Introduction and selection of the foreline pump

Introduction and selection of the foreline pump First, the introduction of the foreline pump: The foreline pump is used to maintain the vacuum level of a vacuum pump before its critical pressure than the previous stage of the vacuum pump. Such as Roots pump configuration before the rotary vane pump or pump is the forefront pump. Second, the purpose of the previous stage pump: The former stage pump in the vacuum unit to see the term more, mainly to help the main pump Roots vacuum pump start. Roots vacuum pump under normal circumstances can not be started under atmospheric pressure, if you want to start the motor power is quite large, and the pump operation is hurt. By using a different fore pump can be applied to different environments. If equipped with a water ring vacuum pump, you can pump out a large amount of condensable vapor and corrosive gases (dilution of corrosive gas by water concentration). At present, the common forecourt pumps are: water ring vacuum pump, rotary vane vacuum pump, vertical oil-free vacuum pump, reciprocating vacuum pump, H, 2H slide valve vacuum pump and so on. Through the choice of different fore pump can get a different degree of vacuum and pumping rate. Third, the choice of the previous stage pump: the choice of the first stage pump: the main system of the vacuum pump selected, the important question is how to match the appropriate pre-pump and pump. Normally, the foreline pump directly affects the pumping performance of the main pump and affects the pumping time and economic efficiency of the vacuum system. Pre-stage pump with the following points should be followed: (1) before the pump should be able to promptly discharge the main pump of the gas flow. (2) The pressure at the outlet of the main pump (such as diffusion pump, oil booster pump, molecular pump and Roots pump) should be lower than the maximum discharge pressure of the main pump. (3) The pre-pump that doubles as the pre-pump should meet the pre-pumping time requirements. Elected oil vapor pump as the main pump, the method with the former pump can be based on empirical standards recommended before the size of the pump to determine. After the pre-stage pump is determined, the pre-stage pipeline valve and pre-pumping pipeline valve can be selected according to the size of the inlet of the foreline pump, so as to determine the size of the part-connecting pipeline. Based on the above determination, can draw a vacuum system design. When the molecular pump as the main pump, its suction capacity is closely related to the suction capacity of the foreline pump. The first stage of the molecular pump needs to maintain the molecular flow state, it can work stably. (4) Select the pump speed before the pump. Roots pump as the main pump, due to Roots pump rotor and rotor, rotor and stator gap between the larger, so it is relatively small gas compression, the general prefix pump to be larger. Oil seal mechanical pump or water ring pump can usually be used as the front-stage pump of Roots pump. The pumping speed of fore-stage pump can be selected according to empirical formula (48). Fourth, the relationship between the Roots pump and the fore pump The performance of the vacuum unit is closely related to the performance of the Roots pump, and Roots pump performance with the previous stage of the pump is different. (1) Because Roots pump rotor and rotor, there is a gap between the rotor and the shell, so there is a regurgitation, and this regurgitation by the inlet pressure and outlet pressure of the impact, even the same Roots Pumps, when used with different forepumps, will have a different pumping rate. Roots pump pumping rate can be determined by the following formula: δ = δ0 (P2 / P1 / K) Where: δ0-design of the pumping rate; P1-inlet pressure; P2- outlet pressure; K- intrinsic constant, Pump rotor shape, the amount of clearance, the rotor peripheral speed and outlet pressure to determine. As can be seen from the above formula, the amount of suction is affected by the ratio of the outlet pressure to the inlet pressure. That is, if the suction rate of the foreline pump is increased, the pumping rate of the Roots pump also increases. (2) The ultimate pressure is determined by the pumping speed of the pump, the return flow of each clearance, the amount of pump leakage, and the amount of deflation on the high vacuum side. That is: P0 = (Q1 + Q2 + Q3) / δ Where: P0 - the ultimate pressure; δ - pumping rate; Q1 - return flow; Q2 - leakage; Q3 - deflation. Among these parameters, Q1 is greatly affected by the exhaust pressure, ie, the ultimate pressure of the foreline pump. The pressure of the Roots pump varies with the saturated vapor pressure of the water ring when the foreline pump is used as a water ring. With the same Roots pump with different front pump performance comparison shows that the former stage of the pump when the ultimate vacuum is higher, the unit of the ultimate vacuum will also increase; two Roots pump in series, you can increase The ultimate vacuum of the unit (essentially the former Roots pump is the fore pump of the next Roots pump), and the performance curve flattens out, ie the range used expands (represented by curves 1 and 2, curves 3 and 5 More). The curves for units 1 and 2 are roughly the same. Similarly, the curves for units 3, 4 and 5 are the same. However, 1,2 units curve and 3,4,5 unit curve is completely different from the two sets of curves. This shows that for the same Roots pump, the choice of different fore pump, the performance curve of the unit is essentially different. This shows that the fore pump on the unit performance has a considerable impact.