How to choose Roots vacuum pump before the pump

Roots vacuum pump can not be used alone, must be equipped with front-stage pump unit, the use of front-stage vacuum pump is to reduce the Roots vacuum pump discharge pressure, the Roots vacuum pump compression ratio is limited, so choose the front vacuum pump speed Importantly, the maximum compression ratio of Roots vacuum pumps varies with the discharge pressure, with a maximum compression ratio of up to 3: 1 at high discharge pressure and a maximum compression ratio of between 1 and 10 mbar at a discharge pressure of 1 mbar = 0.75Torr) up to 50: 1, even higher. Therefore, the choice of pump pumping speed, Roots vacuum pump according to the long-term working pressure range to consider. For example, a system with roots vacuum pump unit pumping, requiring inlet pressure of 1 mbar with 600 liters / s pumping speed and long-term work, asked should choose how much pumping speed of the foreline pump. Assume that with a 30 L / sec foreline pump, the compression ratio K = 600: 30 = 20 and the maximum discharge pressure at K = 20 is 30 mbar. This concludes that it is possible to operate at inlet pressures below 1.5 mbar with a 30 L / s foreline pump, but larger inlet pumps at 1.5 mbar inlet pressure are required. In this case, the working vacuum of 1 mbar is less than 1.5 mbar, so a 30 L / s forepump pump meets the suction requirements of the above vacuum system. In addition should be considered is that the pre-pumping time requirements, if the normal working time is much longer than the pre-pumping time, optional smaller fore pump; if the vacuum chamber is larger, and requires fast pre-pumping, it is necessary to use Larger foreline pump, sometimes in order to give full play to the efficiency of the equipment, the unit is equipped with a larger pre-pumping, stopping it after normal work, but with a smaller fore pump to maintain, this is to save power . Roots vacuum pump unit of the ultimate pressure requirements, but also the choice of the foreline pump must be considered, the ultimate vacuum demanding (up to 1X10-3Pa ~ 1X10-2Pa pressure) generally use double-stage rotary vane seal mechanical pump or Double-stage slide valve mechanical pump as the fore-stage pump. The ultimate vacuum requirements (1X10-2Pa ~ 1X10-1Pa) between the optional single-stage oil seal mechanical pump as the foreline pump, the ultimate vacuum requirements between 133 ~ 1333Pa optional reciprocating vacuum pump or liquid ring vacuum pump as the front Pump. Elected reciprocating or liquid ring rough vacuum pump as the Roots vacuum pump before the pump, the pump speed before the pump is generally Roots vacuum pump 1 / 2-1 / 4, not too large, otherwise it will make the compression ratio too Large, resulting in Roots vacuum pump exhaust temperature is high and can not work properly. Selection of the foreline pump must also be considered whether there is corrosive media being pumped, whether it contains condensable gas, is a large amount or trace. If the corrosive gas, you can not use the oil seal mechanical pump, oil seal mechanical pump is not suitable for pumping gas containing a large amount of condensable steam, otherwise it will seal fluid quickly emulsified, with a small amount of condensable steam pumping gas , It is also necessary to use the oil seal mechanical pump with gas ballast as the foreline pump. In order to obtain a high suction rate at low inlet pressures (1 to 100 Pa), a Roots vacuum pump can be used as a three or four-stage unit consisting of a front-stage pump of another Roots vacuum pump. In this case, a Roots vacuum pump The pumping ratio between the general in 2 ~ 5 between.