18.104.22.168 Backing pump selection
Rotary Vane Pumps
If a negative impact on the function is unlikely 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 hPa over a broad pressure range at constant pumping speed. A Roots vacuum pumping station achieves ultimate pressures of approximately 10-2 hPa 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. Examples of these include alcohols, halogenated hydrocarbons, and light normal paraffin as well as many others besides.
Figure 4.19: Pumping speed of pumping stations with Okta 2000 and various backing pumps
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 vapor 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 hPa can be expected at the liquid ring vacuum pump, and it is then already working in the cavitation range. Cavitation occurs near the ultimate pressure of the pump. The operating fluid vaporizes on the intake side and the vapor bubbles suddenly collapse on the pressure side. This destroys the pump in the long term. A liquid ring pump which operates cavitation-free through an air supply attains an ultimate pressure of approximately 25 to 30 hPa and a combination of a Roots pump and a liquid ring pump achieves a presssure of about 1 hPa. A liquid ring vacuum 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 hPa. If lower pressures need to be achieved, an additional Roots vacuum pump must be connected upstream.
Gas-circulation-cooled Roots vacuum pumps
Since Roots vacuum pumps are technically dry pumps, their use is advisable when pumps with liquid-sealed suction chambers cannot be used.
Their applications include:
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 1,000 hPa to 10-3 hPa, and the individual pump stages can be selected in the ratio of 2:1 to 3:1. 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-1. As stand-alone pumps (see also Chapter 4.4), 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 pressure range of 0.1 to 1,000 hPa. In combination with OktaLine Roots pumps, it is even possible to achieve ultimate pressures of 5 · 10-3 hPa.
Multi-stage Roots Pumps
Multi-stage Roots pumps in the ACP range make for compact pumping stations with a pumping speed of up to 285 m3 · h-1. Combining an ACP backing pump and a Roots pump makes it possible to achieve final pressures of up to 5 · 10-3 hPa.
Corrosive gas versions of Roots pumping stations are described in Chapter 4.6.