< >

       In order to meet the needs of oil-free vacuum cleaning in manufacturing processes such as semiconductor, electronics and chemical industries, more and more dry vacuum acquisition equipment has been developed and widely used. Among them, dry Roots vacuum pump plays an important role in the dry pump market because of its advantages of high pumping speed and loose conditions. Among the dry Roots vacuum pumps, multi-stage Roots vacuum pumps are the main ones. The developed countries of vacuum technology abroad have developed and produced a series of products of multi-stage Roots vacuum pumps.

       The so-called multi-stage is relative to the traditional "single-stage" Roots pump. That is to say, multi-stage Roots rotors are connected in series on the rotor shaft, and the rotors at all levels are separated by the middle wall to form relatively independent pump chambers, which are connected by the air passage between the upper intake port and the lower exhaust port. The diameter and shape of the rotors at all levels are the same, while the width of the rotors at all levels is sometimes different, narrowing in the direction of high pressure side. The exhausted gas enters the pump cavity from the intake port of the first stage. After the first stage compression, it enters the next stage pump cavity through the air passage, and continues to compress. The gas is compressed step by step, and finally discharged from the exhaust day of the last stage. In short, "multi-stage" means "multi-stage rotor", "multi-stage pump chamber", "multi-stage compression".

       Multi-stage Roots pumps can achieve the effect of multiple Roots pumps in series through the series connection of multi-stage rotors, so multi-stage Roots pumps can not have a larger pumping speed than single-stage Roots pumps, and its theoretical pumping speed depends on the theoretical pumping speed of the first stage. But this structure can make Roots vacuum pump achieve a high compression ratio. Therefore, the multi-stage Roots pump can be used as the main pump to discharge air directly, without the need to reconfigure the front pump, so as to avoid the oil and gas pollution caused by the front pump link to the system. Roots pumps of Grade 3-6 can be pumped from atmospheric pressure to the limit pressure of 1Pa. Such a high pressure difference will cause the rotor to overheat and expand, resulting in stuck phenomenon. Therefore, the rotor should be air-cooled in the high-pressure section to control the temperature of the rotor not exceeding its allowable range. The general air-cooling scheme is to make the exhaust gas from all levels flow through the cooler to cool, and then return to the working chamber of the stage. Cooling gas is used to cool the rotor. Of course, this cooling method is at the cost of losing the pumping speed of Roots pump. In order to prevent the lubricating oil of bearings and gears from flowing back into the pump chamber, the pump chamber of multistage Roots pump is separated from the gear box or sealed with a labyrinth structure. In order to improve the oil-free degree, the gear box also has the condition of vacuum pump exhaust.