If you want to obtain a high vacuum, you must use a vacuum unit, and the Roots vacuum pump required by the vacuum unit can not directly discharge the atmosphere. For example, the straight-line atmosphere will form a pressure difference between the inlet and the exhaust port of the Roots vacuum pump, so that the Roots vacuum pump Overloading, such as simply increasing the power of the Roots vacuum pump motor, will cause the Roots vacuum pump to overheat, causing the tiny gap between the Roots vacuum pump rotors to quickly become stuck due to heat shrinkage. In order to ensure that the Roots vacuum pump can reach a higher vacuum, it is necessary to ensure the gap between the rotors of the Roots vacuum pump. Therefore, the Roots vacuum pump must be equipped with a foreline pump. When the foreline pump is used to pump the pressure inside the system to a certain range, the Roots vacuum pump is started, which can prevent the Roots vacuum pump from being overloaded. The foreline pump can use a water (liquid) ring vacuum pump, a rotary vane vacuum pump, a slide valve vacuum pump, a reciprocating vacuum pump and the like which can directly discharge the vacuum pump. Roots vacuum unit Under normal conditions, the use of water ring pump as the foreline pump is more advantageous than other vacuum pumps, mainly because it can remove a large amount of condensable steam, especially when the gas ballast mechanical vacuum pump sweeps the coagulation. Sexual steam can not be enough, or the solvent used can make the pump oil deteriorate and affect the performance, or the time when the vacuum system does not allow oil pollution is more obvious. Roots pump-water ring pump units are commonly used in vacuum systems for chemical, food sublimation, high-altitude simulation experiments. This type of combined unit has the following types. (1) Roots pump-water ring pump: The function of the water ring pump in the unit is to form the required vacuum for the Roots pump, thus requesting the maximum allowable exhaust pressure of the water ring pump, that is to say, on the one hand, try to To improve the ultimate vacuum of the water ring pump, on the other hand, try to improve the maximum allowable exhaust pressure of the Roots pump. Ordinary conditions, the ultimate vacuum of the single-stage water ring pump is not high, and the pre-vacuum requested by the Roots pump currently consumed in China is higher. Therefore, in practice, the single-stage water ring pump is not used as the fore pump of the Roots pump. The two-stage water ring pump that can improve the ultimate vacuum is used as the foreline pump. The two-stage water ring pump can also improve the ultimate vacuum of the unit. The ultimate vacuum of a Roots pump is low, especially when it is combined with a water ring pump, the application range is limited, the ultimate vacuum of the whole unit may be lower, but if two Roots pumps are used The combination of the series and the water ring pump can greatly improve the ultimate vacuum of the unit. Therefore, what is usually seen in this type is that two Roots pumps are connected in series and then a two-stage water ring pump is used as a foreline pump. (2) Roots pump-water ring pump-atmospheric pump unit: Even with a two-stage water ring pump, the ultimate vacuum improvement is only within a certain range, which is limited by the saturated vapor pressure of water. The theoretical ultimate pressure of a water ring pump is the saturated vapor pressure of water. If the influence of factors such as gas backflow is considered, in practice the ultimate pressure of the water ring pump is significantly higher than the saturated steam pressure at the water temperature. In order to improve the ultimate vacuum of the foreline pump, it is also possible to combine the water ring pump with the atmospheric pump. In this way, the ultimate vacuum after connecting the first-stage atmospheric pump can reach 20~30 Torr. If the water ring pump is combined with the secondary 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 for vacuuming systems that require a large amount of water vapor to be disposed of, and which requires a higher ultimate vacuum, for example, in vacuum boring. In a vacuum system that requires a large amount of water vapor to be treated, it is preferable to use a water ring pump, but since the ultimate vacuum is not high, the ultimate vacuum of the entire unit is low. Of course, in a vacuum system requiring a higher degree of vacuum, a mechanical vacuum pump requiring a higher ultimate vacuum is used as a foreline pump. However, due to the large power consumption of the water ring pump, the efficiency is very low, and the noise is high. In the vacuum boring system which requires a long time, the water ring pump is uneconomical as a Roots pump fore pump. Under the above conditions, the gas ball mechanical vacuum pump can be connected in parallel with the water ring pump as the foreline pump of the Roots pump. When the vacuum is boring, first stop the pre-extraction with the water ring pump until the water vapor is greatly reduced, then start the gas-fired mechanical vacuum pump and cut off the water ring pump. If it takes a long time to complete a boring place, less cooling water and power are required. 1) Condenser in front of the unit In order to make the volume of the unit as small as possible, it is possible to try to condense the steam to be pumped before entering the pump unit, so that the remaining is non-condensable gas and a small amount of residual steam. After the gas is cooled, the volume is also reduced at the same pressure. Therefore, the amount of pumping required after condensation is reduced, and accordingly the pump can be selected to be smaller. Which method is more economical? It should be based on its detailed status, exemplified as follows: There are two ways to condense steam: one is to install one unit for cooling, and the other is to install a condenser in the high-pressure stage of the unit so that it can be cooled with ordinary water. The system requires 50 kg of water vapor per hour to be withdrawn, and the volumetric flow rate at a suction pressure of 1 Torr is 50,000 m3/h. 1) To pump the above amount of water vapor, three Roots pumps are required in series, and a water ring pump is used as the unit of the front stage. The total power of the unit is 90 kW. 2) In order to condense the steam before it reaches the vacuum pump, install a condenser at A and a cooling installation with a power of 30,000 kcal/h. At a suction pressure of 1 Torr, the condensation temperature of the water vapor is 3) First pump out the steam with a Roots pump and stop the condensation at a pressure of 45 Torr. The condensation temperature at this pressure is about According to the comparison of the above three groups, the third plan is the best, which can reduce the power consumption of 15kW.