Roots vacuum pump (except straight-line atmospheric Roots pump) need to be combined with the previous stage pump Roots pump vacuum unit used in various fields. According to different uses, Roots pump units commonly used in the former pump rotary vane pump, slide valve pump, water ring pump. The pumping speed of the vacuum unit after the combination of Roots pump and various front-stage vacuum pumps can be calculated and neglected in the following calculation. The effective gas flow rate of Roots pump is: Qe = Qth - Qv (5.2) where Qe is the effective flow of Roots pump, Pa L / s Qth is the geometric flow of Roots pump, Qth is equal to PA Sth Qv a Roots pump leakage backflow Qv = Qv1 + Qv2 Qv1 is due to the Roots pump rotor and the rotor and the pump housing clearance between the formation of gas reflux, Qv1 can be expressed as follows: Qv1 = U (Pv - PA) (5.3) where U - the equivalent conduction of all the clearances in the Roots pump Pv - the exhaust pressure of the Roots pump (pressure of the pump beforehand) PA - the suction pressure of the Roots pump Qv2 is Roots pump rotor adsorption on the high-pressure exhaust side and carry the amount of gas returning to the low-pressure suction side, called back to diffuse gas volume, so there: Qv2 = Sr · Pv (5.4) Where Sr ─ pump pumping diffusion gas equivalent pumping speed. Then equation (5.2) can be expressed as: Qe = Qth - (Qv1 + Qv2) = PA · Sth - [U (Pv - PA) + Sr Pv] Intake line, the gas flow is zero, the front vacuum line pressure and pump inlet pressure ratio of zero flow compression ratio K0 = Pv / PA, the maximum value of the compression ratio with K0max, said the maximum zero flow compression ratio. (5.5) where Qe equals zero (PASth - [U (Pv - PA) + Sr Pv] = 0 (5.6) using Equation (5.6) It is difficult to stop counting K0max. First of all, because K0max is related to the equivalent conduction U, which is related to the rotor clearance in the pump. The rotor clearance is related to a series of factors, such as rotor machining accuracy, pump tolerance and machining accuracy, pump device precision and bearing clearance. In addition, the impact of K0max Sr value and rotor surface accuracy, the appearance of each pump rotor can not be processed in the complete disagreement. Therefore, after all, the K0 curve and the K0max value are obtained through the actual measurement. Roots pumping unit in the practice of pumping the following relationship: Qe = PA · Se = Pv Sv = PA Sth - [U (Pv - PA) + Sr Pv] (5.7) where Se ─ ─ Roots pump unit is valid Pumping speed Sv ─ ─ practice before the pump pumping speed, which changes with the pressure changes (5.7) simplification: (Sth + U) PA = (Sv + U + Sr) Pv (5.10) Then: (5.10) Since Sth "U, let: Kth = Sth / Sv (5.11) According to the formula (5.6) we have: 1 / Ke = 1 / Kth + 1 / K0max (5.12) The volumetric efficiency η represents the ratio of the effective pumping speed Se of the Roots pump (Roots pump) to the theoretical pumping speed Sth of the Roots pump (unit). From equations (5.9) and (5.11) (5.13) Therefore, when K0max, Sth, Sv are known in the unit, η can be obtained through the formula (5.13), then the maximum pumping speed of the Roots pump unit at this time is obtained. Se = ηSth In practice, the effective pumping speed of the Roots pump in the pump unit is the pumping speed of the unit. From the above analysis we can see that it is not only related to the theoretical pumping speed of the Roots pump, but also the maximum zero flow reduction ratio K0max of the pump and the actual pumping speed Sv of the preceding pump. For a particular type of Roots pump, the Sth value has been set and the K0max value has been determined experimentally. Consumers should give a K0 curve for the pump on the product catalog and should specify the range of K0 values. The K0max of the pump consumed must be greater than or equal to the rule value. Roots pump unit before the pump and the main pump ratio is also very important. If the front-stage pump is used improperly, the effective pumping speed of the unit can not meet the requirements of the rules of Roots direct-air pump technical conditions (ZBJ78013 · 2 - 89). Based on common experience, the ratio of the theoretical pumping speed of the Roots pump to the theoretical pumping speed of the fore-stage pump is 5: 1~8: 1.
