The relationship between the Roots pump and the pumping speed of the foreline pump
The relationship between the pumping speed of the Roots pump and the pumping speed of the foreline pump mainly depends on the volumetric efficiency and the pressure difference allowed by the Roots pump. If the volumetric efficiency is low, the Roots pump unit's effective pumping speed
It will drop significantly. If the pressure difference of the Roots pump exceeds the maximum pressure difference allowed, the rotor will overheat and expand due to the compression work and it will be stuck. Because the pump casing is easy to ring around
Heat is still not too hot.
The maximum permissible differential pressure of LUO Si pump is in the range of 40-100 hPa. In the higher pressure range (150 hPa), the allowable pressure difference is slightly higher because of the amount of gas being pumped thanks to the improved rotor.
The cooling. Due to the increase in pressure difference, the required power must be increased.
A special gas cooler is installed in the outlet pipe of the Roots pump. The maximum allowable differential pressure can be increased because the return flow of the cooling gas keeps the temperature of the rotor within the specified range.
Inside.
In the vacuum and high vacuum range, the pumping speed ratio is often 10:1. If there is a large change in the inlet pressure or the pressure in the fore stage becomes smaller, the compression ratio will decrease, so the pumping speed ratio is more than 5:1.
1) When the Roots pump is started at a higher pressure, the pressure drops quickly and reaches the required pressure. Although the maximum permissible pressure difference is exceeded at startup, the overload is short-term (
Within minutes the general Roots pump motor will not have problems.
When the inlet pressure is very low or the pre-stage pump ballast valve is working, a two-stage foreline pump is usually used, otherwise the required fore-stage pressure of the Roots pump is difficult to reach, and, under this pressure, the fore stage
The pumping speed becomes too low. This will cause the compression ratio to be too high and the volumetric efficiency to decrease.
2) When the pump inlet pressure is in the rough vacuum range, this is the maximum allowable pressure difference becomes an important factor.
The pumping speed ratio between the Roots pump and the fore pump is 6:1 when the inlet pressure is 15 hPa, and 2:1 when the inlet pressure is 75 hPa. In the rough vacuum range, the pumping speed ratio between the Roots pump and the fore pump,
It is necessary to determine each situation separately, and generally it is relatively small. And the volumetric efficiency is quite good.
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