Vacuum pump combinations of liquid ring and rotary piston compressors

Is the pressure in the vacuum system less than the pressure which one vacuum pump can reach, connecting two or more vacuum pumps where in series, will enable reaching the required vacuum pressure. By connecting vacuum pumps in series, it is possible to reach pressure in the high vacuum between p = 0, 1 Pa to 10-5 Pa or in the ultrahigh vacuum. For low or medium vacuum with pressures between p = 100 kPa to 0,1 Pa, this is a range of six decimal powers, series combinations of the following vacuum pumps types are used e.g.: piston vacuum pumps in combination with diaphragm vacuum pumps or rotary vane vacuum pumps or liquid ring compressors or rotary piston compressor. The following article will describe the vacuum pump combination with an liquid ring compressor and an rotary piston compressor, which has attracted only a little interest up to day. This combination has importance in process engineering for exhaust of gases and vapours.