Working Principle and Structural Characteristics As mentioned in the “Overview”, the DVBS type Roetz water-ring vacuum pump unit is composed of one or two Roetz vacuum pumps and one screw vacuum pump connected in series. The specific structure is shown in the following figure. Among them, the screw pump is called the pre-stage pump, the Roetz vacuum pump directly connected to the suction system is called the main pump, and the Roetz vacuum pump connected to the pre-stage pump and the main pump is called the intermediate pump.