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Before the roots vacuum pump unit pump _ how to choose
roots vacuum pump cannot be used alone. Must be equipped with backing pump. Before the class is used to reduce the roots vacuum pump vacuum pump discharge pressure. Due to the compression ratio of the roots vacuum pump is restricted, so choose the front-end pumping speed of vacuum pump.
it is important that the roots vacuum pump the maximum compression ratio varies with discharge pressure. Under the high discharge pressure, the maximum compression ratio is as high as 3:1, maximum compression ratio of 1 to 10 mbar. In between, 1 mbar = 0。 75 Torr） To 50:1 between, even higher. Therefore, we should choose according to the scope of the roots vacuum pump long-term work pressure pump pumping speed.
, for example, there is a system using the roots vacuum pump gear pump air, under the inlet pressure of 1 millibars requires 600 litres per second pumping speed, and work for a long time, ask before the pump should be much choice.
the first hypothesis stage pump before use 30 liters per second, when K = 20, compression ratio K = 600:30 = 20, starting from the above, the maximum discharge pressure is 30 millibars.
therefore, when using 30 liters per second stage pump, in less than 1. 5 millibars work under the inlet pressure is feasible, but when in higher than 1. 5 millibars operation under the inlet pressure of pump before use more. In this case, the vacuum for work in 1. 1 under five millibars millibars, so before 30 liters per second stage pump can meet the requirements of the vacuum system of pump.
in addition, you should consider if working time is longer than normal time, you can choose the smaller pump before. If the vacuum chamber is very big and need of pumping air into vacuum state, then you need to use larger backing pump, sometimes in order to better use of the efficiency of the equipment, the device is equipped with a bigger pump, this is stopped during normal operation, and use smaller backing pump for maintenance, to save electricity.
when choosing backing pump, also must consider the ultimate pressure of the roots vacuum pump device. For the ultimate vacuum ( Up to 1 x10 3 ~ 1 x10 - 2 pa pressure) , two stage rotary vane seal mechanical pump or two stage valve mechanical pump as the backing pump. Extreme vacuum need a single-stage oil seal mechanical pump as ( 1 x10 - 2 pa and 1 x10 1Pa） Between the backing pump. Ultimate vacuum requirements between 133 and 1333 pa. You can use the reciprocating vacuum pump or liquid ring vacuum water pump as the front.
was elected with a reciprocating or thick liquid ring type vacuum pump is regarded as the backing pump of the roots vacuum pump, pumping speed of pump before 1/2 - general for roots vacuum pump A quarter, cannot too big, otherwise will exceed the compression ratio. Great, cause the roots vacuum pump exhaust temperature is very high, can't work normally.
when choosing a backing pump must also consider pumping gas in the presence of corrosive medium, whether they contain condensable gases, whether big or small. If the containing corrosive gas, the oil seal mechanical pump cannot be used. Oil seal mechanical pump is not suitable for pumping out contains a lot of gas condensing steam. Otherwise, the sealing liquid will fast emulsion, and will remove contain traceable gas condensing steam. At the same time, also must use a device with a gas ballast oil seal mechanical pump as the backing pump.
in order to lower the inlet pressure ( 1 ~ 100 pa) To acquire high pumping rate, roots vacuum pump can be used as another roots vacuum pump before pump level 3 or 4 units. At this time, the roots vacuum water pump pumping speed between generally between 2 and 5.
in general, choose the backing pump principle is as follows:
1. Needed by the former stage pump operation pre vacuum conditions needed for the main pump.
2。 Allow maximum exhaust pressure in the main pump and backing pump must be timely discharge of main pump discharge the air volume. Before the pump pumping speed must be with the pump in the actual inlet pressure under the same pumping speed.
3。 Before making a pre pump and pump should meet the requirements of the drainage time and vacuum degree.
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