Roots pump is a kind of positive displacement pump. Its pumping speed and limit pressure are constant for various gases. When the molecular weight of the extracted gas (he and H2) is lower than that of nitrogen, a slight increase in the leakage of the gap will cause a slight decrease in the pumping speed.
There is a maximum pumping speed in the range of 1-0.1 HPA of inlet pressure. In the case of higher hole and lower pressure, the pumping speed slightly decreases due to the larger effect of backflow. The cavity of roots pump is dry and oil-free, so the pressure corresponding to the pumping curve is full pressure.
The ultimate full pressure of roots pump unit is related to the type of pre pump selected. Generally, 0.001hpa is affected by the steam pressure of the pump oil of the front stage pump and the steam pressure of the lubricating oil at the gearbox and bearing.
The space with oil on Roots pump shall be connected with the vacuum side of the front stage, and the oil deterioration caused by the load of gear and bearing shall be removed by the front stage pump without any influence on the inlet of roots pump. When working at a higher pressure, a higher pressure difference will send oil through the shaft seal and into the pumped space. In order to avoid this phenomenon, a small vacuum pump can be set in the oil-bearing space to pump out, so as to ensure the cleanness of the pump cavity.
The lubricating oil of roots pump usually uses mechanical pump oil or diffusion pump oil with low steam pressure. Use diffusion pump oil under low load. When the liquid ring pump is used as the front stage pump of roots pump, the pumping speed and limit pressure of roots pump depend on the liquid used in the liquid ring pump. The limiting pressure of water ring pump is 20-30hpa, which depends on the temperature of water supply. The limit pressure of roots pump unit can reach 1 HPA, which is determined by the water vapor from water ring pump. If the front stage pump with gas injector is used, the limit pressure of roots pump unit is 0.1hpa. Because of the operation of the gas ejector, it has a blocking effect on the backflow of water vapor.
Generally speaking, it is more common to use a Roots pump and a single-stage front-end pump group to become a unit, but when lower dangerous pressure and higher pumping speed are required, it is often used to combine multiple roots pumps in series and then add the front-end pump to form a unit.
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