The relationship between the pumping speed of the Roots pump
and the pumping speed of the backing pump mainly depends on the volumetric efficiency and the pressure difference allowed by the Roots pump. If the volumetric efficiency is low, the effective pumping speed of the Roots pump unit will drop significantly. If the pressure difference of the Roots pump exceeds the allowable maximum pressure difference, the rotor will overheat and expand due to the compression work, and it will be stuck. Because the pump casing is easy to dissipate heat to the surrounding environment, it is still not too hot.
The maximum allowable pressure difference range of Luoci pump is 40-100hPa. In the higher pressure range (150hPa), the allowable pressure difference is slightly higher, because the pumped gas volume is large, which improves the cooling of the rotor. Due to the increase in pressure difference, the required power will also increase.
A special gas cooler is installed in the outlet pipeline of the Roots pump, and the maximum allowable pressure difference can be increased at this time, because the return flow of the cooling gas can keep the temperature of the rotor within the specified range.
In the range of vacuum and high vacuum, the pumping speed ratio is mostly 10:1. When the inlet pressure changes greatly or the front stage pressure becomes smaller, the compression ratio will decrease, so the pumping speed ratio should be 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 allowable pressure difference is exceeded when starting, the general Roots pump is overloaded for a short time (within a few minutes). The motor will not have a problem. When the inlet pressure is very low or the backing pump gas ballast valve is working, a two-stage backing pump is usually used, otherwise the backing pressure required by the Roots pump is difficult to achieve, and the pumping speed of the backing pump under this pressure got too low. The compression ratio will be too high and the volumetric efficiency will be reduced.
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 backing pump is 6:1 when the inlet pressure is 15hPa, and 2:1 when the inlet pressure is 75hPa. In the rough vacuum range, the pumping speed ratio between the Roots pump and the backing pump is, To determine each case separately, generally speaking, is relatively small. And the volumetric efficiency is quite good.