Backing pump selection
Rotary vane pumps
If there will be no negative impact on function due to the process, a rotary vane vacuum pump is the most cost-effective backing pump for a Roots vacuum pumping station. Rotary vane vacuum pumps have ultimate pressures of around p < 1 mbar over a broad pressure range at constant pumping speed. A Roots vacuum pumping station achieves ultimate pressures of approximately 10-2 mbar with the gas ballast valve open.
Water vapor can be extracted with these kinds of pumping stations, as well as many solvent vapors and other vapors that have sufficiently high vapor pressures and do not chemically decompose the pump oil (alcohols, halogenated hydrocarbons, light normal paraffins, etc.).
Liquid ring vacuum pumps
Liquid ring vacuum pumps are a suitable solution for extracting vapors that chemically attack and decompose the backing pump oil or that have such low pressure that condensation in the backing pump cannot be avoided, in spite of gas ballast. However they will only achieve an ultimate pressure that is determined by the vapor pressure of the operating fluid. If 15 °C water is used, an ultimate pressure of approximately 20 mbar can be expected at the liquid ring vacuum pump, and it is then already working in the cavitation area.
A cavitation-free liquid ring vacuum pump working through the addition of air achieves an ultimate pressure of approximately 25 to 30 mbar, and a combination of Roots pump and liquid ring pump reaches approximately 1 mbar. A liquid ring pump should not be used with fresh water when evacuating environmentally harmful substances. In this case, a closed circulation system must be provided to advance a suitable operating fluid over a cooled heat exchanger in order to extract the heat of compression.
Liquid ring vacuum pump with gas jet device
The combination of Roots vacuum pump, gas jet device and liquid ring vacuum pump achieves an ultimate pressure of 0.2 mbar. If lower pressures need to be achieved, an additional Roots vacuum pump must be connected upstream.
Gas-cooled Roots vacuum pumps
Since Roots vacuum pumps are technically dry pumps, their employment is advisable when pumps with liquid-tight suction chambers cannot be used.
Their applications include:
- Extracting and compressing helium on cryostats
- Extracting and compressing SF6
- Clean recovery of a wide variety of gases and vapors in a wide variety of processes, e.g. distillation
- Evacuating molecular sieves, etc.
- Pumping down and displacing toxic substances in closed circulation systems
- Evacuating extremely large-volume vessels
Roots pumping stations with gas-cooled Roots pumps can be configured with a wide variety of inlet characteristics. In extreme cases, it is possible to achieve a virtually constant pumping speed throughout the entire pressure range of 1 bar to 10-3 mbar, and the individual pump stages can be selected in the ratio of 2:01 to 3:01.
To do this, however, the Roots vacuum pumps must be equipped with correspondingly powerful motors, and outlet valves to the atmosphere must be provided instead of overflow valves.
With the HeptaDry screw pumps, a complete line of technically dry pumps is available that offer pumping speeds of 100 to 600 m3/ h. As stand-alone pumps (see also Figure 2.10), they cover an extensive pressure range in the low and medium vacuum segments. Due to their internal compression, they can work continuously with relatively low drive power throughout the entire inlet range of 0.1 to 1,000 mbar. In combination with OktaLine Roots pumps, it is even possible to achieve ultimate pressures of 5 ·10-3 mbar.
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Pfeiffer Vacuum GmbH
Berliner Strasse 43
Pfeiffer Vacuum GmbH
T +49 6441 802-0
F +49 6441 802-1202