint imposed by the saturated vapor pressure of water. The theoretical limit pressure of a water ring pump is the saturated vapor pressure of water. If factors such as gas backflow are taken into account, the ultimate pressure of a water ring pump is actually significantly higher than the saturated vapor pressure at that water temperature. To improve the ultimate vacuum of the primary pump, a water ring pump can also be used in combination with an atmospheric pump. http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910313580.gif http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910313621.gif See Figure 2. In this way, 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. (3) Roots pump – water ring pump parallel mechanical vacuum pump: This unit is mainly used in vacuuming systems that require the handling of large amounts of water vapor and high levels of ultimate vacuum, such as in vacuum drying. In vacuum systems that require the handling of large amounts of water vapor, water ring pumps are a suitable choice; however, due to their low ultimate vacuum level, the ultimate vacuum level of the entire system is also low. Although in vacuum systems that require a high degree of vacuum, mechanical vacuum pumps with a high ultimate vacuum are needed as pre-pumps. However, due to the high power consumption, low efficiency, and high noise of water ring pumps, it is not economical to use them as pre-pumps for Roots pumps in vacuum drying systems that require long operation times. Under the above conditions, a gas-loaded mechanical vacuum pump can be connected in parallel with a water ring pump to serve as a pre-pump for a Roots pump. During vacuum drying, pre-evacuation is first carried out using a water ring pump; once the amount of water vapor has been significantly reduced, a mechanical vacuum pump is started and the water ring pump is turned off. In cases where a longer time is required for drying, less cooling water and power are needed, as shown in Figure 3. Operation of Roots pump – water ring pump units （1）Condenser installed in front of the unit http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910313360.gif To minimize the size of the unit, it is possible to have the steam to be pumped condensed before it enters the pump unit; this way, what remains are non-condensable gases and trace amounts of residual steam. After the gas is cooled, its volume also decreases at the same pressure. Therefore, the amount of gas that needs to be pumped out after condensation decreases, 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 by installing a cooling device, and the other is by placing 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 rotary vane pumps connected in series and a water ring pump as a pre-stage is required, with a total power of 90 kW for this unit. 2) 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 location 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 condensing device should be set at -25°C, and two condensers should be installed in parallel. Based on the components of the non-condensable gases, the pumping capacity of the vacuum pump can be reduced to 1000–2000 m3/h, while the power consumption of the entire system (including the condenser) remains at 90 kW. 3) 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 approximately 36°C. This allows the condensation temperature in the condenser to be maintained between 30 and 35°C, and it can be cooled using ordinary cooling water. The condenser is located at B. At this time, the total power consumption is around 75 kW. By comparing the above three methods, it can be seen that the third option is the best, as it can reduce 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 large; once this condensable vapor condenses, the amount of gas that needs to be pumped out by the pump is significantly reduced. Furthermore, whether the steam condenses or not, at the same pressure the volumetric flow rate decreases as the gas temperature drops. For example, gases at temperatures of 200–300°C are not uncommon in chemical processes. If the volume of dry air decreases by about 45% after cooling from 300°C to 50°C, it is then possible to use a vacuum pump system with a smaller capacity. (2) Operating 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 from there it is discharged into the atmosphere. Once the suction pressure of the water ring pump (or, if an atmospheric pump is connected in series, the suction pressure of that pump) reaches the predetermined value for the Roots pump (i.e., the allowable exhaust pressure), the Roots pump is started, and the unit begins to operate properly. 2) 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 being evacuated enters the water ring pump through the bypass valve, while another part enters the water ring pump via the Roots pump itself. As a result, the evacuation rate increases, and the pre-vacuum condition of the Roots pump is quickly achieved. Once the pressure difference between the inlet and outlet becomes smaller, the valve closes automatically (or manually), and the unit begins to operate properly. This method can **reduce the pre-pumping time, but the equipment is more complex. http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910314371.gif (3) 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 the 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: δ = δ0(P2/P1/K), where δ0 is the designed pumping speed ； P1 – Inlet pressure ； P2 – Outlet pressure ； K – an inherent constant, determined by the shape of the pump rotor, the clearance amount, 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, the pumping rate of the Roots pump will also increase. (2) The ultimate pressure is determined by the pump’s pumping speed, the backflow rate in various gaps, the leakage from the pump body, and the gas release rate on the high-vacuum side. That is: P0＝(Q1+Q2+Q3)/δ Where: P0 – ultimate pressure ； δ – pumping rate ； Q1 – Backflow volume ； Q2 – Leakage volume ； Q3 – Air release volume. Among these parameters, Q1 is greatly influenced by the exhaust pressure, that is, the ultimate pressure of the preceding pump. When a water ring pump is used as the preceding pump, the ultimate 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. http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910314797.gif As can be seen from the graph, the higher the ultimate vacuum level of the pre-pump, the higher the ultimate vacuum level of the entire system as well ； When two-stage Roots pumps are used in series, the ultimate vacuum level of the system can be improved (in essence, the first Roots pump acts as a pre-pump for the second one), and 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 the same. Similarly, the curves for units 3, 4, and 5 also share similarities. However, the curves for units 1 and 2 are two completely different sets of curves from those of units 3, 4, and 5. This shows that for the same Roots pump, choosing different forepumps results in fundamental differences in the performance curve of the system. It can be seen that the primary pump has a considerable impact on the performance of the unit. (4) Selection of water ring pumps A water ring pump is one that uses water as the liquid ring. There are many advantages to using water as the liquid ring, such as its low cost, availability, and the fact that it does not pollute the environment. However, it also has a major drawback: due to the high saturated vapor pressure of water, the suction pressure of the water ring pump is also high. At this point, by using a liquid with a reduced saturated vapor pressure as the liquid ring, the ultimate vacuum level of the pump can be improved. If the maximum allowable discharge pressure of the Roots pump in a certain unit is 10 Torr, then an air pump must be added when water is used as the liquid ring in order to serve as a pre-pump for that Roots pump. If mineral oil is used instead as the liquid ring, no additional air pump is needed to function as a pre-pump, which simplifies the setup. (5) 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, the less likely it is that the water ring pump will be used as a pre-pump. If the allowable value for rotary vane pumps is 1–10 Torr, regardless of whether the pressure limit of single-stage or multi-stage water ring pumps is **higher than this value, then it is not possible to use them in combination with such rotary vane pumps; instead, a secondary atmospheric pump is required. If the allowable discharge pressure of the Roots pump is above 100 Torr, then the water ring pump operating at the previous stage can also serve as a pump for the stage preceding the Roots pump, thereby **significantly expanding the application range of the pump at the previous stage. (6) 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 and heating – weaving – finished product. One of the important processes is to dry the slices with dimensions of 5×5×5 mm in order to enable wire drawing. The ideal condition during this process is to have a moisture content of zero; however, since this goal cannot be achieved, the moisture content is required to be no more than 0.02%. If the moisture content exceeds this limit, wire 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 for the Roots pump-water ring pump unit used in the drying process is shown in Figure 7. The technical parameters of the pumps in the unit are shown in the table below. Name, Air Extraction Rate (m3/h), Rotor Diameter (mm), Rotational Speed (r/min), Power Requirement (kW): Rotary Pump 1 – 1400, 160, 1450; 2.2 kW. Rotary Pump 2 – 2200, 102, 2900; 1.3 kW. Two-stage water ring pump – 100; the impeller of the first stage is half the size of that of the second stage. 1450, 5.5 kW. http://www.cntcw.com/Doc/data.WebNoteBooks/20060611162846/20055910314112.gif. A brief description of the functions of the main equipment is as follows: (1) Vacuum valve: By closing the vacuum valve, the unit stops operating, thereby maintaining a certain level of vacuum in the drying system. (2) Automatic pneumatic safety valve: To prevent water in the water ring pump system from flowing back into the Roots pump and vacuum drying system in the event of a sudden shutdown of the unit. (3) Differential pressure valve: When the unit starts operating, the water ring pump is activated first, and a differential pressure gradually develops across the differential pressure 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 the atmosphere gradually flows from the atmospheric pump into the water ring pump, the pressures on both sides of the pressure difference valve decrease gradually, causing the valve to close. The atmospheric pump then starts to operate normally; the function of the pressure difference valve is to shorten the pre-pumping time required before the atmospheric pump can function properly. (4) Water level gauge: Functions 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 rotor of the Roots pump to rotate), and then it is discharged into the atmosphere through the gas-liquid separator. Rotary pump 2 is started when the absolute pressure reaches 50 Torr, and rotary pump 1 is started when the absolute pressure drops to 20 Torr; ultimately, the system pressure can reach 0.088 Torr, and it generally remains below 1 Torr. For the performance parameters of the Roots-water ring vacuum pumps, please refer to: JZJ2S series Roots-water ring vacuum pumps, JZJ2B series Roots-water ring vacuum pumps