The invention is directed to a rough pumping method for a displacement pump as well as to a displacement pump device for establishing a rough differential pressure.
In the present context, a rough differential pressure is understood to be a negative differential pressure in the sense of a rough vacuum or a positive differential pressure in the sense of an application of rough pressure. A typical rough vacuum has a magnitude of up to 500 mbar of differential pressure and typically ranges from 100 to 300 mbar of differential pressure. For a large variety of applications there is a great need for rough vacuum pumps that are mostly designed as single-shaft centrifugal compressors or as side channel blowers. Side channel blowers have a defined volume flow capacity and must continually be operated at a continuously high rotational speed. They operate based on the principle of torque transmission according to Euler's energy equation for compressible fluids. For the generation of a correspondingly low volume flow, side channel blowers must be operated at their full volume flow capacity, even if a large differential pressure exists between the inlet and the outlet of the compressor or blower. The power required by the compressor is proportional to the volume flow capacity, the theoretically required minimum power for compressing and transporting a small gas flow being proportional to the actual volume flow capacity. Due to this difference between the actual power output and the power physically required for compressing the gas, the use of such conventional rough vacuum compressors is inefficient.
Displacement pumps, such as a Roots pump, for example, are particular effective in maintaining low pressures with no large volume flows being conveyed, or in generating small differential pressures. For generating a rough vacuum with a large differential pressure, displacement pumps, such as Roots pumps, for example, are presently not employed.