4.6.1 Design / Operating principle
As with the pumps described in the preceding section, the Roots vacuum pumps described in this chapter belong to the category of technically dry-running rotary displacement vacuum pumps. In this section we are dealing with pumps for corrosive processes and the pumps derived from these for load locks and transfer chambers.
Figure 4.11: Operating principle of a multi-stage Roots pump, process pump
In the pump, two synchronously counter-rotating rotors (1) rotate contactlessly in a single housing (2) (Figure 4.11). The rotors have a figure-eight configuration and are separated from one another by a narrow gap. Four to six pairs of rotors are located on the rotor shafts. Each rotor cavity is separated from the others by stator disks with a gas orifice. The gas conveyed is pumped from the inlet port (3) to the outlet port. The vertical pumping direction is always important in process pumps. The space between the various stages in the outfall channels can be used as particle traps as a result. This is the best way to avoid blockage of the pump.
Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotation speeds of up to 6,000 rpm. The symmetrical distribution of the rotor mass around the shaft axis also results in perfect dynamic balancing, which means that the pump operates extremely quietly in spite of its high speeds.
To avoid the condensation of chemicals in the pump and the silencer, these can be tempered by regulating the cooling water flow or heated electrically with heating sleeves. If the outlet silencer is arranged separately, this requires an additional heating sleeve. Integrating the outlet silencer directly on the pump block not only reduces energy costs by dispensing with an additional heater but also makes installation easier.