Study On Inter-stage Leakage And Pumping Speed Measuring Method Of Trapezoidal Tooth Rotor Screw Vacuum Pump

At present,screw vacuum pump has been widely used in semiconductor,medicine,food,chemical and other industrial fields.Its position continues to be improved in the vacuum pump market.The screw rotor acts as the main part of the screw vacuum pump.The tooth type is with better intermeshing,continuous sealing and stability.With the screw pump continuous work,the pressure at the suction port will continue to decrease and gradually tend to a certain value but the pressure at the exhaust port is always atmospheric pressure.This results in a reverse leakage of the gas from the high pressure side of the exhaust port to the low pressure side of the suction port.The reason why this paper study the inter-stage leakage of screw vacuum pump,mainly because pumping speed can be required in accordance with the theory of design rotor at present.But there is no theory to design rotor according to the vacuum degree requirements and the calculation of inter-stage leakage is just the basis of this design theory.In this paper,aiming at the inter-stage leakage problem of trapezoidal tooth rotor screw vacuum pump,study the inter-stage leakage path,the internal gas flow pattern,the leakage,the proportion of each leakage path to total leakage and a new method for measuring leakage of screw vacuum pump from the structure of the uniform pitch rotor leakage path.Firstly,the structure of the inter-stage leakage path is analyzed in this paper.It is proposed that the leakage path is composed of four parts between the two adjacent suction chambers:Leakage path L1 between the tooth top and the inner wall of the pump chamber,Leakage path L2 between the tooth top and the tooth root,Leakage path L3 between the conjugate tooth surfaces and Leakage path L4 between the two helical surfaces.Then,the length L,depth h and width b of the four kinds of leakage paths are solved.According to the actual working pressure range of the screw pump,this paper mainly investigates the problem of inter-stage leakage of screw pump under viscous flow.Secondly,the L1 path is simplified as a flat path model.L2,L3 and L4 path are simplified into a Laval nozzle model.The leakage formulas of L1,L2,L3 and L4 four paths are deduced respectively.A kind of the common 150-type uniform pitch screw vacuum pump is selected to analyze examples and find out the pressure,temperature,density and leakage.According to the calculated results,it is concluded that the flow in L1 path is mainly the Poiseuille flow,which accounts for the percentage of up to 99%or more.The leakage of L1 path occupies the majority of the whole leakage path,and the percentage is about 76%.The percentage of leakage of the four paths to the total leakage is irrelevant to the position within the pump chamber but only the length and area of the path.Regardless of the value of the gap,the closer path is to the location of the exhaust port and the larger the leakage is and increases the speed faster and faster.But as the gap increases,the amount of leakage in the vicinity of the intake port increases little,and the amount of leakage in the vicinity of the exhaust port increases greatly.It is recommended to select a smaller gap under normal operation.Finally,a new method of dropper-type gas flow measurement based on weighing principle is proposed.It can be directly measured pump effective pumping speed.According to the effective pumping speed that it is equal to the theoretical pumping speed minus the amount of leakage,the leakage can be solved.This paper also uses the test platform in the laboratory to verify its feasibility.The study found that the measuring accuracy of the electronic scale has little effect on the measuring results.The greater the rate of oil in the dropper rise,the greater the impact on the measuring results.Choosing the larger diameter of the dropper and the storage cup can reduce the measuring error.Overall,this method has strong operability,easy to realize automatic measurement,avoids the original measuring method of operational errors and principle error and can accurately measure the effective pumping speed of screw vacuum pump.