# The capacity relationship between roots pump and fore pump

The relationship between roots pumpcapacity and the pumping speed of the former pump depends on the volume efficiency and the pressure difference of the roots pump license. If the volume efficiency is low, the effective pumping speed of the roots pump system will decrease obviously. If the pressure difference of the roots pump exceeds the maximum allowable pressure difference, due to the compression work, the rotor will overheat and expand, and it will be jammed.

The maximum allowable pressure difference range of the roots pump is 40-100hpa, which is slightly higher when the pressure range (150hPa) is high, because the amount of gas pumped is appreciative, and the rotor cooling is improved. Due to the increase of pressure difference, the required power will be increased.

A special gas cooler is installed at the outlet pipe of the roots pump, where the maximum allowable pressure difference can be increased, as the return of the cooling gas can keep the rotor temperature within the specified range. In the vacuum and high vacuum range, the speed ratio is 10:1. When the inlet pressure has a great change or the pressure of the front stage becomes smaller, the compression ratio is reduced. Therefore, the ratio of the extraction speed is 5:1.

1. when the roots pump starts under high pressure, pressure drop quickly and has reached the requirements of the pressure, although more than the maximum permissible when starting pressure difference, but a short time overload (within minutes) general roots pump motor will not occur. The inlet pressure is very low or backing pump gas ballast valve work, usually using two-stage pump, otherwise the roots pump is difficult to achieve the required level before pressure, and, under pressure from the former stage pump suction someone who comported himself too low. Will result in compression ratio too high, volume efficiency reduced.

2.When the inlet pressure of the pump is in the range of coarse vacuum, this is the maximum permissible pressure difference and becomes an important factor. Roots pump and backing pump pumping speed ratio, the inlet pressure is 15 hpa 6:1, constantly to the inlet pressure for 75 hpa is 2:1. Within the scope of the coarse vacuum, roots pump and the backing pump pumping speed ratio, separately for each case to determine, in general, is relatively small. And volume efficiency is pretty good.

Edited by: Lany SHEN (EVP Vacuum Solution)

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