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Due to their low compression ratios, Roots pumps must always be operated as pump combinations<br />
for vacuum generation. Their achievable final pressures will be a function of the<br />
ultimate pressures of the selected backing pumps. Due to gas transport through adsorption,<br />
it is no longer practical to use Roots pumps in the range below 10 - 4 mbar. The behavior of<br />
the pumping speed and ultimate pressure of pumping stations with various backing pumps is<br />
shown in Figure 2.15. The curves clearly show that the pumping speed of this kind of pump<br />
combination rises by a factor of 8 and its ultimate pressure reduces by a factor of 15 relative<br />
to the backing pump.<br />
2.6.3.1 Backing pump selection<br />
Rotary vane pumps<br />
If there will be no negative impact on function due to the process, a rotary vane vacuum<br />
pump is the most cost-effective backing pump for a Roots vacuum pumping station. Rotary<br />
vane vacuum pumps have ultimate pressures of around p < 1 mbar over a broad pressure<br />
range at constant pumping speed. A Roots vacuum pumping station achieves ultimate pressures<br />
of approximately 10 - 2 mbar with the gas ballast valve open.<br />
Water vapor can be extracted with these kinds of pumping stations, as well as many solvent<br />
vapors and other vapors that have sufficiently high vapor pressures and do not chemically<br />
decompose the pump oil (alcohols, halogenated hydrocarbons, light normal paraffins, etc.).<br />
Liquid ring vacuum pumps<br />
Liquid ring vacuum pumps are a suitable solution for extracting vapors that chemically attack<br />
and decompose the backing pump oil or that have such low pressure that condensation in<br />
the backing pump cannot be avoided, in spite of gas ballast. <strong>How</strong>ever they will only achieve<br />
an ultimate pressure that is determined by the vapor pressure of the operating fluid. If 15 °C<br />
water is used, an ultimate pressure of approximately 20 mbar can be expected at the liquid<br />
ring vacuum pump, and it is then already working in the cavitation area.<br />
A cavitation-free liquid ring vacuum pump working through the addition of air achieves an ultimate<br />
pressure of approximately 25 to 30 mbar, and a combination of Roots pump and liquid<br />
ring pump reaches approximately 1 mbar. A liquid ring pump should not be used with fresh<br />
water when evacuating environmentally harmful substances. In this case, a closed circulation<br />
system must be provided to advance a suitable operating fluid over a cooled heat exchanger<br />
in order to extract the heat of compression.<br />
Liquid ring vacuum pump with gas jet device<br />
The combination of Roots vacuum pump, gas jet device and liquid ring vacuum pump achieves<br />
an ultimate pressure of 0.2 mbar. If lower pressures need to be achieved, an additional Roots<br />
vacuum pump must be connected upstream.<br />
Gas-cooled Roots vacuum pumps<br />
Since Roots vacuum pumps are technically dry pumps, their employment is advisable when<br />
pumps with liquid-tight suction chambers cannot be used.<br />
Page 51<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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