Encyclopedia knowledge of Roots vacuum pump
Roots vacuum pump refers to a variable volume vacuum pump in which there are two blade-shaped rotors rotating synchronously in opposite directions in the pump, and there are small gaps between the rotors, between the rotors and the inner wall of the pump casing, which do not contact each other. Roots vacuum pumps have been tested for long-term operation in petroleum, chemical, plastic, pesticide, steam turbine rotor dynamic balance, aerospace space simulation and other devices, so they should be vigorously promoted and applied in China. It is also widely used in petroleum, chemical, metallurgy, textile and other industries. Vacuum pump accessories are used for vacuum pump noise control, vacuum pump silencer. 1. Working principle Roots vacuum pump is a vacuum pump that realizes pumping by moving the gas by the pushing action of a pair of vane rotors in the pump chamber synchronously and counter-rotating. Roots vacuum pump refers to a mechanical vacuum pump with a pair of sole-shaped rotors that rotate synchronously at high speed. This pump cannot be pumped alone. The front stage needs to be equipped with oil seals, water rings, etc. to directly exhaust the atmosphere. Its structure and working principle are similar to the Roots blower, and its suction port is connected to the vacuumed container or the main pump of the vacuum system during operation. There is no contact between the rotor and the rotor, and between the rotor and the pump casing of this vacuum pump, and the gap is generally 0.1 to 0.8 mm; no oil lubrication is required. Rotor profiles include arcs, involutes and cycloids. The ultimate vacuum of the Roots pump depends not only on the structure and manufacturing accuracy of the pump itself, but also on the ultimate vacuum of the backing pump. In order to improve the ultimate vacuum degree of the pump, the Roots pump can be used in series. The working principle of a Roots pump is similar to that of a Roots blower. Due to the continuous rotation of the rotor, the pumped gas is sucked from the air inlet into the space v0 between the rotor and the pump casing, and then discharged through the exhaust port. Since the v0 space is completely closed after inhalation, the gas in the pump chamber is not compressed and expanded. However, when the top of the rotor turns over the edge of the exhaust port, and the v0 space is connected to the exhaust side, due to the high gas pressure on the exhaust side, a part of the gas backrushes into the space v0, causing the gas pressure to suddenly increase. As the rotor continues to rotate, the gas is expelled from the pump. In the pump chamber of the Roots pump, there are two "8"-shaped rotors installed on a pair of parallel shafts perpendicular to each other, and a pair of gear belts with a transmission ratio of 1 act in opposite synchronous rotation. Between the rotors, between the rotor and the inner wall of the pump casing, there is a certain gap, which can realize high-speed operation. 2. Structural composition How the two rotors of the Roots vacuum pump are arranged in the pump body determines the overall structure of the pump. There are generally three schemes for the overall structure layout of Roots vacuum pumps at home and abroad: 1. Vertical: the axes of the two rotors are installed horizontally, but the plane formed by the axes of the two rotors is perpendicular to the horizontal plane. This structure, the inlet and exhaust ports of the pump Set up horizontally, it is more convenient to assemble and connect pipes. But its disadvantage is that the center of gravity of the pump is too high, and the stability is poor at high speed operation, so except for small-sized pumps, not many pumps use this structure. 2. Horizontal type: the axes of the two rotors are installed horizontally, and the plane formed by the axes of the two rotors is in a horizontal direction. The air inlet of the pump with this structure is above the pump, and the exhaust port is below the pump (there are also The opposite of). The exhaust ports below are generally connected in a horizontal direction, so the directions of intake and exhaust are perpendicular to each other. The exhaust port is connected to a three-way pipe to open in two directions, one end is connected to the exhaust pipe, and the other end is dead or connected to the bypass valve. This structure is characterized by a low center of gravity and good stability during high-speed operation. Large and medium-sized pumps at home and abroad mostly use this type of structure. 3. Vertical shaft type: The two rotor axes of some foreign Roots pumps are installed vertically with the horizontal plane. The assembly clearance of this structure is easy to control, the rotor assembly is convenient, and the floor area is small, but the assembly and disassembly of transmission mechanisms such as gears are inconvenient, and the lubricating device is also complicated. When the overall structure is determined, the structure and shape of the pump body itself are determined accordingly. 4. Roots pump with overflow valve: In order to prevent accidents caused by overloading, a relatively reliable safety protector is installed on the Roots pump, that is, an overflow valve is installed on the bypass pipeline. When the exhaust port is at the specified pressure, the relief valve is closed. When the pressure of the exhaust port exceeds the specified pressure, the valve of the overflow valve is automatically pushed open to generate overflow. After the pressure of the exhaust port becomes normal, the overflow valve closes by itself. It can be adjusted automatically and is also the allowable differential pressure device of the pump, so the greatest benefit of the relief valve is that the Roots pump together with the backing pump can run continuously in various pressure ranges. With this design, the pumping stop time of the vacuum container in the rough vacuum state can be shortened by 30~50%. For relatively large pumps, the relief valve is installed on the bypass pipeline outside the pump body, and in relatively small pumps On the other hand, the relief valve is installed in the pump casing. 5. Roots pump with steam condenser: When steam needs to be pumped, the suction unit must be designed with a condenser that can condense steam. This condenser can be installed before or after the pump, not installed in the pump. on the pump body of the Roots pump. Under certain conditions, condensing and escalating heat absorption can reduce the heat generation of the Roots pump. Assuming a duplex condenser is used, the fouling can be removed with a suitable solvent during maintenance and the steam can flow smoothly through the conduit. It can be seen from the characteristic curve that when the ultimate vacuum is reached, the forward air flow through the pump inlet is zero, that is, the actual pumping speed of the pump is zero. Cooling device 1. Air cooling: Roots vacuum pump generates heat due to conveying and compressing gas, and this heat must be dissipated from the rotor to the casing. However, at low pressure, the heat conduction and convection performance of the gas is extremely poor, so that the heat absorbed by the rotor is not easily dissipated, resulting in the rotor temperature being always higher than the shell temperature. Due to the thermal expansion of the rotor, the gap between the rotor and the rotor and between the rotor and the pump casing is reduced, especially when the pressure difference is also high, it is especially serious, and even the rotor is stuck and the pump is damaged. In order to make the Roots pump work under a higher differential pressure to expand the range of use and increase the reliability of the pump, it is necessary to try to dissipate the heat generated by the rotor, that is, to cool the rotor. In order to understand the essence of air cooling, let's first look at the flow of gas on the exhaust side of the Roots vacuum pump. The process of compressing the sucked gas in the Roots vacuum pump is not continuous, but sudden. The suction gas is enclosed in the cavity with the rotation of the rotor, and with the rotation of the rotor, the gas in the cavity is suddenly connected to the exhaust port. Due to the high gas pressure on the exhaust side, the gas at the exhaust port is flushed back into the cavity, and then driven out of the pump with the rotation of the rotor. In such a process, the two rotors perform a total of four exhaust processes in each rotation. From the flow of the above gas, it can be assumed that if the gas flushed into the pump cavity is cold each time, a large amount of heat can be absorbed in the high-temperature pump cavity, and the heat-absorbing gas will continue to compress in the rotor. discharge, so as to achieve the purpose of rotor cooling. Air cooling is to use the above principle. Set up dense cooling fins at the exhaust port of the pump, and the cooling fins are cooled with cold water pipes, or directly install cooling water pipes at the exhaust port of the pump, so that the gas at the exhaust port will be cooled. This cooling method can Effectively dissipate the power generated by the Roots pump rotor in the compressed gas. Moreover, when the exhaust pressure is high, due to the high density of gas molecules, the heat conduction performance is better, and the cooling effect is also better. Using this method can ensure that the pump is operated under a higher pressure difference. The experiment proves that when a Roots pump runs for 6 hours under a pressure difference of 30 Torr, the temperature difference between the rotor and the outer casing is 22 degrees. When it is installed at the exhaust port After the cooler, it runs for a long time under 85Torr pressure difference, and its temperature difference does not exceed 17 degrees. Generally speaking, after the Roots vacuum pump is cooled by air, the pressure difference can be increased by 80 Torr, and it can only reach 15~30 Torr without a cooler. This cooling method is related to the ambient temperature. The higher the ambient temperature, the higher the temperature of the inhaled gas. The cooling effect is not good. In addition, this method can only avoid the high heat generated by the high pressure difference, but cannot prevent the heat generated during the compression process of the pump, which causes the problem of narrowing the gap, so it is limited by the gap of the pump itself. 2. Internal cooling of the rotor: In order to make the Roots vacuum pump work under a higher pressure difference, a more effective cooling method can be adopted, that is, the rotor is cooled by circulating oil, and there are oil holes and oil diameter shaft heads at both ends of the pump shaft. , through the inner wall of the rotor and then discharged from the other end. In addition to cooling the rotor, the cooling oil also lubricates the gears and bearings. This cooling effect is better, and the rotor temperature of the pump is lower than the shell temperature when the pump is running, which is often used in large pumps. For example, when working under a differential pressure of 80 Torr, the rotor temperature of the Roots vacuum pump is 78 degrees lower than that of the casing. It is also found that the heavier the pump load, the larger the gap. This is because the rotor is cooled by oil, the temperature is lower than that of the casing, and the higher the load is. Larger, the more the shell expands, the greater the distance between the shafts, so the gap will increase. Due to the large load, the temperature difference between the rotor and the casing continues to increase, so that the gap continues to increase, which will increase the first countercurrent and cause the pumping speed of the Roots vacuum pump to decrease. In order to overcome this shortcoming, effective measures need to be taken when the Roots pump works under high load. Generally, the casing and rotor of the Roots vacuum pump are cooled by the oil circulation system at the same time. 3. Oil film cooling of the rotor: This cooling method is to connect an oil pipe at the inlet of the Roots vacuum pump, and use the cooling oil dripped evenly to take away the heat of the rotor. The oil passes through the filter and cooler, passes through the well-sealed oil pump, and then sends the oil to the inlet of the pump through the oil pipeline. After the oil drips onto the rotor, it will be on the surface of the rotor as the rotor rotates. This not only takes away the heat of the rotor, but also forms an oil film on the two rotor surfaces to prevent the backflow of gas, but also takes away the fine dust attached to the rotor surface. There is an oil tank at the outlet of the pump to collect waste oil, filter it, cool it and then recycle it. This method works well. However, due to the oil in the pump, the characteristics of the Roots pump that the oil-free vapor contaminates the vacuum system is lost. In addition, the oil has a certain viscosity, which increases a lot of friction to the rotor of the Roots pump rotating at a high speed, which of course increases the power consumption of the pump. The oil used requires that the saturated vapor pressure should be replaced as much as possible. 4. Water cooling: The so-called wet Roots vacuum pump means that the air sucked in by the inter-stage or double-stage pump is compressed and sent through a combined muffler with comprehensive absorption and phase difference. The heat generated by the compressed air is removed by injecting a small amount of water into the pump. The suction water pipe is installed on the suction end of the single-stage or double-stage pump group and connected to the air inlet of the vacuum pump. The water is sucked by the vacuum degree generated by the vacuum pump. The greater the vacuum degree, the higher the amount of water inhaled. A simple adjustment valve can ensure the best inhalation volume. The temperature of the inhaled water should be kept at about 20 degrees, and it should be clean and calcium-free.
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